Research On Active Vibration Isolation And Precision Pointing Control For Spacecraft

Multi degree-of-freedom(DOF) active vibration isolation and precision track-ing/pointing can be widely used in advanced spacecrafts, such as space telescopes, de-fense satellites and space laser communication. Compared with traditional passive isola-tion, active isolation can efficiently suppress random and sinusoid vibrations at low fre-quencies. Moreover, the accurate micro-steering of sensitive payload can also be providedby multi-DOF platform consisting of smart actuators, which overcomes the drawback ofattitude control of spacecraft body. With fast developing of astronautic technologies, itis difficult to satisfy strict requirements in dynamic environment only using current tech-niques of vibration isolation, and their robustness is not sufficient. Moreover, the accuratemodeling and control of smart actuators also needs to develop. In this thesis, multi-DOFactive vibration isolation and precision pointing of sensitive payloads are studied.The analysis indicates that hybrid vibration isolation can provide zero and infinitestiffness. However, this is equivalent to a cancellation approach in control, which needsaccurate value of parameters and its robustness is not adequate. The isolation performanceof random vibrations using PID control for hybrid isolator is not satisfied. Then, withacceleration feedback, multi-objective robust H∞control is designed using loop shapingand weighting function techniques, which can isolate sinusoid and random vibrations withbetter performance. When the parameters of plant are unknown, model reference adaptivecontrol (MRAC) is designed using polynomial method, which has better performance androbustness. On the other hand, the displacements at low frequencies are amplified. Then,with additional displacement feedback, the MRAC-PID composite control suppresses thedisplacements at low frequencies and tracks the reference signal, simultaneously.The hysteresis in smart actuators is one nonlinearity to consider while designing mo-tion controller. The memory characteristics of Preisach hysteresis in smart actuators areinvestigated. Based on singular value decomposition (SVD), the parameter of Preisachhysteresis is identified using least squares method, and its parameter convergence is sat-isfied. However, the computing is time consuming, which is not suitable for realtimeestimation. Thus, SVD online revision is presented, whose initial value is the off-lineidentification. The parameter estimation is computed in realtime, and better accuracy is provided. With the identification result, inverse Preisach feedforward is designed tolinearize the Preisach hysteresis. Moreover, this method is generalized to linear dynam-ics with input Preisach hysteresis. Without the hysteresis measurement, an equivalentPreisach hysteresis is identified using low frequency signal. Then, linear dynamics is alsoidentified. Finally, the inverse Preisach feedforward is used to compensate the hysteresisand the tracking performance is improved.Tip/tilt angular vibration of the payload is adequately suppressed to stabilize laserusing 4-leg platform and MRAC-PID composite control. The dynamic model of Stewartplatform is derived. Then, Multi-input-multi-output (MIMO) robust H∞controller is de-signed. In order to obtain precision pointing and vibration suppression at low frequencies,H∞-PID composite control is designed. Moreover, the MRAC-PID control in chapter 2also obtains similar performance, but it needs more computing ability and high bandwidthcontrol. Image distortion and blur radii due to mechanical vibrations and attitude jitter isalso investigated, and the simulation results indicate that the image quality is improvedby 6-DOF active vibration isolation.The problem of spacecraft precision pointing is investigated, indicating that sensornoise and disturbance suppression needs to be trade-off while designing controller. Eulerangular is used for small angular pointing. The model matching robust H∞controlleris designed for a rigid satellite. However, there is attitude jitter due to measurementnoise. Then, the dynamics of orbit, attitude and ?exible structure of ?exible spacecraftare derived and they couple each other. The attitude control with state and output feedbackis designed for large angular maneuvering. In order to reduce the noise affection and useoutput feedback, an composite differentiator is also designed. To address the problemof attitude jitter, two-stage attitude control is designed. One Stewart platform is used tolocally approach the function of 6-DOF vibration isolation and accurate attitude-steering.Finally, the attitude jitter suppression and fine pointing is obtained.