Observer design with disturbance rejection by acceleration feedforward

This paper addresses observer design of mechanical systems subject to unknown external disturbances and unmodeled dynamics. For fast-acting disturbance forces, realistic and accurate physical models do not always exist, such as for dynamic positioning of marine vessels in ice-infested waters. To accommodate such forces and provide disturbance rejection, a delayed acceleration measurement is used to form an acceleration feedforward which captures the perturbations and enhances the observer phase margins. Through the acceleration measurement, an inherent sensor bias is introduced, creating a pseudo-force in the system. This is tracked and compensates by a bias estimate. The theoretical foundation is exemplified by the derivation of a DP observer and a simulation study featuring an inverted pendulum.