Nonlinear Optimal Control: Concepts, Practicalities and Benefits

Overview

The low-level controllers within a wave energy device regulate the transfer of energy from the wave induced motions of the Wave Energy Converter (WEC), through the Power Take-Off (PTO) subsystems and ultimately onto the electrical grid. The performance of the control is therefore central to extracting as much of the available power as possible over the widest range of sea-states, and at the same time ensuring long term survivability in extreme conditions.

Linear control, whether it is classical control or even linear optimal control has been the mainstay for WEC control strategies. However many of the control challenges are inherently nonlinear which have tended to be addressed in ad-hoc ways resulting in sub-optimality, control system that are laborious to develop and not readily transferrable, and potentially problematic operations if not fully understood or evaluated.

This project demonstrates the feasibility of using nonlinear optimal control methods that have been gaining maturity in other industries (automotive, wind) for wave energy devices. A simplified WEC/PTO simulation was used to evaluate and quantify the potential of nonlinear optimal control and its modelling and sensing requirements. Consideration has been given to how nonlinear control can be extended to the broader range of control challenges, such as optimising fatigue, utilising local energy storage, incorporating incoming wave and sea-state estimates to develop useful control concepts.

Real world wave energy devices have non-ideal characteristics that limit their performance and this project provides a roadmap for the successful adoption of nonlinear control to address those current limitations.

ISC also presented a poster on their Stage 1 Control Systems project at the 2017 WES Annual Conference.