Research On Servo Control System Of Large Telescope

With the development of technology,the resolution and aperture of the telescope has gradually increased.The traditional brush DC motor is not able to meet the system requirements in size,power and other aspects.At present,some famous large telescope has adopted Arc permanent magnetic synchronous motor to drive the telescope,such as GTC,VLT,Subaru and ALMA telescopes.The motor stator of this kind is composed of multiple arc-shaped units,and the permanent magnet synchronous motor has no change brush,resulting in high efficiency and low torque fluctuation,so it can meet the system requirements for accuracily tracking.At present,due to the mature and stable technology of DC motor,telescope servo system based on AC motor has not been used.Therefore,it is of great significance to study how to design a telescope servo control system based on AC motor.In this paper,the principle and implementation of the coordinate transformation of the space vector control method of permanent magnet synchronous motor are expounded firstly.Then the simulation model is established in Matlab / Simulink,and the system step response experiment is carried out.The experiment shows that the servo system based on permanent magnet synchronous motor has the advantages of fast response,small fluctuation,stable and reliable,verifying the feasibility of the vector control method.The traditional servo system based on RS422 / 485 has been compared with the servo control system based on fieldbus,and the advantages of CAN bus are clarified.Then the CANopen protocol based on CAN bus has been studied,including the CAN bus physical layer,packet protocol and application layer.Then the hardware architecture of the servo system based on the field bus and the CANopen master software architecture are designed.The communication test is carried out on the experimental platform.The experimental results show that the servo control system based on CANopen protocol has high real-time reliability and strong reliability,which can meet the system requirements.The system is subjected to sinusoidal sweep test on the experimental platform,and the system frequency is obtained by using the method of discrete Fourier transform.The system model identification is also carried out.Based on the identification model,the velocity loop and position loop of the servo system were designed separately.Because of the problem of phase lag in traditional PI control,it can cause large error during direction change and tracking error.In this paper,the structure of feedforward control is introduced,and the experimental results show that the bandwidth of the system can not be improved by the feedforward control but the phase lag is significantly improved,thus improving the system’s response capability,reducing the tracking error.The main disturbances of the large telescope are analyzed: friction torque,wind load disturbance and torque fluctuation.In order to suppress the influence of external disturbance on the telescope,the control structure with acceleration loop is adopted.Firstly,the influence of the acceleration loop on the disturbance suppression is analyzed,and then an estimator is proposed to combine the low-speed acceleration estimator and the Newton iterative prediction to estimate the system acceleration.Experiments on the experimental platform show that the disturbance suppression ability of the system after the introduction of the acceleration loop is greatly improved.Because of phase lag exists in the proposed acceleration estimator,and the angular acceleration sensor is so expensive.Therefore,from the perspective of control algorithm,a sliding mode control method based on exponential approximation rate is proposed to overcome the influence of parameter change and external disturbance on the system.In order to compensate the friction in real time,a kind of sliding mode observer is proposed to estimate and compensate the disturbance in the system in real time.Experiments on the experimental platform show that the control method based on the exponential approximation rate and the sliding mode observer has greatly decress the error in the process of system operation.Finally,the limitations and shortcomings of the traditional telescope drive with axis single are expounded,and the double motor control model is analyzed.Compared with the single motor drive,the dual motor drive can improve the anti-resonant frequency of the system 2^1/2 times.The dual motor drive control system based on CANopen protocol is implemented on the existing experimental platform,and the synchronization control strategies of multiple motors are compared.The synchronous error of the system is compensated by PD compensator.Experiments show that the double motor control in the bandwidth and disturbance suppression is better than single motor drive are improved.