Research On Technology Of Optical-electric Tracking Based On The Parallel Platform

Space telescopes can approach the diffraction limit because they are not restricted by various conditions on the ground,and are a research hotspot in the field of astronomical observation.With the development and demand of science and technology,future space telescopes require larger aperture and higher resolution.However,the space telescope mounted on the spacecraft will be constantly affected by various external disturbances,which will cause the optical axis of the telescope to shake,making it difficult to obtain clear images.In addition to imaging observations,the optical axis of the telescope needs to be continuously aligned with the observation target for a long time to achieve pointing.Therefore,the space telescope needs to solve two problems for high-precision tracking under the vibration conditions of the spacecraft:one is the problem of vibration isolation,and the other is the problem of high-precision pointing.The Stewart parallel platform has a six-degree-of-freedom motion capability,which can accurately point the observed target while suppressing vibration.Under such research background requirements,this paper focuses on the high-precision pointing control of the Stewart platform based on CCD image measurement.In this subject,the following work was carried out on the Stewart platform of cube configuration.The platform has the following characteristics: the structural length,stiffness and control of the struts are consistent,and the adjacent struts are orthogonal to each other.Using the above characteristics,it is easy to derive the kinematics characteristic equation of the Stewart platform,thereby obtaining the Jacobi matrix,and obtaining the relationship between the posture of the load platform and the elongation of each support.And use Newton-Euler method to get the dynamic model of the platform.According to the dynamic model,the Stewart platform is decoupling analyzed from the two aspects of vibration isolation and pointing,and the decoupling model is obtained,and then the vibration isolation control and the pointing control are explained separately.The first is vibration isolation.The active and passive vibration isolation is discussed.Simulations show that when only passive vibration isolation is used,low frequency and high frequency bands cannot be taken into account.Analyzing the active vibration isolation based on speed feedback,the simulation shows that the Stewart platform has a good follow-up effect on low-frequency vibrations and can isolate highfrequency vibration.In the part of pointing control,the separate method and the integral method are discussed.Both of the two methods can realize the pointing to the target.The idea of the separate method is to directly control the length of the struct,and the idea of the integral method is to change the length of the struct by controlling the position and posture of the load platform.In order to compare the two methods,simulation models based on the separation method and the integral method were built in MATLAB/Simulink respectively.The simulation results show that the two methods have little difference in effect.The integral method adopts a CCD as sensor,which is more suitable for engineering applications.Through the theoretical analysis and simulation of different delay time,it is found that delay has a great influence on the tracking performance.Therefore,a feedforward control method based on Youla parameterization is proposed.Compared with the error suppression capability of traditional feedback control,the proposed method can improve the performance by optimizing the filter.After theoretical and simulation analysis,a third-order low-pass filter Q is adopted,whose low-pass characteristic suppresses the influence of model accuracy identification on system stability to a certain extent.Finally,use the built experimental platform to carry out the frequency response test and target tracking experiment of the Stewart platform based on CCD image measurement to verify the previous theoretical analysis and the proposed control algorithm.For the target tracking experiment,experiments were carried out on the CCD camera horizontal and vertical to the Stewart platform.The results show that the feedforward control method based on Youla parameterization can significantly improve the high-precision tracking ability of the closed-loop system on the target,and the superiority of the integral method when tracking and the correctness of the decoupling Jacobi matrix are also verified.