Research On Image Stabilization Technology Of Diffraction Segmented Telescope

Diffraction segmented telescope is an important development direction to implement ultra-large aperture space telescope.In space,the perturbation generated by the spacecraft propagating to the telescope system through the flexible structure will lead to image jitter and reduce the imaging quality of the system,which brings serious challenges to the high-performance imaging of the diffraction segmented telescope.Therefore,it is necessary to take appropriate image stabilization control methods to reject disturbances,and the image stabilization accuracy directly determines the final imaging performance.Aiming at the problem that the existing image stabilization control methods is limited by the system time delays caused by image sensors,this dissertation studies a set of disturbance rejection methodologies within the Nyquist sampling frequency in the image stabilization control system,and the concrete content is as follows:1.This dissertation investigates the system composition of the diffraction segmented telescope prototype,and analyzes its basic imaging principle.On this basis,this dissertation discusses the source and characteristics of the disturbance in space applications.And the influence of the image jitter caused by the disturbance on the imaging of diffractive segmented telescope is analyzed,including the whole image jitter and sub-mirror jitter.Therefore,a dual-stage control structure is proposed to realize the basic image stabilization of the system.The first stage stabilization is applied to reject the overall image jitter,and the second stage stabilization is applied to reject the residual error of the first stage stabilization and realize the fine stabilization of each sub-mirror.Finally,the image stabilization performance of the dual-stage control is analyzed,and it is deduced that the improvement of the disturbance rejection capacity of the image stabilization system should be carried out under limited closed-loop conditions.2.Aiming at the problem that the time delay limits the disturbance rejection performance of the image stabilization system,this dissertation proposes an error-based Y-K(Youla–Ku(?)era)parameterization control method.Based on the analysis of the closed-loop control performance of the Y-K parameterization in the image stabilization system,it is concluded that the disturbance rejection can be achieved by optimizing the sensitivity function of the control system.The optimal design conditions of rejecting wideband disturbance and narrowband disturbance in low-frequency range are derived,and the effectiveness of the proposed Y-K parameterization method of disturbance rejection in low-frequency range is verified by simulation and experiment.3.To reject the time-varying unknown disturbance in the image stabilization system,this dissertation proposes an adaptive disturbance rejection method based on online closed-loop error estimation.The dissertation compares the classical adaptive control methods,and determinate the indirect adaptive control idea through parameter estimation to realize the adaptive Y-K parameterization,where the parameter update of the controller is determined by the disturbance characteristics.The dissertation analyzes and compares the classical disturbance characteristic estimation methods,and chooses to obtain the disturbance characteristic information through the power spectral density estimation of the online updating closed-loop error sequence.To ensure the timeliness and accuracy of disturbance characteristic estimation,this dissertation analyzes the influencing factors of online power spectral density estimation,and designs an optimal estimation method by searching the maximum amplitude in the power spectral density.Simulation analysis and experimental verification prove the effectiveness of the proposed adaptive disturbance rejection method.4.To reject the mid-/high-frequency disturbance in the image stabilization system,this dissertation proposes a control method based on extended Y-K parameterization.The reason why the basic Y-K parameterization hards to achieve medium and high frequency disturbance rejection is analyzed.The adverse effect of time delays on the characteristic polynomial of the closed-loop transfer function in the Y-K parameterization results in the stability condition of the image stabilization system being unable to guarantee at most frequencies in the mid-/high-frequency range.According to the closed-loop stability condition,the basic Y-K parameterization is extended,and the extended gain factor is derived to meet the basic stability conditions of the system.The optimal stability condition is further derived by the extended time-delay factor.Finally,the effectiveness of the proposed extended Y-K parameterization method for mid-/high-frequency disturbance rejection is verified by simulation analysis and experiments.5.To reject the multi-frequency disturbance in the image stabilization system,this dissertation proposes a parallel structure-based control method.To avoid the adverse effect of time delays on the poles of closed-loop transfer function in the Y-K parameterization,an improved decomposition method is derived.Moreover,aiming at the adverse effect of time delays on optimizing sensitivity function,this dissertation discusses the characteristics of the time-delay function.Due to the physical infeasibility of the lead time-delay compensation,according to the periodic characteristics of the time-delay function,this dissertation proposes a method to achieve time-delay compensation by adding an extra time-delay factor.On the basis of time-delay compensation,the optimized controller design method for multi-frequency disturbance rejection is analyzed.To avoid the problem that the controller order of series design is too high and time delays cannot be compensated,a controller design method based on parallel structure is derived in this dissertation.The simulation analysis and experimental verification show that the proposed control method based on parallel structure can effectively improve the multi-frequency disturbance rejection capacity.In summary,this dissertation figures out the problem of disturbance rejection in the dual-stage control system under the condition of limited image closed-loop bandwidth for a diffraction segmented telescope,and achieves high-precision image stabilization.The specific solution idea is to propose corresponding controller optimization design for different characteristic disturbances based on the Y-K parameterization control structure to achieve complex disturbance rejection up to the Nyquist frequency of image sensors.