Vibration Rejection Of The Tip-tilt Mirror For Telescopes

The optical telescope has been widely used in astronomical observation and spaceoptical communication.However,when the telescope is tracking and imaging,it will be affected by the mechanical structure resonance,wind-shaking,platform vibration,equipment vibration and so on,which will lead to the visual axis jitter,affecting the tracking accuracy,so vibration rejection has become a key technology to improve the performance of the telescope.The application of Tip-Tilt mirror has greatly improved the stability accuracy of telescope system by an order of magnitude and is the key to obtain the core of micro-radian or sub-micro-radian.Therefore,the vibration rejection technology based on Tip-Tilt mirror has become an extremely important research content.Nevertheless,the existing vibration rejection methods are faced with the following three kinds of problems:1.The conventional closed-loop feedback control method based on proportional- integral controller has limited vibration rejection ability.Because the low sampling rate and high delay time of the image sensor makes the telescope has limited closed-loop bandwidth,the conventional feedback control method based on proportional-integral controller is difficult to achieve better vibration rejection effect.2.Using additional sensors(accelerometers,gyroscopes,etc)to measure vibration for realizing feedforward control and optimized control algorithms(LQG,H₂/ H_?,DOB and so on)have improved the vibration rejection ability of the telescope to a certain extent,but it also faces the problems of increasing system complexity or being affected by sensor noise,inaccurate disturbance estimation and inaccurate model identification.3.Disturbances are of many types and complex forms.In the telescope,the vibration is usually in the full frequency,and there may be not only narrow-band vibrations with large-magnitude and narrow-band,but also wide-band vibrations with wide-range and large-energy.To solve the above problems,combining Youla parameterization with disturbance observer,an improved Youla parameterized disturbance observer controller is proposed in this paper,which is based on the error signal,and can transform the vibration rejection problem into the design and parameter optimization of the Q-filter,so it not only simplifies the design difficulty,but also reduces the dependence of the vibration rejection ability of the system on the accurate system model.Then,based on the improved disturbance observer controller,the following three vibration rejection methods are proposed according to the different disturbance types:1.In view of the problem that the narrow-band vibration seriously affects the telescope performance,this paper proposes an effective narrow-band vibration rejection method based on the improved disturbance observer and the narrow- band dual filter.2.Aiming at the problem that wide-band disturbance with wide-range and large- energy in the telescope seriously affects its performance,a wide-band vibration rejection method based on the cascade optimization is proposed based on the narrowband vibration rejection method.3.Vibrations in the telescope may exist in any frequency band.Therefore,based on the enhanced disturbance observer,a full-frequency vibration rejection method combining low-pass filter and narrow-band dual filter is proposed in this paper to improve the full-frequency vibration rejection ability of the system.In addition,based on the theoretical and simulation analysis of the proposed three vibration rejection methods,an experimental verification system for Tip-Tilt mirror based on piezoelectric ceramics was built,and the effectiveness and superiority of the above three methods were verified.Finally,on the basis of summarizing the advantages and disadvantages of the research content,the following three directions for further research are pointed out in this paper:1.Research on vibration rejection methods beyond the bandwidth.The vibration rejection methods studied in this paper are all carried out within the bandwidth of the system.However,the bandwidth of the telescope is limited,so whether vibration rejection methods beyond the bandwidth can be studied on the basis of this study has become a research direction worthy of attention.2.Automatic adjustment and optimization of Q-filter parameters.The parameters of the Q-filter of the vibration rejection method used in this paper are designed for the acquired specific vibration.However,the vibration in the actual system is diverse and easy to change,so further research is needed.Then one of the key points of the next work plan is to automatically adjust and optimize the parameters of Q-filter according to the characteristics of vibration information on the basis of real-time acquisition of vibration information.3.Promote the method to more control systems.Because vibration rejection is a common technical challenge faced by many control systems.The vibration rejection method proposed in this paper requires only a deviation sensor(image sensor)and has low model dependence,so it is easy to design and optimize. Therefore,this method has a certain universality.