Abstract
CCell is a type of oscillating surge wave energy converter (OSWEC) which uses a hydraulic power take-off (PTO). Numerical and experimental studies have demonstrated that an effective PTO control strategy is necessary to achieve good efficiency. The WEC and PTO systems are modelled in a computationally efficient form for the study of feedback control strategies. Nonlinear effects of combined Coulomb, Stribeck and viscous friction are included, along with angular position end stops, PTO torque limits and unidirectional power flow. The control law is based upon the Simple and Effective method, which provides a velocity reference signal designed to keep the WEC velocity in phase with the wave excitation whilst adhering to physical angular position constraints. Two methods are applied for the velocity tracking feedback control component. A fixed gain controller using the State Feedback with Integral Action (SFCIA) architecture is compared to the adaptive Minimal Controller Synthesis (MCS) algorithm. The SFCIA strategy relies on plant estimates while MCS requires no a priori knowledge of the plant dynamics and relies on empirical tuning of a single adaptive effort parameter. Both controllers perform well when the system is excited by a typical irregular sea state, even when faced with plant nonlinearity. The MCS method may be preferable in the face of nonlinearity as it results in a less noisy control signal and is able to rapidly adapt to changing system dynamics. A further practical benefit of MCS is the ease with which it can be applied and tuned, even with no knowledge of the plant dynamics.