Self-Tuning WEC Controller for Changing Sea States

Abstract

A self-tuning proportional-integral control law prescribingmotor torques was tested in experiment on a threedegree-of-freedom wave energy converter. The control objectivewas to maximize electrical power. The control law relied uponan identified model of device intrinsic impedance to generate afrequency-domain estimate of the wave-induced excitation forceand measurements of device velocities. The control law was testedin irregular sea-states that evolved over hours (a rapid, butrealistic time-scale) and that changed instantly (an unrealisticscenario to evaluate controller response). For both cases, thecontroller converges to gains that closely approximate the postcalculatedoptimal gains for all degrees of freedom in a sufficientlyshort-time for realistic sea states. In addition, electricalpower was found to be relatively insensitive to gain tuning overa broad range of gains, implying that an imperfectly tunedcontroller does not result in a large penalty to electrical powercapture. Because the controller relies on an identified model ofdevice intrinsic impedance, the sensitivity of power capture wasevaluated with respect to uncertainty in the constituent termsof intrinsic impedance. Power capture is found to be relativelyinsensitive to uncertainty of 20% in constituent terms of theidentified intrinsic impedance model. An extension of this controllaw that allows for adaptation to a changing device impedancemodel over time is proposed for long-term deployments, aswell as an approach to explicitly handle constraints within thisarchitecture.