Control apparatus and a control method for a servomotor

• US 5,598,077 A
• Filed: 04/18/1995
• Issued: 01/28/1997
• Est. Priority Date: 08/19/1993
• Status: Expired due to Term

• Current Assignee: Fanuc Ltd (Fanuc Corporation)
• Original Assignee: Fanuc Ltd (Fanuc Corporation)
• Inventors: Okita, Tadashi, Iwashita, Yasusuke, Matsubara, Shunsuke
• Primary Examiner(s): Sircus, Brian
• Application Number:
  US08/416,898
• Time in Patent Office: 651 Days
• Field of Search: 318/560-574, 318/630, 318/606-610, 318/807-811, 364/148-151, 364/158, 364/160-166
• US Class Current: 318/568.22
• CPC Class Codes: G05B 19/404 characterised by control arrangements for compensation, e.g. for backlash, overshoot, tool offset, tool wear, temperature, machine construction errors, load, inertia G05B19/19, G05B19/41 take precedence
• CPC Class Codes: G05B 2219/41078 Backlash acceleration compensation when inversing, reversing direction
• CPC Class Codes: H02P 23/24 Controlling the direction, e.g. clockwise or counterclockwise

Abstract

The present invention relates to motor control at the time of reversal of direction of feed axes of a machine tool using a servomotor. The present invention includes a disturbance torque estimating unit for estimating a disturbance torque and estimating a frictional torque. The value in a speed loop integrator is divided into a frictional torque component and an acceleration torque component. An integrator target value for the time of reversal of motor rotation is obtained in accordance with the acceleration torque component and a value obtained by inverting the sign of the frictional torque component. Backlash acceleration correction is effected using a value obtained by applying a certain offset to a speed command so that the integrator target value is reached, as a backlash acceleration value. In estimating the frictional torque, a ratio of torque constant to inertia is estimated automatically, and the frictional torque is estimated by using this ratio. Since proper backlash acceleration correction can be effected, quadrant projections are minimized.

Claims

1. A control apparatus for controlling a servomotor, comprising:

disturbance torque estimating means for estimating the magnitude of a disturbance torque to which the servomotor is subjected;

integrator target value setting means for setting, as a target value for a speed control loop integrator, the sum of the value in the speed control loop integrator at the time of reversal of servomotor rotation and a value obtained by doubling a value which is obtained by inverting the sign of the disturbance torque just before the reversal of motor rotation, which is estimated by said disturbance torque estimating means;

speed command offset applying means for applying an offset to a speed command after the reversal of motor rotation so that the speed control loop integrator can quickly shift to the target value set by said integrator target value setting means; and

means for sending the target value to the servomotor.

2. A control apparatus for a servomotor according to claim 1, wherein said offset is a value obtained by multiplying the difference between said set target value for said integrator and the present value therein by a certain coefficient.

3. A control apparatus for a servomotor according to claim 1, wherein said disturbance torque estimating means includes a term for a ratio of torque constant to inertia as a parameter of a motor model, and the value of said term is modified at every processing period.

4. A servomotor control method applied at the time of reversal of direction of feed axes of a machine tool using a servomotor, said control method for a servomotor comprising the steps of:

estimating a disturbance torque to which the servomotor is externally subjected just before the reversal of servomotor rotation;

dividing the value in a speed loop integrator into a frictional torque component, which is equal to said estimated disturbance torque, and an acceleration torque component;

setting in the speed loop integrator the sum of said acceleration torque component and a value obtained by inverting the sign of said frictional torque component, as an inverted target value, at the time of the reversal of servomotor rotation; and

modifying a speed command by applying a certain offset to the speed command so that the integrator quickly shifts toward the inverted target value.

5. A servomotor control method according to claim 4, wherein said offset is a value obtained by multiplying the difference between said inverted target value set in the speed loop integrator and the present value therein by a predetermined coefficient.

6. A servomotor control method according to claim 4, wherein said estimation of the disturbance torque includes a step of suitably obtaining the ratio of torque constant to inertia of the motor.

7. A servomotor control method applied at the time of reversal of direction of feed axes of a machine tool using a servomotor, said control method for a servomotor comprising the steps of:

estimating a disturbance torque to which the servomotor is externally subjected just before the reversal of servomotor rotation;

dividing the value in a speed loop integrator into a frictional torque component, which is equal to said estimated disturbance torque, and an acceleration torque component;

setting in the speed loop integrator the sum of said acceleration torque component and a value obtained by inverting the sign of said frictional torque component, as an inverted target value, at the time of the reversal of servomotor rotation; and

modifying a speed command by applying a certain offset to the speed command so that the integrator quickly shifts toward the inverted target value,

wherein the addition of said offset to the speed command is repeated a preset number of times, and then said offset is zero thereafter.

8. A servomotor control method applied at the time of reversal of direction of feed axes of a machine tool using a servomotor, said control method for a servomotor comprising the steps of:

estimating a disturbance torque to which the servomotor is externally subjected lust before the reversal of servomotor rotation;

dividing the value in a speed loop integrator into a frictional torque component, which is equal to said estimated disturbance torque, and an acceleration torque component;

setting in the speed loop integrator the sum of said acceleration torque component and a value obtained by inverting the sign of said frictional torque component, as an inverted target value, at the time of the reversal of servomotor rotation; and

modifying a speed command by applying a certain offset to the speed command so that the integrator quickly shifts toward the inverted target value,

wherein said step of suitably obtaining the ratio of torque constant to inertia of the motor includes;

(a) a step of obtaining a variation of a torque command for each of the last two processing periods preceding the present period;

(b) a step of obtaining an estimated speed of the motor from the variation of the torque command obtained in said step (a), an actual speed of the motor for each of the last two processing periods preceding the present period, and an estimated ratio of torque constant to inertia of the motor obtained in the last period;

(c) a step of obtaining an estimated speed error equivalent to the difference between the actual speed of the motor and said estimated speed; and

(d) a step of settling an estimated ratio of torque constant to inertia for the present period in accordance with the variation of the torque command obtained in said (a) and the estimated speed error obtained in (c) so that the difference from the estimated ratio for the period immediately preceding the present period is small when the variation of the torque command obtained in said (a) is small, takes a value substantially proportional to the variation of the torque command before said variation of the torque command reaches a predetermined value, and is saturated and takes a certain fixed value when the variation reaches a value not smaller than said predetermined value, and setting the ratio of torque constant to inertia at a value on which the estimated ratio converges.

9. A servomotor control method applied at the time of reversal of direction of feed axes of a machine tool using a servomotor, said control method for a servomotor comprising the steps of:

estimating a disturbance torque to which the servomotor is externally subjected just before the reversal of servomotor rotation;

dividing the value in a speed loop integrator into a frictional torque component, which is equal to said estimated disturbance torque, and an acceleration torque component;

setting in the speed loop integrator the sum of said acceleration torque component and a value obtained by inverting the sign of said frictional torque component, as an inverted target value, at the time of the reversal of servomotor rotation; and

modifying a speed command by applying a certain offset to the speed command so that the integrator quickly shifts toward the inverted target value,

wherein the estimated ratio b*(i) between the torque constant and inertia of said (d) is obtained as follows;

space="preserve" listing-type="equation">b*(i)=b*(i-1) {β

·

u·

e/(1 β

u).sup.2 },

where u is the variation of the torque command obtained in said step (a), e is the estimated speed error obtained in said step (c), b*(i-1) is the estimated ratio for the preceding period previously obtained in said step (d), and β

is a setting parameter value.