Abstract:
When driving a plurality of paralleled feed drive mechanisms individually by servo motors ( 105, 106 ), torque of the servo motors ( 105, 106 ) is detected, and in dependence on a difference therebetween, a position command of servo motor ( 106 ) at a slave side is corrected, so that torque of the slave side servo motor ( 106 ) is matched to to torque of servo motor ( 105 ) at a master side.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a position control method and a position control system for feed drive equipment, and particularly, to a position control method and a position control system for feed drive equipment provided with a plurality of feed screws for feeding a relatively large movable body. 
     2. Description of the Related Art 
     Among feed drive equipment for feeding a relatively large movable body, such as a worktable of a large-scale tooling machine, there are ones of a system in which a plurality of feed screws are disposed in parallel, and the servo motors are driven individually by servo motors. This system allows motors of relatively low capacity to be used. 
     In feed drive equipment of the above-noted system, a position control is made by distributing an identical position command to a plurality of servo motors, to thereby match rotational positions of a plurality of feed screws with each other. 
     Therefore, if actual pitches are different between feed screw mechanisms, their feeds have a difference therebetween, which causes a couple of forces acting on a movable body in a twisting direction, and lowers a positioning accuracy of the movable body. Further, in the use of servo motors relatively low of capacity, they have to bear loads corresponding to the couple of forces in addition to a load corresponding to a feed of the movable body itself, and may be overloaded. The difference in pitch between the feed screw mechanisms depends on a temperature difference therebetween, as well, and cannot be covered up by simply improving their tooling precision. 
     SUMMARY OF THE INVENTION 
     In this regard, it is possible to detect respective feeds of the feed screw mechanisms, to thereby make a position control for matching the feeds. However, in that case, there is a need for the position control to be a full-closed system in which the feed screw mechanisms are individually provided with position scales. In general, the scales are troublesome to be cleaned if they are made of glass, or unable to render their precision of position high if they are of magnetic type. 
     The present invention is made with the above noted points in view. It is an object of the present invention to provide a position control method and a position control system for drive feed equipment by which, even when an actual pitch difference or temperature difference exists between feed screw mechanisms, it is possible by matching the feeds to keep a positioning accuracy of a movable body and prevent overloading the motors, such that this can be achieved in both of a semi-closed system and a full-closed system, and in the case of a full-closed system, it is unnecessary to employ multiple scales. 
     To achieve the object, according an aspect of the invention, there is provided a position control method for feed drive equipment in which a plurality of feed drive mechanisms disposed in parallel for feeding a movable body are individually driven by servo motors, the position control method comprising: detecting torque of the servo motors, and correcting position commands of the servo motors in dependence on the detected torque so that the servo motors have matching torque. 
     To achieve the object, according another aspect of the invention, there is provided a position control system for feed drive equipment in which a plurality of feed drive mechanisms disposed in parallel for feeding a movable body are individually driven by servo motors, the position control system comprising: a controller for detecting torque of the servo motors, and correcting position commands of the servo motors in dependence on the detected torque so that the servo motors have matching torque. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     The above and further objects and novel features of the present invention will more fully appear from the following detailed description when the same is read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a position control system according to a first embodiment of the invention; 
     FIG. 2 is a block diagram of a position control system according to a second embodiment of the invention; 
     FIG. 3 is a block diagram of a position control system according to a third embodiment of the invention; and 
     FIG. 4 is a perspective view of a tooling machine equipped with a position control system according an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There will be detailed below the preferred embodiments of the present invention with reference to the accompanying drawings. Like members are designated by like reference characters. 
     (First Embodiment) 
     FIG. 1 shows a position control system  1  for feed drive equipment of a semi-closed loop system according to a first embodiment of the invention. The feed drive equipment to which the position control system  1  is applied has: a first feed nut  101  and a second feed nut  102  that are analogous in structure and fixed in parallel, by mount flanges  101   a  and  102   a , to a movable body  100 ; and a first ball screw  103  and a second ball screw  104  that are analogous in structure and screwed in the nuts  101  and  102 , respectively. The first and second ball screws  103  and  104  are arranged parallel to each other. The first ball screw  103  is driven for rotation by a first-axis servo motor  105 , and the second ball screw  104  is driven for rotation by a second-axis servo motor  106 . 
     The first-axis servo motor  105  is provided with a rotary encoder  107  as a motor position detector for detecting a rotational position (motor position) thereof, and the second-axis servo motor  106  is provided with a rotary encoder  108  as a motor position detector for detecting a rotational position (motor position) thereof. 
     The position control system  1  is constituted with a first-axis controller  10 , a second-axis controller  20 , and a correction value calculator  30 . 
     The first axis is now supposed as a master axis, and the second axis is supposed as a slave axis. 
     The first-axis position controller  10  to be for the master axis has a position difference calculator  11 , a position controller  12 , a differentiator  13 , a speed difference calculator  14 , a speed controller  15 , a current command generator  16 , and a servo amplifier  17  for the first-axis servo motor. 
     The position difference calculator  11  inputs a position command, and calculates a difference between a position command value and a position (as a position feedback signal) of the first-axis servo motor  105  detected by the first-axis rotary encoder  107 . The position controller  12  generates a speed command so that the position difference of the first axis becomes a zero. The differentiator  13  differentiates a position signal of the first-axis rotary encoder  107  to generate a speed feedback signal of the first-axis servo motor  105 . The speed difference calculator  14  calculates a difference between a speed command value output from the position controller  12  and a motor speed by the speed feedback signal from the differentiator  13 . The speed controller  15  generates a torque command so that the speed difference of the first-axis becomes a zero. The current command generator  16  receives the torque command from the speed controller  15  and generates a current command depending on the torque command. The servo amplifier  17  receives the current command from the current command generator  16 . 
     The second-axis position controller  20  to be for the slave axis has a position command corrector  31  at a front stage, and further includes a position difference calculator  21 , a position controller  22 , a differentiator  23 , a speed difference calculator  24 , a speed controller  25 , a current command generator  26 , and a servo amplifier  27  for the second-axis servo motor. 
     The position command corrector  31  adds a correction value corresponding to motor torque generated by a later-described correction value calculator  30 , to a position command value (to be identical in value to the position command value given to the first-axis controller  10 ). The position difference calculator  21  calculates a difference between a corrected position command value from the position command corrector  30  and a position (as a position feedback signal) of the second-axis servo motor  106  detected by the second-axis rotary encoder  107 . The position controller  22  generates a speed command so that the position difference of the second axis becomes a zero. The differentiator  23  differentiates a position signal of the second-axis rotary encoder  108  to generate a speed feedback signal of the second-axis servo motor  106 . The speed difference calculator  24  calculates a difference between a speed command value output from the position controller  22  and a motor speed by the speed feedback signal from the differentiator  23 . The speed controller  25  generates a torque command so that the speed difference of the second-axis becomes a zero. The current command generator  26  receives the torque command from the speed controller  25  and generates a current command depending on the torque command. The servo amplifier  27  receives the current command from the current command generator  26 . 
     The correction value calculator  30  generates a correction value (corresponding to motor torque) for correcting a position command to be given to the second-axis servo motor  106 , so that a difference between torque Tm of the first-axis servo motor  105  and torque Ts of the second-axis servo motor becomes a zero. This correction value is passed through a low-pass filter to remove noise components. 
     Thus, the correction value calculator  30  and the position command corrector  30  constitute a position command correction means. 
     According to the present embodiment, at the slave axis side, there is performed a position control by the position command corrected so that the difference between torque Tm of the first-axis servo motor  105  and torque Ts of the second-axis servo motor becomes a zero. Thereby, even if a difference occurs between a pitch error of the feed screw at the first-axis side and a pitch error of the feed screw at the second-axis side or even when a temperature difference develops between the first screw  103  and the second ball screw  104 , there is avoided an occurrence of such an undue force that twists the movable body  100 , with a result that the positioning accuracy of the movable body  100  is improved. Further, the servo motors  105  and  106  are kept from pushing or repulsing each other between master and slave sides, and overloaded states of the servo motors  105  and  106  are avoidable in advance. 
     (Second Embodiment) 
     FIG. 2 shows a position control system  2  for feed drive equipment according to a second embodiment of the invention. This embodiment is different from the first embodiment in that, at the master-axis side, there is added a linear scale  110  for detecting a real movement position of a movable body  100 , and a detection signal thereof is fed back to constitute a full-closed position loop control system. At the slave-axis side, there is constituted a semi-closed position loop like the first embodiment. 
     (Third Embodiment) 
     FIG. 3 shows a position control system  3  for feed drive equipment of a semi-closed loop system according to a third embodiment of the invention. 
     In the position control system  3 , a correction value calculator  40  detects a command value of a torque command output from a speed controller  15 , as torque Tm of a first-axis servo motor  105 , and a command value of a torque command output from a speed controller  25 , as torque Ts of a second-axis servo motor  106 . Then, it calculates an average value of the torque Tm and Ts, and generates a correction value for correcting a position command to be given to the first-axis servo motor  105  and a correction value for correcting a position command to be given to the second-axis servo motor  106 , so that torque of the respective axes match with the average value. 
     A first-axis position controller  10  has at a front stage a position command corrector  41  for adding, to a position command value, the correction value generated by the correction value calculator  40  in correspondence to the motor torque. A second-axis position controller  20  has at a front stage a position command corrector  42  for adding, to a position command value (to be identical in value to the position command value given to the first-axis position controller  10 ), the correction value generated by the correction value calculator  40  in correspondence to the motor torque. 
     According to the present embodiment, the position control is performed by a position command corrected so that both torque Tm of the first-axis servo motor  105  and torque Ts of the second-axis servo motor  106  match with average torque. Thereby, even if a difference occurs between a pitch error of the feed screw at the first-axis side and a pitch error of the feed screw at the second-axis side or even when a temperature difference develops between a first screw  103  and a second ball screw  104 , there is avoided an occurrence of such an undue force that twists a movable body  100 , with a result that the positioning accuracy of the movable body  100  is improved. Further, the servo motors  105  and  106  are kept from pushing or repulsing each other between master and slave sides, and overloaded states of the servo motors  105  and  106  are avoidable in advance. 
     In this embodiment also, the position control system is not simply limited to a semi-closed loop, but is applicable also to a full-closed loop system in which a position loop is made by a feeding back a signal of a real movement position of a movable body detected by a linear scale or the like, or to a hybrid control system by combination of a full-closed system and a semi-closed system. 
     According to any of the first to third embodiments of the invention, even when an actual pitch difference or temperature difference exists between feed screw mechanisms, it is possible by matching the feeds to keep a positioning accuracy of a movable body and prevent overloading the motors. Still more, this can be achieved in both of a semi-closed system and a full-closed system. Yet more, in the case of a full-closed system, it is unnecessary to employ multiple scales. 
     Further, because a command value of a torque command output from a speed controller is detected as torque of the servo motor, there is no need for an extra torque detection means. 
     FIG. 4 is a perspective view of a numerical control tooling machine M in which X-axis, Y-axis, and Z-axis feeds are effected by twin-shaft feed drive equipment Fx, Fy, and Fz each respectively provided with a position control system according to any of the foregoing embodiment. 
     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.