Electricity: motive power systems – Induction motor systems
Patent
1989-10-02
1993-05-04
Ip, Paul
Electricity: motive power systems
Induction motor systems
318723, 318798, H02P 763
Patent
active
052085244
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a current control apparatus for PWM-control of a motor current using a microprocessor.
2. Background Art
In control of a motor, a method of PWM control using velocity information and motor current information is well known. FIG. 4 illustrates an example of a block arrangement of a digital control apparatus for PWM-control of an induction motor.
In FIG. 4, a velocity command signal from a computer enters a velocity controller A. The velocity controller A forms a torque command signal based on an error between the velocity command signal and an actual velocity signal from a velocity detector G connected to an induction motor F, and delivers the torque command signal to a torque controller B. The torque controller B applies a current command signal to a current controller C, where the command signal is compared with actual current obtained by a current detector E. A voltage controller D is driven by the resulting error signal to control the applied voltage of the induction motor F.
When the actual current and the current command signal are compared in the current controller C, current information obtained by AD conversion is already departing from the true value of actual current at the input timing thereof. As a consequence, current overshoot occurs at motor acceleration and current undershoot at motor deceleration. The applicant has already filed a Japanese Patent Application (61-189409) regarding an invention the object of which is to solve the foregoing problem related to control and perform PWM control accurately. The invention disclosed is one in which a PWM command of an inverter which drives an induction motor is outputted by a microprocessor. In this connection, half of a controlled variable (command output) of the preceding cycle is added to a proportion term or proportional integration term for computing the controlled variable of the present cycle in an effort to accurately predict an actually measured value of current required for PWM control of the induction motor. As a result, current overshoot is eliminated and stabilized control can be carried out.
However, with regard to the controlled variable of such a conventional current control loop, an actually measured value of current cannot always be predicted with a method of minimizing error produced by dead time. Consequently, when the gain of the current control loop is made sufficiently high, the amount of the error increases by a wide margin. A problem that results is that the response of the current control loop in PWM control is not sufficiently improved.
SUMMARY OF THE INVENTION
The present invention has been devised to solve the foregoing problems has an object to provide a current control apparatus for PWM control in which a current-change pattern is utilized so that an actual current value in PWM control can be accurately predicted.
In accordance with the present invention, there can be provided a current control apparatus for PWM control in which motor current is subjected to pulse width modulation (PWM) control by a microprocessor, characterized by comprising input means for reading in a motor current value converted into a digital value at a predetermined interval, arithmetic means for predicting an amount of change in the motor current at a present point in time by a current-change pattern decided from motor current information read in previously and a preceding PWM command value, and output means for outputting a PWM command upon correcting the motor current value read in by the amount of change in the motor current, wherein an amount of error caused by a delay in control time required for computing the PWM command is corrected.
Accordingly, the current control apparatus for PWM control according to the invention is such that the amount of change in motor current at the present point in time is predicted based on a current-change pattern stipulated by the commanded voltage value of motor current, with the motor current value read in being corrected.
BRIEF
REFERENCES:
patent: 4611159 (1986-09-01), Kurakake et al.
patent: 4656402 (1987-04-01), Nishikawa
patent: 4914371 (1990-04-01), Shibata et al.
Kono Shinichi
Takahashi Hironobu
Fanuc Ltd.
Ip Paul
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