Abstract:
A double cone stylus includes a shank and a double cone-shaped tip attached to the tip end of the shank, the double cone-shaped tip being a rotationally symmetric body obtained by rotating a triangle having a base extending along the shank around the shank as the symmetric rotation axis. The double cone stylus allows the shape of an inner wall surface of a measurement object with the inner wall surface retracted from an upper reference plane to be measured from above.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The disclosure of Japanese Patent Application No. 2012-232407 filed on Oct. 19, 2012, including specifications, drawings and claims is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD  
       [0002]    The present invention relates to a double cone stylus, to a touch probe, and to a method of calibrating the double cone stylus. The present invention particularly relates to a double cone stylus suitable to be used to measure, from above, the shape of the inner wall of a measurement object with the inner wall retracted from an upper reference plane, to a touch probe having the double cone stylus attached thereto, and a method of calibrating the double cone stylus in order to calibrate the dimensions of the stylus tip. 
       BACKGROUND ART 
       [0003]    In one known three-dimensional coordinate measuring machine (hereinafter may be simply referred to as a coordinate measuring machine or contour measuring machine), the stylus tip (hereinafter may be simply referred to as a tip) of a touch probe is moved along the surface of a measurement object continuously to perform point-to-point measurement at predetermined intervals, whereby the surface contour of the measurement object is measured. Various types of styluses are used for the touch probe, and examples thereof include: a straight stylus shown in  FIG. 1(A)  in which a spherical stylus tip  14  is attached to the tip end (the lower end) of a shank  12 ; a disc stylus shown in  FIG. 1(B)  in which a disc-shaped stylus tip  16  is attached to the tip end of a shank  12 ; and a cylindrical stylus shown in  FIG. 1(C)  in which a cylindrical stylus tip  18  is attached to the tip end of a shank  12 . The present applicant has proposed, in Patent Literature 1, a stylus with a flat end face as a modification of the disc stylus. 
       CITATION LIST 
     Patent Literature 
       [0004]    Patent Literature 1: Japanese Patent No. 3819250 (FIG. 8( a ), paragraph 0060) 
         [0005]    Patent Literature 2: Japanese Patent Application Laid-Open No. 2003-97937 
         [0006]    Patent Literature 3: Japanese Patent Application Laid-Open No. 2004-317159 
         [0007]    Patent Literature 4: Japanese Patent Application Laid-Open No. 2011-85399 
         [0008]    Patent Literature 5: Japanese Patent Application Laid-Open No. 2011-85403 
       SUMMARY OF INVENTION  
     Technical Problem 
       [0009]    However, the conventional styluses have a problem in that although, as exemplified by a straight stylus in  FIG. 2(A) , measurement can be performed on an inner wall surface  8 B of a hole of a measurement object  8  that is perpendicular to a reference plane  8 A (the upper surface in the figure) or has a gradually protruded lower portion, measurement cannot be performed on an inner wall surface  8 C having a retracted lower portion as exemplified in  FIG. 2(B) , on a lower edge  8 D of a hole as exemplified in  FIG. 2(C) , and on an irregular surface  8 E in a lower portion of a hole as exemplified in  FIG. 2(D) . In addition, the disc stylus with a flat end face proposed in Patent Literature 1 has a problem in that measurement cannot be performed on a reference plane  8 A. 
         [0010]    Various methods have been proposed to calibrate the above-described straight stylus, disc stylus, cylindrical stylus, etc. For example, Patent Literatures 2 and 3 each describe a method for calibrating a straight stylus using a reference ball. However, the calibration method proposed by the applicant is not suitable for a novel double cone stylus proposed in this invention. 
         [0011]    In recent years, measurement objects are becoming smaller and finer. In accordance with the trend toward a reduction in size of measurement objects and measurement areas, the burden on the measurer is increasing because the operation of setting a stylus is very difficult and time consuming. In addition, for some measurement objects, the stylus may interfere (collide) with the measurement objects, and this causes the stylus and the measurement objects to be damaged. 
         [0012]    Accordingly, the applicant is developing a coordinate measuring machine including, in addition to a touch probe having a stylus that comes into contact with a measurement object, a vision probe for taking an image of the surface of the measurement object. 
         [0013]    In this coordinate measuring machine, first, an image of the measurement object is acquired using the vision probe, and a measurement start position is specified on the acquired image of the measurement object. Then the moving trajectory of a relative movement mechanism is computed and stored so that the stylus of the touch probe comes into contact with the measurement start position of the measurement object. After completion of the measurement, the relative movement mechanism is actuated, and the stylus of the touch probe is moved along the stored movement trajectory and comes into contact with the measurement start position of the measurement object automatically. 
         [0014]    Since the stylus of the touch probe can be automatically placed at the measurement start position of the measurement object, the measurer is not necessary to place the tip end of the stylus at the measurement start position of the measurement object while the position of the tip end of the stylus relative to the measurement start position of the measurement object is visually checked to adjust the relative position, as in conventional machines. Therefore, the burden on the measurer can be reduced, and the interference (collision) between the stylus and the measurement object can be prevented. 
         [0015]    However, as exemplified in  FIG. 3 , the vision probe  24  and the touch probe  26  having the stylus are disposed offset from each other in a common measurement head  22  because of physical and structural reasons. In this figure, reference numeral  8  represents the measurement object, and  20  represents the measuring machine. 
         [0016]    Therefore, when the vision probe  24  is switched to the touch probe  26  or when the touch probe  26  is switched to the vision probe  24 , control must be performed with the amount of offset between the touch probe and the vision probe calibrated. Thus, it is important to correctly grasp the amount of offset between the touch probe and the vision probe, in order to perform measurement with high accuracy. 
         [0017]    Accordingly, the applicant has proposed, in Patent Literatures 3 and 4, offset calibration methods that can correctly determine the amount of offset. However, these methods cannot be applied to a double cone stylus. 
       Solution to Problem 
       [0018]    The present invention has been made in order to solve the above-described problems in association with the conventional technique, and a first object of the present invention is to provide a double cone stylus suitable to be used for a measurement object having an inner wall retracted from an upper reference plane. 
         [0019]    A second object of the present invention is to provide a touch probe for coordinate measurement. 
         [0020]    A third object of the present invention is to enable calibration of the double cone stylus. 
         [0021]    The present invention achieves the above-described first object by providing a double cone stylus comprising a shank having a tip end and a double cone-shaped tip attached to the tip end of the shank, the double cone-shaped tip being a rotationally symmetric body obtained by rotating a triangle having a base extending along the shank around the shank as a symmetric rotation axis. 
         [0022]    A tip end and/or a circumferential edge of the double cone-shaped tip may be rounded. 
         [0023]    The present invention achieves the above-described second object by providing a touch probe for coordinate measurement or contour measurement, the touch probe comprising the above-described double cone stylus. 
         [0024]    The present invention also provides a coordinate measuring machine or contour measuring machine comprising the above-described touch probe. 
         [0025]    The coordinate measuring machine or contour measuring machine provided by the present invention may further comprise a vision probe. 
         [0026]    The present invention achieves the above-described third object by providing a method of calibrating a double cone stylus included in a coordinate measuring machine or contour measuring machine including a touch probe having the double cone stylus attached to a tip end thereof, the method comprising, when dimensions of a stylus tip of the double cone stylus are calibrated: disposing a master ball with a calibration value in the coordinate measuring machine or contour measuring machine; performing touch measurement on the master ball using the touch probe to acquire a center position and a radius of the master ball; computing a radius of the stylus tip in a horizontal direction; performing touch measurement to determine a position of an apex of the master ball using the touch probe to acquire a height of the apex of the master ball; and computing a radius of the stylus tip in a height direction. 
         [0027]    The radius of the stylus tip in the horizontal direction may be computed by: determining provisional values of the center position of the master ball using values measured near the apex of the master ball; performing measurement on the master ball along search lines extending in a vertical direction around an equator of the master ball that is determined from the measured provisional values of the center position to thereby determine final values of the center position of the master ball and a final value of the radius of the master ball; and computing the radius of the stylus tip in the horizontal direction using the measured final value of the radius of the master ball. The radius of the stylus tip in the height direction may be computed by performing touch measurement at the center position of the master ball to determine the position of the apex of the master ball. 
         [0028]    The present invention also achieves the above-described third object by providing a method of calibrating a double cone stylus included in a coordinate measuring machine or contour measuring machine including a vision probe and a touch probe having the double cone stylus attached to a tip end thereof, the method comprising, when dimensions of a stylus tip of the double cone stylus and an offset between the touch probe and the vision probe are calibrated: disposing a master ball with a calibration value and a ring gauge on the coordinate measuring machine or contour measuring machine; and performing measurement sequentially on the mater ball and the ring gauge using the touch probe and the vision probe to calibrate the dimensions of the stylus tip and the offset between the touch probe and the vision probe. 
         [0029]    An offset distance between the touch probe and the vision probe may be computed by performing measurement on the ring gauge using the vision probe and the touch probe sequentially. 
         [0030]    An offset distance between the vision probe and the touch probe in a horizontal direction may be computed by measuring a circle center position of the ring gauge using the vision probe and measuring the circle center position of the ring gauge using the touch probe. 
         [0031]    An offset distance between the vision probe and the touch probe in a height direction may be computed by measuring a position of an upper surface of the ring gauge using the vision prove and measuring a height when the touch probe touches the upper surface of the ring gauge. 
       Advantageous Effects of Invention 
       [0032]    According to the present invention, the shape of an inner wall surface  8 C of a measurement object  8  with the inner wall surface retracted from an upper reference plane  8 A, a lower edge  8 D of a hole of a measurement object  8 , an irregular surface  8 E of a measurement object  8 , etc. can be measured using a double cone stylus tip  34  from above, as shown in  FIGS. 4(A) to 4(C) . This stylus tip  34  having a tip end  34 A allows measurement from a position above the reference plane  8 A, which is difficult to perform using a disc stylus and the stylus proposed in Patent Literature 1. 
         [0033]    According to the calibration method of the present invention, the radii of the stylus tip can be correctly calibrated in the approach directions (plane and height directions) of the double cone stylus. Therefore, correct measurement results can be obtained by measurement using the double cone stylus. The use of the calibrated offset values allows the position measured using the vision probe to be measured using the touch probe. 
         [0034]    These and other novel features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0035]    The preferred embodiments will be described with reference to the drawings, wherein like elements have been denoted throughout the figures with like reference numerals, and wherein; 
           [0036]      FIGS. 1(A) to 1(C)  are front views illustrating various conventional styluses; 
           [0037]      FIGS. 2(A) to 2(D)  are cross-sectional views illustrating problems in association with a conventional straight stylus; 
           [0038]      FIG. 3  is a perspective view illustrating the tip end of the head of a coordinate measuring machine provided with a touch probe and a vision probe in combination; 
           [0039]      FIGS. 4(A) to 4(C)  are cross-sectional views each illustrating a state in which measurement is performed using the double cone stylus according to the present invention; 
           [0040]      FIGS. 5(A) and 5(B)  are diagrams illustrating the dimensions and shape of the double cone stylus according to the present invention,  FIG. 5(A)  being a perspective view,  FIG. 5(B)  being a vertical cross-sectional view as viewed from the front; 
           [0041]      FIG. 6  is a front view illustrating a modification of the double cone stylus according to the present invention; 
           [0042]      FIG. 7  is a set of front views illustrating a state in which an offset is measured in an embodiment of the calibration method according to the present invention; 
           [0043]      FIG. 8  is a flowchart showing a procedure in the embodiment of the calibration method according to the present invention; 
           [0044]      FIG. 9  is a perspective view illustrating a master ball and a ring gauge used in the embodiment; 
           [0045]      FIG. 10  is a diagram illustrating the definition of the master ball; 
           [0046]      FIG. 11  is a flowchart showing the procedure of calibrating the radii of a stylus tip; 
           [0047]      FIG. 12  is a flowchart showing the procedure of determining the provisional values of the center of the master ball; 
           [0048]      FIG. 13  is a front view illustrating a state in which the provisional values of the center of the master ball are determined; 
           [0049]      FIG. 14  is a plan view of  FIG. 13 ; 
           [0050]      FIG. 15  is a flowchart showing the procedure of determining the final values of the center and radius of the master ball; 
           [0051]      FIG. 16  is a plan view illustrating a state in which the final values of the center and radius of the master ball are determined; 
           [0052]      FIG. 17  is a front view of  FIG. 16 ; 
           [0053]      FIG. 18  is a flowchart showing the procedure of measuring the radius of the stylus tip in a height direction; 
           [0054]      FIG. 19  is a front view illustrating a state in which the radius of the stylus tip in the height direction is measured; 
           [0055]      FIG. 20  is a flowchart showing the procedure of calibration of an offset; 
           [0056]      FIG. 21  is a flowchart showing the procedure of measuring offset distances in the horizontal direction; 
           [0057]      FIG. 22  is a plan view illustrating a state in which the offset distances in the horizontal direction are measured using the ring gauge; 
           [0058]      FIG. 23  is a flowchart showing the procedure of measuring an offset distance in the height direction; and 
           [0059]      FIG. 24  is a front view illustrating a state in which the offset distance in the height direction is measured using the ring gauge. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0060]    Embodiments of the present invention will be described below in detail with reference to the drawings. 
         [0061]    As shown in  FIGS. 5(A) and 5(B) , a double cone stylus  30  in this embodiment includes a shank  32  and a double cone-shaped stylus tip  34  attached to the tip end (the lower end) of the shank  32 . The double cone-shaped stylus tip  34  is a rotationally symmetric body obtained by rotating a triangle having a base extending along the shank  32  around the shank  32  as the symmetric rotation axis. The stylus tip  34  may not be vertically symmetric, and the tip end (the lower end in the figures) and/or the circumferential edge of the stylus tip  34  may be slightly rounded as shown in  FIG. 6 . 
         [0062]    A description will next be given of a calibration method using as an example an image measuring machine, which is one coordinate measuring machine, including a vision probe  24  and a touch probe (TP)  28  equipped with the double cone stylus  30  in combination, as exemplified in  FIG. 7 . 
         [0063]      FIG. 7  illustrates a state in which an offset is measured according to the present invention. The calibration values for the stylus in this embodiment are tip radii Rtipxy and Rtipz shown in  FIG. 5(B)  and offset distances OffsetX, OffsetY, and OffsetZ between the vision probe  24  and the center of the stylus tip  34  of the touch probe  28 . The offset distances are defined with reference to the focal point of an objective lens of the vision probe  24 . 
         [0064]    The procedure of the measurement in this embodiment will be described with reference to  FIG. 8 . 
         [0065]    First, in step  100 , preparation before calibration is performed. Specifically, a master ball (MB)  40  with a calibrated sphere radius exemplified in  FIG. 9  is prepared and placed on a measuring machine  20 . The calibrated sphere radius of the master ball  40  is defined as Rmbcal. A ring gauge  44  is prepared and placed on the measuring machine  20 . In  FIG. 9 , reference numeral  42  represents a support stay for the master ball  40 ,  46  represents a support of the ring gauge  44 , and  48  represents a base for securing these components. The base  48  may be omitted, and the master ball  40  and the ring gauge  44  may be directly placed on the measuring machine  20 . 
         [0066]    Next, the process proceeds to step  200 , and the radii of the stylus tip are calibrated using the master ball  40 . In the following description, the master ball  40  is likened to the Earth, and the equator, northern hemisphere, and southern hemisphere are defined as shown in  FIG. 10 . 
         [0067]    Specifically, according to the procedure shown in  FIG. 11 , the provisional values of the center position of the master ball (Xmbtmp, Ymbtmp, Zmbtmp) are determined in step  210 . 
         [0068]    More specifically, the design value Rtipznom of the radius Rtipz of the stylus tip is set in step  212  in  FIG. 12 . Next, in step  214 , touch measurement is performed on the apex of the master ball  40  using the touch probe (TP)  28  in a manual mode, as shown in  FIG. 13 . In this case, the direction of approach is −Z, and the measured touched point is set to be (Xt, Yt, Zt). 
         [0069]    Next, the process proceeds to step  216 , and touch measurement is performed at five points including the same point as the previously touched point and four points spaced apart from (Xt, Yt) as the center by distance dR, as shown in  FIG. 14 . The direction of approach in this case is −Z, and the measurement points are (Xt−dR, Yt, Zt 1 ), (Xt+dR, Yt, Zt 2 ), (Xt, Yt−dR, Zt 3 ), (Xt, Yt+dR, Zt 4 ), and (Xt, Yt, Zt). 
         [0070]    Then the process proceeds to step  218 , and spherical features are determined with reference to the measurement points in step  216 . The position of the center of the spherical features is set to be (Xmbtmp 0 , Ymbtmp 0 , Zmbtmp 0 ). 
         [0071]    The tip radius Rtipz is thereby corrected, and the provisional values of the center of the master ball (Xmbtmp, Ymbtmp, Zmbtmp) are determined as follows. 
         [0000]      Xmbtmp=Xmbtmp0   (1)
 
         [0000]      Ymbtmp=Ymbtmp0   (2)
 
         [0000]        Zmbtmp=Zmbtmp 0− Rtipznom    (3)
 
         [0072]    Then the process proceeds to step  220  in  FIG. 11 , and the final values of the center position and radius of the master ball  40  are determined. Specifically, the following values are pre-set in step  222  in  FIG. 15 . 
         [0073]    The range of search for the center position Z of the MB: RangeZ 
         [0074]    The measurement pitch in the search for the center position Z of the MB: PitchZ 
         [0075]    The number of search lines for the center position of the MB (3 or larger): NumOfLine (4 in this case) 
         [0076]    Next, in step  224 , the determined (provisional) values of the center position of the master ball  40  are set to be (Xmbtmp, Ymbtmp, Zmbtmp). 
         [0077]    Next, the process proceeds to step  226 . According to the inputted number of search lines (4 in this case), the positions of the search lines  1  to  4  in a vertical direction (height direction) exemplified in  FIG. 16  are determined. 
         [0078]    Next, the process proceeds to step  228 , and touch measurement is performed at the position of the search line  1  within the Z range (RangeZ) near the equator of the master ball, as exemplified in  FIG. 17 . The measurement pitch in this case is PitchZ. 
         [0079]    Next, the process proceeds to step  230 , and the same measurement as that in step  228  is performed at the positions of the search lines  2 ,  3 , and  4 . 
         [0080]    Next, the process proceeds to step  232 , and spherical features are computed with reference to the points measured in steps  228  and  230 . The center position of the spherical features is set to be the spherical center position of the master ball  40  (Xmb, Ymb, Zmb). The radius of the spherical features is set to be the radius Rmbmsr of the master ball. 
         [0081]    Next, the process proceeds to step  240  in  FIG. 11 , and the radius Rtipxy of the stylus tip is computed. 
         [0082]    Specifically, with reference to the determined final value of the radius Rmbmsr of the master ball  40 , the radius Rtipxy of the stylus tip is computed using the following formula. 
         [0000]        Rtipxy=Rmbmsr−Rmbcal    (4)
 
         [0083]    Next, the process proceeds to step  250  in  FIG. 11 , and the radius Rtipz of the stylus tip in the height direction is measured. Specifically, the process proceeds to step  252  in  FIG. 18 , and touch measurement is performed using the touch probe  28  at the spherical center position (Xmb, Ymb) of the master ball  40  measured in step  232  as shown in  FIG. 19  to thereby determine the position Ztop of the apex of the master ball. Next, the process proceeds to step  254 , and the radius Rtipz of the stylus is computed using the following formula. 
         [0000]        Rtipz=Ztop−Zmb−Rmbcal    (5)
 
         [0084]    Next, the process proceeds to step  300  in  FIG. 8 , and the offset is calibrated using the ring gauge  44 , as shown in  FIG. 20 . Specifically, first, offset distances in the horizontal direction are measured in step  310 . More specifically, as shown in  FIG. 21 , fist, the circle center position (Xv, Yv) of the ring gauge  44  is measured using the vision probe  24  in step  312 . Next, in step  314 , the circle center position (Xt, Yt) of the ring gauge  44  is measured using the touch probe  28 , as shown in  FIG. 22 . Next, in step  316 , the offset distances between the vision probe  24  and the touch probe  28  are computed using the following formula. 
         [0000]      (Offset X,  Offset Y )=( Xt−Xv, Yt−Yv )   (6)
 
         [0085]    After completion of step  316 , the process returns to step  320  in  FIG. 20 , and the offset distance in the height direction is measured. Specifically, in step  322  in  FIG. 23 , autofocus (AF) measurement is performed on an upper surface  44 A of the ring gauge  44  using the vision probe  24  to measure height Zv. Next, in step  324 , touch measurement is performed on the upper surface  44 A of the ring gauge  44  using the touch probe  28  as shown in  FIG. 24  to measure height Zt. Next, in step  326 , the offset distance OffsetZ between the vision probe  24  and the touch probe  28  in the height direction is computed using the following formula. 
         [0000]      Offset Z=Zt−Zv+Rtipz    (7)
 
         [0086]    The calibrated radii of the stylus tip and the calibrated offset distances can be obtained in the manner described above. 
         [0087]    In the above embodiment, the coordinate measuring machine used is an image measuring machine, and not only the dimensions of the stylus tip but also the offset between the vision probe and the stylus is calibrated. However, the range of application of the present invention is not limited thereto. The present invention is applicable to general coordinate measuring apparatuses other than the image measuring machine such as three-dimensional coordinate measuring machines with no vision probe, so long as the double cone stylus can be installed, and only the dimensions of the stylus tip may be calibrated. 
         [0088]    It should be apparent to those skilled in the art that the above-described exemplary embodiments are merely illustrative which represent the application of the principles of the present invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and the scope of the invention.