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Hydro-Quebec's Research Institute has designed a robot to perform grinding tasks on underwater structures. This unique system is equipped with direct-drive linear motors, which have many useful dynamic characteristics. Since they lack intrinsic stiffness, however, their robustness to external disturbances must be achieved through the controller. Their lack of stiffness is a major disadvantage, because grinding generates very strong disturbance forces. Moreover, controller performance in such a system is limited by velocity feedback, which is usually derived from position encoder data. Though the state observer is recognized as an effective way to estimate velocity from position feedback without delay, it is not robust when applied to a system sensitive to external disturbances. The dual observer, which combines a state observer and a perturbation observer, aims to solve this problem. The simultaneous estimation of the state and disturbance not only improves state observer robustness, but also helps to compensate for disturbances in the controller. This paper presents the design of a discrete- time state-feedback controller with velocity estimation through a discrete- time dual observer. The design is validated by extensive comparative testing for a task that is as intensive as underwater grinding.
Date of Publication: Feb. 2012