CA1179036A - Method and apparatus for detecting surface defects in mechanical workpieces - Google Patents
Method and apparatus for detecting surface defects in mechanical workpiecesInfo
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- CA1179036A CA1179036A CA000395907A CA395907A CA1179036A CA 1179036 A CA1179036 A CA 1179036A CA 000395907 A CA000395907 A CA 000395907A CA 395907 A CA395907 A CA 395907A CA 1179036 A CA1179036 A CA 1179036A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/84—Systems specially adapted for particular applications
- G01N21/88—Investigating the presence of flaws or contamination
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- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Materials By The Use Of Optical Means Adapted For Particular Applications (AREA)
- Length Measuring Devices By Optical Means (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
The detection of surface defects in mechanical parts is effected by the analysis of diffracted light coming from these defects. The detection method includes the steps of illuminating the surface of the part to be examined with incoherent light and forming a flat image of the said surface in a transparent photosensitive layer of a spatial light modulator. The spatial distribution of the intensity of the incoherent light reflected by the said surface produces a corresponding and proportional spatial distri-bution of values of refractive index in the photosensitive layer. This layer is illuminated with plane polarized, coherent light by frame scanning in elementary areas.
during the scanning operation, the variations in at least one of the polarization components of the coherent light are detected by a matrix of converters after the light has traversed the said transparent photosensitive medium.
The detection of surface defects in mechanical parts is effected by the analysis of diffracted light coming from these defects. The detection method includes the steps of illuminating the surface of the part to be examined with incoherent light and forming a flat image of the said surface in a transparent photosensitive layer of a spatial light modulator. The spatial distribution of the intensity of the incoherent light reflected by the said surface produces a corresponding and proportional spatial distri-bution of values of refractive index in the photosensitive layer. This layer is illuminated with plane polarized, coherent light by frame scanning in elementary areas.
during the scanning operation, the variations in at least one of the polarization components of the coherent light are detected by a matrix of converters after the light has traversed the said transparent photosensitive medium.
Description
I ~ i7~)3~
Field of the Invention `The present invention relates in general to a method of . detecting surface defects in mechanical parts, particularly mechanical parts with curved surfaces.
In particular, the invention relates to amethod based.on the analysis of diffracted light coming from surface defects, this analysis being carried out by observing the changes in the characteristics of a coherent electromagnetic wave in the plane of the spatial frequencies or Fourier plane.
Background of the Invention The analysis of di~fracted light coming from surface defects is a known technique at present in use for detecting and identifying surface defects (cracks, hairline cracks, flaws, cuts,scratches and the like) in mechanical parts. This analysis is normally carried out by illuminating the surface to be examined with even, coherent light and subsequently detecting the spatial intensity distribution of the radiation reflected from the surface subjected to examination.
According to one current implementation, the analysis of the diffracted light coming from the surface defect is carried out by means of a device comprising a first optical transmission sys-tem arranged to irradiate the surface to be examined with even, coherent light, and a second optical receiving system~ for example a visual display or photoelectric detecting matrix symmetrically disposed relative to the first optical system with respect to the direction normal to the surface to be e~amined, and arranged to derive information on the spatial intensity distr.ibution. of the radiation reflected from the surface under examination.
The theorectical basis of this type of analysis is fully discussed, together with several examples of its possible uses, in chapters 4 and 7 of the work ~ ~L79~3~
.
1'Introduction to Fourier Optics" by Joseph W. Goodman-McGraw-Hill editions, 1968.
The analysis of diffracted light coming from surfàce defects, carriedoutbytheprocesses described previously,is limited in use to checking flat mechanical parts and is not suitable for the qualitative checking ~fparts with curved surfaces, particularly of parts which have surfaces which do not possess zero curvature in at least one direction (surfaces with double curvature).
In s,u¢hparts there may be observed a continuous variation, dependent on to the portion examined, in the direction of the normal to the surface, with consequent continuous variation of the direction of propagation of the radiation reflected from the surface under examination. In order to allow the analysis of the diffracted light coming from surface defects in mechanical parts of this type according to the known processess, it is thus necessary to '7track" the radiation reflected from the part in space, as well as to ensure the continuously correct positioning of the analysis device at a rigidly predetermined distance from the surface to be checked.
These operating requirements cannot be achieved in practice outside the laboratory and are totally inapplicable to industrial quality con-trol proc~sses, particularly when this control must be effected on all the parts produced.
Moreover, whenever the,parts to be inspected have a considerable size, for example motor vehicle bodywork or portions thereof subjected to painting treatments or surface protection, the need to scan the entire surface of the part makes it practically impossible to car~ out a qualitative check at a frequency compatible with industrial production times.
Object of the Invention The object of the present invention is to provide a method which enables the quality control of the surface I 17~V3 finish of mechanical parts with curv~d surfaces and/or lar~e dimensions to be carried out rapdily and - precisely.
The present invention has as its subject a method of detecting surface defects in mechanical parts, particularly mechanical parts with curved sur~aces, by the analysis of diffracted light cominq from these surface defects, said method including the steps of:
a) illuminating, with incoherent light, the su~-face of the part in which it is desired to detect defects, b) forming a flat image of the said surface in a transparent photosensitive medium in which the spatial intensity distribution of the incoherent light re~lected from the said surface produces a correspond-ing and proportional spatial distribution of values of the ~efractive index, c) illuminating the transparent photosensitive medium with plane polarized aoherent light by frame scanning in elementary areas, d) during the scanning operation, detecting the varations in at least one of the polarization components of the coherent light after the light has traversed the said transparent photosensitive medium, e) ~leriving from these variations information regarding any surface defects, According to the method specified above, the analysis of the diffracted light coming from the surfacesof a mechanical part is effected by examining a flat image of the surface t~ be checked, thus eliminating the disadvantages described above which occur when the part to be checked has a curved surface, particularly with double curvature.
The invention further relates toapparatus for carry-ing out the method described above, comprising:
3 ~ 3 ~
- a) means for illuminatiny with incoherent light, the surface of the part in which it is desired to detect defect~s, ; b) a spatial light modulator comprising at least one layer of transparent photosensitive material in which a spatial intensity distribution of incoherent light produces a corresponding and proportional spatial distribution of refractive index values, c) an optical system interposed between the surface of the part in which it is desired to detect defects and the spatial light modulator, the said optical system being arranged to form a flat image of this surface on the said layer of transparent photosensitive material, d) a source of plane polarized coherent light, e) means for displacin~ the said coherent light relative to the spatial light modulator so that the light illuminates the said layer of transparent photosensitive material ~y frame scanning in ~lementary areas, f) detector means arranged, during the scanning operation, to detect variations in at least one of the polarization components of the coherent light after its has traversed the said layer of transparent photosensitive material.
Preferably the apparatus according to the invention further includes electronic processing means connected to the detector méans and arranged to derive information relating to any surface defects of the part from the variations in at least one of the polarization components of the coherent light.
Theoretlcal basis of the invention The spatial light modulator (spatial light modulator-SLM-or according to another common expression in the art "light valve") is a device of considerable interest for processing optical systems in "real time".
It is generally constituted by a flat support on which is disposed a transparent layer of a material which can 3 Ç~
modify its transmission characteristics for electromagne--tic waves, in particula~ its refractive index, in dependence on the intensity of incoherent light incident on its surface.
The local variation in the value of the refractive index may result from manifestations of different physical phenomena. In a first class of devices, known in the art by the term "PROM-Pockels Readout Optical Modulator-, a layer of photo-conductive material is interposed between two flat transparent electrodes to which a polarizing voltage is applied by means of an external generator. Under these conditions, the variation in conductivity in dependence on the intensity of light incident on the device induces a proportional variation in the refractive index of the material, based on the effect known as the linear electro-optic effect or Pockels effect, which is able to modify the phase characteristics and hence the polarization of coherent light which is propagated within the material.
More particularly, when the light incident on the device has a non-uniform spatial intensity distribution, the corresponding and proportional spatial distribution of refractive index values constitutes an image, generally of the high definition type, of the source of the incoherentlight radiation which can even be constituted by a partially reflecting object illuminated by a normal incandescent or fluorescent source. This image may be"read" in a non-destructive manner by causing plane polarized coherent light to fall on the device and detec-ting the variation in the polarization characteristics of this coherent light after it has traversed the layer of photosensitive material. The "reading" operation may be effected by frame scanning of the layer of photosensitive material in elementary areas (for example lines) according to the criteriacurrently used in television camera equipment.
The spatial light modulator is thus an optical-optical ~ ~903~
type converter able to convert optical information of an incoherent type into optical information of a coherent type.
The image stored in the device may be cancelled by inverting the polarizing voltage applied to the two transparent electrodes between which the photo-sensitive layer is interposed. Alternatively, cancelling may be achieved by illuminating the photo-sensitive layer of material with high intensity~ spatially uniform incoherent light (floodlight).
In other spatial light modulators which are different from the PROM described above, the variation in the refractive index is achieved by producingmanifestations of theelectro-optic effect in materials such as liquid crystals (SLM), photo-dicroic and ferro-electric materials. There are also spatial light modulators in which the image is stored in the form of deformations in the photo-sensitive material layer, thus modifying the length~of the optical path, and hence the polarization of the plane polarized coherent light used for the reading operation.
Further information on the theoretical basis and criteria of use of spatial light modulators is contained in the article "Spatial Light Modulators"by D. Casasent- Proceedings of the IEEE, Vol, ~5, No. 1, January 1977~ pages 1~3-157, and also in the article "Realtime Spatial Light Modulators" by B. Schneeberger, F. Laeri, T. Tschudi-and F. Mast, Optics Communications, Vol. 31, Nv. 1, October 1979 pages 13-15.
Descriptionofone ~ rred embodiment of the invention A preferred embodiment of the invention will now be described with reference to the appended drawings, provided purely by way of non-limiting example, in which:
Figure 1 is a schematic view of apparatus for carrying out the method according to the invention, and Figure 2 is a schematic representation of the 3 ~ 6 structure of a spatial light modulator used in the apparatus.
.
- ~ Referring to Figure 1, S indicates the surface Of a part to be checked, for example a part of the bodywork of a motor vehicle.
By 10 isshown a normal light source~ for example a tungsten lamp, arranged to illuminate with incoherent light, the surface S of the part to be checkedO
n optical system 12 is arranged to form an ima~e 10 o~ the surface S to be checked on a spatial light moaulator 14. Advantage~usly ! the optical system 12 is constituted by an objective, able to form a reauced image of the surface S on the spatial light modulator 14, making it possible and easy to check qualitatively 15 parts with large dimensions. Prefe~bly the objective is of the type with a laxge depth of field, that is, an objective with a low ratio of the focal length to the diagonal of the image format,such an objective enables thè elimination of effects on the accuracy of 20 the checkwhichresult from variations in distance from the device of different points on the part itself or of different parts which are to be checked one after another.
The spatial light modulator 14, which is of a type 25 known per se, preferably of the PROM type described above, is formed with the structure illustrated schematically in Figure 2. This spatial light modulator 14 basically comprises:
a) a layer 16 of transparent photo-sensitive 30 material in which a spatial intensity distribution of incoherent light induces a corresponding and proportional spatial distribution of values of the refractive index, b) a first flat electrode 18, transparent to incoherent radiation reflecte~ from the surface S of 35 the part to be checked, c) a flat dielectric, semi-transparent mirror20, interposed between the layer of photo-sensitive material 16 and the first flat electrode 18, the reflecting surface 9 ~1 3 ~
~ 8 --of the said dielectric mirror facing the layer of transparent photo-sensitive material 16, d)~ a second flat transparent electrode 22 facing the surface of the layer of transparent photo-sensitive material 16 opposite the flat dielectric mirror 20land e) a supply unit 24 arranged to apply at least two different voltage levels between the first and the second transparent electrode 18, 22 respectively.
The first voltage level corresponds to the conditions in which, according to the method described above, photo-conductivity characteristics are exibited by the layer of transparent photo-sensitive material 16 while the second voltage level is that which induces the cancellation of the image stored in this layer of transparent photo-sensitive material 16.
The supply unit 24 controls the memorising and cancelling functions for the image formed on the layer of photo-sensitive material 16, allowing the sequential examinatio~ of different spatial light intensity distributions corresponding to images of surfaces of n~chanical parts successively ta]cen by the objective 12.
A source of plane polarized coherent light (laser) of known type is indicated 26. The light produced by the source 26 is directed towards the spatial light modulator 14 by means of an optical system which enables this radiation to be displaced relative to the moduLator 14. This optical system comprises:
a~ a first mirror 2~arrangedtodeflect the coherent light produced by the source 26, b) a second mirror 30 arranged to intercept the light deflected by the first mirror 28 and to deflect it in a direction substantially perpendicular to -the surface of the photo-sensitive layer 16 of the spatial light modulator 14, c) a cylindrical lens 32 located between the first and the second mirrors 28, 30 respectively and having its focus F at the point on the first mirror 28 whereat 3 ~.79(~3~ .
. g --the light produced by the source 26 is insident, d) a first drive 34 for oscillating the first mirror~28 about an axis A ~erpendicular to the line L of the cylindrical lens 32 and to the direction of the light which is emitted by the source 26 and passes through the focus F of the lens 32, and e) a second drive 36 for oscillating the second mirror 30 about an axis B which intersects the line L of the cylindrical lens 32 and lies in a plane 10 perpendicular to the axis A.
The plane polarized coherent light is deflected by the second mirror 30 towards the spatial light modulator 14 so that the light, after having traversed the second transparent electrode 22, is incident on 15 the layer of transparent photo-sensitive material 16 in a direction substantially perpendicular to the surface of this layer 16~
After having traversed the layer 16, the coherent light is reflected from the semi-transparent mirror 20, 20 passes again through the layer 16 and leaves the spatial light modulator 14. The coherent light leaving the spatial modulator 14 is reflected from a semi-t~ansparent mirror 38, located between the spatial light modualtor 14 and the second mirror 30andisdeflected towards a 25 normal optical analyser 40 constituted for example by a polarizer.
By 42 is shown schematically a matrix of photoelectric converters which is disposed in series with the analyser 40 and is arranged to produce at the output of each 30 converter an electric signal indicative of the intensity of the light incident on the converter 42. Between the optical analyser 40 and the matrix of photoelectric converters42isinterposed a lens 44 arranged to direct the light leaving the analyser 40 onto the matrix of 35 photoelectric aonverters 42.
- An electronic processing circuit indicated by 46 is fed with the signals output from the matrix of 3 ~
photoelectric conve~ters42.The circuit 46 is arra~ged to form an.axray of numerical values corresponding to the~spatial intensity distribution of the light incident on the matrix of photoelectric converters.42.
The combination comprising the optical analyser -40, the matrix of photoelectric converters,42the lens 44 and the electronic processing circui~ 46 constitute a system arranged, during scanning in elementary areas of the layer of transparent photo-sensitive material 16,to detect variations in the light intensity which occur along the direction of one of the polarization components of the coherent radiation after it has - traversed this layer of transparent photo-sensitive ~ material 16.
According to a simplified embodiment of the apparatus,not illustrated in the drawings, the matrix of photoelectric counters 42 and the electronic processing circuit 46 may be replaced by an ordinary polished screen adapted to allow the observation of the spatial intensity distribution of the coherent light emerging from the optical analyser 40.
According to the embodiment illustrated in Figure 1, the apparatus according to the invention further includes a.logic control unit 48 connected to the electronic processing circuit46,~ the drives 34 and 36, and to the supply unit 24 for the spatial light modulator 14. This logic control unit 48 is preferably constituted by a micro-processor system which is able to follow the operations of the electronic processing circuit 46.
Description of the operation of the device After the suxface S to be checked has been correctly framed and brought into focus by the objective 12, the logic control unit 48 controlsthe supply unit 24 of the spatial light modulator 14 to allow storage of the image of the surface S to be checked in the layel- of transparent photo-sensitive material 16. Simultaneously, ~ ~903~
or after a predetermined period of time, the logic control unit 48 activates the drives 34 and 36 to oseillate the $irst and second mirrors 28, 30 respeetively, starting the scanning in elementary areas of the layer of transparent photo-sensitive material 16.
When, during the scanning operation, the coherent light passes through elementary areas of the layer of transparent photo-sensitive material 16 corresponding to portions of the surface to be checked which are free from de~ects (or, possibly corresponding to surfaces of a sample part free from defects), the light incident on the matrix of photoelectric converters 42 has a spatial intensity distribution which is taken as a reference. For example, when the surfaces are subjeeted to a painting treatment (parts of vehicle bodywork), the reference distribution is comparable to a luminous spot located in the centre of the photoelectric eonverter matrix 42, which corresponds to the origin of the plane of the spatial frequencies (Fourier plane) represented by the surfaee of the matrix of photo--electric converters 42.
The presence of a defect on the surface S to be checked produees a variation in the spatial distribution of the intensity of the light incident on the matrix 25 -of photoelectric conYerters 42,giving this distribution an elongate or irregular geometry, or at least a geo~etry different from that taken as the re~erence.
This variation is detected by the eleetronie processing circuit 46 and signalled to the logic eontrol unit 48 which, being connected to the scanning means (mirrors ~8r 30 and drives 34, 36) identifies the elementary area of the layer of photo-sensitive material 16 and, consequently, the portion of the surface S to be cheeked on whieh the presenee of a defect has been found, emitting a corresponding alarm signal.
The electronic processing circuit 46 is also able to identify, on the basis of algorithms of kno~n type, 1 ~7~V3~
the type oE defect (score, hole, crack etc.) found, trom the particular spatial intensity distribution of ~-he light incident on the matrix of photoelectric converters42~in the presence of the defect.
At the end of the scanning operation, the logic --control unit 48, through the supply unit 24, is arrangèd to cancel the image stored in the spatial light modulator 14, and simultaneously signal its availability to effect a further checking cycle.
The next cheeking cyele may be effeeted on a meehan~ical part different from that checked previously, or on another portion of the part checked during the preceding eycle whenever, as in the case of the quality eontrol of a vehicle body subjected to painting, the 15 dimensions of the part to be checked are considerable so that the part cannot be brought completely within the field of view of the objective 12.
In this ease the apparatus mayusefully be eonneeted to an automatic arrangement for relatively positioning 20the apparatus- itself with respect to the part to be ehecked, whereby to render the carrying through of the checking operation completely automatic.
Naturally, the prinsiple of the invention remaining the same, the details of construction and the embodiments 25 of theapparatus ~ay be varied widely with respect to that described and illustrated, without thereby departing from the seope of the present invention.
Field of the Invention `The present invention relates in general to a method of . detecting surface defects in mechanical parts, particularly mechanical parts with curved surfaces.
In particular, the invention relates to amethod based.on the analysis of diffracted light coming from surface defects, this analysis being carried out by observing the changes in the characteristics of a coherent electromagnetic wave in the plane of the spatial frequencies or Fourier plane.
Background of the Invention The analysis of di~fracted light coming from surface defects is a known technique at present in use for detecting and identifying surface defects (cracks, hairline cracks, flaws, cuts,scratches and the like) in mechanical parts. This analysis is normally carried out by illuminating the surface to be examined with even, coherent light and subsequently detecting the spatial intensity distribution of the radiation reflected from the surface subjected to examination.
According to one current implementation, the analysis of the diffracted light coming from the surface defect is carried out by means of a device comprising a first optical transmission sys-tem arranged to irradiate the surface to be examined with even, coherent light, and a second optical receiving system~ for example a visual display or photoelectric detecting matrix symmetrically disposed relative to the first optical system with respect to the direction normal to the surface to be e~amined, and arranged to derive information on the spatial intensity distr.ibution. of the radiation reflected from the surface under examination.
The theorectical basis of this type of analysis is fully discussed, together with several examples of its possible uses, in chapters 4 and 7 of the work ~ ~L79~3~
.
1'Introduction to Fourier Optics" by Joseph W. Goodman-McGraw-Hill editions, 1968.
The analysis of diffracted light coming from surfàce defects, carriedoutbytheprocesses described previously,is limited in use to checking flat mechanical parts and is not suitable for the qualitative checking ~fparts with curved surfaces, particularly of parts which have surfaces which do not possess zero curvature in at least one direction (surfaces with double curvature).
In s,u¢hparts there may be observed a continuous variation, dependent on to the portion examined, in the direction of the normal to the surface, with consequent continuous variation of the direction of propagation of the radiation reflected from the surface under examination. In order to allow the analysis of the diffracted light coming from surface defects in mechanical parts of this type according to the known processess, it is thus necessary to '7track" the radiation reflected from the part in space, as well as to ensure the continuously correct positioning of the analysis device at a rigidly predetermined distance from the surface to be checked.
These operating requirements cannot be achieved in practice outside the laboratory and are totally inapplicable to industrial quality con-trol proc~sses, particularly when this control must be effected on all the parts produced.
Moreover, whenever the,parts to be inspected have a considerable size, for example motor vehicle bodywork or portions thereof subjected to painting treatments or surface protection, the need to scan the entire surface of the part makes it practically impossible to car~ out a qualitative check at a frequency compatible with industrial production times.
Object of the Invention The object of the present invention is to provide a method which enables the quality control of the surface I 17~V3 finish of mechanical parts with curv~d surfaces and/or lar~e dimensions to be carried out rapdily and - precisely.
The present invention has as its subject a method of detecting surface defects in mechanical parts, particularly mechanical parts with curved sur~aces, by the analysis of diffracted light cominq from these surface defects, said method including the steps of:
a) illuminating, with incoherent light, the su~-face of the part in which it is desired to detect defects, b) forming a flat image of the said surface in a transparent photosensitive medium in which the spatial intensity distribution of the incoherent light re~lected from the said surface produces a correspond-ing and proportional spatial distribution of values of the ~efractive index, c) illuminating the transparent photosensitive medium with plane polarized aoherent light by frame scanning in elementary areas, d) during the scanning operation, detecting the varations in at least one of the polarization components of the coherent light after the light has traversed the said transparent photosensitive medium, e) ~leriving from these variations information regarding any surface defects, According to the method specified above, the analysis of the diffracted light coming from the surfacesof a mechanical part is effected by examining a flat image of the surface t~ be checked, thus eliminating the disadvantages described above which occur when the part to be checked has a curved surface, particularly with double curvature.
The invention further relates toapparatus for carry-ing out the method described above, comprising:
3 ~ 3 ~
- a) means for illuminatiny with incoherent light, the surface of the part in which it is desired to detect defect~s, ; b) a spatial light modulator comprising at least one layer of transparent photosensitive material in which a spatial intensity distribution of incoherent light produces a corresponding and proportional spatial distribution of refractive index values, c) an optical system interposed between the surface of the part in which it is desired to detect defects and the spatial light modulator, the said optical system being arranged to form a flat image of this surface on the said layer of transparent photosensitive material, d) a source of plane polarized coherent light, e) means for displacin~ the said coherent light relative to the spatial light modulator so that the light illuminates the said layer of transparent photosensitive material ~y frame scanning in ~lementary areas, f) detector means arranged, during the scanning operation, to detect variations in at least one of the polarization components of the coherent light after its has traversed the said layer of transparent photosensitive material.
Preferably the apparatus according to the invention further includes electronic processing means connected to the detector méans and arranged to derive information relating to any surface defects of the part from the variations in at least one of the polarization components of the coherent light.
Theoretlcal basis of the invention The spatial light modulator (spatial light modulator-SLM-or according to another common expression in the art "light valve") is a device of considerable interest for processing optical systems in "real time".
It is generally constituted by a flat support on which is disposed a transparent layer of a material which can 3 Ç~
modify its transmission characteristics for electromagne--tic waves, in particula~ its refractive index, in dependence on the intensity of incoherent light incident on its surface.
The local variation in the value of the refractive index may result from manifestations of different physical phenomena. In a first class of devices, known in the art by the term "PROM-Pockels Readout Optical Modulator-, a layer of photo-conductive material is interposed between two flat transparent electrodes to which a polarizing voltage is applied by means of an external generator. Under these conditions, the variation in conductivity in dependence on the intensity of light incident on the device induces a proportional variation in the refractive index of the material, based on the effect known as the linear electro-optic effect or Pockels effect, which is able to modify the phase characteristics and hence the polarization of coherent light which is propagated within the material.
More particularly, when the light incident on the device has a non-uniform spatial intensity distribution, the corresponding and proportional spatial distribution of refractive index values constitutes an image, generally of the high definition type, of the source of the incoherentlight radiation which can even be constituted by a partially reflecting object illuminated by a normal incandescent or fluorescent source. This image may be"read" in a non-destructive manner by causing plane polarized coherent light to fall on the device and detec-ting the variation in the polarization characteristics of this coherent light after it has traversed the layer of photosensitive material. The "reading" operation may be effected by frame scanning of the layer of photosensitive material in elementary areas (for example lines) according to the criteriacurrently used in television camera equipment.
The spatial light modulator is thus an optical-optical ~ ~903~
type converter able to convert optical information of an incoherent type into optical information of a coherent type.
The image stored in the device may be cancelled by inverting the polarizing voltage applied to the two transparent electrodes between which the photo-sensitive layer is interposed. Alternatively, cancelling may be achieved by illuminating the photo-sensitive layer of material with high intensity~ spatially uniform incoherent light (floodlight).
In other spatial light modulators which are different from the PROM described above, the variation in the refractive index is achieved by producingmanifestations of theelectro-optic effect in materials such as liquid crystals (SLM), photo-dicroic and ferro-electric materials. There are also spatial light modulators in which the image is stored in the form of deformations in the photo-sensitive material layer, thus modifying the length~of the optical path, and hence the polarization of the plane polarized coherent light used for the reading operation.
Further information on the theoretical basis and criteria of use of spatial light modulators is contained in the article "Spatial Light Modulators"by D. Casasent- Proceedings of the IEEE, Vol, ~5, No. 1, January 1977~ pages 1~3-157, and also in the article "Realtime Spatial Light Modulators" by B. Schneeberger, F. Laeri, T. Tschudi-and F. Mast, Optics Communications, Vol. 31, Nv. 1, October 1979 pages 13-15.
Descriptionofone ~ rred embodiment of the invention A preferred embodiment of the invention will now be described with reference to the appended drawings, provided purely by way of non-limiting example, in which:
Figure 1 is a schematic view of apparatus for carrying out the method according to the invention, and Figure 2 is a schematic representation of the 3 ~ 6 structure of a spatial light modulator used in the apparatus.
.
- ~ Referring to Figure 1, S indicates the surface Of a part to be checked, for example a part of the bodywork of a motor vehicle.
By 10 isshown a normal light source~ for example a tungsten lamp, arranged to illuminate with incoherent light, the surface S of the part to be checkedO
n optical system 12 is arranged to form an ima~e 10 o~ the surface S to be checked on a spatial light moaulator 14. Advantage~usly ! the optical system 12 is constituted by an objective, able to form a reauced image of the surface S on the spatial light modulator 14, making it possible and easy to check qualitatively 15 parts with large dimensions. Prefe~bly the objective is of the type with a laxge depth of field, that is, an objective with a low ratio of the focal length to the diagonal of the image format,such an objective enables thè elimination of effects on the accuracy of 20 the checkwhichresult from variations in distance from the device of different points on the part itself or of different parts which are to be checked one after another.
The spatial light modulator 14, which is of a type 25 known per se, preferably of the PROM type described above, is formed with the structure illustrated schematically in Figure 2. This spatial light modulator 14 basically comprises:
a) a layer 16 of transparent photo-sensitive 30 material in which a spatial intensity distribution of incoherent light induces a corresponding and proportional spatial distribution of values of the refractive index, b) a first flat electrode 18, transparent to incoherent radiation reflecte~ from the surface S of 35 the part to be checked, c) a flat dielectric, semi-transparent mirror20, interposed between the layer of photo-sensitive material 16 and the first flat electrode 18, the reflecting surface 9 ~1 3 ~
~ 8 --of the said dielectric mirror facing the layer of transparent photo-sensitive material 16, d)~ a second flat transparent electrode 22 facing the surface of the layer of transparent photo-sensitive material 16 opposite the flat dielectric mirror 20land e) a supply unit 24 arranged to apply at least two different voltage levels between the first and the second transparent electrode 18, 22 respectively.
The first voltage level corresponds to the conditions in which, according to the method described above, photo-conductivity characteristics are exibited by the layer of transparent photo-sensitive material 16 while the second voltage level is that which induces the cancellation of the image stored in this layer of transparent photo-sensitive material 16.
The supply unit 24 controls the memorising and cancelling functions for the image formed on the layer of photo-sensitive material 16, allowing the sequential examinatio~ of different spatial light intensity distributions corresponding to images of surfaces of n~chanical parts successively ta]cen by the objective 12.
A source of plane polarized coherent light (laser) of known type is indicated 26. The light produced by the source 26 is directed towards the spatial light modulator 14 by means of an optical system which enables this radiation to be displaced relative to the moduLator 14. This optical system comprises:
a~ a first mirror 2~arrangedtodeflect the coherent light produced by the source 26, b) a second mirror 30 arranged to intercept the light deflected by the first mirror 28 and to deflect it in a direction substantially perpendicular to -the surface of the photo-sensitive layer 16 of the spatial light modulator 14, c) a cylindrical lens 32 located between the first and the second mirrors 28, 30 respectively and having its focus F at the point on the first mirror 28 whereat 3 ~.79(~3~ .
. g --the light produced by the source 26 is insident, d) a first drive 34 for oscillating the first mirror~28 about an axis A ~erpendicular to the line L of the cylindrical lens 32 and to the direction of the light which is emitted by the source 26 and passes through the focus F of the lens 32, and e) a second drive 36 for oscillating the second mirror 30 about an axis B which intersects the line L of the cylindrical lens 32 and lies in a plane 10 perpendicular to the axis A.
The plane polarized coherent light is deflected by the second mirror 30 towards the spatial light modulator 14 so that the light, after having traversed the second transparent electrode 22, is incident on 15 the layer of transparent photo-sensitive material 16 in a direction substantially perpendicular to the surface of this layer 16~
After having traversed the layer 16, the coherent light is reflected from the semi-transparent mirror 20, 20 passes again through the layer 16 and leaves the spatial light modulator 14. The coherent light leaving the spatial modulator 14 is reflected from a semi-t~ansparent mirror 38, located between the spatial light modualtor 14 and the second mirror 30andisdeflected towards a 25 normal optical analyser 40 constituted for example by a polarizer.
By 42 is shown schematically a matrix of photoelectric converters which is disposed in series with the analyser 40 and is arranged to produce at the output of each 30 converter an electric signal indicative of the intensity of the light incident on the converter 42. Between the optical analyser 40 and the matrix of photoelectric converters42isinterposed a lens 44 arranged to direct the light leaving the analyser 40 onto the matrix of 35 photoelectric aonverters 42.
- An electronic processing circuit indicated by 46 is fed with the signals output from the matrix of 3 ~
photoelectric conve~ters42.The circuit 46 is arra~ged to form an.axray of numerical values corresponding to the~spatial intensity distribution of the light incident on the matrix of photoelectric converters.42.
The combination comprising the optical analyser -40, the matrix of photoelectric converters,42the lens 44 and the electronic processing circui~ 46 constitute a system arranged, during scanning in elementary areas of the layer of transparent photo-sensitive material 16,to detect variations in the light intensity which occur along the direction of one of the polarization components of the coherent radiation after it has - traversed this layer of transparent photo-sensitive ~ material 16.
According to a simplified embodiment of the apparatus,not illustrated in the drawings, the matrix of photoelectric counters 42 and the electronic processing circuit 46 may be replaced by an ordinary polished screen adapted to allow the observation of the spatial intensity distribution of the coherent light emerging from the optical analyser 40.
According to the embodiment illustrated in Figure 1, the apparatus according to the invention further includes a.logic control unit 48 connected to the electronic processing circuit46,~ the drives 34 and 36, and to the supply unit 24 for the spatial light modulator 14. This logic control unit 48 is preferably constituted by a micro-processor system which is able to follow the operations of the electronic processing circuit 46.
Description of the operation of the device After the suxface S to be checked has been correctly framed and brought into focus by the objective 12, the logic control unit 48 controlsthe supply unit 24 of the spatial light modulator 14 to allow storage of the image of the surface S to be checked in the layel- of transparent photo-sensitive material 16. Simultaneously, ~ ~903~
or after a predetermined period of time, the logic control unit 48 activates the drives 34 and 36 to oseillate the $irst and second mirrors 28, 30 respeetively, starting the scanning in elementary areas of the layer of transparent photo-sensitive material 16.
When, during the scanning operation, the coherent light passes through elementary areas of the layer of transparent photo-sensitive material 16 corresponding to portions of the surface to be checked which are free from de~ects (or, possibly corresponding to surfaces of a sample part free from defects), the light incident on the matrix of photoelectric converters 42 has a spatial intensity distribution which is taken as a reference. For example, when the surfaces are subjeeted to a painting treatment (parts of vehicle bodywork), the reference distribution is comparable to a luminous spot located in the centre of the photoelectric eonverter matrix 42, which corresponds to the origin of the plane of the spatial frequencies (Fourier plane) represented by the surfaee of the matrix of photo--electric converters 42.
The presence of a defect on the surface S to be checked produees a variation in the spatial distribution of the intensity of the light incident on the matrix 25 -of photoelectric conYerters 42,giving this distribution an elongate or irregular geometry, or at least a geo~etry different from that taken as the re~erence.
This variation is detected by the eleetronie processing circuit 46 and signalled to the logic eontrol unit 48 which, being connected to the scanning means (mirrors ~8r 30 and drives 34, 36) identifies the elementary area of the layer of photo-sensitive material 16 and, consequently, the portion of the surface S to be cheeked on whieh the presenee of a defect has been found, emitting a corresponding alarm signal.
The electronic processing circuit 46 is also able to identify, on the basis of algorithms of kno~n type, 1 ~7~V3~
the type oE defect (score, hole, crack etc.) found, trom the particular spatial intensity distribution of ~-he light incident on the matrix of photoelectric converters42~in the presence of the defect.
At the end of the scanning operation, the logic --control unit 48, through the supply unit 24, is arrangèd to cancel the image stored in the spatial light modulator 14, and simultaneously signal its availability to effect a further checking cycle.
The next cheeking cyele may be effeeted on a meehan~ical part different from that checked previously, or on another portion of the part checked during the preceding eycle whenever, as in the case of the quality eontrol of a vehicle body subjected to painting, the 15 dimensions of the part to be checked are considerable so that the part cannot be brought completely within the field of view of the objective 12.
In this ease the apparatus mayusefully be eonneeted to an automatic arrangement for relatively positioning 20the apparatus- itself with respect to the part to be ehecked, whereby to render the carrying through of the checking operation completely automatic.
Naturally, the prinsiple of the invention remaining the same, the details of construction and the embodiments 25 of theapparatus ~ay be varied widely with respect to that described and illustrated, without thereby departing from the seope of the present invention.
Claims (11)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of detecting surface defects in mechanical parts by analysis of diffracted light coming from these surface defects, said method comprising the steps of:
a) illuminating, with incoherent light, the surface of the part in which it is desired to detect defects, b) forming a flat image of this surface in a transparent photo-sensitive medium in which the spatial intensity distribution of the incoherent light reflected from the said surface produces a corresponding and proportional spatial distribution of the values of refractive index o* said medium, c) illuminating the transparent photo-sensitive medium with plane polarized coherent light by frame scanning in elementary areas, d) during the scanning operation, detecting variations in at least one of the polarization components of the coherent light after the latter has traversed the said transparent photo-sensitive medium, and e) deriving from these variations,information on possible defects in the surface under examination.
a) illuminating, with incoherent light, the surface of the part in which it is desired to detect defects, b) forming a flat image of this surface in a transparent photo-sensitive medium in which the spatial intensity distribution of the incoherent light reflected from the said surface produces a corresponding and proportional spatial distribution of the values of refractive index o* said medium, c) illuminating the transparent photo-sensitive medium with plane polarized coherent light by frame scanning in elementary areas, d) during the scanning operation, detecting variations in at least one of the polarization components of the coherent light after the latter has traversed the said transparent photo-sensitive medium, and e) deriving from these variations,information on possible defects in the surface under examination.
2. Apparatus for detecting surface defects in mechanical parts by analysis of diffracted light coming from these surface defects, said apparatus comprising:
a) means for illuminating, with incoherent light , the surface of the part in which it is desired to detect defects, b) a spatial light modulator comprising at least one layer of transparent photo sensitive material in which a spatial intensity distribution of incoherent light produces a corresponding and proportional spatial distribution of the values of refractive index, c) an optical system interposed between the surface of the part in which it is desired to detect defects and the spatial light modulator, the said optical system being arranged to form a flat image of this surface on the said layer of transparent photo-sensitive material, d) a source of plane polarized coherent light, e) scanning means for displacing the said coherent light relative to the spatial light modulator so that this light illuminates the said layer of transparent photo-sensitive material by scanning in elementary areas, and f) detector means arranged, during the scanning operation, to detect variations in at least one of the polarization components of the coherent light after this light has traversed the said layer of transparent photo-sensitive material.
a) means for illuminating, with incoherent light , the surface of the part in which it is desired to detect defects, b) a spatial light modulator comprising at least one layer of transparent photo sensitive material in which a spatial intensity distribution of incoherent light produces a corresponding and proportional spatial distribution of the values of refractive index, c) an optical system interposed between the surface of the part in which it is desired to detect defects and the spatial light modulator, the said optical system being arranged to form a flat image of this surface on the said layer of transparent photo-sensitive material, d) a source of plane polarized coherent light, e) scanning means for displacing the said coherent light relative to the spatial light modulator so that this light illuminates the said layer of transparent photo-sensitive material by scanning in elementary areas, and f) detector means arranged, during the scanning operation, to detect variations in at least one of the polarization components of the coherent light after this light has traversed the said layer of transparent photo-sensitive material.
3. Apparatus according to Claim 2, further including electronic processing means connected to the detector means and arranged to derive information on any surface defect of the surface under examination from the said variations in at least one of the polarization components of the coherent light.
4. Apparatus according to Claim 2, wherein the spatial light modulator provided with the said layer of transparent photo-sensitive material, comprises:
a) a first flat electrode transparent to the incoherent light reflected from the surface under examination, b) a flat dielectric, semi-transparent mirror interposed between the layer of transparent photo-sensitive material and the first flat electrode, the reflecting surface of the said mirror facing the said layer of transparent photo-sensitive material, c) a second flat electrode transparent to the coherent light and facing the surface of the layer of transparent photo-sensitive material opposite the flat dielectric mirror, and d) a supply unit arranged to apply at least two different voltage levels between the first and second transparent electrodes.
a) a first flat electrode transparent to the incoherent light reflected from the surface under examination, b) a flat dielectric, semi-transparent mirror interposed between the layer of transparent photo-sensitive material and the first flat electrode, the reflecting surface of the said mirror facing the said layer of transparent photo-sensitive material, c) a second flat electrode transparent to the coherent light and facing the surface of the layer of transparent photo-sensitive material opposite the flat dielectric mirror, and d) a supply unit arranged to apply at least two different voltage levels between the first and second transparent electrodes.
5. Apparatus according to Claim 2, wherein the said optical system includes an objective with a large depth of field.
6. Apparatus according to Claim 2, wherein the said scanning means comprise:
a) a first mirror arranged to deflect the light produced by the coherent light source, b) a second mirror arranged to deflect the light deflected by the first mirror in a direction substantially normal to the surface of the layer of transparent photo-sensitive material of the spatial light modulator, c) a cylindrical lens interposed between the first and second mirrors with its focus at the point on the first mirror lit by the light coming from the coherent light source, d) first drive means for oscillating the first mirror about an axis perpendicular to the axis of the cylindrical lens and to the incidence direction of the coherent light passing through the focus of this cylin-drical lens, and e) second drive means for oscillating the second mirror about an axis which intersects the axis of the cylindrical lens and which lies in a plane perpendicular to the axis about which the said first mirror is oscillated.
a) a first mirror arranged to deflect the light produced by the coherent light source, b) a second mirror arranged to deflect the light deflected by the first mirror in a direction substantially normal to the surface of the layer of transparent photo-sensitive material of the spatial light modulator, c) a cylindrical lens interposed between the first and second mirrors with its focus at the point on the first mirror lit by the light coming from the coherent light source, d) first drive means for oscillating the first mirror about an axis perpendicular to the axis of the cylindrical lens and to the incidence direction of the coherent light passing through the focus of this cylin-drical lens, and e) second drive means for oscillating the second mirror about an axis which intersects the axis of the cylindrical lens and which lies in a plane perpendicular to the axis about which the said first mirror is oscillated.
7. Apparatus according to Claim 3, wherein the said detector means comprise:
a) an optical analyser, b) a first optical system arranged to deflect the coherent light leaving the spatial modulator towards the said optical analyser, c) a matrix of photoelectric converters arranged to produce at the output of each converter an electrical signal indicative of the value of the intensity of the light incident on that converter, and d) a second optical system arranged to direct the light leaving the optical analyser onto the matrix of photoelectric converters.
a) an optical analyser, b) a first optical system arranged to deflect the coherent light leaving the spatial modulator towards the said optical analyser, c) a matrix of photoelectric converters arranged to produce at the output of each converter an electrical signal indicative of the value of the intensity of the light incident on that converter, and d) a second optical system arranged to direct the light leaving the optical analyser onto the matrix of photoelectric converters.
8. Apparatus according to Claim 7, wherein the said optical analyster is a polarizer.
9. Apparatus according to Claim 7 or Claim 8, wherein the said first optical system of the detector means is constituted by a semi-transparent mirror.
10. Apparatus according to Claim 7, wherein the said detector means further comprise an electronic circuit fed with the signals output from the matrix of photo-electric converters and arranged to form an array of numerical values corresponding to the spatial intensity distribution of the light incident on the matrix of photoelectric converters.
11. Apparatus according to Claim 10, wherein said detector means further comprises a logic control unit connected to the electronic circuit and the said scanning means.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| IT67187-A/81 | 1981-02-10 | ||
| IT67187/81A IT1143380B (en) | 1981-02-10 | 1981-02-10 | PROCEDURE AND DEVICE FOR THE DETECTION OF SURFACE DEFECTS OF MECHANICAL PARTS, IN PARTICULAR OF PARTS WITH CURVED SURFACE |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1179036A true CA1179036A (en) | 1984-12-04 |
Family
ID=11300313
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000395907A Expired CA1179036A (en) | 1981-02-10 | 1982-02-10 | Method and apparatus for detecting surface defects in mechanical workpieces |
Country Status (6)
| Country | Link |
|---|---|
| CA (1) | CA1179036A (en) |
| DE (2) | DE3204295A1 (en) |
| FR (1) | FR2499718A1 (en) |
| GB (1) | GB2095398B (en) |
| IT (1) | IT1143380B (en) |
| SE (1) | SE448030B (en) |
Families Citing this family (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS58219441A (en) * | 1982-06-15 | 1983-12-20 | Hajime Sangyo Kk | Apparatus for detecting defect on surface of convex object |
| GB2133871A (en) * | 1982-12-23 | 1984-08-01 | Austin Rover Group | Apparatus for a method of inspecting automotive components |
| US4629319A (en) * | 1984-02-14 | 1986-12-16 | Diffracto Ltd. | Panel surface flaw inspection |
| US4920385A (en) * | 1984-02-14 | 1990-04-24 | Diffracto Ltd. | Panel surface flaw inspection |
| GB2159271B (en) * | 1984-04-27 | 1988-05-18 | Nissan Motor | Surface flaw detecting method and apparatus |
| US5206700A (en) * | 1985-03-14 | 1993-04-27 | Diffracto, Ltd. | Methods and apparatus for retroreflective surface inspection and distortion measurement |
| JPH0682102B2 (en) * | 1987-02-27 | 1994-10-19 | 三菱電機株式会社 | Pattern defect inspection device and pattern defect inspection method |
| US5168322A (en) * | 1991-08-19 | 1992-12-01 | Diffracto Ltd. | Surface inspection using retro-reflective light field |
| US5225890A (en) * | 1991-10-28 | 1993-07-06 | Gencorp Inc. | Surface inspection apparatus and method |
| DE4219691A1 (en) * | 1992-06-16 | 1993-12-23 | Siemens Ag | Measuring device for determining direction of polarisation planes of linearly polarised light - has stepper motor with hollow axle, drive circuit with motor positioning accuracy correction stage, analyser and electro-optical detector |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2311287A1 (en) * | 1975-05-12 | 1976-12-10 | Bertin & Cie | Wave motion measuring optical instrument - has laser and light transformer to determine energy spectrum by Fourier transform |
-
1981
- 1981-02-10 IT IT67187/81A patent/IT1143380B/en active
-
1982
- 1982-02-08 GB GB8203572A patent/GB2095398B/en not_active Expired
- 1982-02-08 SE SE8200712A patent/SE448030B/en not_active IP Right Cessation
- 1982-02-09 DE DE19823204295 patent/DE3204295A1/en not_active Ceased
- 1982-02-09 DE DE8203330U patent/DE8203330U1/en not_active Expired
- 1982-02-09 FR FR8202093A patent/FR2499718A1/en active Granted
- 1982-02-10 CA CA000395907A patent/CA1179036A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB2095398A (en) | 1982-09-29 |
| DE8203330U1 (en) | 1985-05-23 |
| SE8200712L (en) | 1982-08-11 |
| IT8167187A0 (en) | 1981-02-10 |
| FR2499718B1 (en) | 1985-04-26 |
| IT1143380B (en) | 1986-10-22 |
| GB2095398B (en) | 1984-08-08 |
| DE3204295A1 (en) | 1982-08-19 |
| FR2499718A1 (en) | 1982-08-13 |
| SE448030B (en) | 1987-01-12 |
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