Title of Invention	PRESSURE SENSOR WITH COMPENSATION FOR NULL SHIFT NON-LINEARITY AT VERY LOW TEMPERATURES.
Abstract	This invention relates to a pressure sensor with compensation for null shift non-linearity at very low temperatures, said sensor comprising strain gauges mounted in respective arms of a Wheatstone bridge, and means for compensating the zero drift of the bridge as a function of temperature, the sensor being characterized in that, in order to compensate for non-linearity in zero drift at very low temperatures, the strain gauge in at least one of the arms of the bridge is in series with a resistor of resistance that is much lower than that of the strain gauge and which is connected in parallel with a compensation circuit comprising a resistive element whose resistance varies as a function of temperature in such a manner as to influence the resistance of the parallel circuit of which it forms a part, in the very low temperature range, in a manner that is substantial and that increases with decreasing temperature.

Title of Invention

PRESSURE SENSOR WITH COMPENSATION FOR NULL SHIFT NON-LINEARITY AT VERY LOW TEMPERATURES.

Abstract

This invention relates to a pressure sensor with compensation for null shift non-linearity at very low temperatures, said sensor comprising strain gauges mounted in respective arms of a Wheatstone bridge, and means for compensating the zero drift of the bridge as a function of temperature, the sensor being characterized in that, in order to compensate for non-linearity in zero drift at very low temperatures, the strain gauge in at least one of the arms of the bridge is in series with a resistor of resistance that is much lower than that of the strain gauge and which is connected in parallel with a compensation circuit comprising a resistive element whose resistance varies as a function of temperature in such a manner as to influence the resistance of the parallel circuit of which it forms a part, in the very low temperature range, in a manner that is substantial and that increases with decreasing temperature.

Full Text	PRESSURE SENSOR WITH COMPENSATION FOR NULL SHIFT NON-LINEARITY AT VERY LOW TEMPERATURES Field of the invention The present invention relates to a pressure sensor of the type comprising strain gauges mounted in respective arms of a Wheatstone bridge. The field to which the invention relates more particularly is that of pressure sensors usable at very low temperatures, typically at temperatures below the boiling rate of nitrogen, and possibly going down to a few Kelvins only. Background of the invention A well-known problem with strain gauge pressure sensors connected as a Wheatstone bridge is that of the zero point of the bridge drifting as a. function of temperature. To resolve that problem, use is generally made of compensation networks connected to the inputs and/or the outputs of the bridge. Those networks which include at least one temperature-sensitive element, are generally designed to provide linear compensation, given that the temperature drift of strain gauge bridges is substantially linear over ordinary temperature ranges. An additional difficulty arises when the pressure sensors are used at very low temperatures. The temperature drift of strain gauge bridges then becomes non-linear, and the non-linearity increases with decreasing temperature. This can be seen from. Figure 1 which shows how the output voltage from a strain gauge bridge made up of thin film nickel-chromium deposits on a silicon substrate varies as a function of temperature, the bridge being at equilibrium (zero output voltage) at a temperature of 22°C. Ordinary compensation networks become ineffective, particularly since the thermistors used generally also have resistance that becomes very high and virtually infinite when the temperature drops below -40°C or -50°C. The problem which the present invention seeks to resolve is that of compensating the non-linearity of . temperature drift, and consequently that of "linearizing" such drift at very low temperatures, in particular at temperatures below the boiling point of nitrogen, i.e. below about -196°C. Brief summary of the invention The problem is resolved by a pressure sensor in which the strain gauge of at least one of the arms of the bridge is in series with' a resistor of resistance that is much less than that of the strain gauge and which is connected in parallel with a compensation circuit comprising a resistive element whose resistance varies as a function of temperature so as to influence the resistance of the resistor with which it is connected in parallel over the very low temperature range in a manner that is substantial and that increases with decreasing temperature. The resistor having the compensation circuit connected in parallel therewith is of resistance that is much less than that of the strain gauge so as to avoid penalizing the magnitude of the operating range and the sensitivity of the bridge. In the present specification, a resistance that is "much less than" another resistance is less than l/20th or even l/100th or even less of the resistance of a strain gauge. According to a feature of the pressure sensor of the invention, the compensation circuit is connected in parallel with a resistor formed by a connection lead connecting the strain gauge to one of the corners of the bridge. Thus, there is no need to modify the bridge in order to connect the compensation circuit thereto. This avoids any instabilities that could be generated if it were necessary to open the bridge for this purpose. In addition, the circuit for compensating non- linearity can thus be placed very close to the bridge, thereby exposing it to exactly the same temperature conditions as the bridge. The strain gauges and the connection leads connecting the strain gauges to the corners of the bridge are preferably constituted by deposits of metal on a substrate. To compensate non-linearity in the very low temperature range, i.e. below -196°C, the resistive element of the compensation circuit must have a resistance such as to have an influence on the resistance of the resistor with which it is connected in parallel, and to do so increasingly with decreasing temperature. To this end, it is preferable for the ratio of the resistance of the resistive element to that of the resistor on which it is connected in parallel to be no greater than 100 when the temperature drops below -196°C, and to decrease thereafter with decreasing temperature. By way of example, such a resistive element can be constituted by a platinum probe. Drief deacriotion of the drawings In the accompanying drawings: Figure 1 shows how the zero drift of a pressure sensor constituted by a strain gauge bridge varies as a function of temperature in the absence of drift compensation; • Figure 2 is an electrical circuit diagram of an embodiment of the pressure sensor of the invention; and • Figure 3 shows how the zero drift of the Figure 1 pressure sensor fitted with a circuit for compensating non-lineax temperature drift in accordance with the invention varies as a function of temperature. Statement of invention Accordingly, the present invention relates to a pressure sensor with compensation for null shift non-linearity at very low temperatures, said sensor comprising strain gauges (J1 to J4) mounted in respective arms of a Wheatstone bridge (10), and means (20) for compensating the zero drift of the bridge as a function of temperature, the sensor being characterized in that, in order to compensate for non-linearity in zero drift at very low temperatures, the strain gauge (Jl, J2, J3, J4) in at least one of the arms of the bridge (10) is in series with a resistor (C) of resistance that is much lower than that of the strain gauge (Jl, J2, J3,. J4) and which is connected in parallel with a compensation circuit comprising a resistive element (P) whose resistance varies as a function of temperature in such a manner as to influence the resistance of the parallel circuit of which it forms a part, in the very low temperature range, in a manner that is substantial and that increases with decreasing temperature. Brief description of the accompanying drawings In the accompanying drawings: Figure 1 shows how the zero drift of a pressure sensor constituted by a strain gauge bridge varies as a function of temperature in the absence of drift compensation; Figure 2 is an electrical circuit diagram of an embodiment of the pressure sensor of the invention; and Figure 3 shows how the zero drift of the Figure 1 pressure sensor fitted with a circuit for compensating non-linear temperature drift in accordance with the invention varies as a function of temperature. Detailed description of a preferred embodiment Figure 2 is a circuit diagram of a pressure sensor comprising four strain gauges Jl, J2, J3, and J4 inserted in respective ones of the four arms of a Wheatstone bridge 10. Each of the strain gauges Jl to J4 is connected to two adjacent corners of the bridge by connection leads c. The strain gauges Jl to J4 and the connection leads c are all formed by deposits of metal on a substrate, e.g. a silicon substrate, forming part of the sensitive element of the sensor. In well-known manner, two of the strain gauges in two opposite arms of the bridge are placed on the substrate so as to be extended when the substrate is subjected to a pressure for measurement, while the other two strain gauges are disposed in such a manner as to be subjected to compression. By way of example, the strain gauges Jl to J4 can be constituted by nickel-chromium deposits that are as identical as possible, while the connection leads c can be constituted, for example, by deposits of gold, likewise as identical as possible. The deposits are implemented as thin films, e.g. by vacuum sputtering, with the bridge being closed by the resulting deposits. Two opposite corners +a and -a of the bridge are connected to power supply terminals +A and -A, while the other two corners +m and -m are connected to measurement terminals +M and -M. An interposed compensation network 12 has one side connected to the corners +a, -a, +m, and -m and has its other side connected to the terminals +A, -A, +M, and -M. The compensation network 10 seeks to compensate the linear drift of the pressure sensor as a function of temperature. It is constituted by a resistive network of conventional type, e.g. as described in document FR—A 2 613 83 3 A circuit 20 for compensating non-linear zero drift of the bridge is connected in parallel with one of the connection leads c connecting one of the strain gauges of the bridge, e.g. the strain gauge J2, to one of the two corners of the bridge situated at the ends of its arm containing the strain gauge, e.g. the corner +m. The compensation circuit 2 0 comprises a resistive element P whose resistance varies as a function of temperature, and in particular in the very low temperature range, i.e., typically, the temperatures lower than the boiling point of nitrogen (-196°C). An adjustable resistor R can be connected in series with the resistive element P, within the compensation circuit, so as to enable the resulting compensation to be adjusted. Connecting the resistive element in parallel with a portion of the total resistance of one of the arms of the bridge thus makes it possible to have a. non-linear influence on the behavior of the bridge, and thus to compensate for its non-linear drift. When such non- linearity increases with decreasing temperature, then the influence of the resistive element P must increase, i.e. its resistance must decrease, so that the ratio of said resistance to the resistance of a connection lead c becomes no greater than 100 when the temperature drops below -196°C, and decreases as the temperature continues to drop below -196°C. Under such circumstances, a resistive element P is used whose temperature coefficient is positive, for example a platinum probe can be used. A pressure sensor as shown in Figure 2 has been made using strain gauges formed by depositing a thin film of nickel-chromium, with each strain gauge having a resistance of 1000 O at ambient temperature (22°C). The connection leads c. are linear thin-film deposits of gold, each having a resistance of 0.6 O at 22°C. Figure 1 shows how the voltage picked up across the corners +m and -m of the bridge varies in the absence of any stress being applied to the sensor, and in the absence of the compensation network 12 and the compensation circuit 20, the bridge being in equilibrium (zero output voltage) at 22°C. It can be seen that the zero drift of the bridge, due essentially to the strain gauges, becomes more and more pronounced as temperature decreases. Down to about. -140°C, drift is substantially linear. At lower temperatures, the non-linearity of the drift becomes more and more perceptible. A compensation circuit constituted by a platinum probe P of resistance equal to 100 O at 0°C connected in series with an adjustable resistance R is connected in parallel with the connection lead made of gold that connects the strain gauge J2 to the corner +m of the bridge. Table 1 below shows the resistances of a connection lead c. and of the probe P, while the adjustable resistance R is zero, and it also shows the equivalent resistance of the parallel circuit formed by P and c., at various different temperatures. The ratio of the resistance of the probe P to that of the connection lead c passes from about 47 to about 7.9 over the range -196°C to -246°C. Including the compensation circuit 20 causes the zero of the bridge to be offset, and the amount of offset increases with decreasing temperature. Table 2 below gives the value of the offset AZ measured as bridge output voltage, while the strain gauges are not subjected to stress (with 800 O strain gauges, an offset of 6.4 O. in any one strain gauge produces an output voltage of 2 0 mV when the bridge is powered at 10 V). The various offsets AZ given in Table 2 correspond to different values of the compensation circuit and to different temperatures. The rows of the table show that the offset can be attenuated by increasing the resistance R or can be emphasized by decreasing the resistance P (e.g. by placing two 100 O probes in parallel). The particular compensation circuit that needs to be selected thus depends on the amount of non-linearity that needs to be corrected. Figure 3 shows how the voltage picked up between the corners +m and -m of the bridge varies under the same conditions as for Figure 1, i.e. no stress on the sensor, bridge at equilibrium at 22°C, and no linear compensation, but with various different compensation circuits 20 having a resistor R in series with two probes P in parallel, each probe P being a platinum probe whose resistance is 100 O at 0°C. It can be seen that compared with curve I showing variation in zero drift in the absence of the compensation circuit 20, non-linearity correction becomes increasingly emphasized with decreasing resistance for the resistor R. In this example, when R = 4 O and with two probes P in parallel, the zero drift of the bridge is linearized down to very low temperatures (about -250°C). The action of the linear compensation network 12 thus makes it possible to compensate for the temperature drift of the bridge zero completely, all the way from ambient temperature to very low temperatures (a few Kelvins). In the above, a circuit for compensating non-linear drift is described as being connected in parallel with one of the connection leads connecting a strain gauge in one of the arms of the bridge to a corner of the bridge. The same effect can be achieved by connecting a non- linear drift compensation circuit in parallel with one and/or both of the connection leads of this arm of the bridge and/or in parallel with one and/or both of the connection leads of the opposite arm. In Figure 1, non-linearity varies in the same direction as the linear component of the drift and it makes it worse, however under other circumstances, non- linearity could vary in the opposite direction. Depending on the direction of this variation, the compensation circuit should be connected either to one or both of a first pair of opposite arms of the bridge, or else to one and/or both of the other two opposite arms of the bridge. A pressure sensor with compensation for null shift non-linearity at very low temperatures, said sensor comprising strain gauges (Jl to J4) mounted in respective arms of a Wheatstone bridge (10), and means (20) for compensating the zero drift of the bridge as a function of temperature, the sensor being characterized in that, in order to compensate for non-linearity in zero drift at very low temperatures, the strain gauge (Jl, J2, J3, J4) in at least one of the arms of the bridge (10) is in series with a resistor (c) of resistance that is much lower than that of the strain gauge (Jl, J2, J3,. J4) and which is connected in parallel with a compensation circuit comprising a resistive element (P) whose resistance varies as a function of temperature in such a manner as to influence the resistance of the parallel circuit of which it forms a part, in the very low temperature range, in a manner that is substantial and that increases with decreasing temperature. A sensor as claimed in claim 1 wherein the compensation circuit (20) is connected in parallel with a resistor constituted by a connection lead (c) connecting the strain gauge to a corner of the bridge. A sensor as claimed in claim 1 wherein the strain gauges (Jl to J4) and the connection leads (c) connecting the strain gauges to the corners of the bridge (10) are constituted by deposits of metal on a silicon substrate. A sensor as claimed in claim 1 wherein the connection leads (c) are made of gold. A sensor as claimed in claim 1 wherein the ratio of the resistance of said resistive element (P) to the resistance of the resistor (c) with which it is connected in parallel is no greater than 100 when the temperature drops below -196C, and thereafter decreases with decreasing temperature. A method as claimed in claim 1 wherein said resistive element (P) is constituted by at least one platinum probe. A method as claimed in claim 1 wherein said compensation circuit (20) includes an adjustable resister (Reconnected in series with said resistive element (P). A pressure sensor substantially as herein described with reference to the accompanying drawings. This invention relates to a pressure sensor with compensation for null shift non-linearity at very low temperatures, said sensor comprising strain gauges mounted in respective arms of a Wheatstone bridge, and means for compensating the zero drift of the bridge as a function of temperature, the sensor being characterized in that, in order to compensate for non-linearity in zero drift at very low temperatures, the strain gauge in at least one of the arms of the bridge is in series with a resistor of resistance that is much lower than that of the strain gauge and which is connected in parallel with a compensation circuit comprising a resistive element whose resistance varies as a function of temperature in such a manner as to influence the resistance of the parallel circuit of which it forms a part, in the very low temperature range, in a manner that is substantial and that increases with decreasing temperature.

Full Text

PRESSURE SENSOR WITH COMPENSATION FOR NULL SHIFT NON-LINEARITY
AT VERY LOW TEMPERATURES
Field of the invention
The present invention relates to a pressure sensor
of the type comprising strain gauges mounted in
respective arms of a Wheatstone bridge.
The field to which the invention relates more
particularly is that of pressure sensors usable at very
low temperatures, typically at temperatures below the
boiling rate of nitrogen, and possibly going down to a
few Kelvins only.
Background of the invention
A well-known problem with strain gauge pressure
sensors connected as a Wheatstone bridge is that of the
zero point of the bridge drifting as a. function of
temperature.
To resolve that problem, use is generally made of
compensation networks connected to the inputs and/or the
outputs of the bridge. Those networks which include at
least one temperature-sensitive element, are generally
designed to provide linear compensation, given that the
temperature drift of strain gauge bridges is
substantially linear over ordinary temperature ranges.
An additional difficulty arises when the pressure
sensors are used at very low temperatures. The
temperature drift of strain gauge bridges then becomes
non-linear, and the non-linearity increases with
decreasing temperature. This can be seen from. Figure 1
which shows how the output voltage from a strain gauge
bridge made up of thin film nickel-chromium deposits on a
silicon substrate varies as a function of temperature,
the bridge being at equilibrium (zero output voltage) at
a temperature of 22°C.
Ordinary compensation networks become ineffective,
particularly since the thermistors used generally also
have resistance that becomes very high and virtually
infinite when the temperature drops below -40°C or -50°C.
The problem which the present invention seeks to
resolve is that of compensating the non-linearity of .
temperature drift, and consequently that of "linearizing"
such drift at very low temperatures, in particular at
temperatures below the boiling point of nitrogen, i.e.
below about -196°C.
Brief summary of the invention
The problem is resolved by a pressure sensor in
which the strain gauge of at least one of the arms of the
bridge is in series with' a resistor of resistance that is
much less than that of the strain gauge and which is
connected in parallel with a compensation circuit
comprising a resistive element whose resistance varies as
a function of temperature so as to influence the
resistance of the resistor with which it is connected in
parallel over the very low temperature range in a manner
that is substantial and that increases with
decreasing temperature.
The resistor having the compensation circuit
connected in parallel therewith is of resistance that is
much less than that of the strain gauge so as to avoid
penalizing the magnitude of the operating range and the
sensitivity of the bridge. In the present specification,
a resistance that is "much less than" another resistance
is less than l/20th or even l/100th or even less of the
resistance of a strain gauge.
According to a feature of the pressure sensor of the
invention, the compensation circuit is connected in
parallel with a resistor formed by a connection lead
connecting the strain gauge to one of the corners of the
bridge.
Thus, there is no need to modify the bridge in order
to connect the compensation circuit thereto. This avoids
any instabilities that could be generated if it were
necessary to open the bridge for this purpose.
In addition, the circuit for compensating non-
linearity can thus be placed very close to the bridge,
thereby exposing it to exactly the same temperature
conditions as the bridge.
The strain gauges and the connection leads
connecting the strain gauges to the corners of the bridge
are preferably constituted by deposits of metal on a
substrate.
To compensate non-linearity in the very low
temperature range, i.e. below -196°C, the resistive
element of the compensation circuit must have a
resistance such as to have an influence on the resistance
of the resistor with which it is connected in parallel,
and to do so increasingly with decreasing temperature.
To this end, it is preferable for the ratio of the
resistance of the resistive element to that of the
resistor on which it is connected in parallel to be no
greater than 100 when the temperature drops below -196°C,
and to decrease thereafter with decreasing temperature.
By way of example, such a resistive element can be
constituted by a platinum probe.
Drief deacriotion of the drawings
In the accompanying drawings:
Figure 1 shows how the zero drift of a pressure
sensor constituted by a strain gauge bridge varies as a
function of temperature in the absence of drift
compensation;
• Figure 2 is an electrical circuit diagram of an
embodiment of the pressure sensor of the invention; and
• Figure 3 shows how the zero drift of the Figure 1
pressure sensor fitted with a circuit for compensating
non-lineax temperature drift in accordance with the
invention varies as a function of temperature.
Statement of invention
Accordingly, the present invention relates to a pressure sensor with compensation for
null shift non-linearity at very low temperatures, said sensor comprising strain gauges (J1
to J4) mounted in respective arms of a Wheatstone bridge (10), and means (20) for
compensating the zero drift of the bridge as a function of temperature, the sensor being
characterized in that, in order to compensate for non-linearity in zero drift at very low
temperatures, the strain gauge (Jl, J2, J3, J4) in at least one of the arms of the bridge (10)
is in series with a resistor (C) of resistance that is much lower than that of the strain
gauge (Jl, J2, J3,. J4) and which is connected in parallel with a compensation circuit
comprising a resistive element (P) whose resistance varies as a function of temperature in
such a manner as to influence the resistance of the parallel circuit of which it forms a
part, in the very low temperature range, in a manner that is substantial and that increases
with decreasing temperature.
Brief description of the accompanying drawings
In the accompanying drawings:
Figure 1 shows how the zero drift of a pressure sensor constituted by a strain gauge
bridge varies as a function of temperature in the absence of drift compensation;
Figure 2 is an electrical circuit diagram of an embodiment of the pressure sensor of the
invention; and
Figure 3 shows how the zero drift of the Figure 1 pressure sensor fitted with a circuit for
compensating non-linear temperature drift in accordance with the invention varies as a
function of temperature.
Detailed description of a preferred embodiment
Figure 2 is a circuit diagram of a pressure sensor
comprising four strain gauges Jl, J2, J3, and J4 inserted
in respective ones of the four arms of a Wheatstone
bridge 10. Each of the strain gauges Jl to J4 is
connected to two adjacent corners of the bridge by
connection leads c.
The strain gauges Jl to J4 and the connection leads
c are all formed by deposits of metal on a substrate,
e.g. a silicon substrate, forming part of the sensitive
element of the sensor. In well-known manner, two of the
strain gauges in two opposite arms of the bridge are
placed on the substrate so as to be extended when the
substrate is subjected to a pressure for measurement,
while the other two strain gauges are disposed in such a
manner as to be subjected to compression.
By way of example, the strain gauges Jl to J4 can be
constituted by nickel-chromium deposits that are as
identical as possible, while the connection leads c can
be constituted, for example, by deposits of gold,
likewise as identical as possible.
The deposits are implemented as thin films, e.g. by
vacuum sputtering, with the bridge being closed by the
resulting deposits.
Two opposite corners +a and -a of the bridge are
connected to power supply terminals +A and -A, while the
other two corners +m and -m are connected to measurement
terminals +M and -M. An interposed compensation network
12 has one side connected to the corners +a, -a, +m, and
-m and has its other side connected to the terminals +A,
-A, +M, and -M. The compensation network 10 seeks to
compensate the linear drift of the pressure sensor as a
function of temperature. It is constituted by a
resistive network of conventional type, e.g. as described
in document FR—A 2 613 83 3
A circuit 20 for compensating non-linear zero drift
of the bridge is connected in parallel with one of the
connection leads c connecting one of the strain gauges of
the bridge, e.g. the strain gauge J2, to one of the two
corners of the bridge situated at the ends of its arm
containing the strain gauge, e.g. the corner +m.
The compensation circuit 2 0 comprises a resistive
element P whose resistance varies as a function of
temperature, and in particular in the very low
temperature range, i.e., typically, the temperatures
lower than the boiling point of nitrogen (-196°C). An
adjustable resistor R can be connected in series with the
resistive element P, within the compensation circuit, so
as to enable the resulting compensation to be adjusted.
Connecting the resistive element in parallel with a
portion of the total resistance of one of the arms of the
bridge thus makes it possible to have a. non-linear
influence on the behavior of the bridge, and thus to
compensate for its non-linear drift. When such non-
linearity increases with decreasing temperature, then the
influence of the resistive element P must increase, i.e.
its resistance must decrease, so that the ratio of said
resistance to the resistance of a connection lead c
becomes no greater than 100 when the temperature drops
below -196°C, and decreases as the temperature continues
to drop below -196°C. Under such circumstances, a
resistive element P is used whose temperature coefficient
is positive, for example a platinum probe can be used.
A pressure sensor as shown in Figure 2 has been made
using strain gauges formed by depositing a thin film of
nickel-chromium, with each strain gauge having a
resistance of 1000 O at ambient temperature (22°C). The
connection leads c. are linear thin-film deposits of gold,
each having a resistance of 0.6 O at 22°C.
Figure 1 shows how the voltage picked up across the
corners +m and -m of the bridge varies in the absence of
any stress being applied to the sensor, and in the
absence of the compensation network 12 and the
compensation circuit 20, the bridge being in equilibrium
(zero output voltage) at 22°C.
It can be seen that the zero drift of the bridge,
due essentially to the strain gauges, becomes more and
more pronounced as temperature decreases. Down to about.
-140°C, drift is substantially linear. At lower
temperatures, the non-linearity of the drift becomes more
and more perceptible.
A compensation circuit constituted by a platinum
probe P of resistance equal to 100 O at 0°C connected in
series with an adjustable resistance R is connected in
parallel with the connection lead made of gold that
connects the strain gauge J2 to the corner +m of the
bridge.
Table 1 below shows the resistances of a connection
lead c. and of the probe P, while the adjustable
resistance R is zero, and it also shows the equivalent
resistance of the parallel circuit formed by P and c., at
various different temperatures. The ratio of the
resistance of the probe P to that of the connection lead
c passes from about 47 to about 7.9 over the range -196°C
to -246°C.
Including the compensation circuit 20 causes the
zero of the bridge to be offset, and the amount of offset
increases with decreasing temperature. Table 2 below
gives the value of the offset AZ measured as bridge
output voltage, while the strain gauges are not subjected
to stress (with 800 O strain gauges, an offset of 6.4 O.
in any one strain gauge produces an output voltage of
2 0 mV when the bridge is powered at 10 V). The various
offsets AZ given in Table 2 correspond to different
values of the compensation circuit and to different
temperatures. The rows of the table show that the offset
can be attenuated by increasing the resistance R or can
be emphasized by decreasing the resistance P (e.g. by
placing two 100 O probes in parallel). The particular
compensation circuit that needs to be selected thus
depends on the amount of non-linearity that needs to be
corrected.

Figure 3 shows how the voltage picked up between the
corners +m and -m of the bridge varies under the same
conditions as for Figure 1, i.e. no stress on the sensor,
bridge at equilibrium at 22°C, and no linear
compensation, but with various different compensation
circuits 20 having a resistor R in series with two probes
P in parallel, each probe P being a platinum probe whose
resistance is 100 O at 0°C.
It can be seen that compared with curve I showing
variation in zero drift in the absence of the
compensation circuit 20, non-linearity correction becomes
increasingly emphasized with decreasing resistance for
the resistor R. In this example, when R = 4 O and with
two probes P in parallel, the zero drift of the bridge is
linearized down to very low temperatures (about -250°C).
The action of the linear compensation network 12 thus
makes it possible to compensate for the temperature drift
of the bridge zero completely, all the way from ambient
temperature to very low temperatures (a few Kelvins).
In the above, a circuit for compensating non-linear
drift is described as being connected in parallel with
one of the connection leads connecting a strain gauge in
one of the arms of the bridge to a corner of the bridge.
The same effect can be achieved by connecting a non-
linear drift compensation circuit in parallel with one
and/or both of the connection leads of this arm of the
bridge and/or in parallel with one and/or both of the
connection leads of the opposite arm.
In Figure 1, non-linearity varies in the same
direction as the linear component of the drift and it
makes it worse, however under other circumstances, non-
linearity could vary in the opposite direction.
Depending on the direction of this variation, the
compensation circuit should be connected either to one or
both of a first pair of opposite arms of the bridge, or
else to one and/or both of the other two opposite arms of
the bridge.
A pressure sensor with compensation for null shift non-linearity at very low
temperatures, said sensor comprising strain gauges (Jl to J4) mounted in
respective arms of a Wheatstone bridge (10), and means (20) for compensating
the zero drift of the bridge as a function of temperature, the sensor being
characterized in that, in order to compensate for non-linearity in zero drift at very
low temperatures, the strain gauge (Jl, J2, J3, J4) in at least one of the arms of the
bridge (10) is in series with a resistor (c) of resistance that is much lower than
that of the strain gauge (Jl, J2, J3,. J4) and which is connected in parallel with a
compensation circuit comprising a resistive element (P) whose resistance varies as
a function of temperature in such a manner as to influence the resistance of the
parallel circuit of which it forms a part, in the very low temperature range, in a
manner that is substantial and that increases with decreasing temperature.
A sensor as claimed in claim 1 wherein the compensation circuit (20) is connected
in parallel with a resistor constituted by a connection lead (c) connecting the
strain gauge to a corner of the bridge.
A sensor as claimed in claim 1 wherein the strain gauges (Jl to J4) and the
connection leads (c) connecting the strain gauges to the corners of the bridge (10)
are constituted by deposits of metal on a silicon substrate.
A sensor as claimed in claim 1 wherein the connection leads (c) are made of gold.
A sensor as claimed in claim 1 wherein the ratio of the resistance of said resistive
element (P) to the resistance of the resistor (c) with which it is connected in
parallel is no greater than 100 when the temperature drops below -196C, and
thereafter decreases with decreasing temperature.
A method as claimed in claim 1 wherein said resistive element (P) is constituted
by at least one platinum probe.
A method as claimed in claim 1 wherein said compensation circuit (20) includes
an adjustable resister (Reconnected in series with said resistive element (P).
A pressure sensor substantially as herein described with reference to the
accompanying drawings.
This invention relates to a pressure sensor with compensation for null shift non-linearity
at very low temperatures, said sensor comprising strain gauges mounted in respective
arms of a Wheatstone bridge, and means for compensating the zero drift of the bridge as
a function of temperature, the sensor being characterized in that, in order to compensate
for non-linearity in zero drift at very low temperatures, the strain gauge in at least one of
the arms of the bridge is in series with a resistor of resistance that is much lower than
that of the strain gauge and which is connected in parallel with a compensation circuit
comprising a resistive element whose resistance varies as a function of temperature in
such a manner as to influence the resistance of the parallel circuit of which it forms a
part, in the very low temperature range, in a manner that is substantial and that increases
with decreasing temperature.

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Patent Number

224155

Indian Patent Application Number

IN/PCT/1999/00091/KOL

PG Journal Number

40/2008

Publication Date

03-Oct-2008

Grant Date

01-Oct-2008

Date of Filing

16-Nov-1999

Name of Patentee

SOCIÉTÉ NATIONALE D'ETUDE ET DE CONSTRUCTION DE MOTEURS D'AVIATION S.N.E.C.M.A.

Applicant Address

2 BOULEVARD DU GÉNÉRAL MARTIAL VALIN 75015 - PARIS

Inventors:

#	Inventor's Name	Inventor's Address
1	JEAN-BERNARD AVISSE	49 RUE SAINT GENES 33480 CASTELNEAU DE MEDOC

PCT International Classification Number

G01L 9/04

PCT International Application Number

PCT/FR99/00637

PCT International Filing date

1999-03-19

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	9703437	1998-03-20	France