JPH04142604A - Control method for servo motor - Google Patents

Abstract

PURPOSE:To improve the stability of a machine in stop and the servo stiffness of the machine in traveling by judging the rotating position of a servo motor, and by lowering a speed loop gain when the rotating position is within the backlash or by raising the speed loop gain when the rotating position is out of the backlash. CONSTITUTION:The acceleration of a servo motor is estimated by a current command to the servo motor, a real acceleration B is obtained from the detected speed of the servo motor, and from comparison of the real acceleration B with the estimated acceleration A, it is judged whether or not the rotating position of the servo motor is within the backlash of the mechanical system to be driven by the servo motor. Further, if the rotating position is within the backlash, gains K2 and K2 are lowered, and if the rotating position of the servo motor is out of the backlash and if the servo motor moves integrally with the machine, gains K1 and K2 of speed loop are made as large as the servo stiffness can be obtained. With this, the stability of servo motor and machine in stop and the servo stiffness of the machine in traveling can be enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a control method for a servo motor that drives a feed axis of a machine tool, an arm of a robot, etc.

Conventional technology In the control of servo motors that drive the feed axes of machine tools, robot arms, etc., conventionally, values are set that provide good response of the control system and servo rigidity (state with high degree of disturbance suppression). The speed loop gain is set and controlled by fixing it to this constant value.

Problems to be Solved by the Invention However, when driving the feed axis of a machine tool or the arm of a robot that is controlled using a servo motor, if the backlash between the servo motor and the machine is large, the servo motor will stop moving within the backlash when the servo motor stops. The motor may vibrate, which may cause vibration or wobbling in the mechanical system. In other words, when positioning the servo motor to the stop position, there is almost no load on the servo motor during backlash, so if the speed loop gain is high and responsiveness is good, the servo motor will pass through the positioning position and correct it. When attempting to do so, the direction of movement is reversed, and this action is repeated, resulting in vibration. The influence of this vibration will also appear in mechanical systems.

In order to avoid this phenomenon, it is effective to lower the speed loop gain, but lowering the speed loop gain lowers the servo rigidity and deteriorates the degree of suppression of disturbances.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a servo motor control method that provides stability when the machine is stopped and high servo rigidity when moving the machine.

Means for Solving the Problems The present invention estimates the acceleration of the servo motor from a current command to the servo motor, determines the actual acceleration from the detected speed of the servo motor, and calculates the acceleration of the servo motor from the ratio of the actual acceleration to the estimated acceleration. It is determined whether or not the rotational position of the servo motor is within the backlash of the mechanical system driven by the servo motor, and when it is within the backlash, the speed loop gain is lowered, and when it is not within the backlash, the speed loop gain is increased.

In particular, when the ratio of the actual acceleration to the estimated acceleration is less than 1, the speed loop gain is set to a large value, and when the ratio exceeds 1, the speed loop gain is set to the set value multiplied by the reciprocal of the ratio. value and decrease the speed loop gain. Furthermore, when the above ratio is larger than the value obtained by dividing the total inertia of the servo motor and mechanical system by the inertia of the servo motor, the gain of the speed loop is set to a value obtained by multiplying the reciprocal of the value by the above setting value, and the gain of the speed loop is Decrease gain. This solved the above problem.

Operation Figure 1 is a block diagram of the speed loop system of a servo motor that drives the feed axis of a machine tool or the arm of a robot, explaining the principle of operation of the present invention. Block 1 is the transfer function of the speed loop compensation circuit; kl is an integral constant, and k2 is a proportionality constant. In addition, block 2 is the torque constant Kt of the servo motor.
Block 3 is a transfer function for the inertia Jm of the servo motor, and block 4 is a transfer function for the machine inertia JL of the feed axis of a machine tool, the arm of a robot, etc.

Block 5 is a transfer function for calculating the estimated acceleration A of the servo motor and the machine, and a torque command T as a current command to the servo motor which is the output of the speed loop compensation circuit 1.
For c, the torque constant Kt of the servo motor is the total inertia of the servo motor and machine (J m + J L
) is used to calculate the estimated acceleration A of the servo motor and machine. Also, block 6
is a transfer function that calculates the actual acceleration B by differentiating the actual speed Vr of the servo motor.

As in the past, the actual speed Vr is subtracted from the speed command Vc output by the position loop control to find the speed deviation, and the speed loop compensation circuit 1 performs proportional-integral control to obtain the torque command Tc.
is determined, and the servo motor is driven using the torque command Tc as a current command. Furthermore, in the present invention, the torque command (current command) Tc is a constant K t/ (Jm+J L )
Find the estimated acceleration A of the servo motor and machine by multiplying by In other words, when the servo motor and the machine are moving as one, this ratio is "1", and in such a case, the speed loop gain kl should be set to a sufficiently high value of 2 to obtain servo rigidity. Make it.

However, when this ratio is greater than "1" and the actual acceleration B of the servo motor is greater than the estimated acceleration A, it is assumed that backlash is present, and the velocity loop gain kl is reduced by 2 to perform velocity loop control.

The value of the velocity loop gain that provides sufficient servo rigidity is k
l, is set to 2, the speed loop gain of the speed loop compensation circuit 1 is the value obtained by multiplying the above value by the variable α, and the mechanical inertia JL that changes due to backlash is set to xJm, and the actual speed Vr is transmitted to the speed command Vc. When calculating the function H(S), the first formula %Formula % Also, when the machine and servo motor are moving together, the machine inertia Jt is β times the motor inertia Jm (J, = βJm). Then, the above ratio is expressed by the following second equation.

B/A I = (Jm+JL)/Jm (1+x)=
(1+β)/(1+x) Furthermore, the gain k1 of the speed loop. 2 to 3rd respectively
Replace the equation as shown in the fourth equation.

kl=klO・(1+β)...(3)k2=
20・(1+β)...(4)And when the servo motor and the machine are moving as one,
The above variable X is β, and the above ratio I B/A l is “1”
becomes. Therefore, as shown in FIG. 2, the above ratio I B/A
When + is "1" or less, α=1. By substituting x=β into the first equation and replacing 2 with the right side of equation 3.4 for kl in the first equation, the first equation becomes the following equation 5.

H(S)=Vr/Vc=(t・k2(1・S+Kt・klo)/(jm
・S2+Kf・k20・s+KikIO)...(5) When the above ratio, B/A I = (1+β)/(1+x), is larger than "1", that is, the actual acceleration B has a larger backlash than the estimated acceleration A. When it is within the range, as shown in Figure 2, substitute α=l A/B = (1+x)/(1+β) into the first equation to reduce the gain of the speed loop (note that 2 is also added to kl). (replace with the right-hand sides of equations 3 and 4). In this case as well, the fifth equation is obtained, and speed loop control with a small gain is obtained without any change in the transfer function H(S).

Further, when the ratio I B/A + is larger than (1+β), the mechanical inertia JL is assumed to be “0”, x=0 is substituted, and the value of α is fixed at 1/(1+β). In this case as well, the transfer function H(S) is expressed by the fifth equation.

In this way, the gain of the speed loop is set small when there is backlash, and the gain of the speed loop is set large when the machine and servo motor are moving together, rather than within backlash, to improve stability when stopped and movement. Controls a servo motor that achieves both servo rigidity and servo rigidity.

Embodiment FIG. 3 is a block diagram of digital servo control implementing an embodiment of the present invention.

10 is a numerical control device (CNC) with a built-in computer that controls machine tools, robots, etc.; 11 is a processor (CNC) of the numerical control device 10 and the digital servo circuit 12;
12 is a digital servo circuit that controls the position, speed, etc. of the servo motor 13 using software;
It consists of a processor (CPU), memory such as ROM, RAM, etc. 13 is a servo motor, and 14 is a detector such as a pulse coder for detecting the position and speed of the servo motor 13.

The configuration of the digital servo control described above is conventionally known, and details thereof will be omitted.

FIG. 4 is a flowchart of speed loop processing to which the present invention is applied, which is executed by the processor of the digital servo circuit 12.

The processor of the digital servo circuit 12 performs position loop control based on the movement command received from the numerical control device 10 via the shared memory 11 and the position of the servo motor 13 detected by the detector 14 to obtain a speed command Vc. Then, the speed loop process shown in FIG. 4 is executed every predetermined period.

First, the speed command Vc calculated by position loop control
At the same time, the actual speed Vr of the servo motor detected by the detector 14 is read (steps Sl, S2), and the same speed loop processing as in the past is performed to obtain the torque command Tc as the current command to the servo motor 13. In the electromagnetic loop, current is controlled using this torque command Tc as a command (step S3).

Next, a constant Kt/(J m
+ J L ) to obtain the estimated acceleration A (step S4). Note that the torque constant Kt and the inertia Jm of the servo motor are known values and constants. Also. Machine inertia JL is the inertia when the servo motor and machine move together, and this is also a known constant, so
Kt/(Jm+JL) is also a constant.

Next, the actual speed of the previous cycle detected in the previous cycle of this speed loop process and stored in the register R (Vr) is subtracted from the actual speed Vr detected in step S2, and the value is used as the value of the previous cycle of this speed loop process. Divided by the period T (differential), the actual acceleration B
(step S5), and the actual speed V detected in the relevant period
r is stored in register R (Vr) (step S6).

Next, the ratio of actual acceleration B to estimated acceleration A, IB/A, is 11.
If it is less than "1", then the variable α is set to "11", the velocity loop gain is set to the set value αkl=kl, and α is set to 2=2 (step S7).
10) End the speed loop processing for the period. Further, when the ratio IB/A is larger than "1", the ratio IB/A is larger than "1".
B/A is the value obtained by dividing the total inertia of the servo motor and mechanical system by the inertia of the servo motor (1 + β) (= (Jm
+JL)/Lm=Jm (1+β)/Jm) is determined in step S8); if not, the variable α
Substituting the reciprocal of the above ratio IA/Bl into , the gain of the velocity loop is l A/B l・kl.

A/B l ·k2 (step 511). Also,
If the above ratio I B/A + is larger than (1+β),
1/(1+β) is assigned to the variable α, and the gain of the velocity loop is set to kl/(1+β) and 2/(1+β) (step S9). In this way, when the velocity loop gain is changed, in the next velocity loop processing cycle, the velocity loop processing in step S3 will be executed using this changed gain.

In the above embodiment, the value of α for switching the gain of the velocity loop was switched as shown in FIG. 2 by the value of the ratio I B /A l, but step S8. 89 may be deleted, and when it is determined in step S7 that the ratio I B/A is greater than "1", the value of the variable α may be set to l A/B l, and the ratio B/AI may be set to 11. '', the gain of the velocity loop may be switched to a preset small gain.

Effects of the Invention The present invention determines whether or not the servo motor position is within the rose flage of the machine based on the ratio of the actual acceleration to the estimated acceleration. When the motor and machine are moving together, the gain of the speed loop is set to a large gain that provides servo rigidity, thereby improving stability when the servo motor and machine are stopped and improving servo rigidity when moving. I can do it.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a speed loop explaining the principle of operation of the present invention, and Fig. 2 shows the relationship between the variable α for switching the gain of the speed loop and the ratio of actual acceleration to estimated acceleration in an embodiment of the present invention. FIG. 3 is a block diagram of digital servo control implementing the same embodiment, and FIG. 4 is a flowchart of speed loop processing in the same embodiment. 1... Transfer function of speed loop compensation circuit, kl... Integral constant, k2... Proportionality constant, 2... Transfer function for torque constant Kt of servo motor, 3... Transfer for inertia of servo motor Function, 4...Transfer function for machine inertia such as the feed axis of a machine tool or robot arm, 5...Transfer function to obtain the estimated acceleration of the servo motor and machine, 6...Differentiate the actual speed of the servo motor 13... a servo motor, 14... a detector that detects position and velocity.

Claims (3)

[Claims] (1) Estimate the acceleration of the servo motor from the current command to the servo motor, find the actual acceleration from the detected speed of the servo motor, and determine the rotational position of the servo motor from the ratio of the actual acceleration and the estimated acceleration. A servo motor control method characterized by determining whether or not a mechanical system to be driven is within backlash, lowering a speed loop gain when the backlash is within the backlash, and increasing the speed loop gain when the backlash is not within the backlash. (2) When the ratio of the above actual acceleration to the above estimated acceleration is 1 or less, the speed loop gain is set to the set value, and when the above ratio exceeds 1, the speed loop gain is set to the above set value multiplied by the reciprocal of the above ratio. 2. The servo motor control system according to claim 1, wherein the servo motor is set to a value. (3) When the ratio of the above actual acceleration to the above estimated acceleration is less than 1, set the speed loop gain to the set value, and when the above ratio is greater than 1, the total inertia of the servo motor and mechanical system is divided by the inertia of the servo motor. If the gain is smaller, set the speed loop gain to the above setting value multiplied by the reciprocal of the above ratio, and if the above ratio is larger than the total inertia of the servo motor and mechanical system divided by the servo motor inertia, the speed loop gain 2. The servo motor control system according to claim 1, wherein the gain is set to a value obtained by multiplying the set value by the reciprocal of a value obtained by dividing the total inertia of the servo motor and the mechanical system by the inertia of the servo motor.