U.S. patent application number 09/835598 was filed with the patent office on 2001-09-13 for shearographic imaging machine.
Invention is credited to Gridley, Jason L., Lindsay, John S..
Application Number | 20010021025 09/835598 |
Document ID | / |
Family ID | 23306635 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021025 |
Kind Code |
A1 |
Lindsay, John S. ; et
al. |
September 13, 2001 |
Shearographic imaging machine
Abstract
The invention relates to an apparatus for performing electronic
shearography on a test object, especially a tire or retread tire.
The apparatus uses a laser light source to illuminate the test
object. An optical element through which electromagnetic radiation
is reflected from the test object is transmitted and forms a random
interference image. The random interference image is electronically
processed to provide a video animation of the effects of stress on
the test object.
Inventors: |
Lindsay, John S.;
(Muscatine, IA) ; Gridley, Jason L.; (Walcott,
IA) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
TWO PRUDENTIAL PLAZA, SUITE 4900
180 NORTH STETSON AVENUE
CHICAGO
IL
60601-6780
US
|
Family ID: |
23306635 |
Appl. No.: |
09/835598 |
Filed: |
April 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09835598 |
Apr 16, 2001 |
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09334311 |
Jun 16, 1999 |
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6219143 |
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Current U.S.
Class: |
356/458 ;
356/520 |
Current CPC
Class: |
G01B 11/162
20130101 |
Class at
Publication: |
356/458 ;
356/520 |
International
Class: |
G01B 009/02 |
Claims
What is claimed is:
1. An apparatus for performing electronic shearography on a test
object comprising: a shearography camera for taking an interference
image of the test object, an image processor coupled to the
shearography camera, the image processor being adapted to receive a
plurality of sequential interference images from the shearography
camera, produce a plurality of sequential shearogram images of the
test object from the interference images and animate the sequential
shearogram images to represent dynamically changing stress states
on the tire, and a display coupled to the image processor for
providing visualization of the animation of the sequential
shearogram images.
2. The apparatus according to claim 1 wherein the image processor
further includes a memory device for storing the shearogram
images.
3. The apparatus according to claim 2 wherein the image processor
is adapted to animate the shearogram images stored in the memory
device.
4. The apparatus according to claim 1 wherein image processor is
adapted to receive interference images from the shearography camera
at a frame rate of at least fifteen frames per second.
5. The apparatus according to claim 1 wherein the image processor
is adapted to animate the shearogram images at an animation rate of
at least fifteen frames per second.
6. The apparatus according to claim 1 wherein the image processor
is adapted to substantially simultaneously animate multiple
shearogram image sequences representative of different sections of
the test object.
7. The apparatus according to claim 1 wherein the image processor
is a computer.
8. A method for analyzing a test object comprising: (a) taking an
interference image of a test object, (b) comparing the interference
image with a baseline interference image to produce a shearogram
image, (c) repeating steps (a) and (b) at varying stress levels to
produce a plurality of shearogram images, and (d) displaying the
plurality of shearogram images at a frame rate fast enough to
generate an animation representative of dynamically changing stress
states on the test object.
9. The method according to claim 8 further including the step of
storing the shearogram images.
10. The method according to claim 8 wherein the frame rate is at
least fifteen frames per second.
11. The method according to claim 8 further comprising
simultaneously displaying multiple shearogram image sequences
representative of different sections of the test object.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATION
[0001] This patent application is a continuation of copending U.S.
patent application Ser. No. 09/334,311 filed Jun. 16, 1999 the
disclosure of which is hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to the field of
nondestructive testing. Specifically, the present invention relates
to the technique of electronic shearography. More specifically, the
present invention relates to the use of electronic shearography to
detect defects in vehicle tires by animating shearograms produced
while the tires undergo a varying stress continuum.
BACKGROUND OF THE INVENTION
[0003] The technique of shearing interferometry, or shearography
involves the interference of two laterally displaced images of the
same object to form an interference image. Conventional
shearographic methods require that a first interference image (or
baseline image) be taken while the object is in an unstressed or
first stressed condition, and another interference image be taken
while the object is in a second stressed condition. Comparison of
these two interference images (preferably by methods of image
subtraction) reveals information about the strain concentrations
and hence the integrity of the object in a single image called a
shearogram. In particular, shearography has been shown to be useful
to detect strain concentrations and hence defects in vehicle tires,
especially retread vehicle tires.
[0004] In conventional electronic shearography, interference images
are stored in a computer memory and are compared electronically to
produce single static shearograms. Because all the data are
processed electronically, the results of the analysis can be viewed
in "real time". "Real time", as used in the prior art, refers to
the ability to view the shearogram nearly instantaneously after the
second interference image has been taken.
[0005] An apparatus and method for performing electronic
shearography is described in U.S. Pat. No. 4,887,899 issued to
Hung. The apparatus described in the cited patent produces an
interference image by passing light, reflected from the test
object, through a birefringent material and a polarizer. The
birefringent material, which can be a calcite crystal splits a
light ray, reflected from the object, into two rays, and the
polarizer makes it possible for light rays reflected from a pair of
points to interfere with each other. Thus, each point on the object
generates two rays, and the result is an interference image formed
by the optical interference of two laterally displaced images of
the same object.
[0006] Prior to the developments disclosed in the Hung patent, the
spatial frequency of the interference image produced in
shearographic analysis was relatively high requiring the use of
high resolution photographic film to record a useful interference
image. The development disclosed in the Hung patent produces an
interference image with a relatively low spatial frequency because
the effective angles between the interfering rays are small.
Therefore, the interference images can be recorded by a video
camera, a video camera normally having much less resolving
capability than a high density or high resolution photographic
film. By storing an interference image of the object in its
initial, unstressed condition, and by comparing that interference
image, virtually instantaneously, by computer with another
interference image taken under a different level of stress, a "real
time" image or shearogram of the resultant strains on the object
can be observed. Each point on the actual interference image is
generated by the interference of light emanating from a pair of
distinct points on the object. Therefore, each pixel of the video
camera is illuminated by light reflected from those two points. If
the overall illumination remains constant, then any variations in
the pixel intensity, in the interference image, will be due only to
changes in the phase relationship of the two points of light.
[0007] When the initial video image of the interference image is
stored, an initial intensity for each pixel is recorded, as
described above. If differential deformations occur in the object,
such deformations will cause changes in the subsequent interference
image. In particular, the intensity of a given pixel will change
according to change in the phase relationship between the two rays
of light, reflected from the two points on the object, which
illuminate the pixel. The phase differences can be either positive
changes, causing the pixel to become brighter or negative changes,
causing the pixel to become darker. Whether the pixel becomes
brighter or darker depends on the initial phase relationship and
the direction of the change of phase. Due to the cyclic nature of
phase interferences, as the deformation of the object continually
increases, the intensity at a given pixel may pass through a
complete cycle. That is, the intensity of the pixel might increase
to a maximum (positive) difference, then return to the original
intensity, and then continue to a maximum (negative) difference,
and so on.
[0008] In systems of the prior art, a single shearogram is derived
from two single static interference images taken at two distinct
stress levels. The single shearogram is then viewed by an operator
for analysis if multiple shearograms are taken, the analysis is
done one shearogram at a time. Thus, the operator attendance time,
required to perform a thorough stress analysis, is substantial.
Further, a single shearogram may falsely show light features that
appear to be defects (referred to as "false positives"). These
"false positives" are caused by different reflective
characteristics on the surface of the test object and appear as
defects when a static shearogram is viewed. Further still, in a
static shearogram some real defects may be "washed out" and thus
not visible (referred to as "false negatives"), at certain
(particularly high) stress levels. These "washed out" effects are
caused by shearographic fringe lines that are not spatially
separated enough to be visibly distinguishable and therefore appear
to be aberrational light effects rather than real defects in the
test object. Thus, a single static shearogram may contain
inaccurate information with regards to the defects actually
present. Furthermore, an operator having to analyze a large number
of shearograms requires a large amount of operator attendance
time.
[0009] There is a need and desire for an improved method of
presentation of shearographic images that provide advantages over
the prior art. There is also a need and desire for a method of
presenting shearographic images that provide improved accuracy,
shorter attendance times by an operator, and shorter overall cycle
times for a test object. Further, there is a need and desire for a
method of presenting shearographic images that reduce the
undesirable effects of false negatives by preventing "wash out" of
larger defects at high stress levels. Further still, there is a
need and desire for a method of presenting shearographic images
that allows real defects to be distinguished over light features
that otherwise may be confused as defects, thereby minimizing false
positives.
SUMMARY OF THE INVENTION
[0010] The present invention relates to an apparatus for performing
electronic shearography on a test object. The apparatus includes a
source of coherent electromagnetic radiation for illuminating the
test object, and an optical element through which electromagnetic
radiation reflected from the test object is transmitted forming an
interference image. A detector converts the interference image into
an electrical signal representative of the interference image. An
animation device is coupled to the detector. The animation device
receives the electrical signal representative of the interference
image. The animation device retains image information derived from
the electrical signals representative of the interference image at
a predetermined frame rate. The animation device compares the
retained interference image information with a baseline
interference image to produce a shearogram image, and the animation
device is adapted to play a series of sequential shearogram images.
A display device is coupled to the animation device, providing
visualization of the sequential shearogram images.
[0011] The present invention further relates to a method of
analyzing a test object. The method includes directing coherent
electromagnetic radiation onto a test object, providing
electromagnetic radiation reflected from the test object to an
optical shearing device, the optical shearing device creating an
interference image, and directing the interference image, emerging
from the shearing device, onto a detector. The method further
includes capturing an electrical signal, communicated from the
detector, in a capture device, the electrical signal being
representative of the interference image, storing interference
image information in a memory device communicated from the capture
device and comparing interference image information stored in the
memory device, to a stored interference image to produce a
shearogram image. The method still further includes repeating the
aforementioned steps at varying stress levels and displaying
shearogram image information at a frame rate.
[0012] The present invention still further relates to an apparatus
for performing electronic shearography on a tire undergoing varying
states of stress. The apparatus includes a source of coherent
electromagnetic radiation for illuminating the tire, a birefringent
material through which electromagnetic radiation reflected from the
tire is transmitted, and a polarizer through which electromagnetic
radiation, emerging from the birefringent material, is transmitted,
the birefringent material and the polarizer cooperating to form an
interference image. The apparatus also includes a video camera, the
video camera converting the interference image to an electrical
signal and a video capture circuit coupled to the video camera, the
capture circuit receiving the electrical signal from the camera,
the electrical signal being representative of the interference
image, the capture circuit retaining image information derived from
the electrical signals representative of the interference image at
a frame rate. Further, the apparatus includes a computer coupled to
the capture circuit, the computer adapted to compare sequential
interference images retained by the capture circuit to a baseline
image to produce a shearogram image, the computer adapted to play
the sequential shearogram images, and the computer including a
display device coupled to the computer providing visualization of
the sequential shearogram images and a memory device, coupled to
the computer, the memory device being adapted to store the
interference image information retained by the capture circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The invention will hereafter be described with reference to
the accompanying drawings, wherein like reference numerals denote
like elements in the various drawings, and;
[0014] FIG. 1 is a schematic block diagram of a shearographic
imaging system;
[0015] FIG. 2 is a schematic diagram of a shearographic imaging
system showing a cross-section of a tire as the test object;
[0016] FIG. 3 is a schematic diagram of a shearographic camera at
two different orientations relative to the tire; and
[0017] FIG. 4 is a graphical representation of the deformation of a
test object, showing the corresponding shearographic fringe pattern
produced.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] The present invention utilizes basic concepts of electronic
shearography. More details of electronic shearography are given in
U.S. Pat. No. 4,887,899, the disclosure of which is incorporated by
reference herein.
[0019] Referring now to FIG. 1, a schematic block diagram of an
arrangement for practicing electronic shearography is depicted.
Coherent electromagnetic radiation or coherent light is produced by
a laser 10, the laser light being directed through a fiberoptic
cable 15 (or alternatively directed by a mirror or a set of mirrors
or provided directly) to a beam expander or illuminator 20. Beam
expander 20 directs the coherent light onto a test object 25. The
surface of test object 25 is illuminated and reflects light into a
shearography camera 30. Shearography camera 30 includes an optical
element 35, a lens 40 for focusing the light, and a detector 45.
Optical element 35 may be a birefringent material and a polarizer,
the birefringent material being a calcite material such as a
Wallestein prism. The optical element is however not limited to a
birefringent material and a polarizer, other elements such as a
defraction grating, a Mickelson mirror, or an appropriate wave
plate may be applied. Further, optical element 35 may contain other
optics, such as, but not limited to a quarter-wave plate. Detector
45 may be a traditional video camera, a digital video camera, a
charge coupled device (CCD), or other photo sensitive detection
equipment.
[0020] The output of detector 45 is coupled to an animation device
such as a computer 50. Computer 50 includes a video capture circuit
55, a central processing unit 60, and a memory 65. Alternatively,
computer 50 may include a logical extractor that is configured to
extract shearographic images from memory in a predetermined manner.
The logical extractor may be embodied in hardware or alternatively
in software within computer 50. Video capture circuit 55 may be a
dedicated video card or a frame grabber preferably capable of
capturing entire video images at a rate of at least 15 frames per
second. However, video capture circuit 55 may be capable of
capturing video images at any suitable rate. Central processing
unit 60 may be any of a number of conventional microprocessors or a
dedicated microprocessor device. Detector 45 is coupled to central
processing unit 60, central processing unit 60 being coupled to
video capture circuit 55 and memory device 65. Central processing
unit 60 is further coupled to a display unit 70, which may be a CRT
(cathode ray tube) display, an LCD (liquid crystal display), or the
like.
[0021] In operation, coherent light emanating from beam expander 20
is reflected from test object 25. Optical element 35 collects the
reflected light from object 25 causing an interference image to be
created. The interference image is focused on detector 45 through
lens 40. Conventionally, a first interference image is taken while
test object 25 is in a first stressed condition, and a second
interference image is taken with object 25 in a second stressed
condition. The two interference images are then compared by a
process of subtracting one image from the other and the shearogram
is created and displayed on a monitor.
[0022] In the present invention, test object 25 undergoes a
sequence of or continuum of varying stress levels. Detector 45
continuously captures the interference image from optical element
35 and communicates the interference image to computer 50, during
the stress cycle. Capture circuit 55 electronically captures entire
interference images at a rate of at least 15 frames per second.
Capture circuit 55 communicates the interference images to central
processing unit 60. Central processing unit 60 compares the
interference image to a baseline interference image of the object
in the unstressed or near unstressed state (or alternatively any
chosen stress state), by a process of subtracting one interference
image from the baseline interference image, thereby forming a
shearogram. Each shearogram image is simultaneously displayed on
display unit 70 and stored in memory device 65. After the series of
varying stress levels has been completed, microprocessor 60 (or
alternatively a logical extractor) recalls the sequence of
shearogram images captured by capture circuit 55 and replays them
in sequence on display unit 70. The sequential display of these
shearogram images, at a rate of at least 15 frames per second,
produces a shearographic animation of the shearograms produced
during or after stressing of test object 25.
[0023] Test object 25 may be a relatively large object, such as a
tire 200, as depicted in FIG. 2. A shearographic camera 230 that is
rotatable within the inside of the bead 202 of tire 200 is depicted
in FIG. 2. (Alternatively, tire 200 may be rotated and camera 230
may be stationary.) Shearographic camera 230 includes a laser 235
producing a coherent beam of light to illuminate the inside of tire
200. Shearographic camera 235 is further coupled to a computer 240
having a display 245, computer 240 and display 245 being used for
data acquisition and animation of the resultant shearographic
images.
[0024] When used for detection of defects in tires or retread
tires, shearographic imaging camera 230 may be positioned inside
the tire depicted as position A in FIG. 3 or outside the tire as
depicted in FIG. 3 by position B. Having shearographic camera 230
in position A allows for detection of defects in the tread area of
tire 200. Having shearographic camera 230 in position B provides
for examination of the bead area and side wall area of tire
200.
[0025] Referring back to FIG. 2, in operation, shearographic camera
230 and tire 200 may be placed into a vacuum chamber capable of
subjecting tire 200 to a vacuum producing stresses on tire 200 by
producing a positive pressure (relative to the pressure inside the
vacuum chamber) in voids within tire 200 causing a bulge 250.
Referring to FIG. 4, the bulge may be caused by a defect 260,
defect 260 possibly being but not limited to a delamination between
two layers of the tire or a void in the molded material. When
subjected to a vacuum, bulge 250 appears because of positive
pressure within the void space of bond 260. The graph of FIG. 4
depicts the slope of bulge 250 by line 270. The graph of FIG. 4
further depicts a fringe pattern, including groups of rings 280 and
290, produced by the differencing of two optical interference
images produced by shearographic camera 230. Fringe patterns 280
and 290 of a shearogram image is produced by computer 240 (by the
method of differencing or by any other image resolving technique)
appear as a set of roughly concentric, substantially circular
fringe lines corresponding to slope 270 of bulge 250. Fringe
patterns 280 and 290 are a contour mapping of the absolute value of
slope 270 of bulge 250. Therefore, because bulge 250 is
substantially symmetric, fringe patterns 280 and 290 appear to be
mirror images of each other.
[0026] Referring back to FIG. 2, in operation, shearographic camera
230 takes a series of interference images that are communicated to
computer 240 while tire 200 undergoes varying vacuum or stress
cycle. In a preferred embodiment tire 200 undergoes a
depressurization cycle and then a pressurization cycle to return
the tire to an unstressed state. Because the field of view of
shearographic camera 230 is limited by the field of view of the
optical elements and by the size of the tire, a tire must be
sectioned into a number of sectors ranging from four to twelve, or
more. In an exemplary embodiment, tire 200 is sectioned into nine
different sectors. Shearographic camera 230 therefore views an area
corresponding to 40.degree. of arc of tire 200. After the
depressurization and pressurization cycle, camera 230 is rotated to
the next sector, there the depressurization and pressurization
cycle is repeated. Computer 240 continues to collect data and may,
in a preferred embodiment, simultaneously display data on display
245 throughout the entirety of the nine sector cycle. The
shearograms are generated and displayed at a rate such that they
appear to be animated.
[0027] Referring now to FIG. 5, a display 300 is depicted, the
display being divided into nine different sectors, each sector 310
corresponding to an approximate 40.degree. arc of the inside of a
tire. Alternatively, however, each sector 310 could correspond to
any specific field of view, of a tire, for a shearographic camera,
such as shearographic camera 230. Computer 240 as depicted in FIG.
2, which may be connected to display 300, is capable of displaying
a plurality of animations simultaneously as depicted in FIG. 5.
FIG. 5 depicts a static screen shot of a typical display, however,
display 300 actually shows animations or sequential imaging of
shearogram images produced by computer 240 at a rate providing an
animated effect and in a preferred embodiment at a rate of 30
frames per second. A display having multiple animation windows or
screen sectors provides the clear advantage that an operator may
observe the animations simultaneously looking for the appearance of
indications of deformations due to defects. This simultaneous
observation permits less attendance time by an operator, therefore
providing substantial time savings without substantial loss of
accuracy. Capturing and providing animation preferably at 30 frames
per second (or alternatively any suitable animation rate) provides
animations that are sufficiently smooth to be useful to an
operator.
[0028] The advantages of animating the sequence of images is that
animation improves accuracy in the detection of defects. Light
effects that would appear as "false positives" in a static
shearogram are not manifested as defects when animated, due to the
absence of apparent motion induced by the animation. A fringe
pattern caused by a real defect will tend to "grow" or "shrink" and
the intensity of fringe lines will appear to cycle during the
animation, due to the continually changing stress state on the test
object. Furthermore, real defects that may be "washed out" in a
static shearogram or even in an integration of multiple
shearographic images, become apparent with animation of the
shearographic images.
[0029] Animation of the shearographic images allows visualization
of defects at a multiplicity of stress states, some of the stress
states may not cause the "washed out" effect and further the
apparent motion created by animation of the images manifests a real
defect as opposed to the light effect. Animation of the shearograms
goes through a substantial continuity of stress states, therefore
defects that may not be present at two chosen stress states become
apparent in the animation. These advantages in animation of the
shearographic images provide better accuracy in detecting defects
and provides for shorter analysis times by an operator.
[0030] It has been recognized that a number of signal processing
techniques, such as, but not limited to the use of fuzzy logic,
neural networks, artificial intelligence, and pattern recognition
techniques, may be applied to perform automatic defect
identification. However, systems such as this tend to be inherently
complex and substantially costly. Therefore, retaining a human
operator, but cutting down on the operators' required attendance
time by providing the operator with numerous simultaneous
animations, has the effect of providing substantial cost
savings.
[0031] Although animation of shearographic images may be preferable
at a rate of at least 15 frames per second, it should be noted that
frame rates of less than 15 frames per second may also be used
effectively, however the animation may appear discretized as
compared to an animation running at least 15 frames per second.
Further, it should be appreciated that frame rates of more than 30
frames per second may be advantageous in specific applications and
may become simpler to implement as microprocessor and video capture
technology is improved.
[0032] It should be appreciated that although a differencing
approach to producing each shearogram is described above, the
methods and apparatuses disclosed may be applied to different image
resolving techniques, including but not limited to continuous
integration. Continuous integration describes the process of taking
a first interference image and subtracting a second interference
image to produce a first shearogram. A third interference image is
taken and subtracted from the first shearogram to produce a second
shearogram. A fourth interference image is then taken and
subtracted from the second shearogram to produce a third
shearogram. This sequence is continued throughout the testing
cycle. The continuous integration technique and other techniques
known to those of ordinary skill in the art, lend themselves to the
animation techniques disclosed above and can be applied thereto
without departing from the spirit and scope of the present
invention.
[0033] The process and apparatus described above should be
appreciated to optimize a number of competing factors associated
with shearographic imaging, especially as applied to the testing
for defects in retread tires (although clearly not limited to this
application). These competing factors include, but are not limited
to, maximizing data, maximizing accuracy, minimizing operator
attendance time, available light wavelengths, object size,
equipment costs, and optical field of view. By animating
shearograms in a plurality of sectors on a display screen, a number
of these competing factors are optimized.
[0034] It is understood that, while the detailed drawings and
examples given describe preferred exemplary embodiments of the
present, they are for purposes of illustration only. The method and
apparatus of the invention is not limited to the precise details
and conditions disclosed. For example, the invention is not limited
to the specific frame rates at which shearographic images are
captured or displayed. Further, the number of sectors of the test
object is completely variable and, the object being tested may be
any of a number of test objects. Still further, the method by which
the test object is placed under stress may be any of a number of
techniques. Still further, other optical systems that produce
interference images, other than shearographic camera 30, may be
applied to produce shearograms. Various changes may be made to the
details disclosed without departing from the spirit of the
invention, which is defined by the following claims.
* * * * *