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State dependent feed-forward control for wave energy conversion

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This paper considers the control of a hydraulic power take-off (PTO) simulation, forming part of a wave energy converter model. The resonant frequency of the device is tuned to the dominant frequency of the incoming wave spectrum by adjusting the PTO damping force, considered the controlled variable. The torque supplied by a generator acts as the control input, with the piston driving the PTO representing a disturbance. Adaptive tuning strategies require high performance actuator control to realise the benefits of optimisation; the work presented here utilises proportional-integral-plus (PIP) control. In their simplest form, PIP controllers cannot account for the interconnected system variables present in the PTO simulation, resulting in relatively poor performance. Therefore, previous research has considered several feed-forward forms of PIP control, in which disturbance is utilised as a measured variable. Although feed-forward controllers offer improved disturbance rejection over standard PIP controllers, when the disturbance signal is sufficiently far from the operating level upon which the controller is based, the response to disturbances deteriorates. For this reason, the present article develops a non-linear feed-forward filter, the gains of which are updated at each sample, resulting in almost complete disturbance rejection.