Robust Active Vibration Control of Flexible Stewart Isolators

Abstract

There are increasing needs of precision pointing and extreme stability for current and future spacecrafts. The James Webb space telescope, terrestrial planet finder, space based laser, space-based interferometer and deep-space laser communication are such examples where the micro-radian pointing and nanometer level of motion stability are required by (Ford, et al, 2005), (Chen, et al, 2004) and (Winthrop, et al 2003). On the other hand, the space systems may contains many vibration sources. A satellite may contain multiple instruments; some of them may use reaction wheels, cryogenic coolers, control moment gyroscopes, solar array drives, stepper motors, and other motion devices. These devices will transmit vibrations. Passive isolation presents a reliable, low cost solution that is effective for attenuating high frequency vibrations, but it is in general not suitable for low frequency vibration isolation, and especially, passive isolation can not provide good trade-off between resonant peak and high frequency attenuation and the trade-off between pointing command keeping and disturbance rejection. (Winthrop, et al 2003) indicates the active vibration control can overcome these limitations. In order to achieve multi-DOF vibration isolation in broadband and precision pointing, the Stewart platform (or hexapod), especially the cubic one, has become one of the most popular approaches as in (Anderson, et al, 2000) and (Thayer, et al, 2002), as shown in Fig.1. The cubic hexapod simplifies the control topologies to allow the decoupled controller designs to be identical for each strut. In order to eliminate the micro dynamics (friction and backlash), flexure joints are generally used as in (Hanieh, 2003). Jet Propulsion Laboratory, Air Force Research Laboratory, Naval Postgraduate School, University of Washington, Free University of Brussels, University of Wyoming, CSA Engineering Inc are very active in this research. Classic control, adaptive control, LQG control, neural control, simple robust control and other control approaches were studied by (Gawronski, et al, 2004), (Joshi, et al, 2005) and (Liu et al, 2008). In this chapter, H∞ and μ controllers are designed for the struts of Stewart platforms, suppressing the overshoot in the neighborhood of resonance frequencies. Then the dynamic model of Stewart isolator is derived, and D-K iteration is used to solve the robust controller, finally, the time domain responses to suppress disturbance are also presented.