Source: http://www.google.com/patents/US5416589?dq=patent:4807115
Timestamp: 2015-01-30 21:08:16
Document Index: 21713630

Matched Legal Cases: ['art. 10', 'art 14', 'art 14', 'art 14', 'art 14', 'art 47', 'art 114', 'art 114', 'art 114', 'art 114']

Patent US5416589 - Electro-optical system for gauging surface profile deviations - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA method and system for gauging deviations of a surface of a test part from a preselected nominal surface profile is disclosed. One embodiment includes a support having a master surface that is substantially a matched or mating surface of the nominal surface profile of the test part and a thin layer...http://www.google.com/patents/US5416589?utm_source=gb-gplus-sharePatent US5416589 - Electro-optical system for gauging surface profile deviationsAdvanced Patent SearchPublication numberUS5416589 APublication typeGrantApplication numberUS 07/960,607Publication dateMay 16, 1995Filing dateOct 13, 1992Priority dateOct 4, 1991Fee statusLapsedPublication number07960607, 960607, US 5416589 A, US 5416589A, US-A-5416589, US5416589 A, US5416589AInventorsCharles D. LysogorskiOriginal AssigneeKms Fusion, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (17), Referenced by (5), Classifications (15), Legal Events (5) External Links: USPTO, USPTO Assignment, EspacenetElectro-optical system for gauging surface profile deviationsUS 5416589 AAbstract A method and system for gauging deviations of a surface of a test part from a preselected nominal surface profile is disclosed. One embodiment includes a support having a master surface that is substantially a matched or mating surface of the nominal surface profile of the test part and a thin layer of an attenuating medium such as a dye liquid between the master and test surfaces. In another embodiment, the interface between attenuation fluid and the air is used as a reference surface thereby eliminating the requirement of the master surface. In both embodiments, electromagnetic radiation is directed through the attenuating medium onto the surface of the test part to be gauged. An image sensor such as a camera is positioned to receive an image of the radiation reflected by the test part surface back through the attenuating medium, with the intensity of such radiation across the image varying as a function of the deviation of the test part surface from the nominal surface profile. The sensor output is digitized to form a series of digital signals indicative of the intensity of radiation associated with each location of the reflected image, and the digitized pixel signals are stored in digital electronic memory and/or displayed on a screen. Computer programming corrects the digitized intensity signals for sensor gain, bias and variations in part reflectivity, and presents a quantitative measurement of the deviations in test surface profile from the master surface profile over the entire surface being measured.
I claim: 1. A system for gauging deviations of a surface on a test part from a preselected nominal surface geometry using electromagnetic radiation, said system comprising:(1) a container adapted to hold the test part; (2) an attenuating fluid within the container such that the attenuating fluid substantially covers the test part surface to be gauged when the test part is placed within the container and such that an attenuating fluid/air interface is formed which is suitable for use as a reference surface; (3) a source of electromagnetic radiation for irradiating the test part surface to be gauged; (4) an image sensor positioned to receive electromagnetic radiation reflected from the test part surface to be gauged that originated from said source, said reflected radiation passes back through the attenuating fluid and across the reference surface prior to being received by said image sensor, the intensity of the reflected radiation varying across the image as a function of the distance between the test part surface and the reference surface, whereby an image of the test part surface to be gauged is formed; (5) a digitizer for converting the image from the image sensor into digital signals indicative of the intensity of the reflected radiation across the image; (6) digital electronic storage coupled to the digitizer for receiving and storing the digital signals; and (7) a calibration arrangement for correcting errors in the image formed by said image sensor; including means for producing a set of correction data and for altering the digital signals in accordance with the correction data. 2. A system as defined in claim 1, wherein the attenuating fluid includes a material for attenuating the electromagnetic radiation as a function of distance the electromagnetic radiation travels through the attenuating fluid.
3. A system as defined in claim 2, wherein the attenuating fluid is a liquid containing a dye and the electromagnetic radiation is in the visible range.
4. A system as defined in claim 2, wherein the attenuating fluid is conductive to microwave radiation and the electromagnetic radiation is in the microwave range.
5. A system as defined in claim 1, wherein the surface of the test part has low reflectivity and the attenuating fluid contains a fluorescent dye responsive to the electromagnetic radiation from the electromagnetic radiation source.
6. A system as defined in claim 1, wherein the attenuating fluid contains a photochromic dye having attenuation characteristics that vary as a function of the intensity of the electromagnetic radiation.
7. A system as defined in claim 1, wherein the image sensor is a CCD sensor having a matrix of image sensing elements in a row-and-column array, each of the image sensing elements receiving a corresponding location of the image such that the resolution of the system depends upon number, size, and spacing between the image sensing elements.
8. A system as defined in claim 7, further comprising a translator for moving the image sensor relative to the reference surface so as to vary the resolution of the system.
9. A system as defined in claim 1, wherein said calibration arrangement is used to correct for non-uniform reflectivity from the surface of the test part, and wherein said calibration arrangement comprises:(1) filtering means adapted to be positioned at a preselected point along the path defined by the electromagnetic radiation as the radiation travels from said source to the test part surface and reflects from the test part surface and travels through said attenuating fluid toward said image sensor between said radiation source and said image sensor for varying the wavelength of radiation collected at said image sensor; and wherein, (2) said storage means stores the digital signals from the image sensor at a minimum of two different wavelengths; (3) said producing means produces the set of correction data based on the digital signals stored from the measurements at the different wavelengths; and (4) means for storing the correction data, whereby the correction data can be used to correct for non-uniform reflectivity from the surface of the test part. 10. A system as defined in claim 1, wherein the container is equipped with vibration-damping means to allow the attenuating fluid to quickly stabilize.
11. A system as defined in claim 1, wherein a second fluid is placed upon the attenuating fluid to increase the resolution of the system where the second fluid floats upon the attenuating fluid.
12. A system for gauging deviations of a surface on a test part from a preselected nominal surface geometry using electromagnetic radiation where the test part surface to be gauged forms a fluid receptacle, said system comprising:(1) an attenuating fluid filling the fluid receptacle such that an attenuating fluid/air interface is formed which is suitable for use as a reference surface; (2) a source of electromagnetic radiation for irradiating the test part surface to be gauged; (3) an image sensor positioned to receive electromagnetic radiation reflected from the test part surface to be gauged that originated from said source, said reflected radiation passes back through the attenuating fluid and across the reference surface prior to being received by said image sensor, the intensity of the reflected radiation varying across the image as a function of the distance between the test part surface and the reference surface, whereby an image of the test part surface to be gauged is formed; (4) a digitizer for converting the image from the image sensor into digital signals indicative of the intensity of the reflected radiation across the image; (5) digital electronic storage coupled to the digitizer for receiving and storing the digital signals; and (6) a calibration arrangement for correcting errors in the image formed by said image sensor; including means for producing a set of correction data and for altering the digital signals in accordance with the correction data. 13. A system as defined in claim 12, wherein the attenuating fluid includes a material for attenuating the electromagnetic radiation as a function of distance the electromagnetic radiation travels through the attenuating fluid.
14. A system as defined in claim 13, wherein the attenuating fluid is a liquid containing a dye and the electromagnetic radiation is in the visible range.
15. A system as defined in claim 13, wherein the attenuating fluid is conductive to microwave radiation and the electromagnetic radiation is in the microwave range.
16. A system as defined in claim 12, wherein the surface of the test part has low reflectivity and the attenuating fluid contains a fluorescent dye responsive to the electromagnetic radiation from the electromagnetic radiation source.
17. A system as defined in claim 12, wherein the attenuating fluid contains a photochromic dye having attenuation characteristics that vary as a function of the intensity of the electromagnetic radiation.
18. A system as defined in claim 12, wherein said calibration arrangement includes filtering means adapted to be positioned at a preselected point along the path defined by the electromagnetic radiation as the radiation travels from said source to the test part surface and reflects from the test part surface and travels through said attenuating fluid toward said image sensor between said radiation source and said image sensor for varying the wavelength of radiation collected in said image sensor; and wherein,said storage means stores the digital signals from said image sensor at a minimum of two wavelengths; and said producing means produces the set of correction data based on the digital signals stored from the measurements at the different wavelengths, whereby the correction data can be used to correct for non-uniform reflectivity from the test part surface. 19. A system as defined in claim 12, wherein the test part is coupled with vibration-dampening means for facilitating a stabilization of the attenuating fluid within the fluid receptacle.
20. A system as defined in claim 12, wherein a second fluid is placed upon the attenuating fluid such that said second fluid floats upon the attenuating fluid for increasing the resolution of the system.
21. A method of gauging deviations of a surface on a test object from a preselected nominal surface geometry using electromagnetic radiation, comprising the steps of:(A) providing a container substantially filled with an attenuating fluid; (B) placing the test object within the container such that the surface of the test object to be gauged is submerged in the attenuating fluid; (C) irradiating the test object surface with the electromagnetic radiation; (D) collecting the electromagnetic radiation that is reflected from the test object surface and passes through the attenuating medium to form an image of the reflected radiation wherein the intensity of the reflected radiation varies across the image as a function of the distance between the test object surface and the reference surface; (E) digitizing the image of the reflected radiation; (F) producing a set of correction data related to errors in the digitized image; and (G) altering the digitized image in accordance with the correction data. 22. The method of claim 21, wherein steps (F) and (G) are performed by the substeps of:positioning a filter in the path of the electromagnetic radiation in order to vary the wavelength of the radiation collected in step (D); storing the digital signals collected at a minimum of two wavelengths; producing the correction data using the digital signals stored from the different wavelength measurements; and using the correction data to correct for non-uniform reflectivity from the test object surface. 23. The method of claim 21 wherein steps (F) and (G) are performed by the substeps of:measuring and storing the intensity of the radiation reflected from the test surface across the image with attenuating fluid having a first concentration of dye present; and using the measured intensity across the image to correct a measurement made with the attenuating fluid having a second concentration of dye present. 24. The method of claim 21, further comprising the step of placing a second fluid upon the attenuating fluid such that the second fluid floats upon the attenuating fluid, prior to irradiating the test object surface, to thereby increase the resolution of the image formed.
25. A method of gauging deviations of a surface on a test object from a preselected nominal surface geometry where the test object surface to be gauged forms a fluid receptacle, comprising the steps of:(A) filling the fluid receptacle with a preselected attenuating fluid; (B) allowing the attenuating fluid to settle such that the interface between the attenuating fluid and the air adjacent the attenuating fluid is suitable for use as a reference surface; (C) irradiating the test object surface with electromagnetic radiation; (D) collecting the radiation reflected from the test object surface and attenuated through the attenuating fluid to form an image of the reflected radiation wherein the intensity of the reflected radiation varies across the image as a function of the distance between the test object surface and the reference surface and wherein there may exist optically generated errors in said image; (E) digitizing the image of the collected radiation; (F) producing a set of correction data related to errors in the digitized image; and (G) altering the digitized image in accordance with the correction data. 26. The method of claim 25, wherein steps (F) and (G) are performed by the substeps of:positioning a filter in the path of the electromagnetic radiation in order to vary the wavelength of the radiation collected in step (D); storing the digital signal collected at a minimum of two wavelengths; producing calibration signals based on the digital signals stored from the different wavelength measurements; and using the calibration signals to correct for non-uniform reflectivity from the test object surface. 27. The method of claim 25, wherein steps (F) and (G) are performed by the substeps of:measuring and storing the intensity of the radiation reflected from the test object surface across the image with an attenuating fluid having a first concentration of dye present; and using the measured intensity across the image to correct a measurement made with the attenuating fluid having a second concentration of dye present. 28. The method of claim 25, further comprising the step of placing a layer of a preselected second fluid on the attenuating fluid such that the second fluid floats upon the attenuating fluid to thereby increase the resolution of the image.
29. The method of claim 25, further comprising the steps of coupling the test object to a vibration damping means and settling the attenuating fluid, using the vibration damping means, prior to performing step (C).
30. A system for gauging deviations of a surface on test part from a preselected nominal surface geometry using electromagnetic radiation, said system comprising:a container adapted to hold the test part; an attenuating fluid within the container such that the attenuating fluid substantially covers the test part surface to be gauged when the test part is placed within the container such that an attenuating fluid/air interface is formed which is suitable for use as a reference surface; a source of electromagnetic radiation for irradiating the test part surface to be gauged; an image sensor positioned to receive electromagnetic radiation reflected from the test part surface to be gauged that originated from said source, said reflective radiation passes back through the attenuating fluid and across the reference surface prior to being received by said image sensor, the intensity of the reflected radiation varying across the image as a function of the distance between the test part surface and the reference surface, whereby an image of the test part surface to be gauged is formed; a digitizer for converting the image from the image sensor into digital signals indicative of the intensity of the reflected radiation across the image; digital electronic storage coupled with the digitizer for receiving and storing the digital signals; a calibration arrangement for correcting errors in the image formed by said image sensor; including means for producing a set of correction data and for altering the digital signals in accordance with the correction data; and a second fluid placed upon the attenuating fluid to increase the resolution of the system where the second fluid floats upon the attenuating fluid. 31. A method of gauging deviations of a surface upon a test object from a preselected nominal surface geometry using electromagnetic radiation, comprising the steps of:(a) providing a container substantially filled with an attenuating fluid; (b) placing the test object within the container such that the surface of the test object to be gauged is submerged in the attenuating fluid; (c) irradiating the test object surface with the electromagnetic radiation; (d) collecting the electromagnetic radiation that is reflected from the test object surface and passes through the attenuating fluid to form an image of the reflected radiation wherein the intensity of the reflected radiation varies across the image as a function of the distance between the test object surface and the reference surface; (e) digitizing the image of the reflected radiation; (f) producing a set of correction data related to errors in the digitized image; (g) altering the digitized image in accordance with the correction data; and (h) placing a second fluid upon the attenuating fluid such that the second fluid floats upon the attenuating fluid, prior to irradiating the test object surface, to thereby increase the resolution of the image formed. 32. A method of gauging the deviations of a surface on a test object from a preselected nominal surface geometry where the test object surface to be gauged forms a fluid receptacle, comprising the steps of:(a) filling the fluid receptacle with a preselected attenuating fluid; (b) allowing the attenuating fluid to settle such that the interface between the attenuating fluid and the air adjacent the attenuating fluid is suitable for use as a reference surface; (c) irradiating the test object surface with electromagnetic radiation; (d) collecting the radiation reflected from the test object surface and attenuated through the attenuating fluid to form an image of the reflected radiation wherein the intensity of the reflected radiation varies across the image as a function of the distance between the test object surface and the reference surface and wherein there may exist optically generated errors in said image; (e) digitizing the image of the collected radiation; (f) producing a set of correction data related to errors in the digitized image; (g) altering the digitized image in accordance with the correction data; and (h) placing a layer of a preselected second fluid on the attenuating fluid such that the second fluid floats upon the attenuating fluid to thereby increase the resolution of the image. Description
This application is a continuation-in-part of U.S. patent application Ser. No. 07/770,885, filed on Oct. 4, 1991, now U.S. Pat. No. 5,289,267.
BACKGROUND OF THE INVENTION It has heretofore been proposed to estimate flatness of a surface on a test part by visually observing reflection through a dye liquid film or layer placed between the test surface and a flat master surface. For example, U.S. Pat. No. 2,695,544 discloses a system consisting of, in order, a pane of glass, a dye layer, and the test part. Light is directed through the pane of glass and into the dye layer. The operator then visually observes the light reflected by the test part surface back through the dye layer and the glass pane. Since the light energy is attenuated as a function of distance traveled through the dye layer, departure of the reflected light from uniform intensity across the image generally indicates a corresponding departure of the test part surface from flatness or parallelism with the surface of the glass pane. This method is limited to a subjective and qualitative estimate of the flatness of the test object. This method is also limited by the visual acuity of the operator which will, of course, vary from operator to operator. This method cannot account for differences in reflectivity of the test part across its surface or for differences in the illumination or for other artifacts. This method is suitable for use only in relatively less-demanding quality control applications where parts are either accepted or rejected depending on their qualitative deviation from a prescribed geometry. This method is generally not suitable for use in the operation, control, and/or modification of a manufacturing process wherein the parts are produced. This method is generally not useful in quality control or other operations where it is necessary to quantitatively determine the deviations of the test part from a prescribed geometry.
SUMMARY OF THE INVENTION The present system for gauging deviations of a surface on a test part from a preselected nominal surface geometry includes a support that is essentially transparent to the electromagnetic radiation used and has a master surface that is substantially a matched or mated surface to the preselected nominal surface geometry of the test part. The terms "matched surface" or "mated surface" as employed in the present application mean that the master surface 18 essentially contains the complement image of the prescribed nominal surface geometry which is the desired profile of the test part such that, when the master surface and the test part are brought into adjacent opposition as shown in FIG. 2, the separation between the master surface and the test part will be essentially uniform across the surfaces. For example, if the nominal surface geometry of the test part is flat, the master surface of the support is likewise flat. If the nominal surface of the test part is of convex curved shape, the master surface of the support is of complementary concave curved shape. An essentially non-scattering or low-scattering attenuating medium is placed on the master surface between the test surface and the master surface, with the test part being supportable on the master surface with the surface of the test part opposed to the master surface. The attenuating medium may be a dye fluid or any appropriate medium (fluid, powder, or gas) providing that the medium attenuates the electromagnetic radiation with minimal scattering, and that the medium freely flows into and substantially fills the voids to be gauged between the master surface and the test surface. The test part may be supported by the attenuating fluid (such as a dye fluid) itself or, preferably, by support shims or other mechanical devices to help ensure nominally uniform spacing between the test and master surfaces. The attenuating medium should substantially fill the spaces to be gauged between the master surface and the test part surface.
Implementation of the present invention provides a two-dimensional image of the test part surface profile in a form suitable for digital manipulation, processing, and analysis purposes within a computer system using appropriate software techniques. The digital image of the test part surface profile or digital data corresponding to the test part surface profile may be readily displayed or plotted in the form of a two-dimensional image illustrating the deviation profile or, with proper computer enhancement, displayed or plotted in the form of a three-dimensional image illustrating the deviation profile. Or cross-sectional views of the deviation profile can readily be obtained through critical surface areas of the test part. The digital image may also be employed using conventional manufacturing process control techniques to automatically correct a part production process to reduce or eliminate profile deviations in the test part or to correct for variations over time in the part production process due, for example, to wear or variations in the cutting process or tooling members. Digital processing and software techniques may be employed to correct for non-uniform illumination of the test part, distortion and/or gain variations in the imaging camera, non-uniformities in surface reflectivity of the test part, variations in dye characteristics across the image, and other artifacts.
As one skilled in the art will realize, the manufacture of master support and its master surface will become more expensive and technically difficult as the dimensions of the test part increases. It would be desirable, therefore, to provide a technique and apparatus wherein the master support and its master surface is not required so that the inventive methods of the present invention can be more easily applied to very large parts such as, for example, automobile doors and the like. The present invention provides such a system. The test part to be evaluated is placed in a container of the attenuating fluid (e.g., dyed fluid) whereby the test part--or at least the surface to be evaluated--is completely submerged in the attenuating fluid. The interface of the attenuating fluid and air is used as the reference surface, thereby eliminating the master support and its master surface. For test parts with a depression, valley, or basin that can receive and hold the attenuation fluid (i.e., a fluid receptacle), the fluid receptacle can be filled with attenuation fluid and the contour of the test part surface within the receptacle can be determined using the attenuation fluid/air interface as the reference surface.
Another object of the present system is to provide a system for gauging deviations of a surface on a test part from a preselected nominal surface geometry using electromagnetic radiation, said system comprising:
(1) a container holding the test part;
(2) an attenuating fluid within the container holding the test part such that the attenuating fluid substantially covers the test part surface to be gauged and such that an attenuation fluid/air interface is formed which is suitable for use as a reference surface;
(3) a source of electromagnetic radiation positioned to direct such radiation through the reference surface and attenuating fluid and upon the test part surface to be gauged;
(4) an image sensor positioned to receive the electromagnetic radiation which is reflected from the test part surface to be gauged and which passes back through the attenuating fluid and across the reference surface, whereby an image of the test part surface to be gauged is formed;
(5) a digitizer for converting the image from the image sensor into digital signals indicative of the intensity of the radiation at locations of the image; and
whereby the intensity of the transmitted radiation varies across the image as a function of the distance of the test part surface to be gauged from the reference surface.
Still another object of the present invention is to provide a method of gauging deviations of a surface on a test part from a preselected nominal surface geometry using electromagnetic radiation, said method comprising:
(1) placing the test part in a container holding an attenuating fluid such that the surface of the test part to be gauged is submerged in the attenuation fluid;
(2) allowing the attenuation fluid to settle such that the interface between the attenuation fluid and the air is suitable for use as a reference surface;
(3) passing electromagnetic radiation through the reference surface and into the attenuating fluid in the direction of the test part surface;
(4) collecting the electromagnetic radiation which is reflected from the test part surface and transmitted from the attenuating fluid to form an image of the transmitted electromagnetic radiation;
whereby the intensity of the transmitted electromagnetic radiation varies across the image as a function of the distance of the test part surface from the reference surface.
Still another object of the present invention is to provide a system for gauging deviations of a surface on a test part from a preselected nominal surface geometry using electromagnetic radiation where the test part surface to be gauged forms a fluid receptacle, said system comprising:
(1) an attenuating fluid filling the fluid receptacle such that an attenuation fluid/air interface is formed which is suitable for use as a reference surface;
(2) a source of electromagnetic radiation positioned to direct such radiation through the reference surface and attenuating fluid and upon the test part surface to be gauged;
(3) an image sensor positioned to receive the electromagnetic radiation which is reflected from the test part surface to be gauged and which passes back through the attenuating fluid and across the reference surface, whereby an image of the test part surface to be gauged is formed;
(4) a digitizer for converting the image from the image sensor into digital signals indicative of the intensity of the radiation at locations of the image; and
(5) digital electronic storage coupled to the digitizer for receiving and storing the digital signals;
Still another object of the present invention is to provide a method of gauging deviations of a surface on a test part from a preselected nominal surface geometry using electromagnetic radiation where the test part surface to be gauged forms a fluid receptacle, said method comprising:
(1) filling the fluid receptacle with an attenuation fluid;
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of an inspection station using the gauging system of the invention.
FIG. 6 is a fragmentary schematic diagram of a modified embodiment of the invention wherein the reference surface is the fluid-air interface.
FIG. 7 is a fragmentary schematic diagram of a modified embodiment of the invention wherein the reference surface is the fluid-air interface and the fluid is contained within a depression or hollow area in the test part.
FIG. 8 is a computer-generated image of a standardized test part generated by the gauging system of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 generally illustrates an inspection station 100 using the gauging system of this invention. The gauging system is shown in more detail and in different embodiments in FIGS. 2 through 5. FIG. 1 shows an inspection station 100 consisting of a test chamber enclosure 102, an electromagnetic source and image sensor compartment 104, and the associated computer work station 106. The computer work station 106is shown with a display screen, input device (i.e., a keyboard), and a cabinet to contain the associated computer hardware, memory, and interface devices. The test chamber enclosure 102 contains the test part 14 and test surface 12 which is to be gauged for deviations from a preselected nominal surface geometry. Using fixture 108, the test part 14 is lowered into or placed in an attenuating medium 20 using fixture 108 such that there is a thin film 24 of the attenuating medium 20 between the test part surface 12 and the master surface 18 of the master support 16. The master support 16 must be transparent to the electromagnetic radiation used. The master support 16, in this case an optical flat, provides the interface between the test chamber enclosure 102 and the electromagnetic radiation source and image sensor compartment 104. Compartment 104 contains the electromagnetic source 32 and the image sensor 34. In FIG. 1, two electromagnetic radiation sources 32 are used. As shown in FIG. 2, only one electromagnetic radiation source can be used; or, if desired, more than two electromagnetic radiation sources can also be used. As explained in more detail below, electromagnetic radiation from the electromagnetic radiation source 32 is directed through the master support 16 and its master surface 18, into the attenuation film 24, onto test part surface 12, and then back through the attenuating film 24 and master support 16 to the image sensor 34. The distance the electromagnetic radiation travels through the attenuation film 24 is generally equal to twice the distance between the surfaces 12 and 18 at any given point on the surface 12. By appropriate manipulation, the electromagnetic image received at image sensor 34 is converted into digital signals suitable for computer manipulation. Using computer and suitable software techniques via the computer work station 106, the deviations of the test part surface 12 from a preselected nominal geometry can be determined and displayed as detailed below.
An attenuation medium 20, preferably a dye liquid, is carried on master surface 18 of support 16. Test part 14 rests on a plurality of shims or spacers 22 that separate master surface 18 from test surface 12 by a nominal distance corresponding to thickness of the shims. It is generally preferred that the shims 22 have the same thickness. In some cases, however, it may be preferred that shims of different thickness are used. Dye liquid 20 thus forms a fluid film or layer 24 between surfaces 12 and 18 and fills the voids and depressions 26, 28, and 30 in surface 12 of test part 14. Generally, the distance between the two surfaces 12 and 18 (i.e., the nominal thickness of the attenuation medium 20) should be minimized. Generally, a separation distance of about 0.01 to 0.05 inches will be satisfactory. Separations of the two surfaces 12 and 18 substantially greater than or less than these limits may, however, be employed.
Camera 34 is connected through suitable digitizing electronics 40 to a computer 42 that includes digital memory 44 for receiving and storing the digitized pixel signals from camera 34. Image data is thus stored as numeric data indicating the intensity of the electromagnetic radiation received for each pixel in the matrix of pixels. Computer 42 also includes a screen 46 for displaying to an operator the stored image of test part surface 12. The stored image or data can be displayed, with suitable computer manipulation or enhancement, as shades of gray or in various colors to illustrate deviations from the prescribed nominal geometry. As shown in FIG. 8, the digital data can also be printed or plotted as desired using suitable computer-graphic techniques. The digital data (in either its raw or manipulated forms) can be stored indefinitely to allow for long-term quality control analysis. Such data might be useful, for example, to study failures of critical components where the actual failed components are not readily available (e.g., satellite malfunctions) or to perform long-term statistical analysis of failure or reject rates to pinpoint and correct manufacturing problems.
FIG. 4 illustrates a modified embodiment of the invention for gauging the profile of a test part 47 having a curved test surface 48. The master surface 50 of support 51 is either machined as a matched or mated surface 48 of the nominal desired geometry into a glass support using, for example, a diamond lathe or is cast into a slab of suitable transparent material. In this connection, it will be appreciated that, although master surface 50 (FIG. 4) or 18 (FIG. 2) is employed as a reference surface for gauging purposes, the master surface need not be an exact replica of the nominal test part surface geometry. Small deviations in profile between the nominal surface geometry and the master reference surface can be accommodated by suitably calibrating computer 42 with a dye liquid between the master surface and the test part surface (i.e., surfaces 18 and 12 in FIG. 2 and surfaces 50 and 48 in FIG. 4) using a test part predetermined to possess a surface of desired nominal contour. Such a test part (i.e., one known to have or specifically manufactured to have the predetermined nominal surface geometry) may be retained as a "standard" for routine calibration purposes. If the amount of light reflected from all points on the standard test object surface during this calibration operation is uniform throughout, the thickness of the dye film is uniform and no corrections need to be made. On the other hand, any deviations between the master reference surface and the opposing surface of the standard part will result in a corresponding variation in intensity at one or more pixels of the reflected surface image. By measuring and storing these pixel signals at all points on the surface image, computer 42 effectively captures the correct profile of the standard part with respect to each opposing or corresponding point on the master surface. The information so obtained can then be employed to offset or bias the corresponding pixel signal or signals during operation of the system so as to accommodate any deviations in the master reference surface.
Im1 =Ii1 exp(-2&#945;1 d)R1 here Ii1 is the effective incident intensity at that pixel location, d is the thickness of the dye layer at that pixel location, and R1 is the reflectivity of the surface at that pixel location. Similarly, the measured intensity of the returned or reflected light Im2 for wavelength λ2 at that same pixel location is given by the equation
Ims =Ii2 exp(-2&#945;2 d)R2 where Ii2 is the effective incident intensity at that pixel location, d is the thickness of the dye layer at that pixel location, and R2 is the reflectivity of the surface at that pixel location. Assuming that the surface reflectivity is independent of wavelength, which is a reasonably good approximation for most metals, R1 equals R2 in the two above equations for each pixel location. The ratio of the measured intensities at the two wavelengths is thus given by the following equation
Im2 /Im1 =(Ii2 /Ii1)exp{-2(&#945;2 -&#945;1)d}
which no longer involves the reflectivities R1 and R2 of the surface. In this equation, all parameters are known except the ratio Ii2 /Ii1 and the distance d to be determined. The ratio Ii2 /Ii1 can be determined using a calibration groove or line of know depth (i.e., a shim 22 could contain a groove of known depth). (Alternatively, a photodiode or other light measuring device can be used to directly measure the intensity of the incident radiation at each wavelength and, therefore, determine the unknown ratio Ii2 /Ii1 in the above equation.) Solving the above equation for d yields the following equation
d=1n{(Im1 Ii2)/(Im2 Ii1)}/{2(&#945;2 -&#945;1)}
for each pixel location, which is independent of the reflectivity of the surface. This method for correcting for differences in reflectivity of the test surface is ideally suited computer manipulation of the digitized data.
As noted, this just described method for correcting for differences in reflectivity requires making measurements at two separate averaged wavelengths λ1 and λ2. In the above described procedure, the filter 52 was moved in and out of the image path between the surface of interest and the camera 34. Other procedures could be used to obtain the data at the two wavelengths. For example, two different filters with different spectral characteristics could be used. Or the filter 52 or different filters could be placed between the light source 32 and the surface of interest. The actual method by which the measurements at the two separate averaged wavelengths λ1 and λ2 are obtained is not critical.
Provision of the test part surface image in digital form suitable for storage and processing in accordance with the present invention readily accommodates calibration. For example, gain associated with each pixel of the surface image can be obtained and employed during operation in a manner analogous to that disclosed in U.S. Pat. No. 4,960,999 which is assigned to the same assignee as the present application and which is hereby incorporated by reference. Because the test part surface may not be uniformly illuminated by the light source 32, or the response of the camera elements may be spatially non-uniform, the system of the present invention preferably includes the capability of correcting for non-uniform illumination and/or detector response. Specifically and as illustrated in FIG. 5, if during a measurement the illumination geometry does not change and the strength of the illumination is held constant, spacial variations in illumination uniformity are accommodated by placing an object 56 having a surface 58 of known uniform reflectance in place of the test object on master surface 18 without the presence of attenuating fluid. The reflected image can then be measured and used to create a two-dimensional map of correction data to normalize the reflected image pixels during a test operation with the fluid in place. This two-dimensional map of correction data need only be reobtained if system geometry or detector characteristics change. For a system in which the test part surface occupies a large portion of the field of view of the camera, the light path through the dye film may not be perpendicular to the master and test surfaces across the entire image. However, such non-uniform optical path lengths can readily be accommodated through calibration techniques and generation of correction maps in a manner similar to that described immediately above as long as the size of the test part and the physical positioning of the light source, test part, and camera remain constant.
As one skilled in the art will realize, the manufacture of master support and its master surface will become more expensive and technically difficult as the dimensions of the test part increases. It would be desirable, therefore, to provide a technique and apparatus wherein the master support and its master surface is not required so that the inventive methods of the present invention can be more easily applied to very large parts such as, for example, automobile doors and the like. This can be accomplished by placing the part to be evaluated in a container of the attenuating fluid (e.g., dyed fluid) whereby the part is completely submerged in the attenuating fluid. FIG. 6 illustrates the gauging apparatus of the present invention where the test part is submerged within the attenuating fluid and the interface of the attenuating fluid and air is used as the reference surface. As shown in FIG. 6, the test part 114 is placed or submerged in the attenuating fluid 120 contained in fluid tank 170. Electromagnetic radiation from electromagnetic radiation sources 132 is passed through the attenuating fluid/air interface 172, through the attenuating fluid, striking the test part surface 112, and then directed to the image sensor 134 (e.g., a camera). The attenuating fluid/air interface acts as, and replaces, the reference or master surface 18 of FIGS. 1, 2, and 5. The electromagnetic radiation is attenuated as it passes through the attenuating fluid and the amount of attenuation is dependent on the distance traveled. By measuring the attenuated electromagnetic radiation reflected from the surface 112, the deviations from a known geometry can be determined using the same procedures as described in this specification. In other words, the data from the image sensor 134 is digitized and then sent to, and manipulated by, a computer (not shown) in the same manner as described above. Various filters (also not shown) can also be used in the same manner as described above.
In operation, test part 114 is placed in the tank 170 of attenuating fluid 120. Once the attenuating fluid surface 172 has settled, the flat surface (i.e., the attenuating fluid/air interface) is suitable for use as a reference or master surface. To decrease the time required for the surface 172 to stabilize and to minimize vibrations of the surface 172 during operation, baffles 174 or other damping devices can be used. For example, the tank 170 could be mounted on a vibration damping pad or support (not shown). Or a thin film of a higher viscosity fluid could be placed on the fluid surface. For example, if water is used as the attenuation fluid 120, a thin layer of oil could be applied to the water surface to reduce waves or ripples. Such a thin film may also serve as an antireflective coating. Examples of suitable attenuating fluid include water, oils, silicone fluids, organic liquids, and the like. Generally fluids of relatively low viscosity and relatively low volatility are preferred. As one skilled in the art will realize, the resolution of the present system using the attenuating fluid/air interface as the reference surface will be dependent, in large part, on the stability of that interface. Thus, measures that enhance that stability, including, for example, minimization of vibrations and reduction of evaporation from the surface, will increase the resolution obtainable and are, therefore, preferred. For attenuating fluids that are relatively volatile, the evaporation can be reduced, and the resolution increased, by applying a thin film of a non-volatile, non-miscible, low density liquid or fluid to the surface of the attenuating fluid. The attenuating effect of such a surface film or layer can be determined during calibration and then accounted for during routine operations.
Once the surface 172 has settled, electromagnetic radiation from source or sources 132 is directed towards the part surface 112 to be evaluated. The electromagnetic radiation passes through the attenuating fluid 120, is reflected off the surface 112, passes again through the attenuating fluid 120, and is collected at the image sensor 134. By measuring the attenuation of the electromagnetic radiation across the surface 112, the surface profile can be determined. If the desired surface profile is flat, the attenuation fluid/air interface can be used as the direct reference surface and deviations from a flat planar surface can be measured directly. If the desired surface geometry deviates from planar, a calibrated or reference test part can be used to calibrate the instrument. The surface profile of the calibrated test part can be stored in the computer and then compared to the data generated from test part 114 to determine deviations from the desired surface profile.
FIG. 7 illustrates the use of the attenuating fluid/air interface as the reference surface where the test part has a depression, valley, or basin that can receive and hold the attenuation fluid (i.e., a fluid receptacle). As shown in FIG. 7, a depression 180 of a test part 114 is filled with attenuation fluid 120. The attenuation fluid/air interface 172 acts as the reference or master surface in the same manner as described for FIG. 6 above. In this manner, the surface profile 112 of the depression 180 can be determined. As one skilled in the art will realize, this modified method can generally only be used to evaluate surfaces within depressions 180 which can contain the attenuating fluid 120. Thus, this modified method is not suitable for evaluation of the flat portions 182 of the test part or other portions that could not hold and contain the attenuating fluid. This method could be used to advantage for large parts where the critical dimensions or profiles of interest are contained, or predominantly contained, within depressions or hollowed-out areas in the test part.
FIG. 8 illustrates the type of results that the present system can generate. A test part was prepared by machining a series of parallel grooves of varying depth in a metal block. In addition, on a portion of the flat surface between two grooves, a shallow, long depression was cut to simulate a surface defect. Using visible light with a dye fluid (i.e., india ink) as the attenuation medium, the image of the surface of this test part was generated using the present invention. A portion of the resulting image is shown in FIG. 8 where the x-axis is labeled 94, the y-axis is labeled 96, and the z-axis is labeled 98. The units for all three axes are given in inches. The five grooves cut in the test surface can clearly be seen: groove 82 is 0.0001 inches deep; groove 84 is 0.0002 inches deep; groove 86 is 0.0003 inches deep; groove 88 is 0.0004 inches deep; and groove 90 is 0.0005 inches deep. The simulated surface defect 92 is seen between grooves 82 and 84. As shown in FIG. 8, depressions as small as 0.0001 inches can readily be observed and measured using the present invention.
where I(x,y) is the uncorrected intensity at point (x,y) and θ is the angle between the camera's optical axis and the light ray from the camera to point (x,y). When the angle θ is small this correction is also small and can, therefore, be disregarded. Thus, with relatively small parts, which can fit into a narrow portion of the camera's optical field, this correction can usually be omitted except where the highest degree of accuracy is needed. Even for larger parts, the camera can be moved relative to the part's surface and multiple images of the surface taken such that all surfaces of interest are contained and recorded within a narrow portion of the camera's optical field.
The deviations from a nominal surface geometry for transparent parts or low reflectivity parts can also measured by first coating the surface to be gauged with a reflective coating. For example, the surface of a glass part could be coated with a thin silver coating. Such a coating could be removed after the measurements are completed (e.g., a silver coating could be removed by an acid wash). As one skilled in the art will realize, such a coating should be as thin as practical to avoid significant loss of resolution which could result from the coating "filling in" or "bridging" depressions and the like in the surface.
The resolution of the system (especially for the non-depth portion) is determined in large part by camera geometry. For example, if a CCD camera with a 512�512 element array were used to image a surface 50 cm�50 cm, each pixel would correspond to about 1 mm2 of the surface. The resolution of the system can be decreased or increased as needed using various techniques. For example, a CCD camera with a larger array could be used. If the image array of such a camera was increased to 1024�1024 elements, each pixel would correspond to about 0.24 mm2 of the same 50 cm�50 cm surface (i.e., approximately four fold increase in resolution). Resolution may also be modified by changing the effective focal length of the camera lens. By moving camera 34 closer to the test surface 12 (i.e., moving the camera in the vertical direction in FIG. 2) will increase the resolution but will decrease the percentage of the test part surface that can be observed with a given measurement. To obtain full analysis or coverage of the test part surface it may be necessary, in such a case, to take multiple measurements for a given part. Such multiple measurements could be made by moving the master surface and test part while holding the camera fixed or, preferable, by moving the camera into the desired positions (i.e., moving the camera in the horizontal direction in FIG. 2) using translator 60 (as shown in FIG. 2) to obtain complete coverage of the test part surface. By combining the measurements, analysis of the entire surface can be obtained. Translator 60 can also be used to vary the distance between the camera 34 and the test part surface 12. If desired, separate translators can be used to control movement of the camera in the vertical and horizontal directions. Preferably the translator 60 or translators are under computer control. The effective focal length and, therefore, camera resolution can also be modified by use of a zoom-type lens on the camera 34. Such a lens would eliminate the need for movement of the camera in the vertical direction. Again, it is preferred that the zoom-type lens is under computer control. For these general purposes, camera 34 is coupled to a translator 60, as shown in FIG. 2, which is controlled by computer 42. Although not shown, similar techniques and equipment can be used in the modified gauging systems shown in FIGS. 6 and 7.
If it is desired to measure the flatness of the surface of a machined part to high tolerances, an optical flat can be used as the master reference surface. In order to protect the optical surface--which is generally relatively expensive--from potential damage caused by placing a machined part in contact with the optical surface, a plurality of thin masks, shims, or spacers 22 may be placed between the two objects. These shims 22 would typically be placed between the optical surface and the surface to be measured at known fixed points at which surface deviations do not need to be measured. Such shims 22 are illustrated in FIG. 2. In some cases it may not be possible to locate the shims at positions where surface deviations do not need to be measured. In such cases, two different measurements can be made with the shims at different positions to obtain complete coverage of the surface of interest. Alternatively, a jig system that contains mechanical stand-offs or a mechanical fixture 108 (see FIG. 1) can be used to hold the part and prevent the object's surface from coming into contact with the surface of the optical flat.
As noted above, optical flats are relatively expensive to prepare and can be damaged if the optical flat and the test part surfaces come into contact. Shims 22, as noted above, are one way to minimize damage to the optical flat used as the master surface. As one skilled in the art will realize, however, the master surface will eventually be damaged during use and the probability of damage will increase as the number of parts tested increases. Another way in which to minimize damage to the optical flat is to simply eliminate its use as the master surface. Rather a commercial-grade glass plate (e.g., float plate glass) can be used as the master surface and the optical flat can be used as a "standard" test part to calibrate the glass plate. By placing the "standard" test part on the master surfaces, preferably with shims 22 supporting the "standard" test part, the differences between the glass plate and the optical flat can be measured and stored in the computer. By measuring actual test parts against the glass plate and using the stored optical flat calibration data, the actual test parts can be compared to the optical flat without exposing the optical flat "standard" test part to potential damage. Recalibration using the "standard" test part will be necessary from time to time to simply check the system's operating characteristics or whenever the glass plate master surface is replaced. In any event, exposure of the relatively expensive optical flat test part to potential damage will be significantly reduced.
Another method for avoiding the preparation and expense of an optical flat or other master surface, and for avoiding damage to such a master system, is to use the system shown in FIGS. 6 and 7 and discussed above wherein the attenuating fluid/air interface is used as the reference or master surface.
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