Method and apparatus for detection of undesirable surface deformities

A method and apparatus for the detection of undesirable surface deformities is disclosed wherein a known (linear) light pattern is reflected off a fabricated part having a nominally smooth surface, the reflected light pattern subsequently being analyzed for distortions from the expected light pattern. The extent of the detected distortions may be quantified by the use of a computer having appropriate software, in order to identify and quantify unacceptable surface deformities.

BACKGROUND OF THE INVENTION 
It has always been desirable to detect and, after detection, possibly 
correct the cause of any undesirable surface deformities that occur during 
the fabrication of any part designed to have a smooth surface. For 
example, in the case of the outer surface of an automobile, any visible 
surface deformities, such as waviness in the surface, or "orange peel" in 
the paint surface, that can be eliminated enhance the aesthetic and 
therefore the commercial value of the entire automobile. The competitive 
need to eliminate undesirable surface deformities in an automobile's outer 
surface becomes particularly important in the case of exterior automobile 
panels fabricated from fiber-reinforced plastics, i.e., fiberglass 
components, wherein the panel may already have a certain degree of 
waviness due to the fiberglass mold conditions, resin cure rates, or other 
conditions of the fabrication process that tend to generate a surface 
waviness. 
One surface deformity measurement system has been developed by the Budd 
Company, Plastic Research and Development Center, Troy, Mich. The Budd 
system moves a height gauge across the surface to be studied, in order to 
attempt to measure the minute dimensions of surface deformities. This 
system is highly susceptible to background vibrations, can only measure 
one linear foot at a time, and is impractically slow in analyzing a large 
area. The system, due to these and other mechanical limitations, has not 
been placed in extensive use. 
Another DiffractoSight system, offered for sale by Diffracto Inc., P.O. Box 
36716, Grosse Point Woods, (Detroit) Mich. 48236, is also available 
wherein light is reflected off the surface of interest, the light then 
being viewed through special lens worn by the viewer. This system merely 
yields a visual representation of any surface deformities, with no means 
of reducing the viewed image of the distorted surface into any type of 
objective format for further study. 
A system needed to be developed, therefore, that presents any undesirable 
deformities in a surface in a readily visible format, the system also 
being capable of reducing the observed deformities into an objective 
format for further analysis. Such a system should identify the location of 
any unacceptable surface deformities, and also perform an objective and 
quantitative analysis as to the extent and magnitude of these deformities. 
SUMMARY OF THE INVENTION 
The apparatus of the present invention analyzes the edges of a light 
pattern that has been reflected from a surface of interest to determine 
the existence of undesirable surface deformities, and visible surface 
coating deficiencies. 
Shifts in the location of the edge from its expected reflected position 
indicate the existence of surface deformities, and the magnitude of the 
shifts can be measured in order to quantify the extent of the surface 
deformity. 
More specifically, the method of the present invention comprises the 
following steps. A light pattern having at least one edge is projected 
onto the surface of interest from a light pattern source. The light 
pattern projected onto the surface is reflected from the surface and then 
received at a light pattern receiver. The reflected light pattern is 
converted by use of a computer and appropriate software into an array of 
pixels wherein each pixel has a value representative of the intensity of 
light received from a portion of the reflected light pattern. The value of 
each pixel is evaluated to define at least one edge of the reflected light 
pattern. And finally, the shape of the defined edge is analyzed to detect 
any undesirable surface deformities. 
The analysis of the edge in a preferred embodiment comprises the steps of 
forming a fitted curve through the edge, measuring the amount of deviation 
of the edge from the fitted curve, and thereafter quantifying the amount 
of deviation of the edge from the fitted curve, to the amount of 
undesirable deformity in the surface. 
The light pattern may be generated by use of a light source having linear 
edges, such as an assemblage of equally spaced and parallel flourescent 
light tubes, although slotted screens illuminated from the back and other 
linear sources are possible. It should be understood that surface 
deformities will become apparent at the high contrast edge(s) of the 
reflected light pattern, and not in the center width area of the reflected 
light pattern, so in the following discussion and description the term 
"edge" will be used extensively. 
It is therefore an object of the invention to quantify the amount of 
undesirable surface deformities. 
It is a further object of the present invention to describe a commercially 
viable surface deformity measurement system. 
It is a further object of the invention, after the deformities in a 
particular surface have been quantified, to compare the quantitative 
measurement of the deformities in one surface to the quantitative 
measurement of deformities in another surface, so as to select one 
manufactured part above another for commercial use and sale, or to compare 
a production manufactured surface to a perfect or desired surface 
painstakingly created from any material that will yield the desired 
surface shape within desired tight tolerances. 
It is a further object of the invention to create a system that will 
optically measure the surface finish of a relatively smooth surface. 
It is a feature of the present invention to generate a known light pattern 
from a light source, reflect the light pattern from the surface of 
interest, receive the reflected light pattern by a camera, and, by the use 
of a computer and appropriate software, convert the reflected light 
pattern into at least one defined edge that may be analyzed so as to 
quantify the amount of deviation of the edge from a particular fitted 
curve mathematically generated through the elements of the edge, the 
fitted curve closely approximating the original edge of the original light 
pattern generated or projected from the light pattern source. 
These and other features, objects and advantages of the present invention 
and become apparent from the following detailed description, wherein 
reference is made to the figures in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 an undesirable surface deformity detection 
apparatus 10 is shown. In general, the apparatus 10 can be seen to 
comprise the following separate elements. Means for projecting a light 
pattern 11 onto surface 12, are provided, such as in a preferred 
embodiment comprising a light pattern source 14 having fluorescent light 
tubes 15 carried by a support frame 16. It should be well recognized that 
many other light pattern sources may be used that would project a light 
pattern having discernable edges upon part 18. Masking may be used over 
the edges of the fluourescent tubes 15 to block out the weaker lighted 
edge portions of the tubes 15 from the projected light pattern 11. 
The light pattern 11 from the fluorescent light tubes 15, when projected on 
the surface 12 of fabricated part 18 causes the surface 12 to be visually 
divided into an alternating series of light areas 19 and dark areas 20. It 
should be well recognized that the contrast of the light intensity of the 
areas 19, 20 is related to the intensity of the projected light pattern 
11, and the intensity of the background lighting projected on the 
fabricated part 18. To enhance the contrast between areas 19, 29, the 
surface 12 may be coated with a "high-lighting" oil, or other suitable 
liquid which will also increase the amount of light reflected from the 
surface 12. 
The fabricated part 18 may comprise any part having a surface 12 wherein it 
is desired to monitor the surface 12 for the existence of any undesirable 
surface deformity 22. In a preferred embodiment, the fabricated part 18, 
may comprise for example a portion of the exterior shell of an automobile, 
the material of the part 18 formed for example from a molded sheet of 
fiber reinforced plastic known as sheet molding compound or "SMC". 
The apparatus 10 can also be seen to include means for receiving a 
reflected light pattern 24 reflected from the part 18 such as by a light 
pattern receiver 26 comprising in a preferred embodiment a camera 27 
located in the path of the reflected light pattern 24 and oriented so as 
to optically receive the reflected light pattern 24. The camera 27 in a 
preferred embodiment comprises a DAGE-MPI CC-68 instrumentation camera 
manufactured by DAGE-MPI Incorporated of Michigan City, Ind., having a 
1,025/60 line resolution with a 2:1 interlace scan and equipped with a 
zoom lens. 
It should be well recognized that other cameras 27 well known to the art 
may also be used to accomplish the teachings of the present invention. 
The apparatus 10 also includes means for generating data 30 (FIG. 5) 
representative of the received reflected light pattern, means for 
evaluating the data 30 in order to define at least one edge 31 (FIG. 5) of 
the reflected light pattern; and means for analyzing the edge 31 to detect 
any undesirable surface deformities 22. These means in a preferred 
embodiment comprise a computer 32 and software compatible with the 
computer 32. 
The computer 32 comprises in a preferred embodiment an IBM PC-AT model, 
having a SONY model GDM-1901-12 model CRT 33 display and a Honeywell 
VGR-5000 printer 34, model no. VGR-5H5HODTO as peripheral equipment. The 
software 35 comprises in a preferred embodiment an "Imagepro" software 
package with "Doctor Halo" graphic software, used in combination with 
"PC-EYE" Video Capture Software, the PC-EYE software available for 
purchase from Chorus Data Systems, Merrimack, N.H., the Image Pro software 
available from Media Cybernetics Incorporated. 
The software 35 may be modified from its purchased format after study of 
the teachings of the present invention, to implement the teachings of the 
invention by those having ordinary skill in the art. 
Referring now to FIG. 2 a reflected light pattern 24 from a substantially 
flat surface 12 is shown wherein the light area 19 is formed in a 
relatively linear straight manner relative to the dark area 20. This is a 
desirable reflected light pattern 24 from a substantially flat surface 12. 
Referring now to FIG. 3, a reflected light pattern 24 is shown from a 
smoothly curved surface 12 wherein the light area 19 is defined in a 
relatively curvilinear manner relative to the dark area 20. Such a display 
forms a convenient man-machine interface for qualitively and subjectively 
inspecting the surfaces of manufactured parts. 
Referring now to FIG. 4 a reflected light pattern 24 is shown from a 
smoothly curved surface 12 having a surface deformity 22A wherein at least 
one of the light areas 19 is no longer defined in a substantially 
uninterrupted curvilinear manner relative to the dark area 20. In other 
words, the projected view of the fluorescent light tubes 15 (FIG. 1) has 
become distorted due to the existence of the undesirable surface deformity 
22A. 
Referring now to FIGS. 5 and 6, the method of detecting undesirable 
deformities in a surface may now be explained in further detail. As 
mentioned earlier the light pattern is projected onto a surface from a 
light pattern source, and a light pattern receiver thereafter receives a 
reflected light pattern from the surface of interest. Thereafter, 
reflected light pattern data 30 is generated by the computer software. In 
the software of the present invention, the reflected light pattern is 
converted into an array of pixels 37 wherein each pixel, (38A, 38B for 
example), has a value representing the intensity of light received from a 
particular portion of the reflected light pattern. 
The values of the pixels 38 in the array 30 are then evaluated to define at 
least one reflected light pattern edge 31 within the pixel array 37. The 
shape of the edge 31, having the functional relationship y=f(x), is then 
analyzed to detect the presence of any undesirable surface deformities in 
the surface of interest. 
The pixel array 37 shown in FIG. 5 can be seen to have a pixel value axis 
40, an x-axis 41, and a y-axis 42. The software assigns a pixel value of 0 
for a received light intensity of "pure black", and a pixel value of 255 
for the light intensity received from a "pure white" areas. As shown 
therefore the dark area pixel value 44 will typically be much less than 
the light area pixel value 45. The dark area pixel value 44 represents the 
dark area 20 shown in FIG. 4, for example, whereas the light area pixel 
value 45 represents the light area 19. It should be well understood that 
pixel scales having other ranges may be used to accomplish the same 
result, such as a scale of 0-63 vs. the 0-255 scale used in the present 
invention. 
As shown in FIG. 5, the value of the pixels varies in a fairly linear 
manner between the dark area and the light area. In other words, a "gray 
area" exists between the dark and light areas. Due to the numerous 
discrete values of each piexel 38 it therefore becomes necessary to define 
the edge 31 with a degree of confidence, recognizing that the real life 
values of each discrete pixel are not so neatly organized as those shown 
in FIG. 5. 
By the method of the present invention, the edge 31 is defined by 
segregating pixels having a selected pixel value 47 or a value greater 
than the selected pixel value 47 into a first group 49 of pixels, and 
thereafter segregating pixels 38 having less than the selected value 47 
into a second group 50 of pixels. The edge 31 of the reflected light 
pattern is thereafter defined at the boundary of the first group 49 and 
the second group 50 of pixels. It has been noted that the width of the 
light/dark intersection is only two or three pixels on a 1024 wide pixel 
screen. 
It is well understood that a mathematical "averaging" process may be used 
to blend together the values of adjacent pixels having widely disparate 
values, in order to create the relatively smooth gray transition area as 
shown in FIG. 5. 
It should be well recognized that many other techniques may be used to 
define an edge 31 within the pixel array 37 representative of the edge 
between the light area 19 and dark area 20 shown in FIG. 4. Mathematical 
filters, called "spatial filters", can be used to adjust the value of each 
pixel in a field as a function of the values of the surronding pixels. 
Such filtering techniques can be used to enhance the sharpness of the 
edge's demarkation. 
The coordinates X.sub.1, Y.sub.1 52 of each pixel along the reflected light 
pattern edge 31 may then be analyzed in the preferred embodiment to detect 
undesirable surface deformities, in the following manner. 
A fitted curve 54, (FIG. 6), may be formed through the edge 31 coordinates 
X.sub.1, Y.sub.1, 52; X.sub.2, Y.sub.2, 53; . . . X.sub.m, Y.sub.m, 56 of 
the rjeflected light pattern by finding coefficients a.sub.o, a.sub.1, 
a.sub.2, . . . a.sub.m, of the equation 
EQU Y=a.sub.o +a.sub.1 X+ . . . +a.sub.i X.sup.i + . . . +a.sub.m X.sup.m. (1) 
It is well recognized that many mathematical techniques, such as the use of 
matrix transforms, are available for determining optimum values for each 
of the coefficients a.sub.i in the above equation. In a preferred 
embodiment the well known Least Squares Method is used. This technique 
generates the curve which will minimize the sum of the squares of the 
values of all the deviations 55 from the generated curve defined by the 
above equation. The technique for calculating the values of these 
coefficients minimizes the value of sum S in the equation 
##EQU1## 
where Y.sub.i ' is the computed value of each point corresponding to the 
measured data point Y.sub.i. To make the required calculations it is first 
necessary to select the form of equation to be used. In this particular 
case a polynomial of M degree having m+1 terms will be described, however, 
it is recognized that a logarithmic or other series of terms could be 
substituted. To generate equations whose solution will meet the above 
minimizing criteria partial derivatives of S are taken in respect to each 
of the coefficients of the selected polynomial. The resulting group of m+1 
normal equations, 
EQU .SIGMA.Y.sub.i =a.sub.o n+a.sub.1 .SIGMA.X.sub.i +a.sub.2 
.SIGMA.X.sub.i.sup.2 + . . . +a.sub.m .SIGMA.X.sub.i.sup.m, (3) 
EQU .SIGMA.X.sub.i Y.sub.i =a.sub.o .SIGMA.X.sub.i +a.sub.1 
.SIGMA.X.sub.i.sup.2 +a.sub.2 .SIGMA.X.sub.i.sup.3 + . . . +a.sub.m 
.SIGMA.X.sub.i.sup.m+1, (4) 
EQU .SIGMA.X.sub.i.sup.m Y.sub.i =a.sub.o .SIGMA.X.sub.i.sup.m +a.sub.1 
.SIGMA.X.sub.i.sup.m+1 +a.sub.2 .SIGMA.X.sub.i.sup.m+2 + . . . +a.sub.m 
.SIGMA.X.sub.i.sup.2m. (5) 
can be solved for values of a.sub.o, a.sub.i, . . . a.sub.m by the 
conventional Gauss elimination technique which succussively determines the 
value of each coefficient. Then the original equation can be solved for 
values of Y.sub.i in respect to its related values of X.sub.i. It is to be 
noted that the original m+1 number of terms used to define the generated 
equation does not restrict the number of X.sub.i, Y.sub.i points which can 
be computed and compared with their related points on the original 
reflected edge. In practice the number of terms selected depends upon the 
intended use of the curve, and more than one equation may be computed to 
satisfy diverse requirements. 
A large number iof terms may be required when the intent of the calculation 
is to define minute discrepancies in the reflected edge such as those 
caused by improperly applied paint or other surface coatings. This small 
scale "roughness", often known as "orange peel", is defined by data 
variations only a few pixels wide and a few high. The resulting calculated 
curve follows the contour of the reflected light edge including those 
deviations from the basic surface curvature caused by any relatively 
large, shallow deformities in the surface. Conversely, when such large 
deformities themselves are being investigated, only a limited number of 
terms is used. For example, if the reflected image from a linear source 
arranged perpendicularly to the axis of symmetry of a surface displaying 
bilateral symmetry is captured, then a second order polynomial curve, 
i.e., a parabola, may be the best choice for the resulting calculated 
curve. 
The deviations from the best fit curve caused by a dent or depression in 
the original smooth surface may imply that there are bumps or raised 
portions of the surface relative to the ideal surface. Therefore, in some 
cases, the best fit curve needs to be adjusted either toward higher or 
lower values of the Y variable. In this latter case, even this use of a 
limited number of terms will not totally eliminate the effect of such 
deformities on the resulting calculated curve 65 of edge 64 as shown in 
FIG. 7. To reduce this effect various known mathematical techniques are 
available. One preferred method is to revalue all point(s) 66 which are 
found to be below the generated curve 65 so that they now lie on this 
generated curve as point(s) 67. Next, a new best fit curve 68 is generated 
using the original measured data except where these modified points have 
been substituted. This will generate a best fit curve 68 having a minimum 
of distortion caused by the discrepancies. It is recognized that a second 
and third repetition of this process may provide additional improvement. 
The final generated curve can then be effectively used for comparison with 
the original data. 
Once a base curve has been defined by its a.sub.i constants, the amount of 
deviation 55 of the edge 31 from the fitted curve can be calculated at 
each selected X.sub.i, Y.sub.i location. It should be well recognized that 
an analysis of these deviations can then be used to establish the 
magnitude of any discrepancies and/or deformities in the surface being 
analyzed and that this analysis can be used to establish the acceptability 
of such defects in respect to the accepted aesthetic norm. 
Different approaches can be used in the presentation of the generated 
surface acceptability data. Each is suitable for use with either the 
"orange peel" or the shallow deformity data. Two preferred approaches are 
described here, however, it is recognized that many variations are 
available to meet each particular user's needs. 
One preferred technique is to provide a single quantitative numerical 
evaluation of the product's surface acceptability. One way to obtain such 
a representative value is to sum the squares of all the individual point 
discrepancies using the previously calculated constants a.sub.i. 
EQU .SIGMA.d.sub.i.sup.2 =.SIGMA.Y.sub.i.sup.2 -(a.sub.o .SIGMA.Y.sub.i 
+a.sub.1 .SIGMA.X.sub.i Y.sub.i + . . . +a.sub.m .SIGMA.X.sub.i.sup.m 
Y.sub.i), (6) 
wherein .SIGMA.d.sub.i.sup.2 equals the summation of the amount of net 
deviation of the edge 31 from the fitted curve 54. The relative 
accetability of this resulting numerical value can then be established by 
comparing it with values obtained from tests of product components of 
known acceptability levels. It is recognized that a relationship can also 
be established between this number and actual measurements of the bumps or 
deressions in surfaces which have been measured using devices such as 
surface profilometers and micrometers. Thus, if shallow or broad 
depressions are being fitted by curve fit routines that closely follow 
these relatively large deformities, then .SIGMA.d.sub.i.sup.2 will be a 
quantitative measure of the small or "orange peel" type of deviation. 
Otherwise, .SIGMA.d.sub.i.sup.2 will represent the major shallow or broad 
deviations. 
A second preferred technique is to provide the user with a visual 
presentation of the generated discrepancy data. As shown in FIG. 8, it 
should be well understood that the computer may be programmed to cause the 
pixels that are located between the fitted curve 54 and the edge 31 to be 
represented graphically on the CRT display 33 as a deviation line 57. 
Furthermore, other pixels located between other fitted curves and edges 
may also be graphically displayed as deviation lines 58, 59, 60, and 61, 
so that a deviation contour map 62 may be presented for the entire part 
18. Separate presentations are used to clearly present both the "orange 
peel" discrepancies and the large, shallow deformities. 
Calibration data generated by prior use of a surface profilometer or 
micrometer may also be referenced by the computer in order to present a 
deviation scale 63, wherein scale 63 may be compared against deviation 
lines 57-61 to determine the dimensions of undesirable surface 
deformities. 
It should be well recognized that many other mathematical methods may be 
used to accomplish the same analytical evaluation, such as for example use 
of the Fourier Series to form a fitted curve through the edge 31. 
Additionally, it should be recognized that separate discrete fitted curves 
may be generated along the length of a single edge. 
The software loaded into the computer, in the preferred embodiment of the 
present invention, performs the above described operations such as 
conversion of the reflected light pattern into an array of pixels, 
evaluation of the pixel values in order to define the edge, and analysis 
of the edge to detect the existence of undesirable surface deformities. It 
is well recognized that many other software combinations, and/or 
combinations of computers may be used to perform these operations. 
Many other variations and modifications may be made in the apparatus and 
techniques hereinbefore described, both by those having experience in this 
technology without departing from the concept of the present invention. 
Accordingly, it should be clearly understood that the apparatus and 
methods depicted in the accompanying drawings and referred to in the 
foregoing description are illustrative only and are not intended as 
limitations on the scope of the invention.