Abstract:
Apparatus for optically measuring a remote object comprises at least one source for projecting a plurality of discrete zones of electromagnetic radiation (for example laser spots) along a projection plane. Imaging apparatus images a plurality of reflections from the remote object. By spatially offsetting the imaging apparatus from the projection plane, easier discrimination and identification of the reflections is made possible.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to triangulation-based non-contact measurement. In particular, this invention relates to multipoint triangulation-based non-contact measurement wherein a plurality of light sources are reflected and imaged on an array.  
       BACKGROUND OF THE INVENTION  
       [0002]     In multipoint triangulation-based sensors, a series of aligned spots reflected from a workpiece are imaged by a camera that lies in the same plane as both the spots and the light sources.  
         [0003]     The camera typically comprises a two-dimensional pixel array onto which the spots are imaged. The range to each of the spots is derived by triangulation from the location of its image on the array.  
         [0004]     As was discussed in U.S. Pat. No. 5,986,745 to Hermary et al., a known limitation of such systems is the difficulty in sorting or discriminating between adjacent spots, for example as a result of the proximity or overlap of imaged spots, or due to dropout of a spot by occlusion from irregular surface characteristics.  
         [0005]     One approach to enhancing reliable discrimination between spots is to more accurately determine the centroids of the spots along the triangulation axis. U.S. Pat. No. 5,056,922 to Cielo et al. discloses the use of an elliptical spot (e.g. the shape characteristic of TEM01 mode laser light) wherein the longitudinal axis of the elliptical spot is perpendicular to the longitudinal axis of the detecting elements of a linear array. This allows more accurate determination of the centroid of the spots and hence, better discrimination between adjacent spots.  
         [0006]     U.S. Pat. No. 5,986,745 to Hermary et al. addresses the problem by using a spatially encoded pattern of light projected onto a workpiece rather than discrete spots.  
         [0007]     U.S. Pat. No. 4,937,445 to Leong teaches the use of a small number of beams, so as to effectively limit the possibility of misidentification of a given spot, while U.S. Pat. No. 4,687,325 to Corby relies on a time-separated series of different patterns of beams.  
         [0008]     It is apparent that the problem of spot discrimination in multipoint triangulation sensing systems is well known and that a variety of approaches are used to address it.  
         [0009]     The object of the present invention is to provide an improved, elegant and simple means of discriminating between successive spots in a multipoint triangulation-based sensor.  
         [0010]     These and other objects of the invention will be better understood by reference to the detailed description of the preferred embodiment which follows.  
       SUMMARY OF THE INVENTION  
       [0011]     In known prior art multipoint triangulation systems, the camera, the set of projected spots and the light sources are all in the same plane such that single reference-axis triangulation takes place.  
         [0012]     According to the invention, the spots are imaged onto a two-dimensional array and the optical and camera set up is selected so as to provide triangulation of the spots effectively about two axes. Preferably one axis is shorter than the other.  
         [0013]     The location of successive spots imaged from a workpiece onto the array will vary less along the shorter triangulation axis than along the longer triangulation axis. Appropriate optical parameters for the shorter triangulation can be selected such that even with the maximum expected variation in height of the workpiece, the spots as imaged along the shorter triangulation axis will be constrained to defined, non-overlapping areas of the array. Thus spot discrimination is significantly improved. The longer triangulation axis is used for dimensional measurement while a combination of the shorter and longer triangulation axes is used for spot discrimination.  
         [0014]     The inventors have also found that the foregoing can be achieved by spatially offsetting the camera from the plane in which all of the aligned spots lie. This results in range triangulation about the longer axis and spot discrimination about a combination of the longer axis and the axis defined by the spatial offset.  
         [0015]     A look-up table may be used to compare the predetermined possible positions of each spot with the actual position of each reflection and thereby correctly associate each reflection with a given projected spot.  
         [0016]     In one aspect the invention comprises optical measuring apparatus comprising at least one source for projecting a plurality of discrete zones of electromagnetic radiation along a projection plane and imaging apparatus for imaging a plurality of reflections of said zones from at least one surface extending through said projection plane, said imaging apparatus being spatially offset from said projection plane. In a more particular aspect, the optical measuring apparatus comprises optical triangulation apparatus, the source comprises a laser and the zones of electromagnetic radiation comprise spots of light.  
         [0017]     In another aspect, the invention comprises optical measuring apparatus for determining spatial or dimensional characteristics of at least one remote object, comprising a plurality of sources for projecting along a projection plane a plurality of discrete zones of light onto said at least one remote object, imaging apparatus for acquiring an image of a plurality of reflections of said zones of light from said at least one remote object, said imaging apparatus being spatially offset from said projection plane, and a processor for outputting from said imaging apparatus data characterizing said image of said plurality of reflections.  
         [0018]     In a further aspect of the invention, there is also provided a record of the possible locations of each reflection in the captured image.  
         [0019]     In yet another aspect, the invention comprises a method of optically determining the spatial or dimensional characteristics of at least one remote object, comprising projecting onto said object a plurality of discrete zones of electromagnetic radiation along a projection plane, said projection plane intersecting at least one surface of said object and acquiring an image of a plurality of reflections of said zones from said at least one remote object from a vantage point that is spatially offset from said projection plane.  
         [0020]     In a further aspect, the method of the invention comprises predetermining a range of possible locations in said image of each of said plurality of reflections, and associating each one of a plurality of said reflections with one of said zones of electromagnetic radiation by reference to the actual location of each of said reflections in said image and said possible locations of each of said reflections.  
         [0021]     In yet a further aspect, the invention uses a two-dimensional array to acquire the image of the reflected spots.  
         [0022]     The foregoing was intended as a broad summary only and of only some of the aspects of the invention. It was not intended to define the limits or requirements of the invention. Other aspects of the invention will be appreciated by reference to the detailed description of the preferred embodiment and to the claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The invention will be described by reference to the detailed description of the preferred embodiment and to the drawings thereof in which:  
         [0024]      FIG. 1   a  is a diagram of a typical optical set up according to the prior art;  
         [0025]      FIG. 1   b  is a view along line  1   b - 1   b  of  FIG. 1   a;    
         [0026]      FIG. 2  is a diagram of a two-dimensional array imaging the spots from the optical set up of  FIGS. 1   a  and  1   b;    
         [0027]      FIG. 3   a  is an optical set up according to the preferred embodiment of the invention;  
         [0028]      FIG. 3   b  is a view along line  3   b - 3   b  of  FIG. 3   a  showing the offset of the camera from the projection plane;  
         [0029]      FIG. 4  is a two dimensional array for imaging the spots from the optical set up of the  FIGS. 3   a  and  3   b;  and,  
         [0030]      FIG. 5  is an illustration of a lookup table according to the preferred embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]      FIGS. 1   a  and  1   b  illustrate an optical set up according to a conventional prior art approach. A plurality of sources of electromagnetic radiation such a laser light sources  10  are aligned and arranged to project beams of light  12 ,  14  onto a workpiece (located generally in zone  16 ) along a common projection plane  18 . It will be appreciated that at least one surface of the workpiece intersects the projection plane  18 .  
         [0032]     Depending on the distance of specific surface points of the workpiece  16  from the light sources  10 , beam  12  may be reflected on the workpiece at points  12   a  (short range) or  12   b  (longer range) and beam  14  may be reflected at points  14   a  or  14   b.    
         [0033]     Camera  20  also lies in plane  18  as will be appreciated by reference to  FIG. 1   b  from which view the laser sources  10  and the camera  20  overlap within the plane  18 . Referring to  FIG. 2 , the reflections of spots  12   a  or  12   b  and  14   a  or  14   b  are imaged onto a two-dimensional array  22  and the distance between the imaged reflections and the light source is determined by the position of the reflections along the horizontal axis of the array. In the event that the reflections are  12   a  and  14   b,  the imaged reflections may be closely adjacent in array  22  or overlap as seen in  FIG. 2 , thereby inducing difficulties in sorting the reflections and associating them with the correct projected spots.  
         [0034]     According to the preferred embodiment of the invention illustrated in  FIGS. 2   a  and  2   b,  the optical set up of  FIGS. 1   a  and  1   b  is modified by spatially offsetting camera  26  from plane  18  by a distance d. As a result of the offset, triangulation is effectively accomplished about two axes, thus resulting in relative displacement of successive reflections imaged on the array  28  along both the horizontal and vertical axes as shown in  FIG. 4 . Discrimination between, for example, points  12   a  and  14   b  therefore becomes significantly easier.  
         [0035]     Dual axis triangulation in the preferred optical set up described above results in different behaviour of the reflections on the imaging array than is the case with prior art optical set ups. As the range to the spot on the workpiece varies, the corresponding reflection is displaced across the array in a generally diagonal path rather than along the horizontal axis.  
         [0036]      FIG. 5  illustrates the areas of possible locations for each reflected spot as predetermined and recorded in a lookup table. For example, the area defined by paralleliped  32  represents the possible locations for a given reflected spot on the array. A record of the areas of possible locations for each reflected spot is stored by means of the lookup table. In use, a processor assesses both row and column centroids for each reflection and such centroids are used to determine range and to assign spot identity. The processor may also output data characterizing the image.  
         [0037]     In the preferred embodiment shown in  FIG. 5 , the areas of possible location of the reflections as mapped onto the array are selected to be laterally contiguous. As a result, the identification of a reflection as belonging to one or the other of the areas can be assessed by a probability function. A reflection in the lateral center of an area can be assigned a 100% probability of being associated with that area while the probability can decrease to zero as the location of the reflected spot crosses an area boundary.  
         [0038]     While the preferred embodiment contemplates a plurality of separate light sources projecting along a common light projection plane, this is not essential to the invention. Provided that the light spots reflected from the workpiece are aligned with one another, the invention will enhance discrimination between the spots. The invention is therefore applicable to beam splitting configurations or configurations where the cameras are not necessarily disposed to project light along common planes.  
         [0039]     It is also within the scope of the invention to use more than a single camera. A first camera could be used to provide an image used principally for range determination of the various spots, while a second camera could be offset from the plane in which the spots lie for the purposes of acquiring an image used primarily for spot discrimination. In such case, it would be necessary to associate the data from the second camera with the data from the first camera in order to correlate the identity of the sorted spots to the spots for which range has been determined.  
         [0040]     It will be appreciated by those skilled in the art that the preferred and alternative embodiments have been described in some detail but that certain modifications may be practiced without departing from the principles of the invention.