Linear and nonlinear control of a power take-off simulation for wave energy conversion.

This article focuses on control of the power take off (PTO) element of a point absorber wave energy converter. The research is based on a nonlinear simulation of a PTO hydraulic circuit, in which the piston velocity and generator torque act as `disturbance' and control actuator variables respectively, whilst the damping force is the controlled output variable. The piston velocity is generated by a hydrodynamic simulation model that reacts to both the damping force and sea wave profile. The damping force set point will be obtained from an associated power capture optimisation module and may be time varying. However, it is clear that such an adaptive tuning system also requires high performance `low-level' control of the device actuators, in order to fully realise the benefits of optimisation. In this regard, the present article illustrates use of the Proportional-Integral-Plus (PIP) control methodology as applied to the PTO simulation. In their simplest linear form, such PIP controllers do not account for the interconnected system variables mentioned above. For this reason, the research also considers `feed-forward' and `state-dependent' forms of PIP control, in which the piston velocity is appended to a non-minimal state space representation of the system