Abstract:
Method and apparatus for calibration of 3-dimensional mapping systems applicable to automotive crash repair and diagnostics in which a calibration assembly provides accurate 3-dimensional optical inputs to a camera unit for calibration of the internal dimensional constants of the unit. The 3-dimensional calibration apparatus employs twin spaced mounting planes for patterns of optical emitters at known spacings and dispositions from the fixed camera unit position whereby tight tolerances in manufacture and assembly enable the apparatus to be more compact than a typical work piece to be mapped.

Description:
BACKGROUND 
     This application relates to a method and apparatus for mapping system calibration. An example of the application of the method is to a three-dimensional mapping system for determining the coordinates in space of identifiable locations on a crashed automotive vehicle. Specific embodiments described below relate to an optical system for three-dimensional automotive vehicle mapping and diagnostics operations. Certain of the broader aspects may be applicable to mapping systems (particularly but not exclusively for automotive diagnostic and repair work) utilizing other energy sources than optical energy. 
     Existing techniques for the calibration of equipment used in relation to dimensional and coordinate mapping and the like operations tend to be based on the obviously applicable technique of carrying out a series of dimensional mapping steps using known dimensional data (whereas in future use of the equipment that dimensional data will be to be determined), so that the unknown parameters, perhaps in relation to the equipment itself, can be accurately determined. 
     Thus, in relation to optical camera equipment, typically the technique is employed of using the optical equipment in a manner corresponding generally closely to that which will be employed when using the equipment in the field, and identifying the coordinates of a series of calibration locations by means of a calibration measurement machine (CMM). 
     Such a technique is somewhat laborious and slow and expensive as a basis for calibration of cameras on a production line, as is required. The step by step procedure is obviously slow. The technique also requires a substantial amount of space, since the calibration locations need to be, generally speaking, disposed at least as far from the camera as will occur in use, and preferably further, in order to enhance the accuracy of the determination. Thus, the technique is relatively slow, relatively costly and relatively inconvenient. An example of a published disclosure relating to in-field calibration of dimensional measuring equipment (applicable to vehicle wheel alignment) is disclosed in WO 08/12503 (Snap-on Technologies Inc). This technique utilizes for calibration purposes moveable targets which are disposable in either of two calibration positions for use in a sequential calibration process. 
     SUMMARY 
     An important aspect is to provide a method and apparatus particularly adapted to the calibration of optical and other mapping systems offering improvements relative to one or more of the matters discussed above, or generally. 
     Specifically in the embodiments, there are disclosed methods and apparatus for accurately determining the dimensional and locational parameters of certain critical internal components of a camera system employed in an optical measurement system used in three-dimensional automotive mapping. In these embodiments, the optical camera apparatus which is to be calibrated is itself manufactured to a relatively very high standard of constructional quality or accuracy, and accordingly the internal dimensions and locational parameters are, in principle, accurately known on the basis of the dimensional data relating to the components used and the assembly techniques employed. These dimensional parameters are, however, only nominal and do not provide a sufficiently accurate basis to be used as the dimensional starting point for calculating the coordinates of points to be mapped, in view of the relatively large distances between the camera and the points to be mapped, as compared with the internal camera dimensions under consideration. 
     In the embodiments described below it is disclosed that the techniques concerned enable the calculation of the relevant internal camera dimensional and locational data for three critical parameters of the camera. In its specific aspects, the invention is well-adapted to enable such data to be calculated. In its broader aspects, the invention provides a basis for very accurate calibration of mapping apparatus without the necessity for, in all cases, the calculation of the specific internal camera dimensions discussed in relation to the described embodiments. 
     In embodiments described below the method and apparatus is well adapted for use by way of calibrating production line equipment on a rapid and convenient basis and without adopting the step-by-step approach which has been necessary in relation to the use of CMM machines as described above. Moreover, in the embodiments described below, the apparatus described is constructed so that, and the corresponding method employs the apparatus so that, the dimensional and locational accuracy which can be readily built into the apparatus (of which only one example may be needed for a camera production line), is built into the apparatus and inherently controls the accuracy of the resultant calibration steps. 
     Thus, in the embodiments described below, we provide the means whereby optical camera apparatus forming part of a three-dimensional mapping system can be readily mounted in relation to the calibration apparatus at a location in which its dimensional position is very accurately known in relation to the relevant parts of the remainder of the apparatus. Then, the apparatus can be employed to provide simultaneously to the camera system energy signals from precisely known dimensional and locationally determined points so that (since these coordinates are known), the internal dimensions and locational data relating to the camera can be determined. Moreover, since the apparatus is extremely simple in construction its cost is relatively low. Also, since the apparatus is itself easily constructed to standards of dimensional accuracy which are better than those required for the mapping procedure itself, the overall dimensions of the apparatus can be significantly smaller than those of the typical three-dimensional article (such as an automobile) to be mapped. 
     In the embodiments, the energy source for calibration of the camera is a series of light-emitting diodes which are mounted on a pair of planar supports or mounting planes which are themselves disposed at spaced apart positions, one in front of the other and both in front of the camera unit. 
     By this arrangement, there is provided, conveniently, an accurately constructed three-dimensional array of energy sources which are precisely positioned and defined with respect to the camera unit, the latter being mounted at a fixed central front location with respect to the mounting planes and being connected thereto at a fixed distance by the structure of the apparatus, constituting spacer means. 
     Thus by adopting a simple mechanical structure providing merely a mounting for the camera module and connection means therefrom to a pair of mounting planes of simple planar construction and supporting, in each case, an array of energy sources, there is provided a simple and relatively cost effective mechanical means for effecting in a largely simultaneous manner the series of steps which have hitherto been necessary to take using CMM apparatus. Moreover, the simple mechanical structure of the apparatus enables the above-discussed provision of accuracy of dimensional construction enabling minimization of space requirements. 
     An important aspect of the embodiments relates to the simplicity of construction arising from the ability to transmit and receive energy signals between the transmitting and receiving energy modules despite the adoption of a simple mechanical construction in which the mounting planes are disposed one in front of the other, with one of these between the other and the camera. 
     The adoption of mounting planes, for example in form of boards or like rigid constructions, in the embodiments enables the precision location and position definition needed for the purposes of the system, without significant cost. However, such construction leads to the need either for an arrangement in which the energy signal can pass around the nearer board in order to reach the camera from the remote board, or else, some other arrangement is needed. 
     In order to allow for the energy signal to pass around the intermediate board, the space implications of the construction would be significant, and such would remove the above-discussed advantage (in terms of space saving) arising from the (effectively) rigid one-piece construction. By the adoption of an arrangement in which window means is provided in the front board and which is penetrable by the transmitted energy signal, and functions as described below, this potential loss of a significant advantage is avoided. 
     As to the window means itself, this is constructed so as to provide firstly the means for permitting the onward transmission of the relevant energy signal so as to permit same to be received and analyzed and its positional data to be processed. For energy transmission purposes, the embodiments provide openings in the forward one of the mounting boards or planes, these openings being positioned in accordance with the principles identified below. Alternatively, the air-gap window openings could be replaced by the provision of energy-permeable (in relation to the relevant wavelength) window elements if such were beneficial for a practical purpose connected with the calibration procedure. However, in the controlled environment of the testing location, such is unlikely to be required. 
     As to the size and position of the window openings in the forward one of the mounting planes, these are chosen on the basis of providing a direct optical path from the individual energy sources to the camera module, taking account of the exact position of the latter, and the angular implications of such. Accordingly, the window openings are generally arranged in a pattern (of openings) formed in the forward mounting plane and corresponding generally to the distribution pattern of the energy emitters on the rearward mounting plane. As to the size of the openings, these are generally a matter of design convenience in accordance with the foregoing principles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments will now be described by way of example with reference to the accompanying drawings in which: 
     FIG. 1 is a sectional view through a camera unit or module taken in a plane transverse to the camera unit&#39;s parallel energy admission slits and disposed generally vertically and centrally of the camera unit; and 
     FIG. 2 is a side elevational view of a 3-dimensional coordinate determination mapping system for automotive crash repair as disclosed in the prior-published WO98/11405 specification, and showing an automotive vehicle mounted on a hoist and having mounted thereon transmitter means which are disposed within good communication distance of corresponding receiver means provided by the camera unit of FIG. 1 together with two associated camera units, and the apparatus being connected to data processing means; 
     FIG. 3A is a rear perspective view of calibration apparatus for the system of FIG. 2; 
     FIG. 3B shows a front perspective view of the apparatus of FIG. 3A; and 
     FIGS. 4,  5  and  6  show, diagrammatically, aspects of the dimensional data and calculations applicable to the calibration process using the apparatus of FIGS.  3 A and  3 B. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIG. 2, apparatus  10  for 3-dimensional coordinate determination for crash repair and diagnostics in relation to an automotive vehicle  12 , comprises coordinate data evaluation apparatus  14  including transmitter means  16 ,  18 ,  19  connected via signal transmission lines  20  and connector junction  22  to data processing means  24  adapted to process data derived from the transmission of an energy signal between said transmitter means  16 ,  18 ,  19  and corresponding receiver means  26  to determine information with respect to the 3-dimensional coordinates of one,  19 , of the transmitter means. Transmitter means  19  is used by an operator  28  in wand fashion to identify locations on vehicle  12  of which the 3-dimensional coordinates are to be mapped. 
     Apparatus  10  as thus-far described is of published construction and operates, for mapping purposes generally as described below. Transmitter means  16 ,  18  and  19  emit energy signals in the optical range, for example by means of light-emitting diodes, and these signals are received by receiver means  26  mounted on a trolley  30  at a fixed position. Data relating to the positions of the transmitters is conveyed to data processing means  24  and, by virtue of data-processing steps based on geometric triangulation, the coordinates of the locations of transmitters  16 ,  18  and  19  can be determined, point  19  being, at any time, one of a large series of points which operator  28  is required to 3-dimensionally map. At least one of the transmitters  16  or  18  is, in the case of a damaged vehicle, located at a reference point, for example an undamaged vehicle location, so as to provide a reference basis for coordination of the mapping operation with the vehicle manufacturer&#39;s own mapping data. 
     Having thus outlined the main features of the method and apparatus as a whole, when used in its intended (non-calibration) field use manner, we now turn to the construction of receiver means with the calibration of which the present application is principally concerned. It is to be understood that, although in this embodiment, camera or receiver means  26  is calibrated using the principles described, in an alternative embodiment the calibration principles can be applied where the energy signal transmission direction is reversed, though such modification might require a different energy signal. 
     Receiver means  26  of FIG. 2 may be a camera including three energy signal sensing modules or camera units (one shown) adapted to receive an optical energy signal and adapted to be mounted at defined positions spaced apart lengthwise of a mounting beam (not shown but disclosed in the aforementioned WO 08/11405), such spacing being a known parameter for the data processing steps carried out by data processing means  24 . 
     One of camera units  32  is shown in FIG. 1, and each camera unit includes a machined, cylindrical, drum-like housing  130  having a circular clamping plate  132  retained by a ring of threaded fasteners  134 . A printed circuit board  136  is retained by clamping plate  132  and associated O-rings  138  in association with the light-sensitive charge-coupled device (ccd) array  140 , which provides an output signal which is fed via printed circuit board  136  to signal transmission lines  20  and data processing means  24 . The ccd array is mounted accurately on the camera unit center line  142 , and is likewise centered thereon. The center line  142  passes concentrically through one of two energy-admitting slits  144 ,  146 . Ccd array  140  is accurately mounted at its indicated location in relation to the structure of the camera unit housing  130 . Light enters the camera unit  32  through an opening  148  in the end of the module, and passes through a glass filter  150  and hence to the slits  144 ,  146  formed in a rectangle of thin foil  152  and aligned with V-slots  154  provided in the end wall  156  of housing  130 . Operation of the system has already been generally described. 
     Critical design distances in camera unit  32  are indicated (inter alia) at  158 ,  160 ,  162 ,  164  and  166 . This dimensional data is to be calibrated to provide accurate dimensional data for processing means  24 , together with the signals provided via transmission lines  20 , whereby mapping of the desired vehicle locations can be carried out for diagnostic and/or repair purposes. 
     Having considered the general mode of operation of apparatus  10  for 3-dimensional coordinate determination in its field use mode as a basis for reference, we turn now to calibration aspects of the use of the system. Before the calibration process, the general arrangement of the apparatus  10  is modified from that which is shown in FIG.  2 . The latter has been provided in order to show the general mode of use of such apparatus. Referring to FIGS. 3A and 3B, for calibration purposes, each of the three camera units  32  of receiver means  26  is individually calibrated, one at a time, such calibration being effected by use of calibration apparatus  200 , and the camera unit being located at a fixed or known location relative to the locations of transmitters  204  throughout the data evaluation calibration steps. 
     Thus, turning to the actual construction of calibration apparatus  200 , as shown in FIGS. 3A and 3B, the apparatus provides fixed and accurately-defined and precisely known locations at  206  for camera  32  and at known distances and spacings therefrom for transmitters  204 . Apparatus  200  comprises a base  208  having supported thereon at fixed positions a camera unit mounting  210  and a first mounting plate  212  and a second mounting plate  214 . The mounting plates serve to support the transmitters  204 , as will be explained. Base  208  simply serves as an inextensible support structure for the camera unit mounting  210  and the first and second mounting plates  212 ,  214 . Camera unit mounting  210  is a simple fixed structure which is dimensionally stable and provides a totally stable and accurately-defined camera unit support location  206  at which camera unit  32  can be mounted in a predetermined and precisely defined position. 
     First and second mounting plates  212 ,  214  are simple planar structures, likewise mounted at precisely defined positions and in parallel vertical attitudes on base  208  (but their positions and attitudes may be varied in relative and absolute terms provided the position data is known). They are constructed as dimensionally stable planar structures from natural or synthetic materials and their front surfaces (toward camera unit mounting  210 ) respectively define mounting planes. The structure is relatively unimportant, provided dimensional stability and accuracy of placement of the transmitters is available. To provide strength and rigidity and structural/dimensional stability there are provided between the first and second mounting plates  212 ,  214  three spacer rods  216  disposed at the corners of a triangle, as shown in FIGS. 3A and 3B. 
     Turning now to the array  202  of transmitters  204 , these are disposed on plates  212 ,  214  in generally uniformly spaced relationship, in straight lines, the lines being disposed generally at right angles. On the front mounting plate  212  the transmitters  204  are disposed in a series of vertical and horizontal lines forming a rectangular block or grid of rows and columns of transmitters, whereas on the rear plate  214 , the transmitters are disposed in a single horizontal line  218  and a single vertical line  220 . 
     Window means  222  is provided in front mounting plate  212  to allow energy signal transmission through the front plate to camera unit  32 . Window means  222  comprises a block  224  of three vertical rows of linearly-disposed apertures  226 , together with a horizontal line  228  of larger apertures comprising a large central somewhat trapezoidal aperture  230  bounded by smaller apertures  232 ,  234 ,  236 . 
     In use apparatus  200  and camera unit  32  are employed in association with data processing means  24  to establish the dimensional parameters of the camera unit  32  in accordance with matters discussed above, and utilizing the known position data relating to transmitters  204  on mounting plates  212 ,  214 , as will be more fully described below. 
     Turning now to the details of the calibration process, the method essentially uses a static array of light sources or transmitters  204  to map pixel space (as read by camera unit  32 ) to angular space (positions to be mapped). The dimensional data determined in the calibration process provides a manufacturing diagnostic for each camera module or unit  32  because the relationship between the camera centroid position and its angular plane or disposition is determined separately for each camera module or unit. 
     In this embodiment, the variables to be determined in the calibration procedure are: 
     a) the distance from the slit (or lens) in camera unit  32  to the CCD array  140 , this being done for each of the three camera modules individually. This is the distance  160  in FIG.  1 . 
     b) the distance between the centers of the slits  144 ,  146 , which distance is identified in FIG. 1 as  164 ; and 
     c) the offset distance between the axis of the slit and the calibration origin of camera  32 , defined by the “zero position transmitter” or light-emitting diode (LED)  204  in the apparatus  200 . 
     These three sets of parameters can be calculated by combining positional data derived from the two static LED arrays provided by mounting plates  212  and  214 , which are at a known spacing. It is implicit in this calculation that all LED  204  positions are known to a high degree of accuracy, namely better than 0.5 mm and preferably better than 0.05 mm, and that the assembly is mechanically stable (less than 0.01 mm movement) in the environment in which it is used. 
     Thus, calibration apparatus  200 , comprising as it does the first and second mounting planes at the front surfaces of the mounting plates  212 ,  214  with their associated LED arrays of transmitters  204 , these planes respectively, are positioned along the y-x plane (top surface of plate  208 ) at x=x 0  and x=x 0 +δx. These two mounting planes define the calibration volume and, as mentioned above, the two plates  212 ,  214  are positioned to a high degree of accuracy, namely 0.5 mm and preferably 0.05 mm, and they must remain mechanically stable (movement less than 0.05 mm) and parallel throughout the measurement procedure. It will thus be appreciated that the procedure is preferably carried out in a thermally controlled enclosure. 
     Calibration of Distance from the Slit to the Ccd Array 
     This value is nominally known for all three camera units  32 , within the mechanical tolerances of the unit assemblies. However the present calibration procedure is intended to enable the determination of these distances by the following steps: 
     i) at known distance x (from the camera unit  32  to, e.g., the front surface of mounting plate  212 ) measure the separation of two LEDs  204  of known separation in the y axis direction (for two of the camera units  32  of receiver means  26 ) or in the z axis direction (for the other camera unit), depending upon the orientation of the slit in each camera unit  32 ; 
     ii) by use of a similar triangles procedure illustrated in FIG. 4, the distance r from the slit to the CCD array (indicated as  160  in FIG. 1) can be determined since x and d (the known horizontal or vertical separation of the LEDs  204  on the mounting plates  212 ,  214  see FIG. 4) are known and A is a camera unit reading of the imaged separation of the LEDs viewed, whereby r is given by the expression: 
     
       
         
           r=xA/d. 
         
       
     
     Since apparatus  200  provides two plates  212 ,  214  of LEDs, there are a number of combinations of LED pairs  204  separated by different distances which can be used in the above manner for this determination and thereby the results can be averaged for further accuracy. 
     Determination of Offset 
     This procedure, illustrated in FIG. 5, determines the displacement δ A,B,C  in the y axis of the outermost LED position (for two of the three camera units  32 , denoted as camera unites A, B, C) from the center of the slit for each of the particular camera units  32 . 
     The displacement δ A,B,C  calculated using a single LED  204  from each of the first and second mounting plates  212 ,  214 . If m and n refer to the m th  LED in the back plate  124  and the n th  LED in the front plate  212  and d is the LED pitch along the y and z axes then the displacement δ m,n   A,B,C  for one of the three camera units under calibration is given by Equation 1 herewith. In Equation (1) r A  is the slit to CCD separation (i.e. distance  160 ) for a camera unit A, and A m,n  is the distance separation in the m th  and n th  LED image positions imaged on the CCD array  150 . As the latter quantity refers to distance, the read out will need to be converted from a pixel number recorded on the array  140 .                  δ     m   ,   n     A     =       {         A     n   ,   m         r   A       -     d        [         (     n   -   1     )       x   0       -       (     m   -   1     )         x   0     +     δ   x           ]         }          {     1       1     x   0       -     1       x   0     +     δ   x             }              
            for                 m     ,                n   =     1                 to                   m   max         ,     n   max               (   1   )                                
     Similarly the offset δ for each of the other two camera unites (B, C) can be determined using Equations 2 and 3 herewith.                  δ     m   ,   n     B     =       {         B     n   ,   m         r   B       -     d        [       n     x   0       -     m     x   +     δ   x           ]         }          {     1       1     x   0       -     1       x   0     +     δ   x             }              
            for                 m     ,     n   =       ±     1   2                       m   max         ,     n   max               (   2   )                   δ     m   ,   n     C     =       {         C     n   ,   m         r   C       -     d        [         n   -     n   max         x   0       -       m   -     m   max           x   0     +     δ   x           ]         }          {     1       1     x   0       -     1       x   0     +     δ   x             }              
            for                 m     ,     n   =     1                 to                   m   max         ,     n   max     ,             (   3   )                                
     For the central camera unit (e.g., B), the center LED of the plates  212 ,  214  is taken as the origin and the offset is measured in the z axis direction. For one of the other two camera units (e.g., C) the offset is measured in the y axis direction. Since the separation of the LEDs is known, the separation of the camera units A and C can be determined. The position of camera unit B denotes the z origin. The relative positions of the camera units A and C should have minimal effect on the accuracy of the calibrated system. 
     Average values of the offset δ can be determined which will allow for correction of systematic errors due to rotation of the camera unit about the z-axis. The effect of dispersion due to the glass filter on the front of the camera units  32  is not significant. 
     Angle Mapping and Slit Separation 
     Having determined the offset δ A,B,C  for each camera unit  32  (A, B, C), the pixel position of each LED in the front mounting plane  212  can be measured. Since the position of each camera unit  32  with respect to each LED  204  is known, the angle subtended by the LEDs  204  from each of the camera unit origins can be calculated and thus the camera calibration of pixels against angles can be determined in accordance with Equations 4, 5 and 6 herewith.                θ   i   A     =         tan     -   1            {       x   0           (     i   -   1     )        d     +     δ   A         }                   for                 i     =     1                 to                   i   max                 (   4   )                 θ   j   B     =         tan     -   1            {       jd   +     δ   B         x   0       }                   for                 j     =     ±     j   max                 (   5   )                 θ   i   C     =         tan     -   1            {       x   0         2      D     -       (     i   -   1     )        d     -     δ   A         }                   for                 i     =     1                 to                   i   max                 (   6   )                                
     Where x, y, z are now calculated with an origin centered at the slit for camera unit A, and 2D which is the separation of the camera units A, C (i.e. outermost camera units) has been calculated from the known LED pitch d and offsets δA and δB (for camera units A and B) in terms of 2D=6d+δA+δC. 
     Camera units A and C will each have a calibration comprising pixel coordinated data for each row of LEDs while camera unit B will have pixel/angle coordinates for each column. This allows calculation of a given angle from a pixel value using linear interpolation (see FIG.  6 ). Thus, if an image is formed at the i th  point, which lies somewhere between the j th  and the j+1 th  calibration points, then the image angle θ i  is given by Equation (7) herewith, where P j  denotes the pixel location of the j th  calibration point and M j  is the pixel location of the data point.                θ   i     =       θ   j     +     {       (         m   j       p     j   +   1         -       p   j       p   j         )          (       θ     j   +   1       -     θ   j       )       }               (   7   )                                
     The x, y, z coordinates can then be calculated from the equations for the intersections of the three planes in accordance with Equations 8, 9 and 10 herewith, where θ A  θ B  and θ C  are the angles subtended by the LEDs with respect to the camera unit origins and 2D is the separation of the camera units A and C.              y   =       2      D       1   +       tan                   θ   A         tan                   θ   C                     (   8   )               x   =     y                 tan                   θ   A               (   9   )               z   =     x                 tan                   θ   B               (   10   )                                
     It should be noted that in an ideal design any movement in the z plane alone should not change the positions of the centroids on the camera units A and C. In reality, mechanical tolerances can rotate the slit which results in a translation of the centroid without a change in the y coordinate. If the effect is large then a second calibration axis may be required so that a 2-dimensional relationship is established between the centroid and the angle of view.