Research On Stabilization And Tracking Technology Of Airborne Star Tracker

The star tracker is mainly used in the aircraft’s inertial navigation / astronomical all-weather combined navigation system.Its function is to compensate the cumulative error of the inertial navigation by calculating the relative positional relationship between the aircraft and the stars,and relying on the binary or multi-star switching.Since the attitude of the aircraft changes continuously during flight and the fuselage will be shaken due to engine and airflow,the star tracker needs to have the ability of carrier disturbance isolation to ensure the quality of the star map and complete the continuous tracking of the target star.Therefore,from a functional point of view,the star tracker can be regarded as a photoelectric stable platform that needs to constantly switch tracking targets.The tracking performance of the star tracker is determined by the boresight stability performance and target tracking performance,and the efficiency of switching and searching the target depends on the boresight stability performance and position control accuracy.This paper will conduct in-depth research on stabilization of the star tracker’s LOS,position control and target tracking,and the performance of star tracker system is improved by improving the control structure..Firstly,the system composition and structure of the star tracker are introduced,and on this basis,the disturbance coupling principle and the relationship between the relative position of the target and the miss distance are analyzed to provide theoretical support for the use of related sensors.According to the functional requirements of the star tracker,the double closed-loop control loops for image stabilization and target switching are proposed,which lays the foundation for the control structure optimization in the following paper.In terms of Los stability,two design schemes are proposed in this paper:1)In this paper,the principle of indirect stabilization scheme used in star tracker is described,and three main problems to be solved in indirect stabilization control are proposed: matched filter,differential velocity measurement noise and disturbance rejection.Then the scheme of active disturbance rejection control(ADRC)with lowpass filter(LPF)is proposed,and the simulation shows that this scheme will reduce the system robustness,which will not only deteriorate the tracking performance of the system but also limit the increase of control gain.In order to prevent the loss of robustness,a noise observer(NOB)based on disturbance observation principle is proposed and combined with ADRC.NOB has three characteristics: firstly,NOB does not need precise model object and can be tuned by parameter tuning;secondly,the filtering function of the NOB can be equivalent to a LPF,so it can replace the LPF to complete the task of high-frequency noise filtering and matching filtering;thirdly,adding the NOB in the system will not reduce the robustness of the system.With the use of ADRC with NOB,the stabilization accuracy of the star tracker has been improved by about 1 time,and the tracking performance has been improved by about 40%.2)Since adding NOB will reduce the disturbance rejection ability of ADRC,and the algorithm complexity is high,a robust reduced order ADRC scheme is proposed.Robust reduced order ADRC is composed of reduced order ADRC and improved noise reduced disturbance observer(INRDOB).The former is mainly used to improve the disturbance rejection ability of ADRC,while the latter is mainly used to improve the robustness and noise rejection ability of the reduced order ADRC,and to complete the matching filtering function at the same time.In addition,the mechanism of improving the robustness of the reduced order system by NRDOB is illustrated by theoretical analysis.Compared with the ADRC with NOB and the classical ADRC,the Los stabilization accuracy of the robust reduced order ADRC is improved by more than one time.To solve the problem that star tracker needs to switch and search target efficiently,an active disturbance rejection position control scheme with sliding mode component is proposed.Firstly,to solve the problem of large disturbance estimation error of extended state observer(ESO)for time-varying disturbance,an improved method based on the total disturbance estimation differential is proposed,which improves the estimation accuracy of low-frequency disturbance by about 12 dB.In order to solve the problem of disturbance rejection and robustness loss caused by the model parameter perturbation and modeling error,the feedback in control law with sliding mode component is proposed,and the time-varying sliding gain is designed by using the differential of disturbance estimation instead of disturbance estimation to avoid the system chattering caused by large disturbance.The experimental results show that the improved ESO has stronger disturbance estimation ability,and the designed sliding mode component is less likely to cause chattering compared with the previous design.After adding sliding mode component,the robustness of the system is significantly improved,the disturbance suppression ability and sinusoidal tracking accuracy are increased by about 1 time,and the commutation error is significantly reduced.Aiming at the problem of disturbance suppression and time delay of off-target quantity in the tracking circuit of star tracker,a tracking control scheme based on predicted-structure ESO is proposed.On the premise of summarizing the state prediction algorithms proposed by the predecessors,and according to the extended state model of system,an extend state prediction algorithm is proposed,which is combined with the ESO to form the predicted-structure ESO.Theoretical analysis and experimental results show that the extended state prediction algorithm can make the system state converge within a smaller error bound,and the predicted-structure ESO can effectively improve the tracking accuracy.