U.S. patent application number 10/784557 was filed with the patent office on 2004-08-26 for portable real-time high-resolution digital phase-stepping shearography with integrated excitation mechanisms.
Invention is credited to Bard, Benjamin A., Lu, Guowen, Newman, John W., Wu, Shudong.
Application Number | 20040165176 10/784557 |
Document ID | / |
Family ID | 32033127 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040165176 |
Kind Code |
A1 |
Bard, Benjamin A. ; et
al. |
August 26, 2004 |
Portable real-time high-resolution digital phase-stepping
shearography with integrated excitation mechanisms
Abstract
A portable nondestructive testing instrument uses high-speed
phase-stepping shearography, and vacuum stressing, to produce
images of disbonds, impact damage, or delaminations, in metal or
composite structures. The invention is especially useful in the
inspection of large areas where only external access is feasible,
such as in large aircraft, space vehicles, boats, or civil
engineered structures having multiple bond lines. The invention
includes a novel combination of components and techniques,
including a high-spatial-resolution CCD sensor, low-voltage
piezoceramic phase stepping, rapid phase stepping, a fast phase
calculation technique, a fast image smoothing technique, and an
implementation of all of the above in a portable unit. Specially
designed timing and control algorithms allow data acquisition,
transfer, calculation, smoothing, and display at rates of up to two
times per second. The invention also includes the above-described
combination, in conjunction with three excitation mechanisms
provided in an integrated portable package.
Inventors: |
Bard, Benjamin A.; (Belmont,
MA) ; Lu, Guowen; (San Jose, CA) ; Wu,
Shudong; (Fremont, CA) ; Newman, John W.;
(Newton Square, PA) |
Correspondence
Address: |
William H. Eilberg
Suite 501 West
Three Bala Plaza
Bala Cynwyd
PA
19004
US
|
Family ID: |
32033127 |
Appl. No.: |
10/784557 |
Filed: |
February 23, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10784557 |
Feb 23, 2004 |
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09537862 |
Mar 28, 2000 |
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6717681 |
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60127229 |
Mar 31, 1999 |
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Current U.S.
Class: |
356/35.5 |
Current CPC
Class: |
G01B 9/02098 20130101;
G01B 11/162 20130101; G01B 9/02054 20130101 |
Class at
Publication: |
356/035.5 |
International
Class: |
G01B 009/02 |
Claims
What is claimed is:
1. A method of nondestructive analysis of a test object,
comprising: a) activating a camera to capture an image of a test
object, and transmitting said image to a memory of a computer, b)
altering a field of view of the camera, after the camera has
completed step (a) but before the camera has acquired any further
image, c) re-activating the camera to capture another image of the
test object, and transmitting said another image to the memory of
the computer, d) deforming the object, e) repeating steps (a)
through (c) while the object is deformed, f) calculating an optical
phase for each portion of each image, and g) displaying an
interference pattern representative of differences between optical
phases of each image.
2. The method of claim 1, wherein steps (a) through (c) are
performed for a plurality of times.
3. The method of claim 2, wherein steps (a) through (c) are
repeated for a plurality of times while the object is undeformed
and while the object is deformed.
4. The method of claim 1, wherein steps (a) through (c) are
performed four times, for both undeformed and deformed states of
the object, and wherein step (g) includes calculating a phase for
each pixel of a final image, according to the following equation: 3
( x , y ) = tan - 1 [ I 8 ( x , y ) - I 6 ( x , y ) I 5 ( x , y ) -
I 7 ( x , y ) ] - tan - 1 [ I 4 ( x , y ) - I 2 ( x , y ) I 1 ( x ,
y ) - I 3 ( x , y ) ] . where I.sub.1 through I.sub.8 are eight
captured images given by: I.sub.1(x,y)-I'(x,y)')+I'(x,y)cos
[.phi.(x,y).vertline.. I.sub.2(x,y)=I'(x,y)+I'(x,y)cos .left
brkt-bot..phi.(x,y)+.pi./2.right brkt-bot..
I.sub.3(x,y)=I'(x,y)+I"(x,y)cos .left brkt-top..phi.(x,y)+.pi.-
.vertline.. I.sub.4(x,y)=I'(x,y)+I"(x,y)cos .left
brkt-bot..phi.(x,y)+3.pi- ./2.vertline..
I.sub.5(x,y)=I'(x,y)+I"(x,y)cos [.phi.(x,y)+.DELTA.(x,y)],
I.sub.6(x,y)=I'(x,y)+I"(x,y)cos .left
brkt-bot..phi.(x,y)+.DELTA.(x,y)+.p- i./2.right brkt-bot.,
I.sub.7(x,y)=I'(x,y)cos [.phi.(x,y)+.DELTA.(x,y)+.pi- .],
I.sub.8(x,y)=I'(x,y)+I"(x,y)cos .left
brkt-bot..phi.(x,y)+.DELTA.(x,y)- +3.pi./2.right brkt-bot.. wherein
I' is a bias intensity, I" is a modulation intensity, .phi. is a
random phase variable due to diffuse reflection of light from the
object, and .DELTA. is a quantity directly proportional to
differential displacement due to deformation.
5. The method of claim 1, wherein the camera includes a
shearography head comprising at least one translatable mirror, and
wherein step (b) is performed by activating a piezoceramic disk
connected to said mirror, wherein movement of the mirror causes a
change in an image captured by the camera.
6. A method of nondestructive testing, comprising: a) stepping a
shearography camera through a plurality of video frames, each frame
comprising an image of a test object, and storing each video frame,
b) repeating step (a) while the test object is deformed, and c)
generating an interference pattern representative of phase
differences caused by deformation of the object, wherein the camera
is stepped between successive video frames.
7. The method of claim 6, wherein the shearography camera includes
a Michelson interferometer having at least one movable mirror, and
wherein step (a) comprises moving said mirror through a
predetermined distance.
8. The method of claim 7, wherein the mirror is connected to a
piezoceramic disk, and wherein step (a) comprises transmitting an
electrical signal to said disk so as to cause movement of the
mirror.
9. A portable real-time high-resolution digital phase-stepped
shearography apparatus comprising: a) a shearography head including
at least one movable mirror and a piezoceramic disk connected to
the mirror, b) a programmed computer connected to said shearography
head, and c) a display, connected to the computer, wherein the
computer is programmed to issue signals to the piezoceramic disk so
as to control movements of the mirror, and wherein the computer is
programmed to store images of a test object captured by the
shearography head, and to analyze said images so as to produce an
interference pattern viewable on the display.
10. The shearography apparatus of claim 9, wherein the computer
comprises means for moving the mirror only while the shearography
head is not acquiring an image.
11. The shearography apparatus of claim 9, wherein the shearography
head is mounted in an enclosure, and wherein the enclosure also
includes at least one excitation mechanism for causing deformation
of a test object.
12. The shearography apparatus of claim 11, wherein the enclosure
includes at least two distinct excitation mechanisms.
13. The shearography apparatus of claim 10, wherein the
shearography head is mounted in an enclosure, and wherein the
enclosure also includes at least one excitation mechanism for
causing deformation of a test object.
14. The shearography apparatus of claim 13, wherein the enclosure
includes at least two distinct excitation mechanisms.
15. The shearography apparatus of claim 9, wherein the shearography
head includes a high-resolution video camera.
16. A portable real-time high-resolution digital phase-stepped
shearography apparatus comprising: a) a shearography head including
a high-resolution video camera and a Michelson interferometer
including at least one movable mirror and a piezoceramic disk
connected to the mirror, b) the shearography head being connected
to a housing, wherein the housing supports at least one excitation
mechanism, c) a programmed computer connected to said shearography
head, and d) a display, connected to the computer, wherein the
computer is programmed to issue signals to the piezoceramic disk so
as to control movements of the mirror, and wherein the computer is
programmed to store images of a test object captured by the
shearography head, and to analyze said images so as to produce an
interference pattern viewable on the display.
17. The shearography apparatus of claim 16, wherein the computer
comprises means for moving the mirror only while the shearography
head is not acquiring an image.
18. The shearography apparatus of claim 16, wherein the housing
comprises an enclosure having a top, the shearography head being
mounted on the top of the enclosure, the enclosure including at
least one mirror and a window.
19. The shearography apparatus of claim 18, wherein the enclosure
has a hole which cooperates with a second hole in the shearography
head, wherein light can pass from the head, to the object, and back
into the head.
20. The shearography apparatus of claim 16, wherein the housing
supports at least two distinct excitation mechanisms.
21. A portable real-time high-resolution digital phase-stepped
shearography apparatus comprising: a) a shearography head including
a high-resolution video camera and a Michelson interferometer
including at least one movable mirror and a piezoceramic disk
connected to the mirror, b) the shearography head being mounted on
an exterior surface of a housing, wherein the housing includes at
least two distinct excitation mechanisms, the excitation mechanisms
being adapted to cause deformation of a test object, c) a
programmed computer connected to said shearography head, and d) a
display, connected to the computer, wherein the computer is
programmed to issue signals to the piezoceramic disk so as to
control movements of the mirror, and wherein the computer is
programmed to store images of a test object captured by the
shearography head, and to analyze said images so as to produce an
interference pattern viewable on the display.
22. The shearography apparatus of claim 21, wherein the computer
comprises means for moving the mirror only while the shearography
head is not acquiring an image.
23. A method of nondestructive testing of an object, comprising: a)
aiming a portable shearography unit at a test object, the
shearography unit including a shearography head, and at least one
excitation mechanism, the shearography unit being connected to a
computer and a display, b) activating the shearography head to
acquire images of the test object while the object is in an
undeformed state, c) actuating the excitation mechanism so as to
deform the object, d) activating the shearography head to acquire
images of the test object while the object is in a deformed state,
e) processing the images acquired in steps (b) and (d) in the
computer so as to generate an interference pattern on the
display.
24. The method of claim 23, wherein steps (b) and (d) include
stepping the shearography head through a plurality of distinct
fields of view, and wherein the stepping is performed only between
successive images.
25. The method of claim 24, wherein the stepping is performed by
transmitting signals to a piezoceramic disk connected to a mirror
in the shearography head so as to move the mirror through a
predetermined distance.
Description
CROSS-REFERENCE TO PRIOR PROVISIONAL APPLICATION
[0001] This application corresponds to U.S. Provisional Patent
Application Serial No. 60/127,229, filed Mar. 31, 1999.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to the field of nondestructive
testing, and, in particular, to the field of speckle-shearing
interferometry, or shearography.
[0003] Nondestructive testing of aircraft structures, such as
honeycombs, has been performed by techniques which include tap
testing (i.e. using an inspector's ear to judge the presence of a
skin-to-core disbond), impedance bond testing, and pulsed-echo
ultrasonics. While effective, these prior art methods are either
subjective, require considerable recalibration with changes in skin
thickness, provide subjective interpretation, or are slow.
Shearography has been found to be a more preferred method of
nondestructive testing.
[0004] Shearography was first introduced by Hung and Taylor, in
"Measurement of slopes of structural deflections by
speckle-shearing interferometry", Experimental Mechanics, vol. 14,
pages 281-285 (1974), and by Leendertz and Butters, in "An
image-shearing speckle-pattern interferometer for measuring bending
moments", J. Phys. E.: Sci. Instrum., vol. 6, pages 1107-1110
(1973). Shearography essentially comprises the formation of an
image comprising two laterally-displaced images of the same
object.
[0005] Shearography is a full-field optical speckle interferometric
procedure which is capable of measuring small deformations of a
surface. These deformations can be produced by several mechanisms,
and are measured for several purposes. Deformation mechanisms
include, among others, vacuum, pressure, microwave, thermal,
vibration, and ultrasonic excitation. Purposes for measurement
include stress analysis, vibration measurement, acoustic and
elastic wave visualization, and other nondestructive testing (NDT)
including detection of flaws in a test object.
[0006] The first practical apparatus for performing shearography
electronically was introduced in 1987, and was based on the
technology described in U.S. Pat. No. 4,887,899, the disclosure of
which is incorporated by reference herein. The form of shearography
described in the above-cited patent, using birefringent optics as a
means of generating a sheared image, provided the first
high-resolution, real-time shearography system which could produce
images of disbonds due to out-of-plane deformations caused by
vacuum stress of the skin of the structure being tested.
[0007] The first portable vacuum stress shearography instruments
used video subtraction to provide rapid imaging of flaws. While
this technology was a major advance in nondestructive testing,
enabling one to perform tests at a rate of 150 square feet per
hour, as compared with 3-8 square feet per hour using prior
methods, the sensitivity of the technology to defects was limited.
Subtraction shearography provides images of flaws only when the
out-of-plane deformation exceeds one-half wavelength in the shear
offset distance, the distance between the sheared images. The
portable subtraction shearography devices of the prior art have
found important uses in the aerospace field, in the inspection of
composite honeycomb structures including radomes, composite
fuselages, wheel doors, close-out panels, and many other
components.
[0008] Portable vacuum stress shearography cameras are subject to
several types of vibration, noise, and mechanical instabilities
that considerably degrade the image quality and the ability to
detect defects in aerospace structures. Not only do these sources
of noise have to be eliminated with techniques that do not add
weight and size to the portable instrument, but any method used to
increase sensitivity must be made to run as fast as possible to
prevent degradation of the images.
[0009] One method of increasing defect sensitivity is the use of
phase-stepping shearography, introduced by Nakadate in 1985. While
Nakadate demonstrated the use of phase-stepping and the increased
defect sensitivity provided by phase map presentations, the
software and hardware available at that time required ten minutes
to yield the phase map image, far too slow for use in a practical
nondestructive testing instrument. In addition, the phase step
process requires capturing consecutive video frames and
considerable arithmetic manipulation of between four and ten images
to calculate the phase map. The level of noise rapidly increases as
the time to perform the calculations increases, dramatically
reducing sensitivity.
[0010] A basic setup of an electronic shearography system is as
follows. Coherent laser light is spread out to illuminate a portion
of the object's surface, e.g. one square foot. The light reflects
from the surface, passes through an optical shearing mechanism, and
then enters a CCD camera. The surface is then deformed by one of
the aforementioned mechanisms, such as heat. As the surface expands
slightly due to the applied heat, the deformation of the surface is
viewed, in real time, on a video monitor. This deformation is
usually not visible to the eye because it is on the order of the
wavelength of the laser light being used, i.e. approximately 250
nm. Deformation of the surface often shows direct evidence of a
subsurface flaw.
[0011] Shearography has evolved over the years from a film-based to
a video-based (electronic, or analog) system, and finally to
systems which store the images in a computer, in digital form. In
its film-based form, shearography is typically limited to an optics
laboratory. In its electronic or digital form, if the system is
made compact, shearography can be removed from the laboratory, and
used in real-world settings. It can survive environmental factors
such as slight heat and vibration fluctuations due to its
particular optical setup.
[0012] As implied above, there are two fundamental modes in which
shearography can be used, namely, speckle correlation fringe
formation due to subtraction, and phase map formation due to phase
stepping. As already noted, phase stepping is beneficial since it
results in increased signal-to-noise ratio (SNR), increased
displacement resolution (resulting in increased flaw detection
sensitivity), quantitative rather than qualitative results, and
other factors. Either of the two modes (fringes or phase stepping)
can be used with either of the three bases (film, electronic,
digital), creating six combinations. Then there are more than seven
known optical shearing mechanisms, and more than six known
phase-stepping methods. The number of possible system
implementations is hence quite high. Furthermore, the
phase-stepping algorithm, of which there are at least ten, the
computational method of implementing the algorithm, and the
consequent speed with which the algorithm is executed are all
important. These directly affect the accuracy and SNR of the final
measurements as well as the rate at which a user can view the
results and make slight changes. Finally, the entire optical system
is either used as a research tool in an optics laboratory on a
vibration-isolation table, or it must be packaged such that it can
be taken into the field and used with several excitation mechanisms
in order to find flaws effectively, efficiently, and
conveniently.
[0013] The present invention provides a high-speed phase-stepping
shearography system which requires less than about one second to
produce an image, and which demonstrates remarkable stability,
sensitivity, and image quality. Compared to the prior art, the
present invention improves defect sensitivity by a factor of 50,
enabling a portable shearography system to compete with
widely-accepted ultrasonic systems, but at a speed of operation
which is about 50 times faster.
SUMMARY OF THE INVENTION
[0014] The system of the present invention includes four
subsystems, namely a shearography head, an enclosure, an excitation
mechanism, and a computational subsystem. The shearography head
preferably uses a Michelson interferometer to generate the sheared
images, and can receive laser light either from an external laser,
coupled by an optical fiber, or from a laser diode which is
internal to the head. The amount of phase stepping is automatically
adjusted by moving one of the mirrors of the Michelson
interferometer, through the use of a piezoceramic disk which is
controlled by a voltage determined by a computer. The amount of
shearing is adjusted by manually tilting a different mirror in the
Michelson interferometer.
[0015] The enclosure includes, at a minimum, a casing having a
transparent window to permit laser illumination and formation of an
image of the test object. In a preferred embodiment, the
shearography head is mounted on the enclosure, the enclosure having
a hole which cooperates with a similar hole in the shearography
head, such that laser light can pass from the head, to the object,
and back into the head, through the cooperation of various mirrors.
The enclosure also includes means for stabilizing the system
relative to the object being tested.
[0016] The excitation mechanism can be a vacuum, or it can be a
thermal or vibration system. In the preferred embodiment, all three
excitation mechanisms (vacuum, thermal, and vibration) are built
into the same enclosure, so that any or all of these mechanisms can
be used without modifying the apparatus. The preferred source of
the vacuum is an external blower which is connected, through
suitable holes in the enclosure, to the interior of the enclosure.
The preferred source of thermal excitation is a grid of thin heated
wires disposed near the transparent window of the enclosure, so as
to be in a position to heat the test object. The preferred source
of vibrational excitation includes a shaker/stinger/plunger
arrangement which is built into the enclosure.
[0017] The computational subsystem includes a programmed computer
which is connected, through appropriate analog-to-digital and
digital-to-analog converters, to the components described above. In
the preferred embodiment, the computer directs a four-step
algorithm which captures and stores images of the test object at
four different positions of the mirror, when the object is both in
the deformed and undeformed states. Comparison of the stored
images, coupled with application of a smoothing algorithm, yields a
pattern that can be viewed, essentially in real time, on a video
display.
[0018] The present invention therefore has the primary object of
providing an apparatus for performing real-time digital
shearography.
[0019] The invention has the further object of providing the
benefits of the phase-stepping technique in real-time digital
shearography for increased sensitivity and signal-to-noise ratio as
well as quantitative rather than qualitative data.
[0020] The invention has the further object of providing real-time
shearography, using a portable unit.
[0021] The invention has the further object of providing a
shearography system which produces a high-resolution image in real
time, and which can conveniently be used in commercial or
industrial environments.
[0022] The invention has the further object of providing an
integrated, compact digital shearography system, wherein a
plurality of mechanisms for excitation are present in a single
housing.
[0023] The invention has the further object of enhancing the
efficiency of nondestructive testing of objects.
[0024] The invention has the further object of making it easier to
search for defects in structures where interior access is difficult
or impractical, such as in aircraft, space vehicles, boats, and
civil-engineered structures.
[0025] The reader skilled in the art will recognize other objects
and advantages of the present invention, from a reading of the
following brief description of the drawings, the detailed
description of the invention, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 provides a schematic diagram of a first embodiment of
the shearography head used in the present invention.
[0027] FIG. 2 provides a schematic diagram of a second embodiment
of the shearography head used in the present invention.
[0028] FIG. 3 provides a side view of the enclosure used in the
present invention, showing the shearography head, in schematic
form, mounted on top of the enclosure.
[0029] FIG. 4 provides a front view of the enclosure of the present
invention, showing components used in generating three different
types of excitation.
[0030] FIG. 5 provides a bottom view of the enclosure of the
present invention, showing other views of the components used in
generating the three different types of excitation shown in FIG.
4.
[0031] FIG. 6 provides a timing diagram showing the various steps
commanded by the computer in performing the method of the present
invention, including the acquisition of an image for each of four
positions of a mirror, in both the undeformed and deformed states
of the test object.
[0032] FIG. 7 provides a reproduction of a photograph showing a
phase map of a thermally deformed honeycomb structure, produced
with the apparatus of the present invention, but having no
smoothing.
[0033] FIG. 8 provides a phase map similar to that of FIG. 7, but
including the effect of nonlinear smoothing.
[0034] FIG. 9 provides a photograph of a prototype system made
according to the present invention.
[0035] FIG. 10 provides a photograph of a portable shearography
unit, made according to the present invention, and being used to
examine a test object.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The present invention comprises four subsystems, namely, a
shearography head, an enclosure, an excitation mechanism, and a
computational subsystem. One embodiment of the invention is in a
single portable hand-held unit, where the head and excitation
mechanisms are integrated with the enclosure which is held by the
user. The head/enclosure/excitation unit is attached to the
computational subsystem. Two slightly different optical setups for
the shearography head can be used in this embodiment. A second
embodiment of the invention has the head inside the enclosure and
mountable on a tripod; for this embodiment, the enclosure is
simplified and excitation mechanisms are applied externally and are
not integrated into the enclosure. Either optical setup for the
shearography head can also be used with this second embodiment. All
described combinations of optical setups and enclosures result in
real-time high-resolution portable digital phase-stepping
shearography. Two particular combinations result in real-time
high-resolution portable digital phase-stepping shearography with
integrated vacuum, thermal, and vibration excitation
mechanisms.
[0037] The shearography head is shown in FIGS. 1 and 2. FIG. 1
shows an embodiment wherein laser light is generated externally of
the head, and conveyed into the head by an optical fiber. FIG. 2
shows an embodiment wherein the laser source is a laser diode
within the head. In both FIG. 1 and FIG. 2, the shearography head
is the portion enclosed by the box.
[0038] In the embodiment of FIG. 1, laser light from coherent laser
source A is connected to armored single-mode or
polarization-maintaining (PM) optical fiber C, through adjustable
optical fiber coupler B. Laser light from the fiber passes through
diverging optics D and illuminates the test object. In the case of
a hand-held unit, approximately one square foot of the object's
surface is illuminated. In the case of a tripod-mounted embodiment,
up to ten square feet can be illuminated, depending on available
coherent laser power. Either the hand-held or tripod embodiment can
be used with either of the embodiments of FIGS. 1 and 2.
[0039] The diverging optics can consist of several elements, but
need at least a simple lens to focus and then diverge the light.
Other elements can consist of custom-made or off-the-shelf
anti-Gaussian lenses to distribute uniformly the illuminating laser
light on the surface of the object.
[0040] Reflected light passes through an optical shearing
mechanism. This mechanism can be one of many, but to meet the
requirements of this invention, the mechanism selected is a
modified Michelson interferometer. The Michelson interferometer
includes two front-silvered mirrors J and H, and a nonpolarizing
beam splitter cube G. Modification of the interferometer comes from
the fact that one of the two mirrors is intentionally misaligned
due to tilting. This tilting can be adjusted, resulting in
adjustable shearing direction and magnitude, by mounting the mirror
on an adjustable kinematic mirror mount I. A further modification
is made to allow phase-stepping to take place.
[0041] The second mirror J is mounted to a piezoceramic (PZT) disk
K, which is attached via two electrical leads to a voltage source.
The PZT disk can require an external voltage amplifier, or be of a
low-voltage variety for convenience; the latter was chosen in this
implementation to promote portability.
[0042] Reflected light finally is imaged by a CCD camera E with
attached imaging lens F. The imaging lens is chosen based on the
field of view to be imaged, as well as the acceptable amount of
transverse distortion. The CCD camera is chosen based on light
sensitivity, noise characteristics, spatial resolution, and data
throughput. Analog or digital CCD cameras can be used. In this
implementation, a high resolution (1024.times.1024 cell) digital
CCD camera was used, but higher resolution devices can also be used
if data throughput is acceptable. This particular camera operates
in "progressive line-scan mode", allowing efficient digitization
and transfer of data.
[0043] In the embodiment of FIG. 2, the laser light is produced by
laser diode A. Light from diode A passes through beam shaping
optics B and collimating optics C, before reaching diverging optics
D. Elements E, F, G, H, I, J, and K are the same as in the
embodiment of FIG. 1.
[0044] The second subsystem of the present invention is the
portable enclosure that houses the shearography head. In the
embodiment where the enclosure is mounted on a tripod and
excitation mechanisms are not integrated, the enclosure is simply a
metal casing with a glass window for the laser illumination and
camera imaging. The casing will also allow controls for camera
focusing and aperture, as well as adjustable shearing (via the
kinematic mount I of FIGS. 1 and 2), to protrude for adjustment by
the user. In the embodiment wherein the enclosure is held by the
user and contains integrated excitation mechanisms, the enclosure
will be as illustrated in FIG. 3.
[0045] FIG. 3 shows a side view of the enclosure A with the
shearography head mounted on top of it. Images of the surface of a
test object are captured by laser light which is fed from the
shearography head B, down through a hole in the enclosure,
reflected from mirror C and again from mirror D, and passed through
window E, which may be made of glass or plastic. The laser light is
reflected off of the surface of the object (not shown in FIG. 3),
and enters the enclosure by passing again through window E. The
light is then further reflected by mirrors D and C, and returns to
shearography head B, where it enters the camera/lens/shearing
combination. Stabilizing feet G will rock about a ball joint (not
shown), allowing the unit to be pushed against the surface being
measured while protecting the window E. Flaps F both keep out stray
ambient light and also help to create a vacuum hold, described
later.
[0046] In the case of a smaller camera and laser diode source, all
components can be contained inside the enclosure itself and do not
need to be mounted on top of the enclosure. In the latter case,
there will be no hole on the top of the enclosure, and at least one
of the two internal mirrors can be omitted.
[0047] The third subsystem of the present invention comprises the
excitation mechanism. Several excitation mechanisms can be used
either independently or in combinations. Sources of these
excitation mechanisms can be built into the enclosure to allow
convenient application at a touch of a switch. This concept is
illustrated in FIGS. 4 and 5.
[0048] The three excitation mechanisms are 1) vacuum, 2) thermal,
and 3) vibration. Vacuum is used in all cases, where tripod
mounting is not used, to hold the shearography head and the
enclosure firmly to the object, but is not varied during the data
acquisition period when used only for this purpose. When used as
such, vacuum is not considered an excitation mechanism but an
integral part of the measurement procedure.
[0049] Vacuum can also be increased slowly during the data
acquisition period to become an excitation source itself. The
vacuum is applied by means of an external vacuum blower which
reduces the pressure in a flexible rubber tube connected to the
bottom of the enclosure. The path of the vacuum is routed through
the enclosure and then through several holes in the glass or
plastic window, evacuating the air lying between the window/flaps
and the object's surface. This arrangement causes the surface of
the object lying just under the window to be pulled toward the
window.
[0050] Thermal excitation is performed by a grid of thin, heated
wires lying between the outside surface of the glass/plastic window
and the leveling feet. The wires heat up at a flip of a switch, and
the surface of the test object thermally deforms. The wires are
thin and will be barely resolved by the imaging system.
Furthermore, due to subtraction algorithms, to be described later,
most visual traces of the wires will be removed
computationally.
[0051] Vibration excitation is performed by a
shaker/stinger/plunger arrangement exiting the enclosure housing.
The shaker is mounted inside the enclosure on a vibration isolation
(dissipative) mounting, and the attached stinger passes through a
hole in the frame of the enclosure. The end of the stinger has an
attached rubber plunger which makes contact with the surface of the
test object. When the shaker is snapped into place, pushing the
enclosure against the test object will also engage the
stinger/plunger. When the shaker is not snapped into place, the
plunger does not make contact with the surface, allowing other
excitation methods to be used.
[0052] FIG. 4 provides a front view of the enclosure with
integrated excitation mechanisms. The figure shows glass or plastic
window A, mounted within metal frame B of the enclosure. Rubber
flaps C function as described above, namely to keep out stray
light, and to enhance the vacuum hold. Stabilizing feet D are
preferably made of plastic. Plunger E is attached to the
shaker/stinger (not shown in FIG. 4). Holes F permit a vacuum to be
applied. Heating wires G provide the desired thermal
excitation.
[0053] FIG. 5 provides a bottom view of the enclosure of FIG. 4. As
in FIG. 4, window A is shown mounted within frame B of the
enclosure. The figure shows rubber flaps C and plastic stabilizing
feet D. The figure also shows shaker E, stinger F, and shaker
mounting G. Plunger H is attached to the shaker/stinger. The grid
of heating wires is shown as element I. Outlet J provides means for
attaching a vacuum hose to the underside of the enclosure.
[0054] The fourth subsystem of the present invention comprises the
computational algorithms for control, calculation, and display.
Operation of the components within the shearography head is
controlled by a host computer. The computer sends calibrated
voltages at specific times to the PZT disk, sends signals to the
camera to begin grabbing frames, receives data from the camera as
images are captured, and can trigger excitation mechanisms to begin
and end. Alternatively, the latter step can be performed manually.
Timing and data throughput are crucial to the success of the data
acquisition. After data is acquired, rapid data processing and
display must be accomplished.
[0055] Several phase-stepping algorithms can be implemented,
including 2-step, 3-step, 4-step, and 5-step methods. The 4-step
algorithm will be illustrated, but the general procedure is the
same for any of them.
[0056] The following steps occur in rapid succession:
[0057] 1. The camera is triggered to capture one frame of the
surface and send the data to the computer.
[0058] 2. While the camera is between two successive video frames,
a signal is sent to the PZT which causes the attached mirror to be
translated by a calibrated distance.
[0059] 3. Once the mirror has finished its travel, the camera is
triggered to capture a second frame and send data to the
computer.
[0060] 4. A second signal is sent to the PZT to translate the
mirror.
[0061] 5. The camera is triggered and a third frame is
captured.
[0062] 6. A third signal is sent to the PZT.
[0063] 7. The camera is triggered and a fourth frame is
captured.
[0064] 8. The optical phase for the first "object state" is
calculated and stored.
[0065] 9. A signal is sent to the PZT to return the mirror to its
initial position.
[0066] 10. The object is deformed, either manually or
automatically, using one or a combination of excitation
methods.
[0067] 11. Once deformation is stopped, or is varying slowly, steps
1 through 7 are repeated.
[0068] 12. The optical phase for the second "object state" is
calculated, subtracted from the phase for the first object state,
and the difference is rectified, converted to 256 values, and
displayed on the screen as a gray scale image.
[0069] The phase calculation occurs at each pixel over all the
images. For the four-step method, the equation is: 1 ( x , y ) =
tan - 1 [ I 8 ( x , y ) - I 6 ( x , y ) I 5 ( x , y ) - I 7 ( x , y
) ] - tan - 1 [ I 4 ( x , y ) - I 2 ( x , y ) I 1 ( x , y ) - I 3 (
x , y ) ] . ( 1 )
[0070] where I.sub.1 through I.sub.8 are the eight captured images,
described by
I.sub.1(x,y)-I'(x,y)')+I'(x,y)cos [.phi.(x,y)].
I.sub.2(x,y)=I'(x,y)+I'(x,y)cos .left
brkt-bot..phi.(x,y)+.pi./2.right brkt-bot..
I.sub.3(x,y)=I'(x,y)+I"(x,y)cos .left
brkt-top..phi.(x,y)+.pi..right brkt-top..
I.sub.4(x,y)=I'(x,y)+I"(x,y)cos .left
brkt-bot..phi.(x,y)+3.pi./2.vertline- ..
I.sub.5(x,y)=I'(x,y)+I"(x,y)cos [.phi.(x,y)+.DELTA.(x,y)],
I.sub.6(x,y)=I'(x,y)+I"(x,y)cos .left
brkt-bot..phi.(x,y)+.DELTA.(x,y)+.pi- ./2.right brkt-bot.,
I.sub.7(x,y)=I'(x,y)cos [.phi.(x,y)+.DELTA.(x,y)+.pi.],
I.sub.8(x,y)=I'(x,y)+I"(x,y)cos .left
brkt-bot..phi.(x,y)+.DELTA.(x,y)+3.p- i./2.right brkt-bot.
[0071] Here, I' is the bias intensity, I" the modulation intensity,
.phi. a random phase variable due to the diffuse reflection of
laser light from the surface, and .DELTA. is a quantity directly
proportional to the differential displacement due to deformation.
The resulting quantity has positive and negative values so it is
rectified and converted to 8-bit (256 values) for gray scale
display. It should be noted that speckle correlation fringe
production due to subtraction is actually a subset of the above
phase calculation equations, and can easily be performed by the
same opto-mechanical and computational components.
[0072] Calculation using Equation (1) can be time consuming. In a
report as recent as a February, 1999 article, a calculation of this
type was described as taking approximately 10 seconds to calculate
one phase map. Several factors allow the current system to
accomplish the same task in less than 500 milliseconds. First, the
actual data acquisition is rapid, due to the tight control of all
the opto-mechanical components and their timed integration due to
computer control. The phase shifting, data acquisition, and data
transfer to the host computer take 132 milliseconds. Then,
calculation of Equation (1) is not performed directly in the host
computer's central processing unit (CPU), but uses a look-up table
with pre-stored values for the tan.sup.-1 function. Calculation and
display hence take approximately 300 milliseconds. Since the frame
grabber, video random access memory (VRAM), signal generator, and
CPU are the fastest available off-the-shelf components, and tight
integration and control algorithms coupled with the use of look-up
tables for phase calculation have been used, the system can hence
capture, calculate, and display up to two phase maps per second.
Details of this process are shown in FIG. 6, which is a timing
diagram showing all events in the phase acquisition, calculation,
and display process.
[0073] After calculation, the phase result displayed on the
computer monitor or video screen shows deformation of the surface
of the test object. An example of such a calculated phase map is
shown in FIG. 7.
[0074] A further calculation can be performed to smooth the
calculated phase for higher quality display and easier visual
location of flaws. Typical smoothing algorithms depend on low-pass
filters, which are time-consuming. In order not to degrade the
real-time display of the phase-stepping system, a "nonlinear
filter" is implemented for fast smoothing. Invalid pixels ("noise")
are identified by failing any one of three criteria: a pixel has a
zero value (null condition), a pixel has the maximum value allowed
by the analog to digital converter of the camera (saturation
condition), or four pixels (from the same (x,y) position in all
four images) have insufficient modulation, where modulation is
defined as: 2 M ( x , y ) = 1 2 [ I 1 ( x , y ) - I 3 ( x , y ) ] 2
+ [ I 4 ( x , y ) - I 2 ( x , y ) ] 2 . ( 3 )
[0075] A threshold value for the modulation test can be found by
simple trial and error, as will be apparent to persons skilled in
the art.
[0076] Once invalid pixels are known, they can be replaced by their
valid neighbors. Bounds exist on this method in order not to
propagate a single valid pixel over the entire image, and these
bounds are checked at each invalid pixel being considered. The
smoothing process takes approximately 200 milliseconds to cover a
1317 by 1035 pixel image. With nonlinear smoothing, the entire
phase measurement and display process takes just over a half
second, still allowing almost two smoothed phase maps per second to
be displayed on the screen. The smoothed version of FIG. 7 is shown
in FIG. 8.
[0077] All of the above-described subsystems were built into a
prototype system, which is shown in FIG. 9. This prototype contains
only vacuum excitation. FIG. 10 shows the vacuum-excitation
shearography head being applied to nondestructive testing of
aircraft.
[0078] As mentioned above, many combinations of optical components,
setups, phase-stepping methods, and algorithms exist. The
originality of the present invention resides, in part, in the
combination of previously-available features never combined before,
in addition to the fact that this combination yields a portable
system and delivers real-time results to the user. For example, a
high-resolution sensor has been combined, in the prior art, with
phase-stepping shearography, but the sensor in the latter system
was digitized film, which required both a stable optics laboratory
and extended time for digitization and calculation. When a
high-resolution sensor was combined, in the past, with
phase-stepping digital shearography, it was removed from the
darkroom, but the wealth of data took up to ten seconds to be
transferred, calculated, and displayed on the screen. Fast
phase-stepping has been used in the prior art, using look-up tables
and integrated timing control, but only in a laboratory setting and
with a low resolution sensor. This is the first time that a
high-resolution sensor has been incorporated into a digital
phase-stepping shearography system that was capable of displaying
up to two phase measurements per second. Hence, this sensor, in
configuration and capabilities, is unique even when mounted on a
tripod and not integrated with a user-held enclosure or excitation
mechanisms.
[0079] Furthermore, the system of the present invention has been
packaged in a user-held enclosure with an integrated vacuum hold,
allowing the entire system to be brought into the field and firmly
mounted to the object being studied. A similar enclosure has been
used with analog subtraction shearography technology, but this is
the first time that the portability, convenience, and vacuum
holding abilities have been combined with high-resolution real-time
digital phase-stepping shearography.
[0080] Still further, the present invention provides a novel
configuration for integrating thermal and vibration excitation into
the portable enclosure. While extremely convenient and effective
for flaw detection, this arrangement has not been suggested or
realized before. It brings the power of three, rather than one,
excitation mechanisms to the aid of portable high-resolution
real-time digital phase-stepping shearography.
* * * * *