Mitigating Force Oscillations in a Wave Energy Converter Using Control Barrier Functions

Abstract

Control barrier functions (CBFs) is a novel feedback control strategy for enforcing safety constraints of mechanical systems. An appealing feature of the CBF method is that the safety objective is defined and enforced independent of the underlying control objective. This enables the merging of CBF-based control with any existing nominal control strategy, by imposing the safety objective as input constraints in a convex optimization problem. CBFs are gaining popularity in the robotics community, in particular for motion control of autonomous vehicles. Yet, limited use of CBFs for mechanical devices such as wave energy converters (WECs) are reported in literature. This paper motivates the use of CBF-based control for constraint satisfaction of WECs, using the Bolt Lifesaver point absorber WEC developed by Fred. Olsen Ltd. as a case study. During initial sea trials of Bolt Lifesaver, large force oscillations were observed in the power take-off unit. The source of oscillations was identified as sudden saturation of the actuator force provided by the generator. Mitigating the undesired response using conventional feedback control is non-trivial, since any such control strategy will attempt to cancel inertia forces, resulting in a reduced stability margin of the system. Using higher order CBF theory, we design a robust controller that ensures safe operation of the device, while minimally interfering with the existing control law optimized for power output. The theoretical results are verified by numerical simulations.