Laser profiler for high precision surface dimensional grouping apparatus and method

A laser optical profiler for determining the shape of a workpiece, the profiler in combination comprising means for projecting a laser probe line at a first fixed focal length on said workpiece at a first angle, means for providing relative movement between said probe line and said workpiece, imaging lens means for focusing on said workpiece at an angle nonspecular with respect to first angle at a second fixed focal length a receiver path to provide an image reflected off said workpiece, the reflected receiver path forming a line essentially coterminous with said probe line, the receiver path having a predetermined transverse width with alpha and beta portions, means to move said receiver path along with said probe line, a beam splitter means for dividing said receiver path into first and second portions, a first linear array of individual detectors associated with said first portion of said receiver path for detecting said alpha portion of said width of said receiver path and generating a signal in response thereto, a second linear array of individual detectors associated with said second part of said receiver path for detecting said beta portion of said width of said receiver path image and generating a signal in response thereto, and means for comparing said response of said first linear array with said response of said second linear array to determine the acceptability of said responses and means for detecting the linear location of the focal point of said probe line with respect to said workpiece whereby the profile of said workpiece is determined.

FIELD OF THE INVENTION 
This invention relates generally to optical measuring devices and 
particularly to a laser measurement system for non-contact optical 
profiling of a workpiece. 
DISCLOSURE STATEMENT 
It is known in the art to project a collimated laser line on a workpiece 
and to sense the reflection thereof in determining the profile of the 
workpiece. 
There were two major systems previously proposed to make optical 
measurements. The first method is shown and described in Balasubramanian 
U.S. Pat. No. 4,355,904, the disclosure of which is incorporated herein. 
The shortcoming of the Balasubramanian method is that it requires a high 
accuracy scanning system and scanning optics sharing a common optical path 
to perform the measurement. The scanning system and optics tend to be 
bulky and high cost. The mechanical scan time is also slow compared to 
that available with the second system which utilizes a linear detector 
array. 
The second system is shown and described in an article titled "Hybird, High 
Accuracy Structured Light Profiler", co-authored by Kevin G. Harding and 
Kenneth Goodson of the Advanced Manufacturing Technologies Laboratory, 
Industrial Technology Institute, Ann Arbor, Michigan 48106 and published 
in SPIE Volume 728 Optics Illumination, and Image Sensing for Machine 
Vision (1986), the disclosure of which is incorporated herein (hereinafter 
referred to as the ITI optical guillotine system). 
The ITI optical guillotine system illuminates the surface to be measured 
using a line of light incident on the subject at 45 degrees from the 
average normal to the surface. Viewing the subject, for example, in the 
case of cylindrical parts, along a direction perpendicular to the axis of 
the cylinder, a line of constant height (constant Y) in space is imaged 
onto a 4000 element linear detector array. The line of light is translated 
in the Z or depth direction (which will move the illumination in the Y 
direction on the object) 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 at specific X locations on the detector are correlated 
with the current Z position as read out from the translation stage 
encoder. The X position is read out directly to one part in 4000 from the 
detector array. 
The distance from the source to the points of intersection with the line on 
the subject being viewed is constant. This means that 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. It is not necessary to translate the source to obtain 
this constant distance relationship. The path length can be maintained 
constant and the motion of the beam isolated from vibrations or wobble in 
the stage through the use of a constant deviation mirror system. It is 
desirable not to move the source since any slight wobble of the source 
will become a large positional error on the subject by the magnification 
of the source to the subject, and as the beam pointing changes. The same 
concern for vibrations and wobble applies to the detector as well. Not 
translating the source also allows for mechanical isolation of the source 
and projection optics to prevent misalignment or failure of the source due 
to mechanical shock from vibrations. 
The depth information (Z value for a given X on the subject) is read out 
from the encoded translation stage. Stages are available with accuracy to 
one micrometer (0.00004 inches), and speeds to ten inches per second (one 
sweep being required to build up a profile). 
The ITI system utilizes a translator and a single linear detector array. By 
using only a single linear array, the ITI system cross-axis dimension must 
be inferred by performing binary or gray scale post processing on the data 
to determine the center pixel illuminated by the laser line as imaged onto 
the linear array. Because of the above, only objects which have highly 
sloped surfaces can be gauged accurately. If the object is flat, with 
respect to the projected laser line, then an unambiguous measurement of 
range cannot be made. Also, because post processing must be performed on 
each line of data, it was required to store data for each cross-axis line 
for each Z-axis position. Since the cross axis resolution may be on the 
order of 4,000 pixels and the Z-axis resolution on the order of 50,000 
pixels, it is evident that the amount of storage or memory required is 
rather large. Also since post-processing would be required on each pixel, 
the amount of processing time is rather large. 
SUMMARY OF THE INVENTION 
To overcome the disadvantaqes of the aforementioned optical measuring 
systems, the present invention is brought forth. A preferred embodiment 
optical profiler of the present invention provides a projector which forms 
a sharply focused laser probe line in front of the device at a known and 
fixed distance. The imaging device comprised of a flat field lens and two 
linear detector arrays combined in a bi-cell type arrangement form an 
imaging plane coincident with the focus position of the laser line which 
is incident at a fixed angle. If an object surface is present at the 
intersection of the laser line plane and the imaging plane, energy will be 
reflected back toward the linear arrays. These arrays in essence are 
aligned parallel to each other and form a bi-cell arrangement such that 
the position of the laser line which is reflected off the part can be 
determined with high accuracy by comparing the relative signal strengths 
from the vertically adjacent pixels on the two linear arrays. 
Since the measurement range is limited to a small distance around the 
intersection of the respective planes, both the projector and the imaging 
device are mounted on a common platform wherein a single axis translator 
moves the intersection line of the laser and of the linear arrays in a 
direction perpendicular to the line and to the workpiece. By moving the 
intersection line of the projector and the imaging planes in a common 
direction, it is possible to treat this intersection line as a common 
probe to measure the surface profile of a part. The measurement accuracy 
of the system along the translation axis is a function of the accuracy of 
the translating device itself. The cross axis accuracy is a function of 
the size of the linear array pixels and the magnification of the receiver 
optics. Since both the translator and the linear array resolution are on 
the order of microns, it is possible to make profiled measurements of 
parts to the same accuracy. 
The invention solves the aforementioned shortcomings (speed, cost, storage 
requirements and post processing requirements) by using a bi-cell imaging 
arrangement to provide unambiguous cross-axis position data. The only 
moving part in the present invention proposed is a reflective device 
consisting of two connected mirrors which moves the intersection line of 
the projector and the receiver planes along the translation axis thus 
reducing cost. The present invention can significantly reduce the post 
processing memory requirements and increases the data processing speed by 
saving neighborhood data only where valid surface data is detected. 
Advantages of the present invention are its ability to produce 
high-accuracy profile measurements using short cycle times in conjunction 
with high data rates, seven (7) megahertz. The preferred embodiment design 
of the present invention has only one moving component, two small mirrors 
joined to provide a single motion, thus making it possible to use a small 
translation system. The measurement method provides for data reduction by 
observing the incoming video data, and making a simple decision as to 
whether the data is valid thereby greatly reducing the post processing and 
data memory storage requirements as mentioned previously. 
The result of the above advantages allows the optical profiler of the 
present invention to make very high resolution measurements at high speeds 
using only one moving component, which was not possible using previous 
profilers. 
Alternatively, the present invention can be described as a scanning laser 
measurement system for measuring depthwise variations of surfaces relative 
to a focal plane of an optical system. Laser light is directed to a test 
surface, generally aligned with respect to the focal plane, through 
focusing optics. Some of the light from a beam spot is retro scattered 
from the test surface and reimaged along a path generally parallel to the 
receiver optical axis. Depthwise variations in the test surface with 
respect to the focal plane cause spatial displacements in the retrobeam 
relative to the receiver optical axis. The retrobeam is directed to a beam 
splitter and two charge coupled device (CCD) line-scan arrays. The arrays 
are used to intercept reflected and transmitted components of the 
retrobeam and the signal intensity of these components is measured and 
used to compute centroid values for the retrobeam on the beam splitter for 
various beam spots on the test surface. The computed centroid values are 
directly proportional to depthwise surface deviations from the focal 
plane. The beam is directed to various points on the test surface by the 
translator optical scanner. If the deviations in the test surface from the 
focal plane are so great that a retrobeam cannot be formed, the entire 
profiler optical system is translated until beam focus can be achieved and 
a retrobeam formed. The extent of translation is a coarse measurement of 
depthwise variations in the test surface relative to the focal plane, 
while the previously mentioned centroid values yield a fine measurement of 
depthwise variations relative to the focal plane. 
It is an object of the present invention to provide an optical profiler 
apparatus and method of utilization thereof. 
Further objects, desires and advantages of the present invention can become 
more apparent to those skilled in the art as the nature of the invention 
is better understood from the accompanying drawings and a detailed 
description.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIGS. 1, 2 and 5, the optical profiler of the present 
invention 7 is contained within a casing 90 which has been removed for 
clarity of illustration. A laser projector 10 is provided along with a 
series of lenses and screens and mirrors to project a focused line 74 on a 
work piece 9 generally transverse to the work piece. The laser can be a 
continuous wave (CW) laser which allows the use of a lower power. 
Alternatively, a switched CW type laser can be used and operated at low 
duty cycle and moderate repetition rates. The switched low duty cycle 
method tends to reduce signal (or image) smear due to line translation. A 
wide range of laser wavelengths may be utilized, however, it has been 
found to be preferable to use a 850 nanometer semiconductor laser. 
Due to the divergent nature of the beam from the laser, a lens 12 is used 
to collimate the beam. A two lens telescope consisting of a first 
achromatic lens 20, pin hole 83, mirror 30 and second achromatic lens 22 
of the telescope are designed to expand, clear up and redirect the laser 
beam on its way to the workpiece. Mirror 32, cylindrical achromatic 24 are 
used to focus the laser beam (in one dimension) into a very narrow line 74 
of laser light at a first predetermined fixed focal length. Mirrors 34 and 
36 direct the transmitted beam to the workpiece. A laser line of 
approximately 30 millimeters.times.15 micrometers will appear on the 
workpiece 9. 
To reiterate the above, after the projected laser line leaves the lens 24, 
the projected laser line will have a fixed focal length. To extend the 
focus of the laser line 74 through the object 9, mirror 34 will be 
translated by platform 80 along line 82. 
Mirrors 31, 33 and 35, and lens 21 and filter 25 provide an imaging lens 
means for a focusing path on the workpiece at an angle 41 (typically 
30.degree.) which is nonspecular with respect to angle 40 (typically 
10.degree.) at a fixed second focal distance relative to the workpiece 9. 
The lens 21 is a high quality 1:1 Nikon copy lens 122 mm focal length 
F5.6. The lens 21 exhibits a modulation transfer function of approximately 
70 line pairs per millimeter of 30% modulation. It is consistent with the 
need for high resolution of the image. Filter 25 is a very narrow ten (10) 
nanometer, band pass filter designed to preclude image interference from 
ambient light. it is consistent with the narrow emission band of the 
laser. 
The receiver path (image) will generally be coterminous with the laser 
probe line where the laser probe line intersects the workpiece 9. The 
laser probe line typically will have a line width of 14 to 20 (15 microns 
as shown in FIG. 4) microns at the workpiece. The receiver path 77 will 
have an alpha portion 73 and a beta portion 71. As illustrated in FIG. 4, 
the alpha portion 73 is especially separated by a spacing 75 of 
approximately 8 microns from the beta portion 71. However, if desired, 
these portions may join without separation. It is desirable to make the 
probe line width as small as possible. However, the size of the pixel 
receivers, expenses of optics, and/or surface texture of the workpiece 9 
limit reductions in probe line width. 
Path (image) 77 is brought through beam splitter 51. The beam splitter 51 
is typically a nitrocellulose type pellicle beam splitter which generally 
splits the beams into equal portions to a first linear detector array 61 
and to a second linear detector array 63. To move the line formed by the 
intersection of the path 77 and the laser line 74 there is provided a 
mirror 31 which is also attached to the translation platform 80 and moves 
along therewith. 
Each individual array of detectors typically has a line-scan CCD type 
detector having approximately 4000 photo detectors (pixels) per linear 
inch. A typical detector array is an NEC U PD 791 4096 array each pixel 
being 7 microns.times.5 microns with 2 microns dead spacing. 
The detectors are typically laterally offset from the center line of the 
path 77 to provide an image with a center line offset from the center line 
of the beam projected laser line by 50% plus or minus 7% of the width of 
the projected laser line. As shown, the alpha 71 and beta portion 73 
center lines are offset 7.5 microns from the path 77 center line and 
provide an image angle of 0.000894 degrees. The offset of the detector 
arrays provides the alpha 71 and beta 73 portions of the reflected beam 
77. 
In operation the distance of the platform 80 to the object 9 is known and 
is fed to a computer. Where the laser probe line intersects the surface 
area of the workpiece 9 an image will be impinged upon the detector arrays 
61,63. In essence the detector arrays 61,63 will be lined up with one 
another. Where there is an acceptable image reading on both aligned and 
corresponding individual detectors (pixels) which are within a ratio 60 to 
40 to one another the data will be stored. It is know that there is a 
location wherein the probe line is intersecting the surface of the 
workpiece. Therefore, to know exactly where the beam is intersecting the 
workpiece it is only necessary to provide a ratio analysis in the response 
of the aligned individual detectors of the first and second linear arrays. 
Those detectors not detecting any radiation or detecting radiation at the 
ratios outside the prescribed limits can be ignored. This allows the 
information on which detectors are being excited by acceptable amounts of 
reflected radiation to be sent to the computer, along with the information 
on the exact location of the translator 80 thus allows the controller to 
provide an accurate profile of the workpiece. At any given time, only 
certain pixel of the linear arrays will be activated depending on the 
profile of the workpiece. In alternative embodiments, alternative 
comparison techniques (such as a normalized ratio) can be used to 
determine acceptability of data based upon the reading of the aligned 
pixel receivers. 
While an embodiment of the present invention has been explained, it will be 
readily apparent to those skilled in the art of the various modifications 
which can be made to the present invention without departing from the 
spirit and scope of this application as it is encompassed by the following 
claims.