Title of Invention	SYSTEM AND METHOD FOR INSPECTION OF CURVED UNIDIRECTIONALLY REGULAR SURFACES
Abstract	The present invention provides a system (10) and method for detection of irregularities on a curved surface (20) having a uniform surface profile along a first direction (24). The proposed system (10) comprises first means (11) for generating a radiation beam (14) having a beam cross-section (13) in the form of a curved line. The radiation beam ( 14) is projected on a strip said curved surface (20). A line-scan camera (30) is used to capture an image (32) of a projection (28) of the beam on said strip of said curved surface (20), the projection (28) of the beam appearing as a straight line from the position of the line-scan camera (30). The curved surface (20) is imparted a motion along said first direction (24) that allows the line-scan camera (30) to capture a series of images (32) of the projections (28) of the beam on successive strips of said curved surface (20). The proposed system (10) comprises second means (31) for extracting ridge information from the images (32) captured by said line-scan camera (30) and reconstructing said ridge information to identify an irregularity on said curved surface (20).

Title of Invention

SYSTEM AND METHOD FOR INSPECTION OF CURVED UNIDIRECTIONALLY REGULAR SURFACES

Abstract

The present invention provides a system (10) and method for detection of irregularities on a curved surface (20) having a uniform surface profile along a first direction (24). The proposed system (10) comprises first means (11) for generating a radiation beam (14) having a beam cross-section (13) in the form of a curved line. The radiation beam ( 14) is projected on a strip said curved surface (20). A line-scan camera (30) is used to capture an image (32) of a projection (28) of the beam on said strip of said curved surface (20), the projection (28) of the beam appearing as a straight line from the position of the line-scan camera (30). The curved surface (20) is imparted a motion along said first direction (24) that allows the line-scan camera (30) to capture a series of images (32) of the projections (28) of the beam on successive strips of said curved surface (20). The proposed system (10) comprises second means (31) for extracting ridge information from the images (32) captured by said line-scan camera (30) and reconstructing said ridge information to identify an irregularity on said curved surface (20).

Full Text	Description System and method for inspection of curved unidirectionally regular surfaces The present invention relates to inspection of irregularities curved surfaces that are unidirectionally regular, i.e. curved surfaces that have a uniform surface profile along one direction. Inspection of irregularities, such as embossments, on various items such as machine parts is a problem of wide interest since it affects quality control and sorting. The need of the hour is such a machine vision system that can map, detect and inspect irregularities such as embossments on not just flat surfaces, but also unidirectionally regular surfaces e.g. tin cans, rubber tires, tubes etc. The system should not only provide accuracy in profiling, but work in cases of low color or intensity contrast at real time and low cost of materials and fabrication. The aforementioned problem may be addressed by a three dimensional profiling system, for example, as disclosed in U.S. Patent No. 5193120. However, such a system is prohibitively slow for machine vision inspection purposes. The aforementioned problem can also be solved using high speed acquisition system and a triangulated laser beam. Prior-art inspection systems based on laser triangulation typically require a high speed area-scan camera and a laser. In such systems, a laser line obtained from a beam shaper or triangulation using optical assemblies is made incident on the inspection surface. A camera scans the laser profile as the object under inspection is made to pass under the incident laser line. The distortion in the laser pattern provides depth information which is subsequently compared with a standard template for inspection purposes. The above solution can be adapted to cases where the object under inspection moves rapidly. This may be done by using line-scan cameras or high speed area-scan cameras. High speed area-scan cameras are expensive and consume huge data transfer or processing rates. Line-scan cameras overcome this disadvantage since a lot of unnecessary information is removed from transfer or processing. However, with line-scan cameras, the surface needs to be flat so that depth artifacts can be mapped to ridges that represent embossments. Many scenarios of inspection of such embossments involve cases where the surface of the inspected item is unidirectionally regular but not necessarily flat. Examples of such surfaces are tin cans, ball shaped and other spherical surfaces. Alternately, adaptive lens steering, for example, as disclosed in U.S. Patent No. 5576948, have also been used where the triangulated laser beam translates adaptively over a test surface to compensate for angular deflections. However, such a system is not just complex, slow and expensive, but cannot be rendered for a machine vision system where space constraints prohibit such motion. The object of the present invention is to provide an improved system for inspection of embossments on curved surfaces that are unidirectionally regular. The above object is achieved by a system in accordance with claim 1 and a method in accordance with claim 10. The underlying idea of the present invention is to leverage the advantage of line-scan cameras to not just flat surfaces, but to curved surfaces that are unidirectionally regular, i.e., having a uniform surface profile along one direction. The above refers to a surface that posses symmetry such that the shape of the surface does not change locally as seen from a certain perspective if the surface is moved in a uniform motion, which may include, for example, rotational or translational motion. In accordance with the present invention, by generating a radiation beam having a beam cross-section in the form of a curved line having an appropriate curvature, it is possible to obtain a projection of the beam on the curved surface that appears as a straight line from the position of the line-scan camera, which would make it possible for the line-scan camera to accurately capture complete ridge information from the image of the projection of the beam on the curved surface. In one embodiment of the present invention, said radiation beam comprises a laser beam. Using a laser beam is useful in inspection of irregularities dark surfaces having very little contrast in intensity. In another embodiment of the present invention, said radiation beam comprises a laser beam. An x-ray beam has lower wavelength and is useful in measuring surface irregularities of extremely small dimensions. To generate a radiation beam having a beam cross-section in the form of a curved line, in a further embodiment of the present invention, said first means further comprises: - a radiation source adapted to emit said radiation beam having an initial beam profile, - a beam shaping aperture having a curvature adapted for re- shaping said radiation beam by altering said initial beam profile such that the re-shaped radiation beam has a cross- section in the form of said curved line. According to a still further embodiment of the present invention, the curvature of said beam shaping aperture is iteratively determined as a function of a curvature of said curved surface. This would provide an appropriate curvature to the beam shaping aperture such that the projection of the radiation beam on the curved surface after passing through the beam shaping aperture would appear as a straight line from the position of the line-scan camera. In an alternate embodiment, said radiation beam is a light beam, wherein first means comprises a video projector adapted for generating a light beam in the form of a light sheet having a constant beam cross-section in the form of said curved line. A video projector can be advantageously- programmed to generate a light sheet having any desired cross-sectional shape, which allows the present invention to be used for any arbitrarily curved surface that has a uniform surface profile along one direction. In an exemplary embodiment, said irregularity is an embossment, wherein said second means is adapted for comparing said reconstructed ridge information to a standard template to determine a machine identifiable pattern of embossment on said curved surface. This is useful in machine vision applications, such as in the' inspection of tires of a vehicle having alphanumeric characters embossed on the sidewalls of the tires. In one embodiment of the present invention, said curved surface is a cylindrical surface. In another embodiment, said curved surface is a sidewall of a tire. Such curved surfaces having a uniform surface profile along one direction, provide an appropriate geometry for realizing the present invention. The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which: FIG 1 is a schematic diagram of a system for detecting irregularities such as embossments on a unidirectionally regular curved surface according one embodiment of the present invention, FIG 2 is a schematic diagram illustrating a beam projection from an arc shaped beam shaping aperture on a flat surface, FIG 3 is a schematic diagram illustrating a beam projection from an arc shaped beam shaping aperture having an appropriate curvature on a curved surface, FIG 4 illustrates an image of a projection of the radiation beam on a strip of the curved surface as captured by a line- scan camera, FIG 5 illustrates a reconstructed image showing embossments on the curved surface, FIG 6 illustrates a use of the present invention for inspecting irregularities on a cylindrical surface along the longitudinal length of the cylindrical surface, and FIG 7 illustrates a use of the present invention for inspecting irregularities on an arbitrarily curved surface. The present invention provides an efficient system and method for detection of irregularities (for example, embossments) on curved unidirectionally regular surfaces. In accordance with the present invention, by generating a radiation beam having a beam cross-section in the form of a curved line having an appropriate curvature, it is possible to obtain a projection of the beam on the curved surface that appears as a straight line from the position of a line-scan camera, which would make it possible for the line-scan camera to accurately capture complete ridge information from the image of the projection of the beam on the curved surface. The present invention would work for arbitrarily curved surfaces that are unidirectionally regular, i.e., having a uniform surface profile along at least one direction, such that surface profiles between successive image frames captured by the line-scan camera remain the same. An exemplary embodiment of the present invention is now described referring to FIG 1. Herein a system 10 is illustrated that is used for inspecting embossments (typically alphanumeric, characters) on the sidewall of a tire, which is a curved surface 20 that has a uniform surface profile along a circumferential direction, indicated by the arrow 24. The direction of curvature of the surface 20 is indicated by the arrow 22. In this example, there is very little contrast of intensity as both the tire surface and the embossments are generally black. Hence, the illustrated embodiment is based on laser triangulation and the use of a line-scan camera 30 to map the ridges on the curved surface 20 into machine identifiable patterns. The system 10 comprises first means 11 for generating a radiation beam 14, which in this case is a laser beam, having a beam cross-section 13 in the form of a curved line (i.e., having a non-zero curvature). To that end, the first means 11 comprises a laser radiation source that emits a laser beam 14 having an initial beam profile that is typically a Gaussian laser beam profile. The laser beam 14 is diverged using a concave lens 16 and made to pass through a beam shaping aperture 16 that re-shapes the laser beam 14 by altering the initial beam profile, such that laser beam 14 now has a beam cross-section 13 in the form of a curved line (i.e., having a non-zero curvature). The laser beam 14 having the above cross-sectional profile is made incident on the curved surface 20 at a non-orthogonal angle a (i.e., a does not equal 90 degrees), wherein a projection 28 of the beam is obtained on a strip of the curved surface 20. The shape of the projection 28 of the beam on the curved surface 20 is a function of the nature of the surface 20 (i.e., the degree of curvature) and the cross-sectional profile of the laser beam 14 that is incident on the surface 20, which in turn is directly related to the shape of the beam shaping aperture 18. This idea is further explained with reference to FIGS 2 and 3. As shown in FIG 2, if a radiation beam is made to pass through an arc-shaped beam shaping aperture 50 and made incident on a flat surface 52, the projection 54 of the beam on a flat surface 52 appears in the shape of a curved line from the position of the line-scan camera. However, as shown in FIG 3, if the surface 62 is curved, then the projection 64 of the beam on the surface 62 may be made to appear as a straight line from the position of the line-scan camera by providing an appropriate curvature to the arc-shaped beam shaping aperture 60. Referring back to FIG 1, the above idea can be incorporated to iteratively determine an appropriate curvature 2 6 for the beam shaping aperture 18 as a function of the curvature 22 of the curved surface 20, such that the projection 28 of the beam on the curved surface 20 appears as a straight line from the position of the line-scan camera 30. This would make it possible for the line-scan camera 30 to accurately capture complete ridge information from the projection 28 of the beam on the curved surface 20. For the purpose of scanning the entire surface area, the curved surface 20 is imparted a motion in the circumferential direction 24 (i.e., about the central axis 21 of the tire), along which the curved surface 20 has a uniform surface profile. The line-scan camera 30 captures a series of images of the projections 28 of the beam on successive strips of the moving surface 20 along the direction 24 of scan. FIG 4 illustrates an image frame 32 captured by the line-scan camera 30 that corresponds to the projection 28 in FIG 1. Since the laser beam is incident on the curved surface at a non-orthogonal angle, the image 32 of the projection of the beam captured by the line scan camera will contain ridges 42, 44, 46, 48 and 50 corresponding to depth variations on the curved surface at the location of the strip on which the beam is projected. Referring back to FIG 1, the proposed system 10 includes second means 31 including, for example, software algorithms running on a computer, for extracting ridge information from the image frames captured by the line-scan camera 30 that scans through the entire surface area of the curved surface 20. As shown in FIG 5, the ridge information thus extracted is then reconstructed to generate an image 34 containing machine identifiable embossment patterns 36, 38 and 48 which may be arrived at by comparison with standard templates. The above described idea may be applicable for measuring any other form of surface irregularities, such as cracks, distortions (for example, thermal distortions) , surface corrosion, leaks, deposits, among others. The above idea would also work any other curved surface which has a uniform surface profile in a particular direction. FIG 6 illustrates an arrangement for a system 80. for inspecting irregularities 84 on a cylindrical surface 82, which, being a curved surface, has a uniform surface profile along a longitudinal direction 86. Means 88 are provided to generate a radiation beam 92 having a beam cross-section :94. in the form of an appropriately curved line, the radiation beam being incident on the surface 82 at a non-orthogonal angle p with the central axis 89 of the cylindrical surface 82. The cylindrical surface is imparted a motion along the direction 86, allowing a line-scan camera 90 to capture a series of images of the projection 96 of the beam on the cylindrical surface, the projection 96 appearing as a straight line from the position of the line-scan camera 90. As explained earlier, the images captured by the line-scan camera 90 are analyzed to extract ridge information, which is then reconstructed to identify the irregularities on the cylindrical surface 82. In still alternative embodiments, the curved surface may include torus shaped surfaces, spheres or even surfaces having complex curved surface profiles (for example, as described referring to FIG 7), all of these surfaces having a uniform surface profile along at least one direction. The present invention may also be equally extended for any other form of radiation beam, depending upon the application. For example, for inspecting surface irregularities of extremely small dimensions, a radiation beam having a low wavelength, for example, an x-ray beam, might be used instead. The above idea also works for regular light beams from an illumination source, such as a video projector. Advantageously, video projectors can be used in case of any arbitrarily curved surface having a complex surface profile that is uniform along one direction. In such cases, the video projector can be programmed using appropriate software algorithms to generate a light sheet in the shape of a complex curve, which when projected on the arbitrarily curved surface, appears as a straight line to a line-scan camera. An example of such an application is illustrated in FIG 7. Referring to FIG 7, a system 100 is used for inspecting an irregularity 112 on a curved surface 102 having an arbitrarily curved complex surface profile.108 that is uniform along the direction 110. Herein radiation beam in the form of a light sheet 114 is generated by a video projector, for example, a liquid crystal display projector 104. The light sheet 114 is incident on the surface 102 at a non- orthogonal angle y. The video projector can be programmed in a manner such that the light sheet 112 generated by the video projector 104 has a cross-section 118 in the form of a curve such that the projection 116 of the light sheet 114 on the arbitrarily curved surface appears as a straight line from the position of the line scan camera 106. For scanning the surface 102, a motion is imparted to the surface 102 along the direction 110, for example by placing the surface 102 on a conveyor belt (not shown), wherein the line-scan camera 10 6 captures a sequence of images of the projection 116 of the light sheet on successive strips on the moving surface 102 along the direction of scan 110. Extraction of. ridge information from the images and reconstruction of the ridge information may be carried out as mentioned above. The proposed system has several advantages. Firstly, re- shaping cross-sectional profile of the radiation beam will remove any referencing issued for depth variation search for any arbitrarily curved, unidirectionally regular surface. Secondly, a line-scan camera without such means for beam re- shaping will not be able to capture such variations and reconstruct the three-dimensional ridge information. Thus, with the aid of such beam re-shaping means, a. line-scan camera with very high frame rates and low cost will be able to capture the profile variations depicted by ridges in order to reconstruct the surface irregularities such as embossments in real time. This is particularly useful in industrial machine vision applications. Such a setup also reduces the need for very high data transfer rates and preemptively discards unnecessary information. Summarizing, the present invention provides a system and method for detection of irregularities on a curved surface having a uniform surface profile along a first direction. The proposed system comprises first means for generating a radiation beam having a beam cross-section in the form of a curved line. The radiation beam is projected on a strip said curved surface. A line-scan camera is used to capture an image of a projection of the beam on said strip of said curved surface, the projection of the beam appearing as a straight line from the position of the line-scan camera. The curved surface is imparted a motion along said first direction that allows the line-scan camera to capture a series of images of the projections of the beam on successive strips of said curved surface. The proposed system comprises second means for extracting ridge information from the images captured by said line-scan camera and reconstructing said ridge information to identify an irregularity on said curved surface. Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined. We claim, 1. A system (10) for detection of irregularities on a curved surface (20) having a uniform surface profile along a first direction (24), comprising: - first means (11) for generating a radiation beam (14) having a beam cross-section (13) in the form of a curved line, said radiation beam (14) being projected on a strip said curved surface (20), - a line-scan camera (30) adapted to capture an image (32) of a projection (28) of the beam on said strip of said curved surface (20), said projection (28) of the beam appearing as a straight line from the position of the line-scan camera (30), wherein said curved surface (20) is imparted a motion along said first direction (24), allowing said line-scan camera (30) to capture a series of images (32) of the projections (28) of the beam on successive strips of said curved surface (20), and - second means for extracting ridge information from the images (32) captured by said line-scan camera (30) and reconstructing said ridge information to identify an irregularity on said curved surface (20). 2. The system (10) according to claim 1, wherein said radiation beam (14) comprises a laser beam. 3. The system (10) according to claim 1, wherein said radiation beam (14) comprises an x-ray beam. 4. The system (10) according any of the preceding claims, wherein said first means (11) further comprises: - a radiation source (12) adapted to emit said radiation beam (14) having an initial beam profile, - a beam shaping aperture (18) having a curvature (26) adapted for re-shaping said radiation beam (14) by altering said initial beam profile such that the re- shaped radiation beam has a cross-section (13) in the form of said curved line. 5. The system (10) according to claim 4, wherein said curvature (2 6) of said beam shaping aperture (18) is iteratively determined as a function of a curvature (22) of said curved surface (20). 6. The system according to claim 1, wherein said radiation beam is a light beam, wherein first means comprises a video projector adapted for generating a light beam in the form of a light sheet having a constant beam cross-section in the form of said curved line. 7. The system (10) according to any of the preceding claims, wherein said irregularity is an embossment, wherein said second means (31) is adapted for comparing said reconstructed ridge information to a standard template to determine a machine identifiable pattern of embossment (36, 38, 40) on said curved surface (20). 8. The system according to any of the preceding claims, wherein said curved surface is a cylindrical surface. 9. The system according to any of the preceding claims, wherein said curved surface is a sidewall of a tire. 10. A method for detection of irregularities on a three- dimensional test surface, comprising: - generating a radiation beam (14) having a beam cross-section (13) in the form of a curved line and projecting said radiation beam (14) on a strip said curved surface (20), - positioning a line-scan camera (30) to capture an image (32) of a projection (28) of the beam which appears as a straight line from the position of the line-scan camera (30), - imparting a motion to said curved surface (20) along said first direction (24), - capturing a series of images (32) of the projections (28) of the beam on successive strips of said curved surface (20) by aid line-scan camera (30), - extracting ridge information from the images captured by said line-scan camera (30), and - reconstructing said ridge information to identify an irregularity on said curved surface (20). 11. The method according to claim 10, wherein generating the radiation beam having a beam cross-section in the form of said curved line further comprises passing a radiation beam (14) emitted from a radiation source (12) through a beam shaping aperture (18) having a curvature (26) adapted for re-shaping said radiation beam (14) by altering said initial beam profile such that the re-shaped radiation beam (14) has a cross- section (13) in the form of said curved line. 12. The method according to claim 11, comprising iteratively determining said curvature (26) of said beam shaping aperture (18) as a function of a curvature (22) of said curved surface (20). 13. The method according to any of claims 10 to 12, wherein said irregularity is an embossment, said method comprising comparing said reconstructed ridge information to a standard template to determine a machine identifiable pattern of embossment (36, 38, 40) on said curved surface (20). 14. A system or method substantially as herein above described in the specification with reference to the accompanying drawings. The present invention provides a system (10) and method for detection of irregularities on a curved surface (20) having a uniform surface profile along a first direction (24). The proposed system (10) comprises first means (11) for generating a radiation beam (14) having a beam cross-section (13) in the form of a curved line. The radiation beam ( 14) is projected on a strip said curved surface (20). A line-scan camera (30) is used to capture an image (32) of a projection (28) of the beam on said strip of said curved surface (20), the projection (28) of the beam appearing as a straight line from the position of the line-scan camera (30). The curved surface (20) is imparted a motion along said first direction (24) that allows the line-scan camera (30) to capture a series of images (32) of the projections (28) of the beam on successive strips of said curved surface (20). The proposed system (10) comprises second means (31) for extracting ridge information from the images (32) captured by said line-scan camera (30) and reconstructing said ridge information to identify an irregularity on said curved surface (20).

Full Text

Description
System and method for inspection of curved unidirectionally
regular surfaces
The present invention relates to inspection of irregularities
curved surfaces that are unidirectionally regular, i.e.
curved surfaces that have a uniform surface profile along one
direction.
Inspection of irregularities, such as embossments, on various
items such as machine parts is a problem of wide interest
since it affects quality control and sorting. The need of the
hour is such a machine vision system that can map, detect and
inspect irregularities such as embossments on not just flat
surfaces, but also unidirectionally regular surfaces e.g. tin
cans, rubber tires, tubes etc. The system should not only
provide accuracy in profiling, but work in cases of low color
or intensity contrast at real time and low cost of materials
and fabrication.
The aforementioned problem may be addressed by a three
dimensional profiling system, for example, as disclosed in
U.S. Patent No. 5193120. However, such a system is
prohibitively slow for machine vision inspection purposes.
The aforementioned problem can also be solved using high
speed acquisition system and a triangulated laser beam.
Prior-art inspection systems based on laser triangulation
typically require a high speed area-scan camera and a laser.
In such systems, a laser line obtained from a beam shaper or
triangulation using optical assemblies is made incident on
the inspection surface. A camera scans the laser profile as
the object under inspection is made to pass under the
incident laser line. The distortion in the laser pattern
provides depth information which is subsequently compared
with a standard template for inspection purposes.

The above solution can be adapted to cases where the object
under inspection moves rapidly. This may be done by using
line-scan cameras or high speed area-scan cameras. High speed
area-scan cameras are expensive and consume huge data
transfer or processing rates. Line-scan cameras overcome this
disadvantage since a lot of unnecessary information is
removed from transfer or processing. However, with line-scan
cameras, the surface needs to be flat so that depth artifacts
can be mapped to ridges that represent embossments. Many
scenarios of inspection of such embossments involve cases
where the surface of the inspected item is unidirectionally
regular but not necessarily flat. Examples of such surfaces
are tin cans, ball shaped and other spherical surfaces.
Alternately, adaptive lens steering, for example, as
disclosed in U.S. Patent No. 5576948, have also been used
where the triangulated laser beam translates adaptively over
a test surface to compensate for angular deflections.
However, such a system is not just complex, slow and
expensive, but cannot be rendered for a machine vision system
where space constraints prohibit such motion.
The object of the present invention is to provide an improved
system for inspection of embossments on curved surfaces that
are unidirectionally regular.
The above object is achieved by a system in accordance with
claim 1 and a method in accordance with claim 10.
The underlying idea of the present invention is to leverage
the advantage of line-scan cameras to not just flat surfaces,
but to curved surfaces that are unidirectionally regular,
i.e., having a uniform surface profile along one direction.
The above refers to a surface that posses symmetry such that
the shape of the surface does not change locally as seen from
a certain perspective if the surface is moved in a uniform
motion, which may include, for example, rotational or
translational motion. In accordance with the present

invention, by generating a radiation beam having a beam
cross-section in the form of a curved line having an
appropriate curvature, it is possible to obtain a projection
of the beam on the curved surface that appears as a straight
line from the position of the line-scan camera, which would
make it possible for the line-scan camera to accurately
capture complete ridge information from the image of the
projection of the beam on the curved surface.
In one embodiment of the present invention, said radiation
beam comprises a laser beam. Using a laser beam is useful in
inspection of irregularities dark surfaces having very little
contrast in intensity.
In another embodiment of the present invention, said
radiation beam comprises a laser beam. An x-ray beam has
lower wavelength and is useful in measuring surface
irregularities of extremely small dimensions.
To generate a radiation beam having a beam cross-section in
the form of a curved line, in a further embodiment of the
present invention, said first means further comprises:
- a radiation source adapted to emit said radiation beam
having an initial beam profile,
- a beam shaping aperture having a curvature adapted for re-
shaping said radiation beam by altering said initial beam
profile such that the re-shaped radiation beam has a cross-
section in the form of said curved line.
According to a still further embodiment of the present
invention, the curvature of said beam shaping aperture is
iteratively determined as a function of a curvature of said
curved surface. This would provide an appropriate curvature
to the beam shaping aperture such that the projection of the
radiation beam on the curved surface after passing through
the beam shaping aperture would appear as a straight line
from the position of the line-scan camera.

In an alternate embodiment, said radiation beam is a light
beam, wherein first means comprises a video projector adapted
for generating a light beam in the form of a light sheet
having a constant beam cross-section in the form of said
curved line. A video projector can be advantageously-
programmed to generate a light sheet having any desired
cross-sectional shape, which allows the present invention to
be used for any arbitrarily curved surface that has a uniform
surface profile along one direction.
In an exemplary embodiment, said irregularity is an
embossment, wherein said second means is adapted for
comparing said reconstructed ridge information to a standard
template to determine a machine identifiable pattern of
embossment on said curved surface. This is useful in machine
vision applications, such as in the' inspection of tires of a
vehicle having alphanumeric characters embossed on the
sidewalls of the tires.
In one embodiment of the present invention, said curved
surface is a cylindrical surface. In another embodiment, said
curved surface is a sidewall of a tire. Such curved surfaces
having a uniform surface profile along one direction, provide
an appropriate geometry for realizing the present invention.
The present invention is further described hereinafter with
reference to illustrated embodiments shown in the
accompanying drawings, in which:
FIG 1 is a schematic diagram of a system for detecting
irregularities such as embossments on a unidirectionally
regular curved surface according one embodiment of the
present invention,
FIG 2 is a schematic diagram illustrating a beam projection
from an arc shaped beam shaping aperture on a flat surface,

FIG 3 is a schematic diagram illustrating a beam projection
from an arc shaped beam shaping aperture having an
appropriate curvature on a curved surface,
FIG 4 illustrates an image of a projection of the radiation
beam on a strip of the curved surface as captured by a line-
scan camera,
FIG 5 illustrates a reconstructed image showing embossments
on the curved surface,
FIG 6 illustrates a use of the present invention for
inspecting irregularities on a cylindrical surface along the
longitudinal length of the cylindrical surface, and
FIG 7 illustrates a use of the present invention for
inspecting irregularities on an arbitrarily curved surface.
The present invention provides an efficient system and method
for detection of irregularities (for example, embossments) on
curved unidirectionally regular surfaces. In accordance with
the present invention, by generating a radiation beam having
a beam cross-section in the form of a curved line having an
appropriate curvature, it is possible to obtain a projection
of the beam on the curved surface that appears as a straight
line from the position of a line-scan camera, which would
make it possible for the line-scan camera to accurately
capture complete ridge information from the image of the
projection of the beam on the curved surface. The present
invention would work for arbitrarily curved surfaces that are
unidirectionally regular, i.e., having a uniform surface
profile along at least one direction, such that surface
profiles between successive image frames captured by the
line-scan camera remain the same.
An exemplary embodiment of the present invention is now
described referring to FIG 1. Herein a system 10 is
illustrated that is used for inspecting embossments

(typically alphanumeric, characters) on the sidewall of a
tire, which is a curved surface 20 that has a uniform surface
profile along a circumferential direction, indicated by the
arrow 24. The direction of curvature of the surface 20 is
indicated by the arrow 22. In this example, there is very
little contrast of intensity as both the tire surface and the
embossments are generally black. Hence, the illustrated
embodiment is based on laser triangulation and the use of a
line-scan camera 30 to map the ridges on the curved surface
20 into machine identifiable patterns.
The system 10 comprises first means 11 for generating a
radiation beam 14, which in this case is a laser beam, having
a beam cross-section 13 in the form of a curved line (i.e.,
having a non-zero curvature). To that end, the first means 11
comprises a laser radiation source that emits a laser beam 14
having an initial beam profile that is typically a Gaussian
laser beam profile. The laser beam 14 is diverged using a
concave lens 16 and made to pass through a beam shaping
aperture 16 that re-shapes the laser beam 14 by altering the
initial beam profile, such that laser beam 14 now has a beam
cross-section 13 in the form of a curved line (i.e., having a
non-zero curvature). The laser beam 14 having the above
cross-sectional profile is made incident on the curved
surface 20 at a non-orthogonal angle a (i.e., a does not
equal 90 degrees), wherein a projection 28 of the beam is
obtained on a strip of the curved surface 20.
The shape of the projection 28 of the beam on the curved
surface 20 is a function of the nature of the surface 20
(i.e., the degree of curvature) and the cross-sectional
profile of the laser beam 14 that is incident on the surface
20, which in turn is directly related to the shape of the
beam shaping aperture 18. This idea is further explained with
reference to FIGS 2 and 3. As shown in FIG 2, if a radiation
beam is made to pass through an arc-shaped beam shaping
aperture 50 and made incident on a flat surface 52, the
projection 54 of the beam on a flat surface 52 appears in the

shape of a curved line from the position of the line-scan
camera. However, as shown in FIG 3, if the surface 62 is
curved, then the projection 64 of the beam on the surface 62
may be made to appear as a straight line from the position of
the line-scan camera by providing an appropriate curvature to
the arc-shaped beam shaping aperture 60.
Referring back to FIG 1, the above idea can be incorporated
to iteratively determine an appropriate curvature 2 6 for the
beam shaping aperture 18 as a function of the curvature 22 of
the curved surface 20, such that the projection 28 of the
beam on the curved surface 20 appears as a straight line from
the position of the line-scan camera 30. This would make it
possible for the line-scan camera 30 to accurately capture
complete ridge information from the projection 28 of the beam
on the curved surface 20. For the purpose of scanning the
entire surface area, the curved surface 20 is imparted a
motion in the circumferential direction 24 (i.e., about the
central axis 21 of the tire), along which the curved surface
20 has a uniform surface profile. The line-scan camera 30
captures a series of images of the projections 28 of the beam
on successive strips of the moving surface 20 along the
direction 24 of scan. FIG 4 illustrates an image frame 32
captured by the line-scan camera 30 that corresponds to the
projection 28 in FIG 1. Since the laser beam is incident on
the curved surface at a non-orthogonal angle, the image 32 of
the projection of the beam captured by the line scan camera
will contain ridges 42, 44, 46, 48 and 50 corresponding to
depth variations on the curved surface at the location of the
strip on which the beam is projected.
Referring back to FIG 1, the proposed system 10 includes
second means 31 including, for example, software algorithms
running on a computer, for extracting ridge information from
the image frames captured by the line-scan camera 30 that
scans through the entire surface area of the curved surface
20. As shown in FIG 5, the ridge information thus extracted
is then reconstructed to generate an image 34 containing

machine identifiable embossment patterns 36, 38 and 48 which
may be arrived at by comparison with standard templates.
The above described idea may be applicable for measuring any
other form of surface irregularities, such as cracks,
distortions (for example, thermal distortions) , surface
corrosion, leaks, deposits, among others. The above idea
would also work any other curved surface which has a uniform
surface profile in a particular direction. FIG 6 illustrates
an arrangement for a system 80. for inspecting irregularities
84 on a cylindrical surface 82, which, being a curved
surface, has a uniform surface profile along a longitudinal
direction 86. Means 88 are provided to generate a radiation
beam 92 having a beam cross-section :94. in the form of an
appropriately curved line, the radiation beam being incident
on the surface 82 at a non-orthogonal angle p with the
central axis 89 of the cylindrical surface 82. The
cylindrical surface is imparted a motion along the direction
86, allowing a line-scan camera 90 to capture a series of
images of the projection 96 of the beam on the cylindrical
surface, the projection 96 appearing as a straight line from
the position of the line-scan camera 90. As explained
earlier, the images captured by the line-scan camera 90 are
analyzed to extract ridge information, which is then
reconstructed to identify the irregularities on the
cylindrical surface 82. In still alternative embodiments, the
curved surface may include torus shaped surfaces, spheres or
even surfaces having complex curved surface profiles (for
example, as described referring to FIG 7), all of these
surfaces having a uniform surface profile along at least one
direction.
The present invention may also be equally extended for any
other form of radiation beam, depending upon the application.
For example, for inspecting surface irregularities of
extremely small dimensions, a radiation beam having a low
wavelength, for example, an x-ray beam, might be used
instead. The above idea also works for regular light beams

from an illumination source, such as a video projector.
Advantageously, video projectors can be used in case of any
arbitrarily curved surface having a complex surface profile
that is uniform along one direction. In such cases, the video
projector can be programmed using appropriate software
algorithms to generate a light sheet in the shape of a
complex curve, which when projected on the arbitrarily curved
surface, appears as a straight line to a line-scan camera. An
example of such an application is illustrated in FIG 7.
Referring to FIG 7, a system 100 is used for inspecting an
irregularity 112 on a curved surface 102 having an
arbitrarily curved complex surface profile.108 that is
uniform along the direction 110. Herein radiation beam in the
form of a light sheet 114 is generated by a video projector,
for example, a liquid crystal display projector 104. The
light sheet 114 is incident on the surface 102 at a non-
orthogonal angle y. The video projector can be programmed in
a manner such that the light sheet 112 generated by the video
projector 104 has a cross-section 118 in the form of a curve
such that the projection 116 of the light sheet 114 on the
arbitrarily curved surface appears as a straight line from
the position of the line scan camera 106. For scanning the
surface 102, a motion is imparted to the surface 102 along
the direction 110, for example by placing the surface 102 on
a conveyor belt (not shown), wherein the line-scan camera 10 6
captures a sequence of images of the projection 116 of the
light sheet on successive strips on the moving surface 102
along the direction of scan 110. Extraction of. ridge
information from the images and reconstruction of the ridge
information may be carried out as mentioned above.
The proposed system has several advantages. Firstly, re-
shaping cross-sectional profile of the radiation beam will
remove any referencing issued for depth variation search for
any arbitrarily curved, unidirectionally regular surface.
Secondly, a line-scan camera without such means for beam re-
shaping will not be able to capture such variations and

reconstruct the three-dimensional ridge information. Thus,
with the aid of such beam re-shaping means, a. line-scan
camera with very high frame rates and low cost will be able
to capture the profile variations depicted by ridges in order
to reconstruct the surface irregularities such as embossments
in real time. This is particularly useful in industrial
machine vision applications. Such a setup also reduces the
need for very high data transfer rates and preemptively
discards unnecessary information.
Summarizing, the present invention provides a system and
method for detection of irregularities on a curved surface
having a uniform surface profile along a first direction. The
proposed system comprises first means for generating a
radiation beam having a beam cross-section in the form of a
curved line. The radiation beam is projected on a strip said
curved surface. A line-scan camera is used to capture an
image of a projection of the beam on said strip of said
curved surface, the projection of the beam appearing as a
straight line from the position of the line-scan camera. The
curved surface is imparted a motion along said first
direction that allows the line-scan camera to capture a
series of images of the projections of the beam on successive
strips of said curved surface. The proposed system comprises
second means for extracting ridge information from the images
captured by said line-scan camera and reconstructing said
ridge information to identify an irregularity on said curved
surface.
Although the invention has been described with reference to
specific embodiments, this description is not meant to be
construed in a limiting sense. Various modifications of the
disclosed embodiments, as well as alternate embodiments of
the invention, will become apparent to persons skilled in the
art upon reference to the description of the invention. It is
therefore contemplated that such modifications can be made
without departing from the spirit or scope of the present
invention as defined.

We claim,
1. A system (10) for detection of irregularities on a
curved surface (20) having a uniform surface profile
along a first direction (24), comprising:
- first means (11) for generating a radiation beam
(14) having a beam cross-section (13) in the form of
a curved line, said radiation beam (14) being
projected on a strip said curved surface (20),
- a line-scan camera (30) adapted to capture an image
(32) of a projection (28) of the beam on said strip
of said curved surface (20), said projection (28) of
the beam appearing as a straight line from the
position of the line-scan camera (30),
wherein said curved surface (20) is imparted a
motion along said first direction (24), allowing
said line-scan camera (30) to capture a series of
images (32) of the projections (28) of the beam on
successive strips of said curved surface (20), and
- second means for extracting ridge information from
the images (32) captured by said line-scan camera
(30) and reconstructing said ridge information to
identify an irregularity on said curved surface
(20).
2. The system (10) according to claim 1, wherein said
radiation beam (14) comprises a laser beam.
3. The system (10) according to claim 1, wherein said
radiation beam (14) comprises an x-ray beam.
4. The system (10) according any of the preceding claims,
wherein said first means (11) further comprises:
- a radiation source (12) adapted to emit said
radiation beam (14) having an initial beam profile,
- a beam shaping aperture (18) having a curvature (26)
adapted for re-shaping said radiation beam (14) by
altering said initial beam profile such that the re-

shaped radiation beam has a cross-section (13) in
the form of said curved line.
5. The system (10) according to claim 4, wherein said
curvature (2 6) of said beam shaping aperture (18) is
iteratively determined as a function of a curvature
(22) of said curved surface (20).
6. The system according to claim 1, wherein said radiation
beam is a light beam, wherein first means comprises a
video projector adapted for generating a light beam in
the form of a light sheet having a constant beam
cross-section in the form of said curved line.
7. The system (10) according to any of the preceding
claims, wherein said irregularity is an embossment,
wherein said second means (31) is adapted for
comparing said reconstructed ridge information to a
standard template to determine a machine identifiable
pattern of embossment (36, 38, 40) on said curved
surface (20).
8. The system according to any of the preceding claims,
wherein said curved surface is a cylindrical surface.
9. The system according to any of the preceding claims,
wherein said curved surface is a sidewall of a tire.
10. A method for detection of irregularities on a three-
dimensional test surface, comprising:
- generating a radiation beam (14) having a beam
cross-section (13) in the form of a curved line and
projecting said radiation beam (14) on a strip said
curved surface (20),
- positioning a line-scan camera (30) to capture an
image (32) of a projection (28) of the beam which
appears as a straight line from the position of the
line-scan camera (30),

- imparting a motion to said curved surface (20) along
said first direction (24),
- capturing a series of images (32) of the projections
(28) of the beam on successive strips of said curved
surface (20) by aid line-scan camera (30),
- extracting ridge information from the images
captured by said line-scan camera (30), and
- reconstructing said ridge information to identify an
irregularity on said curved surface (20).
11. The method according to claim 10, wherein generating
the radiation beam having a beam cross-section in the
form of said curved line further comprises passing a
radiation beam (14) emitted from a radiation source
(12) through a beam shaping aperture (18) having a
curvature (26) adapted for re-shaping said radiation
beam (14) by altering said initial beam profile such
that the re-shaped radiation beam (14) has a cross-
section (13) in the form of said curved line.
12. The method according to claim 11, comprising
iteratively determining said curvature (26) of said
beam shaping aperture (18) as a function of a
curvature (22) of said curved surface (20).
13. The method according to any of claims 10 to 12, wherein
said irregularity is an embossment, said method
comprising comparing said reconstructed ridge
information to a standard template to determine a
machine identifiable pattern of embossment (36, 38,
40) on said curved surface (20).

14. A system or method substantially as herein above
described in the specification with reference to the
accompanying drawings.

The present invention provides a system (10) and method for
detection of irregularities on a curved surface (20) having a
uniform surface profile along a first direction (24). The
proposed system (10) comprises first means (11) for
generating a radiation beam (14) having a beam cross-section
(13) in the form of a curved line. The radiation beam ( 14) is
projected on a strip said curved surface (20). A line-scan
camera (30) is used to capture an image (32) of a projection
(28) of the beam on said strip of said curved surface (20),
the projection (28) of the beam appearing as a straight line
from the position of the line-scan camera (30). The curved
surface (20) is imparted a motion along said first direction
(24) that allows the line-scan camera (30) to capture a
series of images (32) of the projections (28) of the beam on
successive strips of said curved surface (20). The proposed
system (10) comprises second means (31) for extracting ridge
information from the images (32) captured by said line-scan
camera (30) and reconstructing said ridge information to
identify an irregularity on said curved surface (20).

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=9CUr6yA5wqdVyyYq9+8WEA==&loc=wDBSZCsAt7zoiVrqcFJsRw==

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Patent Number

272957

Indian Patent Application Number

415/KOL/2009

PG Journal Number

19/2016

Publication Date

06-May-2016

Grant Date

05-May-2016

Date of Filing

06-Mar-2009

Name of Patentee

SIEMENS INFORMATION SYSTEMS LTD.

Applicant Address

43, SHANTIPALLY, E. M. BYPASS RASHBEHARI CONNECTOR, KOLKATA

Inventors:

#	Inventor's Name	Inventor's Address
1	VARUN AKUR VENKATESAN	#286, SECTOR 5, HSR LAYOUT, 560102 BANGALORE
2	GARIMELLA PADMA MADHURI	22/26(1), EAST END C MAIN, JAYANAGAR 9TH, 560069 BANGALORE
3	VENKATESH BAGARIA	1179, FIRST FLOOR, 11TH CROSS, 22B MAIN, HSR 560034

PCT International Classification Number

G01B11/25; G01B11/24

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA