GB2100424A - Methods and apparatus for scanning an object - Google Patents

Abstract

Apparatus for scanning an object 16 with a laser beam 28, 52 at normal incidence, in a series of arcuate scans, comprises an optical system 40, 50, 56 mounted on a support 42 for rotation by a motor 20, the lens 56 enabling light reflected from the illuminated area to reach a detector 72, and a conveyor belt 72 for translating the object to provide the second component of scan. Lens 56 and mirror 66 are apertured for passage of the incident beam. Outputs from a clock, a reference detector 62 and a conveyor position encoder are combined to provide the relative position of the scan on the object. The detection arrangement may be replaced by an annular photodetector on the support 42. To scan the interior of a brake drum, prism 50 is omitted (Figure 7). In a further arrangement a head, comprising a laser diode at the centre of a photodiode array and both behind an apertured lens, is rotated to provide the arcuate scan component (Figure 8). <IMAGE>

Description

SPECIFICATION Methods and apparatus for scanning an object The present invention relates to surface scanning of parts and more particularly concerns improved scanning methods and apparatus to provide precision information concerning surface characteristics, configuration, orientation and discontinuities.

Electro-optical parts inspection and identification broadly involves the collection and analysis of light reflected from the surface of an object being monitored. In general, prior scanning apparatus, such as the video camera for example, involve the equivalent of a point scanning source.

Light is transmitted to and received from the part at varying angles for different points of the object being scanned. Such arrangements require fixed positioning and orientation of the part being scanned, so that the part is usually mounted in a fixture that predetermines position and orientation with respect to the scanning device. A point scanning source, such as that providing a conical scan for example, has an illuminating beam that strikes different portions of the scanned surface at different angles. Surface elevation characteristics, such as cavities or protuberances, will refiect differently in different orientations and different angles of illumination, so that reflection intensities afford less useful information.

For optimum precision in identification and measurement of surface detail, for improved repeatability of measurement, and for greater freedom from orientation and position restraint, all points on the surface of the part should be illuminated by light beams that are at all times parallel to one another, or always normal to a selected plane through the part. For example, such an orthogonally directed scan is required for measurement of part dimensions in a plane normal to the illuminating beam and for measuring surface elevation features in directions parallel to the beam. With such a perpendicular pattern of parallel scanning beams there is available a considerably greater flexibility in part position and orientation relative to the scanner, and reflections from surface areas of unique roughness configurations will have greater uniformity and repeatability.The lack of orthogonal scanning imposes substantial limitations on usefulness of the scanning.

In those prior art scanning devices employing a projected scanning beam and a receiver for collecting light reflected from the object, the receiver must be large enough to receive light reflected from all areas of the object that are illuminated during the entire scan. In such arrangements the size of the part that can be scanned is relatively small, being limited by practical and economic considerations that limit the size of a light receiver such as an optical lens, a collecting mirror, or an array of detectors or optical fibres.

Accordingly, it is an object of the present invention to provide scanning apparatus and methods that avoid or minimize above-mentioned problems.

According to the present invention a method of scanning an object comprises: projecting an energy beam at the object, whereby energy of the beam is reflected from the object, moving the beam across the object in a scan pattern, receiving the reflected energy in a relatively narrow area extending along the scan pattern, and relatively moving the object and the pattern.

Preferably the energy beam is projected substantially normal to the object, throughout the scan pattern.

Preferably the step of receiving energy comprises moving an energy receiver along the narrow area.

Preferred scanning apparatus according to the invention comprises: a movably mounted support, means for effecting relative scanning motion of said support and an object to be scanned, means for projecting an energy beam from said support in directions substantially normal to an object to be scanned, said beam having a scanning motion relative to the object to be scanned, a receiver mounted to said support for motion therewith relative to the object to be scanned, and means for moving the object relatively to the pattern produced by the scanning motion.

Preferably the receiver has an energy receiving axis coaxial with the beam, the receiving axis having a scanning motion relative to the object to be scanned, whereby the receiver will receive energy of the beam reflected from the object through an area centred upon the projected beam.

Preferably the support is mounted for rotation about an axis, the receiver comprises a lens mounted on said support and aligned with said projected beam, and the beam passes through a hole through the lens.

The accompanying drawings show examples of apparatus according to the invention. In these drawings: Figure 1 is a pictorial illustration of a first apparatus; Figure 2 is a simplified side elevation and section of the apparatus of Figure 1; Figure 3 is an enlargement of part of Figure 2; Figure 4 diagrammatically depicts the circular scan pattern that moves repetitively across the object being monitored; Figure 5 illustrates geometry that defines coordinates of the circular scan; Figure 6 is a block diagram of electronic components used to generate signals defining beam intensity; Figure 7 illustrates a modified apparatus; and Figure 8 illustrates another modification, in which the laser, detector and collecting lens are rotated together.

The apparatus illustrated in Figure 1 comprises a support structure 10 fixedly supported above and adjacent to a conveyor 12 having a movable belt 14 on which is placed an object 1 6 that is to be scanned. The object to be scanned can be of many different sizes, shapes and construction, being generally illustrated as a transmission stator. The conveyor is driven by a motor 17 to move the belt and the object from left to right as viewed in Figure 1, close to and directly beneath the scanning means, and entirely across the scan pattern thereof.

The support structure comprises a rigid base 1 8 on which is fixedly mounted a motor 20 having a hollow vertical shaft 22 that is rotated at high speed by the motor. Fixedly mounted to an upstanding sidewall 24 is an energy beam generator in the form of a laser 26 that generates and projects a very small cross-sectional area light beam 28 to a first 900 reflecting prism 30 mounted at the upper end of an upstanding rear wall 32 of structure 10. Prism 30 turns the light beam through 900 to a second 900 reflecting prism 34, also fixed to the upper end of wall 32, and positioned in alignment with the centre of the hollow motor shaft, whereby the beam is reflected downwards through the centre of the shaft 22.At the bottom of the shaft is mounted a third 900 reflecting prism 40 (Figure 3) which again turns the light beam at an angle of 900 so that the beam is now directed perpendicular to the axis of rotation of the motor shaft.

The scanner support structure includes an enlarged lower section 42 of generally inverted dish-shaped configuration having a downwardly facing end closed and sealed by a high strength rigid protective plate 44. Plate 44 is preferably made of a completely transparent material but may be made of any suitable opaque material provided that a transparent annular area 46 completely around the lower section 42 of the support structure is formed in the bottom plate.

Fixedly connected to the end of the hollow motor shaft 22 is a rotatable disc 48, part of which acts as a rotatable arm, in the radially outer end of which is mounted a fourth 900 reflecting prism 50, positioned to receive the light beam from prism 40 and turn it through 900 along the path indicated by reference character 52.

Accordingly, as the motor shaft 22 is rotated, prisms 40 and 50 and disc 48 rotate about the shaft axis, causing the projected laser beam 52 to scan in a right circular cylindrical pattern centred on the shaft axis and having a radius equal to the radial displacement of the reflective prism 50 from the shaft axis. The arrangement provides an orthogonal scan, with the scanning beam always exiting parallel to the rotation axis and normal to the part-supporting surface of conveyor belt 14.

A lens 56 having an axial hole 58 extending completely therethrough is fixedly mounted in a support 60 that is fixed to the end of the rotating arm. The lens and its hole are coaxial with the projected beam 52, which passes freely through the lens. The lens is focused on the point of impingement of the beam upon the object being scanned.

A reference generator in the form of a lightsensitive diode or equivalent 62 is fixed to the bottom of plate 44 in the path of the projected beam 52 so as to be illuminated momentarily by the beam during each cycle of its rotation.

Light projected from the rotating energy beam 52 is reflected at any instant from a very small area of the object upon which the beam impinges, and some of this reflected light is collected by the lens 56, which collimates the collected light and transmits it back to the reflecting prism 50. The collimated retro-reflected light is then retrodirected along several legs of the outgoing laser beam path, from the prism 50 back to prism 40 and then upwards along and through the hollow motor shaft. However, between the upper end of the motor shaft and the reflecting prism 34 there is mounted a 900 turning mirror 66 that has a small central hole 68 through which the laser beam 28 passes without disturbance.The small hole 68 in the reflector 66 does not significantly affect the reflection by this mirror of the received collimated reflected energy, which is directed to a detector 72 that provides an output signal on an output lead 74 having a magnitude directly related to the intensity of the light received thereby.

The belt 14 of conveyor 12 is entrained over a second roller 80 at the end opposite the motor, and a conveyor position detector 82, such as a conventional incremental shaft encoder, is mounted to the roller so as to provide from the detector encoder 82 a series of pulses each of which denotes an increment of rotation of the roller and thus an increment of motion of the conveyor belt 14.

As can be seen in Figure 4, the laser beam 52 moves in a cylindrical scan pattern and scans a circular path 86 that crosses the part 1 6 as the latter moves relative to the scan path 86 in the direction indicated by arrow 88. The receiving lens 56 also moves in a circular path and collects light over successive portions of a narrow annular area 90. As the part 1 6 moves across the scan path 86, the beam makes many passes across the part. With the relative dimensions of scan and part as illustrated in Figure 4, such dimensions being merely exemplary, the part is initially scanned repetitively by the left-hand side (as viewed in Figure 4) of the scan path and then it is scanned repetitively by the right-hand side of the scan pattern.

The diameter of the lens 56 is effectively the diameter of the receiver. The scanning motion of the lens enables its diameter to be less than the diameter of the cylindrical scan. In fact the area of the receiver need be no greater than the area within which the beam 52 is reflected at any instant.

As an alternative to a lens carried by a rotating beam, use may be made of a narrow annular receiver mounted on the scanner support structure, in a position corresponding to the narrow circular path 86 scanned by the beam 52.

From the pulse trains provided at the outputs of reference detector 62 and conveyor position encoder 82, the position of the area of the part illuminated by the beam at each of a large number of points in its scan is determined. Geometry and equations for identifying beam position are basically the same as those described in the Profile Scanning Apparatus of U.S. Patent 4 1 22 525, differing primarily in that instead of reading out position coordinates solely on intersection of the beam with a part boundary, a clock is employed to read out signals identifying position coordinates at selected fixed time intervals.

Geometry of the point position identification is illustrated in Figure 5. Part 1 6 travels, together with a moving coordinate system XY, relative to the laser scan of radius R. Detector 62 provides a reference at point D, which lies on a radius at an angle m with respect to the Y axis. The position of any point of the beam scan at an angle Oi with respect to the scan radius through the reference point is identified by the coordinates x1, y1, in the moving coordinate system defined by the following equations: x1=Xc-Rsin (8,-a) (1) y1=R-R cos (8,--m) (2) where Xc is the X coordinate of the scan centre.

Measurements are based upon pulses produced by a fixed repetition rate pulse generator so that cg=K, x K2 where K, is the number of such pulses that occur in the time required for the beam to travel through the reference angle a, and K2 is the angular distance through which the beam travels along the scan pattern in the interval between two successive pulses.Accordingly, equations (1) and (2) become xi=Xc-R sin (N1-K1)K2 (3) y,=R 1 -cos (N1-K1)K2 (4) in which N, is the number of pulses occurring in the time required for the beam to travel from the reference point D to the point x,y. Thus, equations (3) and (4) define the coordinates of points in the beam scan in terms of fixed quantities R, K, and K2 and variable quantities Xc and N. Xc is the quantity obtained from the incremental encoder 82 that signals position of the conveyor, and N, is determined by count of pulses of the pulse train to a given point.

Illustrated in Figure 6 is an electronic circuit that will generate signals defining beam intensity and coordinates at selected clock intervals. Beam reference sensor 62 generates pulses that are sent to reset a counter 92 having a counting input on line 94 from a system clock 96. Each count of the counter is clocked into and stored in storage register 98 which, accordingly, provides outputs respectively representing successive clocked angular positions of the beam in its rotating scan.

The output of conveyor position detector 82 is fed to a second counter 100 of which the outputs are fed to a storage register 102 at intervals determined by the timing pulses from clock 96.

Thus, register 102 stores signals representing successive positions of the conveyor and thus, successive positions of the part, in the direction of conveyor travel at successive clock periods. The output 1 of the reflection detector 72 is fed through a gate 104 under control of the system clock 96, and all the signals are fed to a data processor 106 (Figure 1). The signals from storage 98 and 102 and from the reflection detector 72, which, of course, may be stored and used for manual computation and plotting of reflected energy intensity at different coordinate positions, are preferably handled by digital computation. Details of the computation and data processing form no part of this invention.

The analog reflection intensity signals may be digitized and stored together with position coordinate information and then compared to similar stored intensity and position signals that have been previously generated on a scan of a part of a known configuration. The comparison will indicate the correspondence of the newly scanned part with the reference part.

Alternatively, or, in addition, the stored information representing intensity and coordinate position may be fed to an oscilloscope 108 (Figure 1) to provide a visual display of the scanned object. Accordingly, it will be seen that the signals generated by the reflected light intensity detector 72, the beam reference sensor 62, and the conveyor position detector 82 collectively define intensity of light reflected from a number of points on the object and also define the relative positions of such points, therefore enabling a plot of intensity over the area of the object to be made. This information is readily available from the output reflected beam intensity, the output N, indicating radial angle of the beam (position of the beam in its circular scan) and the linear displacement Xc relative to the reference system (the conveyor system).This is done for a large number of points as the part is moved through the cylindrical beam scan.

In an exemplary embodiment the disc rotates at 1 800 revolutions per minute, and the conveyor travels at 1.25 inches (32 mm) per second, so that the part advances approximately 0.042 inches (1.1 mm) during each beam rotation.

However, as mentioned previously, the part is scanned twice, by the left segment of the circular beam as the part enters the scan, and by the right segment of the beam scan pattern as the part leaves the scan, thus improving the resolution.

This apparatus provides a true twodimensional or orthographic view.

It will be readily appreciated that many modifications may be made in the apparatus employed to provide this orthogonal scanning. For example, as illustrated in Figure 7, with minor modification the principles of the present invention may be employed to scan the interior of a cylindrical part such as the interior surface of a brake drum 116. In this arrangement, the laser 126 directs its narrow beam through the hollow shaft 122 of a motor 120 via a pair of 900 turning prisms 130, 134 and through the aperture of a turning mirror 166. Light passing through the shaft 122 is reflected by a third turning mirror or prism 140 at the bottom of the hollow shaft, this prism, in this embodiment, being the final prism from which the rotating and scanning beam 142 is projected.The beam is projected in a direction perpendicular to the axis of rotation and perpendicular to the interior surface of the part 11 6 that is being scanned, thereby scanning a circular path around the interior of the drum in a plane that is always normal to the drum surface and normal to the axis of rotation. In effect, the beam 142 scans along successive radii of a circle lying in a plane normal to the brake drum surface and normal to the rotational axis. Light reflected from the drum surface is received by a collector lens 1 56 having an aperture therethrough through which the beam 142 passes without disturbance. Lens 156 is fixed mounted to the shaft 122 to rotate together with the turning prism 140.Light collected by the lens 156 is retro-reflected through the hollow shaft via prism 140 to the mirror 1 66 from which it is reflected to a light intensity detector 1 72.

To enable the scanning plane to move relative to the drum surface, the drum 11 6 is mounted upon a vertically movable table 114 that is driven vertically by a rack and gear 11 5, 11 7, and a motor 11 8, and guided in its motion by means of guides 121, 123. A vertical position encoder and a beam angular reference generator (not shown) are provided to establish position of the beam relative to the drum surface during the scan.

Schematically illustrated in Figure 8 are portions of still another modification of the scanner means. In this arrangement a small laser 1 80, such as a continuous wave laser diode of the GOLS series made by General Optronics of South Plainfield, New Jersey, United States of America, is mounted in the centre of an array 1 82 of photo diodes to project a beam 1 84 through a central hole in a lens 1 86. The laser, photo diode array, and lens are all mounted at the end of a rotating scanner support arm 1 88 that itself is mounted for rotation about an axis indicated at 1 90.

Suitable electrical leads (not shown) are carried through the rotating arm for providing power to the laser and transmitting intensity signals from the photodiode array. Light from the laser 180 is transmitted through the hole in the lens 186 to the surface of a part to be scanned. Light reflected from the part is collected by the lens and transmitted to the photo diode detector to provide the desired intensity signals. Use of a larger photo diode detector array provides more information by collating reflected light from a larger area. A Fresnel lens 186, aligned with the projected beam and detector, is preferred for the larger detector.

In place of the diode array 180, use may be made of a solid optical energy receiver. If this is moving close to the object to the scanned, no lens 186 is needed.

It is contemplated that the laser 26 of Figures 1,2 and 3 be replaced by a laser distance measuring system such as, for example, the Laser Measurement System 5501A of Hewlett Packard.

This system employs two laser beams of different wave lengths and both interferometry and Doppler techniques to determine distance to a beam reflecting surface that is moving towards or away from the laser beams. Such an arrangement when employed in the rotating scanner described herein, provides quantitative measurement of surface elevational configuration.

For example, assuming the scanning beam, which is moving in its scan pattern along the surface of a part, crosses a portion having an elevation change. As the horizontally scanning beam moves to a point on the surface of a greater elevation it is reflected from a lesser distance from the laser receiver. In effect, distance to the reflecting surface has changed thus enabling a Doppler measurement of the elevation.

For inspection of surface features of parts of contoured surfaces it is desirable to maintain the orthogonal relation between the energy beam and the surface being measured. Accordingly, for such an application the final turning prism 50 and lens 56 of Figures 1, 2 and 3, or equivalent components of other embodiments, are mounted with one or two degrees of pivotal freedom relative to the scanner support arm, so that the direction of the projected beam 52 can be automatically changed to maintain the beam perpendicular to the surface of the part. This arrangement employs pivotal mounting of the mirror and lens, and a servo system that senses departure of reflected light from maximum intensity to control the projected beam direction so as to maximize intensity.

A relatively large solid-state light detector may be arranged with different segmental areas that yield mutually distinct signals in response to received light, so as to give added information.

For example, if the portion of the object surface upon which the laser beam impinges is slightly tilted so that maximum intensity of the reflected beam will be angularly shifted from the beam axis, such a segmented receiver will provide information concerning such angular shifting, including the direction in which the intensity maximum is shifted, and thus provide information indicative of both reflectivity and inclination or other characteristics of the surface of the object being scanned. The beam axis may be angularly shifted in a selected search pattern and those positions yielding greater intensity are noted, remembered and employed to shift the beam axis so as to obtain a beam axis orientation that produces maximum reflection intensity, which is the desired condition of perpendicularity of the beam to the surface being scanned.

Other methods may be employed to maintain the scanning laser beam in a condition of perpendicularity to the surface being scanned. For example, the solid receiver may be formed with a reticular pattern that is established to yield a direction for the sensor to be moved to achieve a decreased angular deviation from the surface normal. Alternatively, a circular distribution of concentric sensor rings will yield a smaller or larger angle sensed, depending upon which of the concentric sensor or receiver circles produce an output signal. This allows the system to sense small changes in the deviation from the surface normal. Thus, selection of a smaller radius of such receiver rings provides a collecting element sensitive to small angles of deviation from surface normal. Conversely, a larger radius yields a broader response to angular deviation.

Such a system more readily detects surface blemishes or discontinuities. Such discontinuities appear as a sharp drop in intensity to the detector, but of a duration short enough to be filtered from the operation of the servo system that controls beam direction so as to maximize beam intensity over a somewhat longer period.

Claims (27)

Claims

1. A method of scanning an object comprising: projecting an energy beam at the object, whereby energy of the beam is reflected from the object, moving the beam across the object in a scan pattern, receiving the reflected energy in a relatively narrow area extending along the scan pattern, and relatively moving the object and the pattern.

2. A method according to claim 1, wherein the energy beam is projected substantially normal to the object, throughout the scan pattern.

3. A method according to claim 1 or claim 2, wherein the step of receiving energy comprises moving an energy receiver along the narrow area.

4. A method according to claim 3, wherein the step of moving the beam comprises projecting the beam through the receiver as the receiver moves along the area.

5. A method according to any of claims 1 to 4, wherein the area is annular, the step of receiving energy includes moving a lens in a circular path around the area, and the projected beam is passed through the lens orthogonally towards the object as both the beam and receiver move in a circular path.

6. A method according to any of claims 1 to 5, wherein the scan pattern comprises elements of a right circular cylinder.

7. A method according to any of claims 1 to 5, wherein the scan pattern comprises radii of a circle.

8. Scanning apparatus comprising: a movably mounted support, means for effecting relative scanning motion of said support and an object to be scanned, means for projecting an energy beam from said support in directions substantially normal to an object to be scanned, said beam having a scanning motion relative to the object to be scanned, a receiver mounted to said support for motion therewith relative to the object to be scanned, and means for moving the object relatively to the pattern produced by the scanning motion.

9. Apparatus according to claim 8, wherein the receiver has an energy receiving axis coaxial with the beam, the receiving axis having a scanning motion relative to the object to be scanned, whereby the receiver will receive energy of the beam reflected from the object through an area centred upon the projected beam.

10. Apparatus according to claim 8, or claim 9, wherein the support is mounted for rotation about an axis.

11. Apparatus according to any of claims 8 to 10, wherein the receiver comprises a lens mounted on said support and aligned with said projected beam.

1 2. An apparatus according to claim 11, wherein a hole extends through the lens and wherein the projected energy beam passes through the hole.

13. An apparatus according to claim 10, wherein the energy beam is projected in scanning directions substantially parallel to the axis.

14. An apparatus according to claim 13, wherein the beam moves in a cylindrical scan having a diameter greater than the diameter of the receiver.

1 5. An apparatus according to claim 10, wherein the energy beam is projected in scanning directions perpendicular to the axis.

1 6. An apparatus according to claim 9, wherein the energy beam is projected through the receiver.

1 7. An apparatus according to claim 10, including carrier means for moving an object relative to said rotating beam along a plane normal to the axis, and wherein the energy beam is projected in scanning directions normal to the plane.

18. An apparatus according to claim 8, wherein the receiver comprises a light sensitive device mounted on the support, and the means for projecting an energy beam comprises a laser mounted on the support for projecting a light beam in alignment with the device.

19. An apparatus according to claim 10, comprising: an apparatus support, a motor mounted on the apparatus support and having a hollow shaft, a rotating scanner support fixed to the motor shaft for rotation therewith and extending radially outwards from the shaft, a laser mounted on the apparatus support, and means for directing light from the laser through the hollow shaft and through the scanner support, for projection parallel to the motor shaft from an area of the scanner support radially outwardly displaced from the shaft.

20. An apparatus according to claim 19, wherein the means for directing light includes a reflector mounted at a radially outward end of the scanner support, and the receiver comprises a lens mounted adjacent to the reflector and positioned to pass the projected light through a portion thereof.

21. An apparatus according to claim 20, including a light detector mounted on the apparatus support and means for directing reflected light received by the lens to the detector.

22. An apparatus according to claim 21, including reference detector means for generating a signal representing the rotational position of the scanner support.

23. An apparatus according to claim 22, including a part carrier, means for moving the carrier in a direction perpendicular to the motor shaft, and means for generating a signal representing the position of the carrier.

24. An apparatus according to claim 19, wherein the means for directing light comprises a first reflector at one end of the shaft, a second reflector at a radially outward end of the scanner support, and a third reflector at the other end of the shaft, the third reflector being apertured to pass a light beam from the laser without reflection to the first reflector, and the receiver comprises a lens for passing collected light to the second reflector, and includes a light detector mounted on the apparatus support, positioned to receive light from the lens, reflected by the second and first reflectors to the third reflector and reflected from the third reflector.

25. An apparatus according to claim 19, including a light detector, and wherein the means for directing light includes means for retrodirecting reflected light from the receiver to the detector through the hollow shaft and along a part of the path of light from the laser through the scanner support.

26. A method according to claim 1 comprising: rotating the projected energy beam about an axis of rotation in a scan pattern centred upon the axis, moving the object to be scanned relative to the scan pattern in a plane perpendicular to the axis, mounting an energy collecting lens in alignment with the projected energy beam, the lens being focused upon the point of impingement of the beam upon the object, rotating the lens together with the rotating energy beam, and detecting intensity of reflected energy collected by said lens.

27. An apparatus according to claim 8, substantially as described with reference to Figures 1 to 4, Figure 7, or Figure 8 of the accompanying drawings.