Abstract:
Form measuring instrument includes: first measuring means which moves contact piece from first position in parallel with second axis to trace surface of workpiece, measure amount of displacement of contact piece, to obtain first profile; second placing means which rotates workpiece about first axis by 90 degrees to place workpiece at second position from first position; second measuring means which moves contact piece from second position in parallel with second axis to trace surface of workpiece, measure amount of displacement of contact piece, to obtain second profile; extremum position calculating means which fits circles to first and second profiles and calculate positions, in direction parallel with second axis, of first and second extremums indicating circles&#39; extremums; and moving means which moves workpiece in direction parallel with second axis and direction parallel with third axis such that positions, in direction parallel with second axis, of first and second extremums become 0.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims the benefit of priority from prior Japanese Patent Application No. 2009-120705, filed on May 19, 2009, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a form measuring instrument such as a roundness measuring machine which measures a displacement in synchronization with an angular position of rotation of a workpiece that is rotating about a specified rotation axis, a form measuring method, and a program. 
         [0004]    2. Description of the Related Art 
         [0005]    Conventionally, there has been known a roundness measuring machine which receives a workpiece on its table, rotates the table, and makes a measuring probe (a contact piece) scan the surface of the workpiece (see JPH5-231864A, JP2551698B). Such a roundness measuring machine has to have the contact piece calibrated in an X-axis direction and a Y-axis direction, so that the center of the workpiece may be aligned (centered) with the rotation axis of the table. 
         [0006]    Currently, centering is performed by best-fitting data obtained by concentrically measuring an aspheric workpiece to aspheric surface design values. At this time, if the off-centering value of the workpiece is large, a radial position that is outside the range of a design value might be measured or an error contained in a detected value might become large. Therefore, the workpiece might stick out from the measuring region which the machine is guaranteed. In this case, the workpiece and the measuring machine might be damaged. 
       SUMMARY OF THE INVENTION 
       [0007]    A form measuring instrument according to the present invention is a form measuring instrument operative to rotate a workpiece about a first axis, and measures a displacement of a surface of the workpiece at each angular position of rotation of the workpiece, the workpiece being a rotating object, the instrument comprising: a contact piece having a tip capable of contacting with the workpiece; first placing means operative to place the workpiece at a first position; first measuring means operative to move the contact piece from the first position in parallel with a second axis perpendicular to the first axis such that the contact piece traces the surface of the workpiece, thereby to measure an amount of displacement of the contact piece at each position along the second axis to obtain a first profile; second placing means operative to rotate the workpiece about the first axis by 90 degrees to place the workpiece at a second position from the first position; second measuring means operative to move the contact piece from the second position in parallel with the second axis such that the contact piece traces the surface of the workpiece, thereby to measure an amount of displacement of the contact piece at each position along the second axis to obtain a second profile; extremum position calculating means operative to fit circles to the first profile and second profile respectively, and calculate positions, in a direction parallel with the second axis, of a first extremum and a second extremum indicating extremums of the respective circles; and moving means operative to move the workpiece in the direction parallel with the second axis and in a direction parallel with a third axis perpendicular to the first axis and second axis, such that the positions, in the direction parallel with the second axis, of the first extremum and second extremum become 0. 
         [0008]    A form measuring method according to the present invention is a form measuring method involving use of a form measuring instrument operative to rotate a workpiece about a first axis, and measure a displacement of a surface of the workpiece at each angular position of rotation of the workpiece, the workpiece being a rotating object, the form measuring instrument including a contact piece having a tip capable of contacting with the workpiece, the method comprising: placing the workpiece at a first position; moving the contact piece from the first position in parallel with a second axis perpendicular to the first axis such that the contact piece traces the surface of the workpiece, thereby to measure an amount of displacement of the contact piece at each position along the second axis to obtain a first profile; rotating the workpiece about the first axis by 90 degrees to place the workpiece at a second position from the first position; moving the contact piece from the second position in parallel with the second axis such that the contact piece traces the surface of the workpiece, thereby to measure an amount of displacement of the contact piece at each position along the second axis to obtain a second profile; fitting circles to the first profile and second profile respectively, and calculating positions, in a direction parallel with the second axis, of a first extremum and a second extremum indicating extremums of the respective circles; and moving the workpiece in the direction parallel with the second axis and in a direction parallel with a third axis perpendicular to the first axis and second axis, such that the positions, in the direction parallel with the second axis, of the first extremum and second extremum become 0. 
         [0009]    A form measuring program according to the present invention is a form measuring program involving use of a form measuring instrument operative to rotate a workpiece about a first axis, and measure a displacement of a surface of the workpiece at each angular position of rotation of the workpiece, the workpiece being a rotating object, the form measuring instrument including a contact piece having a tip capable of contacting with the workpiece, the program controlling a computer to: place the workpiece at a first position; move the contact piece from the first position in parallel with a second axis perpendicular to the first axis such that the contact piece traces the surface of the workpiece, thereby to measure an amount of displacement of the contact piece at each position along the second axis to obtain a first profile; rotate the workpiece about the first axis by 90 degrees to place the workpiece at a second position from the first position; move the contact piece from the second position in parallel with the second axis such that the contact piece traces the surface of the workpiece, thereby to measure an amount of displacement of the contact piece at each position along the second axis to obtain a second profile; fit circles to the first profile and second profile respectively, and calculate positions, in a direction parallel with the second axis, of a first extremum and a second extremum indicating extremums of the respective circles; and move the workpiece in the direction parallel with the second axis and in a direction parallel with a third axis perpendicular to the first axis and second axis, such that the positions, in the direction parallel with the second axis, of the first extremum and second extremum become 0. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an appearance perspective diagram showing a schematic configuration of a form measuring instrument according to an embodiment. 
           [0011]      FIG. 2  is a block diagram showing a configuration of an arithmetic processing unit  31 . 
           [0012]      FIG. 3  is a flowchart showing an operation of the form measuring instrument according to an embodiment. 
           [0013]      FIG. 4  is a schematic diagram showing step S 103  of  FIG. 3 . 
           [0014]      FIG. 5  is a schematic diagram showing steps S 104  and S 105  of  FIG. 3 . 
           [0015]      FIG. 6  is a schematic diagram showing step S 106  of  FIG. 3 . 
           [0016]      FIG. 7  is a diagram for explaining steps S 103  and S 105  of  FIG. 3 . 
           [0017]      FIG. 8  is a diagram for explaining an effect of the form measuring instrument according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]    An embodiment of the present invention will be explained with reference to the drawings. 
       Embodiment 
     Configuration of Form Measuring Instrument According to Embodiment 
       [0019]    First, the configuration of a form measuring instrument according to an embodiment will be explained with reference to  FIG. 1 .  FIG. 1  is an appearance perspective diagram of a form measuring instrument (roundness measuring machine) according to an embodiment. 
         [0020]    The form measuring instrument rotates a workpiece  4  about a specified rotation axis and measures a displacement of a surface of the workpiece  4  at each angular position of rotation of the workpiece  4 . In the present embodiment, the workpiece  4  is a convex aspheric lens, which is a rotating object. 
         [0021]    As shown in  FIG. 1 , the form measuring instrument includes a measuring unit  1  and an arithmetic processing device  2  connected to the measuring unit  1  via a drive control unit  1   a . The measuring unit  1  includes a base mount  3 , a table  5  provided on the base mount  3  and on which a workpiece  4  is placed, a displacement detecting unit  6  configured to detect a displacement of the workpiece  4  placed on the table  5 , and an operation section  7  used for operating them. 
         [0022]    The table  5  drives a disk-shaped stage  11  to rotate by means of a rotation drive unit  12  that is positioned below the stage  11 , thereby to rotate the workpiece  4  placed on the stage  11 . Adjustment knobs  13  are provided on the side surface of the rotation drive unit  12  at generally 90-degree intervals in the circumferential direction. Operating these adjustment knobs  13  allows for manual centering and leveling of the stage  11 . That is, the stage  11  is constructed to be adjustable in X-axis, Y-axis, and Z-axis directions. The stage  11  is also constructed such that it is centered and leveled by a control unit  41 , which will be described later. The X-axis, the Y-axis, and the Z-axis are perpendicular to one another. 
         [0023]    The displacement detecting unit  6  is constructed as follows. That is, a column  21  that extends upward stands on the base mount  3 , and a slider  22  is mounted on the column  21  in a way to be able to move up and down. A stylus  23  is attached to the slider  22 . The stylus  23  can be driven in horizontal (X-axis, Y-axis) and vertical (Z-axis) directions, and has a contact piece  24  provided at the end. The contact piece  24  can bring its tip into contact the workpiece. The column  21 , the slider  22 , and the stylus  23  constitute a contact piece driving means. 
         [0024]    By moving the slider  22  and the stylus  23  to make the contact piece  24  scan (trace) the surface of the workpiece  4  in the X-axis direction while rotating the table  5 , it is possible to obtain an amount of displacement of the contact piece  24  at each position in the X-axis direction as measurement data (profile). 
         [0025]    The arithmetic processing device  2  acquires measurement data obtained by the displacement detecting unit  6 . The arithmetic processing device  2  includes an arithmetic processing unit  31  configured to execute arithmetic processing, an operation section  32 , and a display screen  33 . The arithmetic processing device  2  is configured to be able to control the operation of the measuring unit  1  like the operation section  7 . 
         [0026]    Next, with reference to  FIG. 2 , the configuration of the arithmetic processing unit  31  will be explained. As shown in  FIG. 2 , the arithmetic processing unit  31  mainly includes a control unit (CPU: Central Processing Unit)  41 , a RAM (Random Access Memory)  42 , a ROM (Read Only Memory  43 , an HDD (Hard Disk Drive)  44 , and a display control unit  45 . In the arithmetic processing unit  31 , code information and positional information entered from the operation section  32  are input to the control unit  41  via an I/F  46   a . The control unit  41  executes various processes in accordance with a macro program stored in the ROM  43  and various programs that are loaded onto the RAM  42  from the HDD  44  via an I/F  46   b.    
         [0027]    The control unit  41  controls the measuring unit  1  via an I/F  46   c  in accordance with a measurement execution process. The HDD  44  is a recording medium that stores various control programs. The RAM  42  stores various programs and provides a work area for various processes. The control unit  41  displays a measurement result, etc. on the display screen  33  via the display control unit  45 . 
         [0028]    The control unit  41  reads out various programs from the HDD  44  and executes the following operation shown in  FIG. 3  by executing the programs. 
       [Operation of Form Measuring Instrument According to Embodiment] 
       [0029]    Next, with reference to the flowchart shown in  FIG. 3 , an operation of the form measuring instrument according to an embodiment for centering and leveling an aspheric workpiece by best-fit method will be explained. First, after roughly centering and leveling the workpiece  4 , which is an aspheric workpiece and placed on the stage  11 , the control unit  41  receives via the operation section  32  an input of measurement conditions, e.g., a radial position at which the measurement is started, a measurement length, an auto-set level for effectively using a measuring range, etc. (step S 101 ). Next, the control unit  41  places the workpiece  4  at a first position P 1  in accordance with the input measurement conditions (step S 102 ). Specifically, the control unit  41  sets the angular position of rotation of the stage  11  to “0 degree”. 
         [0030]    Then, as shown in  FIG. 4 , the control unit  41  moves the contact piece  24  from the first position P 1  by a specified distance in the X-axis direction such that the contact piece  24  traces the surface of the workpiece  4 , thereby to measure the shape of the surface to obtain a first profile L 1  (direct measurement) (step S 103 ). The first profile L 1  represents an amount of displacement of the contact piece  24  at each position in the X-axis direction from the first position P 1 . 
         [0031]    Next, as shown in  FIG. 5 , the control unit  41  rotates the workpiece  4  about the Z-axis by 90 degrees to place the workpiece  4  at a second position P 2  from the first position P 1  (step S 104 ). Specifically, the control unit  41  sets the angular position of rotation of the stage  11  to “90 degrees”. Then, the control unit  41  moves the contact piece  24  from the second position P 2  by a specified distance in the X-axis direction such that the contact piece  24  traces the surface of the workpiece  4 , thereby to measure the shape of the surface to obtain a second profile L 2  (direct measurement) (step S 105 ). The second profile L 2  represents an amount of displacement of the contact piece  24  at each position in the X-axis direction from the second position P 2 . 
         [0032]    Then, the control unit  41  fits circles to the first profile L 1  and the second profile L 2  respectively in accordance with least square approach, and calculates the positions, in the X-axis direction, of the maximum values (extremums) M 1  and M 2  of the circles (step S 106 ). For example, as shown in  FIG. 6 , the control unit  41  best-fits a function f(x) having an arc shape to measurement values D that constitute the first profile L 1  (or the second profile L 2 ) in accordance with least square approach, and obtains the maximum value M 1  or M 2  from the function f(x). By executing such a process, it is possible to suppress influence of a measurement value D that contains an error due to noise, etc. 
         [0033]    Next, the control unit  41  moves the workpiece  4  in the X-axis direction and in the Y-axis direction such that the positions of the maximum values M 1  and M 2  in the X-axis direction become 0 (step S 107 ). With this, the control unit  41  completes the operation. 
         [0034]    Next, with reference to  FIG. 7 , the processes of steps S 103  to S 105  described above will be explained in detail. In  FIG. 7 , it is assumed that when the workpiece  4  is set at the first position P 1 , the center C of the workpiece  4  is at a position that is away from the rotation axis O (Z-axis) by a distance cx in the X-axis direction and by a distance cy in the Y-axis direction. (a 1 ) and (a 2 ) of  FIG. 7  show the measurement at the first position P 1  (step S 103 ). (a 1 ) of  FIG. 7  is an X-Y plane diagram, and (a 2 ) of  FIG. 7  is an X-Z plane diagram. (b 1 ) and (b 2 ) of  FIG. 7  show the measurement at the second position P 2  (step S 105 ). (b 1 ) of  FIG. 7  is an X-Y plane diagram, and (b 2 ) of  FIG. 7  is an X-Z plane diagram. 
         [0035]    By the measurement at step S 103  shown in (a 1 ) of  FIG. 7 , the first profile L 1  shown in (a 2 ) of  FIG. 7  is measured. The maximum value M 1  of the circle fitted to the first profile L 1  is at the position cx in the X-axis direction as shown in (a 2 ) of  FIG. 7 . That is, by this measurement, the control unit  41  can specify an amount of deviation cx of the center C of the workpiece  4  from the rotation axis O in the X-axis direction. 
         [0036]    By the measurement at step S 105  shown in (b 1 ) of  FIG. 7 , the second profile  12  shown in (b 2 ) of  FIG. 7  is measured. The maximum value M 2  of the circle fitted to the second profile L 2  is at the position cy in the X-axis direction as shown in (b 2 ) of  FIG. 7 . That is, by this measurement, the control unit  41  can specify an amount of deviation cy of the center C of the workpiece  4  from the rotation axis O in the Y-axis direction. 
       [Advantages of Form Measuring Instrument According to Embodiment] 
       [0037]    Next, with reference to  FIG. 8 , an advantage of the form measuring instrument according to the embodiment will be explained.  FIG. 8  shows an example where the contact piece  24  is made to trace the workpiece  4  while the workpiece  4  is rotated about the Z-axis, whereby a displacement of the surface of the workpiece  4  at each angular position of rotation of the workpiece  4  is measured and a profile L is obtained. In this case, before the processes of steps S 101  to S 107  shown in  FIG. 3  described above are executed, the profile L is not fully contained within a measuring region AR as shown in (a) of  FIG. 8 . The measuring region AR is a region in which it is guaranteed that a stable measurement can be performed. That is, when the profile L is not fully contained within this region, an error in the amount to be detected will become larger, and the contact piece  24  and the workpiece  4  might be damaged. 
         [0038]    On the other hand, after the processes of steps S 101  to S 107  shown in  FIG. 3  described above are executed, the profile L is fully contained within the measuring region AR as shown in (b) of  FIG. 8 . That is, even when the off-centering value is large, the form measuring instrument according to the present embodiment can perform centering by a stable measurement with the above-described processes. 
         [0039]    Further, conventional form measuring instruments perform centering by measuring at least five pieces of concentric measurement data. As opposed to this, through steps S 101  to S 107  of the present embodiment, the measurement includes only two direct measurements that are performed in the X-axis direction over a specified distance. Therefore, centering can be performed in a short time. Furthermore, the processes of the present embodiment are effective when the circumference of the workpiece  4  cannot be measured due to the shape of the workpiece  4  or setting conditions. 
       Other Embodiment 
       [0040]    The embodiment of the form measuring instrument having been described, the present invention is not limited to the embodiment described above, but various alterations, additions, substitutions, etc. can be made within the scope of the spirit of the invention. For example, in the embodiment described above, the workpiece  4  is a convex aspheric workpiece such as a convex lens, etc. However, the workpiece  4  may be a concave aspheric workpiece such as a concave lens, etc. In the case of a concave lens, the control unit  41  may calculate the minimum values of the circles that are fitted to the first and second profiles L 1  and L 2  at step S 106 .