Abstract:
A system for integrating dispersed point-clouds of an object is provided. The system includes a fixture for fixing an object, a measurement machine to scan all surfaces of the object and a computer. The fixture, which has three reference balls, is  360 -degree rotatable. The computer includes a point-cloud reading module, a sphere fitting module, a computing module, a matching module and an aligning module. The system utilizes three reference objects to integrate dispersed point-clouds of multiple scans of the object, restore original space location relations of the point-clouds, so as to obtain a complete space point-cloud of the object with simple operation and higher precision. A related method is also provided.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to measuring systems and methods, and more particularly, to a system and method for integrating dispersed point-clouds of an object. 
     DESCRIPTION OF RELATED ART 
     A point-cloud, often created by probing a surface of an object utilizing a three-dimensional (3-D) measurement machine, is a set of 3D points describing the outlines and/or surface features of the object. Generally, multiple scans, or even hundreds of scans, are required in order to obtain information of all surfaces of the object (multiple-angles data acquisition). In order to obtain a complete 3-D model/space point-cloud, multiple point-clouds from the multiple scans are aligned with each other and merged into an integrated point-cloud of the object. 
     At present, methods for aligning the multiple point-clouds requires delicate and complicated equipments, or accurate data acquisition procedures, such as presetting some reference points on the object before scanning, identifying common reference points in 3D point-clouds at different angles, ascertaining locations of the common reference points, computing coordinate transformation relations of the 3D point-clouds based on the locations of the common reference points, and aligning the 3D point-clouds according to the coordinate transformation. A predetermined condition of the method is that the reference points are represented by characteristics of the object, so it&#39;s inevitable that the method would have errors. 
     What is needed, therefore, is a system and method for aligning multiple dispersed point-clouds of an object with simpler operations and higher precision, so as to restore original space location relations of the multiple dispersed point-clouds, and obtain an integrated point-cloud of the object. 
     SUMMARY OF THE INVENTION 
     A system for integrating dispersed point-clouds of multiple scans of an object in accordance with a preferred embodiment includes a computer, a measurement machine connected to the computer, and a fixture for locating an object to be scanned. The fixture includes three reference balls. The computer includes: a point-cloud reading module for reading a point-cloud of the object and references point-clouds of the three reference balls corresponding to each of the scans; a sphere fitting module for fitting a group of three spheres which forms a scalene triangle, according to the reference point-clouds of each of the scans; a computing module for computing a location of each sphere and a distance between two adjacent spheres in each scalene triangle; a matching module for selecting one scan of the object as a base scan and selecting the scalene triangle corresponding to the base scan as a base triangle, mapping each sphere of other scalene triangles corresponding to other scans with a corresponding base sphere in the base triangle, based on the distance between two adjacent spheres in each scalene triangle; and an aligning module for aligning each sphere in each of other scalene triangles to the corresponding base sphere based on a location of the sphere and a mapping relation with the base sphere, obtaining transformation matrixes of the alignment, and for aligning the point-clouds of the object corresponding to other scans to the point-cloud of the object corresponding to the base scan according to the transformation matrixes, to obtain an integrated point-cloud of the object. 
     Another preferred embodiment provides a method for integrating dispersed point-clouds of multiple of an object by utilizing the above system. The method includes the steps of: (a) fixing the object on a rotatable fixture holding three reference balls; (b) rotating the fixture to scan all surfaces of the object, and scanning the three reference balls once when scanning each surface of the object; (c) reading a point-cloud of the object and reference point-clouds of the three reference balls corresponding to each scan towards the object; (d) fitting a group of three spheres which forms a scalene triangle, according to the reference point-clouds of the three reference balls corresponding to each scan towards the object; (e) computing a location of each sphere and a distance between two adjacent spheres in each scalene triangle; (f) selecting one scan of the object as a base scan and the scalene triangle corresponding to the base scan as a base triangle, mapping each sphere in other scalene triangles with a corresponding base sphere in the base triangle, based on a distance between two adjacent spheres in each scalene triangle; (g) aligning each sphere in each of other scalene triangles to the corresponding base sphere based on a location of the sphere and a mapping relation with the base sphere; (h) obtaining transformation matrixes of the alignment; and (i) aligning the point-clouds of the object corresponding to other scans to the point-cloud of the object corresponding to the base scan according to the transformation matrixes, to obtain an integrated point-cloud of the object. 
     The invention computes a complete space point-cloud of an object by integrating space point captures of the object. The space point captures of the object are integrated by aligning triangles formed by reference point-clouds of three reference balls in each of the captures. 
     Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of the preferred embodiment and preferred method of the present invention with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a system for integrating dispersed point-clouds of multiple scans of an object according to a preferred embodiment; 
         FIG. 2  is a block diagram illustrating main function modules of a computer in  FIG. 1 ; 
         FIG. 3  is a flowchart of a preferred method for integrating dispersed point-clouds of multiple scans of an object; and 
         FIG. 4  to  FIG. 6  show an aligning process of two groups of fitted spheres. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a schematic diagram illustrating a system for integrating dispersed point-clouds of multiple scans of an object according to a preferred embodiment. The system typically includes a fixture  10 , a measurement machine  20 , and a computer  30 . 
     The fixture  10  can be rotated 360 degrees, and is configured for receiving an object A to be scanned and exposing all surfaces of the object A to be scanned by the measurement machine  20 . There are three reference balls a, b, and c on the fixture  10 . In this embodiment, the three reference balls, which are of the same size and made of china, form three corners of a scalene triangle. In other embodiments, the three reference balls may be of different sizes and made of other materials that provide smooth and bright surfaces. 
     The measurement machine  20  is used for scanning the surfaces of the object A to obtain space point captures. Each space point capture includes a point cloud of the object A and a reference point-cloud of each of the three reference balls corresponding to one scanned surface of the object A. 
     The computer  30  is used for reading the point-cloud of the object A and reference point-clouds of the three reference balls corresponding to the scanned surface of the object A, fitting a group of three spheres of the three reference balls a, b and c according to the reference point-clouds of the three reference balls, and aligning the point-clouds of the object A to obtain an integrated point-cloud of the object A by utilizing the group of spheres (detailed description is given in  FIG. 2 ). 
       FIG. 2  is a block diagram illustrating main function modules of the computer  30 . The computer  30  includes a point-cloud reading module  310 , a sphere fitting module  320 , a deleting module  330 , a computing module  340 , a matching module  350 , an aligning module  360 , and a point-cloud outputting module  370 . 
     The point-cloud reading module  310  is programmed for reading the point-clouds of the object A and the reference point-clouds of the three reference balls corresponding to each scanned surface of the object obtained by the measurement machine  20 . For example, the point-cloud reading module  310  reads three reference point-clouds “scan 0 ”, “scan 1 ”, “scan 2 ” of the three reference balls a, b, and c corresponding to a top surface of the object A, and reads three reference point-clouds “scan 3 ”, “scan 4 ”, “scan 5 ” of the three reference balls a, b, and c corresponding to a back side of the object A. 
     The sphere fitting module  320  is programmed for fitting a group of three spheres according to the reference point-clouds of the three reference balls corresponding to each scanned surface of the object A. For example, the sphere fitting module  320  fits a first group of three spheres Q 1 , Q 2 , and Q 3  according to the three reference point-clouds “scan 0 ”, “scan 1 ”, “scan 2 ”, and fits a second group of three spheres M 1 , M 2 , and M 3  according to the three reference point-clouds “scan 3 ”, “scan 4 ”, “scan 5 ”, by applying the least square method. 
     The deleting module  330  is programmed for deleting messy points surround each fitted sphere. Furthermore, the deleting module  330  is programmed for deleting all the spheres and all the reference point-clouds of the three reference balls a, b and c, after obtaining the integrated point-cloud of the object A. 
     The computing module  340  is programmed for computing a location of each of the spheres in the space point captures, and computing a distance between adjacent spheres of the group of spheres, such as 
     |Q 1 Q 2 |, |Q 2 Q 3 |, |Q 1 Q 3 |, and |M 1 M 2 |, |M 2 M 3 |, |M 1 M 3 |. 
     The matching module  350  is programmed for selecting one of the space point captures of the object A as a base space point capture and selecting the group of three spheres corresponding to the base space point capture as base spheres, mapping each sphere in other groups corresponding to other space point captures to a corresponding base sphere according to the distance between adjacent spheres in each group. Suppose that, taking the first group of spheres Q 1 , Q 2 , and Q 3  as base spheres, because the three reference balls a, b, c on the fixture  10  form a scalene triangle, that is to say, 
     |ab|≠|bc|≠|ac|, 
     the spheres in each group also form a scalene triangle, namely, 
     |Q 1 Q 2 |≠|Q 2 Q 3 |≠|Q 1 Q 3 |, and |M 1 M 2 |≠|M 2 M 3 |≠|M 1 M 3 |. 
     If |Q 1 Q 2 |=|M 1 M 2 |, |Q 2 Q 3 |=|M 2 M 3 |, and |Q 1 Q 3 |=|M 1 M 3 |, 
     the sphere M 1  maps the sphere Q 1 , the sphere M 2  maps the sphere Q 2 , and the sphere M 3  maps the sphere Q 3 . 
     The aligning module  360  is programmed for aligning each sphere in the other groups of three spheres to each corresponding base sphere based on a location of the sphere and a matching relation with the corresponding base sphere, and reading transformation matrixes of the alignment. For example, the aligning module  360  aligns the scalene triangle M 1 , M 2 , M 3  to the scalene triangle Q 1 , Q 2 , Q 3  by translating and rotating the scalene triangle M 1 , M 2 , M 3  until the spheres M 1 , M 2 , M 3  superposes the spheres Q 1 , Q 2 , Q 3  respectively, and reads a translating matrix and a rotating matrix. Furthermore, the aligning module  360  is programmed for aligning the point-clouds of the object A of other space point captures to the base point-clouds of the object A of the base space point capture according to the transformation matrixes, such as the translating matrix and the rotating matrix, to obtain the integrated point-cloud of the object A. 
     The point-cloud outputting module  370  is programmed for outputting the integrated point-cloud of the object A into a CAD system. 
       FIG. 3  is a flowchart of a preferred method for integrating dispersed point-clouds of multiple scans of an object. In step S 100 , an object A is fixed on a fixture  10 . The fixture  10  can be rotated 360 degrees to expose all surfaces of the object A to be scanned by the measurement machine  20 . There are three reference balls a, b and c on the fixture  10 . In this embodiment, the three reference balls, which are of the same size and made of china, form three corners of a scalene triangle. In other embodiments, the three reference balls may be of different sizes and made of other materials that provide smooth and bright surfaces. 
     In step S 102 , the fixture  10  is rotated to expose all surfaces of the object A to be scanned by the measurement machine  20 . The measurement machine  20  obtains space point captures of the object A and the three reference balls a, b, and c. Each space point capture includes a point cloud of the object A and a reference point-cloud of each of the three reference balls corresponding to one scanned surface of the object A. 
     In step S 104 , the point-cloud reading module  310  reads point-clouds of the object A and reference point-clouds of the three reference balls corresponding to each scanned surface of the object obtained by the measurement machine  20 . For example, the point-cloud reading module  310  reads three reference point-clouds “scan 0 ”, “scan 1 ”, “scan 2 ” of the three reference balls a, b, and c corresponding to a top surface of the object A, and reads three reference point-clouds “scan 3 ”, “scan 4 ”, “scan 5 ” of the three reference balls a, b, and c corresponding to a back side of the object A. 
     In step S 106 , the sphere fitting module  320  fits a group of three spheres according to the reference point-clouds of the three reference balls corresponding to each scanned surface of the object A. For example, the sphere fitting module  320  fits a first group of three spheres Q 1 , Q 2 , and Q 3  according to the three reference point-clouds “scan 0 ”, “scan 1 ”, “scan 2 ”, and fits a second group of three spheres M 1 , M 2 , and M 3  according to the three reference point-clouds “scan 3 ”, “scan 4 ”, “scan 5 ”, by applying the least square method. 
     In step S 108 , the deleting module  330  deletes messy points surround each of the fitted spheres. 
     In step S 110 , the computing module  340  computes a location of each sphere, and computes a distance between each adjacent spheres in the each group such as 
     |Q 1 Q 2 |,|Q 2 Q 3 |,|Q 1 Q 3 |, and |M 1 M 2 |,|M 2 M 3 |,|M 1 M 3 |. 
     In step S 112 , the matching module  350  selects one of the space point captures of the object A as a base space point capture and selects the group of three spheres corresponding to the base space point capture as base spheres, maps each sphere in other groups corresponding to other space point captures to a corresponding base sphere according to the distance between adjacent spheres in each group. Suppose that, taking the first group of spheres Q 1 , Q 2 , and Q 3  as base spheres, because the three reference balls a, b, c on the fixture  10  form a scalene triangle, that is to say, 
     |ab|≠|bc|≠|ac|, 
     the spheres in each group also form a scalene triangle, namely, 
     |Q 1 Q 2 |≠|Q 2 Q 3 |≠|Q 1 Q 3 |, and |M 1 M 2 |≠|M 2 M 3 |≠|M 1 M 3 |. 
     If |Q 1 Q 2 |=|M 1 M 2 |, |Q 2 Q 3 |=|M 2 M 3 |, and |Q 1 Q 3 |=|M 1 M 3 |, 
     the sphere M 1  maps the sphere Q 1 , the sphere M 2  maps the sphere Q 2 , and the sphere M 3  maps the sphere Q 3 . 
     In step S 114 , the aligning module  360  aligns each sphere in the other groups of three spheres to each corresponding base sphere based on a location of the sphere and a matching relation with the corresponding base sphere, and reading transformation matrixes of the alignment. For example, the aligning module  360  aligns the scalene triangle M 1 , M 2 , M 3  to the scalene triangle Q 1 , Q 2 , Q 3  by translating and rotating the scalene triangle M 1 , M 2 , M 3  until the spheres M 1 , M 2 , M 3  superposes the spheres Q 1 , Q 2 , Q 3  respectively, detailed description is as that: (a) translating the first triangle M 1  M 2 M 3  until the sphere (or vertex) M 1  superposes the sphere (or vertex) Q 1  of the second triangle Q 1 Q 2 Q 3  (shown in  FIG. 5 ), to obtain a first transformation matrix D 1 ; (b) taking the vertex Q 1  as a rotating origin, a normal vector of the plane Q 1 Q 2 M 2  as a first rotating axis, and a first inner angle between a first side Q 1  Q 2  and a second side Q 1  M 2  as a first rotating angle, to rotate the third side M 1  M 2  until the third side M 1  M 2  superposes the first side Q 1 Q 2 , to obtain a second transformation matrix D 2 ; (c) taking the vertex Q 1  as the rotating origin, the first side Q 1 Q 2  as the second rotating axis, and a second inner angle between the second side Q 1  M 2  and a fourth side Q 1  Q 3  as a second rotating angle, to rotate a fifth side M 1 M 3  until the fifth side M 1 M 3  superposes the fourth side Q 1 Q 3  (shown in  FIG. 6 ), to obtain a third transformation matrix D 3 . 
     In step S 116 , the aligning module  360  reads transformation matrixes of the alignment, such as the first translating matrix D 1 , the second transformation matrix D 2  and the third transformation matrix D 3 . 
     In step S 118 , the aligning module  360  aligns the point-clouds of the object A of other space point captures to the base point-clouds of the object A of the base space point capture according to the transformation matrixes, such as the transformation matrixes D 1 , D 2 , and D 3 , to obtain an integrated point-cloud of the object A. 
     In step S 120 , the deleting module  330  deletes all the point-clouds of the three reference balls a, b and c, and all the spheres. 
     In step S 122 , the point-cloud outputting module  370  outputs the integrated point-cloud of the object A to a CAD system. 
     Although the present invention has been specifically described on the basis of a preferred embodiment and preferred method, the invention is not to be construed as being limited thereto. Various changes or modifications may be made to the embodiment and method without departing from the scope and spirit of the invention.