Optical inspection of lacquer and carbon deposits

An improved method and apparatus for classifying and quantifying lacquer and carbon deposits on internal combustion engine pistons. In the preferred embodiment of the invention, a piston is mounted on a rotating means and is rotated to expose the entire surface of the piston to a video imaging system. The piston is illuminated with indirect lighting in order to minimize reflections and to enhance the contrast of the video image. The video imaging system is comprised of a video camera which employs a charged coupled device (CCD) sensor and data storage for storing digital video produced by the camera. A microprocessor is operable to control operation of the camera and to process the stored data according to an algorithm to classify the video image into one of six categories.

FIELD OF THE INVENTION 
The present invention relates generally to the field of optical inspection 
systems and, more particularly, to a novel and improved technique for 
classifying and quantifying lacquer and carbon deposits on internal 
combustion engine pistons. 
BACKGROUND 
One of the problems encountered in systems employing hydrocarbon fuels is 
the build up over time of thermal oxide derived varnish-like lacquer 
deposits on the surfaces of combustion chambers and components of the fuel 
distribution network. One of the commonly used methods for rating an 
engine lubricant involves examination of a test piston which has been 
subjected to many hours of operation in a running engine. Lubricant 
efficacy can be measured, in part, by measuring the amount of lacquer and 
carbon which has been deposited on the piston surface (lands) and in the 
ring grooves. Currently, this evaluation procedure is done manually, using 
human judgement to classify the deposit color and coverage. 
Currently lacquer deposits are categorized according to six classifications 
based on color: (1) clean (shiny aluminum, no deposits); (2) very light 
amber lacquer; (3) light amber lacquer; (4) amber lacquer; (5) dark brown 
lacquer; and (6) black lacquer. Each lacquer classification is slightly 
darker than the previous one, beginning with no lacquer deposit (clean) 
and ending with black (class 6 above). Under standards issued by the 
Coordinated Research Council (CRC) rating specification any lacquer 
deposit appearing to have a color value falling between two classes is 
given the higher class categorization. One of the difficulties with 
current inspection techniques is the use of subjective judgment on the 
part of the operator who classifies the lacquer deposit based on his 
individual perception of the "best match" to the color standard. 
From the above discussion, it is clear that the prior art lacks an 
objective, precise and repeatable evaluation technique for evaluating 
lacquer and carbon deposits on surfaces of internal combustion engine. A 
method and apparatus of the present invention, discussed in more detail 
below, provides an efficient and effective inspection technique overcoming 
the difficulties of the prior art. 
SUMMARY OF THE INVENTION 
The present invention provides an improved method and apparatus for 
classifying and quantifying lacquer and carbon deposits on internal 
combustion engine pistons. In the preferred embodiment of the invention, a 
piston is mounted on a rotating means and is rotated to expose the entire 
surface of the piston to a video imaging system. The piston is illuminated 
with indirect lighting in order to minimize reflections and to enhance the 
contrast of the video image. The video imaging system is comprised of a 
video camera which employs a charged coupled device (CCD) sensor and data 
storage for storing digital video images produced by the camera. A 
microprocessor is operable to control operation of the camera and to 
process the stored data according to an algorithm to classify the video 
image into one of six categories.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is an illustration of an internal combustion engine piston of the 
type used to measure carbon and lacquer deposits in the inspection system 
of the present invention. The piston is comprised of a plurality of 
"lands" 12a-12d and a plurality of "grooves" 14a-14c. The upper land area 
illustrated by reference numeral 12a is often referred to as the "crown" 
land of the piston. 
A rating reference chip 16, shown in FIG. 2, is used to calibrate the 
imaging system of the present invention, as will be discussed in greater 
detail below. This reference chip 16 is constructed from small strips of 
anodized aluminum, with each strip representing the equivalent lacquer 
color level corresponding to the CRC lacquer classification system. The 
reference chip 16 shown in FIG. 2 contains reference strips corresponding 
to each of the CRC rating categories: (1) clean (shiny aluminum, no 
deposits); (2) very light amber lacquer; (3) light amber lacquer; (4) 
amber lacquer; (5) dark brown lacquer; and (6) black lacquer. In the 
system, a video image of the reference chip is used to determine the 
reflectance intensity breakpoints to distinguish one lacquer class from 
another. 
The inspection system of the present invention is shown in FIGS. 3 and 4. 
The piston 10 is mounted on an appropriate means for rotating the piston 
to expose all of its surface area to the inspection system. The rotating 
means can be in the form of conventional electric motor, which rotates the 
piston at a constant velocity, or a "stepper" motor, which rotates the 
piston in discrete increments. The rotating means includes a platform 
which provides control of the piston (vertical) z-axis. The Z-axis control 
is used to position the piston inspection areas into the camera 
field-of-view. The field of view is either a land section or a groove. The 
rotational axis allows the piston surface to be "unwrapped" by 
successively acquiring images of the piston groove or land surface during 
rotation. The rotational speed of this axis is dependent on the required 
image capture and processing time. Typically the rotation speed will vary 
from 1/4 rpm to 60 rpm. 
The piston is illuminated by a plurality of lights, illustrated by light 
sources 18a and 18b. In the embodiment illustrated in FIG. 3, the light 
sources 18a and 18b are fluorescent lights. Optimum lighting is crucial in 
order to accurately separate the various lacquer color levels into 
distinct categories. It has been determined that an indirect diffuse 
lighting method produces the most uniform image with the greatest 
contrast. The geometry of the lighting used in the preferred embodiment is 
shown in FIG. 3. The light produced by the sources 18a and 18b is 
reflected by a cylindrical diffusing white reflector 20. This arrangement 
increases the uniformity of the lighting and minimizes shadows. The 
reflector 20 has an aperture 22 therein to receive the lens 24 of camera 
26. Operation of the camera 26 is controlled by a microprocessor 28 which 
processes the video signal from the camera and stores the resulting data 
in data storage 30. Results of the data processing are displayed on a 
appropriate output device 32 which can be a conventional computer printer 
or a video display. The processing steps used to analyze the reference 
data and the video signal will be discussed in greater detail below. 
An alternate embodiment of the lighting arrangement for the invention 
system is shown in FIG. 4. In this embodiment, a translucent panel 36 is 
placed between the light sources 18a and 18b and the piston 10. The 
translucent panel 36 is provided with an aperture 38 to receive the lens 
24 of camera 26. 
The camera 26 is a high resolution, black and white, camera employing a 
charge coupled device (CCD) sensor to obtain a digital representation of 
the piston reflectance. Each point on the piston surface under inspection 
is represented by an image picture element (pixel) with a value 
proportional to the reflected light intensity corresponding to that point. 
The pixels may take on integer values from 0 (black) to 255 (white). It is 
possible to use cameras having various spatial resolutions, for example, 
256 by 240 or 512 by 480. In an alternate embodiment of the invention 
system, the CCD camera is replaced by a line scan camera which generates 
only a line of pixel data. In this embodiment, an image is created by 
passing the piston 10 past the camera at a controlled rate. The linescan 
camera is available at a variety of spatial resolutions. Excellent results 
were obtained in the invention system with a linecamera having a 
resolution of 512 pixels per line. Although the system can be operated 
with the camera axis centered on the piston axis, the camera 26 of the 
preferred embodiment is directed slightly off-center (approximately 1 
centimeter) from the piston axis, as shown in FIGS. 3 and 4. This offset 
eliminates shadows caused by reflection of the camera lens on the shiny 
piston surface. 
The reference chip 16, shown in FIG. 2, is imaged in order to calibrate the 
video system for lighting compensation and to create the table which maps 
image reflectance levels into lacquer classifications. One graylevel 
histogram is generated for each reference strip (six altogether) and a 
table created from this information is stored by microprocessor 28 in data 
storage 30. The microprocessor uses the processing steps discussed below 
to remap the pixel graylevels corresponding to the piston video signal 
into one of the six lacquer levels stored in memory. 
There are several methods which are known in the art for generating the 
table breakpoints. For example, the successive graylevel breakpoints can 
be calculated as: 1) the histogram peak; 2) the midpoint between 
successive histogram peaks; 3) the midpoint between successive histogram 
means; 4) the overlap point between successive histogram tails; 5) linear 
combinations of the points listed above in methods (1)-(4). Once the upper 
and lower graylevel boundaries have been computed for each lacquer 
classification, the piston is imaged while rotating and each pixel (having 
an integer value ranging from zero to 255) is mapped into a lacquer 
category, i.e. a value from one to 6. The percentage coverage for each 
lacquer classification is then computed for each land and groove using 
standard percentage equations. 
The processing steps implemented by the microprocessor 28 can be understood 
be referring to the flowchart shown in FIG. 5. In step 100, the system is 
started. In step 110, a video image is obtained of the reference chip set. 
In step 114, the reference chip histogram is computed and the map decision 
breakpoints are determined according to one of the methods discussed 
hereinabove. These points delineate the chip graylevel boundaries. In step 
116, an image is obtained of the piston land or groove to be rated. In the 
preferred embodiment of the invention, the piston circumference is divided 
into 18.degree. sectors. In step 118, a data window is positioned around 
the land or groove sector of the image just obtained. In step 120, each of 
the pixels in the windowed region is sorted into one of six categories 
based on the decision point table determined in step 114. For purposes of 
this sorting procedure, an allowance for image saturation error is made. 
Saturated black is defined as a pixel value less than 5 and saturated 
white is defined as a pixel value greater than 250. In step 122, the 
system computes the percent of each of the six nonsaturated values found 
inside the rating widow. In step 124, a decision is made as to whether the 
entire piston has been surveyed. If the entire piston has not been 
surveyed, the piston is rotated by a predetermined increment in step 126 
and steps 116 through 124 are repeated. If the determination of step 124 
indicates that the entire piston has been surveyed, a report is generated 
based on the cumulative ratings for each sector in step 128. 
One of the novel features of the invention system is the use of laser 
profilometry to determine the depth of carbon deposits in the piston 
groove. FIG. 6 is an illustration of the geometry of a laser profilometry 
system. The piston groove is illustrated by upper groove edges 40a and 40b 
and by lower groove edges 42a and 42b, shown in phantom. A beam of laser 
light is illustrated by the generally rectangular profile 44. The camera 
is aimed at the piston at an angle, with the field of view shown in FIG. 
6. The thickness of the deposit 46 in the groove can be calculated using 
the data processing algorithm described below. 
The angle .theta. between the incident laser beam and the camera field of 
view can vary over a fairly wide range so long as the angle is 
substantially greater than 0.degree.. Excellent results can be obtained 
with a .theta. angle of approximately 45.degree.. From the camera 
viewpoint, the laser line defines the groove height profile from which 
carbon thickness can be measured. Given the offset distance of the camera 
and the camera-laser angle, it is straightforward to calculate the given 
profile for a given point in the groove. The rotating platform provides 
the facility to move the piston through 360.degree., and collect depth 
information at an arbitrary dense set of points. Although the invention 
system has been described in connection with the measurement of deposit 
depths in piston grooves, it can also be adapted to measure deposit 
thickness on the piston lands. 
The laser line may be generated by three different methods: (1) A point 
source of laser light can be spread in one direction by using a 
cylindrical lens. (2) A point source of laser light can be directed toward 
an oscillating mirror. This mirror is typically mounted on a galvanometric 
movement which is driven by a sinusoidal voltage. In this way, the light 
is rapidly swept back and forth over the area of interest (the groove). If 
the camera has the proper aperture speed, the point source will appear as 
a line of light. The mirror oscillation frequency is typically 600 Hz. (3) 
A point source of laser light can be directed toward a rotating polygon. 
The polygon will cause the point source of light to be scanned along a 
line (similar to (2) above). However, the scan is linearly directed from 
one line endpoint (A) to the other endpoint (B), rather than sinusoidally 
directed from A to B to A (back and forth). 
Suitable lasers are available in a wide variety of wavelengths. However, in 
the preferred embodiment, HeNe lasers in the visible and infrared range 
were used. An optical filter was found to be very useful to subdue ambient 
room noise. This filter may be a bandpass, centered at the laser 
frequency, or an optical longpass filter which passes the laser light. 
The processing steps implemented by the microprocessor 28 for using laser 
profilometry to determine the depth of carbon deposits in the piston 
grooves can be seen by referring to FIG. 7. In step 200, the system is 
started. In step 210, the initial position of the piston is defined to be 
zero degrees (0.degree.). In step 212, a data window is defined around the 
portion of the piston groove to be surveyed. In step 214, the piston 
groove is illuminated with a structured light profile and an image of the 
piston groove is obtained. In step 216, the light beam image is reduced to 
produce a profile line which is one pixel wide. Next, in step 218, for 
each row in the image, the distance in pixels is computed from the left 
most image edge to the profile point. This distance will have a digital 
value between 0 and 255. In step 220, this point is stored as a graylevel 
in a column in another image frame buffer corresponding to the profile 
image row. In step 222, a determination is made as to whether the entire 
groove has been processed. If the entire groove has not been processed, 
the piston is rotated by a predefined increment in step 224 and steps 214 
through 222 are repeated. In the preferred embodiment of the invention, 
the rotation increment is approximately 2.degree.. If a determination is 
made that the entire groove has been processed, the amount of carbon in 
the fill groove space is calculated in step 226. In the preferred 
embodiment of the invention, 100% of the groove gap space is defined as 
90% of the camera horizontal resolution which allows for piston-to-camera 
placement error. Thus, the percentage fill in the groove is determined as 
a ratio of the maximum number of pixels possible in the corresponding 
video image. In step 228, the groove is rated by subtracting a clean 
piston range image and, in step 30, the pixels are placed in bins 
corresponding to the CRC categories. CRC rating specifications require 
that carbon depth must be categorized as clean, light (between 0 and 25% 
fill), medium (greater than 25% and less than 100% fill), and heavy (100% 
fill). This categorization can be done with the computed carbon profile 
data. The profile data may also be used directly for a more accurate 
carbon volume measurement. 
Although the method and apparatus of the present invention has been 
described in connection with the preferred embodiment, it is not intended 
to be limited to the specific form set forth herein, but on the contrary, 
it is intended to cover such modifications, alternatives and equivalents 
as can reasonably be included within the spirit and scope of the claims.