Title of Invention

ROTATIONAL ANGLE SENSING DEVICE

Abstract

Abstract A yoke main body (21) and a bent piece (31) of a first yoke segment (4, 206) may be opposed to a yoke main body (22) and a bent piece (32), respectively, of a second yoke segment (5, 207). The bent pieces (31, 32) of the First and second yoke segments (4, 5) hold the rotational angle sensor (3) There between in a plate thickness direction of the bent pieces (31, 32). A linear Stance, which is measured between a rotational axis of a magnet (203) at a Reference position (C) and a furthermost point (225) of an outer surface of the magnet (203) relative to a magnetic sensing element of a sensing unit (204), •nay be generally equal to a linear distance between the rotational axis of the nagnet (203) at the reference position (C) and a distal end (238, 239, 248, 249) Df a yoke opening end portion (237, 247) of the yoke segment (206, 207).

Full Text

ROTATIONAL ANGLE SENSING DEVICE Description The present invention relates to a rotational angle sensing device. As shown in FIG. 22, a previously proposed rotational angle sensing device described in US-5164668B includes a magnet 101, a rotational angle sensor 102 and an open type yoke. The magnet 101 is fixed to an axial end of a rotatable shaft of a sensing object, such as a throttle valve. The rotational angle sensor 102 senses a rotational angle of the sensing object by using an output change characteristic of a magnetic sensing device (Hall IC) of the rotational angle sensor 102 with respect to a rotational angle of the magnet 101. The open type yoke forms a magnetic circuit (for example, an open magnetic path) in corporation with the magnet 101 and the rotational angle sensor 102. The rotational angle sensor 102 has the magnetic sensing device fixed on a flat board 103. In addition, the open type yoke includes two magnetic bodies, i.e., first and second magnetic bodies 104, 105. The first and second magnetic bodies 104, 105 are arranged symmetrically about an imaginary center plane, which is perpendicular to a rotational axis of the rotatable shaft of the sensing object. Each of the first and second magnetic bodies 104, 105 includes a yoke main body 111 and a projection 112. The yoke main body 111 forms a gap between the yoke main body 111 and the magnet 101. The projection 112 projects toward a side of the rotational angle sensor 102 from an end edge of the yoke main body 111. The first and second magnetic bodies 104, 105 are parallel to each other in a plate thickness direction thereof and are parallel to a rotational axis of the sensing object. Furthermore, the first and second magnetic bodies 104, 105 are respectively opened at one side thereof. Distal end portions of the projections 112 of the first and second magnetic bodies 104, 105 are opposed with each other in such a manner the distal end portions of the projections 112 are spaced from each other by a magnetic flux sensing gap. In addition, the magnet 101 is rotatable relative to the yoke main body 111 of each of the first and second magnetic bodies 104,105. The rotational angle sensor 102 is disposed inside the magnetic flux sensing gap formed between the opposing portions of the respective projections 112, and a gap is formed between the rotational angle sensor 102 and the opposing portion of each of the projections 112. In the rotational angle sensing device described in US-5164668B is provided with the open type yoke of the open magnetic path type having the symmetrical configuration. In a case of such a symmetrical magnetic body (open type yoke), it is difficult to highly accurately manage a gap between the magnet 101 and the open type yoke (the first and second magnetic bodies 104, 105) and a gap between the rotational angle sensor 102 and the open type yoke (the first and second magnetic bodies 104, 105) simultaneously. Therefore, these gaps may disadvantageous^ vary from product to product, and thereby characteristics may vary from product to product. Also, in the case of an open type yoke of the open magnetic path type (the first and second magnetic bodies 104, 105), when an external magnetic field or an external magnetic field source (for example, alternator (AC generator) mounted in a vehicle or the like) or a magnetic body (for example, fastening bolt or bracket of iron based metal or the like) is disposed close to the rotational angle sensor 102, it raises the following disadvantage. That is, due to the influence from the external magnetic field or the magnetic body to the magnetic sensing device, the output change characteristic of the magnetic sensing device with respect to the rotational angle of the magnet 101 largely changes. In addition, in a case of the open type yoke of the open magnetic path type (the first and second magnetic bodies 104, 105), the distal end portion (yoke opening end portion) of the yoke main body 111 of each of the first and second magnetic bodies 104, 105 is opened. Therefore, the following disadvantage may occur. That is, the magnetic sensing device tends to be easily influenced by radio, wave noises, so that the output change characteristic of the magnetic sensing device with respect to the rotational angle of the magnet 101 largely changes. Other disadvantages of the previously proposed rotational angle sensing device will now be described in view of US-6707292B and JP-2005-345250A. With reference to FIG. 23A, the rotational angle sensing device disclosed in US-6707292B includes a magnet 301 and a rotational angle sensing unit 302. The magnet 301 is fixed to a rotor, which is rotated upon rotation of the sensing object, such as a throttle valve. The rotational angle sensing unit 302 forms a magnetic circuit in corporation with the magnet 301. The rotational angle sensing unit 302 includes two yoke segments (stator core) 303 and a magnetic sensing device 304 (e.g., a Hall IC) having a magnetic sensing element. The yoke segments 303 are divided such that the yoke segments 303 are symmetrical about an imaginary center plane, which includes a reference line that connects between a center of the magnetic sensing device 304 and a rotational center of the magnet 301 and which also includes a rotatable axis of the sensing object. The magnetic sensing device 304 changes its output according to a density of a magnetic flux, which passes through a magnetic flux sensing gap that is formed between opposing portions 311 of the yoke segments 303. In response to a rotational angle of the magnet 301, the density of the magnetic flux, which passes through the magnetic flux sensing gap, changes, i.e., the density of the magnetic flux, which flows across the magnetic sensing device 304 arranged in the magnetic flux sensing gap, changes. In response to the change in the density of the magnetic flux, the output of the magnetic sensing device 304 changes. The rotational an§le sensing unit 302 senses the rotational angle of the sensing object based on the output of the magnetic sensing device 304. Here, as shown in FIG. 23A, the yoke segments 303 are symmetrically arranged open type yoke segments. A top end portion of the yoke segments 303 includes the opposing portion 311. The opposing portions 311 of the yoke segments 303 are opposed with each other and are spaced from each other by a distance of the magnetic flux sensing gap. In addition, as shown in FIG. 23A, in the case of US-6707292B, each yoke segment 303 includes a yoke opening-side extension portion 312. The yoke opening-side extension portion 312 extends from a lower end of the opposing portion 311 and forms a predetermined air gap relative to the magnet 301. Each of the yoke opening-side extension portion 312 includes a linear portion (a shoulder) 313, a turned portion 314 and a vertical portion (a linear portion) 315. The linear portion 313 extends linearly from the lower end of the opposing portion 311 away from the magnetic sensing device 304 in the left or right direction in the drawing. The turned portion 314 is bent generally at a right angle from the end of the linear portion 313. The vertical portion 315 extends linearly from the lower end of the turned portion 314 toward a distal end surface of a yoke opening end portion 316. However, in the rotational angle sensing device described in US-6707292B, the magnetic sensing device 304 generates an output, which is bulged and has a flex point between an intermediate angle (e.g., 40 degrees) and a maximum angle (e.g., 80 degrees) of the magnet 301 in the operable angular range. This is due to the following fact. That is, in the state where the rotational angle of the magnet 301 is held to the maximum angle, the vertical portion 315, which forms the air gap between the magnetic pole surface of the magnet 301 and the vertical portion 315, extends linearly from the lower end of the turned portion 314 to the distal end surface of the yoke opening end portion 316. That is, even when the magnet 301 rotates from the intermediate angle to the maximum angle in the operable angular range, the air gap formed between the magnetic pole surface of the magnet 301 and an inner side surface of the vertical portion 315 shows no abrupt increase. In consequence, the magnetic sensing device 304 generates the output, which is bulged and has the flex point. Accordingly, the linearity of the output change characteristic of the magnetic sensing device 304 with respect to the rotational angle of the magnet 301 is decreased or deteriorated, thereby disadvantageous^ decreasing the detection accuracy of the rotational angle of the sensing object. In view of the above disadvantage, the rotational angle sensing device described in JP-2005-345250A is provided with the symmetrical open type yoke segments, which are reversely warped, as shown in FIG. 23B for the purpose of improving the linearity of the output change characteristic of the magnetic sensing device 304 with respect to the rotational angle of the magnet 301, thereby improving the detection accuracy of the rotational angle. The yoke opening-side extension portion 312 of each of the yoke segments 303 includes a linear portion 313 and a turned portion 319. The turned portion 319 is bent in a reversely warped shape from an end of the linear portion 313 toward a reversely warped portion 321. In addition, each of the reversely warped portions 321 has an arcuate portion in a reversely warped shape, which is convex to the magnet side. Further, each of the reversely warped portions 321 of the yoke segments 303 described in JP-2005-345250A is arcuately curved from a reference position of the magnet 301, at which the air gap between the warped portion 321 and the magnet 301 is minimum, and extends by a predetermined arcuate length from the reference position of the magnet 301 toward the lower end portion of the turned portion 319 on the magnetic sensing device side. Also, the warped portion 321 is arcuately curved from the reference position of the magnet 301 and extends by a predetermined yoke opening-side length (6 mm) from the reference position of the magnet 301 on the side opposite from the magnetic sensing device. Since the rotational angle sensing device described in JP-2005-345250A is provided with the yoke segments 303 including the turned portions 319 and the reversely warped portions 321, when the magnet 301 is rotated by a predetermined rotational angle from the minimum state of the air gap formed between the magnet 301 and the yoke segment 303 toward a direction of increasing the air gap, the air gap abruptly increases. Accordingly, the linearity of the output change characteristic of the magnetic sensing device 304 with respect to the rotational angle of the magnet 301 is improved between the intermediate angle and the maximum angle in the operable angular range, making it possible to improve the detection accuracy in the rotational angle of the sensing object. However, the rotational angle sensing device described in JP-2005-345250A defines the yoke opening-side length of the reversely warped portion 321 of each of the yoke segments 303 as 6 mm. In this case, with reference to FIGS. 16 and 17B, when the rotational angle of the magnet 301 is the minimum angle (0 degree) in the operable angular range, that is, when an axis in a plate length direction (a plate longitudinal direction) of the magnet 301 and an axis in the plate longitudinal direction of the magnetic sensing device 304 are positioned on a straight line, a magnetic circuit part (A) is formed to create the magnetic flux in the route of one magnetic pole of the magnet 301 (N pole), the reversely warped portion 321 of the left side yoke segment 303, the turned portion 319 of the left side yoke segment 303, the linear portion 313 of the left side yoke segment 303 and the other magnetic pole of the magnet 301 (S pole) in that order. In addition, a magnetic circuit part (B) is formed to create the magnetic flux in the route of the N pole of the magnet 301, the reversely warped portion 321 of the right side yoke segment 303, the turned portion 319 of the right side yoke segment 303, the linear portion 313 of the right side yoke segment 303 and the S pole of the magnet 301 in that order. At this point, since the magnetic flux does not pass through the magnetic flux sensing gap, an output value of the magnetic sensing device 304 becomes nearly zero, as shown in FIG. 18. Next, when the magnet 301 rotates about the rotational axis (rotational center) in the left direction (the counterclockwise direction) in the drawing by 40 degrees from the state of the rotational angle of 0 degree to set the rotational angle of the magnet 301 to the intermediate angle (40 degrees) in the operable angular range, the magnetic circuit part (A) is formed to create the magnetic flux in the route of the N pole of the magnet 301, the reversely warped portion 321 of the left side yoke segment 303 and the S pole of the magnet 301 in that order. In addition, the magnetic circuit part (B) is formed to create the magnetic flux in the route of N pole of the magnet 301, the reversely warped portion 321 of the right side yoke segment 303, the turned portion 319 of the right side yoke segment 303, the linear portion 313 of the right side yoke segment 303, the opposing portion 311 of the right side yoke segment 303, the magnetic sensing device 304, the opposing portion 311 of the left side yoke segment 303, the linear portion 313 of the left side yoke segment 303, the turned portion 319 of the left side yoke segment 303, the reversely warped portion 321 of the left side yoke segment 303, and the S pole of the magnet 301 in that order. At this point, although the magnetic flux passes through the magnetic flux sensing gap, the magnetic flux also flows across the magnetic circuit part (A) which has no substantial influence on the output of the magnetic sensing device 304. Therefore, the amount of the magnetic flux, which passes through the flux sensing gap, is reduced. Thus, the density of the magnetic flux, which passes through the magnetic sensing device 304, is reduced. As a result, as shown in FIG. 18, the output value of the magnetic sensing device 304 is slightly reduced from an ideal output value. Accordingly, in the rotational angle sensing device described in JP-2005-345250A, the linearity of the output change characteristic of the magnetic sensing device 304 with respect to the rotational angle of the magnet 301 is reduced at the intermediate angle (40 degrees) in the operable angular range. Next, when the magnet 301 rotates about the rotational axis (rotational center) in the left direction (the counterclockwise direction) by 40 degrees from the state of the rotational angle of 40 degrees to set the rotational angle of the magnet 301 to the maximum angle (80 degrees) in the operable angular range, the magnetic circuit part (A) is formed to create the magnetic flux in the route of the N pole of the magnet 301, the reversely warped portion 321 of the right side yoke segment 303, the reversely warped portion 321 of the left side yoke segment 303, and the S pole of the magnet 301 in that order. In addition, the magnetic circuit part (B) is formed to create the magnetic flux in the route of the N pole of the magnet 301, the reversely warped portion 321 of the right side yoke segment 303, the turned portion 319 of the right side yoke segment 303, the linear portion 313 of the right side yoke segment 303, the opposing portion 311 of the right side yoke segment 303, the magnetic sensing device 304, the opposing portion 311 of the left side yoke segment 303, the linear portion 313 of the left side yoke segment 303, the turned portion 319 of the left side yoke segment 303, the reversely warped portion 321 of the left side yoke segment 303, and the S pole of the magnet 301 in that order. At this point, although the magnetic flux passes through the magnetic flux sensing gap, the magnetic flux also flows in the magnetic circuit part (A), which has no substantial influence on the output of the magnetic sensing device 304. Therefore, the amount of the magnetic flux, which passes the magnetic flux sensing gap, is reduced. Thus, the density of the magnetic flux, which passes across the magnetic flux sensing element 304, is reduced. As a result, as shown in FIG. 18, the output value of the magnetic sensing device 304 is reduced largely from the the ideal output value. Accordingly, in the rotational angle sensing device described in JP-2005-345250A, the linearity of the output change characteristic of the magnetic sensing device 304 with respect to the rotational angle of the magnet 301 is reduced at the maximum angle (80 degrees) in the operable angular range. Accordingly, in the rotational angle sensing device described in JP-2005-345250A, the amount (the magnetic flux leak amount) of the magnetic flux, which flows in the magnetic circuit part (A) that is not related to the magnetic sensing device 304, is relatively large in the detectable angular range, which is generally from the intermediate angle to the maximum angle in the operable angular range, and the output of the magnetic sensing device 304 is reduced from the ideal output value. Therefore, for the purpose of improving the linearity of the output change characteristic of the magnetic sensing device 304 with respect to the rotational angle of the magnet 301 to improve detection accuracy of the rotational angle, it is required to increase a size (or a magnetic force strength) of the magnet 301. When this requirement is satisfied, an entire size of the rotational angle sensing device may be disadvantageously increased, and a mountability of the rotational angle sensing device to the vehicle may be disadvantageously deteriorated. The present invention addresses the above disadvantages. Thus, it is an objective of the present invention is to provide a rotational angle sensing device, which alleviates or reduces variations from product to product by sandwiching a rotational angle sensor between bent pieces of two yoke segments of an open type yoke and thereby to alleviate or reduce variations in characteristics from product to product. It is another objective of the present invention to provide a rotational angel sensing device, which effectively limits influences of radio wave noises, influences of an external magnetic filed and influences of a magnetic body onto an open type yoke or a rotational angle sensor. It is another objective of the present invention to provide a rotational angle sensing device, which increases an output of a magnetic sensing element without an increase in a size and magnetic force strength of a magnet. It is another objective of the present invention to provide a rotational angle sensing device, which improves linearity of an output change characteristic of a magnetic sensing element with respect to a rotational angle of a magnet to improve detection accuracy of a rotational angle of a sensing object. To achieve the objectives of the present invention, there is provided a rotational angle sensing device, which includes a magnet, a plate shaped rotational angle sensor and an open type yoke. The magnet is fixed to a rotatable shaft of a sensing object. The plate-shaped rotational angle sensor includes a magnetic sensing element, which senses a magnetic flux emitted from the magnet. The rotational angle sensor senses a rotational angle of the sensing object by using an output change characteristic of the magnetic sensing element with respect to a rotational angle of the magnet. The open type yoke is made of a magnetic material and has an opening on one side of the yoke. The yoke concentrates the magnetic flux emitted from the magnet onto the rotational angle sensor. The yoke includes first and second yoke segments, which are formed separately. Each of the first and second yoke segments forms an air gap relative to the magnet and includes a yoke main body and a bent piece. The bent piece is bent relative to the yoke main body at a predetermined bent angle in each of the first and second yoke segments. The yoke main body and the bent piece of the first yoke segment are opposed to the yoke main body and the bent piece, respectively, of the second yoke segment. The bent pieces of the first and second yoke segments hold the rotational angle sensor therebetween in a plate thickness direction of the bent pieces. To achieve the objectives of the present invention, there is also provided a rotational angle sensing device, which includes a magnet, a rotational angle sensing unit. The magnet is rotated synchronously upon rotation of a sensing object and is magnetized in a radial direction that is perpendicular to a rotational axis of the sensing object. The rotational angle sensing unit forms a magnetic circuit in corporation with the magnet and senses a rotational angle of the sensing object. The rotational angle sensing unit includes a magnetic sensing element and first and second yoke segments. An output of the magnetic sensing element changes according to a density of a magnetic flux that passes through a magnetic flux sensing gap formed in the magnetic circuit. The first and second yoke segments are arranged symmetrically about an imaginary center plane, which includes a reference line that connects between a center of the magnetic sensing element and a rotational center of the magnet and which also includes the rotational axis of the sensing object. The magnetic sensing element is placed in the magnetic flux sensing gap formed between the first and second yoke segments at one side of the first and second yoke segments. Each of the first and second yoke segments includes a yoke opening end portion, which forms a predetermined air gap relative to the magnet and which is located on the other side of the yoke segment that is opposite from the magnetic sensing element. A position of a rotational axis of the magnet in a direction parallel to the reference line is set at a reference position, at which the air gap between at least one of the first and second yoke segments and the magnet is minimum. When the magnet is held in a predetermined rotational angle, which causes generation of a maximum output from the magnetic sensing element in an operable angular range of the sensing object, a linear distance, which is measured in the direction parallel to the reference line between the rotational axis of the magnet at the reference position and a furthermost point of an outer surface of the magnet relative to the magnetic sensing element, is generally equal to a linear distance in the direction parallel to the reference line between the rotational axis of the magnet at the reference position and a distal end of the yoke opening end portion of at least one of the first and second yoke segments. The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: FIGS. 1A to 1C are schematic diagrams, each showing an entire construction of a rotational angle sensing device according to a first embodiment of the present invention; FIGS. 2A to 2C are schematic diagrams, each showing a modification of the major construction of the rotational angle sensing device according to the first embodiment; FIG. 3 is a schematic diagram showing an entire construction of an intake module of a rotational angle sensing device according to a second embodiment; FIG. 4 is a perspective view showing an intake air temperature sensor according to the second embodiment; FIG. 5 is a perspective view showing an intake air pressure sensor according to the second embodiment; FIG. 6 is a perspective view showing an entire construction of an intake module of a rotational angle sensing device according to a third embodiment; FIG. 7 is a bottom view showing an injection opening of thermosetting resin according to the third embodiment; FIG. 8 is a perspective view showing a major construction of the intake module according to the third embodiment; FIG. 9 is a perspective view showing a major construction of the rotational angle sensing device according to the third embodiment; FIG. 10 is a front view showing a major construction of the rotational angle sensing device according to the third embodiment; FIG. 11 is a perspective view showing a major construction of the rotational angle sensing device according to a fourth embodiment; FIG. 12 is a perspective view showing a rotational angle sensing device according to a fifth embodiment; FIG. 13 is a cross sectional view showing an intake module according to the fifth embodiment; FIG. 14 is a cross sectional view taken along line XIV-XIV in FIG. 13; FIG. 15 is a front view showing a rotational angle sensing device according to the fifth embodiment; FIG. 16 is a graph showing a relation among a density of a magnetic flux across a Hall IC, a rotational angle of a magnet and a yoke opening-side length for illustrating the fifth embodiment and and a previously proposed technique; FIG. 17A is an explanatory diagram showing a flow of a magnetic flux in the fifth embodiment; FIG. 17B is an explanatory diagram showing a flow of a magnetic flux in the previously proposed technique; FIG. 18 is a graph showing a relationship between an output change characteristic of a Hall IC and a rotational angle of a magnet for illustrating the fifth embodiment and the previously proposed technique; FIG. 19A is an explanatory diagram showing a flow of a magnetic flux at 0 degree for illustrating the fifth embodiment and the previously proposed technique; FIG. 19B is an explanatory diagram showing a flow of a magnetic flux at 40 degrees for illustrating the fifth embodiment and the previously proposed technique; FIG. 19C is an explanatory diagram showing the flow of the magnetic flux at 80 degrees for illustrating the fifth embodiment and the previously proposed technique; FIG. 20 is a front view showing a rotational angle sensing device according to a sixth embodiment; FIG. 21 is a front view showing a rotational angle sensing device according to the sixth embodiment; FIG. 22 is a front view showing an entire construction of a previously proposed rotational angle sensing device. FIG. 23A is a perspective view showing another previously proposed rotational angle sensing device; and FIG. 23B is a perspective view showing another previously proposed rotational angle sensing device. (First Embodiment) FIGS. 1A to 2C show a first embodiment of the present invention. Specifically, FIGS. 1A to IC are diagrams showing an entire construction of a rotational angle sensing device. FIG. 2A to 2C are diagrams showing a modification of the major construction of the rotational angle sensing device. A control system of an internal combustion engine (engine control system) according to the present embodiment includes an electronically controlled fuel injection system, an intake module (intake air quantity control cylinder four-cycle gasoline engine for a motorcycle: hereinafter referred to as engine). The intake module is incorporated into an intake system of the engine. The ECU controls the electronically controlled fuel injection system and the intake module. The electronically controlled fuel injection system is a system, which pressurizes fuel (for example, gasoline) at a predetermined pressure by an electric fuel pump and supplies the pressurized fuel to an injector (electromagnetic fuel injection valve) through a fuel filter, thereby injecting the fuel at optimal timing. The intake module of the present embodiment is an intake air quantity control apparatus (intake passage opening/closing device or system), which controls an intake air quantity, i.e., a quantity of intake air supplied into the combustion chamber of the engine based upon an operating amount (hereinafter, referred to as a throttle operating amount) of a throttle operating component (e.g., a throttle lever or a throttle handle of a vehicle, such as a motorcycle), which is operated by a driver. It should be noted that the throttle operating amount corresponds to an amount of depression (accelerator operating amount) of an accelerator pedal, which is depressed by a driver in a case of a four-wheeled vehicle. The intake module includes a throttle body (not shown), a throttle valve (sensing object not shown) and a rotational angle sensing device. The throttle body is incorporated into an intermediate part of an engine intake pipe connected to an intake port of the engine. The throttle valve is received in the throttle body in such a manner that the throttle valve is rotatable to open and close a throttle bore described below. The rotational angle sensing device senses a rotational angle of the throttle valve. The throttle body is integrally formed from a non-magnetic material (e.g., a resin material, such as thermoplastic resin or the like). The throttle body includes a cylindrical throttle bore wall (hereinafter referred to as cylindrical portion) and two cylindrical bearing potions. An intake passage (hereinafter referred to as the throttle bore) having a circular cross section is formed in the cylindrical throttle bore wall. The cylindrical bearing portions are provided at two opposed sides of the cylindrical throttle bore wall, which are opposed to each other in an axial direction that is perpendicular to an intake flow direction of the air in the throttle bore of the cylindrical portion. In addition, the throttle valve is connected integrally to a shaft 1, which extends linearly in the axial direction described above. The throttle valve controls an intake air quantity, i.e., a quantity of intake air supplied into the combustion chamber of the engine through changing of the rotational angle of the throttle valve within an operable angular range that is between a fully closed position, at which the intake air quantity is minimized, and a fully opened position, at which the intake air quantity is maximized. The shaft 1 is a valve shaft that rotates integrally with the throttle valve. Two opposed axial ends of the shaft 1 are rotatably received in the two bearing portions, respectively, at the opposed sides of the cylindrical portion of the throttle body. One of the opposed axial ends (hereinafter, simply referred to as one end) of the shaft 1 penetrates through the cylindrical portion of the throttle body and projects outward from the throttle body. Furthermore, the other end of the shaft 1 penetrates through the cylindrical portion of the throttle body and projects outward from the throttle body. In addition, an accelerator lever is fixed to the other end of the shaft 1 by, for example, a metal bending process. A wire cable, which is driven synchronously with the throttle operating component (e.g., the throttle lever or the throttle handle), is connected to the accelerator lever. The intake module of the present embodiment has a non-contact type rotational angle sensing device (throttle opening degree sensing device). The rotational angle sensing device converts a throttle opening degree, which corresponds to a rotational angle (valve opening degree) of the throttle valve that is opened and closed in accordance with the throttle operating amount implemented by the driver, into an electrical signal to inform the opening degree of the throttle valve to the ECU. The rotational angle sensing device of the present embodiment includes a thin-plate shaped magnet (a permanent magnet: hereinafter referred to as a magnet) 2, a rotational angle sensor (hereinafter, referred to as a throttle opening degrefe sensor) 3, an open type yoke (a magnetic body of an open magnetic path type) and a casing. The magnet 2 is fixed to the one end of the shaft 1 of the throttle valve. The throttle opening degree sensor 3 includes a non-contact type magnetic sensing element, which senses the magnetic flux emitted from the magnet 2. The open type yoke concentrates the magnetic flux emitted from the magnet 2 on the throttle opening degree sensor 3. The casing receives the throttle opening degree sensor 3 and the open type yoke. The open type yoke includes first and second yoke segments 4, 5, which are formed as two plate-shaped yoke segments of the same type. A magnetic flux sensing gap is formed between the first and second yoke segments 4, 5 to receive the throttle opening degree sensor 3 therein. Here, the ECU of the present embodiment performs a fuel injection quantity control operation to control a valve-opening period of the injector in such a manner that a corresponding fuel injection quantity, which corresponds to an output of the throttle opening degree sensor 3, i.e., the electrical signal outputted from the throttle opening degree sensor 3, is supplied to each corresponding cylinder of the engine. In addition, the casing, which receives the throttle opening degree sensor 3 and the open type yoke, includes an intake module cover (a sensor cover, a magnetic shield cover) 11, a plate 12 and a housing 14. The plate 12 is fitted into the intake module cover 11. The housing 14 is mounted on an outer wall surface of the cylindrical portion of the throttle body. Shaft receiving holes 15, 16 penetrate through the plate 12 and the housing 14, respectively, in the axial direction of the shaft 1 of the throttle valve. The magnet mounting portion of the shaft 1 penetrates through the bearing receiving hole, which is formed in the bearing portion at the one side of the cylindrical portion of the throttle body, the shaft receiving hole 16, which is formed in the housing 14, and the shaft receiving hole 15, which is formed in the plate 12, so that the magnet mounting portion of the shaft 1 projects into the interior of the intake module cover 11 (e.g., the magnet mounting portion of the shaft 1 being disposed in a sensor receiving space 17 in the interior of the intake module cover 11). In this way, the magnet mounting portion of the shaft 1 is rotatably received in the interior of the intake module cover 11. The magnet 2 forms a magnet rotor, which rotates relative to the throttle opening degree sensor 3 and the open type yoke. The magnet 2 is held by and is fixed to the one end (the magnet mounting portion) of the shaft 1 of the throttle valve, so that the magnet 2 is rotated synchronously with rotation of the throttle valve, which serves as the sensing object. Specifically, the magnet 2 is held by and is fixed in a linear groove, which is formed in the shaft 1 of the throttle valve, by using fastening means, such as an adhesive or bonding. The magnet 2 has a square (or rectangular) shape when the magnet 2 is viewed in a direction perpendicular to a plane of FIG. 1A. More specifically, the magnet 2 is a cuboid-shaped permanent magnet, which stably produces a long lasting magnetic force. Furthermore, the magnet 2 is made of a rare-earth magnet (e.g., a samarium-cobalt (Sm-Co) magnet or a neodymium (Nd) magnet), an alnico magnet or a ferrite magnet. An N pole and an S pole are magnetized in the magnet 2 in such a manner that opposed ends of the magnet 2, which are opposed to each other in a longitudinal direction of the magnet 2, have the opposite polarities, respectively. In addition, the magnet 2 is magnetized to implement parallel magnetization in such a manner that lines of magnetic force in the magnet 2 are parallel to each other. Furthermore, the magnet 2 is magnetized in a radial direction, which is perpendicular to the rotational axis (rotational center axis) of the shaft 1 of the throttle valve. In this way, the magnetizing direction (the longitudinal direction) of the magnet 2 coincides with a diametrical direction, which is perpendicular to the rotational axis of the shaft 1 of the throttle valve. Furthermore, the magnetized surface (magnetic pole surface) at the one longitudinal side of the magnet 2 forms the N pole, and the magnetized surface (magnetic pole surface) at the other longitudinal side of the magnet 2 forms the S pole. Here, the magnet 2 is rotatable about the rotational center thereof within an operable angular range that is between the fully closed position and the fully opened position of the throttle valve, particularly in a magnet receiving space (magnet receiving portion) 19 formed in the interior of the open type yoke (between the first and second yoke segments 4, 5). In addition, in the present embodiment, when the throttle valve is placed in the fully closed position, the rotational angle of the magnet 2 becomes a minimum angle (for example, 0 degree) in the operable angular range (detectable angular range) of the throttle valve. When the throttle valve is placed in an intermediate position, the rotational angle of the magnet 2 becomes an intermediate angle (for example, 45 degrees) in the operable angular range of the throttle valve. When the throttle valve is placed in the fully opened position, the rotational angle of the magnet 2 becomes a maximum angle (for example, 90 degrees) in the operable angular range of the throttle valve (refer to FIGS. IB and 1C). The throttle opening degree sensor 3 of the present embodiment is disposed in the magnetic flux sensing gap, which is formed between the first and second yoke segments 4, 5. Furthermore, the throttle opening degree sensor 3 includes a Hall IC, which senses the magnetic flux (a density of the magnetic flux) emitted from the magnetized surface at one side of the magnet 2. The Hall IC is an IC (integrated circuit), which includes a Hall element(s) and an amplifier circuit. The Hall element(s) serves as a non-contact type magnetic sensing element(s), an output of which changes according to a change in the magnetic flux density, i.e., the density of the magnetic flux (density of the magnetic fiux passing through the Hall IC) passing through the magnetic flux sensing gap. The amplifier circuit amplifies the output of the Hall element(s). The Hall IC outputs an electrical voltage signal in accordance with the density of the magnetic flux (density of the magnetic flux passing through the Hall IC) passing through the magnetic flux sensing gap. It should be noted that the Hall IC may have a function of externally executing electrical trimming of a correction program(s) for an output gain adjustment, an offset adjustment and a temperature characteristic correction with respect to the magnetic flux density and may also have a self-diagnosis function for diagnosing, for example, breaking of wires or short-circuit. The Hall IC is sealed inside the resin housing (sealing member), which forms a main body (weight portion) of the throttle opening degree sensor 3. The resin housing is formed into a cuboid shape (thin plate-shape) and has opposed joint surfaces at opposed sides thereof, which are opposed with each other in a plate thickness direction of the resin housing. The opposed joint surfaces of the resin housing are in direct close contact with the first and second yoke segments 4, 5, respectively. A lead terminal group (a group of lead terminals) 3a, which serves as a sensor lead terminal group, extends out of the resin housing, which receives the Hall IC therein. In addition, the lead terminal group 3a of the throttle opening degree sensor 3 includes a single output-side lead terminal (a sensor output terminal), a single ground (GND)-side lead terminal (a sensor GND terminal) and a single power source-side lead terminal (a sensor power source terminal). The throttle opening degree sensor 3 is arranged in the magnetic flux sensing gap in such a manner that a perpendicular line, which extends through the rotational center of the magnet 2 and is perpendicular to the rotational axis (rotational center axis) of the shaft 1 of the throttle valve, passes through the center of the Hall IC. That is, the throttle opening degree sensor 3 is arranged in the magnetic flux sensing gap in such a manner that rotational center of the magnet 2 and the center of the Hall IC are positioned substantially on the same axis (same line). Here, when the magnet 2 is placed in such a manner that the longitudinal direction (the magnetizing direction) of the magnet 2 coincides with the direction of the magnetic flux sensing gap between the first and second yoke segments 4, 5, the rotational angle of the magnet 2 becomes the minimum angle (e.g., 0 degree) in the operable angular range. Furthermore, when the magnet 2 is placed in such a manner that the longitudinal direction (the magnetizing direction) of the magnet 2 is perpendicular to the direction of the magnetic flux sensing gap, the rotational angle of the magnet 2 becomes an angle (e.g., 90 degrees) in the operable angular range. In such a case, when the rotational angle of the magnet 2 becomes 90 degrees, the magnetic flux density, i.e., the density of the magnetic flux, which passes through the magnetic flux sensing gap, shows the maximum value, and the Hall IC outputs the maximum output value in the operable angular range. Furthermore, when the rotational angle of the magnet 2 becomes 0 degree, the magnetic flux density, i.e., the density of the magnetic flux, which passes through the magnetic flux sensing gap, shows the minimum value, and the Hall IC outputs the minimum output value in the operable angular range. In addition, the throttle opening degree sensor 3 has two magnetism-sensing surfaces at opposed sides of the throttle opening degree sensor 3, which are opposed to each other in plate thickness direction (hereinafter, simply referred to as a thickness direction) of the throttle opening degree sensor 3. Further, the throttle opening degree sensor 3 is arranged in the magnetic flux sensing gap in such a manner that the throttle opening degree sensor 3 is inclined by a predetermined inclination angle relative to a perpendicular plane that is perpendicular to the rotational axis (rotational center axis) of the shaft 1 of the throttle valve. Therefore, the plane of each of the opposed magnetism-sensing surfaces of the throttle opening degree sensor 3 is inclined by the predetermined angle relative to the perpendicular plane that is perpendicular to the rotational axis of the shaft 1 of the throttle valve. The open type yoke of the present embodiment includes the two divided yoke segments, i.e., the first and second yoke segments 4, 5, which are formed as the thin-plate shaped yoke segments of the same kind and which are opposed to each other while having the magnet receiving space 19 therebetween. The first and second yoke segments 4, 5 are formed to have a predetermined shape. Furthermore, the first and second yoke segments 4, 5 are made of a magnetic material (e.g., iron) and form one set of plate-shaped yoke segments (magnetic bodies) for concentrating the magnetic flux emitted from the magnet 2 on the throttle opening degree sensor 3, particularly on the Hall IC (magnetic sensing element of the non-contact type). Each of the first and second yoke segments 4, 5 includes a yoke main body 21, 22 and a holding piece (a bent piece) 31, 32. The holding pieces 31, 32 of the first and second yoke segments 4, 5 hold the throttle opening degree sensor 3 therebetween in the thickness direction of the throttle opening degree sensor 3. 'An inner side surface of the first yoke segment 4 and an inner side surface of the second yoke segment 5 are opposed to each other in a plate thickness direction (hereinafter, simply referred to as a thickness direction) of the first and second yoke segments 4, 5 while the magnet 2 and the throttle opening degree sensor 3 are interposed between the inner side surface of the first yoke segment 4 and the inner side surface of the second yoke segment 5. In each of the first and second yoke segments 4, 5, a base end (a magnet side end, i.e., a yoke opening end portion 23) of the yoke main body 21, 22 forms a maximum width portion of the yoke segment 4, 5 where a plate width of the yoke segment 4, 5 is maximum. Also, in each of the first and second yoke segments 4, 5, a distal end (a throttle opening degree sensor 3 side end, i.e., a distal end of a sensor mounting part 33) of the holding piece 31, 32 forms a minimum width portion of the yoke segment 4, 5 where the plate width of the yoke segment 4, 5 is minimum. The plate width P of the yoke segment 4, 5 at this minimum width portion is equal to or greater than the plate thickness Q of the magnet 2. Each of the first and second yoke segments 4, 5 is formed in such a manner that the plate width thereof decreases in a stepwise manner or decreased continuously from the yoke opening end portion 23 toward the distal end of the sensor mounting part 33. Specifically, each of the first and second yoke segments 4, 5 is tapered, so that the magnetic flux is converged from the yoke opening end portion 23 toward the distal end of the sensor mounting part 33 or toward the magnetic flux sensing gap. The yoke main body 21, 22 of each yoke segment 4, 5 is symmetrical about an imaginary center plane, which includes a center axis (reference line) connecting between the rotational center of the magnet 2 and the thickness center of the throttle opening degree sensor 3 in the thickness direction of the throttle opening degree sensor 3, and which also includes the rotational axis of the shaft 1 of the throttle valve. Each yoke main body 21, 22 forms a predetermined air gap between the magnet 2 and the yoke main body 21, 22, and the yoke main bodies 21, 22 are opposed to each other. In addition, the yoke main bodies 21, 22 are opposed to each other in such a manner that the yoke main bodies 21, 22 are spaced from each other by a non-circular magnet receiving space 19, which rotatably receives the one end of the shaft 1 of the throttle valve and the magnet 2. The yoke main body 21, 22 of each yoke segment 4, 5 includes a plate-shaped yoke opposing portion 24 and a plate-shaped linear portion 25. The yoke opposing portions 24 of the yoke main bodies 21, 22 are opposed to each other at the end (the throttle opening degree sensor 3 side end), which is opposite from yoke opening end portion 23. In each yoke main body 21, 22, the linear portion 25 is bent relative to the yoke opposing portion 24 toward the magnet 2. The yoke opposing portion 24 of each yoke main body 21, 22 is a rectangular plate that is parallel to a radial perpendicular line, which is perpendicular to the rotational axis of the shaft 1 of the throttle valve. The holding piece 31, 32 is connected to one of two lateral edges of the yoke opposing portion 24, which are opposed to each other in a plate width direction (hereinafter, simply referred to as a width direction) of the yoke opposing portion 24. The linear portion 25 of each yoke main body 21, 22 includes the plate-shaped yoke opening end portion 23 at the opening side of the linear portion 25. The yoke opening end portion 23 is a magnet opposing portion, which is located at the opening side of each yoke main body 21, 22 and which forms a minimum air gap relative to the corresponding one of the opposed ends (opposed magnetized surfaces) of the magnet 2 that are opposed to each other in the magnetizing direction of the magnet 2 upon positioning of the magnet 2 and the shaft 1 at the maximum angle or therearound (e.g., 90 degrees or therearound) in the operable angular range of the throttle valve. The linear portion 25 of each yoke main body 21, 22 extends linearly from the yoke opening end portion 23 to the yoke opposing portion 24 in such a manner that the linear portion 25 is inclined by the predetermined inclination angle relative to the radial perpendicular line, which is perpendicular to the rotational axis of the shaft 1 of the throttle valve. Furthermore, the linear portion 25 is inclined in such a manner that the gap between the yoke opposing ends 24 is greater than the gap between the yoke opening end portions 23. Specifically, each yoke main body 21, 22, particularly the linear portion 25 of the yoke main body 21, 22 is inclined to satisfy the following condition. That is, when the magnet 2 rotates from the maximum angle, at which the minimum air gap is formed between the linear portion 25 and the magnet 2, toward the angular position where the minimum angle is made, the air gap between the linear portion 25 and the magnet 2 gradually increases. In each of the first and second yoke segments 4, 5, the holding piece 31, 32 is bent at a predetermined bending angle (an obtuse angle larger than a right angle) relative to the corresponding yoke main body 21, 22. In each of the first and second yoke segments 4, 5, the holding piece 31, 32 is connected to the one of the two lateral edges of the yoke opposing portion 24, which are opposed to each other in the width direction of the yoke opposing portion 24, through a bending portion 34, which is bent in a substantially V-ietter (or substantially U-letter) shape at an obtuse angle larger than a right angle. Here, it should be noted that the one of the two lateral edges of the yoke opposing portion 24 of the first yoke segment 4 is opposite from the one of the two lateral edges of the yoke opposing portion 24 of the second yoke segment 5. Each holding piece 31, 32 includes a linear portion 35, which extends linearly from the bending portion 34 toward the distal end of the sensor mounting part 33 in such a manner that the linear portion 35 is inclined relative to the perpendicular line, which is perpendicular to the plane of each of the yoke main bodies 21, 22. In addition, the sensor mounting part (the yoke opposing portion) 33 is provided to the distal end of the linear portion 35 of each holding piece 31, 32, so that the sensor mounting parts 33 of the holding pieces 31, 32 are opposed to each other via the magnetic flux sensing gap. Each holding piece 31, 32 is formed by bending a projection piece, which projects in the width direction of the yoke opposing portion 24 from the one of the two lateral edges of the yoke opposing portion 24, which are opposed to each other in the width direction of the yoke opposing portion 24. Here, this projection piece is bent about the edge of the yoke opposing portion 24 toward the throttle opening degree sensor side (toward one side in a plate thickness direction of the yoke opposing portion 24). A bending angle of each holding piece 31, 32 relative to the corresponding yoke main body 21, 22 is set such that the throttle opening degree sensor 3 is placed within the plate widthwise dimension (plate width extent) of each yoke main body 21, 22. The bending angle of each holding piece 31, 32 relative to the corresponding yoke main body 21, 22 is set to be generally the same for the respective holding pieces 31, 32. Furthermore, in each holding piece 31, 32, an opposed surface of the sensor mounting part 33, which is opposed to the throttle opening degree sensor 3, is used as a contact surface, which directly contacts the opposed one of the magnetism-sensing surfaces of the throttle opening degree sensor 3. The sensor mounting part 33 of each holding piece 31, 32 serves as a sensor holding portion, which securely holds the throttle opening degree sensor 3, for example, through an adhesive or bonding upon clamping the throttle opening degree sensor 3 between the sensor mounting parts 33 of the holding pieces 31, 32, Here, the sensor mounting part 33 of the holding piece 31, which is placed on the front side in FIG. IB, forms a sensor upper-side mounting portion that holds and presses the throttle opening degree sensor 3 from the front side in the thickness direction of the throttle opening degree sensor 3 in such a manner that the sensor upper-side mounting portion closely or tightly contacts the opposed magnetism-sensing surface of the throttle opening degree sensor 3. Furthermore, the sensor mounting part 33 of the holding piece 32, which is placed on the back side in FIG. IB, forms a sensor lower-side mounting portion that holds and presses the throttle opening degree sensor 3 from the back side in the thickness direction of the throttle opening degree sensor 3 in such a manner that the sensor lower-side mounting portion closely or tightly contacts the opposed magnetism-sensing surface of the throttle opening degree sensor 3. The magnetic flux sensing gap is a gap, which has a constant width or distance between the sensor mounting part 33 of the holding piece 31 of the first yoke segment 4 and the sensor mounting part 33 of the holding piece 32 of the second yoke segment 5. The throttle opening degree sensor 3 is arranged in the magnetic flux sensing gap in such a manner that the rotational center of the magnet 2 and the thickness center of the Hall IC are generally located along the same axis (same line). The magnetic flux sensing gap is disposed in an intermediate part of a magnetic circuit, which is formed by the magnet 2, the throttle opening degree sensor 3 and the first and second yoke segments 4, 5. The intake module cover 11 has a relatively thin wall and is formed into a container shape by a magnetic material (e.g., an iron based metal material that contains, for example, 80% nickel). The intake module cover 11 forms a sensor receiving space 17 between the intake module cover 11 and an upper end surface of the plate 12 in FIG. 1A. Further, the tubular wall (side wall) 41 is formed integrally with the intake module cover 11 to surround the outer peripheral edges of the plate 12. A housing 14 side end (a lower end in FIG. 1A) of the tubular wall 41 is closed by a plate base of the plate 12. An opposite end (an upper end in FIG. 1A) of the tubular wall 41, which is opposite from the housing 14 side end of the tubular wall 41, is closed by a top wall plate (top wall) 42 that covers an upper portion of the sensor receiving space 17. The tubular wall 41 of the intake module cover 11 has an opening, which is opened externally. A connector 13, which is formed integrally with the plate 12 and will be described in detail below, is securely fitted into the opening of the tubular wall 41. Here, epoxy thermosetting resin (dielectric mold resin) is filled in the interior of the the intake module cover 11, i.e., in the sensor receiving space 17 of the intake module cover 11, to which the the plate 12 and the connector 13 are installed. The thermosetting resin is a seal member (potting material), which seals each lead terminal of the lead terminal group 3a of the throttle opening degree sensor 3, each connector terminal of a connector terminal group 13a of the connector 13 and a plurality of conductors (conductor (copper wire) having insulation coating, conductive plate or the like). The conductors electrically connect between the lead terminals of the lead terminal group 3a of the throttle opening degree sensor 3 and the connector terminals of the connector terminal group 13a of the connector 13. The tubular wall 41 of the intake module cover 11 has two concave portions (anchoring portions) 43, 44. The concave portions 43, 44 project toward a center of the sensor receiving space 17. The concave portions 43, 44 extend parallel to an extending direction of two sensor holding portions 51, 52, which are formed integrally in the plate 12. The two concave portions 43, 44 increase a contact area between the thermosetting resin and the tubular wall 41 of the intake module cover 11 to control a linear expansion movement of the thermosetting resin and electrical components sealed inside the thermosetting resin caused by a difference in linear expansion coefficient. In place of the two concave portions 43, 44, the tubular wall 41 of the intake module cover 11 may have a plurality of convex portions, which are formed by, for example, outwardly punching the tubular wall 41. Furthermore, a anchoring portion, such as a concave portion (or convex portion), which is used to anchor the thermosetting resin, may be formed along the entire circumference of the sensor receiving space 17 to limit the linear expansion movement of the thermosetting resin by increasing the contact area relative to the thermosetting resin. In this case, for example, anchoring portions, such as concave portions (convex portions), may be arranged one after another at predetermined intervals in the circumferential direction. In addition, as shown in FIGS. 2A to 2C, no anchoring portion may be provided in the tubular wall 41 of the intake module cover 11. The plate 12 is integrally formed from a non-magnetic material (e.g., a resin material, such as thermoplastic resin). The plate 12 includes the plate base and plate thick portions 53, 54. The plate base is installed in such a manner that the plate base closely contacts a top end surface of the housing 14. Each of the plate thick portions 53, 54 has a plate thickness that is greater than that of the plate base. The plate thick portions 53, 54 include convex-shaped sensor holding portions (yoke holding portion) 51, 52, respectively, which are disposed in such a manner that the plate thick portions 53, 54 project upward from a reference plane that extends along the top end surface of the plate base. The sensor holding portions 51, 52 include concave fitting grooves 61, 62, respectively, into which the yoke main bodies 21, 22 of the first and second yoke segments 4, 5 are securely fitted by, for example, press fitting. It should be noted that the yoke main bodies 21, 22 of the first and second yoke segments 4, 5 may be fixed into the fitting grooves 61, 62, respectively, through an adhesive or bonding. Furthermore, the shaft receiving hole 15 is formed in the plate base of the plate 12, particularly at a location between the two sensor holding portions 51, 52. A single connector housing of the connector 13 is formed integrally at a side of the plate 12. The connector 13 receives the connector terminal group 13a, which corresponds to the lead terminal group 3a that is pulled out of the main body (resin housing) of the throttle opening degree sensor 3. The connector 13 is a device, which includes a terminal base and a rectangular tubular connector shell. The terminal base holds the connector terminal group 13a. The rectangular tubular connector shell is disposed outside of the terminal base. The connector 13 connects between an ECU-side wiring harness and the throttle opening degree sensor 3 mounted on the plate 12. The housing 14 is a .die cast product or an aluminum mold, which is made of an aluminum alloy mainly containing aluminum and is formed into a predetermined shape by the aluminum alloy. The housing 14 serves as a bracket for mounting the plate 12 and the intake module cover 11 onto an outer wall surface of the cylindrical portion of the throttle body. In addition, the plate 12 is mounted on the top end surface (housing top end surface) of the housing 14. Furthermore, the shaft receiving hole 16 is formed through the housing 14. The flanges 63 are formed integrally in the housing 14. The tubular wall 41 of the intake module cover 11 is fixed to the flanges 63 by the metal bending process for bending the tubular wall 41 against the flanges 63. In the present embodiment, the intake module cover 11 is fixed to the housing 14 by using fastening means (e.g., the metal bending process) in a state where the joint end surface (inner peripheral surface) of the tubular wall 41 of the intake module cover 11 is in surface-to-surface contact with the joint end surface (outer peripheral surface) of the respective flanges 63 of the housing 14. Next, an operation of the intake module cover, which includes the rotational angle sensing device of the present embodiment, will be briefly described with reference with FIGS. 1A to 2C. When the throttle operating component (e.g., the throttle lever or the throttle handle) is operated by the driver, the accelerator lever, which is connected to the throttle operating component through the wire cable, is rotated. Therefore, the throttle valve is rotated about the center axis (rotational axis) of the shaft 1 in accordance with the throttle operating amount caused by the driver. Thereby, the throttle bore, which is communicated with the combustion chamber of the engine, is opened at the corresponding degree, so that the engine rotational speed is changed to a corresponding speed, which corresponds to the throttle operating amount caused by the driver. Here, at the time of operating the engine at the idling speed, i.e., at the time of fully closing the throttle valve, the rotational angle of the magnet 2 becomes the minimum angle (e.g., 0 degree) in the operable angular range of the throttle valve. In this state, the center line of the magnet 2, which extends in the longitudinal direction of the magnet 2, coincides with the center line of the throttle opening degree sensor 3, which extends through the thickness center of the throttle opening degree sensor 3. In this state, the magnetic circuit is formed to create the flow of the magnetic flux through one of the magnetic poles (e.g., the N pole or S pole) of the magnet 2, the holding piece 31 of the first yoke segment 4 (specifically, through the linear portion 35 and the bent portion 34), the yoke main body 21 of the first yoke segment 4 (specifically, through the yoke opposing portion 24, the linear portion 25 and the yoke opening end portion 23), and the other one of the magnetic poles (e.g., the S pole or N pole) of the magnet 2 in this order. Also, the magnetic tircu'rt is formed to create the flow of the magnetic flux through the N pole (or the S pole) of the magnet 2, the holding piece 32 (specifically, through the linear portion 35 and the bent portion 34), the yoke main body 22 (specifically, through the yoke opposing portion 24, the linear portion 25 and the yoke opening end portion 23) of the second yoke segment 5, and the S pole (or the N pole) of the magnet 2 in this order. At this time, the magnetic flux, which is emitted from the one of the magnetic poles of the magnet 2, does not pass through the magnetic flux sensing gap. Thus, the output of the Hall IC of the throttle opening degree sensor 3 with respect to the rotational angle of the magnet 2 becomes the minimum output value (nearly zero) in the operable angular range of the throttle valve. In addition, when a driver operates the throttle operating component to open the throttle valve to an intermediate position between the fully closed position and the fully opened position, the rotational angle of the magnet 2 becomes an intermediate angle (e.g., 45 degrees) in the operable angular range of the throttle valve. That is, the magnet 2 is rotated about the rotational center gf the magnet 2 by 45 degrees in the counterclockwise direction in FIG. IB or 2B from the position of zero degree, so that the rotational angle of the magnet 2 becomes 45 degrees. At this time, the magnet 2 is positioned relative to the magnetizing direction (the longitudinal direction) of the magnet 2 in such a manner that the density (magnetic flux density) of the magnetic flux, which passes through the magnetic flux sensing gap and thereby crosses the Hall IC, eaches a middle level. In such a case, the magnetic circuit is formed to create the flow of the nagnetic flux through the N pole (or the S pole) of the magnet 2, the yoke main xxfy 21 of the first yoke segment 4 (specifically, through the linear portion 25 ind the yoke opening end portion 23), and the S pole (or the N pole) of the nagnet 2 in this order. Furthermore, the magnetic circuit is formed to create the low of the magnetic flux through the N pole (or the S pole), the yoke main body >1 of the first yoke segment 4 (specifically, through the linear portion 25 and the 'oke opposing portion 24), the holding piece 31 of the first yoke segment 4 specifically, through the bent portion 34, the linear portion 35 and the sensor nounting part 33), the magnetic flux sensing gap (the throttle opening degree ;ensor 3), the holding piece 32 of the second yoke segment 5 (specifically, the ;ensor mounting part 33, the linear portion 35 and the bent portion 34), the yoke nain body 22 of the second yoke segment 5 (specifically, through the yoke >pposing portion 24, the linear portion 25 and the yoke opening end portion 23), ind the S pole (or the N pole) of the magnet 2 in this order. Thereby, the output of the Hall IC of the throttle opening degree sensor 3 vith respect to the rotational angle of the magnet 2 becomes a middle level jetween the minimum output value and the maximum output value in the )perable angular range of the throttle valve. In addition, when the driver operates the throttle operating component to >pen the throttle valve to the fully opened position, the rotational angle of the magnet 2 becomes the maximum angle (e.g., 90 degrees) in the operable angular range of the throttle valve. That is, the magnet 2 is rotated about the rotational center of the magnet 2 by 45 degrees in the counterclockwise direction in RG. IB or 2B from the position of 45 degrees, so that the rotational angle of the magnet 2 becomes 90 degrees. At this time, the center line of the magnet 2, which extends in the longitudinal direction of the magnet 2, becomes perpendicular to the center line of the throttle opening degree sensor 3r which extends through the thickness center of the throttle opening degree sensor 3 (see FIG. IB and 1C). In such a case, the magnetic circuit is formed to create the flow of the magnetic flux through the N pole (or the S pole), the yoke main body 21 of the first yoke segment 4 (specifically, through yoke opening end portion 23, the linear portion 25 and the yoke opposing portion 24), the holding piece 31 of the first yoke segment 4 (specifically, through the bent portion 34, the linear portion 35 and the sensor mounting part 33), the magnetic flux sensing gap (the throttle opening degree sensor 3), the holding piece 32 of the second yoke segment 5 (specifically, the sensor mounting part 33, the linear portion 35 and the bent portion 34), the yoke main body 22 of the second yoke segment 5 (specifically, through the yoke opposing portion 24, the linear portion 25 and the yoke opening end portion 23), and the S pole (or the N pole) of the magnet 2 in this order. Therefore, almost all of the magnetic flux emitted from the magnetic pole surface of the magnet 2 passes through the magnetic flux sensing gap, so that the output of the Hall IC of the throttle opening degree sensor 3 with respect to the rotational angle of the magnet 2 becomes the maximum output value in the operable angular range of the throttle valve. Thus, in response to the change in the rotational angle of the magnet 2, the density of the magnetic flux, which passes through the magnetic flux sensing gap and thereby crosses the Hall IC, changes, so that the output of the Hall IC changes accordingly. Thereby, the throttle opening degree sensor 3 senses the throttle opening degree, which corresponds to the rotational angle of the throttle valve, through use of the change characteristic of the output (hereinafter, referred as an output change characteristic) of the HaH IC with respect to the rotational angle of the magnet 2. Further, the ECU, which receives the electrical signal (throttle opening degree signal) outputted from the Hall IC of the throttle opening degree sensor 3, computes a control target value (fuel injection timing and fuel injection quantity), which is required by the electronically controlled fuel injection system. The ECU indirectly computes the intake air quantity based on an intake pipe pressure measured at a location downstream of the throttle valve through an intake air pressure sensor. Then, the ECU computes a basic injection time period (a basic injection quantity) based on the above computed intake air quantity and a measured engine rotational speed. Then, the ECU determines a final injection time period (a fuel injection quantity, a target injection quantity) in view of the above basic injection time period and a correction amount (ah injection quantity correction amount). The correction amount is determined based on the output value of the Hall IC of the throttle opening degree sensor 3. Furthermore, the ECU optimizes the fuel injection timing (injection timing, target injection timing) in such a manner that the fuel injection is terminated before an intake stroke of the engine. ■(■»* Now, advantages of the first embodiment will be described. As described above, in the rotational angle sensing device of the present embodiment, the throtde opening degree sensor 3 is sandwiched from the opposite sides in the thickness direction of the throtde opening degree sensor 3 by the holding pieces 31, 32 of the first and second yoke segments 4, 5 of the open type yoke. That is, the main body (resin housing) of the throttle opening degree sensor 3 is sandwiched between the sensor mounting part 33 of the holding piece 31 of the first yoke segment 4 and the sensor mounting part 33 of the holding piece 32 of the second yoke segment 5. Therefore, a gap is eliminated between each magnetism-sensing surface of the throttle opening degree sensor 3 and the corresponding opposed surface of the holding piece 31, 32 of the first or second yoke segment 4, 5. As a result, it is no longer required to accurately manage such a gap. In consequence, variations in the gap among products can be eliminated, so that characteristic variations among the products can be eliminated. That is, the output change characteristic of the Hall IC with respect to the rotational angle of the magnet 2 can be stabilized, and thereby variations in sensing accuracy among the products can be limited. Furthermore, the bending angle of each holding piece 31, 32 relative to the corresponding yoke main body 21, 22 of the yoke segments, 5 is set as the obtuse angle in such a manner that the throttle opening degree sensor 3 is placed within the plate widthwise dimension of each yoke main body 21, 22. In this way, an increase in the plate width dimension of the product can be limited. Thereby, a mounting space of the product in the vehicle can be easily secured. In each of the first and second yoke segments 4, 5, the base end (the magnet side end, i.e., the yoke opening end portion 23) of the yoke main body 21, 22 forms the maximum width portion of the yoke segment 4, 5 where the plate width of the yoke segment 4, 5 is maximum. Also, in each of the first and second yoke segments 4, 5, the distal end (the throttle opening degree sensor 3 side end, i.e., the distal end of the sensor mounting part 33) of the holding piece 31, 32 forms the minimum width portion of the yoke segment 4, 5 where the plate width of the yoke segment 4, 5 is minimum. The minimum width portion of the holding piece 31, 32 has the plate width, which is equal to or larger than the plate thickness of the magnet 2. In this way, the magnetic flux, which is emitted from the magnet 2, can be efficiently concentrated on the throttle opening degree sensor 3, particularly the Hall IC, so that the magnetic flux can be effectively applied across the the Hall IC. As a result, the output of the Hall IC is advantageously increased. In addition, the thin-plate shaped throttle opening degree sensor 3 is -inclined within the plate width (within the yoke height) of the yoke opening end portion 23, which forms the maximum width portion of the yoke segment 4, 5, and the bending angle of the holding piece 31, 32 is set to be the same in the first and second yoke segments 4, 5. Thereby, components (i.e., the plate-shaped yoke segments) of the open type yoke become the common components. That is, the open type yoke (the first and second yoke segments 4, 5) is formed by combining the plate-shaped yoke segments (magnetic bodies) of the same kind, and thereby the components can be used in common, thus reducing the costs. Each of the first and second yoke segments 4, 5 is formed in such a manner that the plate width thereof decreases in a stepwise manner or decreases continuously from the yoke opening end portion 23 toward the distal end of the sensor mounting part 33. Specifically, each of the first and second yoke segments 4, 5 is tapered, so that the magnetic flux is converged from the yoke opening end portion 23 toward the distal end of the sensor mounting part 33 or toward the magnetic flux sensing gap. Therefore, even when a size of the throttle opening degree sensor 3 is small, the magnetic flux, which is emitted from the magnetized surface (pole surface) of the magnet 2, can be efficiently applied to the throttle opening degree sensor 3, particularly the Hall IC. That is, the magnetic flux, which is emitted from magnetized surface (pole surface) of the magnet 2, can be effectively concentrated on the throttle opening degree sensor 3, particularly the Hall IC. Thus, the magnetic flux emitted from the magnet 2 can be effectively applied across the Hall IC, and thereby the output of the Hall IC can be advantageously increased. As a result, it is possible to achieve a minimum profile of the product, which includes the throttle opening degree sensor 3 and the open type yoke. In the rotational angle sensing device of the present embodiment, the intake module cover 11, which forms the sensor receiving space 17 between the intake module cover 11 and the top end surface of the plate base of the plate 12, is made of the magnetic material (the iron based metal material). Thereby, even when an external magnetic field or an external magnetic field source (for example, an alternator or the like) and a magnetic body (an iron screw) are placed in close proximity to the rotational angle sensing device, the magnetism from the external magnetic filed source and the magnetic body can be absorbed by the intake module cover 11, which is made of the magnetic body. As a result, influences of the external magnetic field or the magnetic body on the throttle opening degree sensor 3, particularly the Hall IC are limited or reduced. Thus, a change in the output change characteristic of the Hall IC with respect to the rotational angle of the magnet 2 can be limited. That is, a product quality can be improved. In the rotational angle sensing device of the present embodiment, the intake module cover 11 is made of the magnetic material, which has the relatively small electrical resistance, and the housing 14 is made of an aluminum alloy, which has the relatively small electrical resistance. Furthermore, a volume of the housing 14 is made larger than a volume of the intake module cover 11. Also, the joint end surface (inner peripheral surface) of the tubular wall 41 of the intake module cover 11 is in surface-to-surface contact with the joint end surface (outer peripheral surface) of the respective flanges 63 of the housing 14. Thereby, the radio wave noises, which approach the intake module cover 11, is released from the joint end surface of the tubular wall 41 of the intake module cover 11, which has the relatively small electrical resistance, to the flange 63 of the housing 14, which has a relatively large volume. Therefore, influence from the external magnetic field source and the magnetic body to the throttle opening degree sensor 3, particularly the Hall IC is limited. Thus, it is possible to effectively limit the change in the output change characteristic of the Hall IC with respect to the rotational angle of the magnet 2. That is, a quality of the product can be improved without an increase in the size of the product and without deteriorating the mountability of the product. In the rotational angle sensing device of the present embodiment, the flanges 63 of the housing 14, which is made of the aluminum alloy, are fixed to the tubular wall 41 of the intake module cover 11, which has the small linear expansion coefficient, by the metal bending process. Thus, the linear expansion movement of the epoxy thermosetting resin, which is filled inside the intake module cover 11, can be effectively limited. In addition, in the rotational angle sensing device of the present embodiment, the two concave portions 43, 44, which project toward the central portion of the sensor receiving space 17, are formed in the tubular wall 41 of the intake module cover 11. In consequence, the epoxy thermosetting resin, which is filled inside the intake module cover 11, is held by the two concave portions 43, 44, and thereby the linear expansion movement of the inner components (e.g., the throttle opening degree sensor 3 and the open type yoke) sealed in the thermosetting resin can be minimized. Thus, the output change characteristic of the Hall IC with respect to the rotational angle of the magnet 2 can be stabilized, and thereby variations in sensing accuracy among the products can be limited. Further, an electrical conduction failure, such as breaking of conductive wires, which electrically connect between the lead terminal group 3a of the throttle opening degree sensor 3 and the connector terminal group 13a of the connector 13, can be limited, so that the reliability of the throttle opening degree sensor 3 can be improved. That is, a product quality can be improved. Also, it is possible to limit an occurrence of an adverse phenomenon (migration), which is caused by separation of the thermosetting resin sheaths (covers) from the conductors that electrically connect between the lead terminal group of the throttle opening degree sensor 3 and the connector lead terminal group of the connector 13. When the thermosetting resin sheaths are separated from the conductors, it may cause a deterioration of the electrical insulation between the lead terminals of the lead terminal group of the throttle opening degree sensor 3, a deterioration of the electrical insulation between the plurality of the conductors as well as a deterioration of the electrical insulation between the connector terminals of the connector terminal group of the connector 13. (Second Embodiment) FIGS. 3 to 5 show a second embodiment of the present invention. Specifically, FIG. 3 is a diagram showing an intake module, which has a rotational angle sensing device according to the second embodiment. FIG. 4 is a diagram showing an intake air temperature sensor according to the second embodiment. FIG. 5 is a diagram showing an intake air pressure sensor according to the second embodiment. * The intake module of the present embodiment includes the rotational angle sensing device (see the first embodiment), the intake air temperature sensor 6 and the intake air pressure sensor 7. The rotational angle sensing device includes the magnet 2, the throttle opening degree sensor 3 and the open type yoke. The intake air temperature sensor 6 measures the temperature (intake air temperature) of the intake air, which is supplied to the combustion chamber of the engine. Then, the intake air temperature sensor 6 coverts the measured intake air temperature into an electrical signal and supplies it to the ECU. The intake air pressure sensor 7 measures the pressure (intake air pressure) of the intake air, which is supplied to the combustion chamber of the engine. Then, the intake air pressure sensor 7 converts the measured intake air pressure into an electrical signal and supplies it to the ECU. The intake air temperature sensor 6 includes a temperature sensing element such as a thermistor, in which a resistance value changes in accordance with a change in the intake air temperature. The intake air temperature sensor 6 includes a thermistor portion 71, which has a distal end that is exposed in the intake passage. The thermistor of the thermistor portion 71 is sealed in epoxy resin. Further, two terminals 73 are sealed in a resin housing (sealing member) 72, which forms a main body of the intake air temperature sensor 6. The thermistor is fixed (electrically connected) between one ends of these terminals 73. Furthermore, the other ends of the terminals 73, which are opposite from the thermistor, extend out of the resin housing 72 and form a lead terminal group 6a. This lead terminal group 6a includes a single output side lead terminal (temperature sensor output terminal), which is connected to an output side of the thermistor, and a single power source side lead terminal (temperature sensor power source terminal), which is connected to a power source side of the thermistor. The intake air pressure sensor 7 includes a pressure sensing element (e.g., a piezoresistive element) and a pressure sensing circuit (e.g., an amplifier circuit). The pressure sensing element converts an intake air pressure, which is introduced from an air introducing passage (a sensing port), into an electrical signal. The pressure sensing circuit amplifies the electrical signal, which is supplied from the pressure sensing element. The pressure sensing element and the pressure sensing circuit are sealed in a resin housing (sealing member) 74, which forms the main body of the intake air pressure sensor 7. A lead terminal group 7a extends out of the resin housing 74, which receives the pressure sensing element and the pressure sensing circuit. This lead terminal group 7a includes a single ground (GND)-side lead terminal (pressure sensor GND terminal), a single output-side lead terminal (pressure sensor output terminal), and a single power source-side lead terminal (pressure sensor power source terminal). The ground (GND)-side lead terminal is connected to a ground terminal of the pressure sensing circuit. The output-side lead terminal is connected to an output terminal of the pressure sensing circuit. The power source-side lead terminal is connected to a power source terminal of the pressure sensing circuit. In addition, the main body of the rotational angle sensing device (the yoke main bodies 21, 22 of the first and second yoke segments 4, 5 of the open type yoke), the main body of the intake air temperature sensor 6 and the main body of the intake air pressure sensor 7 are securely held by the plate 12. This plate 12 is fitted into the intake module cover 11 of the present embodiment. A single connector 13 is formed integrally at a side portion of the plate 12. A connector terminal group 13a is received in the connector 13 and is provided to correspond with the lead terminal group 3a of the main bodies (resin housings) of the throttle opening degree sensor 3, the lead terminal group 6a of the intake air temperature sensor 6 and the lead terminal group 7a of the intake air pressure sensor 7. The tubular wall 41 of the intake module cover 11 has an opening 45, which is opened externally. The connector 13 is fluid-tightly fitted into the opening 45 of the tubular wall 41 in such a manner that the connector 13 projects outwardly from an outer wall surface of the tubular wall 41 of the intake module cover 11. The connector 13 is a device, which includes a terminal base and a rectangular tubular connector shell. The terminal base holds the connector terminal group 13a. The rectangular tubular connector shell is disposed outside of the terminal base. The connector 13 connects an ECU-side wiring harness to the lead terminal group 3a of the throttle opening degree sensor 3, of the intake air temperature sensor 6 and of the intake air pressure sensor 7, which are mounted on the plate 12. The connector terminal group 13a includes first to fifth connector terminals (sensor-side connector terminal, external connection terminal and terminal), which are electrically connected to the lead terminals of the lead terminal group 3a of the throttle opening degree sensor 3, the lead terminals of the lead terminal group 6a of the intake air temperature sensor 6 and the lead terminals of the lead terminal group 7a of the intake air pressure sensor 7 through multiple conductors. The first connector terminal is electrically connected to the output-side lead terminal of the lead terminal group 6a of the intake air temperature sensor 6. The second connector terminal is electrically connected to the output-side lead terminal of the lead terminal group 3a of the throttle opening degree sensor 3. The third connector terminal is electrically connected to the GND-side lead terminal of the lead terminal group 7a of the intake air pressure sensor 7 and also to the GND-side lead terminal of the lead terminal group 3a of the throttle opening degree sensor 3. The fourth connector terminal is electrically connected to the output-side lead terminal of the lead terminal group 7a of the intake air pressure sensor 7. The fifth connector terminal is electrically connected the power source-side lead terminal of the lead terminal group 6a of the intake air temperature sensor 6, the power source-side lead terminal of the lead terminal group 7a of the intake air pressure sensor 7 and the power source-side lead terminal of the lead terminal group 3a of the throttle opening degree sensor 3. Here, like in the first embodiment, epoxy thermosetting resin is filled in the interior of the intake module cover 11, i.e., in the sensor receiving space 17. The thermosetting resin is a seal member, which seals each lead terminal of the lead terminal group 3a of the throttle opening degree sensor 3, each lead terminal of the lead terminal group 6a of the intake air temperature sensor 6, each lead terminal of the lead terminal group 7a of the intake air pressure sensor 7, the multiple conductors and each connector terminal of the connector terminal group 13a of the connector 13. Here, the ECU includes a microcomputer of a. known structure having a CPU, a storage device (e.g., memories, such as a ROM and RAM), an input circuit and an output circuit. The CPU performs various control operations and computing operations. The storage device stores various programs and data. The ECU electronically controls the injectors according to the control programs or control logics stored in the memory when an ignition switch (not shown) is turned on (IG ON). The ECU forcefully terminates the above control operations according to the control programs or control logics when the ignition switch (not shown) is turned off (IG OFF). Furthermore, the sensor signals, which are outputted from the throttle opening degree sensor 3, the intake air temperature sensor 6 and the intake air pressure sensor 7, undergo analog-to-digital conversion through an A/D converter and are thereafter supplied to the microcomputer of the ECU. In addition, the sensor signals, which are outputted from various other sensors, undergo analog-to-digital conversion through the A/D converter and are thereafter supplied to the microcomputer of the ECU. These other sensors include, for example, a crank angle sensor, which senses a rotational angle of the crankshaft of the engine, and an intake air quantity sensor, which senses an intake air quantity supplied into the combustion chamber of the engine. Next, an operation of the intake module of the present embodiment, which is installed to the intake pipe of the engine will be briefly described with reference with FIGS. 3 to 5. When the throttle operating component (e.g., the throttle lever or the throttle handle) is operated by the driver, the accelerator lever, which is connected to the throttle operating component through the wire cable, is rotated. When the accelerator lever is rotated, the shaft 1, which is connected to the accelerator lever, is rotated. In consequence, the throttle valve rotates about the rotational axis of the shaft 1 in accordance with the throttle operating amount caused by the driver. Thereby, the intake passage, which is communicated with the combustion chamber of the engine, is opened at the corresponding degree, so that the engine rotational speed is changed to a corresponding speed, which corresponds to the throttle operating amount caused by the driver. At this time, the the ECU, which receives the sensor signals from the various sensors (e.g., the throttle opening degree sensor 3, the intake air temperature sensor 6 and the intake air pressure sensor 7), computes a control target value, which is required by the electronically controlled fuel injection system. The ECU indirectly computes the intake air quantity based on the intake pipe pressure measured at the location downstream of the throttle valve through the intake air pressure sensor 7. Then, the ECU computes the basic injection time period based on the above computed intake air quantity and the measured engine rotational speed. Then, the ECU determines the final injection time period (the fuel injection quantity) in view of the above basic injection time period and a correction amount. The correction amount is determined based on the sensor signals of the various sensors (e.g., the intake air temperature sensor 6 and the Hall IC). Furthermore, the ECU optimizes the fuel injection timing in such a manner that the fuel injection is terminated before the intake stroke of the engine. (Third Embodiment) FIGS. 6 to 10 show a third embodiment of the present invention. Specifically, FIG. 6 is a diagram showing an entire structure of an intake module according to the third embodiment. FIG. 7 is a diagram showing a resin injection opening (port) for injecting thermosetting resin according to the third embodiment. FIG. 8 is a diagram showing a major structure of the intake module according to the third embodiment. FIGS. 9 to 10 are diagrams showing a major structure of the rotational angle sensing device according to the third embodiment. The intake module of the present embodiment includes the rotational angle sensing device, the intake air temperature sensor 6 and the intake air pressure sensor 7. The rotational angle sensing device includes the magnet 2, the throttle opening degree sensor 3 and the open type yoke. The intake air temperature sensor 6 measures the temperature (intake air temperature) of the intake air, which is supplied to the combustion chamber of the engine. Then, the intake air temperature sensor 6 coverts the measured intake air temperature into an electrical signal and supplies it to the ECU. The intake air pressure sensor 7 measures the pressure (intake air pressure) of the intake air, which is supplied to the combustion chamber of the engine. Then, the intake air pressure sensor 7 converts the measured intake air pressure into an electrical signal and supplies it to the ECU. Further, the resin injection port 57 is formed in the plate 12 of the present embodiment. Epoxy thermosetting resin (mold resin) 10 is injected into the sensor receiving space 17 through the resin injection port 57. Accordingly, each lead terminal of the lead terminal group 3a of the throttle opening degree sensor 3, each connector terminal of the connector terminal group 13a of the connector 13 and the multiple conductors are insert molded by the dielectric mold resin 10, which is injected in to the sensor receiving space 17 through the resin injection port 57. Here, the multiple conductors may be conductors (e.g., copper wires), each of which has a dielectric sheath (cover), or conductive plates. These multiple conductors electrically connect the lead terminals of the lead terminal group 3a of the throttle opening degree sensor 3 to the connector terminals of the connector terminal group 13a of the connector 13. In the open type yoke of the present embodiment, each of the first and second yoke segments 4, 5 includes a bent portion 26 in the yoke main body 21, 22. The bent portion 26 is arcuately bent toward the magnet 2 side. In addition, two mounting portions 55 are integrally formed at an outer peripheral part of the tubular wall 41 or at the plate base of the plate 12. The mounting portions 55 contact the top end surfaces of the flanges 63 of the housing 14. The mounting portions 55 of the intake module cover 11 or of the plate 12 are securely fixed to the top end surfaces of the flanges 63 of the housing 14 by fastening screws (for example, iron based magnetic bodies) 64. Furthermore, the plate base of the plate 12 includes a convex-cylindrical portion 56. The cylindrical portion 56 rotatably receives the magnet mounting portion of the shaft 1 of the throttle valve. The cylindrical portion 56 is located between the two plate thickness portions 53, 54 (yoke holding portions 51, 52) and forms the magnet receiving portion 19 to rotatably receive the magnet 2. In the drawings, number 65 indicates a circular through hole, which receives the corresponding fastening screw 64. Here, in the rotational angle sensing device of the present embodiment, the fastening screws (for example, iron based magnetic bodies) 64 are placed near the throttle opening degree sensor 3 and the two yoke segments 4, 5. However, when the intake module cover 11 is made of the magnetic material (iron based metal material), magnetic influences of the fastening screws (e.g., iron based magnetic bodies) 64 can be absorbed by the intake module cover 11. As a result, the magnetism influences of the fastening screws (e.g., iron based magnetic bodies) 64 on the throttle opening degree sensor 3, particularly the Hall IC are limited or reduced. Thus, a change in the output change characteristic of the Hall IC with respect to the rotational angle of the magnet 2 can be limited. (Fourth Embodiment) FIG. 11 shows a fourth embodiment of the present invention. Specifically, FIG. 11 is a diagram showing a major construction of a rotational angle sensing device according to the fourth embodiment. The open type yoke of the present embodiment includes the two divided yoke segments, i.e., the first and second yoke segments 4, 5. These yoke segments 4, 5 are formed as two different types of plate shaped yoke segments, which are opposed to each other while having the magnet receiving space 19 therebetween. Each of the first and second yoke segments 4, 5 is formed to have a corresponding predetermined shape. Furthermore, the first and second yoke segments 4, 5 are made of a magnetic material (e.g., iron) and form one set of plate-shaped yoke segments (magnetic bodies) for concentrating the magnetic flux emitted from the magnet 2 on the throttle opening degree sensor 3, particularly on the Hall IC (magnetic sensing element of the non-contact type). The first yoke segment 4 includes a yoke main body 21 and a holding piece 91. The yoke main body 21 is opened at one side. The holding piece 91 is bent by a predetermined bent angle relative to the yoke main body 21. The holding piece 91 of the first yoke segment 4 is connected to one of two lateral edges of the yoke opposing portion 24, which are opposed to each other in the width direction of the yoke opposing portion 24, through a bent portion 34, which is bent in a substantially V-letter (or substantially U-letter) shape at an obtuse angle that is larger than a right angle. The holding piece 91 of the first yoke segment 4 is formed by bending a projection piece, which projects in the width direction of the yoke opposing portion 24 from the one of the two lateral edges of the yoke opposing portion 24, which are opposed to each other in the width direction of the yoke opposing portion 24. Here, this projection piece is bent about the edge of the yoke opposing portion 24 toward the throttle opening degree sensor side (toward one side in the thickness direction of the yoke opposing portion 24). In this way, the sensor mounting part (yoke opposing portion) 33 of the first yoke segment 4 is formed at a back surface side of the holding piece 91 of the first yoke segment 4. The second yoke segment 5 includes a yoke main body 22 and a holding piece 92. The yoke main body 22 is opened at one side. The holding piece 92 is bent by a predetermined bent angle relative to the yoke main body 22. The holding piece 92 of the second yoke segment 5 is connected to one of two lateral edges of the yoke opposing portion 24, which are opposed to each other in the width direction of the yoke opposing portion 24, through a bent portion 34, which is bent in a substantially V-letter (or substantially U-letter) shape at a generally right angle. Here, it should be noted that the one of the two lateral edges of the yoke opposing portion 24 of the second yoke segment 5 and the one of the two lateral edges of the yoke opposing portion 24 of the first yoke segment 4 are located on the same lateral side. The holding piece 92 of the second yoke segment 5 is formed by bending a projection piece, which projects in the width direction of the yoke opposing portion 24 from the one of the two lateral edges of the yoke opposing portion 24, which are opposed to each other in the width direction of the yoke opposing portion 24. Here, this projection piece is bent about the edge of the yoke opposing portion 24 toward the throttle opening degree sensor side (toward one side in the thickness direction of the yoke opposing portion 24). In this way, the sensor mounting part (yoke opposing portion) 33 of the second yoke segment 5 is formed at a front surface side of the holding piece 92 of the second yoke segment 5. It should be noted that the sensor mounting part 33 of each of the first and second yoke segments 4, 5 has the plate width, which is smaller than the plate width of the bent portion 34. (Fifth Embodiment) FIGS. 12 to 19C show a fifth embodiment of the present invention. FIG. 12 is a diagram showing a rotational angle sensing device. FIGS. 13 and 14 are diagrams showing an intake module. A control system of an internal combustion engine (engine control system) according to the present embodiment includes an electronically controlled fuel injection system, an intake module and an engine control unit (ECU). The electronically controlled fuel injection system injects fuel into a combustion chamber of the internal combustion engine of a vehicle, such as a motorcycle. The intake module is incorporated into an intake system of the engine. The ECU controls the electronically controlled fuel injection system and the intake module. The electronically controlled fuel injection system is a system, which pressurizes fuel (for example, gasoline) at a predetermined pressure by an electric fuel pump and supplies the pressurized fuel to an injector (electromagnetic fuel injection valve) through a fuel filter, thereby injecting the fuel at optimal timing. The intake module is an intake air quantity control apparatus (intake passage opening/closing device or system), which controls an intake air quantity, i.e., a quantity of intake air supplied into the combustion chamber of the engine based upon an operating amount (hereinafter, referred to as a throttle operating amount) of a throttle operating component (e.g., a throttle lever or a throttle handle of a vehicle, such as a motorcycle), which is operated by a driver. It should be noted that the throttle operating amount corresponds to a an amount of depression (accelerator operating amount) of an accelerator pedal, which is depressed by a driver in a case of a four-wheeled vehicle. The intake module includes a housing, a throttle valve (a sensing object) 201 and a shaft 202. The housing is installed to an intermediate part of an engine intake pipe, which is connected to an intake port of the engine. The throttle valve 201 is received in the housing in such a manner that the throttle valve 201 is rotatable to open and close a throttle bore described below. The shaft 202 is a valve shaft, which is rotated integrally with the throttle valve 201. The housing includes a cylindrical throttle body 211, which forms a part of the engine intake pipe. The throttle body 211 includes a throttle bore wall (hereinafter referred to as cylindrical portion) 213 and two cylindrical bearing portions 214. An intake passage (hereinafter referred to as the throttle bore) 212 having a circular cross section is formed in the cylindrical portion 213. The cylindrical bearing portions 214 are provided at two opposed sides of the cylindrical portion 213, which are opposed to each other in an axial direction (axial direction of the shaft 202) that is perpendicular to an intake flow direction of the air in the throttle bore 212 of the cylindrical portion 213. The intake module of the present embodiment has the throttle valve 201 of a butterfly type, which is rotatably received in the cylindrical portion 213 of the throttle body 211 to open and close the throttle bore 212. The throttle valve 201 is joined integrally to the shaft 202, which extends in the axial direction of the shaft 202 (a diametrical direction of the throttle valve 201). The throttle valve 201 controls an intake air quantity, i.e., a quantity of intake air supplied into the combustion chamber of the engine by changing the rotational angle of the throttle valve 201 within an operable angular range that is between a fully closed position, at which the intake air quantity is minimized, and a fully opened position, at which the intake air quantity is maximized. The shaft 202 is made of a non-magnetic material (e.g., a non-magnetic metal material) and is formed into a generally cylindrical body. Two opposed ends of the shaft 202, which are opposed to each other in the axial direction of the shaft 202, are rotatably received by the bearing portions 214, which are provided at the opposed ends, respectively, of the cylindrical portion 213 of the throttle body 211. One axial end of the shaft 202 extends through the bearing portion 214 at the right side in FIG. 13 and is rotatably received in an interior (a hollow portion 216) of a cylindrical shaft fitting portion 215, which is connected to the bearing portion 214. A generally linear groove 217 is formed in the one end of the rotatable shaft 202 to extend in a direction perpendicular to the rotational axis (rotational center axis) of the shaft 202. The other end of the shaft 202 penetrates through the bearing portion 214 at the left side in FIG. 13 and projects outward from the throttle body 211. An accelerator lever 218 is fixed to the other end of the shaft 202 by, for example, a metal bending process. A wire cable, which is driven synchronously with a throttle operating component (e.g., a throttle lever or a throttle handle), is connected to the accelerator lever 218. Furthermore, a return spring 219 is mounted between the cylindrical portion 213 of the throttle body 211 and the accelerator lever 218. The return spring 219 applies a spring load against the accelerator lever 218 to urge the throttle valve 201 toward a fully closed position of the throttle valve 201. Here, the throttle body 211 is resin-molded integrally from a nonmagnetic material (e.g., a resin material, such as thermoplastic resin). A sensor holding portion 221 is formed integrally in the bearing portion 214 of the throttle body 211 at the right side in FIG. 13 to securely hold a throttle opening degree sensing unit (a rotational angle sensing unit) 204 described below. A sensor insert hole 222, which has a generally rectangular shape, is formed in the sensor holding portion 221 on a lateral side of a mounting portion of the throttle opening degree sensing unit 204 (particularly, a mounting portion of a Hall IC 205) to insert the throttle opening degree sensing unit 204 (particularly, the Hall IC 205) at a predetermined location (particularly, a magnetic flux sensing gap) of the sensor holding portion 221. The sensor insert hole 222 is opened at the right end in FIG. 13. An opening of the sensor insert hole 222 serves as a sensor insert port, through which the Hall IC (a magnetic sensing device having a magnetic sensing element, such as a Hall element) 205 is inserted into the predetermined location (particularly, the magnetic flux sensing gap) of the sensor holding portion 221. The opening of the sensor insert hole 222 is closed by installing a cover plate (sensor cover) 223, which is made of a resin material, to an outer wall surface of the sensor holding portion 221 through a heat welding process or the like after installation of the Hall IC 205 to the predetermined location of the sensor holding portion 221. In this way, it is possible to limit intrusion of foreign substances, such as water, into the interior of the sensor holding portion 221. The intake module of the present invention includes a non-contact type rotational angle sensing device (throttle opening degree sensing device). The rotational angle sensing device converts a rotational angle (a valve angle, a throttle opening degree) of the throttle valve 201 into an electrical signal to inform the opening degree of the throttle valve 201 to the ECU. The rotational angle sensing device of the present embodiment includes a thin-plate shaped magnet (permanent magnet: hereinafter referred to as a magnet) 203 and the throttle opening degree sensing unit 204. The magnet 203 is fixed to the one end of the shaft 202 of the throttle valve 201. The throttle opening degree sensing unit 204 senses a rotational angle of the throttle valve 201 by using an output change characteristic of the Hall IC 205 (described later) with respect to the rotational angle of the magnet 203. Here, the ECU of the present embodiment performs a fuel injection quantity control operation to control a valve-opening period of the injector in such a manner that a corresponding fuel injection quantity, which corresponds to an output of the throttle opening degree sensing unit 204, i.e., the electrical signal outputted from the throttle opening degree sensing unit 204, is supplied to each corresponding cylinder of the engine. The magnet 203 forms a magnet rotor, which rotates relative to the throttle opening degree sensing unit 204 and the throttle body (housing) 211 and is securely held at the one end of the shaft 202 of the throttle valve 201 to rotate upon rotation of the throttle valve 201 that serves as the sensing object. Specifically, the magnet 203 is securely held in the linear groove 217, which is formed in the shaft 202 of the throttle valve 201, by using fastening means, such as an adhesive or bonding agent. The magnet 203 has a square (or rectangular) shape when the magnet 203 is viewed in a direction perpendicular to a plane of FIG. 13. More specifically, the magnet 203 is a cuboid-shaped permanent magnet, which stably produces a long lasting magnetic force. Furthermore, the magnet 203 is made of, for example, a rare-earth magnet (e.g., a samarium-cobalt (Sm-Co) magnet or a neodymium (Nd) magnet), an alnico magnet or a ferrite magnet. An N pole and an S pole are magnetized in the magnet 203 in such a manner that opposed ends of the magnet 203, which are opposed to each other in a plate longitudinal direction of the magnet 203, have the opposite polarities, respectively. In addition, the magnet 203 is magnetized to implement parallel magnetization in such a manner that lines of magnetic force in the magnet 203 are parallel to each other. Furthermore, the magnet 203 is magnetized in a radial direction, which is perpendicular to the rotational axis (rotational center axis) of the shaft 202 of the throttle valve 201. In this way, the magnetizing direction (the longitudinal direction) of the magnet 203 coincides with a diametrical direction, which is perpendicular to the rotational axis of the shaft 202 of the throttle valve 201. Furthermore, the magnetized surface (magnetic pole surface) at the one longitudinal side of the magnet 203 forms the N pole, and the magnetized surface (magnetic pole surface) at the other longitudinal side of the magnet 203 forms the S pole. Here, the magnet 203 is rotatable about the rotational center thereof within an operable angular range that is between the fully closed position and the fully opened position of the throttle valve 201 in the hollow portion 216, particularly in a magnet receiving space (magnet receiving portion) 224 formed in the interior of the throttle opening degree sensing unit 204. Therefore, in the present embodiment, as shown in FIGS. 17A and 19A, when the throttle valve is placed in the fully closed position, the rotational angle of the magnet 203 becomes a minimum angle (e.g., 0 degree) in the operable angular range (detectable angular range) of the throttle valve 201. Furthermore, as shown in FIGS. 17A and 19B, when the throttle valve 201 is placed in an intermediate position, i.e., a middle position, the rotational angle of the magnet 203 becomes an intermediate angle, i.e., a middle angle (e.g., 40 degrees) in the operable angular range of the throttle valve 201. Furthermore, as shown in FIGS. 17A and 19B, when the throttle valve 201 is placed in the fully opened position, the rotational angle of the magnet 203 becomes the maximum angle (e.g., 80 degrees) in the operable angular range of the throttle valve 201. When the rotational angle of the magnet 203 becomes the maximum angle, an edge (a lowest end of the magnet) 225 of an outer surface of the magnet 203 is placed at a furthermost location, which is furthermost from the Hall IC 205 that serves as the magnetic sensing element described below. The throttle opening degree sensing unit 204 of the present embodiment includes the Hall IC 205 and a non-cylindrical stator core, which forms a stator that is fixed to the throttle body (housing 211). The Hall IC 205 senses a density of a magnetic flux, which is emitted from the magnetic pole surface of the magnet 203. The stator core includes two divided portions, which are symmetrical with each other and hold the Hall IC 205 therebetween. Specifically, the two divided portions of the stator core are symmetrical to each other about an imaginary center plane, which includes a reference line R that connects between the rotational center of the magnet 203 and the thickness center of the Hall IC 205 in the thickness direction of the Hall IC 205, and which also includes the rotational axis (rotational center axis) of the shaft 202 of the throttle valve 201. Hereinafter, one of the two divided portions of the stator core will be referred to as a first yoke segment (a first stator yoke segment) 206, and the other one of the two divided portions of the stator core will be referred to as a second yoke segment (a second stator yoke segment) 207. The first and second yoke segments 206, 207 are magnetic bodies that are used to concentrate the magnetic flux, which is emitted from the magnetized surface (magnetic pole surface) of the magnet 203, onto the Hall IC 205. The Hall IC 205, which serves as the rotational angle sensor, is an integrated circuit (IC), which includes the Hall element(s) and an amplifier circuit. The Hall element(s) serves as a non-contact type magnetic sensing element(s), an output of which changes according to a change in the magnetic flux density, i.e., the density of the magnetic flux (density of the magnetic flux passing through the Hall IC 205) passing through the magnetic flux sensing gap, which is formed in the magnetic circuit, specifically between the first yoke segment 206 and the second yoke segment 207. The amplifier circuit amplifies the output of the Hall element(s). The Hall IC 205 outputs an electrical voltage signal in accordance with the density of the magnetic flux (density of the magnetic flux passing through the Hall IC 205) passing through the magnetic flux sensing gap. It should be noted that the Hall IC 205 may have a function of externally executing electrical trimming of a correction program(s) for an output gain adjustment, an offset adjustment and a temperature characteristic correction with respect to the magnetic flux density and may also have a self-diagnosis function for diagnosing, for example, breaking of wires or short-circuit. The Hall IC 205 is placed in the magnetic sensing gap in such a manner that the center of the Hall IC 205 is located along the perpendicular line, which is perpendicular to the rotational axis (rotational center axis) of the shaft 202 of the throttle valve 201. Specifically, the Hall IC 205 is placed in the magnetic sensing gap in such a manner that the center of the magnet 203 and the center of the Hall IC 205 are substantially positioned on the same line (reference line R). In addition, the Hall IC 205 includes two opposed magnetism-sensing surfaces (two magnetically sensitive surfaces), each of which has a constant width. The magnetism-sensing surfaces are parallel to the imaginary center plane, which includes the reference line R that connects between the rotational center of the magnet 203 and the thickness center of the Hall IC 205 in the thickness direction of the Hall IC 205, and which also includes the rotational axis (rotational center axis) of the shaft 202 of the throttle valve 201. These magnetism-sensing surfaces are provided to two opposed sides of the Hall IC 205, which are opposed to each other in the thickness direction (left-to-right direction in FIG. 15) of the Hall IC 205. Here, in the present embodiment, the Hall IC 205 is inserted into the sensor insert hole 222 and is fitted into the magnetic flux sensing gap, which is formed between the first yoke segment 206 and the second yoke segment 207. In this way, the Hall IC 205 is installed in a predetermined location of the sensor holding portion 221 of the throttle body 211. Thereby, the Hall IC 205 is fitted into and is positioned in the magnetic flux sensing gap, and lead lines of the Hall IC 205 (two output terminals and a single power supply terminal) are electrically and mechanically connected to connector pins (terminals not shown), which are insert-molded in the sensor holding portion 221, by a connecting means (e.g., resistance welding). The first and secopd yoke segments 206, 207 are symmetrical open type yoke segments and are made of a magnetic material, such as iron (magnetic body). Each of the first and second yoke segments 206, 207 includes an opposing portion (a vertical portion) 231, 241 at one end side (an upper end side in FIG. 12). The vertical portions 231, 241 are opposed to each other while the magnetic flux sensing gap is interposed therebetween. The Hall IC 205 is placed in the space (magnetic flux sensing gap) between the vertical portions 231, 241. Two bent portions 232, 242 are provided to lower ends of the vertical portions 231, 241, respectively, in FIG. 12. The bent portions 232, 242 are bent substantially at right angles (L-letter shape) in opposite directions, respectively. The bent portions 232, 242 are connected to opposing-side ends of yoke opening-side extension portions 233, 343 respectively, described below. Furthermore, the first and second yoke segments 206, 207 have yoke opening end portions 237, 247, respectively. Each of the yoke opening end portions 237, 247 forms a predetermined air gap (e.g., a minimum air gap) etween the yoke opening end portion 237, 247 and a corresponding one of the pposed ends (opposed magnetized surfaces) of the magnet 203 that are pposed to each other in the magnetizing direction of the magnet 203 upon ositioning of the magnet 203 at the maximum angle or therearound (e.g., 80 egrees or therearound) in the operable angular range of the throttle valve 201. "he yoke opening end portions 237, 247 are opposed to each other while the ion-circular magnet receiving space (magnet receiving space) 224 is interposed herebetween. The magnet receiving space 224 rotatably receives the one end if the shaft 202 of the throttle valve 201 and the magnet 203. The yoke opening md portions 237, 247 will be described in detail latter. Furthermore, the first and second yoke segments 206, 207 include the roke opening-side extension portions 233, 343, respectively. The yoke opening-;ide extension portions 233, 343 extend distally from the bent portions 232, 242, vhich form the lower ends of the vertical portions 231, 241 in FIG. 12, toward he yoke opening end portions 237, 247, respectively. The yoke opening-side extension portions 233, 343 are connected to lower ends of the vertical portions >31, 241 through the bent portions 232, 242 in FIG. 12. Furthermore, each of he yoke opening-side extension portions 233, 343 forms the corresponding air jap between the yoke opening-side extension portion 233, 343 and the :orresponding one of the opposed ends (opposed magnetized surfaces) of the magnet 203 that are opposed to each other in the magnetizing direction of the magnet 203 throughout the entire operable angular range of the throttle valve 201. Each of the yoke opening-side extension portions 233, 343 includes a linear portion (shoulder) 234, 244, a U-shaped portion (turned portion) 235, 245 and an arcuate portion 236, 246. The linear portion 234, 244 extends linearly from the lower end of the corresponding vertical portion 231, 241 away from the Hall IC 205 in the left or right direction in FIG. 12. The U-shaped portion 235, 245 extends from a left or right end of the corresponding linear portion 234, 244 in a direction away from the Hall IC 205 and is curved into an inverted U-shape. The arcuate portion 236, 246 extends from the corresponding U-shaped portion 235, 245 in the direction away from the Hall IC 205 and is curved into an arcuate shape. The two vertical portions 231, 241 are Hall IC holding portions, which contact the magnetism-sensing surfaces, respectively, of the Hall IC 205 and are opposed to each other while the magnetic flux sensing gap is interposed therebetween. Each of the vertical portions 231, 241 has a plate width, which is smaller than the rest of the open type yoke segment. Furthermore, the vertical portions 231, 241 are in parallel with the imaginary center plane, which includes the reference line R that connects between the rotational center of the magnet 203 and the thickness center of the Hall IC 205 in the thickness direction of the Hall IC 205, and which also includes the rotational axis (rotational center axis) of the shaft 202 of the throttle valve 201. In addition, the vertical portions 231, 241 are bent substantially at the right angles from the ends of the linear portions 234, 244, which are located on the magnetic flux sensing gap side of the two linear portions 234, 244, away from the rotational center axis of the shaft 202 and of the magnet 203. The first and second yoke segments 206, 207 are insert-molded inside the sensor holding portion 221 in such a manner that at least the opposed surfaces of the vertical portions 231, 241 are exposed in the sensor insert hole 222. The linear portions 234, 244 extend generally linearly away from each other in the perpendicular direction (the left-to-right direction in FIG. 12), which is generally perpendicular to the above imaginary center plane. In addition, each of the linear portions 234, 244 is disposed at the most distant position in the perpendicular direction from the one end (N pole) of the magnet 203 in the first or second yoke segment 206, 207, so that a largest air gap is formed between the linear portion 234, 244 and the one end (N pole) of the magnet 203. Each of the U-shaped portions 235, 245 is an arcuately turned portion, which is bent at a generally acute angle from the left or right end of the corresponding linear portion 234, 244 toward the corresponding arcuate portion 236, 246 in FIG. 12 and is connected to the upper end of the corresponding arcuate portion 236, 246. Each of the U-shaped portions 235, 245 forms a maximum air gap between the U-shaped portion 235, 245 and the corresponding end of the magnet 203 among all of the air gaps formed between the corresponding end of the magnet 203 and the inner peripheral surface of the first or second yoke segment 206, 207. Each of the U-shaped portions 235, 245 is formed into the inverted U-shape, which is convexed in a direction away from the magnet 203 and extends from the left or right end of the corresponding linear portion 234, 244 toward the corresponding arcuate portion 236, 246. Furthermore, each of the arcuate portions 236, 246 is formed into an arcuate shape, which is curved convexiy toward the magnet 203 and extends from the lower end of the corresponding U-shaped portion 235, 245 toward the side opposite from the the Hall IC 205. Here, in the first yoke segment 206 of the present embodiment, the linear portion 234, the U-shaped portion 235 and the arcuate portion 236 form a generally reversed 7-shaped body portion. In the second yoke segment 207 of the present embodiment, the linear portion 244, the U-shaped portion 245 and the arcuate portion 246 form a generally 7-shaped body portion. Furthermore, in the first yoke segment 206, the linear portion 234 and the U-shaped portion 235 forms a generally oshaped body portion. Also, in the second yoke segment 207, the linear portion 244 and the U-shaped portion 245 forms a generally >-shaped body portion. The magnetic flux sensing gap is a gap, which is formed between the vertical portion 231 of the first yoke segment 206 and the vertical portion 241 of the second yoke segment 207 and extends linearly from a magnet side end thereof toward a side away from the magnet 203 in a radial direction relative to the rotational axis (rotational center axis) of the shaft 202 of the throttle valve 201 while maintaining a constant width of the gap. The Hall IC 205 is placed in the magnetic flux sensing gap in such a manner that the center of the magnet 203 and the center of the Hall IC 205 are located along the same line R. The magnetic flux sensing gap is positioned relative to the magnetizing direction of the magnet 203 in the middle of the magnetic circuit, which is made of the magnet 203 and the first and second yoke segments 206, 207, to satisfy the following positional relationship. That is, when the magnet 203 is rotated to a location where the air gap between the magnet 203 and the yoke opening end portion 237, 247 is minimum, the density of the magnetic flux, which flows across the opposed magnetism-sensing surfaces of the Hall IC 205, becomes relatively large. Here, when the magnet 203 is placed in such a manner that the longitudinal direction (the magnetizing direction) of the magnet 203 coincides ■ Next, details of the yoke opening-side extension portions 233, 343 of the first and second yoke segments 206, 207, particularly the arcuate portions 236, 246 and the yoke opening end portions 237, 247 will be described in detail with reference to FIGS. 12 to 19C. Here, FIG. 15 is a diagram showing the flow of the magnetic flux when the rotational angle of the magnet 203 is held in the Ttaximum angle in the operable angular range. Each of the yoke opening-side extension portions 233, 343 includes the arcuate portion 236, 246, which is convexed toward the magnet 203, at the yoke opening end side of the first or second yoke segment 206, 207. In the present embodiment, each arcuate portion 236, 246 is arranged at the yoke opening end side of the first or second yoke segment 206, 207 from the lower end of the corresponding U-shaped portion 235, 245 to the distal end surface 238, 248 of the corresponding yoke opening end portion 237, 247. That is, each of the yoke opening end portions 237, 247 includes the arcuate portion 236, 246. Here, the arcuate portions 236, 246 serve as gathering portions which are arranged around the magnet 203 to gather the magnetic flux of the magnet 203. Also, each of the arcuate portions 236, 246 forms the corresponding predetermined air gap relative to the magnet 203 on the side opposite from the Hall IC 205. In addition, the arcuate portions 236, 246 and the U-shaped portions 235, 245 are arranged to satisfy the following condition. That is, when the magnet 203 is rotated by a predetermined angle from the location where the air gap between the corresponding one of the opposed end surfaces of the magnet 203 (the magnetized end surfaces that are opposed to each other in the magnetizing direction of the magnet 203) and the inner surface of the corresponding first or second yoke segment 206, 207 is minimum toward a location where the air gap is increased, this air gap is rapidly increased. Furthermore, the arcuate portions 236, 246 are arcuately curved in such a manner that the distance between the arcuate portions 236, 246 increases toward the lower ends of the U-shaped portions 235, 245 of the first and second yoke segments 206, 207 in the drawings. In the rotational angle sensing device of the present embodiment, the yoke opening-side extension portions 233, 343 of the first and second yoke segments 206, 207, particularly, the arcuate portions 236, 246 and the yoke opening end portions 237, 247 are positioned relative to the magnet 203 to implement the positional relationship shown in FIGS. 12 to 15. Here, in a plane of FIG. 15, a direction of Y coincides with a direction of the reference line R, and a direction of X is perpendicular to the direction of Y In FIG. 15, the position of the rotational axis of the magnet 203 in the direction of Y is set to a predetermined position (hereinafter, referred to as a reference position) C, at which the air gap between the one of the opposed end surfaces of the magnet 203 (opposed magnetized end surfaces of the magnet 203) and the inner surface of the corresponding adjacent one of the the first and second yoke segments 206, 207 as well as the air gap between the other one of the opposed end surfaces of the magnet 203 and the inner surface of the corresponding adjacent one of the first and second yoke segments 206, 207 are both minimum while the magnet 203 is held in the maximum rotational angle (e.g., 80 degrees) in the operable angular range of the magnet 203 for implementing the maximum output of the Hall IC 205 in the operable range. In FIG. 15, for descriptive purpose, there is indicated a line S, which extends in the direction of X that is perpendicular to the rotational axis of the magnet 203 at the reference position C and is also perpendicular to the reference line R, i.e., perpendicular to the direction of Y. At this line S, a yoke opening-side length, which is a distance measured from the line S on the yoke opening side (lower side in FIG. 15), is 0 (zero) mm. Furthermore, a distance (J) measured in the direction of Y between the rotational axis of the magnet 203 at the reference position C and the edge 225 of the magnet 203 (the lowest end of the magnet 203, i.e., the furthermost point of the magnet 203 that is furthest from the Hall IC 205), i.e., a distance (J) between the line S and the edge 225 of the magnet 203, is generally equal to a distance (L) measured in the direction of Y between the rotational axis of the magnet 203 at the reference position C and the distal end surface (the opening-side yoke distal end surface) 248 of the adjacent yoke opening end portion 247, which is adjacent to the edge 225, i.e., a distance (L) between the line S and the distal end surface 248 of the yoke opening end portion 247. Here, when a distance measured in the direction of Y between the edge 225 of the magnet 203 and the distal end surface 248 of the yoke opening end portion 247 is in a range of 0 mm to 1.5 mm, desirably, in a range of 1.1 mm to 1.2 mm, the distance measured in the direction of Y between the rotational axis of the magnet 203 at the reference position C and the edge 225 of the magnet 203 is considered to be generally equal to the distance measured in the direction of Y between the rotational axis of the magnet 203 at the reference position C and the distal end surface 248 of the adjacent yoke opening end portion 247. This range may vary based on a thickness of the magnet 203. That is, when the plate thickness of the magnet 203 is increased, this range may be increased. Here, the distal end surface 248 of the yoke opening end portion 247 is defined as the distal end surface 248 of the adjacent yoke opening end portion 247 since the distal end surface 248 of the yoke opening end portion 247 is placed adjacent to and is opposed to the lower one of the opposed magnetic pole surfaces of the magnet 203 at the time of positioning the magnet 203 at the maximum angle in the operational angular range (the maximum output state of the Hall IC 205 in the operable angular range). Furthermore, now it is assumed that the position of the rotational axis of the magnet 203 in the direction of Y is set to the predetermined position (the reference position) C, at which the air gap between the one of the opposed end surfaces of the magnet 203 (opposed magnetized end surfaces of the magnet 203) and the inner surface of the corresponding adjacent one of the the first and second yoke segments 206, 207 as well as the air gap between the other one of the opposed end surfaces of the magnet 203 and the inner surface of the corresponding adjacent one of the first and second yoke segments 206, 207 are both minimum while the magnet 203 is held at the angle (e.g., 90 degrees), which is greater than the operable angular range of the magnet 203. As described above, the line S extends perpendicular to the rotational axis of the magnet 203 at the reference position C and also perpendicular to the reference line R, i.e., perpendicular to the direction of Y. At this line S, the yoke opening-side length, which is a distance measured from the line S on the yoke opening side (lower side in FIG. 15), is 0 (zero) mm. In this state, a portion of each yoke segment 206, 207 is arcuately curved (inversely curved) in a direction away from the reference line R and extends by a predetermined arcuate length from the line S toward the lower end of the U-shaped portion 235, 245 of the yoke segment 2t)6, 207. Furthermore, the yoke opening end portion 237, 247 of each yoke segment 206, 207 is arcuately curved (inversely curved) in a direction away from the reference line R and extends by a predetermined arcuate length from the line S toward the distal end surface 238, 248. Here, a yoke opening-side length (L) is defined as a linear distance measured in the direction of Y between the line S (the reference position C) and the distal end surface 238, 248 of the yoke opening end portion 237, 247 (a length of each of the yoke opening end portions 237, 247 measured in the direction of Y). Alternatively, the distance between the line S (the reference position C) and the distal end surfaces 238, 248 may be change to a length of the arc of each of the yoke opening end portions 237, 247. Also, the yoke opening end portion 237 may be set as a reference position (L=0 mm), and the yoke opening end portion 247 may be lengthened by an amount of the predetermined yoke opening-side length (L). Next, a first experiment will be described. In the first experiment, the rotational angle (degrees) of the magnet 203 and the length of the yoke opening end portion 237, 247 of each of the first and second yoke segments 206, 207 are changed, and a change in the density of the magnetic flux, which passes through the magnetic flux sensing gap, i.e., the density of the magnetic flux, which flows across the Hall IC (IC portion) 205, is observed. Specifically, in the first experiment, the length (the yoke opening-side length L) of the yoke opening end portion 237, 247 of each of the first and second yoke segments 206, 207 relative to the rotational angle (degrees) of the magnet 203 is changed in a range of -1.5 mm to +6.0 mm, and the density of the magnetic flux across the Hall IC 205 is monitored. A result of this first experiment is indicated by a graph in FIG. 16. Here, the magnet 203, which is used in the experiment 1, has the same standard as the magnet of the fifth embodiment. Specifically, each of a plate thickness and a plate width of the plate-shaped magnet 203 is 1.5 mm, and a plate length of the magnet 203 is 5.4 mm. The magnet 203 is magnetized in the longitudinal direction of the magnet 203. The longitudinal direction of the magnet 203 coincides with (corresponds to) the radial direction that is perpendicular to the rotational axis (the rotational center axis) of the shaft 202 of the throttle valve 201. The plate width of the magnet 203 may be lengthened or shortened relative to the plate thickness of the magnet 203. The yoke opening-side extension portions 233, 343 of the first and second yoke segments 206, 207, particularly, the arcuate portions 236, 246 and the yoke opening end portions 237, 247 may have a plate width of about 1.5 mm, which is generally the same as that of the magnet 203. The yoke opening-side length (L) of the fifth embodiment is 1.1 mm, and the yoke opening-side length (L) of the previously proposed technique (see FIG. 23B) is 6 mm. As is clearly understood from the graph of FIG. 16, in the case where the rotational angle of the magnet 203 is 10 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than a range of 0 mm to 3 mm, the density of the magnetic flux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 10 degrees, the most efficient result (i.e., achievement of the maximum density of the magnetic flux that passes through the magnetic flux sensing gap, resulting in achievement of the maximum output of the Hall IC 205) is obtained when the yoke opening-side length (L) is 2.5 mm. Furthermore, in the case where the rotational angle of the magnet 203 is 20 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than the range of 0 mm to 3 mm, the density of the magnetic fiux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 20 degrees, the most efficient result is obtained when the yoke opening-side length (L) is 3 mm. Furthermore, in the case where the rotational angle of the magnet 203 is 30 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than a range of 0.5 mm to 3 mm, the density of the magnetic flux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 30 degrees, the most efficient result is obtained when the yoke opening-side length (L) is 3 mm. Furthermore, in the case where the rotational angle of the magnet 203 is 40 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than a range of 0.5 mm to 3 mm, the density of the magnetic flux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 40 degrees, the most efficient result is obtained when the yoke opening-side length (L) is 2.5 mm. Furthermore, in the case where the rotational angle of the magnet 203 is 50 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than the range of 0 mm to 3 mm, the density of the magnetic flux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 50 degrees, the most efficient result is obtained when the yoke opening-side length (L) is 1.5 mm. Furthermore, in the case where the rotational angle of the magnet 203 is 60 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than the range of 0 mm to 3 mm, the density of the magnetic flux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 60 degrees, the most efficient result is obtained when the yoke opening-side length (L) is 2 mm. Furthermore, in the case where the rotational angle of the magnet 203 is 70 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than the range of 0 mm to 3 mm, the density of the magnetic flux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 70 degrees, the most efficient result is obtained when the yoke opening-side length (L) is 2 mm. Furthermore, in the case where the rotational angle of the magnet 203 is 80 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than the range of 0 mm to 3 mm, the density of the magnetic flux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 80 degrees, the most efficient result is obtained when the yoke opening-side length (L) is 1.5 mm. Furthermore, in the case where the rotational angle of the magnet 203 is 90 degrees in the operable angular range, when the yoke opening-side length (L) is greater than or smaller than the range of 0 mm to 3 mm, the density of the magnetic flux is relatively small. Thus, the output of the Hall IC 205 becomes relatively small. In the case where the rotational angle of the magnet 203 is 90 degrees, the most efficient result is obtained when the yoke opening-side length (L) is 1.5 mm. Thus, as clearly understood from FIG. 16, in the case where the yoke opening-side length (L) is in the range of 0.5 mm to 3 mm, the density of the magnetic flux is increased regardless of the rotational angle of the magnet 203, thereby resulting in the improved efficiency. Furthermore, in the case where the yoke opening-side length (L) is in a range of 0.7 mm to 3 mm, the density of the magnetic flux is increased regardless of the rotational angle of the magnet 203, thereby resulting in the improved efficiency. Particularly, in the case where the yoke opening-side length (L) is in a range of 1.1 mm to 2 mm, the highest efficiency is obtained. Thereby, in the fifth embodiment, as shown in FIG. 17A, the yoke opening-side length (L) is set to be 1.1 mm. Furthermore, in the rotational angle sensing device of the present embodiment, the yoke opening-side length (L) is set to the most appropriate value, so that the the length of the yoke opening end portion 237, 247 of each of the first and second yoke segments 206, 207, i.e., the yoke opening-side length (L) is set such that a magnetic resistance of the magnetic circuit part (A), which is not related to the magnetic flux sensing gap, becomes larger than a magnetic resistance of the magnetic circuit part (B), which is related to the magnetic flux sensing gap in a range between the intermediate angle (e.g., 40 degrees) and the maximum angle (e.g., 80 degrees) in the operable angular range. In this way, the required output of the Hall IC 205 can be obtained in the operable angular range of the throttle valve 201 without increasing the size or magnetic force of the magnet 203. Here, as shown in RGS. 17A-17B and 19A-19C, the magnetic circuit part (A) is defined as a magnetic circuit part, in which the magnetic flux, which is emitted from the one of the opposed magnetic pole surfaces of the magnet 203 opposed to each other in the longitudinal direction of the magnet 203, does not pass through the magnetic flux sensing gap (Hall IC 205). Furthermore, the magnetic circuit part (B) is defined as a magnetic circuit part, in which the magnetic flux, which is emitted from the one of the opposed magnetic pole surfaces of the magnet 203 opposed to each other in the longitudinal direction of the magnet 203, passes through the magnetic flux sensing gap (Hall IC 205). Next, an operation of the intake module cover, which includes the rotational angle sensing device of the present embodiment, will be briefly described with reference with FIGS. 12 to 19C. When the throttle operating component (e.g., the throttle lever or the throttle handle) is operated by the driver, the accelerator lever 218, which is connected to the throttle operating component through the wire cable, is rotated. Therefore, the throttle valve is rotated about the center axis (rotational axis) of the shaft 202 of the throttle valve 201 in accordance with the throttle operating amount caused by the driver. Thereby, the throttle bore 212, which is communicated with the combustion chamber of the engine, is opened at the corresponding degree, so that the engine rotational speed is changed to a corresponding speed, which corresponds to the throttle operating amount caused by the driver. Further, the ECU, which receives the electrical signal (throttle opening degree signal) outputted from the Hall IC 205 of the throttle opening degree sensing unit 204, computes a control target value (fuel injection timing and fuel injection quantity), which is required by the electronically controlled fuel injection system. The ECU indirectly computes the intake air quantity based on the intake pipe pressure measured at the location downstream of the throttle valve 201 through, for example, an intake air pressure sensor. Then, the ECU computes the basic injection time period (basic fuel injection quantity) based on the above computed intake air quantity and the measured engine rotational speed. Then, the ECU determines a final injection time period (a fuel injection quantity, a target injection quantity) in view of the above basic injection time period and a correction amount (an injection quantity correction amount). The correction amount is determined based on the output value of the Hall IC 205 of the throttle opening degree sensing unit 204. Furthermore, the ECU optimizes the fuel injection timing (injection timing, target injection timing) in such a manner that the fuel injection is terminated before an intake stroke of the engine. Next, a second experiment will be described. In the second experiment, the rotational angle of the magnet 203 is changed from the minimum angle (e.g., 0 degree) to the maximum angle (e.g., 80 degrees) in the operable angular range through the intermediate angle (e.g., 40 degrees), and a change in the output of the Hall IC 205 is observed. In the second experiment, the output of the Hall IC 205 is monitored while the rotational angle of the magnet 203 is changed. A result of the second experiment is shown in the graph of FIG. 18. In the graph of FIG. 18, a bold solid line indicates an experimental result of the rotational angle sensing device of the fifth embodiment, in which the yoke opening-side length (L) is set to be 1.1 mm. Furthermore, a fine bent solid line indicates an experimental result of the previously proposed technique (the rotational angle sensing device disclosed in JP-2005-345250A), in which the yoke opening-side length (L) is set to be 6 mm. Here, in the rotational angle sensing device of the fifth embodiment, each of the plate thickness and the plate width of the magnet 203 is 1.5 mm, and the plate length of the magnet 203 is 5.4 mm. Furthermore, the magnet 203 is a cuboid-shaped permanent magnet, which is magnetized in the direction parallel to the longitudinal direction of the magnet 203. Also, the first and second yoke segments 206, 207 of the throttle opening degree sensing unit 204 are the open type yoke segments, one side of which is opened. At the time of idling operation of the engine, i.e., at the time of fully closing the throttle valve 201 (i.e., time of setting 0% throttle opening degree), the rotational angle of the magnet 203, which is fixed to the one end of the shaft 202 of the throttle valve 201, becomes the minimum angle (e.g., 0 degree) in the operable angular range (detectable angular range) of the throttle valve 201. In such a case, the axial line, which extends in the longitudinal direction of the magnet 203, and the axial line, which extends in the longitudinal direction of the Hall IC 205, are placed on the same straight line (see FIGS. 17A and 19A). In a case where the yoke opening-side length (L) is set to be 1.1 mm, as shown in FIGS. 12, 15, 17A and 19A, the magnetic circuit part (A) is formed to create the flow of the magnetic flux through one of the magnetic poles (e.g., the N pole or S pole) of the magnet 203, the yoke opening-side extension portion 233 of the first yoke segment 206 (specifically, through the distal end surface 238 of the yoke opening end portion 237, the arcuate portion 236, the U-shaped portion 235 and the linear portion 234) and the other one of the magnetic poles (e.g., the S pole or N pole) of the magnet 203 in this order. Furthermore, the magnetic circuit part (B) is formed to create the flow of the magnetic flux through the N pole (or the S pole) of the magnet 203, the yoke opening-side extension portion 243 of the second yoke segment 207 (specifically, through the distal end surface 248 of the yoke opening end portion 247, the arcuate portion 246, the U-shaped portion 245 and the linear portion 244) and the S pole (or the N pole) of the magnet 203 in this order. At this time, the magnetic flux does not pass through the magnetic flux sensing gap (the Hall IC 205), so that the output of the Hall IC 205 becomes substantially 0 (zero), as shown in the graph of FIG. 18. Alternatively, the rotational angle of the magnet 203 may become such that the density of the magnetic flux that passes through the magnetic flux sensing gap (density of the magnetic flux that flows across the Hall IC 205) is relatively small. Thus, in the case where the rotational angle of the magnet 203 becomes the minimum angle (e.g., 0 degrees) in the operable angular range (detectable angular range) of the throttle valve 201, the output of the Hall IC 205 with respect to the rotational angle of the magnet 203 becomes substantially 0 (zero), as clearly understood from FIG. 18. Then, when the magnet 203 is rotated by 40 degrees in the counterclockwise direction about the rotational center thereof from the rotational angle of 0 degree, the magnetic circuit part (A) is formed to create the flow of the magnetic flux through the N pote (or the S pole) of the magnet 203, the yoke opening-side extension portion 233 of the first yoke segment 206 (specifically, through the distal end surface 238 of the yoke opening end portion 237, the arcuate portion 236) and the S pole (or the N pole) of the magnet 203 in this order, as indicated in FIGS. 12, 15, 17A and 19B. Furthermore, the magnetic circuit part (B) is formed to create the flow of the magnetic flux through the N pole (or the S pole) of the magnet 203, the yoke opening-side extension portion 243 of the second yoke segment 207 (specifically, through the distal end surface 248 of the yoke opening end portion 247, the arcuate portion 246, the U-shaped portion 245 and the linear portion 244), the bent portion 242, the vertical portion 241, the Hall IC 205, the vertical portion 231, the bent portion 232, the yoke opening-side extension portion 233 of the first yoke segment 206 (specifically, through the linear portion 234, the U-shaped portion 235 and the arcuate portion 236) and the S pole (or the N pole) of the magnet 203 in this order. At this time, the rotational angle of the magnet 203 may become such that the density of the magnetic flux that passes through the magnetic flux sensing gap (density of the magnetic flux that flows across the Hall IC 205) is generally an intermediate level. In this way, the output of the Hall IC 205 with respect to the rotational angle of the throttle vale 1 and of the magnet 203 is increased linearly according to the amount of change in the rotational angle, as shown in FIG. 18. Next, in a case where the magnet 203 is further rotated by 40 degrees in the counterclockwise direction about the rotational center thereof from the rotational angle of 40 degrees, the magnetic circuit part (B) is formed to create the flow of the magnetic flux through the N pole (or the S pole) of the magnet 203, the yoke opening-side extension portion 243 of the second yoke segment 207 (specifically, through the arcuate portion 246, the U-shaped portion and the linear portion 244), the bent portion 242, the vertical portion 241, the Hall IC 205, the vertical portion 231, the bent portion 232, the yoke opening-side extension portion 233 of the first yoke segment 206 (specifically, through the linear portion 234, the U-shaped portion 235 and the arcuate portion 236) and the S pole (or the N pole) of the magnet 203 in this order, as indicated in FIGS. 12, 15, 17A and 19C. As shown in FIG. 15, the magnetic circuit part (A) is created. However, the yoke opening-side length (L) is set to be 1.1 mm. This yoke opening-side length (L) is substantially smaller than that of the previously proposed technique (the yoke opening-side length (L) being 6 mm), and the magnetic resistance of the magnetic circuit part (A) becomes substantially larger than that of the magnetic circuit part (B). Thus, a majority of the magnetic flux, which is emitted from the magnetic pole surface of the magnet 203, flows through the magnetic circuit part (B). Now, advantages of the fifth embodiment will be described. As described above, in the rotational angle sensing device of the present embodiment, the yoke opening-side extension portions 233, 343 are provided in the first and second yoke segments 206, 207 to form the predetermined air gap (the variable air gap that becomes narrower upon increasing of the rotational angle of the magnet 203) relative to the magnet 203. Furthermore, the arcuate portions 236, 246 and the yoke opening end portions 237, 247 are provided in the yoke opening-side extension portions 233, 343 to create the minimum air gap relative to the magnet 203 when the magnet 203 is placed generally in the maximum angle in the operable angular range of the throttle valve 201. In the rotational angle sensing device of the present embodiment, the first and second yoke segments 206, 207 are positioned relative to the magnet 203 to satisfy the following condition. That is, in the state where the Hall IC 205 implements its maximum output in the operable angular range, i.e., in the state where the rotational angle of the magnet 203 becomes the maximum angle (e.g., 80 degrees) in the operable angular range, the distance measured in the direction of Y parallel to the reference line R between the rotational axis of the magnet 203 at the reference position C and the edge 225 of the magnet 203 is generally equal to the distance measured in the direction of Y parallel to the reference line R between the rotational axis of the magnet 203 at the reference position C and the distal end surface 248 of the adjacent yoke opening end portion 247, which is adjacent to the edge 225. Furthermore, the yoke opening-side length (L) of each of the first and second yoke segments 206, 207 is set to be the most appropriate value (e.g., 1.1 mm). In this way, the linear distance measured in the direction of Y parallel to the reference line R between the rotational axis of the magnet 203 at the reference position C and the distal end surface 238, 248, i.e., the yoke opening-side length (L), which corresponds to the length of each of the yoke opening end portions 237, 247, is substantially reduced in comparison to the previously proposed technique (the one with 6 mm of the yoke opening-side length (L)). Thereby, in the operable angular range of the throttle valve 201, particularly in the angular range between the intermediate angle and the maximum angle in the operable angular range, the magnetic resistance of the magnetic circuit part (A), which does not have substantial influence on the output of the Hall IC 205, i.e., which is not related to the output of the Hall IC 205 is increased in comparison to the magnetic resistance of the magnetic circuit part (B), which have the substantial influence on the output of the Hall IC 205. In this way, the magnetic flux, which is emitted from the one of the opposed magnetic pole surfaces of the magnet 203 that are opposed to each other in the longitudinal direction of the magnet 203, can be concentrated on the magnetic circuit part (B). That is, the magnetic flux, which flows through the yoke opening-side extension portions 233, 343 of the first and second yoke segments 206, 207, is concentrated in the magnetic flux sensing gap to effectively apply it to the Hall IC 205, so that the output of the Hall IC 205 is increased in comparison to the previously proposed technique. In this way, the required output of the Hall IC 205 can be obtained in the operable angular range of the throttle valve 201 without increasing the size or the magnetic force of the magnet 203. Thus, it is possible to limit an increase in the entire size of the rotational angle sensing device, so that the installation space of the rotational angle sensing device can be relatively easily obtained or found. Furthermore, the linearity of the output change characteristic of the Hall IC 205 with respect to the rotational angle of the magnet 203 can be improved without increasing the size or magnetic force of the magnet 203. Thus, it is possible to increase the detection accuracy of the rotational angle of the throttle valve 201. Furthermore, in the rotational angle sensing device of the present embodiment, the first and second yoke segments 206, 207 are opened on the side opposite from the Hall IC in such a manner that the first and second yoke segments 206, 207 are symmetrical to each other about the imaginary center plane, which includes the reference line R that connects between the center of the Hall IC and the center of the magnet 203, and which also includes the rotational axis (rotational center axis) of the shaft 202 of the throttle valve 201. In the case where the output of the Hall IC 205 is in the maximum output state in the operable angular range of the throttle valve 201, when the rotational angle of the magnet 203 becomes the maximum angle (e.g., 80 degrees) in the operable angular range, the distance measured in the direction of Y parallel to the reference line R between, the rotational axis of the magnet 203 at the reference position C and the edge 225 of the magnet 203 is generally equal to the distance measured in the direction of Y parallel to the reference line R between the rotational axis of the magnet 203 at the reference position C and the distal end surface 248 of the adjacent yoke opening end portion 247, which is adjacent to the edge 225. When the above construction is adapted, as shown in FIG. 18, the output (e.g., an intermediate output value) of the Hall IC 205 at the intermediate angle of the magnet 203 in the operable angular range of the fifth embodiment is larger than the output of the Hall IC 205 at the intermediate angle of the magnet in the operable angular range of the previously proposed technique by a predetermined amount (an efficiency a at the intermediate angle). Furthermore, as shown in the graph of FIG. 18, the output (e.g., the maximum output value) of the Hall IC 205 at the maximum angle of the magnet 203 in the operable angular range of the fifth embodiment is larger than the output of the Hall IC 205 at the maximum angle of the magnet in the operable angular range of the previously proposed technique by a predetermined amount (an efficiency p at the maximum angle). Here, when the operable angular range (the detectable angular range) of the throttle valve 201 is increased, the efficiency a at the intermediate angle in the operable angular range can be reduced in comparison to the efficiency (3 at the maximum angle in the operable angular range. In this way, as shown in FIG. 18, the linearity of the output change characteristic of the Hall IC 205 with respect to the rotational angle of the magnet 203 can be improved, so that the detection accuracy of the rotational angle of the throttle valve 201 can be improved over the entire operable angular range (over the entire detectable angular range) of the throttle valve 201. (Sixth Embodiment) FIGS. 20 and 21 show a sixth embodiment of the present invention. Specifically, FIGS. 20 and 21 are diagrams showing a rotational angle sensing device according to the sixth embodiment. In the fifth embodiment, the distal end surfaces 238, 248 of the yoke opening end portions 237, 247 of the first and second yoke segments 206, 207 are formed in such a manner that the end surfaces 238, 248 are parallel to the direction of X, i.e., the horizontal line (the perpendicular line) S that passes through the rotational center of the magnet 203 and is perpendicular to the imaginary center plane, which includes the reference line R that connects between the center of the Hall IC and the center of the magnet 203, and which also includes the rotational axis (rotational center axis) of the shaft 202 of the throttle valve 201. In the present embodiment, unlike the fifth embodiment, the distal end surfaces (opening-side yoke distal end surfaces) 238, 248 of the yoke opening end portions 237, 247 of the first and second yoke segments 206, 207 are slightly inclined relative to the direction of X, i.e., the horizontal line (the perpendicular line) S. In the rotational angle sensing device of the present embodiment, the first and second yoke segments 206, 207 are positioned relative to the magnet 203 to satisfy the following condition. First, the position of the rotational axis of the magnet 203 in the direction of Y is set to the predetermined position (hereinafter, referred to as a reference position) C, at which the air gap between the one of the opposed end surfaces of the magnet 203 (opposed magnetized end surfaces of the magnet 203) and the inner surface of the corresponding adjacent one of the the first and second yoke segments 206, 207 as well as the air gap between the other one of the opposed end surfaces of the magnet 203 and the inner surface of the corresponding adjacent one of the first and second yoke segments 206, 207 are both minimum while the magnet 203 is held in the maximum rotational angle (e.g., 80 degrees) in the operable angular range of the magnet 203 for implementing the maximum output of the Hall IC 205. Then, in the state where the Hall IC 205 implements its maximum output in the operable angular range, i.e., in the state where the rotational angle of the magnet 203 becomes the maximum angle (e.g., 80 degrees) in the operable angular range, the distance measured in the direction of Y parallel to the reference line R between the rotational axis of the magnet 203 at the reference position C and the edge 225 of the magnet 203 is generally equal to the distance measured in the direction of Y parallel to the reference line R between the rotational axis of the magnet 203 at the reference position C and a distal end edge (opening-side yoke distal end) 249 of the arcuate portion 246 of the adjacent yoke opening end portion 247, which is adjacent to the edge 225. Furthermore, in the rotational angle sensing device of the present embodiment, in the state where the rotational angle of the magnet 203 is held at the angle (e.g., 90 degrees), which is greater than the operable angular range of the magnet 203, a portion of each yoke segment 206, 207 is arcuately curved (inversely curved) in a direction away from the reference line R and extends by a predetermined yoke opening-side length (L) from the line S (the reference position C) toward the distal end surface 238, 248 or the distal end edge 239, 249 of the yoke opening end portion 237, 247. In the present embodiment, the yoke opening-side length (L) is set to be a value, which is the same or similar to the value of the yoke opening-side length (L) described above with reference to the fifth embodiment. In this way, advantages similar to those discussed in the fifth embodiment can be achieved. Now, modifications of the above embodiments will be described. In the first to sixth embodiments, the rotational angle sensing device of the present invention is applied to the throttle opening degree sensing device, which senses a throttle opening degree that corresponds to the rotational angle of the throttle valve. Alternatively, the rotational angle sensing device of the present invention may be applied to an accelerator opening degree sensing device, which senses an accelerator opening degree that corresponds to an amount of depression of an accelerator pedal. In addition, the rotational angle sensing device of the present invention may be applied to a rotational angle sensing device, which senses a rotational angle of a valve (valve body of an air flow quantity control valve, such as an exhaust gas recirculation quantity control valve), which opens and closes a fluid flow path formed in a housing. Further, the rotational angle sensing device of the present invention may be applied to a device, which drives the throttle valve through use of a drive source (e.g., an electric motor) to open and close the throttle valve. In the first to sixth embodiments, the plate-shaped or column-shaped magnet(s) 2, 203 is used. Alternatively, the magnet(s) may be an elongated magnet, a needle-shaped magnet or a bar-shaped magnet depending on a need. In particular, when the opposed ends of the magnet, which are opposed to each other in the longitudinal direction of the magnet and are respectively magnetized with the opposite polarities, are made thinner (i.e., made to have a lower profile), linearity of the output voltage of the Hall IC (linearity of the output change characteristic of the Hall IC) with respect to the rotational angle of the magnet 2, 203 can be advantageously improved. It should be noted that the above magnet(s) 2, 203 may be replaced with a resin magnet(s), which is made by sintering powder of polyamide resin (PA), Nd, Fe, B. Further alternatively, an electromagnet(s), which generates a magnetomotive force upon energization, may be used in place of the magnet(s) 2, 203. Further alternatively, a magnet rotor(s), which includes a permanent magnet and a rotor core (magnetic body), may be used in place of the magnet(s) 2, 203. In the first to fourth embodiments, the operable angular range of the sensing object is set in the range of 0 degree to 90 degrees. Alternatively, the operable angular range of the sensing object may be set in a range of -45 degrees to +45 degrees or in a range of -90 degrees to 0 degree. In addition, in the first to fourth, the operating direction of the throttle valve is set to be the counterclockwise direction about the rotational center of the magnet 2 in the drawings. Alternatively, the opening direction of the throttle valve may be set to be the clockwise direction about the rotational center of the magnet 2 in the drawings. Furthermore, the operable angular range of the sensing object may be increased from that of the first and second embodiments. In such a case, the operable angular range of the sensing object may be set to be in a range of 0 degree to 80 degrees or in a range of -80 degrees to +80 degrees. In the fifth and sixth embodiments (as well as first to fourth embodiments), the Hall IC 205 is used as the magnetic sensing element of the non-contact type. Alternatively, the Hall element itself or a magnetoresistance element may be used as the magnetic sensing element of the non-contact type. In the fifth and sixth embodiments, each of the first and second yoke segments 206, 207 is configured to extend arcuately by the predetermined yoke opening-side length (L) from the line S (the reference position C) toward the distal end surface 238, 248 of the yoke opening end portion 237, 247 away from the Hall IC 205. Alternatively, each of the first and second yoke segments 206, 207 may be configured to extend linearly by the predetermined yoke opening-side length (L) from the line S (the reference position C) toward the distal end surface 238, 248 of the yoke opening end portion 237, 247 away from the Hall IC 205. That is, each of the first and second yoke segments 206, 207 may be configured to linearly extend from the line S (the reference position C) in a tangential direction, which is tangent to the arcuate portion 236, 246. In the fifth and sixth embodiments, the operable angular range of the sensing object is set in the range of 0 degree to 80 degrees. Alternatively, the operable angular range of the sensing object may be set in a range of -40 degrees to +40 degrees or in a range of -80 degrees to 0 degree. In the fifth and sixth embodiments, the valve opening direction of the throttle valve 201 is the counterclockwise direction about the rotational center of the magnet 203 in the drawings. Alternatively, the valve opening direction of the throttle valve 201 may be changed to a clockwise direction about the rotational center of the magnet 203 in the drawings. Furthermore, the operable angular range of the sensing object may be increased from that of the fifth and sixth embodiments. In such a case, the operable angular range of the sensing object may be set to be in a range of 0 degree to 90 degrees or in a range of -80 degrees to +80 degrees. V In the above embodiments, any one or more components of each embodiment may be combined with any one or more components of any one of the remaining embodiments within a scope and spirit of the present invention. Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Claims 1. A rotational angle sensing device comprising: a magnet (2) that is fixed to a rotatable shaft (1) of a sensing object; a plate-shaped rotational angle sensor (3) that includes a magnetic sensaig element, which senses a magnetic flux emitted from the magnet (2), wherein the rotational angle sensor (3) senses a rotational angle of the sensing object by using an output change characteristic of the magnetic sensing element with respect to a rotational angle of the magnet (2); and an open type yoke (4, 5) that is made of a magnetic material and has an opening on one side of the yoke (4, 5), wherein: the yoke (4, 5) concentrates the magnetic flux emitted from the magnet (2) onto the rotational angle sensor (3); the yoke (4, 5) includes first and second yoke segments (4, 5), which are formed separately; each of the first and second yoke segments (4, 5) forms an air gap relative to the magnet (2) and includes a yoke main body (21, 22) and a bent piece (31, 32); the bent piece (31, 32) is bent relative to the yoke main body (21, 22) at a predetermined bent angle in each of the first and second yoke segments (4, 5); the yoke main body (21) and the bent piece (31) of the first yoke segment (4) are opposed to the yoke main body (22) and the bent piece (32), respectively, of the second yoke segment (5); and the bent pieces (31, 32) of the first and second yoke segments (4, 5) hold the rotational angle sensor (3) therebetween in a plate thickness direction of the bent pieces (31, 32). 2. The rotational angle sensing device according to claim 1, wherein the rotational angle sensor (3) is placed in a magnetic flux sensing gap, which is formed between the bent pieces (31, 32) of the first and second yoke segments (4, 5). 3. The rotational angle sensing device according to claim 1 or 2, wherein the bent piece (31, 32) is bent toward the rotational angle sensor (3) at one of opposed first and second lateral edges of the yoke main body (21, 22), which are opposed to each other in a plate width direction of the yoke main body (21, 22) in each of the first and second yoke segments (4, 5). 4. The rotational angle sensing device according to any one of claims 1 to 3, wherein the bent angle of each bent piece (31, 32) is set such that the rotational angle sensor (3) is held within a plate width extent of each yoke main body (21, 22). 5. The rotational angle sensing device according to any one of claims 1 to 4, wherein the bent angle of each bent piece (31, 32) is an obtuse angle that is larger than a right angle. 6. The rotational angle sensing device according to any one of claims 1 to 5, wherein the bent angle of the bent piece (31, 32) of the first yoke segment (4) is generally the same as the bent angle of the bent piece (31, 32) of the second yoke segment (5). 7. The rotational angle sensing device according to any one of claims 1 to 6, wherein: the magnet (2) is a plate-shaped magnet that is magnetized in a plate longitudinal direction thereof; and each of the first and second yoke segments (4, 5) has a plate width that is equal to or larger than a plate thickness of the plate-shaped magnet (2). 8. The rotational angle sensing device according to any one of claims 1 to 7, wherein each of the first and second yoke segments (4, 5) has a decreasing plate width that progressively decreases from a magnet side end of the yoke segment (4, 5) toward a sensor side end of the yoke segment (4, 5). 9. The rotational angle sensing device according to any one of claims 1 to 8, wherein: the rotational angle sensor (3) has first and second magnetism-sensing surfaces, which are opposed to each other in a thickness direction of the rotational angle sensor (3) and contact the bent pieces (31, 32), respectively, of the first and second yoke segments (4, 5); and a plane of each of the first and second magnetism-sensing surfaces of the rotational angle sensor (3) is tilted at a predetermined angle relative to a perpendicular plate, which is perpendicular to a rotational axis of the sensing object. 10. The rotational angle sensing device according to any one of claims 1 to 9, wherein the magnetic sensing element of the rotational angle sensor (3) is sealed in a sealing member, which forms a main body of the rotational angle sensor (3). 11. The rotational angle sensing device according to any one of claims 1 to 10, wherein the rotational angle sensor (3) includes a lead terminal group (3a) that extends out of the magnetic sensing element thereof. 12. The rotational angle sensing device according to any one of claims 1 to 11, further comprising: a plate (12) that is made of a non-magnetic material and includes at least one yoke holding portion (51, 52), which securely holds the yoke (4, 5); and a cover (11) that is made of a magnetic material and forms a sensor receiving (17) space between the cover (11) and the plate (12) to receive the rotational angle sensor (3) and the yoke (4, 5). 13. The rotational angle sensing device according to claim 12, wherein: the rotational angle sensor (3) includes a lead terminal group (3a) that extends out of the magnetic sensing element thereof; and thermosetting resin (10) is filled in an interior of the cover (11) to seal the lead terminal group (3a). 14. The rotational angle sensing device according to claim 13, wherein the cover (11) includes at least one anchoring portion (43, 44), to which the thermosetting resin (10) is anchored. 15. The rotational angle sensing device according to claim 13 or 14, wherein: the plate (12) includes a connector (13) that includes a connector terminal group (13a), which corresponds to the lead terminal group (3a) of the rotational angle sensor (3); and a plurality of conductors, which electrically connect between the lead terminal group (3a) of the rotational angle sensor (3) and the connector terminal group (13a) of the connector (13), is sealed in the thermosetting resin (10). 16. The rotational angle sensing device according to any one of claims 12 to 15, further comprising a housing (14), with which the cover (11) forms a surface-to-surface contact and to which the cover (11) is fixed, wherein the housing (14) is made of a metal material, which includes aluminum as its main component; 17. A rotational angle sensing device comprising: a magnet (203) that is rotated synchronously upon rotation of a sensing object (201) and is magnetized in a radial direction that is perpendicular to a rotational axis of the sensing object (201); and a rotational angle sensing unit (204) that forms a magnetic circuit in corporation with the magnet (203) and senses a rotational angle of the sensing object (201), wherein: the rotational angle sensing unit (204) includes: a magnetic sensing element, an output of which changes according to a density of a magnetic flux that passes through a magnetic flux sensing gap formed in the magnetic circuit; and first and second yoke segments (206, 207) that are arranged symmetrically about an imaginary center plane, which includes a reference line (R) that connects between a center of the magnetic sensing element and a rotational center of the magnet (203) and which also includes the rotational axis of the sensing object (201); the magnetic sensing element is placed in the magnetic flux sensing gap formed between the first and second yoke segments (206, 207) at one side of the first and second yoke segments (206, 207); each of the first and second yoke segments (206, 207) includes a yoke opening end portion (237, 247), which forms a predetermined air gap relative to the magnet (203) and which is located on the other side of the yoke segment (206, 207) that is opposite from the magnetic sensing element; a position of a rotational axis of the magnet (203) in a direction parallel to the reference line (R) is set at a reference position (C), at which the air gap between at least one of the first and second yoke segments (206, 207) and the magnet (203) is minimum; when the magnet (203) is held in a predetermined rotational angle, which causes generation of a maximum output from the magnetic sensing element in an operable angular range of the sensing object (201), a linear distance, which is measured in the direction parallel to the reference line (R) between the rotational axis of the magnet (203) at the reference position (C) and a furthermost point (225) of an outer surface of the magnet (203) relative to the magnetic sensing element, is generally equal to a linear distance in the direction parallel to the reference line (R) between the rotational axis of the magnet (203) at the reference position (C) and a distal end (238, 239, 248, 249) of the yoke opening end portion (237, 247) of at least one of the first and second yoke segments (206, 207). 18. The rotational angle sensing device according to claim 17, wherein the yoke opening end portions (237, 247) of the first and second yoke segments (206, 207) are opposed to each other in such a manner that a magnet receiving space (224), which receives the magnet (203), is held between the yoke opening end portions (237, 247) of the first and second yoke segments (206, 207). 19. The rotational angle sensing device according to claim 17 or 18, wherein the yoke opening end portion (237, 247) of at least one of the first and second yoke segments (206, 207) extends by a predetermined yoke opening-side length (L) from the reference position (C) on a side opposite from the magnetic sensing element. 20. The rotational angle sensing device according to claim 19, wherein the yoke opening-side length (L) of the yoke opening end portion (237, 247) of the at least one of the first and second yoke segments (206, 207) is a length measured from the reference position (C) to the distal end (238, 239, 248, 249) of the yoke opening end portion (237, 247) of the at least one of the first and second yoke segments (206, 207). 21. The rotational angle sensing device according to daim 19 or 20, wherein: the magnet (203) is a plate-shaped magnet that has a plate thickness of 1.5 mm and is magnetized in a plate longitudinal direction of the magnet (203); the yoke opening-side length (L) of the the yoke opening end portion (237, 247) of the at least one of the first and second yoke segments (206, 207) is greater than 0 mm and is equal to or less than 3. 0 mm. 22. The rotational angle sensing device according to any one of claims 19 to 21, wherein the yoke opening-side length (L) of the yoke opening end portion (237, 247) of the at least one of the first and second yoke segments (206, 207) is set such that a magnetic resistance of a first magnetic circuit part (A), which is not related to the magnetic flux sensing gap, is larger than a magnetic resistance of a second magnetic circuit part (B), which is related to the magnetic flux sensing gap, in a range from an intermediate angle to a maximum angle in the operable angular range of the sensing object (201). 23. The rotational angle sensing device according to any one of claims 17 to 22, wherein the magnet (203) is magnetized in such a manner that lines of magnetic force in the magnet (203) are parallel to each other. 24. The rotational angle sensing device according to any one of claims 17 to 23, wherein: each of the first and second yoke segments (206, 207) has a vertical portion (231, 241); and the vertical portions (231, 241) of the first and second yoke segments (206, 207) are parallel to the center plane and are opposed to each other such that the magnetic flux sensing gap is held between the vertical portions (231, 241) of the first and second yoke segments (206, 207). 25. The rotational angle sensing device according to any one of claims 17 to 23, wherein: each of the first and second yoke segments (206, 207) includes a vertical portion (231, 241) and a yoke opening-side extension portion (233, 243); the vertical portions (231, 241) of the first and second yoke segments (206, 207) are opposed to each other such that the magnetic flux sensing gap is held between the vertical portions (231, 241) of the first and second yoke segments (206, 207); and the yoke opening-side extension portion (233, 243) extends from an end of the vertical portion (231, 241) to the distal end of the yoke opening end portion (237, 247) in each of the first and second yoke segments (206, 207). 26. The rotational angle sensing device according to claim 25, wherein the yoke opening-side extension portion (233, 243) includes an arcuate oortion (236. 246), which is curved convexly toward the magnet (203). 27. The rotational angle sensing device according to claim 26, wherein the yoke opening-side extension portion (233, 243) further includes: a linear portion (234, 244) that extends lineariy from the end of the vertical portion (231, 241) in a direction away from the magnetic sensing element; and a turned portion (235, 245) that is curved into an inverted U-shape and extends from an end of the linear portion (234, 244) to the arcuate portion (236, 246). 28. The rotational angle sensing device according to any one of claims 17 to 27, wherein the center of the magnetic sensing element is located along a perpendicular line, which extends through the rotational center of the magnet (203) and is perpendicular to the rotational axis of the sensing object (201). 29. The rotational angle sensing device according to any one of claims 17 to 28, wherein the magnetic sensing element has first and second magnetism- sensing surfaces, which are parallel to the center plane.

Documents:

1741-CHE-2007 AMENDED CLAIMS 02-05-2011.pdf

1741-che-2007 form-3 02-05-2011.pdf

1741-CHE-2007 POWER OF ATTORNEY 02-05-2011.pdf

1741-CHE-2007 AMENDED PAGES OF SPECIFICATION 25-01-2012.pdf

1741-che-2007 amended claims 20-06-2011.pdf

1741-CHE-2007 AMENDED CLAIMS 25-01-2012.pdf

1741-che-2007 correspondence others 20-06-2011.pdf

1741-CHE-2007 CORRESPONDENCE OTHERS 25-01-2012.pdf

1741-CHE-2007 EXAMINATION REPORT REPLY RECEIVED 02-05-2011.pdf

1741-CHE-2007 FORM-3 28-03-2012.pdf

1741-CHE-2007 CORRESPONDENCE OTHERS 13-01-2012.pdf

1741-CHE-2007 CORRESPONDENCE OTHERS 28-03-2012.pdf

1741-CHE-2007 FORM-3 13-01-2012.pdf

1741-che-2007-abstract.pdf

1741-che-2007-claims.pdf

1741-che-2007-correspondnece-others.pdf

1741-che-2007-description(complete).pdf

1741-che-2007-drawings.pdf

1741-che-2007-form 1.pdf

1741-che-2007-form 18.pdf

1741-che-2007-form 3.pdf

1741-che-2007-form 5.pdf

1741-che-2007-other document.pdf