SYSTEM AND METHOD FOR ILLUMINATING OBJECTS FOR VISION SYSTEMS

Abstract: An illumination system in an automatic vision system employing an electronic camera (17) with a lens system (18) defining a vertical optical axis, illuminates an object (16) under analysis with a cone or arc segment of light. Light from optical fibres (38) forming a ring source is directed toward an annular parabolic surface (44) on a light collector ring (42) that substantially collimates the light and fans it out radially out toward a toroidal reflecting surface (48) on an encompassing ring (46), the relative position of which determines the angle of incidence of a cone of light formed to fall on the object (16). To change the angle of incidence while maintaining focus, a differential drive mechanism (50) moves the parabolic collector ring (42) and the toroidal angle generator ring (46) in differential fashion concurrently.

Description: The invention relates to a vision system of the kind comprising a lens systems and an illuminating system, the lens system defining a vertical optical axis, the vision system being suitable for inspection, measurement or recordation of a product arranged along the optical axis, the illuminating system comprising a remote light source coupled to a flexible optical fiber harness terminating in a light ring for generating a hollow cylinder or sheath of light concentric with the optical axis and directed toward the product, a first ring element concentric with the optical axis and defining an aperture between the lens system and the product and having an outer reflecting surface for deflecting the cylinder or sheath of light in a first direction, and a second ring element having a reflecting surface disposed in the path of the light reflected from the first reflecting surface for deflecting the light onto the product. The rapid growth of automatic measurement techniques for precision products, ranging from mechanical parts made to very narrow tolerances to minute VLSI semiconductor products, has led to the generation of a number of systems for automatic inspection of such parts and components. In these systems, an article to be checked or measured is imaged, generally through a high magnification optical system, on an electronic image pickup device, such as a Vidicon, CCD array, or other image-to-signal converter. The video signals can be processed, utilizing adapted software and a microprocessor or minicomputer, to analyze the image and particular parts of the image. Image transformation techniques can be used to rotate, zoom and translate the image, pattern recognition techniques can be employed to compare the image to a predetermined standard, flaws can be identified, dimensions and spacings can be measured. These functions can be performed with high reliability and freedom from error when the image is adequately defined. The signals derived from scanning of the optical image, however, depend on the physical and optical characteristics of the object under examination. While the eye can readily perceive differences in color and texture, and the eye can also adjust for reflections and other effects and a human operator can make adjustments dependent on overall perceptions, processing of the image signal depends essentially upon the nature of the variations in the signal. Precise location of an edge of a given surface, for example, requires a high signal contrast between the surface and its background. If the diameter of an aperture is being measured, for example, abrupt variations in signal magnitude enable the vision system to give a reading of the dimension that is much more rapid, accurate and reliable than any system requiring operator judgement. Precision components, however, are three-dimensional in character and it is often desirable to inspect or measure specific attributes at different levels. If a hole is tapered, for example, and the hole is concentric with a vertical axis, the taper can be determined with accuracy by taking hole diameter readings at each end, for which purpose uniform illumination is not optimal. This illustrates the general problem of so illuminating an object under inspection that highlights and shadows can be used to best advantage in displaying the features and surfaces that are to be inspected or measured. The vision systems of the class described herein are best exemplified by the View Engineering Model 1200, a microprocessor-based system which makes non-contact measurements in three dimensions of complex parts and to an accuracy of the order of 6 µm (0.00025"). In this system, object illumination may be provided by a light source disposed around the objective lens of the magnification system for the camera, or coaxially through the objective. The capability for enhancing different characteristics at different times has been limited except that limited advantages have been derived by using discreetly differing light sources in particular instances. Exemplary of limited prior art illuminators is German patent DE-A-2 535 556 which discloses an illuminator apparatus for surgical lighting providing shadow reduction and uniform lighting to an illuminated area. Substantially collimated light rays impinge upon a first reflector which reflects the rays radially outward to a radially-spaced reflector. Radially-spaced reflector reflects such impinging light rays in a fixed angled relationship to an illuminated zone located beneath the convex reflector. Adjustment of the first reflector along the axis of symmetry with respect to a fixed radially-spaced reflector alters the cross-sectional area of the illuminated zone. UK patent application GB-A-2 072 825 discloses a surgical operation lamp comprising a light source which emits a pencil of rays which are converted by a lens into a parallel pencil of rays which appear on a reflecting surface of a reflection body. The parallel pencil of rays impinging on the reflecting surface are reflected to a reflector from whence it is reflected onto an operating field. The reflection body is attached to a retaining rod having an axis which is concentric with the optical axis of light source. Reflection body and retaining rod obstruct the optical axis between the light source and the operating field making it unsuitable for an axial vision system. U.S. patent 4,109,304 discloses a device for coherent lighting of objects, wherein across the path of a coherent light beam of parallel rays emitted by a laser, there is arranged a means to transform said coherent light beam of parallel rays into an infinite number of parallel-ray light beams travelling at an angle to one another, whereby there is produced an effect of light equivalent to that of a spatially extended source of coherent light. The lenses and reflectors are disposed in a fixed relation so as to make the device for coherent lighting of objects not suitable for an axial viewing system in which contrast adjustment and a fixed focal plane are desired. APPLIED OPTICS, Vol. 24(1985), pages 1560-62 teaches a variety of illuminator configurations utilizing commercially available fiber optic rings. It does not however teach a fiber optic ring having a plurality of arc segments which are independently controllable to provide illumination from different directions as well as from all directions around the axis. An illumination system which can efficiently an economically provide different, controllable, illumination of an object under study is not limited to use with vision systems of the type generally described. It can also be employed in microscopy, microphotometry, and microphotography, where the part being examined is viewed under some substantial magnification and image enhancement is desirable for specific purposes. According to the present invention, a vision system of the kind defined hereinbefore at the beginning is characterised in that a differential drive mechanism is adapted to move the first and second ring elements at differential rates of movement in the axial direction and the reflecting surfaces of the ring elements are curved in axial cross-section so that the incident angle of the light deflected onto the product by the second ring element can be varied while maintaining a constant focal point. In a preferred embodiment of the invention, the illuminating system employs a light source providing a hollow cylindrical sheet of diverging light, together with a pair of curved ring reflectors. One of the reflectors collects and collimates the cylindrical sheet of light and directs it as a lateral fan to an adjacent reflector ring having a curved surface. The angle of deflection from the second reflector forms a cone whose angle of incidence varies in accordance with the relative position of the two rings. When moving the two reflecting rings toward or away from the object at differential rates, the cone of illuminating light continues to focus at a given plane but with a varying included angle that changes the highlights, the shadows and contrasts in the image through a wide range. A feature of the embodiment is that the focal plane can be changed through a wide range. Furthermore, the ring light source generating the sheet of light is advantageously divided into separate arc segments which can be illuminated in different combinations so as to achieve further versatility in presentation of the image. In a specific example of a system in accordance with the invention, a vision system has an optical axis along which a camera views an object through a magnification system. X, Y, and Z axis positioning is provided between the camera and the item being examined. A ring light source comprising a cylindrical array of optical fibers, energizable in quadrants, is disposed concentric with the optical axis, encompassing the lens magnification system. The optical fibers in the ring light source generate individually diverging beams forming a hollow cylindrical sheet of light directed downwardly toward the object under examination. The light is reflected off a reflecting surface of parabolic cross section on the inner ring, substantially radially outwardly toward a curved reflecting surface on the second outer ring. The second reflecting surface is a toroidal segment that has a circular cross section to direct the light down to a common focal point, dependent upon the region of incidence on the second reflecting surface. A differential drive mechanism is coupled to the Z axis carriage to move the first and second reflector rings in the same direction, but at different rates of movement, such that the focal length changes along with the angle of incidence, and the focal point remains fixed. The mechanism moves separately from the focusing of the camera, so as to provide greater versatility. In this arrangement, differential motion is preferentially provided by a double rack system. Drive and idler gears between opposing racks enable the two racks to be moved in the selected ratio by a single motor, and without requiring external bearings. Brief Description of the Drawings A better understanding may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a perspective view, partially broken away, of a vision system incorporating an illumination system in accordance with the invention; Fig. 2 is a perspective fragmentary view of a portion of the arrangement of Fig. 1; Fig. 3 is a side sectional view of a portion of the structure of Fig. 2; Fig. 4 is a fragmentary view of a portion of a differential drive mechanism used in the mechanism of Figs. 2 and 3; Fig. 5 is an end view of the structure of Fig. 4; and Fig. 6 is a simplified depiction of variations in the angle of incidence with different positions of the reflectors in the system of Figs. 1-5. Detailed Description of the Invention A vision system 10 in accordance with the invention, as seen in Figs. 1 and 2, comprises a base console 12 having a temperature stable flat granite table 13 on which a stage 14 movable in the Y direction under control of a drive 15, comprising a motor and lead screw is mounted to place a product 16, indicated only generally, to be inspected at a given position in a viewing zone. A vertically mounted camera 17 points down at the viewing zone through a magnifying lens system 18. The camera 17 is mounted on a Z axis support 19 that is driven in the vertical direction by a Z axis motor 20 so that the camera can focus on particular regions and surfaces. The camera 17 and Z axis support 19 are stably mounted on an X axis carriage 22 that is mounted on a stable vibration free bridge 23 spanning the viewing region above the platform 14. The carriage 22 is translated in the horizontal plane by a drive 24 comprising a motor and lead screw. Both the drives 15 and 24 as well as the Z axis drive motor 20 are controlled by signals from a processor 25, which is typically programmed to position the product 16 at given X and Y axis positions relative to the camera 17 and also focus on a particular level at the object. An illumination system 30 having elements concentric with the optical axis is mounted conjointly with the camera 17 and lens system 18 on the Z axis support 19. The illumination system 30, referring now to Figs. 2-5 as well as Fig. 1, includes a remote light source 32 on the console coupled through a flexible optical fiber harness 34 having many individual optical fibers to a light ring 36 that is disposed about the lower end of the lens system 18 and concentric with the optical axis. As seen in the sectional view of Fig. 3, the ends of the individual optical fibers 38 are fanned out from a parallel core into a cylindrical pattern in which the fibers are parallel and adjacent around the cylinder. The light emanating from the ends of the fibers 38 collectively generates a hollow cylinder or sheath of light directed down toward the product 16. However, the light from each fiber end diverges at an angle of about 62° in this instance, this angle varying in dependence on the precision of the end flat. The remote light source 32 has four separate sources in this example, each controlling light to a different quadrant at the illuminator 30 and each controlled by the processor 25. Below the light ring 36, a first reflector ring 42 is movably mounted along the optical axis and includes a first reflector surface 44 which is parabolic in cross section. The focal point of the parabola is chosen relative to the angle of divergence of light from the fibers, to provide substantial collimation. Because the parabolic reflector surface 44 is in the path of the downwardly directed light cylinder, it deflects the light radially outwardly from the circumference of the reflector surface 44 to form what may be called a fan of light concentric with the optical axis of the camera 17. Thus the first reflector ring 42 acts as a light collector as well as a light path diverter because of the parabolic surface. The first reflector ring is in the form of a surface of revolution whose body includes a waist section for receiving the light ring 36. The outwardly directed light pattern impinges on a second reflector ring 46 outside the first ring 42, and relatively movable with respect to it along the Z axis. The reflecting surface 48 of the second reflector ring 46 is of circular cross section, as seen in Fig. 3. A circular or spherical segment is adequate for present purposes, although a hyperbolic curve would be preferred where higher precision is needed. The position at which the substantially horizontal fan beam from the first reflector ring 42 impinges on the curved second reflector surface 48 determines the angle of deflection downward toward a focal point, thus acting as an angle generator. The parabolic reflector surface 44 substantially collimates the light from the optical fibers 38 that fans out toward the curved surface 48. A differential drive mechanism 50, best seen in Figs. 2, 4 and 5, is mounted on the X axis carriage 22, and consequently is independent of the Z axis position of the camera 17 and lens system 18. A motor 52 (Fig. 2) independently drives the mechanism 50 so as to control the position of the second ring 46. The positions of both rings 42, 46 are changed concurrently in a certain proportionality which maintains focus as well as changing the angle of incidence. To this end, a pair of rack and pinion mechanisms are mounted in side-by-side relation, with a first rack 54 having a given tooth pitch being coupled to the second ring 46 by an angle bracket 55 and a second rack 56 having an equal pitch being coupled to the first ring 42 by an extending arm 57. The racks 54, 56 are offset in adjacent planes within an encompassing housing 59 attached to the Z axis support 19 (Fig. 1). Parallelism is maintained by drive and idler gears coupled within the housing at spaced apart regions between the racks 54, 56. The drive gears, as seen in Figs. 4 and 5, include a first gear 62 coupled to the shaft of the motor 52 and engaging the first rack 54. The first gear 62 drives an adjacent second gear 64 in its plane, which is clustered with a larger third gear 66 in the plane of the second rack 56, and meshing with the teeth in the second rack 56. The first and third gears 62, 66 generate a differential movement determined by the ratios of the diameters of the three gears 62, 64 and 66. With the arrangement shown in the present embodiment, the first ring 42 is moved faster than the second ring 46. Two idlers 68, 69 are arranged to engage the racks 54, 56 to maintain parallelism without bearings. With this arrangement, the racks 54, 56 reciprocate within the housing 59 and the needed motions are effected without the necessity for bearing supports. As seen in Fig. 5, the racks 54, 56 are attached to the slider rods 70 which slide within bushings 72 in the housing. In the operation of the system of Fig. 1, the sequence of operation for examination of a product 16 will typically commence with an operator controlling positioning of the Y axis stage 14 and the X axis carriage 22 to view different portions of the product 16, as these sequences and positions are stored in the processor memory. At each position focal planes are selected for the lens system 18 and camera 17 by positioning the Z axis support 19, and then a preferred angle of incidence is chosen for light from the illumination system 30. As the necessary set points for each position are sequentially selected by the operator, the sequence is stored in memory for that product 16. As each like product 16 is placed on the stage 14 in position for analysis, software for the system runs through the sequence in conventional fashion, focusing on and illuminating each selected point or region of the part. If the taper in the hole of the product 16 is to be measured, let it be assumed that the upper surface is a larger diameter and the lower surface is a smaller diameter. The camera 17 and lens 18 are then focused on one end or the other, and the illumination system 30 is shifted to give an optimum angle of incidence for a sharp transition in light contrast between the inner diameter of the hole and the surrounding background. Then the other end of the hole can be examined in similar fashion, with the focus end angle of incidence changed to show the transition most clearly. As best seen in Fig. 6, the illuminating light from the ends of the optical fibers 38 in the light ring 36 provides a 360° closed sheath of light, but the light is somewhat divergent. Consequently, the parabolic first reflector surface 44 collects the light and substantially collimates it, directing it toward the second, angle generator surface 48, from whence it is directed as a cone toward a focal point. As the angle of incidence is changed, the two reflector rings 42, 46 are moved together, but with varying gap between them. Thus the light from the first, collector surface 44 impinges on a different region of the curved, angle generator surface 48, to change the angle of incidence but maintain the same focal point. By controlling the remote light source 32 to illuminate different groups of the optical fibers 38, in quadrants around the light ring 36, the product 16 can be illuminated from different sides, as well as angles, improving the versatility of the system. While there have been described and illustrated in the drawings, various forms and variations in accordance with the invention, it will be appreciated that the invention is not limited thereto but encompasses all exemplifications with the scope of the appended claims.

Claims: A vision system (10) comprising a lens system (18) and an illuminating system (30), the lens system (18) defining a vertical optical axis, the vision system (10) being suitable for inspection, measurement or recordation of a product (16) arranged along the optical axis, the illuminating system (30) comprising a remote light source (32) coupled to a flexible optical fiber harness (34) terminating in a light ring (36) for generating a hollow cylinder or sheath of light concentric with the optical axis and directed toward the product (16), a first ring element (42) concentric with the optical axis and defining an aperture between the lens system (18) and the product (16) and having an outer reflecting surface (44) for deflecting the cylinder or sheath of light in a first direction, and a second ring element (46) having a reflecting surface (48) disposed in the path of the light reflected from the first reflecting surface (44) for deflecting the light onto the product (16), characterised in that a differential drive mechanism (62,64,66,52,54,56) is adapted to move the first and second ring elements (42,46) at different rates of movement in the axial direction and the reflecting surfaces (44,48) of the ring elements (42,44) are curved in axial cross-section so that the incident angle of the light deflected onto the product (16) by the second ring element (46) can be varied while maintaining a constant focal point. A vision system (10) according to claim 1, characterized in that the first reflecting surface (44) has an outer surface of parabolic cross section and the second reflecting surface (48) has a substantially circular cross section. A vision system (10) to claim 1, characterized in that the differential drive mechanism comprises a pair of parallel racks (54,56) having facing teeth of equal pitches and drive gear means (62,66) disposed between the racks (54,56) and engaging the teeth, and drive motor means (52) coupled to the drive gear means (62,66). A vision system (10) according to claim 1, characterized in that the ring light (36) comprises a plurality of arc segments independently energizable to provide illumination from different directions as well as from all directions about the product (16). A vision system (10) according to claim 1, characterized in that the system further comprises an image camera (17) directed along the optical axis and toward the product. A vision system (10) according to claim 1, characterized in that in an axial plane a focal point of the first reflecting surface (44) is selected to collimate the light rays impinged upon it and to reflect substantially parallel light rays onto the second ring element (46). A vision system (10) according to claim 1, characterized by: a stable base (12,13) having a horizontal upper surface; a stable bridge (23) spanning the upper horizontal surface in an X axis direction and overlying a viewing zone on the base (12,13); platform means (14) on the upper surface of the base (12,13) for receiving a product to be viewed, the platform means (14) including Y axis drive means (15) for providing horizontal movement in the Y direction; X axis carriage means (22) mounted on the bridge (23) and including X axis drive means (24) for providing horizontal movement in the X direction; Z axis carriage means mounted on the X axis carriage means and including drive means (20) for providing vertical movement; and electronic camera means (17), including magnification means (18), mounted on the Z axis carriage means and oriented along an optical axis to view the viewing zone; and in that the means (38) providing a light pattern is disposed concentrically about the electronic camera means (17) and the optical axis for providing a cone of light on the product to be viewed; and the means (52,54,56) coupled to move the ring elements (42,46) is mounted on the Z axis carriage means. A vision system (10) according to claim 7, characterized in that the system (10) includes processor means (25) coupled to control the X axis drive (24), Y axis drive (15), Z axis drive (20) and means (52) coupled to move the ring elements (42,46) in a selected sequence of positions to focus the camera means (17) on selected positions on the product and to concurrently illuminate the product with light at preferred angles of incidence. A method of examining a product with a vision system to enhance automatic recognition of distinctive features to be measured or identified automatically by illuminating the product with a hollow cylinder or sheath of light reflected by a first reflector ring and a second reflector ring, the light being concentric with the optical axis of the vision system, characterized in that the cylinder or sheath of light is focused on a selected focal point relative to the product to be analyzed, and that the first and second reflector rings are differentially driven at different rates of movement in the axial direction so that the incident angle of the light deflected onto the product by the second element is varied while maintaining a constant focal point. A method according to claim 9, characterized by the steps of changing the selected focal plane of the product to be analyzed, and concurrently changing the angle of incidence of the said deflected light for best illumination.

Inventors: WEISNER, RALPH MARK
Applicants: VIEW ENGINEERING, INC.
Application year: 1986
Main classification: G01B11
IPC codes: G03B15, G01B11