JPH03110603A - Backlash correction control system for servo motor - Google Patents

Abstract

PURPOSE:To reduce the burden of the control of backlash acceleration and to prevent quadrant projection and inward overcutting by raising the speed loop gain by a set prescribed extent in the period when the extent of backlash acceleration is added to a speed command to control the speed. CONSTITUTION:As for a transfer function 2, the value of a parameter alpha is set to '0' at the time of not correcting the backlash, and the value of the parameter alpha is set to a set value at the time of correction to raise the gain of the speed loop. When the sign of a position command Pc is changed, a speed command Vc is multiplied by (1+alpha) in accordance with the set parameter alpha, and a backlash acceleration extent BA is added to the multiplication result, and therefore, the integration value of an integrator is quickly inverted to output a torque command value Tc which overcomes the frictional resistance to drive a servo motor in the inverse direction. Thus, quadrant projection and overcutting are reduced to reduce the burden of backlash acceleration even if the extent of backlash acceleration is not exactly set at every machine.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a control system for a servo motor that drives a feed axis of a machine tool table, etc., and in particular, the invention relates to a control system for a servo motor that drives a feed axis of a machine tool table, etc. The present invention relates to a servo motor control method that reduces quadrant protrusions that occur.

When reversing the driving direction of a servo motor that drives a table, etc. in a conventional machine tool, the machine usually cannot be reversed immediately due to the backlash and friction of the feed screw. Therefore, when a machine tool performs arc cutting or the like, if the quadrant changes, a protrusion (hereinafter referred to as quadrant protrusion) is generated on the cutting arc surface. For example, perform arc cutting on a workpiece on a plane with two axes, X and Y, and set the X axis in the plus direction.
If the quadrant changes while the Y-axis is moving in the negative direction, and the Y-axis continues to be driven in the negative direction and the X-axis is switched to be driven in the negative direction, the speed for the Y-axis remains the same as before. However, since the positional deviation of the X-axis becomes "0", the torque command value becomes small, the servo motor cannot reverse immediately due to friction, and the backlash of the feed screw that feeds the table causes the table to Since the movement of the workpiece cannot be immediately reversed, the movement of the workpiece in the X-axis direction cannot follow the movement command and is delayed. As a result, quadrant protrusions are generated on the cutting arc surface.

Conventionally, in order to eliminate or reduce this quadrant protrusion, position backlash correction was performed on the position signal when the moving direction was reversed, and an appropriate value (acceleration amount) was added to the speed command to accelerate the servo motor in the reverse direction. This is called backlash acceleration, which reduces the quadrant protrusion.

After backlash acceleration, position feedback pulses are integrated, and when the integrated value exceeds a certain value, control is performed such as stopping backlash acceleration.

Problems to be Solved by the Invention However, since the amount of backlash and the magnitude of friction differ depending on the machine, there is a problem that it is difficult to find the optimal amount of backlash acceleration.

When the amount of backlash acceleration is small, quadrant protrusions occur as described above. Furthermore, if the amount of backlash acceleration is too large, the inverted shaft will move more than the backlash and friction torque can be corrected, causing the cutting arc surface to cut inward. Therefore, there is a difficult problem in that the amount of backlash acceleration must be set strictly for each machine. Further, there is a problem in that when conditions such as cutting speed change, the optimum amount of backlash acceleration also changes.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a backlash correction control system for a servo motor that can reduce the burden of controlling backlash acceleration and prevent quadrant protrusions and excessive inward cutting.

Means for Solving the Problems The present invention performs speed control by increasing the period speed loop gain by a predetermined amount during which the amount of backlash acceleration is added to the speed command. In particular, during the period of adding the backlash acceleration amount, the backlash acceleration amount is added to the speed deviation obtained by subtracting the actual speed of the servo motor from the speed command multiplied by a predetermined set value, and this added value is integrated. The value obtained by multiplying this integral value by the integral gain is used as the output of the integrator of the speed loop to increase the gain of the speed loop and perform speed control, thereby making it possible to perform cutting without quadrant protrusions or excessive depth of cut.

For example, assume that the servo motor is being driven in the forward direction. Then, just before the quadrant changes and the drive direction of the servo motor is switched to the negative direction, the position deviation value is "
Then, the actual speed of the servo motor becomes slow, and the output of the integrator of the speed loop becomes the main proportion of the torque command value to the servo motor.Then, the position deviation value approaches "0". At the point when , the servo motor comes to a stop state due to friction.Therefore, backlash correction is performed by adding the position backlash correction amount to the error counter and adding the backlash acceleration amount to the speed command. As shown above, a positive integral value remains in the velocity loop integrator, and in order to cancel this positive integral value and further drive the servo motor in the negative direction, the integral value of the integrator must be made faster. It is necessary to output a torque command value to the servo motor to overcome static friction by setting it to a negative value.Therefore, the present invention increases the gain of the speed loop during the backlash correction period (the period in which the backlash acceleration amount is added). and quickly increase the integral value of the integrator,
A negative integrator output is generated to overcome static friction, which is used as a torque command value to drive the servo motor. As a result, it is not necessary to strictly set the backlash acceleration amount, and the burden of the backlash acceleration amount can be reduced.

In addition, even if the amount of backlash acceleration is too large and overshoot occurs when the servo motor reverses, the position error will be reversed due to this overshoot, and the inverted value will be quickly integrated into the integrator due to the increased gain. Since the integral value of is reduced, overshoot can be quickly corrected, and it is also possible to prevent inward cuts from occurring on the machined surface.

Embodiment FIG. 1 is a block diagram of an embodiment of the present invention, showing an example in which speed loop control is IP controlled (integral, proportional control). The difference from the conventional servo system block diagram is that a transfer function numbered 2 is added.

That is, KP in transfer function 1 is the position gain in the position loop, and in transfer function 2, the value of parameter α is set to 0 when backlash correction is not performed, and the value of parameter α is set to 0 when backlash correction is performed. It is set to a set value and increases the gain of the velocity loop. Transfer functions 3 and 4 are transfer functions of the integrator of the velocity loop, and 1 represents the integral gain. In transfer function 5, 2 represents the proportional gain of the speed loop, and transfer function 6 represents the current loop.
Normal gain is "1". In addition, transfer function 7 represents the transfer function of the servo motor, Jm is motor inertia, and K
t is a torque constant. The transfer function 8 is a transfer function that integrates the speed of the motor to determine the position.

When backlash correction is not performed, the value of the parameter α is set to "0", and the servo motor is controlled in the same manner as in the past.

When backlash correction is not performed, the position feedback signal Pf detected by a detector such as a pulse coder attached to the servo motor is subtracted from the position command Pc output from the numerical control device to obtain the position deviation, and the position gain The speed command Vc is obtained by multiplying by KP, and the speed deviation is obtained by subtracting the feedback signal Vf of the actual speed of the motor detected by a pulse coder etc. from the speed command Vc.If it is not during backlash correction, the parameter α=0, Since the rush acceleration amount BA=O, the above velocity deviation is integrally accumulated), the integral value is obtained, and the value obtained by multiplying the integral gain by 1 is obtained by multiplying the velocity feedback signal Vf by the proportional gain by 2. The torque command value Tc is determined by subtracting the torque command value Tc, which is passed to the current loop 6 to control the servo motor.

Also, when the sign of the position command Pc changes, that is, when the rotation direction of the servo motor is reversed, when the position backlash correction amount is output from the numerical control device and the position deviation becomes "0", at this time The backlash correction amount is added to the positional deviation, and the set backlash acceleration amount is also added. The operation up to now is the same as the conventional backlash correction, but in the present invention, the value of the parameter α is further set to the set value to increase the gain of the speed loop. Immediately before the rotation direction of the servo motor is reversed, the rotation speed of the servo motor approaches "0",
The servo motor is driven by the torque command value Tc based on the output of the integrator of the speed loop, and when the servo motor stops rotating, it stops with the output torque of the servo motor being canceled by frictional resistance. That is, the output of the integrator is an output that attempts to drive the servo motor in the same direction as before (for example, an output in the positive direction). Therefore, when the position backlash correction amount is added to the position deviation and the backlash acceleration amount is added, the position deviation is multiplied by the position gain KP to obtain the speed command, and the speed is determined by the set parameter α. Since the command is multiplied by (1+a) (speed feedback signal is "0") and the backlash acceleration amount is added to this, the integral value of the integrator rapidly reverses, overcomes the frictional resistance, and moves in the reverse direction (for example, negative A torque command value TC sufficient to drive the servo motor in the direction) is now issued. As a result, the rotational direction of the servo motor is rapidly reversed, and the number of quadrant protrusions is reduced. In addition, even if the backlash acceleration amount is set a little large and the motor speed when the servo motor is reversed becomes faster than the speed command Vc, the gain is increased by the parameter α, so the integral value of the integrator will rapidly change. , the overshoot is immediately improved and there is less inward cutting of the cutting surface.

FIG. 2 is a block diagram of an NC machine tool implementing an embodiment of the present invention, in which numeral 11 is a numerical control device with a built-in computer (hereinafter referred to as CNC), numeral 12 is a shared memory,
It serves to pass device commands and the like configured from the CNC 11 to the digital servo circuit 13, and to pass signals output from the digital servo circuit 13 to the CNC II. Further, the digital servo circuit 13 is a processor (CPU)
.

It has ROM, RAM, etc., and executes processing of the servo circuit that drives the servo motor of the machine tool 14 using software,
Drive and control the servo motor for each axis.

In addition, N with a digital servo circuit as shown in Figure 2
Since the C machine tool is already known, a detailed explanation thereof will be omitted.

FIG. 3 is a flowchart of a speed loop process executed at predetermined intervals by the processor of the digital servo circuit 13 in the same embodiment.

First, as a backlash correction command from the CNC 11,
It is determined whether or not the position backlash correction amount has been sent (step S1). If not, the timer T
is "0" or not (step S6), timer T is "0" or not.
0'', subtract the speed feedback signal Vf from the pulse coder etc. from the speed command Vc calculated in the position loop to find the speed deviation, add this speed deviation to the register R (1) for integrating, and calculate the integral value ■. (Step S9).

Then, the torque command value Tc is obtained by subtracting the value obtained by multiplying the speed feedback signal Vf by the proportional gain by 2 from the value obtained by multiplying the integral value I stored in this register R(1) by the integral gain by 1. The command value Tc is transferred to the current loop in step S10 (511), and the processing of the speed loop in the corresponding cycle is ended. That is, velocity loop processing similar to the conventional one is performed.

On the other hand, when a backlash correction command is output from the CNCII and detected in step S1, the processor determines whether the sign of the position deviation value stored in the error counter has been reversed (step S2). In other words, if the sign of the backlash correction amount is positive, it is determined whether the position deviation value is "0" or a positive value, and if the sign of the backlash correction amount is negative, the position deviation value is "0". ” or a negative value, and if the sign of the positional deviation value is not reversed, the above-described processes of steps 86 and 89 to 811 are performed. If the sign of the position deviation value is reversed, the timer T is set to the set value, the position backlash correction data (correction amount) is added to the error counter, and the backlash correction data is cleared (steps 83 to S5). ).

Next, it is determined whether the timer T is "0" or not, and since it is not "0", the value obtained by subtracting the speed feedback signal Vf from the speed command Vc is multiplied by the value obtained by adding "1" to the setting parameter α, and the value is integrated. In addition, the set backlash acceleration amount BA is added to the register R(1) as a device, and "1" is subtracted from the timer T (steps S6 to S88). Then, the torque command value Tc is obtained by subtracting the value obtained by multiplying the speed feedback signal Vf and the proportional gain by 2 from the value obtained by multiplying the integral gain by 1 and the value of the register R(1), and the torque command value Tc is transferred to the current loop.Step 310 , 811), ' of the period
End speed loop processing.

In the next cycle, the backlash correction data is
5, the process moves from step S1 to step S6, and if the timer T is "0", step S7. S8. S.I.O.

The process of S11 is performed. That is, until the time set in the timer T is reached, backlash acceleration correction is performed and the gain is increased by a parameter of 6 (step 87) to perform the integration process.

Then, when the timer T reaches "0", the process moves from step S6 to step S9 and returns to normal speed loop processing.

The above is the operation processing of this embodiment. In the above embodiment, the speed loop is controlled by IP control (integral, proportional control).
Although we have shown an example using PI control (proportional, integral control), the same applies to PI control (proportional, integral control). What is necessary is to multiply it by (1+α) and add the backlash acceleration amount during the set backlash correction period.

Further, the amount of backlash acceleration may be added to the speed command value.

Effects of the Invention In the present invention, when the rotational direction of the servo motor is reversed, the gain of the speed loop is increased, the integral value output of the integrator of the speed loop is rapidly reversed, and the torque command value is rapidly reversed to control the servo motor. Since it is driven, the delay of the servo motor for reversing is reduced, and the number of quadrant protrusions can be reduced. Moreover, since the gain of the speed loop has increased, even if the backlash acceleration is too large and the servo motor speed overshoots during reversal, the overshoot is quickly stopped, so the cutting surface Inward cutting is also reduced, and quadrant protrusions and excessive cutting can be reduced without strictly setting the amount of backlash acceleration for each machine, and the burden of backlash acceleration can be reduced.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a servo system in one embodiment of the present invention, Fig. 2 is a block diagram of a CNC machine tool implementing the same embodiment, and Fig. 3 is a block diagram of a digital servo circuit processor in the same embodiment. It is a flowchart of speed loop processing. KP...Position gain, α...Parameter, B
A...backlash acceleration amount, kl...integral gain, k2...proportional gain.

Claims (1)

[Claims] (1) In a backlash correction control method for a servo motor that drives the feed axis of a machine tool, speed control is performed by increasing the speed loop gain by a predetermined amount during the period in which the amount of backlash acceleration is added to the speed command. A backlash correction control method for servo motors. (2) In a backlash correction control method for a servo motor that drives the feed axis of a machine tool, during the period in which backlash acceleration is added, the speed deviation obtained by subtracting the actual speed of the servo motor from the speed command is multiplied by a predetermined value. A servo motor characterized in that a backlash acceleration amount is added to the value obtained by the above, the added value is integrated, and the value obtained by multiplying this integrated value by an integral gain is set as the output of the integrator of the speed loop. Backlash correction control method.