The electromagnetic torque of any motor is generated by the interaction of the main magnetic field and the armature magnetic field. The main magnetic field and the armature magnetic field of the DC motor are 90° out of each other in space, so they can be independently adjusted; the main magnetic field and the armature magnetic field of the AC motor are not perpendicular to each other and affect each other. Therefore, the torque control performance of the AC motor has been poor for a long time. After long-term research, the current AC motor control has constant voltage-to-frequency ratio control, vector control, and direct torque control.
First, constant voltage frequency ratio control
Constant voltage ratio control is an open loop control. It is controlled by the space vector pulse width modulation to the desired output voltage uout according to the given system, so that the motor runs at a certain speed. In some places where the dynamic performance requirements are not high, the open-loop variable voltage frequency conversion control method is simple and is still widely used in general speed control systems. However, due to the steady state model of the motor, the ideal dynamic control performance cannot be obtained. Therefore, it must be based on the dynamic mathematical model of the motor. The dynamic mathematical model of a permanent magnet brushless motor is nonlinear and multivariable. It contains the product term of ω and id or iq. Therefore, in order to obtain accurate dynamic control performance, ω and id, iq must be decoupled. In recent years, various nonlinear controllers have been studied to solve the nonlinear characteristics of permanent magnet brushless motors.
Second, vector control
High-performance AC speed control systems require the support of modern control theory. For AC motors, the most widely used vector control scheme is currently used. Since 1971, Siemens AG, Germany. Blaschke proposed the principle of vector control, which is very popular. Therefore, it is studied in depth.
The basic idea of ​​vector control is to simulate the control law of DC motor torque on an ordinary three-phase AC motor. The field oriented coordinates are decomposed into the excitation current component and the torque current component by vector transformation. The two components are made perpendicular to each other, independent of each other, and then separately adjusted to obtain as good dynamic characteristics as a DC motor. Therefore, the key to vector control is the control of stator current amplitude and spatial position (frequency and phase). The purpose of vector control is to improve torque control performance, and the final implementation is control of id, iq. Since the physical quantity on the stator side is the amount of alternating current, the space vector rotates at a synchronous speed in the space, so adjustment, control, and calculation are inconvenient. Vector control is required by means of complex coordinate transformation, and the dependence on the motor parameters is very large, and it is difficult to ensure complete decoupling, which greatly reduces the control effect.
Third, direct torque control
The vector control scheme is an effective AC servo motor control scheme. However, because it requires a complicated vector rotation transformation, and the mechanical constant of the motor is lower than the electromagnetic constant, it cannot respond quickly to the torque in the vector control. In response to this shortcoming of vector control, German scholar Depenbrock proposed a control scheme with fast torque response characteristics in the 1980s, namely direct torque control (DTC). The control scheme abandons the control idea and current feedback link of decoupling in vector control, adopts the method of stator flux linkage, and uses the discrete two-point control to directly adjust the stator flux linkage and torque of the motor, which has a simple structure. The torque response is fast and so on. DTC was first used in induction motors. In 1997, L Zhong et al. modified the DTC algorithm and used it for permanent magnet brushless motor control. At present, relevant simulation and experimental research have been carried out.
The DTC method achieves double closed loop control of flux linkage and torque. After obtaining the flux linkage and torque value of the motor, the DTC can be performed on the permanent magnet brushless motor. Figure 2 shows the block diagram of the DTC scheme of the permanent magnet synchronous motor. It consists of permanent magnet brushless motor, inverter, torque estimation, flux linkage estimation and voltage vector switch table, among which ud, uq, id, iq are voltage and current in static (d, q) coordinate system. Component.
Although the research on DTC has made great progress, it is not mature enough in theory and practice, such as low-speed performance and load capacity, and it has high real-time requirements and large calculation amount.
Fourth, decoupling control
After the mathematical model of the permanent magnet brushless motor is coordinate-transformed, there is still coupling between id and id, and independent adjustment of id and iq cannot be realized. If you want to achieve good dynamic and static performance of the permanent magnet brushless motor, you must solve the decoupling problem of id, iq. If the control id is constant to 0, the equation of state for the permanent magnet brushless motor can be simplified as follows:
At this time, there is no coupling relationship between id and iq, Te=npψfiq, and the linearization of torque can be realized by independently adjusting iq. A decoupling control with id constant of 0 can be used, and voltage type decoupling and current type decoupling can be used. The former is a complete decoupling control scheme, which can be used to completely decouple id and iq, but the implementation is more complicated; the latter is an approximate decoupling control scheme, the control principle is: properly select the parameters of the id loop current regulator , so that it has a considerable gain, and always make the controller's reference input command id * = O, you can get id ≈ id * = 0, iq ≈ iq * o, thus obtaining the approximate decoupling of the permanent magnet brushless motor . Figure 3 shows a permanent magnet brushless motor based on vector control and id*=O decoupling control.
Although the current-mode decoupling control scheme cannot be completely decoupled, it is still an effective control method. As long as a better processing method is adopted, high-precision torque control can be obtained. Therefore, there are many current-mode decoupling control schemes used in engineering. However, current-type decoupling control can only achieve static decoupling of motor current and speed. If dynamic coupling is achieved, it will affect the control precision of the motor. In addition, current-mode decoupling control introduces a hysteresis power factor by keeping one of the coupling terms constant.
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