Disturbance attenuation in precise hexapod pointing using positive force feedback

Negative feedback control improves system performance by high loop gain. However, it is well known that the achievable performance is limited theoretically by right half complex plane (RHP) poles, RHP zeros, time delays, etc. of the plant models, and practically by the input constraints as well as model uncertainties. Though positive feedback has the potential to improve the system performance by direct subtraction of the measured external disturbances from the plant inputs, it is notorious for its sensitivity to measurement errors and lack of robustness. Inappropriate use of positive feedback control could deteriorate the performance, or even result in instability. This paper presents a new combined control strategy for two degrees-of-freedom precise hexapod pointing control, in which positive force feedback (PFF) is used to reject payload disturbances, and extra acceleration feedback loops are added to enhance the PFF loop robustness as well as decrease its sensitivity. Also, a new method for avoiding destructive interference in parallel controller design using a sequential loop closure technique is proposed. Since the payload disturbances cannot be measured directly due to sensor mounting difficulties, an estimate is constructed based on the hexapod model and used for PFF. The algorithm is implemented on one of the University of Wyoming (UW) hexapods, and experimental results demonstrate that pointing errors caused by the payload disturbances are decreased, despite residual coupling and disturbance estimation errors. (c) 2005 Elsevier Ltd. All rights reserved.