JP3308656B2 - Servo motor control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to control of the position of a feed axis, a robot arm, and the like of a machine tool driven by a servomotor, and more particularly, to a method of controlling a servomotor by semi-closed loop control.

[0002]

2. Description of the Related Art As a method of driving a machine such as a feed axis of a machine tool or a robot arm with a servomotor and controlling its position, the position itself of the machine driven by the servomotor is detected and feedback controlled. There are a closed-loop control method and a semi-closed-loop control method in which the rotational position of a servomotor is detected and the position is feedback-controlled to control the position of the machine. In the semi-closed loop system, the mechanical system has rigidity, and the position of the servomotor and the position of the machine correspond to each other and there is no shift, and the position of the machine is controlled by the position of the servomotor.

[0003]

However, there are cases where the mechanical system has low rigidity and it can be considered that the servomotor and the machine are spring-connected. FIG. 4 is a diagram schematically showing such a coupling state between the motor and the machine. The motor and the machine are connected by a spring and are affected by frictional force. Therefore, assuming that the influence of the frictional force is sufficiently small for the spring, the amount of expansion and contraction of the spring is proportional to the force applied to the mechanical inertia, and the force is proportional to the magnitude of the acceleration. As a result, if the spring is weak (mechanical rigidity is weak), the position of the motor and the position of the machine are shifted when the acceleration is large, and the trajectory of the machine, that is, the machining trajectory for a machine tool, , The movement locus of the robot arm deviates from the command locus.

[0004] Therefore, conventionally, in a machine tool or the like, in order to maintain the accuracy of the processing shape, processing is performed at a low processing speed and a low acceleration.

Accordingly, an object of the present invention is to provide a method of controlling a servomotor which can improve the accuracy of the movement trajectory of a machine driven by the servomotor even when the rigidity of the mechanical system is low in the control by the semi-closed loop system. Is to provide.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a method for controlling a servomotor in a system for controlling a machine position by a semi-closed loop system .
In order to compensate for the deformation of the mechanical system in accordance with the acceleration, the speed command output by the position loop control is corrected to an offset amount proportional to the acceleration of the movement command to obtain a speed command. Is controlled. In particular, the acceleration of the movement command is obtained by obtaining a difference between the movement command outputted in the predetermined period and the movement command of the previous period, and the offset is obtained by multiplying the acceleration by a predetermined coefficient.

[0007]

When the machine and the servomotor are connected by a spring, the amount of expansion and contraction of the spring is determined by the mechanical inertia. This mechanical inertia force is proportional to the magnitude of the acceleration. Therefore, the offset amount proportional to the acceleration of the movement command is corrected to the speed command obtained by the ordinary position loop control, and the servo motor is controlled by the corrected speed command, so that the position of the servo motor is excessively increased during acceleration / deceleration. In this case, the trajectory accuracy is improved by compensating for the difference and canceling out the amount of expansion and contraction of the spring.

[0008]

FIG. 1 is a block diagram of a servo motor control system according to an embodiment of the present invention. In the figure, 1 is a term showing a transfer function of the position gain Kp in the position loop, 2 is a term of the speed loop, 3 is a term showing the servomotor, and 4 is a transfer function for obtaining the position by integrating the output speed of the servomotor. Term. Reference numeral 5 denotes a machine driven by the servomotor 3, and the servomotor 3 and the machine 5 are schematically illustrated as being connected by the spring 6 as described above. In addition, 7 means a term of friction.

The above-described configuration is the same as the conventional method of controlling a servo motor of the semi-closed loop system, but the configuration 8 is added to the present invention. That is, the position command (movement command) is differentiated twice to determine the acceleration of the position command (movement command), and the acceleration is multiplied by a coefficient α determined by the rigidity of the mechanical system (magnitude of the spring constant) to determine the offset amount. The difference is that this offset amount is added to the speed command.

The position error E is obtained by subtracting the position of the servomotor from the position command (movement command) output from a control device for controlling a machine such as a numerical control device. A speed command (Kp × E) in the position loop control is obtained by multiplying E by the position gain Kp. Further, in the present invention, an offset amount is obtained by multiplying the acceleration of the position command (movement command) obtained by differentiating the position command (movement command) twice by a setting coefficient α, and this offset amount is calculated by the speed command (Kp × E) to obtain a speed command to the speed loop. In the speed loop, as in the conventional case, a speed loop control such as PI control is performed based on the input speed command to obtain a torque command, and the servo motor 3 is driven.

When the acceleration of the position command (movement command) is large, for example, when the movement is started from a stopped state, a large force acts on the spring system of the joint between the machine and the servomotor due to the mechanical inertia, and the spring Will expand and contract. However, in the present invention, since the offset amount proportional to the acceleration is added to the speed command obtained by the ordinary position loop control, the torque command is excessively increased and the servo motor is moved extra. Therefore, the position of the servomotor is corrected by an amount corresponding to the amount by which the spring (mechanical system) expands and contracts due to the mechanical inertia. Therefore, the deviation of the mechanical position from the command position due to the expansion and contraction of the spring is corrected.

[0012] The extra amount of the offset amount added to the speed command causes an extra movement, which then occurs as a positional deviation and is corrected. With this correction, the displacement of the mechanical spring is also returned and returned. Therefore, these are canceled and the error between the command position and the machine position is reduced. As a result, in the case of a machine tool, a machining shape error is reduced, and machining accuracy is improved. In addition, if the servomotor drives the robot arm or the like, the accuracy of the tip trajectory of the robot arm is improved.

FIG. 2 is a block diagram of a servo motor control system of the machine tool for performing the control shown in FIG.
Numeral 10 denotes a numerical controller for controlling a machine tool, 11 denotes a position command and various control signals output from the numerical controller 10 to a processor of a digital servo circuit 12, and various control signals from the digital servo circuit 12, The shared memory and the digital servo circuit 12 used to transfer the status signal to the numerical controller 10 are constituted by a processor, a memory such as a ROM and a RAM, and control the servo motor described above by software. is there. Reference numeral 13 denotes a servo amplifier which is configured by an inverter or the like and drives the sabo motor 3. 14 is the position of the servo motor 3
It is a position / speed detector such as a pulse coder that detects speed. The configuration of the control unit of the machine tool or the like shown in FIG. 2 is conventionally known, and a detailed configuration is omitted.

The above-described digital servo circuit 1
The processor 2 of FIG.
Is performed to control the servo motor shown in FIG. First, as in the conventional case, a position command (movement command) for each distribution cycle (ITP cycle) sent from the numerical controller 10 via the shared memory 11 is converted to an incremental movement command MCMD for each position / speed loop processing cycle. (Step S1). Since this movement command is a movement amount within a predetermined cycle for each position / speed loop processing cycle, it means a speed. Next, a value obtained by subtracting the position feedback amount Pfb fed back from the position / speed detector 14 as the movement amount of the servo motor in the previous cycle from the movement command MCMD is added to the register storing the position deviation E, The position deviation E is updated (step S2). In FIG. 3, the position deviation in the previous cycle is represented by E (n-1), and the position deviation in the cycle is represented by E (n). Note that in FIG. 3, n means the period, and (n-1) means the previous period.

The position error E (n) thus obtained is multiplied by the position gain Kp to obtain a speed command by ordinary position loop processing.
The speed command VCMD (n) is obtained by adding a value obtained by subtracting the previous cycle MCMD (n-1) from the CMD (n) by the set coefficient α, that is, an offset amount (step S3). Since the movement commands MCMD (n) and MCMD (n-1) are movement commands of an incremental amount for each position / speed loop processing cycle, they mean a movement amount within a predetermined time, that is, a speed. Move command difference [MCMD
(N) -MCMD (n-1)] means a speed difference, that is, a position command acceleration. Therefore, the process of step S3 means that a value obtained by multiplying the acceleration of the position command by the coefficient α is added to the speed command as an offset amount. The coefficient α is experimentally obtained and set based on the strength of the mechanical system (the magnitude of the spring multiplier).

Based on the obtained speed command VCMD (n), a speed loop process similar to the conventional one is executed to obtain a torque command (step S4), and the obtained torque command is transferred to a current loop (step S5). The processing of the position / velocity loop processing cycle ends. The above-described steps S1 to S5 are repeatedly executed for each position / speed loop processing cycle to control the servomotor.

FIGS. 6 and 7 show the actual machine position and position command (movement command) as shown in FIG. 5 when the position command (movement command) is viewed in terms of acceleration. FIG. 6 shows a difference from a theoretical value calculated from a position command to a tip of the machine as a temporary delay system having no spring expansion and contraction (no torsion). FIG. 6 shows a conventional servomotor control to which the present invention is not applied. Is the difference between the methods.
FIG. 7 shows the difference when the present invention is applied. FIG.
In FIG. 7, the horizontal axis represents time, while the vertical axis represents the difference between the above positions, and the unit varies depending on the speed of the input position command. Therefore, as shown in FIG. Is a comparison of the difference between the conventional method and the present invention as a pattern of the difference in position when is input.

As can be seen by comparing FIGS. 6 and 7,
When the present invention is applied, the difference between the theoretically obtained position and the actual position is smaller, and the mechanical system twists (stretches and shrinks).
It can be seen that the displacement caused by the above is improved.

[0019]

According to the present invention, when controlling a servomotor of a low rigidity machine by a semi-closed loop method, deviation from a command position caused by excessive torsion of a mechanical system due to acceleration during acceleration / deceleration. To improve the accuracy of the machined shape when applied to the control of a servomotor that drives the feed axis of a machine tool, and to improve the trajectory accuracy when applied to a servomotor that drives a robot arm. Can be.

[Brief description of the drawings]

FIG. 1 is a block diagram of a servo motor control system according to an embodiment of the present invention.

FIG. 2 is a block diagram of a servo motor control unit of the machine tool that implements the embodiment.

FIG. 3 is a flowchart of a process performed by a processor of the digital servo circuit in the embodiment.

FIG. 4 is a diagram schematically showing a coupling state between a motor and a machine.

FIG. 5 is a diagram when a position command (movement command) input to see the effect of the present invention is viewed in terms of acceleration.

FIG. 6 shows a conventional servo motor control method in which the actual machine position when the position command of the pattern in FIG. 5 is commanded and the time from the position command to the tip of the machine are calculated as a temporary delay system with no spring expansion and contraction. It is a figure which shows the difference with a theoretical value.

7 shows an actual machine position when a position command of the pattern shown in FIG. 5 is issued by the servo motor control method of the present invention;
It is a figure which shows the difference from the theoretical value calculated from the position command to the front-end | tip of a machine as a temporary delay system without expansion and contraction of a spring.

[Explanation of symbols]

 3 Servo motor 5 Machine 6 Spring 7 Friction 10 Numerical controller 11 Shared memory 12 Digital servo circuit 13 Servo amplifier 14 Position / speed detector

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI H02P 5/00 H02P 5/00 G

Claims (2)

(57) [Claims] To 1. A for controlling the position of a machine driven by the servo motor, the control method of the servo motor for feedback control of the rotational position of the servomotor, acceleration
Compensates for mechanical system deformation according to acceleration generated with speed
To reason, a velocity command to correct the offset amount that is proportional to the acceleration of the movement command to the speed command output by the position loop control, the control method of the servo motor for controlling the servo motor by the speed command, which is the corrected. 2. An acceleration is obtained by obtaining a difference between a movement command of the movement command outputted in a predetermined cycle and a movement command of a preceding cycle, and the offset is obtained by multiplying the acceleration by a predetermined coefficient. 2. The servo motor control method according to claim 1, wherein a speed command corrected by adding to the speed command output by the position loop control is obtained.