AC servo motor drive control strategy
The AC servo motor model represented by permanent magnet synchronous motor is a strongly coupled, time-varying nonlinear system. The control strategy is complex, so the performance of the AC servo system is directly related to the control strategy it adopts. Excellent control strategy can not only make up for the lack of hardware design, but also further improve the performance of the system. The control strategy plays a vital role in the AC servo. The requirements of the control strategy of the high-performance AC servo system can be summarized as follows: not only the system has fast dynamic response and high dynamic and static precision, but also the system is insensitive to parameter changes and disturbances.
The control strategy of representative permanent magnet synchronous motor has the traditional control strategy represented by the speed open loop constant voltage ratio (u/f=constant) control, the classic pid control, the field oriented control (vector control), and the direct torque. Control, sliding mode variable structure control, adaptive control, nonlinear feedback linearization theory, etc. represent modern control strategies and intelligent control represented by fuzzy control and neural network control.
Traditional control strategy
(1) Constant voltage ratio control
The constant voltage frequency ratio control with stator voltage drop compensation ensures that the air gap flux of the synchronous motor is constant, and the adjustment frequency is given to synchronously change the rotational speed of the motor. This control strategy is open-loop control, which only controls the air gap flux of the motor, cannot adjust the torque, and is prone to problems such as rotor oscillation and out-of-step. At the same time, since the constant voltage frequency ratio control is based on the steady state model of the motor, its dynamic control performance is not high, and it is not suitable for servo drive control occasions with high performance requirements.
In order to achieve good dynamic performance, it must be based on the dynamic mathematical model of the motor. The dynamic mathematical model of the AC permanent magnet synchronous motor is a nonlinear, strongly coupled, time-varying multivariable system. To get good control performance, decoupling control of angular velocity and current is required, that is, vector control technology.
(2) Classic pid control
The pid controller uses the proportional, integral, and differential functions to calculate the control error to control the controlled object. The pid controller is currently the most widely used regulator. It has the advantages of simple structure, good stability, reliable operation and convenient adjustment. It has always been one of the main technologies of industrial control and can satisfy most servo control applications.
However, there are still some problems in the three-loop pid adjustment control mode of the classic AC servo synchronous motor. For example, the adjustment of the regulator parameters is cumbersome and the error is large, and the dependence on the system model and parameters is strong. In some high-precision applications, it is very Difficult to meet system requirements.
(3) Magnetic field orientation control (id=0)
The vector control is built on the accurate mathematical model of the controlled object, so that the AC motor control is controlled by the external macroscopic steady state control to the transient control of the electromagnetic process inside the motor. The vector control transforms the nonlinear variable of the complex coupling inside the AC motor into a DC variable (current, flux linkage, voltage, etc.) whose relative coordinate system is stationary through coordinate transformation, realizes the approximate decoupling control, and finds the constraint condition to obtain a certain The optimal control strategy of the target, id=0 control is a specific control strategy of vector control. The permanent magnet synchronous motor cross-axis current decoupling is realized in the rotor coordinate system. Due to the existence of id and iq dual current closed loop, the motor is made. The iq current dynamically follows the system torque reference (te=ktiq, kt is the motor torque coefficient) to realize the motor electromagnetic torque control. This control strategy allows the motor system to have better output torque linearity and maximum linear torque. At the same time, since all currents are used to generate electromagnetic torque, the motor overload capability can be fully utilized to improve the starting and braking speed of the motor, and the motor has excellent starting and braking performance.
Vector control technology has experienced more than 20 years of research and perfection, and its performance in the speed control system is excellent. Whether it is in low speed (constant torque control mode) or high speed (constant power control mode), its anti-interference characteristics Both the braking characteristics and the steady speed characteristics meet or exceed the DC speed control system. However, the vector control model and algorithm are more complicated. It is necessary to perform coordinate transformation when implementing. It is difficult to ensure the complete decoupling of the voltage and current of the motor system in the direct and intersecting axes, which will affect the dynamic and efficiency of the motor system.
