JPH02275507A - Feedforward control system - Google Patents

Abstract

PURPOSE:To accurately execute the interpolation outputting of a numerical control part and to execute feedforward control by the interpolation output by providing the feedforward control system with an interpolator for outputting the an interpolation output less than an interpolation pulse unit, a feedforward control element for forming a feedforward item, an integer forming circuit, and a servo circuit. CONSTITUTION:An interpolation output including a decimal of the interpolator 11 is added to the output of the feedforward control element 12 by an adder 13. Namely, the output of the adder 13 indicates the value obtained by adding a feedforward item to the interpolation output of the interpolator 11, and since the output includes a decimal point, the output is converted into an integer pulse in each interpolation by the integerinzing circuit 14 and the integer pulse is sent to the servo circuit 20. Namely, an accurate feedforward item can be found out by the element 12 arranged in a numerical control part 10 by using the interpolation output of the interpolator including a decimal less than the output pulse unit. Consequently, accurate feedforward control can be attained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a feedforward control method for controlling a servo motor using a numerical control unit (CNC), and particularly to a feedforward control method having a feedforward control element in the numerical control unit. Regarding.

[Conventional technology]

Generally, a servo motor is controlled by a numerical control device using feedback control. However, in controlling the servo motor of a numerical control device, feedback control alone is not sufficient to more accurately follow a commanded path. This is because, with only feedback control, the tool path will deviate from the commanded path at corners, etc. of the tool path by the delay of the servo system. That is, feedback control alone cannot compensate for the effects of transient characteristics of the servo system.

For this reason, when controlling the servo motor of a numerical control device,
In addition to feedback control, feedforward control is performed to achieve more precise positioning control. Feedforward control in such numerical control devices was performed on the servo circuit side.

FIG. 5 is a block diagram of a feedforward control method in a conventional numerical control device. The numerical control unit 40 includes an interpolator 41 that performs interpolation for each axis.

In reality, there are multiple axes, and the configurations of the other axes are the same, so only one axis is shown in the figure. The output pulses of the interpolator 41 are output to the servo circuit 50.

The output pulse is input to the adder 51, and the adder 51 subtracts the feedback pulse Pf from the servo motor 30 from the output pulse and inputs it to the proportional element 52. Pg is position loop gain. The output of proportional element 52 is input to adder 53.

On the other hand, the output pulse is input to the feedforward element 54. α is a feedforward coefficient, which determines the degree to which feedforward is applied.

When α=0, it means that no feedforward control is performed, and when α=1, in a continuous system, the servo delay is theoretically 0. In the servo circuit 50, the differentiated input from the numerical control section 40 is multiplied by α and added to the adder 53. S means differentiation.

The output of the adder 53 is added with the feedforward term, and is given to the servo motor 30 as a speed command VC. The servo motor 30 rotates according to this speed command. Integral element 31 corresponding to the characteristics of the servo motor 30
is the position of the servo motor as the position feedback pulse Pf,
It is fed back to the adder 51.

FIG. 6 is a diagram showing an example of a pulse train of output pulses received by the servo circuit 50. The horizontal axis shows time, and the vertical axis shows the number of output pulses output from the numerical control section 40 to the servo circuit 50 every one interpolation period Δ.

[Problem to be solved by the invention]

It is assumed that in the section of block N1, a speed corresponding to 2°25 pulses per interpolation period is commanded, and in block N2, a speed corresponding to 3.5 pulses per one interpolation period is commanded. The output pulses are output from the numerical control section 40 to the servo circuit 50 in pulse units for each -interpolation period.
Two or three pulses are periodically output in block N1, and three or four pulses are output in block N2.

When feedforward control is performed by the servo circuit 50, the pulses shown in FIG. Although there is no change, it is assumed that there has been a speed change every -interpolation period, and the feedforward control element is operated.

Conversely, at time T9, although there was originally a speed change of 1.25 pulses, it is assumed that there is only a speed change of I pulse, and feedforward control is not performed correctly.

That is, since the servo circuit 5o receives commands from the numerical control unit 40 in pulse units, even when there is no speed change, it treats the dispersion in the command for one pulse as a speed change and performs feedforward control. This lowered positioning accuracy and increased machining errors.

The present invention has been made in view of these points, and it is an object of the present invention to provide a feedforward control method in which the interpolation output of a numerical control section is precisely performed and the fieldforward control is executed using the interpolation output. shall be.

[Means for Solving the Problems] In order to solve the above problems, the present invention provides an interpolator that outputs an interpolation output of an interpolation pulse unit or less in a feedforward control method for controlling a servo motor in a numerical control device. a feedforward control element that generates a feedforward term from the interpolation output from the interpolator; an integerization circuit that converts the output obtained by adding the feedforward term to the interpolation output into an integer; and an output of the integerization circuit. Accordingly, there is provided a feedforward control method characterized by having a servo circuit that controls a servo motor.

[Effect]

The numerical control section is provided with an interpolator that can perform interpolation in units including small units smaller than the output pulse unit. Using the interpolation output of this interpolator, an accurate feedforward term is determined by a feedforward control element provided in the numerical control section.

This feedforward term is added to the interpolation output and output to the servo circuit.

This prevents negative effects of feedforward control due to apparent speed changes in pulse output for each interpolation unit.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 1 is a block diagram of feedforward control according to the present invention. The numerical control unit 10 includes an interpolator 11 that performs interpolation for each axis. In reality, there are multiple axes, and the configurations of the other axes are the same, so only one axis is shown in the figure. Interpolator 11
The command is given in units of 1 μm, but when linear interpolation or circular interpolation is performed, the interpolation output to each axis for each constant pulse interpolation unit becomes a numerical value including decimals. The interpolator 11 outputs an interpolated output including this decimal number.

This interpolated output is input to the feedforward control element 12. The feedforward control element 12 multiplies the differentiated input by α and outputs it to the adder 13. α is a feedforward coefficient, which determines the degree to which feedforward is applied.

When α=0, it means that no feedforward control is performed, and when α=1 and it is a continuous system, the servo delay is theoretically 0. In reality, α is set to a value between 0 and 1. This depends on the servo motor and servo circuit to be controlled.
Determined experimentally.

S means differentiation, and Pg is the position gain of the servo circuit 20.

An adder 13 adds the interpolation output including decimals from the interpolator 11 and the a value from the feedforward control element 12 . That is, the output of the adder 13 is the interpolation output of the interpolator 11 plus a feedforward term. Since this output includes a decimal point, it is converted into an integer pulse for each interpolation unit by the integer conversion circuit 14 and sent to the servo circuit 20. In addition,
A flowchart of the feedforward control of the numerical control unit 10 and details of the integer conversion circuit 14 will be described later.

The output pulses of the numerical control section 10 are sent to the servo circuit 20 at every interpolation period. This output pulse is input to the adder 21 of the servo circuit 20. The adder 21 subtracts the feedback pulse Pf from the servo motor 30 from the output pulse and inputs it to the proportional element 22.

Pg is the position loop gain of the proportional gain element. The output of the proportional element 22, ie, the speed command VC, is sent to the servo motor 30.

The servo motor 30 rotates according to this speed command Vc. An integral element 31 corresponding to a speed detector provided in the servo motor 30 feeds back the position of the servo motor to the adder 21 as a position feedback pulse Pf.

FIG. 2 is a flowchart of feedforward control of the numerical control section. In the figure, the number following S indicates the step number.

[S1] Interpolation is performed by the interpolator 11.

This interpolation output is performed to units including decimals.

[S2] A feedforward term is determined using the feedforward element 12.

[S3] Adder 13 adds the feedback term to the interpolated output.
Add with .

[S4] The interpolation output is converted into an integer by the integer conversion circuit 14.

[S5] Output the output pulse converted into an integer to the servo circuit 20.

FIG. 3 is a diagram showing an example of the interpolated output received by the feedforward control element. The horizontal axis shows time, and the vertical axis shows the number of pulses. The number of pulses is output as an interpolation output from the interpolator 11 every interpolation period Δ, but since the interpolator 11 outputs an interpolation output that includes decimals, the number of pulses is always 2.2 in block NA.
5 pulses, 3.5 pulses in block NB, and 6th pulse
There is no variation within the same block as shown in the figure, and feedforward control is performed only at T9 and not at other locations, so the problem as shown in FIG. 6 is eliminated.

FIG. 4 is a detailed diagram of the integer conversion circuit. The integer conversion circuit 13 includes an input register 14, a decimal register 15, an adder 16, and an adder 17.

The interpolated output from interpolator 11 is stored in input register 14. The interpolation output has an integer part of 14a and a decimal part of 14a.
It is stored in b. The decimal part 14b is added to the contents of the decimal register 15 in an adder 16, and if there is an overflow, the overflow is added to the integer part 14a of the input register in the adder 1.
7 and output. If there is no overflow,
Only the integer part of input register I4 is output. The result of addition between the decimal part 14b of the input register 14 and the decimal register 15 is newly stored in the decimal register 15.

〔Effect of the invention〕

As explained above, in the present invention, the interpolation output including decimal numbers is output by the interpolator of the numerical control section, and the feedforward control is performed using the interpolation output, so that accurate feedforward control is possible. The trajectory accuracy of the numerical control device is improved and the machining accuracy is improved.

[Brief explanation of drawings]

Fig. 1 is a block diagram of the feedforward control of the present invention, Fig. 2 is a flowchart of the feedforward control of the numerical control section, Fig. 3 is a diagram showing interpolation outputs received and taken by the feedforward control element, Fig. 4 is A detailed diagram of the integer conversion circuit, No. 5 is a block diagram of feedforward control in a conventional numerical control device, and FIG. 6 is a diagram showing an example of a pulse train received and taken by the servo circuit. Numerical control section Interpolator Feedforward control element Adder Integerization circuit Servo circuit Adder Proportional element Servo motor

Claims (3)

[Claims] (1) In a feedforward control method for controlling a servo motor in a numerical control device, an interpolator that outputs an interpolation output of an interpolation pulse unit or less, and a feed that generates a feedforward term from the interpolation output from the interpolator. A forward control element, an integer conversion circuit that converts into an integer an output obtained by adding the feedforward term to the interpolation output, and a servo circuit that receives the output of the integer conversion circuit and controls a servo motor. Feedforward control method. (2) The feedforward control system according to claim 1, wherein the feedforward control element is comprised of a differential element that differentiates the interpolation output. (3) The integer conversion circuit includes an input register, a decimal point register, and a first circuit that adds the contents of the input register and the decimal register.
2. The feedforward control system according to claim 1, further comprising: an adder, and a second adder that adds the overflow of the first adder and the integer part of the input register.