Application of Anti-Windup (AW) techniques to the control of Wave Energy Converters (WEC)

This thesis considers control system enhancement for Wave Energy Converters (WECs), of the point-absorber type, used for water desalination. The thesis makes several contributions. Firstly, it is shown that a type of nonlinear control system previously used in the literature provides global stability guarantees for this type of WEC in the absence of input constraints. Following this, several anti-windup techniques for a certain class of nonlinear systems with input constraints are developed; a nonlinear Internal Model Control (IMC) compensator, a linear reduced-order compensator and a linear sub-optimal performance compensator. It is shown how these anti-windup strategies are natural generalisations of those found elsewhere in the literature and how all of these compensators can be designed such that global exponential stability of the class of systems considered is guaranteed. Finally, the thesis describes the application of these anti-windup techniques to a nonlinear simulation model of a WEC system where their benefits are clearly demonstrated. It is shown that these compensators improve the performance of the WEC during periods of saturation and, moreover, that the sub-optimal compensator can achieve desirable tracking without causing any damage to the desalination equipment. These results demonstrate the benefit of anti-windup for WEC control and imply potential savings in terms of operation and maintenance costs, thereby contributing to the potential commercialisation of such devices.