Note: Descriptions are shown in the official language in which they were submitted.
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HIGH ACCURACY STR~CTUR~D LIGHT PROFILER
Background of the Invention
This invention relates to a noncontacting optical
gaugi~g system and particularly tn such a ~ystem which
illuminates a target s~rface with a line of light which is
evaluated to provlde a measure of the surface contour of the
target surface.
Optically based gauging systems ~are presently employed
in industry for evaluating the profile ~bape of workpieces
such as turbine blades, gears, helical threads, etc. These
devices have inherent advantages over contacting-type
mechanical gauges in that they can generally operate at
greater speeds and are not subjected to mechanical wear. In
one example of an optical gauge according to the prior art,
a line or sheet of light is projected onto the object to be
characterized. ~The illuminated portion of the object is
viewed with a two-dimensional video camera along an axis at
some different angle than the illumination ~ beam.
Accordingly, the line of light illuminates a profile of a
cross section of the part which is viewed by the camera,
just as if the part had been sliced alDng the light beam.
Points nearer to the light source will b~ illuminated t~ one
side of the field bf view of the came~a, while fuxther
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points will be seen illuminated on the other side of the
camera' 5 field of view.
The above described optical gauging techniques have a
number of significant limitations. In many instances, it is
desira~le to evaluate a workpiece surface along a particular
cross section, such as perpendicular to the axls of symmetry
of a turned workpiece, or along the chord o~ a turbine
engine blade, etc. To characterize such a cross section in
one view normally requires the illuminating line of light to
be brought in precisely along the specific cross section
plane. To evaluate the workpiece contour, the camera views
the surface at an angle from the axis of illumination; which
produces magnification errors across the surface ~the so
called "keystone effect") which complicates data processing.
Moreover, viewi~lg the plane of interest off axis requires
the viewing system to have a depth of fieid adequate to
encompass the depth of interest, which may be difficult to
achieve for some workpieces. Illumination systems for such
devices also have their own limitations. To provide
resolution, the depth of focus of the illuminating line of
light must be adequate to encompass the depth of interest.
This requirement leads to a greater line width, thus
sacrificing accuracy. Finally, those systems are further
limited by `the pixel resolution of the video image
processor.
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Alternate gauging approaches such as coor~inate
measuring machines (CMMs) obtain their accuracy by means of
a high precision encoded translation stage. The operational
speed of such systems is, however, limited by the use of
contact probes which requires the machine to stop and very
slowly approach each measurement point. As a hybrid
approach, noncontact triangulation probes have been attached
to CMMs, but the measurements are still strictly done for
selected individual points on the workpiece.
In view of the foregoing, there is a need to provide an
optical gauging system which overcomes the depth~of-field
and resolution limitations of prior art optical systems and
which does not possess magnification variations along the
evaluated~image which can complicate image processlng. It
is further desirable to provide such a device which provides
rapid gauging time and high measurement accuracy and
resolution.
Summary of t_e Invention
An optical gauging system achieving the above-merrtioned
desirable features is provided in accordance with this
invention. The illumination system of this invention
illuminates the surface to be characterized with a line of
light which illuminates the surface of interest which may be
incident to the subject at some angle from the plane of the
cross section of interest. The subject is viewed by a
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viewing system which images a line of constant range onto a
high resolution linear detector array. In one embodiment,
the optical axis of the viewing system lies within the plane
of the cross section of interest. The illuminating line of
light and the focus of the viewing system are then
translated in the depth direction with respect to the
surface using a position encoded translation stage. As the
line of light sweeps past the section on the subject which
is imaged onto the detector array, the points of
intersection as indicated by the illuminated spots on the
workpiece surface are correlated with the position of the
translation stage. Accordingly, data are provided giving
the relationship between the stage position and elements of
the array`whicn are illuminated, which are related to the
surface contour and tilt of the target surface.
In the system according to this invention, the distance
from the illuminating source to the points of intersection
with the line on the subject being viewed (i.e. range) is
constant. Accordingly, a sharply focused line can be used
to illuminate the subject and the line will always~be at
best focus when it intersects the region on the subject
being viewed by the detector array. Similarly, the distance
from the detector array to the point of intersection of the
illuminating ray is also constant, thus enabling the viewing
system to operate wijthin a narrow depth of field at best
focus. Preferably, the light source and detector array are
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not translated to obtain this constant distance
relationship. Instead, the path leng~hs can be maintained
constant and any motion of the beam and array isolated from
vibrations or wobble of the translation stage through
fixedly mounting the light source and array, and moving only
the optical elements of the system. Several of the
embodiments of the present invention disclose various means
for translating the i.lluminating beam and focus of the
viewing system along the workpiece surface being
characterized in a manner which isolates any wobble or
vibration of the translation stage.
Additional benefits and advantages of the present
invention will become apparent to those skilled in the art
to which 'this invention relates upon a reading of the
described preferxed embodiments of this i.nvention taken in
conjunction with the accompany drawings.
Description of Drawings
Figure l i~ a pictorial concept drawing illustratlng a
first embodiment of the present invention which discloses
the general principles of the optical gauging systems
according to this invention.
Figure 2 is a graph showing an illustrative
relationship between the translation stage position and
vertical offset as indicated by the output from the detector
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array which provides a curve related to the profile shape of
the workpiece being imaged.
Figure 3 is an optical gauging system in accordance
with a second embodiment of this invention in which all the
components of the gauge are movable with the translation
stage.
Figure 4 is an optical gauging system in accordance
with a third embodiment of this invention utilizing a
translation stage with a separately movable follower mirror
which enables both the light source and detector array to be
fixed to a support structure.
Figure 5 is an optical gauging system in accordance
with a fourth embodimen~ of this invention enabling the
light source and detector array to be fixed to a support
structure and utilizing a pair of pentapri~ms in the
translation stage which isolates translation stage
vibrations.
Flgure 6 is a pictorial vlew of a pentaprism
lllustratlng its ability to isolate stage wobble for the
gauging system 6hown in Figure 5.
Figures 7 through 10 illustrate the intersection of the
illuminating line of light and the line imaged by the
detector array as they cut into a workpiece along the plane
of interest.
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Detailed Description of the Invention
A high accuracy structured light profiler in accordance
with the first embodiment of th~s invention is shown in
schematic fashion in Figure 1, and is generally designated
there by reference number 10. Profiler 10 primarily
consists of illumination system 12 and viewing system 14.
Profiler 10 i8 shown being employed to characterize the
shape of an exemplary generally cube shaped workpiece 16.
Profilers in accordance with this invention however, are
adapted for characterizing the surface shapes of numerous
prismatic or contoured workpieces. Translation stage 1~ is
employed to move the optical axes of both illumination
system 12 and viewing system 14.
As shown ln Figure 1, illumination system 12 provides a
line of light 20 on the workpiece which may be formed by a
sheet of light 21 bounded by a pair of separated rays, as
shown in Figure 1. As shown, the angle of incidence between
sheet of light 21 and the surface of workpiece 16 being
characterized is approxlmately 45 degrees from the plàne of
the cross section of interest. Other angles of intersection
may, however, be used. The angle is non-critical since it
is necessary only to produce a line which is imaged at a
fixed range. In fact, it is possible to have the optical
axes of both illumi~ation system 12 and viewing system 14
coaxial by causing the focuses to be coincident. Various
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sou ces of light may be employed to form line of light 20,
including in a slit illuminated by a white light source.
Preferably, however, a diode (or other) laser 1s used which
may be pulsed to freeze any motion of the beam while
measurements are being made. The illumination system 12
includes optical elements which image the llght source onto
the cross section of workpiece 16 of interest so as to
produce a narrow line width in the dimension perpendicular
to that cross section, while defocusing or spreading out the
source in the dimension which is in the plane of the
subject's cross section~ Line of light 20 may be formed,
for example, by an optical scanner such as a rotating mirror
and a focu~ing lens, or by varlous passive optical
approaches such as cylindrical lenses, etc.
The viewing system 14 principally comprises linear
detector array 22 and imaging lens 24. As shown in Figure
1, the intersection of line of light 20 presented by
illumination system 12, and the line imaged by detector
array 22 forms a line designated by reference number 26.
Line 26 is maintained at a constant distance or range from
translation stage 18. The workpiece 16 and translation
stage 18 are relatively positioned so that line 26
intersects the workpiece surface at one or more points. The
points of intersection are illuminated and are, therefore,
imaged onto detector aFray 22. By maintaining workpiece 16
stationary and moving translation stage 18 in the direction
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of the arrows hown ln Figure 1, the position of line 26 can
be varied; thus, cutting a slice into workpiece 16 along the
cross section of interest. This slicing action is
illustrated by Figures 7 through 10, which show line 26
progressively movlng into the workpiece, shown there a~ a
turbi~e engine blade. Accordingly, various points of the
workpiece surface become imaged onto detector array 22
throughout the range of movement of translation stage 18.
Detector array 22 preferably includes a large number of high
resolution individual light sensitive regions. Signal
processing 32 associated with array 22 locates the center
element which is illuminated (or all of the illuminated
points) as translation s~age 18 is moved. The position of
stage 18 is determined and outputted to signal processor 32
by stage position encoder 30.
Figure 2 provides an exemplary trace resulting from
operation of profiler 10 relating the position of the
elements of detector array 22 illuminated, which are plotted
along the ordinate, versus the position of stage 18, which
is plotted along the abscissa. The characteristic curve 28
is thus derived which is related to the contours of the
workpiece 6urface being characterized.
Structured light profiler 10 provides a sharply focused
line which is used to illuminate the subject, and the line
is always at best fo)cus when it intersects the region on the
subject being imaged by detector array 22. Accordingly, the
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only limitation to the depth of field of the device is
related to the range of motion of translation stage 18. The
system further provides a large field of view coverage
limited only by the length of line of light 20 and the
resolutlon across that line determined by the pixel number
of array 22. The system is able to measure flat surfaces
when they are tilted relative to the viewing axi 8 and also
can be used to characterize highly curved surfaces.
Furthermore, through slight modification, the system can be
adapted to evaluate multiple cross sections of a part simply
by adding multiple detection modules.
The information from light profiler 10 further permits
the radius of curvature at a single point on the workpiece
to be determined without the need for a high density of data
points around the specific region. The curvature
information i9 provided in the form of a center pixel
location and the total number of contiguous pixels on array
22 being illuminated. This information is a direct measure
of surface curvature.
A structured light profiler in accordance with a second
embodiment of this invention is shown in Figure 3 and is
generally designated there by reference number 40. For this
embodiment and those described hereinafter, elements which
are identical to those described in connection with first
embodiment will be designated by like reference numbers.
For this embodiment, laser 42 is employed as a light source
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for illumination system 41 having an optical axis 43. Laser
42 may be a pulsed diode laser producing light ~n the near
infrared region. In one experimental embodiment, a laser 42
was used which produced about 300 milliwatts (peak power)
and was pulsed at about 50 kilohertz with a pulse duration
of less than a microsecond to freeze the motion of the beam
while measurements are taken. The output from laser 42 is
focused onto pinhole 44 by lens 46. The point source from
pinhole 44 is then reflected off steering mirror 48 and
passes through cylindrical lens 50 which spreads the light
in one planar ,direction. The ray is thereafter directed
through collimating lens 52, reflected off mirror 54 and
sent through focusing lens 56. This arrangemen~ has been
found to produce an illuminated line on the subject
workpiece 16 at the measurement points having a width of
approximately 0.001 inches.
Viewing system 60 of profiler 40 includes imaging lens
24 which focuses the line of intersection 26 with line of
light 20 onto detector 58. This embodiment differs from the
first with respect to the type of detector used. Detector
58 includes a single light sensitive element and optical
elements which scans across line 26. Outputs from the light
sensitive element and the phase of the scanning mechanism
gives the location of illuminated spots across line 26.
In accordance wilth this embodiment, translation stage
64 of profiler 40 supports each of the above-mentioned
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elements of viewing system 60 and illumination system 41.
Translation stage 64 is movable in the direction of the
arrows shown in Figure 3 to cause the imaged line 26 formed
by the intersection of line of light 20 and that imaged onto
detector 58 to be translated into the cross section being
characterized on workpiece 16, which is graphically
represented by reference number 66. In other respects,
profiler 40 operates in a manner identical to that of
profiler 10 as previously described.
Figure 4 illustrates a structured light profiler
according to a third embodiment which is generally
designated by reference number 80. Profiler 80 differs
principally from the above-described embodiments in that
laser source 42 and detector array 22 are fixedly mounted to
a support structure and do not move with translation stage
82. This system provides the advantage of eliminating
errors associated with vibration of the light source or
detector when they are mounted onto the translation stage.
Such mechanical isolation is very desirable since any slight
wobble of the source would become a large posltional error
on the subject due to beam pointing changes. ~y not
translating the source, mechanical isolation of the source
and projection optics is provided which also prevents
optical misalignment or failure of the source due to
mechanical shock or vibrations. Profiler 80 illustrates the
use of the concepts of this inveniton in evaluating the
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tooth profile shape of gear 108, which is another of the
wide spectrum of workpieces which may be evaluated by this
invention.
For profiler 80, illumination system 84 includes laser
source 42 having lts output passing through focusing lenses
86 and cylindrical lens 88, and is further directed off
mirror 90. Two large mirrors 92 and 94 are oriented at 90
degrees to each other, and are fixed to translation ~tage 82
to move along the dimension of the optical axis of viewing
system 96, which is designated by the arrows in Figure 4. A
separate smaller mirror 98 is provided as a constant
deviation device. Mirror 98 or a constant deviation optical
element is affixed to a precision sliding stage 102. For
this embodiment, it is necessary to linearly move sliding
stage 102 at twice the rate of movement of translation stage
82 in order that illumination and viewing systems 84 and 96
maintain proper alignment. This tracking may be
accomplished by attaching stage 102 to a steel band which is
fixed at one location to a support structure and circles
around two wheels which are flxed to translation stage 82.
~iewing system 96 of profiler 80 includes auto-focus right
angle prism 104 which is af~ixed to and movable with
translation stage 82. A fixed mirror 106 directs the
received light beam through imaging lens 24 onto detector
array 22.
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In operation of profiler 80, the focused light from
laser 42 i8 incident on mirror 92 at 45 degrees which
directs the light onto mirror 94 at a 45 degree inc~dent
angle. The second mirror directs the light to mirror 98
affixed to sliding stage 102 at an incident angle of 72.5
degrees, which causes the light to be directed toward
workpiece 16 at a 45 degree angle from the plane of the
cross section of interest. Sliding stage 102 causes mirror
98 to track the output beam from large mirror 94. This
arrangement causes the line of light 20 to translate with
the line imaged by viewing system 96. The principal
advantage of using the mirror system described above for
profiler 80 is that the mirrors ar~ arranged in a constant
deviation configuration such that translatlon stage 82 as a
whole can wobble without changing the angle of the beams.
Another advantage is that the beam translates at twice the
rate of stage movement and thus covers twice the distance
that the stage travels. Moreover, there are additional
disadvantages with moving the light source which are
obviated by this embodiment, such as problems associated
with stability, wiring and vibration induced failure.
Prism 104 attached to translation stage ~2 is used to
move the focus of viewing system 96 along the optical axis
of a viewing system at the subject so as to keep the viewing
system focus in coincidence with the focus of illumination
system 84. As stage 82 moves, line 26 moves along the path
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of the optical axis of viewing system 96, and the focus of
viewing system 96 is maintained at that line.
Yet another embodiment of a structured light profiler
according to this invention is illustrated in Figuxe 5 and
is generally designated by reference number 120. For this
embodiment, translation stage 122 carries a pair of
pentaprisms 124 and 126 associated with illumination system
128 and viewing sys~em 130, respectively. For this
embodiment, laser 42 and array 22 are affixed to an
associated support structure. As pentaprism 124 is
translated relative to the input ~eam from laser 42, the
output beam translates by 2 times the distance which the
prism has moved. Similarly, the line imaged onto detector
array 22 eracks the movement of the illumination beam by
passing through pentapri6m 126. The constant deviation
effect provided by pentaprisms 124 and 126 provides the
benefits of isolating motion of ~tage 122, while magnifying
the translation of the stage to provide a larger measurement
range. Figure 6 graphically illustrates the characteristlcs
of pentaprism 124 (or 126) which provides isolation from
wobble of stage 122. As pentaprism 124 is rotated to the
phantom line position in Figure 6, the position of the
emitted beam does not vary. With this embodiment, to move
the projected line as desired, translation stage 122 is
moved at 45 degrees~to the plane of the cross section of
interest on workpiece 16, as shown by the arrows in Figure
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5. As stage 122 is moved from the full line posltion to the
phantom line position shown in Figure 5, line 26 moves into
the w~rkpiece in the range designated by the letter "Z".
While the above description constitutes the preferred
embodiments of the present invention, it will be appreciated
that the invention ,is susceptible to modification, variation
and change without departing from the proper scope and fair
meaning of the accompanying claims.
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