Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1984-07-23
1986-09-09
Smith, Jr., David
Electricity: motive power systems
Induction motor systems
Primary circuit control
318811, H02P 540
Patent
active
046111590
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
Background of the Invention
This invention relates to an AC control system in which a velocity loop computation and a current loop computation for an AC motor are performed by a microprocessor to control the AC motor. More particularly, the invention relates to an AC motor control system capable of enhancing the response characteristics of the velocity and current loops.
An arithmetic circuit such as a microprocessor has recently come to be employed for servo-controlling an AC motor. It is required that the microprocessor execute at least a velocity loop computation, in which a current command is computed from an offset velocity between a commanded velocity and the actual velocity of the AC motor, and a current loop computation, in which a command for application to the current drive circuit of a servomotor is computed based on a difference between a current command and the armature current of the servomotor.
In order to obtain a desirable servomotor operating characteristic, it is required that the response characteristic of the current loop be quicker than that of the velocity loop. Since there is interference between current and velocity ascribable to a back electromotive force in a servomotor, the velocity loop and current loop computations cannot be rendered independent of each other and both computations must be executed at a predetermined sampling period. The result is a burden upon the microprocessor in terms of processing time.
Moreover, even if the velocity computation is executed at a period which is longer than that at which the current loop computation is executed, the computation for obtaining the effective current requires a long period of time when performed at the time of the velocity loop computation. A problem that results is a deterioration in the response of the velocity loop.
Summary of the Invention
An object of the present invention is to provide an AC motor control system capable of enhancing the response characteristic of a current loop and, moreover, of a velocity loop.
An AC motor is controlled by providing a sensor for sensing the velocity of the AC motor, a sensor for sensing an actual current flowing into the AC motor, a power drive circuit for the AC motor, and a control unit for performing a velocity loop computation to derive a current command from an offset velocity between a velocity command for the AC motor and the sensed actual velocity, and for performing a current loop computation to obtain an offset current between the current command and the sensed actual current, the offset current obtained by the control unit being applied to the power drive circuit to control the AC motor. In the current loop computation, a velocity compensation signal is obtained by amplifying the sensed actual velocity by a predetermined magnification, the command for the power drive circuit is compensated by the velocity compensation signal, and the current loop computation is executed at a sampling period shorter than that at which the velocity loop computation is executed. At the time of a velocity loop computation, the actual current at the relevant sampling instant is estimated from the current command value of the previous sampling, and a current command is computed from the estimated actual current and the offset current.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing the construction of a synchronous motor;
FIG. 2 is a block diagram of conventional servo control;
FIG. 3 is a view for describing velocity and current on the basis of the conventional control system;
FIG. 4 is a block diagram of servo control according to the present invention;
FIG. 5 is a circuit diagram of an embodiment of the present invention;
FIG. 6 is a view showing the construction of a principal portion of FIG. 5;
FIG. 7 is a view for describing the operation of the arrangement shown in FIG. 6; and
FIGS. 8 and 9 are views for describing operation according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention will now be described in detail in a
REFERENCES:
patent: 4458192 (1984-07-01), Sakamoto et al.
patent: 4477762 (1984-10-01), Karakake et al.
patent: 4575667 (1986-03-01), Kurakake
Le-Huy et al., "Microprocessor Control of a Current-Fed Synchronous Motor Drive", Conference: Industry Applications Society IEEE-IAS Meeting, Cleveland, OH, U.S.A. (Sep. 30-Oct. 4, 1979).
Kurakake Mitsuo
Sakamoto Keiji
Fanuc Ltd.
Smith Jr. David
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