Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1997-05-22
1999-09-21
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
H02P 318
Patent
active
059558636
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an electric current control method for an AC servomotor to be used as a drive source in a machinery such as a machining tool and an industrial machine, or a robot.
BACKGROUND ART
FIG. 10 is a block diagram showing a control system of a conventional AC servomotor. In this control system, a position feedback value detected by an encoder, etc. is subtracted from a position command to obtain a position deviation, and the obtained position deviation is then multiplied by a position gain in term 1 to obtain a speed command by a position loop control. A speed feedback value is subtracted from the speed command to obtain a speed deviation, and a speed loop process of a proportional-plus-integral control is performed in term 2 to obtain a torque command (current command). Further, a current feedback value is subtracted from the torque command and a current loop process is performed in term 3 to obtain a voltage command of each phase. Based on the voltage commands, the AC servomotor M is controlled by a PWM control, etc.
In controlling a three-phase AC servomotor in the above-mentioned control system, an alternating current control method for controlling currents of three phases individually in a current loop. In this current control method, a torque command (current command) obtained by the speed loop process is multiplied by each of sine waves which are shifted by an electrical angle of 2.pi./3 for U, V and W phases, respectively from a rotor position .theta. of the servomotor detected by the encoder, to obtain a current command of each phase. Then, current deviations are obtained by subtracting actual currents Iu, Iv, Iw detected by current detectors from the three current commands, respectively, and a proportion-plus-integral (PI) control for currents of the individual phases is performed to output command voltages Eu, Ev, Ew for the respective phases to the power amplifier. In the power amplifier, PWM control is performed by an inverter, etc. to provide currents Iu, Iv, Iw for the individual phases to flow in the servomotor M, thus driving the servomotor M. As a result, a current loop is formed as the innermost minor loop of the position and speed loops, and this current loop controls a current flowing in each phase of the AC servomotor.
In the above method for controlling the currents of the three phases separately, since the frequency of each current command rises as the rotational speed of the motor increases to cause the gradual phase lag of the current, the reactive component of current increases to rise a problem that torque cannot be generated with good efficiency. Also, since the controlled variable is alternating current, even in a steady state in which the rotational speed and the load are constant, deviations such as a phase lag with respect to the command, attenuation of the amplitude, etc. occur, making it difficult to attain torque control comparable to that attainable with a direct-current motor.
As a solution to the above problems, a DQ control method is known in which the three-phase current is converted into a two-phase, i.e., D- and Q-phase, in direct-current coordinate system through a DQ conversion, and then the individual phases are controlled by direct-current components.
FIG. 11 illustrates a control system in which an AC servomotor is controlled through the DQ conversion. It is assumed that the D-phase current command is "0", and that the current command for Q-phase is a torque command outputted from the speed loop. In a converter 9 for converting the three-phase current to a two-phase current, D- and Q-phase currents Id and Iq are obtained by using actual currents of u-, v- and w-phases of the motor, and the phase of the rotor detected by a rotor position detector 7, and the currents thus obtained are subtracted from the command values of the respective phases, to obtain D- and Q-phase current deviations. In current controllers 5d and 5q, the respective current deviations are subjected to proportional and integral control, to obtain
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Dong Seong Oh et al: "New Rotor Parameter Estimation for a Flux and Speed Control of Induction Machine Considering Saturation Effects", Proceedings of the International Conference on Industrial Electronic Control and Instrumentation (IECON), Kobe, Oct. 28-Nov. 1, 1991, vol. 1, No. Conf. 17, Oct. 28, 1991, pp. 561-566. Institute of Electrical and Electronics Engineers.
Le-Huy et al.: "Analysis and Implementation of a Real-Time Predictive Current Controller for Permanent-Magnet Synchronous Servo Drives", IEEE Transactions on Industrial Electronics, vol. 41, No. 1, Feb. 1994, pp. 110-117.
Iwashita Yasusuke
Kawamura Hiroyuki
Fanuc Ltd.
Lockett Kim
Shoop Jr. William M.
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