Research On AC Servo System Of Low Speed For The TMT Tertiary Mirror System

With the improvement of resolution and aperture of the large telescope, the traditional ground based or space telescope platform which detect objectives of deep space with low speed tracking mode can’t satisfy the design requirement of the image quality and stability in a new generation of large aperture telescope. TMT is a large optical/infrared telescope, and the aperture of its primary mirror is thirty meters, the main aim is to detect the galaxy and research the unknown star and planet. When tracking the astronomy objective, the servo system of TMT tertiary mirror system must achieve precisely tracking, and the control speeds need to be several arcseconds per second, the ultra low tracking speed is the major challenge and difficulty in the design. The extremely high requirements will be need for the selection of the drive way, the identification accuracy of the control model, control strategy design, disturbance analysis, low speed valuation, the resolution of the current and positon sensors and so on. Based on such background, this paper will study on the design of the TMT prototype control system and the realization of low speed tracking.Firstly, it is a research for the current situation of the optical, radio and space telescopes over the world, according to their control system design, tracking speed and the tracking accuracy under low speeds are compared with TMT tertiary mirror system, leads to complex development trend of large telescope, as well as for the research on this task is of great significant.The prototype of the TMT tertiary mirror system will use the way of direct drive based on the permanent magnet synchronous torque motor, with its advantages of small size and large driven torque, PMSM is suitable for the use of large load torque, and have made a wide range of applications in the large aperture telescope servo control system at home and abroad. For the research of control method, this paper will use the vector control to realize a decoupling control of PMSM, the basic idea is using the method of coordinate transformation, which make the control of AC motor equivalent to DC motor. A simulation model is built based on PMSM to realize the vector control method.The prototype of TMT tertiary mirror system is used for the validation of the design method, the first resonant frequency of 25 Hz is obtained through modeling to analyze the interactions between prototype and the structural dynamic. In order to improve the precision of pointing and low speed tracking, and to meet the design requirements of tertiary mirror system, sin swept is used to test the frequency characteristics for open loop and speed close loop system of the prototype, the structure resonance frequency and the bandwidth are estimated according to the frequency characteristic test results. The accuracy of the plant model is the main factor for the low speed performance of the control system, the system model will be identified based on the test result of frequency characteristics, state space model is obtained and will be used for preliminary design of the control system.Due to the nonlinear and strong coupling of AC servo system, making some traditional control strategies unsatisfactory. In this paper, control strategies are designed for the current, speed and position loop for the prototype of tertiary mirror system, in order to meet the requirements of rapidity and accuracy, vector control method is selected for the current loop control, the response speed of the system is improved. A speed detection method based on observer has been used for the speed loop, it can overcome the external disturbance and at the same time can effectively undermine the system noise, improving the speed resolution, the motor motion speed can achieve a higher control precision in the steady state. LQG control method is used for low speed tracking, and the position error can meet the low velocity tracking and jitter requirements of tertiary mirror system.This paper analyzes the disturbance factors through modeling for impacting the low speed tracking precision of tertiary mirror servo system, LuGre friction model is established based on the bearing friction torque, also build the wind load disturbance model. In the simulation, wind load and current loop output are the driven torques for the servo system, according to the simulation results, wind load will effect the system pointing accuracy, which is a foundation for the following design and interference analyses of the control system.Finally, in order to improve the low speed tracking and the image quality for the astronomical objectives, the analysis is needed for the low speed performance and the position tracking error of the prototype. A method for jitter test is presented by using a high-resolution encoder, the jitter test results and its power spectrum distribution are obtained under different tracking speeds and directions by tracking the position slope curve, it also can be done as a standard for the evaluation of the steady performance and tracking performance under low speeds. Experimental results show that the position tracking jitter is less than 10.5mas RMS, the servo system of the prototype has very good performance and tracking accuracy. Prototype jitter test results can also fit with the jitter requirement of TMT tertiary mirror system.