| As the core unit of the direct drive transmission mechanism, the permanent magnet synchronous linear motor(PMLSM) which has the advantages of high thrust density, high precision, high velocity and fast dynamic response ability is widely used in lithography, high-grade CNC machine tool and other precision manufacturing equipment. Because the linear motor is the power source of the micro-nano movement, the thrust quality is the core of the precision motion system. In the precision PMLSM control system, the thrust is controlled by adjusting the currents of the motor drive system. Therefore, among the position loop, velocity loop and current loop, the current closed-loop as the inner loop links up the control system with the motor, and its performance directly determines the final technical indicators of the motor control system.Because the output thrust of PMLSM can be seen two parts, electromagnetic thrust and thrust ripple, the purpose of regulating motor current mainly considers from two aspects in the high performance current loop. One is to control the electromagnetic thrust through the high performance current control strategy. Compared with the common rotary motor, the direct driving model, strong coupling and parameter uncertainty of PMLSM make high demands on the bandwidth of the current closed-loop system, decoupling and robustness, respectively, so the high performance current control strategy becomes a necessary condition to meet these requirements. On the other hand, due to the specificity of the PMLSM structure, the end and cogging effects make an additional ripple in the output thrust, therefore, in the process of regulating the current, the thrust ripple should be suppressed by compensating the corresponding current. Based on the above analyses, in face of the urgent need for the current regulation technology, the current control strategy and thrust ripple compensation method are researched in this paper.The thrust ripple includes high order harmonic components in the PMLSM with the rectangular structure. To suppress the ripple, the high performance requirement of the control system is proposed, but the ripple is not easy to be compensatd by using control method separately, therefore, this paper suppresses high order harmonic components of the thrust ripple by the permanent magnets(PMs) skewing method, and on the basis of the analysis results, the motor model is built to lay the foundation for subsequent studies. Firstly, based on the permanent magnet equivalent magnetization method, through combining the analytical model of skewed permanent magnet with the air gap relative permeability distribution function, the precise gap magnetic field of skewed iron core PMLSM considering the pole-to-pole leakage flux is deduced. According to the maxwell tensor method, under the no-load condition, the electromagnetic thrust analytic expression is acquired, and reasons for the thrust ripple are analyzed. Then harmonic components of the thrust ripple are compared between the rectangular and skewed PM structure, and high order harmonic components can be suppressed effectively by designing an optimal skewing length. Based on the principle of coordinate transformation, the motor mathematical model considering the parameter perturbation under two phase rotating coordinate system is established. Combining with ripple analysis results, the dynamics model of the linear motion containing the thrust ripple is derived.In the current control system, due to advantages of the predictive control strategy, this method is studied in this paper. For the time delay and parameter disturbance issues in the predictive current control, some high performance current control methods are proposed. Firstly, a discrete mathematical model of the linear motor is derived based on the voltage space vector control technology, and the principle of the predictive current control is analyzed. The time delay issue is solved by calculating the voltage command during the next sampling period, and in view of the voltage command limit and the position hysteresis problems, the corresponding optimization method is presented. Then the influence of parameter variations is analyzed, and the robustness of the current loop is improved by proposing a disturbance state observer, in addition, combined with the time delay compensation method, a high performance predictive current controller is achieved by injecting the observed results into the controller in the form of the feedforward. To improve the applicability and simplicity of the predictive current control, through reconstructing the motor vectors, the time delay and parameter disturbance issues can be simultaneously overcome by this integrated strategy.In the thrust ripple compensation method, Jacobian linearization observer is proposed to suppress the low order and other remainder harmonic components after the structure optimization, and the equivalent compensation current is converted to suppress the thrust ripple through the feedforward method. Firstly, each harmonic component of the thrust ripple is defined as a part of the state variable, and then the system state equation is establised with the dynamic equation. Due to the nonlinear of the state equation, Jacobian linear state equation of the thrust ripple is deduced by the first-order linearization mehotd. To be realized in a digital system easily, the linear observer considering the discretiation deviation is required by using the mean state variable. Based on the Lyapunov stability theorem, to meet conditions such as the system stability and the fast convergence of the observed error, the reasonable parameters of the observer are obtained. The estimated thrust ripple is injected to the control system in a feedforward way, and the velocity fluctuation caused by the ripple disturbance is suppressed.A precise PMLSM testing platform based on the aerostatic guideway is established, and the accurate testing scheme of the thrust ripple is given. To develop a high performance digital driving controller, the hardware structure of key circuits and the software process of the control system are designed. At last, the proposed thrust ripple suppression method and two predicdive current control strategies are experimented on this testing platform, and experimental results are shown to demonstrate the effectiveness and correctness of the proposed schemes. |
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