JPS58195483A - Servo control device for motor - Google Patents

Abstract

PURPOSE:To preferably control a control system by applying the output of high resolution detecting means to a frequency/analog signal converter as it is in frequency, and dividing the frequency of the output signal of detecting means and applying it to a phase comparison system. CONSTITUTION:A DC servo motor SWM is driven by a current which is switched by a motor drive circuit DRV that receives signals from two control systems. A PWM control system is composed of a sawtooth wave oscillator STU, F/V converter FVC, an error amplifier EVA, operational amplifiers ICLa, IC4a, IC4b, IC4d, etc., and a phase comparison control system is composed of Schmitt triggers ST1, ST2, a frequency divider DVU, a phase comparator IC7, an inverter IC8a, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a servo control device for an electric motor, and in particular, to a servo control device for an electric motor, which obtains a feedback signal with a frequency corresponding to the operation of the electric motor, and converts the signal into F-V (or F-I). A device that servo-controls an electric motor using a first control system that performs control by comparing a signal with a predetermined reference signal, and a second control system that performs control by comparing the phase of a feedback signal with the phase of a reference pulse. Regarding.

For example, in the case of a copying machine, when the drive system uses a single servo motor as the drive source, the load is connected via various clutches, etc., so the magnitude of the load varies greatly depending on whether the clutches are turned on or off. . However, in the case of a copying machine or the like, if the photosensitive drum, optical scanning system, etc. are not driven at a constant speed, image jitter etc. will occur and the quality of the recorded image will deteriorate. In such applications, the servo drive system consists of a control system that performs phase comparison control to respond to rapid speed changes, and a control system that outputs pulses in response to motor rotation to maintain the average speed at a predetermined speed. It is preferable to drive with two control systems, one that performs F/V conversion of the output voltage and performs control according to the output voltage. By the way, in this type of motor control, due to the influence of the motor's electrical time constant, even if a predetermined voltage is applied to the motor, the predetermined current does not flow immediately, as shown in Figure 1, and the value of the current changes. gradually increases with a specific exponential curve determined by the motor's time constant. Moreover, when the voltage is removed, the current immediately becomes O. When performing phase/phase ratio throttle control, the phase difference ΔΦ corresponding to the difference between the target speed and the actual speed is detected, and the phase difference ΔΦ
Outputs an error signal according to the However, as mentioned above, it takes time for the current to rise due to the time constant of the motor, so the relationship between the level of the error signal and the driving force is not proportional in the region where the phase difference ΔΦ is small as shown in FIG. For the second reason, if the period of phase comparison is shortened, the width of the phase comparison output pulse becomes narrower, and the power supplied to the motor becomes insufficient compared to the theoretical value. The period of phase comparison is the period of pulses from a rotary encoder connected to the motor shaft when the motor is rotated at a predetermined speed.
In order to lengthen the phase comparison period, it is necessary to widen the width of the pulses output by the rotary encoder. However, widening the amplitude of the pulses output by the rotary encoder means lowering the resolution of the rotary encoder, so if the phase comparison period is lengthened, The accuracy of the other control system that performs control becomes low. In other words, if the phase comparison period is made longer so that one control system (phase comparator/control system) can perform highly accurate control, the accuracy of the other control system will decrease, and the accuracy of the other control system will decrease. If the resolution of the rotary encoder is increased in order to increase the resolution, the control of the phase comparison control system will become insufficient.

In conventional devices, in order to avoid such inconveniences, the target speed must be set so low that the time constant of the motor does not become a problem.

An object of the present invention is to provide a servo control device for an electric motor in which both a phase comparison control system and a control system using signals converted from frequency/analog signals can perform preferable control.

In order to achieve the above object, in the present invention, the resolution of the detection means such as a rotary encoder is increased, and the output signal of the detection means is applied to the single frequency/analog signal converter at the same frequency, and the output signal of the detection means is applied to the phase comparison system. applies a signal obtained by frequency-dividing the output signal of the detection means. The division ratio is determined depending on the resolution of the detection means, the rotational speed (target speed) of the motor, the electrical time constant of the motor, and the like. According to this, there is no need to lower the resolution of the detection means, and the period of phase comparison can be arbitrarily set by setting the division ratio, so the degree of freedom in design increases.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. FIG. 3a is a block diagram showing one embodiment of the present invention, and FIG. 3b is a circuit diagram showing a specific configuration of the phase comparator (IC7) in FIG. 3a. This will be explained with reference to FIGS. 3a and 3b. First, to explain the outline, DC servo motor SV
M is driven by current switched by a motor drive circuit DRV that receives signals from two control systems. A control circuit CPU outputs a signal for turning on and off the motor drive. A disk DK having slits formed at equal intervals around the periphery is fixed to the drive shaft of the servo motor SVM, and a light-shielding photosensor PS is arranged across the slit positions of the disk DK. This photo sensor PS
and disk DK act as a so-called rotary encoder.

The STU is a sawtooth wave oscillation circuit, and always outputs a linear wave having a predetermined frequency and a constant amplitude. In this embodiment, the frequency f1 is set to IKHz. STU is 2
It consists of two operational amplifiers IC1a and IC1b, an iris capacitor and resistor that determine the oscillation time constant, a Zener diode that regulates the amplitude of the signal, and the like. FVU is an F/1 frequency/voltage conversion circuit. Integrated circuit IC3 is AD537 manufactured by Analog Devices, and is an IC for F/V conversion.
It is. FVU consists of four AND gates and an operational amplifier IC4 connected as a voltage follower, centered around IC3.
a, a low-pass filter made up of a resistor and a capacitor, etc. The AD537 is equipped with a terminal (fourth pin) for outputting a reference voltage, and in this embodiment, the reference voltage is used to obtain a speed command voltage at the output terminal of the operational amplifier IC1d. The speed command voltage can be adjusted using a semi-fixed resistor VRI. A signal obtained by amplifying the output signal of FVC to an operational amplifier IC4b, and an error amplifier EVA configured by the above-mentioned speed command voltage to an operational amplifier IC4c.
is applied to. The two signals applied to the EVA are
They are added together as a current through a resistor, and the difference between the two signals appears at the output as an error signal vb. The wave signal Va and the error signal vb of the STU output are each applied to an operational amplifier, -I Clc, that is, a comparator, one or more wave oscillators STU, an F/V converter FVC,
Error amplifier EVA, operational amplifier IC1a, IC4a, I
C4b, IC4d, etc. constitute one control system, that is, a PWM (pulse width modulation) control system.

The control circuit CPU generates a reference pulse having a constant period, that is, a second pulse.
This reference pulse is applied to the reference signal input terminal R of the phase comparator IC7 via a Schmitt trigger ST2 provided for waveform shaping. The frequency of the second signal is set to 1.6 KHz. The output signal of the photosensor Ps is the Schmitt trigger ST1
The signal is applied to the branch unit DVU via the DVU. Frequency divider DV
U is two D type flip-flops IC6a and I
C6b, and outputs a pulse signal with a period four times that of the input signal'. CK is the clock input terminal, PR
is the preset input terminal and CLi*clear input terminal. P
The R and CL ends are not used, so they are pulled up.
Input a signal to the clock input terminal CK and set the duty to 50.
% signal is obtained.

The pulse signal at the divided period DVU output end is sent to the phase comparator 1C7.
is applied to the V input terminal of. The signal at the up output terminal U of the phase comparator IC7 is passed through the inverter IC8a and the diode D3 to the resistor R11 together with the PWM signal VC.
The signal at the down output end of the 0 phase comparator IC7, which is applied to the motor drive circuit DRV, is passed through the diode D4.
is applied to the input terminal of The signals at the output terminals U and D of the phase comparator IC7 are sent to the inverters IC8b, IC8c, and I.
The pulse is applied to the light emitting diode LED1 via C8d, etc., and the LED1 emits light when the phase of the reference pulse and the feedback signal are different. Schmitt trigger STI, ST2. Frequency divider DVU, phase comparator IC7
, inverter IC8a, etc. constitute one control system, that is, a phase comparison control system. The motor drive circuit DRV includes a transistor Q2. It is composed of Q3, Q4, etc. When the input terminal (base) of transistor Q2 is at high level, power transistor Q4 is turned on and current flows to servo motor SVM, and when the input terminal of Q2 is at low level, transistor Q4 is turned off and current to servo motor SVM is cut off. be done. The input terminal of the motor drive circuit DRV can be at a high level if the on/off signal of the control circuit CPU is at a low level L,
That is, when the input terminal of the inverter IC8e is at a low level and the output terminal D of the phase comparator IC7 is at a high level. In this state, when the PWM signal Vc becomes high level or the output terminal U of the phase comparison amount IC7 becomes low level, the base terminal of transistor Q2 becomes high level and the servo motor SVM
A current flows through. Even when the PWM signal Vc becomes a high level, when the output end of the phase comparator IC7 becomes a low level, current flows through the diode D4, so that the base end of the transistor Q2 becomes a low level.

FIG. 4a is a time chart showing the operation of the PWM control system. The PWM control system will be explained with reference to FIGS. 3a and 4a. The sawtooth wave signal Va is a signal that always has a constant amplitude and a constant period. The error signal Vb is a voltage proportional to the speed command voltage output from the IC1d and the frequency of the feedback signal from the photosensor PS output according to the rotational speed of the servo motor SVM. Therefore, comparator IC1
The signal outputted by c is a signal with a constant period and whose pulse width changes depending on the voltage of the error signal vb. In other words, when the rotational speed of the servo motor SVM is lower than the target speed, the frequency of the feedback signal from the PS is low, so the output voltage of the F/V converter FVC becomes low and the pulse width of the PWM signal becomes long. is accelerated by applying a large amount of electric power. If the servo motor rotation speed is higher than the target speed.

The frequency of the feedback signal from the PS increases, and F
Since the output voltage of VC becomes higher and the voltage of the error signal vb becomes higher, the pulse width of the PWM signal becomes shorter and the remoter, which requires less power to be applied to the servo motor, is decelerated. In this way, the PWM control system controls the pulse width of the signal so that the speed of the servo motor becomes the target speed.

FIG. 4b is a time chart showing the operation of the phase comparison control system. The operation of the phase comparison control system will be explained with reference to FIGS. 3a and 4b. The feedback signal from the photosensor PS is divided into 1/4 frequency by the frequency divider DRV and applied to the terminal V of the phase comparator IC7. Compare the phase of the reference signal applied to R, and if the phase of the signal at the input terminal V lags behind the phase of the reference signal, output a pulse signal (valid at low level) to the output terminal U, If the phase of the signal at the input terminal V is ahead of the phase of the reference signal, a pulse signal is output at the output terminal.

FIG. 4c is a time chart showing the relationship between the output signals of the PWM control system and the phase comparison control system, the signals applied to the motor drive circuit, and the waveforms of the current flowing through the servo motor SVM. Figure 3a.

This will be explained with reference to FIGS. 3b and 4c. When the speed of the servo motor SVM is slow and a phase lag occurs, the output end of the phase comparator IC7 becomes high level H, and the output end U
A low level L pulse signal having a pulse width corresponding to the delay phase difference ΔΦ is generated. In addition, the PWM control system is independent of the control of the phase comparison system and uses a photo sensor! It outputs a PWM signal according to the frequency of the signal from the tube. In this state, an on-energizing voltage (H) is applied to the vector of the transistor Q2 in the period in which the PWM signal is at a high level H, and in the period in which the output terminal U of the IC7 is in a low level, and during these periods, the servo motor SVM A current flows through the servo motor, and the servo motor is accelerated by both the phase comparison system and the PWM system. When the speed of the servo motor SVM increases and the phase of the feedback signal advances, a low level L signal with a pulse width corresponding to the phase advance angle ΔΦ is generated at the output end of the phase comparator IC7.

At this time, the PWM control system applies the PWM signal vc generated by the system to the resistor R11, but when the D of IC7 is at the low level L, the diode D4 is forward biased and the pace end of the transistor Q2 is lowered. Since the level is set to L, the current of the servo motor is cut off. That is, when the motor speed is high, the motor current flows only when the PWM signal Vc is at a high level and D of the phase comparator IC7 is at a high level, so the servo motor is decelerated by the amount of phase control. When the on/off signal of the control circuit CPU is at a high level (drive prohibition level), the inverter I
The output terminal of C8e becomes low level.

Since the base potential of transistor Q2 is forced to a deactivated level, the motor does not operate. The phase comparison control system follows the motor operation quickly, so when switching the load by turning the clutch on and off, as in the case of the drive system of a copying machine, for example, the phase comparison control system follows the motor operation quickly. In response, the phase comparison control system is activated to quickly perform acceleration or deceleration for speed correction.

In the above embodiment, an example was explained in which the servo system was configured with two, a phase comparison control system and a PWM control system, but the output of a 1-frequency analog signal converter may be configured with a control system other than PWM. However, other control systems may be combined. In the embodiment, the frequency division ratio of the frequency divider DVU is set to 4, but the value of 2 is not particularly limited. This frequency division ratio is
It can be arbitrarily set depending on the rotation speed of the servo motor, the time constant of the servo motor, the resolution of the rotary encoder (in the embodiment, the number of slits in the disk), etc. However, it is not preferable to make the 1 division ratio too large because the number of phase comparisons will be reduced, and it is appropriate to set the division ratio to about 4.

As described above, according to the present invention, the servo control system is configured so that the phase comparison control system operates preferably without reducing the accuracy of the control system using the frequency/analog signal converter.

[Brief explanation of the drawing]

Fig. 1 is a time chart showing the relationship between the applied voltage for motor control and the motor drive current, and Fig. 2 is a graph showing the relationship between the phase difference ΔΦ in phase control and the torque generated by the motor.
3a and 3b are block diagrams showing the electric circuit configuration of one embodiment, FIG. 4a is a time chart showing the operation of the PWM control system, FIG. 4b is a time chart showing the operation of the phase comparison control system, and FIG. FIG. 4c is a time chart showing the waveforms of output signals of each control system and signals for driving the motor. CPU: Control circuit DVU: Branch unit (branch circuit) EV
A: Error amplifier FVC: F/V conversion circuit (signal converter) ST1, ST2
:Schmitt trigger STU: Sawtooth wave oscillation circuit IC7: Phase comparator SVM
: Servo motor DRV: Motor drive circuit DK: Disk PS: Photo sensor (detection means) Patent applicant Ricoh Co., Ltd.

Claims (3)

[Claims] (1) Detection means that outputs a signal with a frequency corresponding to the rotational speed of the electric motor; A signal converter connected to the detection means that converts the frequency into an analog signal; The output signal of the signal converter is converted into a first reference signal A first control means for performing control by comparing the output signal with the second reference signal; a frequency dividing circuit connected to the detection means; a second control means for generating a signal according to the phase difference between the output signal of the frequency dividing circuit and the second reference signal; A servo control device for an electric motor, comprising: a control means; and a motor drive means for applying electric power to the electric motor according to an output signal of the first control means and an output signal of the second control means. (2) The servo control device for an electric motor according to claim 1, wherein the first control means is a pulse width modulation circuit that generates a pulse width modulation circuit having the same period as the first reference signal. (3) The servo control device for an electric motor according to claim (2), wherein the first reference signal is a serpentine wave.