Abstract:
The invention aims to suppress or prevent the vibration that can occur due to the intervention of a low-rigidity part between detector and rotating body. An angular acceleration sensor for detecting the rotational angular acceleration of a rotating body is provided, and an angle/angular speed command value is compensated in accordance with the value of the angular acceleration.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus for controlling a rotary shaft in a machine tool or the like. 
   2. Description of the Related Art 
   When controlling a rotary shaft in a machine tool, it is general practice to perform feedback control by detecting the rotational angle/rotational speed of a rotating body and by computing the amount of control of a servo motor from the difference between the detected angle/speed and its command value. 
   However, in the control of an indexing table in a machine tool, for example, the rotating body may vibrate due to the presence of a low-rigidity part such as a joint connecting between the rotating body and the detector. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to suppress or prevent the vibration that can occur due to the intervention of a low-rigidity part between the detector and the rotating body. 
   According to the present invention, there is provided a rotary shaft control apparatus comprising: a servo motor; a rotating body driven by the servo motor; a detector for detecting at least one of the rotational angle and rotational angular speed of the rotating body; a control processor for computing the amount of control of the servo motor, based on a detection value fed from the detector and on a command value for at least one of the rotational angle and rotational angular speed of the rotating body; an angular acceleration detector for detecting rotational angular acceleration of the rotating body; and a compensation calculator for compensating at least one of the rotational angle command value, the rotational angular speed command value, and an electric current command value for the servo motor, in accordance with the rotational angular acceleration detected by the angular acceleration detector. 
   Preferably, the angular acceleration detector is mounted on the rotating body. 
   The angular acceleration detector may be constructed by combining an angular speed detector for detecting the angular speed and a differentiator for obtaining the rotational angular acceleration by differentiating the detected rotational angular speed. Here, instead of the angular acceleration detector and in addition to the detector for detecting at least one of the rotational angle and rotational angular speed of the rotating body, an angular speed detector may be provided on the rotating body, and the compensation calculator may calculate the angular acceleration by differentiating the angular speed value and compensate the command value accordingly. 
   As the torque acting on the rotary shaft is proportional to the angular acceleration, the vibration can be suppressed or prevented by detecting the angular acceleration and compensating the command value accordingly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing one example of a rotary shaft control apparatus according to the present invention; 
       FIG. 2  is a diagram showing another example of the rotary shaft control apparatus according to the present invention; and 
       FIG. 3  is a diagram showing the details of control processing performed in a control processor  18 . 
       FIG. 4  is a diagram showing another example of the rotary shaft control apparatus according to the present invention; and 
       FIG. 5  is a diagram showing the details of an alternative control processing performed in a control processor  18 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows one example of a rotary shaft control apparatus according to the present invention. This rotary shaft control apparatus is used, for example, to control an indexing table in a machine tool. 
   In  FIG. 1 , a rotating body  10  is connected to a servo motor  12  to which an angle/angular speed detector  14  such as an encoder is also connected. The angular speed is obtained, for example, from the time differentiation of the angle that the encoder outputs. 
   An angular acceleration sensor  16  for detecting the rotational angular acceleration of the rotating body  10  is mounted on the rotating body  10 . A control processor  18  performs processing for compensating an angle/angular speed command value in accordance with an angular acceleration detection value fed from the angular acceleration sensor  16 , in addition to the conventional processing for calculating an electric current control value for the servo motor  12  from the angle/angular speed command value and angle/angular speed detection value and for supplying the electric current control value to the servo motor  12 . The details of the control processing performed in the control processor  18  will be described later. Instead of detecting the angular acceleration by using the conventional angular acceleration sensor such as shown in  FIG. 1 , the angular acceleration may be obtained, for example, from the time differentiation of the angular speed detection value detected by the angle/angular speed detector  14 . 
     FIG. 2  shows another example of the rotary shaft control apparatus according to the present invention. This example differs from the example of  FIG. 1  in that the servo motor  12  and the rotating body  10  are connected via a worm  20  and a gear  22 . 
     FIG. 3  is a block diagram showing the details of the control processing performed in the control processor  18  shown in  FIG. 2 . 
   In  FIG. 3 , the angle detection value  23  detected by the encoder connected to the servo motor  12  is used as a position feedback signal and, from the difference ( 25 ) between this signal and a position command, a position control processor  24  computes a speed command using, for example, a known PID operation. Next, the detected angular speed value  26  is used as a speed feedback signal and, from the difference ( 27 ) between this signal and the speed command, a speed control processor  26  computes an electric current command using, for example, a known PID operation. Further, the electric current of the servo motor  12  is detected and, from the difference ( 30 ) between the electric current detection value  28  and the electric current command, an electric current control processor  32  computes an electric current control value using, for example, a known PID operation, and supplies the control value to the servo motor  12 . 
   The value of the rotational angular acceleration of the rotating body  10  detected by the angular acceleration sensor  16  mounted on the rotating body  10  is used for the compensation ( 34 ) of the position command, the compensation ( 36 ) of the speed command, and the compensation ( 38 ) of the electric current command. The calculation for the compensation is carried out in a calculator  40  using, for example, the following equation.
 
(Compensated command value)=(Command value before compensation)+(Coefficient)×(Angular acceleration detection value)
 
   The coefficient in the above equation is determined by trial and error so that the vibration of the rotating body is suppressed or eliminated. 
     FIG. 4  shows another example of the rotary shaft control apparatus according to the present invention. This example differs from the examples of  FIGS. 1 and 2  in that angular acceleration of the rotating body  10  is obtained by using a unit  17  that differentiates a detected rotational angular speed of the rotating body  10 . 
     FIG. 5  is a block diagram showing the details of the alternative control processing performed in the control processor  18  shown in  FIG. 4 . Instead of detecting angular acceleration by using the angular acceleration sensor  16 , as shown in  FIGS. 1 and 2 , the unit  17  combines an angular speed sensor  17 A forb detecting angular speed of the rotating body  10  and a differentiator  17 B for obtaining rotational angular acceleration by differentiating the detected rotational angular speed. Like the angular acceleration sensor  16 , the angular speed sensor  17 A is mounted on the rotating body  10 . The differentiator  17 B may be integrated together with the angular speed sensor  17 A (as shown in  FIG. 5 ), or may be integrated together with the calculator  40 .