Electricity: motive power systems – Limitation of motor load – current – torque or force
Patent
1997-05-13
1999-03-02
Cabeca, John W.
Electricity: motive power systems
Limitation of motor load, current, torque or force
318609, 318805, 318601, 388815, 388806, H02P 5408
Patent
active
058776036
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an electric current control method for an AC servomotor used as a driving power source for machinery such as machine tools and industrial machines or for robots.
BACKGROUND ART
FIG. 7 is a block diagram showing a conventional AC servomotor control system. In this control system, a position deviation is obtained by subtracting a position feedback value detected by an encoder or the like from a position command, and a speed command is obtained by a position loop process in which the position deviation is multiplied by a position gain in term 1. A speed deviation is obtained by subtracting a speed feedback value from the speed command, and a speed loop process for 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, then a current loop process is performed in term 3 to obtain voltage commands for individual phases, and PWM control or the like is performed to control an AC servomotor M.
To control a three-phase AC servomotor by the control system described above, there is known an alternating-current control method in which currents of three phases are separately controlled in the current loop. In this current control method, the 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 an encoder or the like, to obtain the current command for respective phases. Then, current deviations are obtained by subtracting actual currents Iu, Iv and Iw of the three phases, which are detected by current detectors, from the obtained three current commands, and proportional-plus-integral (PI) control or the like is performed by current controllers for respective phases, to thereby output command voltages Eu, Ev and Ew for the phases to a power amplifier. The power amplifier performs PWM control by means of inverters etc., so that currents Iu, Iv and Iw of the individual phases are fed to the servomotor M to drive the same. In this way, a current loop is formed as an innermost minor loop in the position and speed loops, and this current loop controls the currents of three phases to be sent to 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 30 method is known wherein the three-phase current is converted into a two-phase, i.e., d- and q-phase, direct-current coordinate system through a DQ conversion, and then the individual phases are controlled by direct-current components.
FIG. 8 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
REFERENCES:
patent: 4680525 (1987-07-01), Kobari et al.
patent: 5083039 (1992-01-01), Richardson et al.
patent: 5652495 (1997-07-01), Narazaki et al.
patent: 5777447 (1998-07-01), Okano
Endo Yuichi
Iwashita Yasusuke
Okamoto Takashi
Uchida Hiroyuki
Cabeca John W.
Fanuc Ltd
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