Study On Robust Control For NC Rotary Table Ring Torque Motor Direct Drive Servo System

Based on the project supported by National Natural Science Foundation of China "Study on Numerical Control(NC) Machine Tool Rotary Feeding Precision Direct Drive Technique and Its Control Strategy"(No.50375102), for the characteristic of the ring permanent magnet torque motor used in the direct drive NC rotary table servo system suffering from the effect of parameters variations and external load disturbances, H_∞robust control is combined with other control methods and two control strategies are proposed to enhance servo performances of the rotary table system in this thesis.In various NC rotary tables, the traditional drive mode, which is generally the rotating servo electrical machine with worm wheel, worm pair and gear pair mechanism, results in slow motion response, bad dynamic stiffness and other nonlinear errors due to mechanical transmission chain, thus it is very difficult to implement high precise processing. The rotary table servo system adopting direct drive technique is no medium transmission, and has large thrust, quick response, high dynamic stiffness and position precision. However, the system is more sensitive to uncertainties such as load disturbances and inertia variations, which greatly reduces servo performances. Designing a servo controller is required to improve robustness, tracking performance and dynamic servo stiffness.For real systems with complex cases such as uncertainties, an important control strategy, H_∞robust control is presented. But H_∞control is conservative, and considers no robust performance but robust stability.μtheory remedies the shortages of H_∞control, and it simultaneity gives attention to robust stability and robust performance in system designs, yet it has slow convergence speed and large calculation, and is difficultly applied to actual processings. Hence, aμ-H_∞speed controller is designed through combining H_∞design methodology without orthogonality assumptions with D-K iteration inμsynthesis in this thesis. The orthogonality assumptions is removed to greatly condense the upper bound ofμvalue and to reduce the searching time and scope. Compared with the traditional H_∞controller, this speed controller is better in robustness, and its acquisition is faster than that of theμcontroller.Moreover, dynamic servo stiffness directly affects attenuation capability of rotary table direct drive systems against dynamic disturbances. Based on advanced PDFF control configuration, H_∞design method is adopted to enhance interference rejection performance and response speed to angle displacement input signal, thus to improve the dynamic servo stiffness of the system.Type RM410/100 ring torque motor produced by Germany CyTec company is considered as the controlled object, its mathematical model is established, and it is necessary to simulate and analyze above two control strategies using MATLAB software. Simulation results show that the proposed H_∞controllers meet the requirement of robustness and rapidity for high-speed high-precision rotary table servo system.