Abstract:
A target that is usable in a position determination system such as, for example, a wheel alignment system, is structurally stable over wide temperature ranges, protected from humidity and chemical contamination, and not subject to breakage. A layered target structure includes a substrate board, an electro-reflective layer formed on the substrate board, a transparent sheet overlaying the electro-reflective layer, and an opaque patterned layer between the electro-reflective layer and the transparent sheet. The layered target structure may be secured a support assembly.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/730,830, filed Oct. 28, 2005, entitled “RETRO-REFLECTIVE TARGET WAFER FOR IMAGE ALIGNMENT,” the disclosure of which also is entirely incorporated herein by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The present disclosure relates to visual position determination systems, more particularly to retro-reflective target wafers used in vision imaging.  
       BACKGROUND  
       [0003]     Certain types of position determination systems, such as wheel alignment systems, use a vision imaging system that employs optical sensing devices to determine the positions of various target devices. A wheel alignment system of this type is capable of obtaining alignment information about a vehicle, such as toe, camber, ride height, toe curve, tilt angle, and the angular relationship of the vehicle&#39;s body relative to the vehicle&#39;s wheels.  
         [0004]     The wheels of a motor vehicle may be aligned in a number of ways. For example, an operator or an alignment technician can use a vision imaging system, such as a computer-aided, three-dimensional (3D) machine vision that employs image sensing devices, such as cameras, to determine the positions of various target devices. Although such vision imaging systems are typically used for alignment purposes, these systems can also be used to obtain other positional and angular orientation information about a motor vehicle. Examples of such apparatus and methods are disclosed in U.S. Pat. No. 5,724,743, entitled “Method and Apparatus for Determining the Alignment of Motor Vehicle Wheels,” issued to Jackson, et al. on Mar. 10, 1998, U.S. Pat. No. 5,535,522, entitled “Method and Apparatus for Determining the Alignment of Motor Vehicle Wheels,” issued to Jackson, et al. on Jul. 16, 1996, and U.S. Pat. No. 6,526,665, entitled “Glint-Resistant Position Determination System, issued to Jackson on Mar. 4, 2003, each incorporated herein by reference.  
         [0005]     A computer is often used in conjunction with such vision imaging systems to calculate the orientation of the target device by identifying certain geometric characteristics on the target device. The computer takes perspective measurements and compares these measurements with the true image previously pre-programmed into the memory of the computer.  
         [0006]     An example of a typical target device that can be used in a wheel alignment system is illustrated in  FIG. 1 . Target device  64  consists of a flat plate with a pattern of two or more differently sized circles  62 ,  63  marked in a pre-determined format thereon. Although a specific pattern is shown, a large number of different patterns can be used on the target device  64 . In practice, a mathematical representation, or data corresponding to a true image (i.e. an image taken by viewing the target device perpendicularly to its primary plane) and the dimensions of the target device are preprogrammed into the memory of a computer so that, during the alignment process, the computer has a reference image to which the viewed perspective images of the target devices can be compared.  
         [0007]     The computer calculates the orientation of the target device  64  by identifying certain geometric characteristics on the target device. The computer takes perspective measurements and compares these measurements with the true image previously pre-programmed into the memory of the computer.  
         [0008]     The computer could, for example, calculate the center of each of the circles  62   a ,  62   b  by means of centroiding. This is a method commonly used by image analysis computers to determine the positioning of the center point or centerline of an object. Once the center points of the two circles  62   a ,  62   b  have been determined, the distance between the two can be measured. This process is then repeated for other circles in the pattern on the target device  64 . These distances can then be compared to the true distances (i.e. non-perspective distances) between the respective centers. Similarly, the angle to the horizontal (or vertical) of the line joining the two centers can be determined. A calculation can then be made of the orientation of the target device  64 .  
         [0009]     Other methods of calculation can be used to determine the orientation of the target device  64 . For example, the camera could sight onto only one of the circles, for example the circle  63 , and by using the perspective image thereof (the distorted ellipse), calculate the orientation of that circle and, therefore, the orientation of the target device  64 , as more fully described in U.S. Pat. Nos. 5,535,522 and 5,724,743.  
         [0010]     Existing targets commonly contain retro-reflective sheets sandwiched between front and back sheets of glass for protection from humidity and chemical contamination. The retro-reflective sheets generally comprise a rather fragile material. Neither glass layer adheres to the retro-reflective sheet. The combination is glued at its edges to a housing using hard epoxy glue to provide a seal and permanent attachment to the housing.  
         [0011]     Wheel alignment systems are used predominantly in establishments such as automobile service centers, tire dealer shops, garages, repair shops, and the like. Such environments can subject the systems to a wide range of temperature, e.g., in the order of zero degrees F. to one hundred thirty degrees F. System targets additionally are often exposed to extremes of humidity and various harmful chemical products. Rough handling of the targets when being positioned on or removed from wheels is commonplace. Such handling of the target may cause the glue bond to loosen and allow humidity to wrinkle the retro-reflective sheet that is positioned between the two layers of glass. Resulting distortion can significantly affect the accuracy of the positioning system. Moreover, glass coverings of existing targets, as well as fragile retro-reflective material, can easily be broken if mishandled or dropped, rendering the targets unusable. Impact can be transferred through perimeter epoxy glue, imparting stresses to the glass elements with consequent breakage.  
         [0012]     The need remains for alignment retro-reflective targets that are structurally stable over wide temperature ranges, that are protected from humidity and chemical contamination, and are better protected from breakage.  
       SUMMARY OF THE DISCLOSURE  
       [0013]     The subject matter described herein overcomes these shortcomings. The use of glass layers, which are subject to breakage, has been eliminated. A physically strong, thermally stable, board is provided as a base to which thin retro-reflective material is adhered. Such adhesion ensures that the retro-reflective material becomes a composite of the backing material. The retro-reflective material will not be subject to wrinkling or damage from humidity. A transparent material, having a high resistance to breakage, is applied as an outer layer and is glued to the retro-reflective material. The combination can then be glued along its periphery to a supporting structure to provide a seal against humidity and contamination.  
         [0014]     The board may comprise a printed circuit board such as, for example, a board known in the industry as FR-4 board. Other thermally stable materials, such as aluminum plate, copper plate, etc., can be used in lieu of the board. The outer layer may comprise an acrylic sheet A target pattern may be photo exposed on a thin opaque layer of photo sensitive epoxy glue, or equivalent, formed on the inner surface of the outer sheet. Target images, such as circles, are thus formed and surrounded by the opaque material. An additional thin layer of glue may be used to bond the outer layer to the retro-reflective sheet.  
         [0015]     Alternatively, a photo etching process to deposit opaque epoxy ink may be carried out directly on the face of the retro-reflective material after it has bonded to the board. The outer layer may thereafter be glued with epoxy to the retro-reflective sheet. If a retro-reflective material and epoxy ink are used that are not easily scratched or damaged, an outer layer of protection may be omitted.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Implementations of the present invention are illustrated by way of example, by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements.  
         [0017]     A typical prior art wheel alignment system target device is illustrated in  FIG. 1 .  
         [0018]      FIG. 2  is an exploded view of a target composite in accordance with the present invention.  
         [0019]      FIG. 3  is a more detailed plan view of a target configuration shown in  FIG. 2 .  
         [0020]      FIGS. 4A and 4B  are exploded views, respectively, of rear and front targets and support structure in accordance with the present invention.  
         [0021]      FIG. 5  is a partial sectional view of a target and support structure of  FIGS. 4A and 4B .  
         [0022]      FIGS. 6A-6B  are illustrative views of an embodiment for mounting a target to a support structure. 
     
    
     DETAILED DESCRIPTION  
       [0023]     A composite target  10  is shown in  FIG. 2 . A thermally stable and structurally sturdy board  12  serves as a backing for a thin, highly retro-reflective, sheet  14 . Retro-reflective sheet  14  may be adhered to the board  12 . Outer layer  16  is a scratch resistant, non-breakable transparent material. Layer  12  may be composed of a printed circuit board material, such as FR-4, or other stable materials, such as aluminum plate and copper plate. Layer  16  may comprise an acrylic sheet or an equivalent material.  
         [0024]     Layer  16  contains the target circles shown in more detail in the plan view of  FIG. 3 . The target circle configuration shown may represent a front wheel target. A large circle  17  is shown encircling a smaller circle  19 , which can provide a reference for the system. The particular pattern used may be varied, as long as the system can derive sufficient information to identify the target with the wheel to which it is to be attached and to configure the coordinate system accordingly. A rear wheel target configuration may contain a different reference icon, such as a large circle without an inner circle.  
         [0025]      FIGS. 4A and 4B  are exploded views, respectively, of rear and front composite targets  10  and support structure  20 .  FIG. 5  is a partial sectional view of target and support structure configuration taken, for example, at a section A-A of  FIG. 4A . Support structure  20  contains a ledge  22  along its peripheral region upon which the composite target  10  is seated. Raised outer edge  24  surrounds target  10  for setting its position on the structure. Composite target  10  is secured at its peripheral edges to support structure  20  along the juncture of ledge  22  and edge  24  by a thin layer  26  of epoxy glue or equivalent adhesive.  
         [0026]     Manufacture of the composite target  10  may be performed by several alternative processes. In one such process, a target pattern is formed on the inner surface of a transparent acrylic sheet  16 , or equivalent. A thin layer of opaque, photo sensitive epoxy glue, or equivalent material, is applied to the inner surface and may be allowed, at least somewhat, to cure. Application of the material may be performed, for example, by a silk screen process.  
         [0027]     A master mask template, which has a highly accurate negative target pattern, is overlaid on the thin layer of photosensitive material on the inner surface of sheet  16 . For this material a commercially available substance, for example, PSR-4000/CA-40, produced by Taiyo, may be utilized. This substance is a two part liquid photo imageable solder mask that has no odor, has a fast photo speed, and is designed to produce a high-resolution image. An interferrometric plotter may be used to obtain the highly accurate negative pattern. The master template, for example, may comprise a glass sheet with the negative pattern formed by chrome or equivalent substance interferrometrically plotted onto the glass. Light, such as ultraviolet light, is applied to the thin photosensitive layer through the mask plate. Areas not masked by the master template are exposed to the light and fully cure. Areas masked by the template do not cure. Uncured areas are washed away during a chemical rinse.  
         [0028]     The rear surface of retro-reflective sheet  14  may contain adhesive with a removable backing strip. Sheet  14  may be adhered to board  12  by first aligning these sheets and applying a thin adhesive backer strip to the board  12  to preserve the alignment. The assembly may then be fed through rollers while the backing sheet is removed from sheet  14 . Light roller pressure is applied while avoiding trapping of visible air bubbles under sheet  14 . Removal of the backing strip permits application of the retro-reflective material sheet  14  securely to the board  12  without wrinkling, while providing protection against damage from humidity.  
         [0029]     A thin layer of glue may be used to bond the patterned outer sheet  16  to the retro-reflective sheet  14  after the latter has bonded with board  12 . An opaque epoxy glue or equivalent may be used. The glue may be applied to the inner surface of patterned sheet  16  by a silk screen process. A silk screen pattern permits the glue to be applied only to the dark areas of sheet  16 . For a circle target pattern, such as illustrated in  FIGS. 2 and 3 , the silk screen pattern contains circles that overlap the target circles. The silk screen circles are sufficiently larger than the target circles to prevent the glue from overlapping the target pattern, thus maintaining the accuracy of the pattern. After screening the parts are allowed to air dry until a desired tackiness is achieved. Sheets  14  and  16  are then glued together. A jig may be used to align the two sheets. While aligned, sheet  14 , which had been silk screened with the glue, is lightly pressed onto sheet  16 . Constant pressure is maintained while the glue dries. Heat may be applied to speed the drying process.  
         [0030]     The three layer target composite is then placed on ledge  22  of the support structure  20  and glued at its periphery to the edge  24  of the support frame. Epoxy or urethane glue, or equivalent, may be used to form a sealed beading  24  at the periphery of the target composite sheet.  
         [0031]     As an alternative process, retro-reflective material  14  may first be adhered to the board  12 . Next, a photo etching process can be carried out directly on the face of layer  14 . Photo sensitive material may be applied to layer  14  and then exposed to light through a mask template. After development of the pattern on sheet  14 , a layer of glue can be applied through a silk screen process to sheet  14  in a manner such as previously described. Outer layer  16  is then glued to the layer  14 .  
         [0032]     In another embodiment, the outer layer  16  can be deleted by the use of a retro-reflective material that is not easily scratched or otherwise damaged. With such an arrangement, the retro-reflective material  14  is adhered to the thermally stable plate  12 . Then, a photo etching process deposits a layer of black opaque ink directly on the face of the retro-reflective material. The resulting composite may then be glued at its edges to the supporting structure  20 .  
         [0033]     Various alternatives may be used in lieu of peripherally applied adhesive for retention of the composite target  10  to the support structure  20 . One such alternative comprises provision of target frame and seal members, as illustrated in  FIGS. 6A-6D . As shown in  FIG. 6A , the composite target  10  is positioned on ledge  22 , which is recessed within the edges  24  of the support structure  10 . Ledge  22  contains mounting holes  28 . Overlapping the periphery of the target  10  is a seal  30 , a bottom corner portion of which is shown in  FIG. 6D . The seal comprises a top layer  32  that overlaps the target and a side layer  34  that surrounds the edge of the target.  
         [0034]     A frame  36 , partially shown in a bottom view in  FIG. 6C , comprises a top surface  38  and side surfaces  40 . The side surfaces comprise through holes  42 . Frame  36  is configured to overlap the seal  30  and target  10 , with through holes  42  in alignment with through holes  28  of ledge  22 . In fabrication of the completed structure, screw fasteners or the like retain ledge  22 , seal  30  and frame  40  in tight engagement with housing  44 , as shown in  FIG. 6B . Screws may be inserted through holes in the bottom of the housing (not shown) and retained in aligned holes within the top portion of the housing.  
         [0035]     In this disclosure there are shown and described only preferred embodiments of the invention and but a few examples of its versatility. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For example, although the invention has been described in the context of wheel alignment, it is applicable to a variety of other position determination systems. The particular target configurations disclosed are merely exemplary. The invention may be used, for example, with different circular, geometrical or other arbitrarily configured targets.