Research On Second Order Sliding Mode Control For Direct Drive NC Rotary Table Servo System

In recent years, direct drive NC is one of hot-spots on the field study of multi-axis Linkage machine tools. The work presented in this paper is supported by the sub-projects of the National Science and Technology Major Projects (No.2009ZX04010-031-02), and Foundation of Liaoning Province Education Department (No.2009A554).The paper researched on ring permanent magnet torque motor servo system used in direct drive NC. And based on sliding mode control, for the characteristic of direct drive technology and the requirement of robustness and tracking performance for the servo system in NC, several theories and methods, such as high-order sliding mode and improved disturbance observer, are combined to solve some uncertainties problem of NC servo system such as model parameters variations and load disturbances. The main contents are as follow:Based on analyzing the uncertainties of NC servo system, the speed controller of direct drive NC servo system is proposed which used by second order sliding mode for the effects of parameters variations and load disturbances. The sliding surfaces of speed controller is designed, and the control law is designed by super twisting algorithm of second order sliding mode, which needs not to know the information of the sliding mode time derivative. The control method ensures a high strong robustness of the servo system for parameters variations and load disturbances and chattering phenomenon is reduced.When the uncertainties of the model are very big, particularly, parameters variations and load disturbances are increased, in order to further increase the restraining disturbances and tracking performance, the control method combing second order sliding mode and improved disturbance observer was applied to NC servo system. Suboptimal algorithm based on the second order sliding mode control (SMC) is proposed as the speed controller of the servo system, which through the continuous control measure the sliding mode and its derivative approach zero in finite time. Then improved disturbance observer is designed based on the sliding mode controller, which compensates the external disturbances and the model uncertainties. This method effectively restrains the servo system performance of the uncertainties such as model parameters variations and load disturbances, and enhances the tracking performance of the servo system.