Electricity: motive power systems – Positional servo systems – With protective or reliability increasing features
Patent
1987-06-25
1988-09-20
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
With protective or reliability increasing features
318565, 425162, 425167, 264 403, G05B 902
Patent
active
047728302
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and an apparatus for controlling the torque of a servomotor, in which a driving force applied actually to a load driven by the servomotor is controlled directly, and more particularly, to feedback control of a driving force applied to a load in pressure control, robot control, etc., in an injection-molding machine driven by a servomotor, and other systems of control of this type.
BACKGROUND ART
In controlling a driving force applied to a load by using a servomotor, the output torque of the servomotor is conventionally controlled by torque restriction. Since a motor output, however, is generally applied to the load through a transmission mechanism, the driving force applied actually to the load is controlled only indirectly, according to the prior art method of controlling the motor output. This entails the following awkward situations.
FIG. 2 is a block diagram of a basic circuit of a conventional control circuit for controlling a servomotor using a permanent-magnet synchronous motor. In FIG. 2, symbol E designates a three-phase power source. Reference numeral 3 denotes a rectifier circuit; 4, a transistor inverter; and 1, a transistor PWM control circuit. Also, symbol M designates the permanent-magnet synchronous motor, while numeral 2 denotes a rotor position detector, such as a pulse encoder, for detecting the position of a rotor of the permanent-magnet synchronous motor M.
The transistor PWM control circuit 1 compares a speed command value Vo from a control unit with a present speed Vs of the rotor, which is obtained from a rotor position S detected by the rotor position detector 2. Transistors TA to TF of the transistor inverter 4 are turned on or off to control currents flowing through the U-, V-, and W-phase windings of the permanent-magnet synchronous motor M, thereby controlling the rotating speed of the motor M. If the output torque of the motor M is to be controlled, the transistor PWM control circuit 1 is arranged as shown in FIG. 3.
In FIG. 3, numeral 5 denotes a signal processing circuit; 6 and 7, ROMs; and 8, a differential amplifier. The signal processing circuit 5 delivers a voltage Vs, indicative of the present rotor speed, in accordance with the rotor position detection output S. The ROMs 6 and 7 store a group of U- and W-phase command values to be delivered, so as to correspond to individual rotor positions, in order to make the phase of the resultant current, flowing in the U, V, and W phases, perpendicular to that of the main flux of a magnetic field generated by the rotor. The differential amplifier 8 amplifies the difference between the voltage Vo, indicative of the speed command, and the voltage Vs, indicative of the present speed, from the signal processing circuit 5, and delivers an amplified difference signal. Numeral 9 denotes a filter which has a frequency characteristic such that the gain is lowered at high frequencies, and is increased at low frequencies. Zener diodes ZD1 of the filter 9 serve to clamp the peak voltage. Numerals 50 and 52 designate a D/A converter and a clamping circuit, respectively. The D/A converter 50 serves to convert a torque limiting command PL, as a digital signal, into an analog signal. The command PL, which is supplied from a numerical control unit (not shown) or the like, is used to set the value of the driving force to the load. If an input Vr to an amplifier 51, that is, a voltage Vr corresponding to the difference between the speed command Vo from the filter 9 and the present speed Vs, exceeds a predetermined voltage +Vc or -Vc, which corresponds to a torque limiting command PV, in the form of an analog signal, from the D/A converter 50, the clamping circuit 52 clamps the voltage Vr to the voltage +Vc or -Vc. Numerals 10 and 11 denote multiplying digital-to-analog converters. The converter 10 multiplies a voltage VE, delivered from the amplifier 51, by the U-phase command value delivered from the ROM 6. Likewise, the converter 11 multiplies the voltage VE by the W-phase command value
REFERENCES:
patent: 3936713 (1976-02-01), Hunkar
patent: 4094481 (1978-06-01), DeWalt
patent: 4531081 (1985-06-01), Liesegang
patent: 4682089 (1987-07-01), Tamari
Kobari Katsuo
Kobayashi Toshio
Fanuc Ltd.
Shoop Jr. William M.
Young Brian
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