| This dissertation investigates the use of Positive Force Feedback (PFF) in precise hexapod pointing. It is well known that (negative) feedback control performance is limited theoretically by RHP (right half complex plane) poles, RHP zeros, time delays, etc. of the plant transfer functions, and practically by the (actuator) input constraints as well as model uncertainties. PFF can compensate for the detrimental effects of load dynamics, some nonlinearities such as Coriolis terms, model uncertainties and exogenous force/torque, by subtracting the disturbance forces at the plant inputs. With real world non-ideal force/torque actuators, a PFF controller can be designed to accommodate the actuator's dynamics. This dissertation develops new methods to make PFF an effective method for some practical control problems.; When accurate measurement of the disturbances is available, it is shown that the analysis and design of motion control systems with PFF can be put into the mu framework, and thus the tracking controller and the force/torque feedback controller can be synthesized simultaneously using commercially available mu tools. Some practical issues, which are independent of design methods, such as the robust stability of the PFF loop and the bandwidth requirements for the PFF controller, are also discussed. It is concluded that, outside the system's effective tracking bandwidth, it is not wise to use model-based "perfect" PFF for a "big" load due to the lack of robustness of the PFF loop against the actuator uncertainties. PFF can be effectively used inside the tracking bandwidth to enhance performance.; Two degree-of-freedom precise pointing control algorithms are aimed at tracking while suppressing disturbances. Though positive feedback has the potential to improve the system performance by direct subtraction at the plant inputs, it is sensitive to measurement errors and lacks robustness to model uncertainties. This means that if the disturbances cannot be measured accurately, direct feedforward of the inaccurate information could deteriorate the performance, or even result in instability. This dissertation presents a new combined control strategy for two degree-of-freedom precise hexapod pointing control, in which PFF is used to attenuate 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 feedback system 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's (UW) hexapods, and experimental results demonstrate that pointing errors caused by the payload disturbances are decreased, despite residual coupling and disturbance estimation errors. |
