CURRENT CONTROL DEVICE FOR PWM CONTROL

Abstract: PCT No. PCT/JP89/00428 Sec. 371 Date Oct. 2, 1989 Sec. 102(e) Date Oct. 2, 1989 PCT Filed Apr. 21, 1989 PCT Pub. No. WO89/10655 PCT Pub. Date Nov. 2, 1989.A current control apparatus is provided for PWM-control of motor current using a microprocessor. Since motor current enters a control processor as a discrete value, a delay in control always arises. In prior art, it is not possible to accurately predict the actual current value needed for PWM control; as a result, the gain of the current control loop cannot be made large enough. Accordingly, a change pattern of A/D-converted motor current is utilized to estimate what the motor current will be at the present point in time, and current control is carried out based on this motor current. As a result, the delay is made equivalently small and the gain of the current control loop can be freely set, thus improving the response of the current control loop.

Description: This invention relates to a current control apparatus for PWM-controlling a motor current using a microprocessor. 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-controlling 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 JP-A-630 48 196 regarding an invention the object of which is to solve the foregoing problem related to control, and to 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. The present invention has been devised to solve the foregoing problems and its object is 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 is provided a current control apparatus for PWM control in which current in a motor is subjected to pulse width modulation control by a microprocessor according to the difference between a current command value and a feedback current value at a predetermined sampling instant, wherein the amount of error in the feedback current value caused by the delay in control time required by the microprocessor for computing a PWM voltage command value is corrected, the apparatus comprising: input means for reading the feedback motor current value converted into a digital value at a predetermined sampling instant, arithmetic means for predicting the amount of change in the motor current at the present sampling instant with respect to the last read feedback current value, said predicted amount of change in current being determined in dependence upon a previous PWM voltage command value; and output means for determining and outputting said PWM voltage command by correcting said last read current value by said predicted amount of change in the feedback current value. 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 Description of the Drawings Fig. 1 is a block diagram illustrating an embodiment of the present invention, Fig. 2 is a flowchart illustrating a processing procedure for outputting a corrected PWM command, Fig. 3 is an explanatory view illustrating current control timing, and Fig. 4 is a block diagram of a digital control apparatus for PWM-controlling an induction motor. An embodiment of the present invention will now be described in detail with reference to the drawings. Fig. 1 illustrates the simplified construction of a current control apparatus for PWM control in which a three-phase induction motor 1 is subjected to inverter control. A three-phase AC power supply is converted into direct current by a rectifier 2, and the resulting DC voltage Vdc is applied to three pairs of switching transistors. Motor currents Iu, Iv, Iw flow into respective windings of the induction motor 1 in accordance with a PWM command signal outputted by a control circuit 3 constituted by a microprocessor. In these phases, the U- and V-phase currents Iu, Iv are detected by a current detector and are fed back to the control circuit 3 as current feedback signals. The control circuit 3 receives current command voltage signals Vuc, Vvc, Vwc from a velocity computing unit (not shown) as inputs thereto and applies predetermined PWM processing between these signals and the current feedback signals. In the present embodiment, a corrected PWM command signal is formed in accordance with a motor current-change pattern, as set forth below. Specifically, in a case where the potential of its own phase and the potentials of the other two phases are equal, the motor current does not change. If the potential is the same as that of a first but different from that of a second of the other two phases, or if the potential is different from those of both of the other two phases, each current value varies as follows: In the former case, the magnitude ΔI of the change in current is decided by load impedence and inductance. In other words, since the DC voltage Vdc is expressed by Vdc = L(dI/dt) the magnitude ΔI of the change in current is as follows: ΔI = (1/L) x Vdc x ΔT In the latter case, the magnitude of the change in current is 2ΔI. The reason is that the magnitude of the change in current is proportional to the number of phases having a potential different from its own and to the time. If L, V are assumed to be constant and the substitution (1/L) x Vdc = K₁ is made, then ΔT can be replaced by differences between the PWM command values Vuc, Vvc, Vwc. Therefore, it is possible to estimate and calculate the magnitude ΔI of the change in current in each phase as follows: ΔIu = K₁(Vuc-Vvc) + K₁(Vuc-Vwc) ΔIv = K₁(Vvc-Vuc) + K₁(Vvc-Vwc) ΔIw = K₁(Vwc-Vuc) + K₁(Vwc-Vvc) The processing steps for predicting and computing the amount of change in motor current by the control circuit 3 in order to output the corrected PWM commands will now be described with reference to the flowchart of Fig. 2. Data at the timing of pulse Pn-1 sampling the feedback current is assumed to be Id(n-1). First, a motor current value Id converted into a digital value is detected (step a), then the amount of change ΔI(n-1) in motor current based on the Id read in is computed (step b) in accordance with Eq. (1). These items of data Id(n-1) and ΔI(n-1) are added to arrive at a predicted value of motor current at the present point in time, namely at the next sampling pulse Pn (step c). This is regarded as a feedback current value If(n). If the error between If(n) and a current command Ic(n) inputted to the control circuit is computed at this time (step d), the PWM command signal becomes one in which the delay in control time necessary for this computation is corrected. Fig. 3 illustrates the timing waveforms in a case where current control is performed by comparison with current command voltages Vuc, Vvc, Vwc, with the PWM carrier wave being a triangular wave. When the current command voltages Vuc, Vvc, Vwc enter the control circuit 3, a delay due to processing time occurs in the control circuit 3 with regard to feedback current which enters at the actual current sampling timings n-1, n ..., as shown in (a) of Fig. 3. However, if a motor current-change pattern is utilized based on a transistor on/off signal shown in (c) of Fig. 3, the actual current shown in (d) of Fig. 3 can be predicted. Accordingly, the current feedback signal approaches the true value and the processing time lag when current control is perforemd becomes equivalently smaller, thus making it possible to freely set the gain of the current control loop. Though an embodiment of the present invention has been described, the invention it not limited thereto but can be modified in various ways without departing from the scope of the claims. With the described and illustrated current control apparatus for PWM control according to the present invention, it is possible to neglect the control time delay so that the gain of the current control loop can be freely set, thus improving the response of the current control loop.

Claims: A current control apparatus for PWM control in which current Iu, Iv, Iw in a motor (1) is subjected to pulse width modulation (PWM) control by a microprocessor (3) according to the difference between a current command value Ic(n) and a feedback current value If(n) at a predetermined sampling instant Pn, wherein the amount of error in the feedback current value If(n) caused by the delay in control time required by the microprocessor (3) for computing a PWM voltage command value Vuc, Vvc, Vwc is corrected, the apparatus comprising: input means (E) for reading the feedback motor current value Id(n-1) converted into a digital value at a predetermined sampling instant P(n-1), arithmetic means (3) for predicting the amount of change ΔI(n-1) in the motor current I(n) at the present sampling instant Pn with respect to the last read feedback current value Id(n-1), said predicted amount of change in current being determined in dependence upon a previous PWM voltage command value Vuc, Vvc, Vwc ; and output means for determining and outputting said PWM voltage command Vuc, Vvc, Vwc by correcting said last read current value Id(n-1) by said predicted amount of change ΔI(n-1) in the feedback current value.

Inventors: KONO SHINICHI FANUC MANSION HA, TAKAHASHI HIRONOBU FANUC DAI-, KONO, SHINICHI FANUC MANSION HARIMOMI 7-203, TAKAHASHI, HIRONOBU FANUC DAI-3 VILLA-KARAMATSU
Applicants: FANUC LTD, FANUC LTD.
Application year: 1989
Main classification: H02P5
IPC codes: H02P27