Abstract:
A numerically controlled machine tool in which a numerical control program acquired from a reading and interpreting unit of a numerical control device is executed by a distribution interpolating unit and servo control units, to drive a feed shaft configured from a coarse movement mechanism and a fine movement mechanism, causing a tool to move relative to a workpiece, and thereby machining the workpiece, wherein the difference between a movement command for the feed shaft, and an output value which varies on the basis of said movement command is obtained, a movement command for the coarse movement mechanism is generated on the basis of said movement command, and a movement command for the fine movement mechanism is generated on the basis of said difference.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage patent application of International Patent Application No. PCT/JP2014/076220, filed on Sep. 30, 2014, which is hereby incorporated by reference in the present disclosure in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method of controlling a feed axis of a machine tool and a numerically controlled machine tool configured to carry out the feed axis controlling method. 
       BACKGROUND OF INVENTION 
       [0003]    In the control of a feed axis of a machine tool, when carrying out machining commands, impacts may be generated at parts of the machine tool due to sudden changes in the velocities of the feed axes. In order to reduce such impacts, an acceleration/deceleration control for feed axes is performed. The acceleration/deceleration control includes for example a post-interpolation acceleration/deceleration control wherein move commands, from a distributing and interpolating section of a numerical control device, are passed through an acceleration/deceleration filter, whereby a feed axis is accelerated or decelerated. 
         [0004]    Patent Literature 1 describes an example of a post-interpolation acceleration/deceleration control for a numerically controlled machine tool, wherein acceleration/deceleration curve parameters, corresponding to cutting feed rates of the numerically controlled machine tool are determined, whereby the acceleration and deceleration for the cutting feed is controlled based on the determined acceleration/deceleration curve parameters. 
         [0005]    Patent Literature 2 describes a numerical control device including a post-interpolation acceleration/deceleration processing section for performing post-interpolation acceleration/deceleration processing on the move commands from an interpolation processing section, and axis servo-controlling sections for performing servo control for the respective feed axes, on the basis of the move commands after the post-interpolation acceleration/deceleration processing, whereby a post-interpolation acceleration/deceleration processing section performs velocity control with allowable inward-turning amount. 
         [0006]    Further, Patent Literature 3 describes a precision positioning control apparatus comprising a composite servo system, which includes a coarse positioner and a fine positioner, wherein the sum of the displacements of the coarse positioner and the fine positioner is detected so that the sum is compared with the displacement command, the difference of which is supplied to adjusters of the coarse positioner and the fine positioner, whereby the adjusters output signals to be input into the coarse positioners and the fine positioners, respectively. 
       PRIOR ART DOCUMENTS 
       [0007]    Patent Literature 1: JP-A-2001-312309 
         [0008]    Patent Literature 2: JP-A-2001-312309 
         [0009]    Patent Literature 3: JP-A-H08-241128 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Pre-interpolation acceleration/deceleration controls, as described in Patent Literatures 1 and 2, involve a problem that the acceleration/deceleration time is elongated, resulting in the longer machining time. Further, post-interpolation acceleration/deceleration controls involve a problem that when two or more feed axes are simultaneously controlled in order to machine for example a corner portion in the X-Y plane, the machining error is increased since the actual tool path extends along an inner arcuate course compared with a tool path based on the move commands before the acceleration/deceleration control. 
         [0011]    Composite servo systems, as described in Patent Literature 3, involve a problem that mutually interfering forces are generated between the coarse and fine positioners due to the movement relative to each other, resulting in errors in the servo-control. 
         [0012]    The invention is directed to solve the problems of the prior art, and the object of the invention is to provide a method of controlling a feed axis and a numerically controlled machine tool configured to carry out the feed axis controlling method, improved to reduce the impact generated by the changes in the acceleration and deceleration of the feed axis, and to machine a workpiece at high speed and with high accuracy. 
         [0013]    In order to achieve the above described object, according to the invention, a method of controlling a feed axis of a machine tool, comprising obtaining differences between move commands and output values changeable based on move commands, generating move commands for the coarse motion mechanism based on the move commands, and generating move commands for the micro-motion mechanism based on the differences is provided. 
         [0014]    Further, according to the invention, a method of controlling a feed axis of a machine tool configured to drive the feed axis composed of a coarse motion mechanism and a micro-motion mechanism so as to move a tool and a workpiece relatively to each other, whereby to machine the workpiece, comprising generating move commands for the coarse motion mechanism by passing move commands for the feed axis through a filter adapted to make acceleration continuous, driving the coarse motion mechanism by the move commands for the coarse motion mechanism, obtaining move commands for the micro-motion mechanism based on the differences between the move commands for the feed axis and the move commands for the coarse motion mechanism, and driving the micro-motion mechanism by the obtained move commands for the micro-motion mechanism is provided. 
         [0015]    The coarse motion mechanism may be driven by the move commands for the feed axis, wherein the move commands for the micro-motion mechanism may be obtained based on the differences between the move commands for the feed axis and the feedback signals for the coarse motion mechanism so as to drive the micro-motion mechanism by the obtained move commands for the micro-motion mechanism. 
         [0016]    Furthermore, a numerically controlled machine tool configured to run a numerical control program, from a reading and interpreting section of an NC device, in a distributing section and a servo-control section so as to drive a feed axis, composed of a coarse motion mechanism and a micro-motion mechanism, so that a tool and a workpiece are moved relative to each other, whereby the workpiece is machined, performing obtaining differences between move commands and output values changeable based on move commands, generating move commands for the coarse motion mechanism based on the move commands, and generating move commands for the micro-motion mechanism based on the differences. 
         [0017]    The numerically controlled machine tool may comprise a filter adapted to make the acceleration according to the move commands continuous, means adapted to generate move commands for the coarse motion mechanism through the filter, and means adapted to generate move commands for the micro-motion mechanism based on the differences between the move commands for the feed axis and the move commands for the coarse motion mechanism. 
         [0018]    According to the invention, the accelerations connoted in the move commands, which accelerations are suppressed by the acceleration/deceleration section in the prior art, are compensated by driving the micro-motion device so that the acceleration and deceleration of the respective feed axes can be increased, enabling the machining accuracy and the cutting efficiency to be increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a front view showing an example of a numerically controlled machine tool to which the feed axis controlling method of the invention is applied. 
           [0020]      FIG. 2  is a partially enlarged illustration of a part of the numerically controlled machine tool of  FIG. 1 . 
           [0021]      FIG. 3  is a schematic block diagram showing an example of a control system carrying out the feed axis controlling method of the invention. 
           [0022]      FIG. 4  is a schematic illustration showing tool paths for cutting a corner portion in the X-Y plane. 
           [0023]      FIG. 5  is a control block diagram showing a servo-controlling device according to a first embodiment of the invention. 
           [0024]      FIG. 6  is a schematic illustration showing deviation of tool path when machining along an arcuate or curved tool path. 
           [0025]      FIG. 7  is a control block diagram showing a servo-controlling device according to a second embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    With reference to the attached drawings, a preferred embodiment of the invention will be described below. 
         [0027]      FIG. 1  is a front view showing an example of a numerically controlled machine tool to which a method of controlling a feed axis of the present invention is applied.  FIG. 2  is a partially enlarged illustration of a part of the numerically controlled machine tool of  FIG. 1 . 
         [0028]    In  FIG. 1 , the numerically controlled machine tool  10  comprises a bed  12  providing a base, a column  14  provided on the top of the bed  12  for moving in the horizontal left-and-right direction (X-axis direction), a Y-axis slider  16  mounted to the column  14  for moving in the vertical up-and-down direction (Y-axis direction), a headstock  20  mounted to the Y-axis slider  16  for moving in the up-and-down direction, and a spindle head  22 , mounted to the headstock  20  for moving in the horizontal left-and-right direction, for supporting a spindle  24  for rotation about a rotational axis extending in the horizontal front-and-rear direction. 
         [0029]    The column  14  has guide blocks  28  slideable on a pair of X-axis guide rails  26  extending in the horizontal left-and-right direction (the X-axis direction) along the top of the bed  102 , and therefore is provided for reciprocating along the X-axis guide rails  26 . A ball screw  36  ( FIGS. 5 and 6 ), extending in the X-axis direction, and an X-axis servomotor  28  coupled to an end of the ball screw  36  are provided on the bed  102  as an X-axis feed device for reciprocally driving the column  14  along the X-axis guide rails  26 . A nut  46  ( FIGS. 5 and 7 ), engaging the ball screw  36 , is mounted to the column  14 . Further, an X-axis scale  48  ( FIGS. 5 and 7 ), for measuring the X-axis coordinate position of the column  14 , is mounted to the bed  12 . 
         [0030]    The Y-axis slider  16  is provided on a front face of the column  14  for reciprocating along a pair of Y-axis guide rails (not shown) extending in the vertical direction (the Y-axis direction). A pair of left-and-right ball screws  30  extending in the Y-axis direction and Y-axis servomotors  32  coupled to ends of the ball screws  30  are provided on the column  14  as a Y-axis feed device for reciprocally driving the Y-axis slider  16  along the Y-axis guide rails. A nut (not shown), engaging the ball screw  30 , is mounted to the Y-axis slider  16 . Further, a Y-axis scale (not shown), for measuring the Y-axis coordinate position of the Y-axis slider  16 , is mounted to the column  14 . 
         [0031]    The headstock  20  is provided so as to be finely movable in the Y-axis direction relative to the column  14  via a Y-axis micro-motion device  40 , while the spindle head  22  is provided so as to be finely movable in the X-axis direction relative to the headstock  20  via an X-axis micro-motion device  42 . Although the configurations of the Y-axis micro-motion device  40  and the X-axis micro-motion device  42  are selected based on the required accuracies and the weights of the headstock  20  and the spindle head  22 , they may be formed by linear motors or piezoelectric devices. In  FIGS. 1 and 2 , broken lines indicate the scopes of micro-motion in the Y- and X-axis directions. The micro-motion mechanism, driven by the X-axis micro-motion device  42  and the Y-axis micro-motion device  40 , is configured to have inertia smaller than and rigidity greater than a coarse motion mechanism. 
         [0032]    Further, the numerically controlled machine tool  10  comprises a position sensor (not shown) for detecting the relative position in the Y-axis direction of the headstock  20  relative to the Y-axis slider  16  and a position sensor  44  (refer to  FIGS. 5 and 7 ) for detecting the relative position in the X-axis direction of the spindle head  22  relative to the headstock  20 . 
         [0033]    With reference to  FIG. 3 , illustrating a schematic block diagram showing a control system for carrying out the method of controlling a feed axis of the invention, a control system  50  comprises a reading and interpreting section  52 , a distributing and interpolating section  54  and X-, Y- and Z-axis servo-controlling sections  56 ,  58  and  60 . The reading and interpreting section  12  reads and interprets a machining program fed from for example a CAM device (not shown) to output move commands to the distributing and interpolating section  54 . The move commands include amounts of feed and feeding rates in the X-, Y- and Z-axis directions. 
         [0034]    The distributing and interpolating section  54  performs the interpolation operation on the received X-, Y- and Z-axis move commands to output position commands, corresponding to interpolation functions and feed rates, to the servo-controlling sections  56 ,  58  and  60  of the respective axes. The servo-controlling sections  56 ,  58  and  60  output electric current values for driving the respective X-, Y- and Z-feed axes of the machine tool  10 , based on the received position commands for the respective X-, Y- and Z-axes, to the servomotors  28  and  32  and the micro-motion device  40  and  42  of the X- and Y-axes, respectively. 
         [0035]      FIG. 4  illustrates a tool path when cutting a corner portion in the X-Y plane by using a tool for example a ball end mill. When machining such a corner portion, the position commands output from the distributing and interpolating section  54  are generally passed through a filter adapted to make acceleration continuous, in order to reduce the vibrations and impacts which may be generated when a moving part such as the column  14 , the Y-axis slider  16 , headstock  20 , the spindle head  22  and the spindle  24  moves, and to ensure the continuity of the accelerations of the commands. This changes tool path TP based on the position commands from the distributing and interpolating section  54  to tool path TP′ extending along an inner arcuate course as shown by the broken line in  FIG. 4 . The invention approximates the tool path TP′, which is based on the position commands after filtered, to the tool path TP which is based on the position commands from the distributing and interpolating section  54  by using the micro-motion devices. 
         [0036]    As a preferred embodiment of the invention, a servo-control device  100 , forming the X-axis servo-controlling section  56  of  FIG. 3 , will be described below. It may be understood that the Y-axis servo-controlling section  58  and the Z-axis servo-controlling section  60  can be similarly formed by the servo-control device  100 . 
         [0037]    With reference to  FIG. 5 , the servo-control device  100  according to a first embodiment of the invention includes, similar to conventional servo-control devices, an acceleration/deceleration filter  102  for position commands Xs from the distributing and interpolating section  54 , a subtractor  104  for comparing the position commands from the acceleration/deceleration filter  102  and position feedback signals from the X-axis scale  48 , a position controller  106  for performing a differential operation on the outputs from the subtractor  104 , a subtractor  108  for comparing the outputs from the position controller  106  and velocity feedback signals from the rotary encoder of the X-axis servomotor  28 , a velocity controller  110  for performing a differential operation on the signals from the subtractor  108 , a current controller  112  for controlling the electric current output to the X-axis servomotor  28  based on the outputs from the velocity controller  110 , a velocity feedforward controller  114  and an acceleration feedforward controller  116  for generating velocity feedforward values and acceleration feedforward values based on the position commands from the acceleration/deceleration filter  102 . 
         [0038]    The servo-control device  100  further includes subtractor  118  for comparing the position commands from the distributing and interpolating section  54  and the position commands from the acceleration/deceleration filter  102 , a subtractor  120  for comparing the outputs from the subtractor  118  and the position feedback signals from the position sensor  44 , a micro-motion position controller  122  for performing a differential operation on the outputs from the subtractor  120 , a subtractor  124  for comparing the outputs from the micro-motion position controller  122  and the signals from the position sensor  44  after the differential operation, a micro-motion velocity controller  126  for performing a differential operation on the signals from the subtractor  124 , a micro-motion current controller  128  for controlling the electric current output to the X-axis micro-motion device  42  based on the outputs from the micro-motion velocity controller  126 , a micro-motion velocity feedforward controller  130  and a micro-motion acceleration feedforward controller  132  for generating micro-motion velocity feedforward values and micro-motion acceleration feedforward values based on the outputs from the subtractor  118 . 
         [0039]    In this embodiment, the X-axis micro-motion device  42  is controlled so as to compensate the differences between the position commands from the distributing and interpolating section  54  and the outputs from the acceleration/deceleration filter  102 . Therefore, according to this embodiment, it is possible to approximate the tool path TP′ based on the position commands after being filtered to the tool path based on the position commands from the distributing and interpolating section  54 . Further, in the prior art, large accelerations connoted in the position commands are suppressed by a filter. In this embodiment, the suppressed accelerations are compensated by driving the micro-motion devices, enabling the servomotors  28  and  32  of the X- and Y-axes to be increased, whereby the machining accuracy and the cutting efficiency can be increased. 
         [0040]    Further, even if a tool path TP extends along an arc or a curve as shown in  FIG. 6 , the actual tool path may be deviated from the tool path TP, which is based on the position commands from the distributing and interpolating section, as shown by broken line TP′, since delays of the feed axes are caused by changes in the friction acted on the moving parts of a machine tool. 
         [0041]    With reference to  FIG. 7 , a servo-controlling device  200  according the a second embodiment of the invention includes, similar to conventional servo-controlling devices, a subtractor  202  comparing the position commands X 0  from the distributing and interpolating section  54  and the position feedback signals from the X-axis scale  48 , a position controller  204  for performing a differential operation on the outputs from the subtractor  202 , a subtractor  206  for comparing the outputs from the position controller  204  and the velocity feedback signals from the rotary encoder  28   a  of the X-axis servomotor  28 , a velocity controller  208  for performing a differential operation on the outputs from the velocity controller  208 , a current controller  210  for controlling the electric current output to the X-axis servomotor  28  based on the outputs from the velocity controller  208 , a velocity feedforward controller  212  and an acceleration feedforward controller  214  for generating velocity feedforward values and acceleration feedforward values based on the position commands X 0  from the distributing and interpolating section  54 . 
         [0042]    The servo-controlling device  200  further includes a subtractor  218  for comparing the outputs from the subtractor  202  and the position feedback signals from the position sensor  44 , a micro-motion position controller  220  for performing a differential operation on the outputs from the subtractor  218 , a subtractor  222  for comparing the outputs from the micro-motion position controller  220  and the signals from the position sensor  44  after the differential operation, a micro-motion velocity controller  224  for performing a differential operation on the signals from the subtractor  222 , a micro-motion current controller  226  for controlling the electric current to be output to the X-axis micro-motion device  42  based on the outputs from the micro-motion velocity controller  224 , a micro-motion velocity feedforward controller  228  and a micro-motion acceleration feedforward controller  230  for generating micro-motion velocity feedforward values and micro-motion acceleration feedforward values based on the outputs from the subtractor  202 . 
         [0043]    The servo-controlling device  200  can effectively reduce machining errors due to changes in the friction acted on the moving parts of a machine tool, as shown in  FIG. 6 . Such errors are small, and therefore the micro-motion device  42  may be a driving device comprising piezoelectric devices, instead of linear motors. In this case, mutually interfering forces may be generated, via the piezoelectric devises, between the Y-axis slider  16 , providing a coarse motion mechanism, and the spindle head  22  or the headstock  20 , providing a micro-motion mechanism. Therefore, in this embodiment, the servo-controlling device  200  feeds the mutually interfering forces, acting on the coarse motion mechanism and the micro-motion mechanism, forward to the current controller  210  and the micro-motion current controller  226 . For this purpose, an interference acceleration feedforward controller  216 , for generating interference acceleration feedforward values based on the outputs from the subtractor  202 , and a micro-motion interference acceleration feedforward controller  232 , for generating micro-motion interference acceleration feedforward values based on the position commands X 0  from the distributing and interpolating section  54 , are provided. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  Numerically Controlled Machine Tool 
           12  Bed 
           14  Column 
           16  Y-axis slider 
           20  Headstock 
           22  Spindle Head 
           24  Spindle 
           40  Y-axis Micro-motion Device 
           42  X-axis Micro-motion Device 
           100  Servo-Controlling Device 
           102  Acceleration/Deceleration Filter 
           114  Velocity Feedforward Controller 
           116  Acceleration Feedforward Controller 
           130  Micro-motion Velocity Feedforward Controller 
           132  Micro-motion Acceleration Feedforward Controller 
           212  Velocity Feedforward Controller 
           214  Acceleration Feedforward Controller 
           216  Interference Acceleration Feedforward Controller 
           228  Micro-motion Velocity Feedforward Controller 
           230  Micro-motion Acceleration Feedforward Controller 
           232  Micro-motion Interference Acceleration Feedforward Controller