JP2008228419A - Torque control method of motor based on model prediction control - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem, wherein there is proposed a technique for speedily changing voltage phase, on the basis of a numerical expression of a motor as a technique for enabling high-performance torque (current) control, even in a motor drive region where conventional vector control is difficult to be applied, however, since this technique is based on a numerical expression model at a stationary period, restrictions may be imposed on its current (torque) response, and the optimization of a switching vector to be selected cannot be compensated, even at the stationary time. <P>SOLUTION: On the basis of a control system for controlling the rotation operation of the motor, the current flowing in the motor, the position of a rotor magnetic pole, and the rotation speed information, there is selected, as a solution for the optimization of problem, the switching vector in which an error of a future torque (current) behavior, calculated on the basis of a motor model whose transient state is taken into consideration is minimized, with respect to a torque (current) command value necessary for controlling the rotation operation of the motor with high performance, and thereby high-response torque (current) control is achieved, even in a speed range which may be the problem by inputting the switching vector into the control system. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a motor torque (current) control technology included in household appliances or widely used for industrial purposes.

The demand for permanent magnet synchronous motors capable of high-efficiency operation is increasing even for high-speed drive applications as the performance of permanent magnets improves and the price decreases. In the triangular wave comparison PWM method in which the carrier is compared and the switching vector is determined, it is difficult to realize the desired control performance in principle when the carrier frequency and the drive frequency are close to each other.

Furthermore, as the speed electromotive force increases with respect to the output voltage upper limit determined from the link voltage, the voltage margin becomes smaller and voltage saturation is likely to occur during current transient response. However, the conventional triangular wave comparison PWM method has the above characteristics. Since the control system is not designed in consideration, proper switching cannot be performed, and control performance is deteriorated. In the worst case, control may be disabled. These problems are that the characteristics of the inverter (such as the upper voltage limit) are not taken into account when designing the control system.

As a problem solving method described above, for example, there is the following Non-Patent Document 1. Here, even in the drive region where the above problem occurs, high current (torque) control performance is realized by changing the voltage phase at high speed based on the mathematical model of the motor.

Nakazawa, Toda, Yasuoka: “Induction motor vector control in fixed voltage mode”, IEEJ Transactions, Vol.118-D No.9, 1998

However, since the above-mentioned non-patent document is a method based on a mathematical model in a steady state, its current (torque) response is limited. Further, the optimality of the selected switching vector is not compensated even in the steady state.

Therefore, an object of the present invention is to realize a new current (torque) control method that takes into account the characteristics and transient state of the inverter and compensates for the optimality in the selection of the switching vector.

  The motor current (torque) control method according to the present invention includes a first step of deriving time series of all switching modes in a rotational operation control system of a three-phase AC drive motor using an inverter, and a current flowing through the motor. A second step for deriving a future current for each time series derived in the first step based on information on the rotor magnetic pole position and the rotational speed, and an optimum switching vector from the future current value derived in the second step. This is a torque control method for a motor characterized by comprising a third step for deriving and a fourth step for introducing the switching vector derived in the third step into the rotational motion control system. Furthermore, the motor torque control method is characterized in that the motor is a permanent magnet synchronous motor.

  According to the present invention, the current (torque) control in the drive region, which is difficult to realize with the conventional method, can be realized as the optimum control in consideration of the characteristics of the inverter and the transient response. ) Control performance can be expected.

FIG. 1 shows a schematic diagram of a motor drive system. The motor is controlled by turning on and off a total of six switches in the inverter section. When one of the upper and lower switches is turned on, the other is turned off. Therefore, there are 8 switching modes.

First, step 1 will be described. In this method, a mathematical model of the motor is used, and an optimal switching vector is selected in consideration of future current (torque) behavior. Equation 1 shows a mathematical model of the motor in the dq coordinate continuous system.

In Equation 1, i _d and i _q are a d-axis current and a q-axis current, respectively. R _a is the armature winding resistance, L _d and L _q are the d-axis and q-axis inductances, v _d and v _q are the d-axis and q-axis voltages, and ω _re and φ are the motor rotational angular frequency and linkage, respectively. Represents the number of magnetic flux.

Equation 3 is obtained by discretizing Equation 1 using Equation 2 with T _s as the sampling time.

In addition
n = 0,1, .., 7, and the input voltage vector v ⁿ _dq (t) means the n-th vector.
Next, the second step will be described. In Equation 3, if a certain input voltage vector is given, the current behavior of one step future can be obtained. When this operation is repeated once more, the current behavior of the future in two steps is obtained as shown in Equation 4.

Therefore, if the number of prediction steps is determined in advance, the current behavior for all input sequences can be derived by the above iterative calculation. Therefore, the input sequence closest to the next obtained current behavior and the desired current command is uniquely determined using the evaluation function shown in Formula 5.

Formula 5 calculates and adds the sum of squares of errors (Δi _dq ) with the command values of the d-axis and q-axis currents for each prediction. By using this evaluation function, switching capable of minimizing the current error is realized (third step).
FIG. 2 shows a block diagram of a motor drive system including a current control system using model predictive control (fourth step). In FIG. 2, characters (ω _re ^* and i _dq ^* ) including “*” on the right shoulder represent command values.

  Figure 3 shows the equipment used in this example. In the future, interior permanent magnet synchronous motors (IPMSM), which are expected to be used in various fields such as automobiles and railway main machines or air conditioner compressors, were controlled. The motor frame is supported by anti-vibration rubber, and the load-side frame is fixed to the base using bolts. Here, a rotary encoder was used to detect the rotor position.

Here, when the embedded magnet synchronous motor (IPMSM) is driven at a constant speed of 1500 [rpm] and 0.02 [Nm] using model predictive control, the load changes to 3.5 [Nm] stepwise. The results of d-axis and q-axis current responses are shown in FIG. For reference, the simulation results are also shown in FIG.

  From Fig. 4 (a), the actual current follows the command value without step-out because of switching due to the nature of the inverter even in response to sudden load fluctuations.

  4 (a) and 4 (b), there is a difference between the amplitude values of the pulsation components of the d-axis and q-axis currents in the simulation and the experimental results. Possible causes of this are the effects of dead time, inductance changes due to magnetic saturation, and modeling errors in spatial harmonics.

  This method determines the switching mode based on the mathematical model of the motor. Therefore, the above modeling error directly leads to deterioration of control performance. However, if further studies can model the motor characteristics more accurately, the control performance of this method will increase dramatically.

According to the present invention, the improvement of the current (torque) response of the permanent magnet synchronous motor, the reduction of the cooling device with respect to the switching element caused by the reduction of the switching frequency by devising the constraint condition or the evaluation function, the efficiency increase by the loss reduction, etc. Contributing to industrial development is extremely important.

It is a schematic diagram showing a motor drive system. It is a block diagram which shows the control system of this invention. This is an experimental apparatus used in this example. It is the experimental result (a) and simulation result (b) by this invention.

Claims (2)

In the rotational operation control system of a three-phase AC drive motor using an inverter, the first step for deriving the time series of all switching modes and the information on the current flowing through the motor, the rotor magnetic pole position and the rotational speed A second step for deriving a future current for each time series derived in one step, a third step for deriving an optimum switching vector from a future current value derived in the second step, and a third step. And a fourth step of introducing the switching vector into the rotational motion control system. The motor control method according to claim 1, wherein the motor according to claim 1 is a permanent magnet synchronous motor.

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