Abstract:
The thickness of a workpart ( 10 ) is measured to a high degree of accuracy by passing a coherent light beam ( 20 ) through an aperture ( 16 ) in the workpart ( 10 ). The aperture ( 16 ′) can alternatively be created between an edge of the workpart ( 10 ) and an external reference plate ( 30 ). The light is diffracted on the far side of the workpart ( 10 ) and its diffraction pattern captured by a CCD camera ( 22 ). The captured image is analyzed by a computer ( 24 ) which compares the captured diffraction pattern to a stored referenced value to determine whether the thickness of the workpart ( 10 ) is within an acceptable range. The method is capable of returning measurements with micron or submicron resolution, and is a robust process readily adaptable to high volume production quality control applications.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for measuring the thickness of objects by diffracting a coherent beam of light through an aperture or across an edge and comparing the diffraction pattern to a reference diffraction pattern. 
     2. Related Art 
     In the manufacturing environment, there is often a need to measure the thickness of manufactured components or workparts with a high degree of accuracy. In some applications, thickness measurements with micron or submicron resolution are necessary. 
     In situations requiring high resolution thickness measurements, it has been proposed to use the technique of laser triangulation, in which measurements are calculated based on the reflection of multiple laser beams off the surface. One problem with optical measurement methods results from the so-called “noise” created by unwanted light reflections. Such noise can produce false positive and/or false negative measurements. In high-volume mass production manufacturing operations, quality control standards often require defective parts statistics less than one or two parts per million. When the difference between an acceptable and unacceptable workpart is a thickness measurement in the range of a few microns, achieving and maintaining consistent production quality can be a challenge. 
     Accordingly, there is a need for a thickness measuring technique which is highly accurate, robust, not sensitive to light noise created from unwanted reflections, and adaptable to high production volume settings. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     The invention comprises a method for measuring the thickness of a workpart comprising the steps of: forming an aperture through a workpart, passing a light beam through the aperture to create a light diffraction pattern on the far side of the workpart, capturing the diffraction pattern, and measuring the captured diffraction pattern to determine the thickness of the workpart. The measure of the diffraction pattern is indicative of the workpart thickness so that acceptable diffraction pattern measurements can be associated with workpart thicknesses in an acceptable range. 
     According to another aspect of the invention, a method for measuring the thickness of a workpart comprises the steps of: providing a test piece of known thickness, forming an aperture through the test piece, passing a light beam through the aperture to create a light diffraction pattern on the far side of the test piece, capturing the diffraction pattern, measuring the captured diffraction pattern to establish a reference diffraction pattern, and associating the reference diffraction pattern with the known thickness of the test piece. An aperture is then formed through a workpart of unknown thickness and a light beam passed through the aperture of the workpart to create a light diffraction pattern on the far side of the workpart. The diffraction pattern is captured and then compared to the referenced diffraction pattern to determine whether the thickness of the workpart is equal to the known thickness of the test piece. 
     According to yet another aspect of the invention, a method of inspecting the thickness of a plurality of workparts traveling in a predetermined path comprises the steps of: conveying a plurality of workparts along a predetermined path, wherein the workparts are of unknown thickness and each have an aperture of identical dimensions formed therethrough. The method continues with the step of directing a light beam at the workparts in sequence so that the light beam passes through the aperture of each workpart in succession to project a distinctive light diffraction pattern for each workpart on the far side of the workpart. The diffraction pattern is captured and then measured to determine the thickness of the workpart. 
     This invention, which operates on the principle of analyzing light that is passed through an aperture, rather than reflected off a surface, provides higher signal-to-noise ratios thus making the thickness measurements more robust. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view demonstrating how a coherent beam of light is passed through an aperture in a workpart and captured on a CCD camera and finally processed by a computer; 
         FIG. 2  is a perspective view of an exemplary workpart fitted with an aperture through which a coherent beam of light may be passed; 
         FIG. 3  is a cross-sectional view of the workpart taken along lines  3 - 3  of  FIG. 2 ; 
         FIGS. 4A through 4C  represent a sector of exemplary diffraction patterns captured by the CCD camera, with  FIG. 4B  representing a ten micron change in thickness from  FIG. 4A  and  FIG. 4C  representing a ten micron change from  FIG. 4B ; 
         FIG. 5  is a graph comparing the intensity values for each of the images presented in  FIGS. 4A through 4C  as a function of distance from the center of the diffraction pattern; 
         FIG. 6  is a simplified view showing a method for inspecting the thickness of sequentially manufactured workparts in a high volume production setting; and 
         FIG. 7  is a schematic view of an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout several views, a workpart according to the subject invention is generally shown at  10 . The workpart  10  may comprise any article of manufacture, however, is preferably of the type having a generally planer and consistent cross-sectional thickness in the region to be measured. Also, the workpart  10  is of the type whose dimensional thickness must be measured to exacting tolerances. The method of this invention is well suited to measuring workpart thicknesses on the order of a few microns or even less. 
     For the purposes of example only, and not to be in any way limiting, the workpart  10  may comprise an engine bearing of the type positioned between the crankshaft and either the main bearings or the connecting rods. An engine bearing is a good example of a workpart for this invention, because such engine bearings must be manufactured to exacting tolerances. Furthermore, engine bearings, like many workparts, are manufactured as a composite comprising a top material layer  12  and an underlying substrate  14 . Quite often, during the workpart manufacturing process, the techniques used to apply the top layer  12  require exacting controls. Any one of a number of parameter changes can result in the top layer  12  being deposited or applied too thickly or too thinly. In the example of powder coating, the top layer  12  can achieve an unacceptable deposition thickness if the spray equipment or other handling or post treating steps are not followed precisely. This can result in a finished workpart  10  which has a thickness out of tolerance. 
     Depending on the particular type of workpart  10  being measured, there is typically an opportunity to form an aperture  16  through the workpart  10  at some location in which a thickness measurement is desired. In one preferred embodiment of this invention, the aperture  16  must be formed fully through the thickness the workpart  10  to be measured. Thus, if the workpart  10  is composed of multiple layers, such as a top layer  12  and a substrate  14 , the aperture  16  must form a complete pass-through, fully open from one surface to the other. The aperture  16  may be of any appropriate shape, including circular or otherwise, but is shown in the figures taking the form of an elongated slit which has traditional acceptance in the field of diffraction optics. Furthermore, it is not necessary that the aperture  16  be fully bounded on all sides by remaining portions of the workpart  10 . Instead, the aperture  16  can take the form of a notch or cut propagating inwardly from an edge of the workpart  10 . In some applications, the aperture  16  may even take the form of a protruding obstacle. This is because the purpose of the aperture  16  is to interact with light waves to create a diffraction effect. And, it is well known that diffraction effects can occur when incident light waves interact with either obstacles or apertures of finite size. When the light waves bend around the obstacles or spread out from the aperture  16 , waves propagate outwardly resulting in a diffraction pattern which can be analyzed. 
     Referring to  FIG. 1 , a device for creating a beam of light is generally indicated at  18 . Preferably, this device consists of a laser or other device capable of producing coherent light, i.e., radiant electromagnetic energy of the same or nearly the same wavelength and with definite phase relationships between different points in the field. When the laser  18  is energized, it emits a light beam  20  which is directed at the aperture  16  in the workpart  10 . When the light waves interact with the aperture  16 , the light waves spread out from the aperture in the familiar manner of water waves, creating a diffraction effect on the far side of the workpart  10 . This diffraction effect is represented by the spreading of the light beam  20  in  FIG. 1 . 
     A device, such as a CCD camera  22 , is positioned in the path of the diffracted light beam  20  to capture the diffraction pattern. In the case of CCD cameras, this diffraction pattern is captured on a piece of silicon called a charge-coupled device, i.e., CCD. This silicon wafer is a solid-state electronic component which is usually micro-manufactured and segmented into an array of individual light-sensitive cells. The CCD camera  22  thus collects the diffraction signature produced by the laser light  20  passing through the aperture  16 , with each light sensitive cell registering a given intensity of light at a given spatial position. These relationships, i.e., intensity as a function of position, can be readily plotted and graphed. A device may be operatively coupled to the CCD camera  22  for receiving, processing, and presenting the intensity vs. position data from the captured diffraction pattern. This device may preferably be a computer which is then programmed to determine the thickness of the workpart  10  by measuring the intensity and position values. Because the diffraction pattern is indicative of the workpart  10  thickness, changes in the measured diffraction pattern are useful to conclude whether the thickness of the workpart  10  may be out of tolerance. 
     Instead of measuring the diffraction pattern per se, the computer  24  can determine workpart  10  thickness by comparing the captured diffraction pattern to one or more reference patterns contained in recorded memory. For example,  FIGS. 4A through 4C  represent the diffraction patterns for three different workparts  10 , each containing an aperture  16  of identical dimensions. However, the thickness of the workpart  10 , in  FIG. 4A  is ten microns smaller than the thickness of workpart  10  in  FIG. 4B . And again, the workpart  10  associated with  FIG. 4B  is ten microns thinner than the workpart  10  associated with the diffraction pattern of  FIG. 4C . 
     By comparing the diffraction patterns in  FIGS. 4A ,  4 B, and  4 C, it is evident that the thickness of the workpart  10  responsible for producing the diffraction pattern of  FIG. 4A  is ten microns thinner than the workpart  10  that produced the diffraction pattern of  FIG. 4B , and 20 microns thinner than the workpart  10  which produced the diffraction pattern in  FIG. 4C . Additional useful information can be gleaned by comparing the intensity values of the images corresponding to  FIGS. 4A ,  4 B, and  4 C in graph form. A graph showing these intensity value comparisons as a function of distance is provided in  FIG. 5 . 
     Using either a comparison technique or a measurement technique, the computer  24  analyzes the captured diffraction pattern produced by the workpart  10  to determine whether the thickness of the workpart  10  is within or outside of an acceptable range. Of course, the breadth of an acceptable range is determined by the intended application of the workpart  10 . 
     According to this comparison technique for determining workpart  10  thicknesses, a test piece is provided having a known thickness. The test piece is preferably identical in all respects to a workpart  10 . An aperture is formed through the workpiece which is dimensionally identical to the aperture  16  in the workpiece  10 . A light beam  20  from the laser  18  is passed through the aperture in the test piece to create a light diffraction pattern on the far side of the test piece. This diffraction pattern is captured by the CCD camera  22  and then measured and/or stored in the computer memory. This capture diffraction pattern (and/or measurement characteristics) are associated with the known thickness of the test piece. Thus, when an actual workpart  10  is measured using this diffraction pattern capturing technique, the captured diffraction pattern from the workpart  10  is compared to the referenced diffraction pattern created by the test piece and a determination made by the computer  24  as to whether the thickness of the workpart  10  is equal to that of the test piece. 
     Referring now to  FIG. 6 , a method for inspecting the thickness of a plurality of workparts  10  traveling along a predetermined path is shown. The predetermined path preferably comprises a material handling device  26 , which in the example of  FIG. 6  is a simple conveyor belt. Alternatively, the material handling device  26  can be a carrousel or any other type unit which moves the workparts  10  in a predictable path. In this scenario, the material handling device  26  may usher workparts  10  directly from a manufacturing operation so that their thicknesses can be determined for quality control purposes. The laser  18  is positioned adjacent the material handling device  26  so that its light beam  20  is directed at the passing array of workparts  10 . The light beam  20  is focused to pass through the aperture  16  in every workpart  10  passing by, such that the orientation of the workparts  10  is important. Alternatively, if a statistical sampling of thicknesses is sufficient, only a given number of workparts  10  per thousand need be provided with an aperture  16  and measured according to this method. 
     The CCD camera  22  is placed on the opposite side of the material handling device  26 , ready to receive the diffraction pattern emerging from the far side of the workparts  10  as they cross the light beam  20 . The computer  24  quickly analyzes the distinctive diffraction pattern for each workpart  10  and makes a measurement determination as to whether the thickness of the workpart  10  is within an acceptable range. If not within an acceptable range, the computer  24  may be coupled with a reject device  28  which diverts a workpart  10  away from the predetermined path of the material handling device  26 . 
     While the example of  FIG. 6  suggests that the laser  18  and light beam  20  remain stationary while the workparts  10  are conveyed therepast, other arrangements and configurations are possible. For example, the laser  18  and CCD camera  22  can move with the workparts  10  as they travel their predetermined path, or the laser  18  and CCD camera  22  can move a predetermined path while the workparts  10  remain stationary. 
     Those skilled in the art will appreciate other configurations as well. For example, in  FIG. 7  another preferred embodiment of the invention is shown in which the aperture  16 ′ is formed between an outer edge of the workpart  10  and a reference plate  30 . The reference plate  30  is, in this example, held stationary while the workparts  10  are conveyed along a material handling device  26 ′. However the reverse motions are equally possible, as well as the possibility for both workpart  10  and reference plate  30  to be in motion or stationary at the same time. In any event, the light beam  20  is directed at the aperture  16 ′ and a diffraction pattern is thereby created on the opposite side. Using any known technique, the captured diffraction pattern can be analyzed to determine whether the thickness of the workpart  10  is within or outside of an acceptable range. This alternative embodiment has the advantage of obviating the need to form a hole or notch in the workpart  10  in situations when such is not convenient. 
     The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims.