Research On Cascade Multi-loop Control And Vibration Suppression Of Tip-Tilt Mirror

Tip-Tilt mirrors adopt linear motors(voice coil motor,piezoelectric ceramic)with high precision and fast response,and the elastic support mode,which can produce the closed-loop performance with high bandwidth and high resolution.Therefore,Tip-Tilt mirrors play an important role in photoelectric tracking system requiring the tracking performance at the level of microradian or sub-microradian.In the compound axis photoelectric tracking system,the functions of Tip-Tilt mirror system are to correct the coarse tracking residual and restrain the disturbance of the platform.Considering the detection accuracy and target recognition,image sensors(CCD,CMOS)are usually used as precision tracking detectors.The time delay of the control system is caused by the integral imaging and image processing of the image sensor,which limits the closedloop bandwidth of the Tip-Tilt mirror system.Especially,this issue occurs under the condition of vibrations,because dynamic behaviors of vibrations are complex and the models are difficult to be obtained accurately.Another challenging issue comes from the measurement of vibrations and its extraction for the closed-loop control.Therefore,the control method of Tip-Tilt mirror is particularly important.For an image-based TipTilt mirror tracking control system,the most effective method enhances the tracking performance is to increase the control gain for a high bandwidth.However,the time delay of the Tip-Tilt mirror control system will restrict the control system bandwidth,which will limit the closed-loop performance of the control system.The time delay in the system mainly includes two aspects: the first is the image sensor sampling frequency,because image sensor imaging needs a large number of integration time,and the other is the time required for data transmission.However,it is difficult to reduce the time delay of the tracking loop,because too short integration time will affect the image quality,and the time required to process and transmit data is difficult to be reduced.Further,analysis of the characteristics of Tip-Tilt mirrors show that Tip-Tilt mirror is a 0-type system.The traditional control method uses the position sensor such as eddy current sensor to form position internal loop.The bandwidth of the controlled system is expanded and the characteristic of the controlled system is improved.The expanded bandwidth to facilitate the design of the tracking loop controller.Although the traditional control method can add an integral function to the tracking loop,the type of the Tip-Tilt mirror control system is upgraded to 1-type.The system always has deviation in tracking rate signal,which limits the tracking accuracy of the Tip-Tilt mirror.Therefore,a rate-position cascade multi loop control method is proposed to improve the tracking accuracy and tracking accuracy for the Tip-Tilt mirror system.For such a 0-type system,adding a rate loop can theoretically improve the type of the controlled system from 0-type to 1type,and the system can be upgraded to 2-type after the tracking loop completes closed-loop.In this paper,the linear encoder is used as the inner loop sensor,which has high precision and resolution.It can not only measure the position directly,but also get the rate signal by the difference of the position signal.The rate open-loop transfer function of the Tip-Tilt mirror features differential characteristics.When the rate inner loop is implemented,an integrator is introduced into the original position loop.A PI(Proportional-Integral)controller can stabilize the position loop,leading to two integrators to be in the tracking loop.Thus,the low-frequency performance can be improved compared to the original control method.To further improve the vibration suppression ability of the control system,feedforward control is introduced on the basis of cascaded multi-loop control.The feedforward method used is rate feedforward.The stability of the system will not be affected by the feedforward path.The stability of the system will not be affected by the feedforward path,and the vibration rejection ability can be improved by the feedforward path.The proposed method is analyzed in theory and simulation,and then an experimental platform is built for verification.Compared with different control methods,the simulation analysis and experimental results show that the speed position loop control method can effectively improve the system's error suppression ability in the low-frequency band to more than 6dB;the simulation results show that the feedforward control can improve the system's disturbance suppression ability to more than 19 dB.