JPS61256414A - Position controller - Google Patents

Abstract

PURPOSE:To improve the positioning accuracy by obtaining the optimum gain with control in response to the change of the frictional force in terms of time and therefore reducing the variance of the positioning accuracy in terms of time. CONSTITUTION:A mobile base 1 is driven by a DC servo motor 2 and a pole screw 3 and the position of the base 1 is detected by a laser interference length measuring machine 4. While a subtraction circuit 8 obtains the deviation between between a target position command signal 6 given from an electronic computer 5 and the present position signal 7 detected by the machine 4. The clock signal 14' given from a clock generator 14 is applied to a minute position command generating circuit 15. Then a minute position command 15' given from the circuit 15 is applied to an addition circuit 16 together with the position deviation signal 9 given from the circuit 8. The signal 14' is processed by a logical circuit 18 and a gain control counter 19 and applied to an operational amplifier 11. Then the gain of the amplifier 11 is controlled to an optimum level in response to the variance of the frictional force in terms of time. Thus the variance of the positioning accuracy is reduced in terms of time and therefore the positioning accuracy is improved with a position controller.

Description

[Detailed description of the invention] [Field of application of the invention] The present invention relates to a positioning control method for a servo system.

In particular, the present invention relates to a positioning servo control device suitable for cases where extremely high positioning accuracy is required, such as on a stage of a reduction projection exposure apparatus.

[Background of the invention]

In order to improve the positioning accuracy of closed-loop position control consisting of a position detector, a rotary wheel for calculating the deviation from the target position, a servo motor, and a drive system using a feed screw, the gain of the closed loop must be increased.

That is, sufficient gain is required to generate a torque that moves the movable base against the frictional force present in the guide portion, feed screw, etc. of the movable base in response to minute positional deviations.

However, increasing the gain makes the control system unstable,
...It cannot be made larger than the limit because it will cause tinging and oscillation. Therefore, efforts are being made to increase the gain as much as possible to avoid pinching and oscillation.

Publicly known example, Japanese Patent Publication No. 58-47043, position control method is as follows:
The gain is set high enough to overcome the static friction force only during startup, and the gain is switched to a low gain after startup to obtain stable running and positioning operations. The positioning accuracy in this method is determined by the lower gain mentioned above and the dynamic friction force, so this gain is Kd to obtain high positioning accuracy.

It is necessary to increase the size to the maximum extent within the range where the control system is stable.

In addition, related application 1% Application No. 59-65785 describes a position control device that provides bending support to the amplifier, generates torque that cancels out the frictional torque even in the case of minute deviations, and maintains stable operation due to low gain while driving. It is designed to allow running.

In these conventional techniques, torque due to frictional force is assumed in advance, and optimal gain characteristics are set in advance in accordance with this. However, the frictional force changes depending on the location of the sliding part and changes over time.

Therefore, the positioning accuracy of the movable base changes depending on the location of the movable base and also changes over time.

By reducing variations in positional accuracy and temporal variations in positioning accuracy, one positioning accuracy can be significantly improved.

That is, in the conventional technology, there is a large variation in positioning accuracy due to local and temporal changes in frictional force, so there is a drawback that further improvement in accuracy cannot be expected.

CObject of the Invention] The present invention performs control such that the gain is optimized according to changes in frictional force depending on location, (l! temporal changes in frictional force, It is an object of the present invention to provide a means for improving positioning accuracy by reducing variations.

[Summary of the invention]

In the present invention, the gain of the amplifier is set to a low value that allows the servo system to operate stably until the target position is reached, and then the gain is gradually increased while observing the response of the servo system. This is a position control method. and,
A means for observing the response of this servo system is at the heart of the invention.

That is, in the vicinity of the target position, minute position commands corresponding to the desired positioning accuracy are alternately given in both positive and negative directions, and the response of the servo system to this position command is counted by pulses fed back from the 5-position detector. This is what we try to observe.

Based on this observation result, the gain is gradually increased, and the increase in gain is stopped when it is confirmed that the servo system is moving reliably following minute commands in both the positive and negative directions. Obtain the optimal adjustment value.

[Embodiments of the invention]

FIG. 1 is a drawing showing an embodiment of the present invention.

The moving table 1 is driven by a DC servo motor 2 and a ball screw 3. The position of the moving table 1 is detected by a laser interferometer 4 . Electronic computer 5.

A target position command signal 6 is sent out to command the target position. The deviation between this target position command signal 6 and the current position signal 7 detected by the laser interferometric length measuring device 4 is digitally calculated by a subtraction circuit 8. The calculation result, ie, position deviation 9, rotates the DC servo motor 2 in the direction in which the position deviation becomes smaller via the DA converter 10, the calculation amplifier 011, and the servo amplifier 12. The tacho generator 13 is used to stabilize these servo systems and improve their control performance.

Clock generator 14. Micro position command generation circuit 15, addition circuit 16. Reversible counter 7, logic mouth', ), butterfly 18. Counter for amplifier gain control9. The rising/falling detection circuit 20 is a part necessary for implementing the present invention, and will be described in detail below.

The clock generator 14 generates clock pulses of a constant frequency to the minute position command generation circuit 15. Reversible counter 7 UP
It is supplied to the side power logic circuit 18.

The minute position command generation circuit 15 is a circuit that generates a minute position command corresponding to a desired positioning accuracy, and is controlled by a clock signal to alternately generate minute position commands in both positive and negative directions. This minute position command is added to one position deviation 9 in the addition circuit 16, but when the servo system is running with a large position deviation, it does not have any effect on the running.

The rising and falling edges of the least significant bit of the current position signal 7 are detected by the rising/falling detection circuit 20 and added to the DOWN side input of the reversible counter 17 .

By observing the contents of the reversible counter 17 at regular intervals, the traveling state of the movable platform can be determined.

For example, when the servo system is running at high speed with a large positional deviation, the number of pulses applied to the DOWN side is greater than the number of pulses applied to the UP side, and the content of the reversible counter becomes a negative value for each observation. shows.

When the positional deviation becomes small and the movable base stops due to the frictional force existing in the guide part of the movable base, ball screw, etc., the U
Because the pulse is applied only to the P side.

The contents of the counter indicate a positive value for each observation.

In the above state, when the gain of the operational amplifier 11 is increased little by little, a torque is finally generated that overcomes the frictional force at the positional deviation at that time, and the movable table moves in a direction that reduces the deviation.

If the same thing is repeated, eventually a state will be reached where the content of the reversible counter becomes O every time the observation is made. This means that the servo system reliably follows minute position commands in both the positive and negative directions, +1 → 0 → +
1 → 0 → Hit... Indicates that the old movement is being performed. At this time, if the optimum value scale of the gain moves reliably following this, the least significant bit 7' of the current position signal will become <
Since the +1 and 00 states are repeated, when the rising and falling states are detected, a signal 20' corresponding one-to-one with the clock signal 14' is obtained. 14' is the UP side of the reversible counter.

20' is added to the same <DOVVN side, so if its contents are observed at regular intervals, it becomes 0.

The logic circuit 18 and the gain control counter 19.

The gain of the operational amplifier 11 is increased in small increments, and each time the result is observed as the contents of the reversible counter 17, thereby determining whether or not the gain should be further increased.

Logic circuit 18Vi resets the reversible counter at fixed time intervals, reads the contents after a fixed period of time has elapsed after the reset, makes a logical judgment according to the contents, and controls the gain control counter 1.
9 controls the gain of the operational amplifier 11. The logical judgments made by the logic circuit 18 and its operations are determined as shown in the table 91 of FIG. 1, for example.

FIG. 4 shows an embodiment of a circuit for controlling the gain of the operational amplifier 11 using the gain control counter 19 shown in Table 1. The ratio of the four input resistors 11-1 to 11-4 of the operational amplifier 11 is 1:
2:4:8, and by opening and closing the analog switch 11-5 according to the 4-bit output of the gain control counter, the gain changes in 16 steps.

In this way, when positioning the movable table in a place where the frictional force is large, the gain of the amplifier is increased.

In locations where the frictional force is small, positioning control is performed with a small gain.

〔Effect of the invention〕

According to the present invention, control is performed such that the gain is optimized in accordance with changes in frictional force depending on location5 and changes over time.
This has the effect of reducing the variation in accuracy between positioning operations, thereby improving the positioning efficiency.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention. FIG. 2 is a diagram showing the states of various parts of the servo system when the optimum gain is obtained by the control of the present invention, and FIG. 3 is a circuit diagram showing an example of means for controlling the gain of the operational amplifier. 1...Moving table, 2...DC servo motor, 3...
Ball screw, 4... Laser interference length measuring device; 5... Electronic computer. 6...Target position command signal, 7...Current position signal, 7
'...Same lowest pit, 8...Subtraction circuit, 9...
Position error signal, 10...DA converter, 11... Arithmetic amplifier, 12... Servo amplifier, 13... Tacho generator. 14...Clock generation 6.14'...Clock signal, 15...Minimum position command generation circuit, 15'...Same command. 16... Addition circuit, 17... Addition circuit, 18...
Logic circuit, 19... Gain control counter, 20...
・Rise and fall detection circuit, 20'...same output.

Claims (1)

[Claims] 1. Servo system consisting of a servo motor that drives the controlled object to the target position, a position detector that detects the current position of the controlled object, and an amplifier that amplifies the deviation between the current position and the target position and drives the servo motor. In particular, in the position control method where the gain of the amplifier is set to a low value that allows the servo system to operate stably until the target position is reached, and the gain is increased near the target position, it is difficult to achieve the desired positioning accuracy. A minute position command generation circuit that alternately generates corresponding positive and negative position commands, a pulse counting circuit that detects that the servo system reliably follows this position command, and a gain that gradually increases according to the count value of this pulse counting circuit. A servo system position control device characterized by being provided with a variable amplification degree amplifier that makes it possible to