JPS58195482A - Servo control system for motor - Google Patents

Abstract

PURPOSE:To reduce the variation in the speed of a motor by setting the reference signal frequency of a phase comparison control system to become the prescribed range in which the influence of the electric time constant of the motor is less and setting the reference frequency of a PWM control system to match the frequency. 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 signal which drives or stops the motor is outputted from a control circuit CPU. A PWM control system is composed of a sawtooth wave oscillator STU, F/V converter FVC, an error amplifier EVA, operational amplifiers IC1a, 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. The period T of a reference signal of the PWM control system is set in the range of T2<=T1<=3.T2 for the period T2 of the reference signal of a phase comparison control system.

Description

Detailed Description of the Invention The present invention relates to servo control of electric motors, and in particular obtains a feedback signal with a frequency corresponding to the operation of the electric motor, and compares the level corresponding to the signal with a reference signal to perform PWM (pulse width modulation). A first control system that performs control and a second control system that performs phase comparison control by comparing the phase of the feedback signal with the phase of a reference pulse switch the current applied to the motor to servo control the motor. Regarding control.

For example, in the case of a copying machine, when a single servo motor is used as the drive source in the drive system, 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, unless the photosensitive drum, optical scanning system, etc. are driven at a constant speed, the quality of the recording 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: a control system that performs F/V conversion of the output voltage and performs PWM (pulse width modulation) control according to the output voltage.

By the way, in this type of motor control, due to the influence of the electrical time constant of the motor, even if a predetermined voltage is applied to the motor, the predetermined current does not flow immediately, as shown in Figure 1a.
The value of the current gradually increases in a certain exponential curve determined by the time constant of the motor. Moreover, when the voltage is removed, the current immediately becomes O. When performing phase comparison control, a phase difference corresponding to the difference between the target speed and the actual speed is detected, and
An error signal corresponding to the phase difference is output. However, as described 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 a region where the phase difference is small as shown in FIG. 1b. Therefore, if the period of phase comparison is shortened, the width of the phase comparison output pulse becomes narrower, and the electric power supplied to the motor becomes insufficient compared to the theoretical value. A predetermined reference signal is pulse width modulated (PW) using the output signal of the phase comparison control system and the signal obtained by converting the frequency of the wave pack signal into an analog signal.
M) In a control system that switches the current applied to the motor using both signals, if the rising current is insufficient compared to the theoretical value and the falling current is equal to the theoretical value, the control becomes unstable. In other words, as shown in Figure 2, when the motor speed is higher than the target value, the phase comparison control system cuts off the motor current for a period of ΔΦ corresponding to the phase advance in order to slow down the speed.
Even if voltage is applied to the motor by PWM signal after the elapsed time,
Since the rise of the current is slow, the power applied to the motor is controlled by phase comparison control so that the power applied to the motor is much higher than the phase lead;
This causes the motor speed to drop below the target value. When the motor speed falls below the target value, the 1-phase comparison control system applies voltage to the motor for a period of ΔΦ corresponding to the phase delay, but since the specified current does not flow immediately at this time, The power generated will be insufficient to correct the speed of the motor to a predetermined value. In other words, the speed does not stabilize at the target speed forever, and the speed always fluctuates between above and below the target speed. In the case of the drive system of a copying machine, speed fluctuations like the one shown in 2 appear as image jitter in recorded images.

In addition, in this type of control system that controls the power applied to the motor by switching, when the output signals of the PWM control system and the phase comparison control system are at the non-energizing level, no power is supplied to the motor, so during the non-energizing level period. If is made too long, the motor will follow changes in the energization level and cause speed changes.

Furthermore, in this type of composite control that performs PWM control and phase control, if the period of phase control is made larger than the period of PWM control, the effect of the phase comparison control system becomes smaller and it becomes impossible to follow fast speed changes. .

In this way, various factors cause speed changes when it comes to motor servo control, so when manufacturing a conventional servo control device, a test device is created and operation experiments are repeated with that device, and the frequency etc., and set each parameter to obtain the most preferable state. However, this takes a lot of time to design, and if you change the motor etc. to a different type or one made by another company, you will have to conduct the experiment again.

One object of the present invention is to provide a servo control device for an electric motor with small speed fluctuations, and another object is to provide a servo control device for an electric motor that is easy to design.

In order to achieve the above object, the present invention uses PWM
The period T1 of the reference signal of the control system is set in the range T2≦Tl≦3·T2 with respect to the period T2 of the reference signal of the phase comparison control system. Accordingly, the reference signal frequency of the phase comparison control system is set so that the period ΔΦ of the phase comparison control system is within a predetermined range where the influence of the electrical time constant of the motor is small, and the above-mentioned signal frequency is set accordingly. If the reference frequency of the PWM control system is set within the range, preferable constant speed control can be performed. Experiments have revealed that when the frequency ratio is set within the above range, good speed control can be achieved even in control where the phase comparison period is set relatively short. Further, it is clear that the control is performed well from the fact that the fluctuation in the amplitude of the current waveform shown in FIG. 4C is smaller than that of the waveform shown in FIG. The experiment was conducted using an apparatus configured as shown in Examples described below. The motor used in the device was a DC motor 80WTO-M type manufactured by Hitachi. The specifications of the motor are shown in Table 1 below.

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 a current switched by a motor drive circuit DRV that receives signals from two control systems. The control circuit CPU outputs signals 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 between the slits of the disk DK. This photosensor PS and disk DK act as a so-called rotary encoder. In this embodiment, when the servo motor SVM operates at the target speed of 3900 rpm, a 6.5 KHz pulse signal is obtained as the output of the photosensor PS. The STU is a sawtooth wave oscillation circuit, and always outputs a sawtooth wave having a predetermined frequency and a constant amplitude, that is, a first reference signal. In this embodiment, the frequency f1 is 1KH.
It is set to z. STU has two operational amplifiers IC1a
and IC1b, a capacitor and resistor that determine the oscillation time constant, a Zener diode that regulates the amplitude of the signal, etc.

FVU is an F/1 frequency (voltage) conversion circuit.

The integrated circuit IC3 is AD537 manufactured by Analog Devices, Inc., and is an IC for F/V conversion. The FVU includes IC3 as the center, four AND gates, an operational amplifier IC4a connected as a voltage follower, and a low-pass filter composed of resistors and capacitors. AD53
Reference numeral 7 includes 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 the FVC by an operational amplifier IC4b and the aforementioned speed command voltage are applied to an error amplifier EVA constituted by an operational amplifier IC4c. The two signals applied to the EVA are summed as a current through a resistor, and the difference between the two signals appears at the output terminal as an error signal vb. STU output sawtooth wave signal Va and error signal vb
are respectively applied to the operational amplifier IC1c, that is, the comparator. Sawtooth oscillator STU, F/
V converter FVC, error amplifier EVA, operational amplifier IC1
a, IC4a, IC4b, 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 this 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. Brancher DV
U is two D type flip props IC6a and I
C6b, and outputs a pulse signal with a period four times that of the input signal. CK is a clock input terminal, PR is a preset input terminal, and CL is a clear input terminal. Since the PR and CL terminals are not used, they are pulled up, and a signal is input to the clock input terminal GK to obtain a signal with a duty of 50%.

The pulse signal at the divided period DVU output end is the phase comparator IC7.
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 C8a and the diode D3 to the resistor R11 together with the PWM signal VC.
is applied to. The signal at the down output end of the phase comparator IC7 is sent to the motor drive circuit DRV via 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,
The pulse is applied to the light emitting diode LED1 via the IC8d, etc., and the LED1 emits light when the phase of the reference pulse and the feedback signal are different. The above Schmitt trigger ST1. ST2, frequency divider DVU, phase comparator IC
7. The inverter IC 8a and the like constitute one control system, that is, a phase comparison control system. The motor drive circuit DRV includes a transistor Q2. It consists 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 because the on/off signal of the control circuit CPU is at a low level L, that is, the input terminal of the inverter IC8e is at a low level, and the output terminal of the phase comparator IC7 is at a low level. It's time for a high level. In this state, when the PWM signal Vc becomes high level or the output terminal U of the phase comparator IC7 becomes low level, the base terminal of transistor Q2 becomes high level and current flows to the servo motor SVM. Even when the PWM signal Vc is at a high level, when the output terminal of the phase comparator IC7 is at a low level, a current flows through the diode D4, so that the base terminal of the transistor Q2 is at 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. When the rotation speed of the servo motor is higher than the target speed, the frequency of the feedback signal from the PS increases, the output voltage of the FVC increases, and the voltage of the error signal vb increases, so the pulse width of the PWM signal increases. The remoter, which is shorter and requires less power to be applied to the servo motor, is slowed down. 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 frequency-divided to 1/4 by the divider DRV and applied to the terminal V of the phase comparator C7. The phase comparator IC7 compares the phase of the signal at the input terminal (2) and the reference signal applied to the input terminal R, and if the phase of the signal at the input terminal V lags behind the phase of the reference signal, the phase comparator IC7 A pulse signal (valid at low level) is output to U, and if the phase of the signal at input terminal V is ahead of the phase of the reference signal, a pulse signal is output to 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. Further, the PWM control system outputs a PWM signal according to the frequency of the signal from the photosensor PS, regardless of the control of the phase comparison system. In this state,
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 IC7 is at a low level, an on-energizing voltage (H) is applied to the base of the transistor Q2, and a current flows through the servo motor SVM during these periods. 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 at the timing when the D of the IC7 is at the low level L, the diode D4 moves in the forward direction to
Since the base terminal of the transistor Q2 is turned to a low level 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 output terminal of the inverter IC8e becomes a low level, and the base potential of the transistor Q2 is forcibly set to the deactivation level.
Motor does not work. The phase comparison control system follows the motor operation quickly, so when switching the load by turning on and off the clutch, as in the case of the drive system of a copying machine, for example, it is necessary to switch the load.

The phase comparison control system operates in response to load changes during conversion,
Acceleration or deceleration for speed correction is performed quickly.

In the above embodiment, the frequency division ratio of the frequency divider DVU is set to 4, but the integer value is not particularly limited. This frequency division ratio can be arbitrarily set depending on the rotational speed of the Homare motor, the time constant of the servo motor, the resolution of the rotary encoder (in the embodiment, the number of slits in the disk), and the like. But 1
It is not preferable to make the frequency division ratio too large because it reduces the number of phase comparisons, and it is appropriate to set the frequency division ratio to about 4.

[Brief explanation of drawings]

Figure 1a is a time chart showing the relationship between the applied voltage for motor control and the motor drive current, Figure 1b is a graph showing the relationship between the phase difference ΔΦ in phase control and the torque generated by the motor, and Figure 2 is a graph showing the relationship between the control signal and the motor drive current. Figures 3a and 3b are block diagrams showing the electric circuit configuration of one embodiment. Figure 4a is a time chart showing the operation of the PWM control system. Figure 4b is a phase comparison control system. 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: Frequency divider (branch circuit) EV
A: Error amplifier FVC: F/V conversion circuit (signal converter) ST1, ST
2: Schmitt trigger STU: Sawtooth wave oscillation circuit IC7: Phase comparator SVM
: Servo motor DRV: Motor drive circuit DK: Disk PS: Photo sensor (detection means)

Claims (3)

[Claims] (1) Obtain a feedback signal with a frequency corresponding to the operating speed of the electric motor, and compare the level according to the feedback signal with the level of the first reference signal of a predetermined frequency,
generating a first control signal whose pulse width changes; comparing the phases of the feedback signal and a second reference signal; generating a second control signal according to the result; In servo control that turns on and off the energization of the electric motor according to the second control signal, the period T of the first reference signal
1 with respect to the period T2 of the second reference signal °゛T2≦T1
A servo control method for an electric motor, characterized in that the servo control method is set within a range of ≦3・T2. (2) A servo control system for an electric motor according to claim (1), wherein the first reference signal is a letter wave. (3) A servo control system for an electric motor according to claim (1), wherein the level corresponding to the feedback signal is obtained by frequency/analog signal conversion of the feedback signal.