the acquisition, tracking and pointing (ATP) mechanism plays a more and more important role in the space laser communication field. Its tracking performances determine the establishment of an optical communication link to a great extent. Thus, the tracking accuracy, rapidity and stability of the ATP system have always been hot issues. Two types of strategies are usually considered to improve the tracking performances. One is to improve the mechanical gimbal structure and the other is to improve the tracking control method. The latter is considered in this paper and a novel back-stepping time-varying sliding mode control strategy is proposed. The kinematic model of the two-dimensional photoelectric ATP mechanism is built up first and from this model it is concluded that the two axes can be controlled separately. Next the mathematical model of PMSM which is served as the execution unit of ATP mechanism is built up and the impact of friction torque is discussed either. Based on these analyses, the back-stepping time-varying sliding mode controller with an observer of load torque is designed and verified in MATLAB/SIMULINK. The simulation results of the new controller are compared in detail with the traditional PID controller's and it is evident that the tracking and pointing performances can be improved effectively by using the back-stepping time-varying sliding mode controller.
['Xiangdong Liu', 'Xiaorui Zhao', 'Zhen Chen']
A PMSM back-stepping time-varying sliding mode control system based on two-dimensional spaceborne ATP mechanism