Abstract:
There is described a scanner having a frame with a support, two lasers attached to the frame and two optical sensors attached to the frame, the lasers and optical sensors being positioned and oriented to reduce shadowing effects for the cameras and for the laser lines while covering close to 100% of the surface of an object to be imaged and reducing the scanning time.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 USC §119(e) of U.S. Provisional Patent Application No. 60/989,880 filed on Nov. 23, 2007, the contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of three-dimensional surface measuring scanners, such as those for dental applications, medical applications, automotive applications, and other areas where a 3D surface is measured and reproduced virtually. 
       BACKGROUND OF THE INVENTION 
       [0003]    Multiple scanners, such as those in the dental industry, are based on laser triangulation. In these systems, a laser beam is projected on an object to be digitized, such as a dental casting, for example. By imaging the diffuse reflectance of the laser beam on the object, the distance to the object is measured and a three-dimensional (3D) image of the object can be reconstructed. 
         [0004]    Numerous dental scanners include a single sensor to take images of a dental casting in order to create the 3D image. The sensor is displaceable in order to take images of different views of the dental casting. Then the dental scanner creates a 3D image of the dental casting from the different views and using data processing. The sensor is usually moved by means of a motor and the exact position of the sensor must be known when the images of the dental casting are taken. Because the movements of a motor are not always perfectly reproducible, an error on the sensor position can be introduced which affects the quality of the scan and periodic calibration of the scanner is needed to reduce the error on the sensor position. 
         [0005]    Therefore, there is a need for an improved scanner. 
       SUMMARY 
       [0006]    There is described a scanner having a frame with a support, two lasers attached to the frame and two optical sensors attached to the frame, the lasers and optical sensors being positioned and oriented to reduce a shadow effect for the cameras while covering close to 100% of the surface of an object to be imaged. The present scanner is not to be limited to dental applications. 
         [0007]    According to a broad aspect, there is provided a scanner system for scanning an object comprising: a frame comprising a support for receiving the object, the support having a rotational axis extending vertically therefrom; a first laser and a second laser attached to the frame and emitting a first beam of light centered on a first illumination axis and a second beam of light centered on a second illumination axis, respectively, the first laser being positioned and oriented such that the first beam of light illuminates at least a first region of the object when received on the support, the second laser being positioned and oriented such that the second beam of light illuminates at least a second region of the object when received on the support, the first illumination axis and the second illumination axis defining an illumination plane and intersecting the rotational axis at a first intersection angle and a second intersection angle, respectively, the first intersection angle being comprised in a first range from about −15 degrees to about −5 degrees or from about 5 degrees to about 15 degrees, and the second intersection angle being comprised in a second range from about −65 degrees to about −55 degrees or from about 55 degrees to about 65 degrees; and a first optical sensor and a second optical sensor attached to the frame and having a first field of view centered on a first sensing axis and a second field of view centered on a second sensing axis, respectively, the first optical sensor being positioned and oriented on one side of the illumination plane such that at least the first region of the object is within the first field of view and the second optical sensor being positioned and oriented on another side of the illumination plane such that at least the second region of the object is within the second field of view. 
         [0008]    A field of view of a sensor is defined as the angular extent of the observable world that can be measured by the sensor. The field of view is also called the angular field of the sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
           [0010]      FIG. 1  is a perspective view of a scanner comprising a rectangular frame, in accordance with an embodiment; 
           [0011]      FIG. 2  illustrates the positioning of lasers with respect to rotational axis, in accordance with an embodiment; 
           [0012]      FIG. 3  illustrates the positioning of cameras with respect to an illumination axis, in accordance with an embodiment; 
           [0013]      FIG. 4  is a perspective view of a scanner comprising two scanning modules, in accordance with an embodiment; 
           [0014]      FIG. 5  is a perspective view of a scanning module comprising a single camera, in accordance with an embodiment; 
           [0015]      FIG. 6  is a perspective view of a scanning module comprising two cameras, in accordance with an embodiment; and 
           [0016]      FIG. 7  is a perspective view of a scanner comprising a ball joint plate for tilting an object to be scanned, in accordance with an embodiment. 
       
    
    
       [0017]    It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
       DETAILED DESCRIPTION 
       [0018]      FIG. 1  illustrates one embodiment of a scanner  10  for digitizing an object in order to obtain a 3D representation of this object. The scanner  10  comprises a frame  12 , a first laser  14  emitting a first beam of light, a second laser  16  emitting a second beam of light, a first camera  18  having a first field of view, and a second camera  20  having a second field of view. The frame  12  also comprises a support  22  adapted to receive the object to be scanned. The object is received on the top surface of the support  22  which is rotatable about a rotational axis  24  which extends perpendicularly from the top surface. The lasers are used for illuminating the object and the cameras are used for imaging the diffuse reflectance of the laser beam on the object. By rotating the object, it is possible to scan substantially the whole surface of the object. 
         [0019]    The first laser  14  is positioned and oriented such that its emitted beam of light illuminates at least one region of the object when this object is positioned on the support and such that an illumination axis  26  on which the beam of light of the first laser  14  is centered intersects the rotational axis  24 . The second laser  16  is positioned and oriented such that the beam of light emitted by the second laser  16  illuminates at least one region of the object positioned on the support  22  and such that an illumination axis  28  on which the beam of light emitted by the second laser  16  is centered intersects the rotational axis  24 . In one embodiment, the regions of the object illuminated by the lasers  14  and  16  are substantially the same. In another embodiment, these regions are different. 
         [0020]    The axes  24 ,  26 , and  28  are all comprised in a single plane defined as the illumination plane as illustrated in  FIG. 2 . The illumination axis  26  and the rotational axis  24  intersect at an angle α while the illumination axis  28  and the rotational axis  24  intersect at an angle β. Throughout the description, angles are positive if measured anticlockwise according to arrow  30 . When referring to angles between an illumination axis  26 ,  28  of a laser and the rotational axis, the angles are measured from the laser axis  26 ,  28  to the rotational axis  24 . As a result, in the embodiment illustrated in  FIG. 2 , the angle α is positive while the angle β is negative, when looking at the scanner  10  in accordance with arrow A illustrated in  FIG. 1 . It should be understood that the angle α will be negative while the angle β will be positive if the scanner  10  is viewed in accordance with arrow B. 
         [0021]    The angle α is comprised in a first range from about minus sixty-five degrees to about minus fifty-five degrees or from about fifty-five degrees to about sixty-five degrees while the angle β is comprised in a range from about minus fifteen degrees to about minus five degrees or from about five degrees to about fifteen degrees. As a result, the lasers  14  and  16  can be positioned on different sides of the rotational axis  24 , as illustrated in  FIG. 2 , or on the same side of the rotational axis  24 . The particular values for the angles α and β allow for the reduction/elimination of shadows caused by the laser lines on the object due to the particular shape of the object preventing the laser beam from illuminating a complete line of the object. In one particular embodiment, the angle α is about minus sixty degrees or about sixty degrees and the angle β is about minus ten degrees or about ten degrees. 
         [0022]    Referring back to  FIG. 1 , the first camera  18  is positioned on a first side of the illumination plane while the second camera  20  is positioned on the other side of the illumination plane. While the cameras  18  and  20  are positioned on the right and on the left of the illumination plane, respectively, when one looks at the scanner  10  according to arrow C, it should be understood that the positions of the cameras  18  and  20  can be reversed. This particular positioning of the camera allows a reduction of laser triangulation shadowing effects. These shadowing effects occur when the camera cannot see the region illuminated by the laser because of the shape of the object, such as a relief. The first camera  18  is oriented such that the region of the object illuminated by the first laser  14  is within the field of view of the first camera  18 . The second camera  20  is oriented such that the region of the object illuminated by the second laser  16  is within the field of view of the second camera  20 . 
         [0023]    In one embodiment, the first camera  18  is positioned such that its axis  32  is in a plane comprising the axis  26  of the first laser  14  and orthogonal to the illumination plane  33 . The plane comprising the axis  26  and orthogonal to the illumination plane  33  is referred to as the first orthogonal plane. The second camera  20  is positioned such that its axis  34  is in a plane comprising the axis  28  of the second laser  16  and orthogonal to the illumination plane  33 . The plane comprising the axis  28  and orthogonal to the illumination plane  33  is referred to as the second orthogonal plane. 
         [0024]    In one embodiment, the first camera  18  is positioned such that the angle ω between the axis  32  and the illumination plane  33  is about thirty degrees or about minus thirty degrees whether or not the axis  32  is comprised in the first orthogonal plane, as illustrated in  FIG. 3 . The second camera  20  is positioned such that the angle φ between the axis  34  and the illumination plane  33  is about minus thirty degrees or about thirty degrees whether or not the axis  34  is comprised in the second orthogonal plane. It should be understood that if the angle ω is positive, then the angle φ is negative and vice versa. This configuration allows for the reduction of shadowing effects. 
         [0025]    In another embodiment, the first camera  18  is positioned such that the angle ω is about forty-five degrees or about minus forty-five degrees whether or not the axis  32  is comprised in the first orthogonal plane while the second camera  20  is positioned such that the angle φ is about minus forty-five degrees or about forty-five degrees whether or not the axis  34  is comprised in the second orthogonal plane. It should be understood that if the angle ω is positive, then the angle φ is negative and vice versa. These particular camera positions provide an improved measurement accuracy as a wide vision angle is provided to the cameras  18 ,  20 . 
         [0026]    In one embodiment, the distance between the object/support and the cameras and lasers, referred to as the working distance, is defined as a function of the sensor-lens-object-motion combination. The camera is chosen as a function of parameters such as the quality of signal, frame rate speed, and the like. The lens is chosen is chosen to mediate between the object and the image (camera sensor) also taking into consideration parameters such as resolution and approximate working distance to allow for the range of the motion system. The working distance can be optimized after some testing such as determining the best resolution as a function of the working distance. 
         [0027]    In one embodiment, the lasers  14  and  16  illuminate substantially the same region of the object and this region is within the field of view of both the first and the second cameras  18  and  20 . In another embodiment, the first laser  14  and the second laser  16  illuminate different regions of the object. The first camera  18  is positioned such that the region of the object illuminated by the first laser  14  is within the field of view of the camera  18  while the second camera  20  is positioned such that the region of the object illuminated by the second laser  16  is within the field of view of the camera  20 . The first laser  14  and the first camera  18  form a first scanning assembly while the second laser  16  and the second camera  20  form a second scanning assembly. Alternatively, each one of the cameras  18  and  20  may be positioned and adapted to comprise the whole object in their field of view. 
         [0028]    In one embodiment, positioning the lasers  14  and  16  having particular values for angles α and β and positioning the cameras having particular values for the angles φ and ω allows the digitization of the whole object following a 360 degrees rotation with the utilization of only two lasers and two cameras. Furthermore, the particular positions and orientations of the different components also allow the reduction of the shadowing effects and the scanning time required to digitize the whole object. 
         [0029]    In one embodiment, the scanner  10  is a dental scanner for scanning dental castings, prostheses, crowns, bridges, and other dental objects. The angles α and β are equal to about ±10 degrees and about ±sixty degrees, respectively, while the camera angles φ and ω are both equal to about ±thirty degrees, with angle φ being positive if ω is negative, and vice versa. 
         [0030]    In one embodiment, the support  22  comprises two translation stages in addition to the rotation stage allowing the rotation about the rotational axis  24 . The translation stages allow moving the object to be scanned in a horizontal plane in the event the size of the object is too large to allow a complete scan of the object following a 360 degrees rotation. 
         [0031]    It should be understood that the scanner  10  can comprise further lasers and cameras in addition to the lasers  14  and  16 , and the cameras  18  and  20 . 
         [0032]      FIG. 4  illustrates one embodiment of a scanner  50  comprising a circular frame  52 , a first scanning module  54 , a second scanning module  56 , and a support assembly for receiving an object to be scanned. The support assembly comprises a motorized rotational stage  58  having a rotational axis  60 . Two motorized translation stages  62  and  63  are mounted on the rotational stage  58 . 
         [0033]    The first scanning module  54  comprises a laser  64  emitting a laser beam  66  and a camera  68  having an angular field or field of view  70 , as illustrated in  FIGS. 4 and 5 . The second scanning module  56  comprises a second laser  72  emitting a laser beam  74  and a second camera  76  having an angular field  78 . The lasers  64 ,  72 , and the cameras  68 ,  76  are fixedly secured into their respective scanning module  54 , such that no relative movement occurs between the laser  64 ,  72  and the camera  68 ,  76 . 
         [0034]    The laser axis on which the beam  66  is centered and the laser axis on which the beam  74  is centered intersect the rotational axis  60  and these three axes are part of a single plane, namely the illumination plane. The angle between the axis of the laser  64  and the rotational axis  60  is about ten degrees while the angle between the axis of the laser  72  and the rotational axis  60  is about sixty degrees. Both scanning modules  54  and  56  are positioned on a same side with respect to the rotational axis  60 . 
         [0035]    In the scanner  50 , the axis of the camera  68  is in a first plane comprising the axis of the laser  64  and orthogonal to the illumination plane while the axis of the camera  76  is in a second plane comprising the axis of the laser  72  and orthogonal to the illumination plane. When looking at the scanner  50  according to arrow D, the angle between the axis of the camera  68  and the axis of the laser  64  is about thirty degrees while the angle between the axis of the camera  76  and the laser  72  is about −thirty degrees such that each camera  68 ,  76  is located on a different side of the illumination plane. 
         [0036]    In one embodiment, the scanner  50  is about twenty-eight inches high while the circular basis  52  has a diameter of about twenty-two inches. The working distance between the camera lens/laser and the center of the measurement field is from about six inches to about seven inches. 
         [0037]    While the scanning modules  54  and  56  each comprise a single camera  68 ,  76 , it should be understood that a scanning module may comprise two or more cameras, as illustrated in  FIG. 6 . The scanning module  80  comprises a single laser  82  and two cameras  84  and  86 . In one embodiment, the scanning modules  54 ,  56  are each replaced by a scanning module  80  in the scanner  50 . Using scanning module  80  allows a further reduction of the shadow effects. 
         [0038]    In one embodiment, the scanning modules  54  and  56 , and the rotational stage  58  are fixedly secured to the frame  52  while the lasers  64 ,  72  and the cameras  68 ,  76  are fixedly secured into their respective scanning modules  54 ,  56  such that the axis of the lasers  64  and  72 , the axis of the cameras  68  and  76 , and the rotational axis  60  have a fixed relative position. Having the axis of the cameras and the lasers in a fixed position relative to the rotational axis  60  which is fixed relative to the frame  52  ensures that the elements of the scanner  50  will not be out of alignment because of vibrations, for example. As a result, the number of recalibrations of the scanner  50  needed after its fabrication is greatly reduced. Any mechanical means and/or fabrication techniques for fixedly securing the different elements together can be used. For example, welding, precision machining such as electro-discharge machining, and adhesives can be used for fixedly securing the elements together and ensuring that no axis will move with respect to the other axes. 
         [0039]      FIG. 7  illustrates the scanner  50  in which the support assembly is further provided with a ball joint plate  88  and a dental casting holder  90  to receive the object to be scanned. The association of the ball joint plate  88  and the dental casting holder  90  allows tilting of the dental casting in any direction. 
         [0040]    The frame can have any shape allowing the appropriate positioning and orientation of the lasers and cameras. The frame can be made from any solid material such as aluminum, for example. In one embodiment, the frame can be a box-like frame that can be closed during scanning for avoiding any interference between the light emitted by the lasers and the ambient light surrounding the scanner. Alternatively, the scanner can be an “open” scanner such as scanner  50 . In this case, the lasers and cameras are adapted to minimize the influence of the surrounding light on the measurements. For example, high power lasers and high sensitivity cameras can be used. 
         [0041]    In one embodiment, the scanner comprises vibration reducing elements. For example, high quality cross-roller bearings may be inserted in the rotational stage in order to reduce vibrations during rotation of the object. 
         [0042]    Any laser emitting light detectable by the cameras can be used. For example, the lasers can emit visible light such as light at 660 nm, infrared light, and the like. The beam of light emitted by the lasers can have any profile, such as a Gaussian profile. Additional optics may be used for obtaining a structured light beam. 
         [0043]    While the present description refers to cameras for detecting the light reflected by the lasers, it should be understood that any optical sensor adapted to detect the reflected light can be used. A charge-coupled discharge (CCD) camera, a complementary metal-oxide-semiconductor (CMOS) active-pixel sensor, and the like are examples of optical sensors that can be used in the scanner. In one embodiment, the camera is a CMOS based 1 megapixel monochrome 10 bit digital camera. 
         [0044]    In one embodiment, the scanner further includes an electronic module for controlling the motorized translation and rotation stages, the lasers and the cameras. 
         [0045]    In one embodiment, a computer is used to operate the scanner and generate the 3D image of the scanned object. In this case, the CPU of the computer is connected to the electronic module. The commands entered in the computer are sent to the electronic module which executes the tasks related to the commands. While the object is rotated, the cameras take images of the object illuminated by the lasers. The CPU receives the images of the object taken by the cameras. From these images, the CPU creates a 3D image of the object using laser triangulation. It should be understood that any laser triangulation methods known by a person can be used to generate the 3D image of the object from the received images taken by the cameras. 
         [0046]    In one embodiment, the object is rotated twice from zero degrees to 360 degrees. During the first rotation, images of the object illuminated by a first laser are taken by a first camera. The images are sent to the CPU and stored in a memory. During the second rotation, the object is illuminated by a second laser and images are taken by a second camera. This second set of images is then sent to the CPU and stored in the memory. Alternatively, profiles of the object are extracted “on the fly” from the received images and these profiles are stored in memory. 
         [0047]    The embodiments described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.