Abstract:
A stereo camera system suitable for use in high performance computer vision systems has two or more electronic cameras. Each camera has an assembly comprising a light sensing array and a lens assembly which are each directly mounted to a rigid mounting member. The rigid mounting member may be, for example, a thick metal plate penetrated by a plurality of apertures which provide light paths between the lens assemblies and the light sensing arrays. Stereo camera systems according to the invention are more rugged than current stereo camera systems. Calibration is maintained for longer periods of time. Relative motion between the two or more cameras is virtually eliminated. Relative motion between the optical components of individual cameras is virtually eliminated.

Description:
TECHNICAL FIELD 
     This invention relates to stereo camera systems in which images obtained by two or more spaced apart cameras are used to obtain three-dimensional information about objects in the fields of view of the cameras. The invention has particular application to stereo camera systems for use in high accuracy computer vision applications. 
     BACKGROUND 
     Stereo camera systems are used to acquire three-dimensional information about objects. Stereo camera systems are used, for example, in computer vision systems, 3-dimensional tracking applications, object dimensioning applications, object tracking applications, and so on. Typical stereo camera systems include at least two, and typically three or more, electronic cameras which are mounted at spaced apart locations. The electronic cameras have overlapping fields of view. A computer connected to receive images from each of the cameras can compare the images to derive three-dimensional information about objects in the field of view. Information such as the distances to the objects and the sizes, dimensions and orientations of the objects can be determined by triangulation. 
     A stereo camera system is typically calibrated by placing a known object in the field of view of the cameras. Computer software which receives images from the cameras can determine the precise relationship between the cameras from the images of the known object and also compensate for distortions introduced by the lenses. After the stereo camera system has been calibrated then the computer can be used to obtain information about objects whose positions or configurations are not known. 
     Currently available stereo camera systems use small cameras which have arrays of light sensing elements such as charge coupled devices (“CCDs”), CMOS sensors or the like. A typical camera, as is used in a typical stereo camera system, comprises a circuit board on which a light sensing array is mounted. A lens is supported above the light sensing array by a lens holder. The entire camera is mounted by the circuit board to a suitable support in a desired position relative to other cameras in the stereo camera system. 
     A problem with such existing stereo camera systems is that the calibration of the systems can degrade over time. For a stereo camera system to remain perfectly calibrated the fields of view of the cameras must not move relative to one another. In a conventional stereo camera system there is potential for movement in the mounting between the lens holders and the circuit boards, in the mounting between the circuit boards and the substrate or frame to which the circuit boards are attached and there is potential for movement of the frame itself, The multiple interfaces between components in a conventional stereo camera system make it highly likely that vibration, shocks or the like will cause the field of view of one or more cameras in a stereo camera system to shift over time. If this happens then calibration will be lost. 
     This problem has not been recognized in the prior art because prior art stereo camera systems have either been laboratory type systems not subjected to shocks or vibrations or have been used in situations where highly accurate calibration has not been regarded as being critical. The inherent susceptibility of current stereo camera systems to losing calibration has therefore not been recognized as a particular problem. 
     Tocher, U.S. Pat. No. 5,483,336 discloses a stereoscopic optical rangefinder system. The system includes two arrays of photosensitive elements which each have associated focussing optics. A separate optical system corrects for relative motions of the arrays and the focussing optics which might be caused by mechanical shock or thermal vibrations. The Tocher system adds expense and complication. It is not clear whether the Tocher system can compensate for the effects of distorted lenses or rotations of light sensing arrays relative to one another. 
     There is a need for stereo camera systems having two or more cameras which can be used for extended periods in environments where they are exposed to vibration and or shock without losing calibration. 
     SUMMARY OF THE INVENTION 
     This invention provides a stereo camera system in which a rigid mounting member, which preferably comprises a metal plate, directly supports both a lens assembly and a light sensing assembly for each of two or more cameras. The mounting member holds the lens assemblies and light sensing assemblies rigidly relative to one another. 
     One aspect of the invention provides a stereo camera system comprising rigid mounting member having a front face and a rear face; a plurality of lens assemblies rigidly affixed to the mounting member, the lens assemblies each comprising a lens; and, a plurality of light sensing assemblies rigidly affixed to the mounting member. The mounting member is penetrated by a plurality of apertures which extend between the front and rear faces. The apertures provide paths for carrying light which is focussed by the lens assemblies onto the light sensing assemblies. Each of the lenses is located to focus light through one of the apertures onto one of the light sensing assemblies. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     In drawings which illustrate non-limiting embodiments of the invention: 
     FIG. 1 is a partially sectional view through a stereo camera system according to a first prior art design; 
     FIG. 2 is a sectional view through one camera in a stereo camera system according to a second prior art design; 
     FIG. 3 is an exploded view of a stereo camera system according to a preferred embodiment of this invention; 
     FIG. 4 is a front view of a mounting member of a stereo camera system according to the invention; and, 
     FIG. 5 is a section through a stereo camera system according to a second embodiment of the invention in which several light sensing arrays are mounted on a single circuit board. 
    
    
     LIST OF REFERENCE NUMERALS 
       10  stereo camera system 
       12 ,  12 A base plate 
       14  camera 
       16 ,  16 A circuit board 
       18  light sensor 
       19  set screw 
       20  lens housing 
       22  lens 
       24  lens holder 
       28  spacer 
       30  screw 
       32  aperture 
       50  stereo camera system 
       51  lens assembly 
       52  mounting member 
       53  light sensor assembly 
       54  aperture 
       55  threaded hole 
       56  front surface 
       57  cavity 
       58  pad 
       59  mounting hole 
     DETAILED DESCRIPTION 
     Prior Art 
     FIG. 1 shows a prior art stereo camera system  10 . System  10  has a base  12  to which are mounted two or more electronic cameras  14 . Each camera  14  comprises a circuit board  16  to which a light sensing array  16 , which is typically a chip containing a CCD array, is mounted. A lens housing  20  contains one or more focussing lenses  22 . Lens housings  20  are threadedly mounted in lens holders  24  so that the positions cf lenses  22  relative to light sensing arrays  18  can be adjusted to focus light onto the light sensing arrays  18 . A set screw  19  is typically provided to lock the lens housing  20  in position after the lens has been focussed. 
     Lens holders  24  support lens housings  20  and also block light which does not pas, through lenses  22  from illuminating light sensing arrays  18 . Each circuit board  16  is spaced away from base plate  12  by suitable spacers  28 . Several suitable fasteners such as screws  30  hold circuit boards  16  in place on base  12 . Cables (not shown for clarity) supply electrical power to and carry video signals from cameras  14  to a computer or other suitable processing system (not shown). 
     Cameras  14  are typically provided in the form of a separate module which is affixed to base  12  by screws  30 . Such cameras are readily available from various suppliers of electronic equipment. As shown in FIG. 2, Many such cameras have a stack of two or more circuit boards in place of the single circuit board  16  shown in FIG.  1 . Stereo camera systems made with cameras having multiple circuit boards are typically even more prone to falling out of calibration than the stereo camera system of FIG.  1 . 
     It can be appreciated that the design of the system  10  shown in FIG. 1 permits slight movements of circuit board  16  from side to side relative to base  12  and also permits movements of lens holders  20  relative to light sensing arrays  18  and base  12 . Further the base plates  12  typically used for supporting cameras in a stereo camera system are not completely rigid. 
     FIG. 2 shows a section through one camera in another type of prior art stereo camera system in which cameras  14  are mounted behind a base plate  12 A. In FIG. 2, parts which are similar to the parts of FIG. 1 are given like reference numbers. The system  10 A of FIG. 2 has cameras  14  mounted behind a base  12 A. Each camera  14  is mounted adjacent an aperture  32  in base  12 A with long screws  30  which extend through elongated spacers  34 . The arrangement of system  10 A of FIG. 2 provides even more opportunity for relative motion of cameras  14  and for shifts in the images received by cameras  14  than does the system of FIG.  1 . 
     This Invention 
     The inventors have determined that the performance of a three-camera stereo camera system can become substantially degraded if any of the cameras moves by more than about ½ pixel relative to any of the other cameras. At currently available camera resolutions each camera should preferably not move relative to the other cameras by more than about 0.001 centimeters from the position it had when it was calibrated. 
     FIG. 3 shows a stereo camera system  50  according to the invention. System  50  has a rigid mounting member  52 . A lens assembly  51  comprising a lens housing  20  and a light sensor assembly  53  for each of two or more cameras are each directly affixed to mounting member  52 . Mounting member  52  both supports lens assemblies  51  relative to light sensing assemblies  53  and rigidly holds the components of each one of cameras  14  relative to the components of other ones of cameras  14 . 
     Mounting member  52  is preferably fabricated from a unitary piece of material. Most preferably mounting member  52  is fabricated from a metal which has a coefficient of expansion which is less than about 1.3×10 −5  inches per degree Fahrenheit. Aluminum has been found to work well because it is reasonably low in density and can be readily machined. Mounting member  52  is most preferably in excess of 1.0 centimeters thick and is preferably in the range of 15 mm thick to about 25 mm thick. A satisfactory prototype has been made in which mounting member  52  is fabricated from 17 mm thick Aluminum plate. In addition to providing rigidity, this thickness permits mounting member  52  to completely replace the lens holders  24  which are used in conventional prior art stereo camera systems. If the material used for mounting member  52  is very stiff then portions of mounting member  52  may be reduced in thickness as long as mounting member  52  remains rigid enough to hold lenses  22  and light sensing arrays  18  in position to within the desired tolerance under the worst expected operational conditions. 
     As shown in FIG. 4, mounting member  52  is penetrated by an aperture  54  at the location of each camera. Apertures  54  provide paths for light to pass through lenses  22  in lens assemblies  51  and on to light sensing arrays  18  in light sensing assemblies  53 . 
     In the preferred embodiment illustrated in the drawings, each aperture  54  has a round threaded portion  55  extending to the front surface  56  of mounting member  52 . Lens assemblies  53  each comprise a lens housing  20  which has a thread matching the thread of threaded portion  55  and is received within threaded portion  55 . Lenses  22  in lens housings  20  (not shown in FIGS. 3 and 4) are preferably bedded in epoxy or otherwise immovably mounted within lens housings  20  as is known in the art. 
     Preferably threaded portions  55  are sufficiently deep to receive substantially the entire length of lens housings  20  and lenses  22  have focal lengths such that lens housings  20  are received substantially entirely within threaded portions  55  when lenses  22  are properly focussed. This supports lens housings  20  against torsional loads that could be caused if system  50  is dropped or otherwise jarred. Where mounting member  52  comprises a thick plate of rigid material, threaded portions  55  may be machined in holes bored in mounting member  52 . Preferably the threads on both lens housings  20  and mounting member  52  are precisely machined so that lens  22  will remain stationary relative to mounting member  52  even if system  50  is jarred. 
     In the illustrated embodiment, each light sensing assembly  53  comprises a light sensing array  18  rigidly mounted to a circuit board  16 . The light sensing array may be connected to circuit board  16  by way of leads from light sensing array  18  soldered to the circuit board  16 . 
     Circuit boards  16 , which carry light sensing arrays  18 , are directly affixed to rigid mounting member  52 . Cavities  57  at the rearward sides of apertures  54  communicate with threaded portions  55 . Cavities  57  are dimensioned accommodate light sensing arrays  18 . Preferably each circuit board  16  is affixed to mounting member  52  with three or more fasteners which are spaced apart around the periphery of a cavity  57 . Most preferably the fasteners are screws received in threaded holes in mounting member  52 . Preferably but optionally circuit boards  16  are also adhered to mounting member  52  with epoxy or another suitable adhesive. 
     Circuit boards  16  may be standard fiberglass circuit boards but should have a thickness sufficient to provide a degree of rigidity about the same as, or greater than, that of a fiberglass circuit board having a thickness of 0.082 inches. 
     After circuit boards  16  have been affixed to rigid mounting member  52  then lens housings  20  can be adjusted for focus. When proper focus has been achieved a locking material such as LOCTITE™ adhesive or an epoxy may be introduced between the threads of lens housings  20  and mounting member  52  to hold lens housings  20  in focus. The use of a locking material instead of a set screw to hold lens housings  20  in position permits lens housings  20  to be locked in place without disturbing the focus of lenses  22 . When a set screw is used to hold a lens housing in position, as shown in FIG. 1, tightening the set screw can cause the lens housing to shift slightly in its lens holder. This can alter the lens focus. 
     Rigid mounting member  52  holds each of lens housings  20  precisely spaced from other ones of lens holders  20  and also maintains each light sensing array  18  held rigidly with respect to its lens housing  20 . Threaded mounting holes  59  may be provided in rigid mounting member  52  to allow system  50  to be supported in a desired location. 
     Instead of supplying a separate circuit board  16  for each camera  14 , a single circuit board  16 A to which two or more light sensor arrays  18  are affixed at spaced apart locations may be used. As shown in FIG.  5 . Preferably circuit board  16 A is secured with three or more screws  30  at spaced apart locations around the periphery of each cavity  57  so that there is no possibility that light sensing arrays  18  will move significantly relative to threaded apertures  54  in any direction. When a single circuit board is used, as shown in FIG. 5, mounting member  52  preferably has a pad  58  on its rear surface surrounding each cavity  57 . Other portions of mounting member  52  are recessed so that they do not cause any short circuits in or mechanically interfere with components on circuit board  16 A. 
     Each of the optical components of each camera in system  50  is rigidly affixed to the same mounting member  52 . This provides a stereo camera system  50  which is much less susceptible to being disturbed by shock or vibration than are prior art stereo camera systems. Furthermore, where lens assemblies  51  are substantially received within mounting member  52 , the lens assemblies are protected from side impact. System  50  does not require a case which is completely rigid because the case does not play a significant role in maintaining the relative positions of lens assemblies  51  and light sensing assemblies  53 . 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example, while the foregoing description has described lens housings  20  as having threads which are received in threaded holes  54 , lens housings  20  could be directly attached to mounting member  52  in another suitable manner. For example, lens housings could be provided with flanges (not shown) which are attached to the front face of mounting member  52  with screws or lens housings could be welded or rigidly adhesively affixed to mounting member  52 . What is required is that mounting member  52  should be rigid and both a lens or lens housing and a light sensing array be directly attached to a rigid mounting member  52 . 
     While mounting member  52  has been described as having threads formed in it to receive lens housings  20 , the threads could be formed on threaded inserts which are rigidly affixed to mounting member  52  by a suitable process. For example, threaded inserts could be glued or welded into rigid member  52  or threaded inserts could be pressed into holes in rigid member  52 . Conventional lens holders  24  could even be rigidly attached to a rigid member  52  to receive lens housings  20  and circuit boards  16 . 
     Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.