Research On Two-Degree-of-Freedom Robust Tracking Control For High Precision Permanent Magnet Linear Synchronous Motor

Taking the project supported by National Natural Science Foundation of China (The research of high precision micro-feed servo system driven permanent-magnet linear AC motor) as background, this dissertation researches on the permanent magnet synchronous linear motor(PMLSM) servo feeding system used in modern advanced NC machine tool. For the characteristic of direct drive and the double high requirements of robustness and tracking performance for high precision servo system in NC machine tool, it is very difficult to obtain dual gains between rejecting disturbance and tracking performance using traditional single degree of freedom controller, therefore in the dissertation the proposed scheme is to design a novel two-degree-of-freedom(2-DOF) controller to ensure the robustness and high precision and high speed positioning and tracking performance of the system, on the basis of the feedback control, combining with feedforward control.Robust feedback controller and feedforward controller are designed respectively in 2-DOF control system. Robust feedback controller ensures the stability and the robust restraining disturbance ability of the system, and robust feedforward controller solves mainly the problem of the tracking ability of the system output. 2-DOF control can design and adjust controllers independently to obtain the best state of the restraining disturbance ability and tracking performance. Robust feedback controller consists of the following elements: a disturbance observer in the velocity loop, a velocity P controller( PI controller), a PD feedback controller and a repetitive controller in the position loop. Robust feedforward controller adopts zero phase error tracking controller (ZPETC). According to the characteristic of the controlled plant and controllers, the external disturbance modes and the requirement of servo drive, the controllers are designed.For the parameters uncertainties for PMLSM servo system and nonlinear disturbances including the friction, a disturbance observer is utilized in the velocity loop to eliminate and attenuate the bad influences of the factors. A PD controller is used in the position loop to construct an asymptotically stable system. However, the periodic force disturbance is generated by specially existing end effect of PMLSM, which can not be rejected effectively by the disturbance observer. For it, a repetitive controller is designed in the position loop to restrain the periodic disturbances. The performance of restraining disturbance is improved. To enhance high-speed response and high accuracy positioning and tracking performance, a ZPETC is added to the closed-loop feedback control system as feedforward controller.Combining ZPETC with robust feedback controller, a novel 2-DOF control system is designed to solve the conflict between the fast tracking performance and the interference rejection in the linear servo system. ZPETC depends on the model of the system and is sensitive to the parameters changes of the system. For it, the variable gain ZPETC is presented to overcome the influences modeling error and the parameters changes. Thus the robustness of ZPETC is greatly enhanced. ZPETC is used to compensate for the phase error, but this will also cause gain error. To improve the tracking performance of the ZPETC, the optimal design scheme of feedforward controller based on L₂-norm is proposed. The optimal digital pre-filter (DPF) is designed by selecting proper objective function and cascaded to ZPETC. The scheme can ensure the phase error of the system to be zero, at the same time, the gain performance of the system is greatly improved. The tracking precision is enhanced.Finally, for the proposed methods, the simulations and experiments are conducted, which verify the feasibility of the theory studies and simulation results of the proposed control methods.