A Self-Tuning WEC Controller for Changing Sea States

Abstract

A self-tuning proportional-integral control law prescribing
motor torques was tested in experiment on a three
degree-of-freedom wave energy converter. The control objective
was to maximize electrical power. The control law relied upon
an identified model of device intrinsic impedance to generate a
frequency-domain estimate of the wave-induced excitation force
and measurements of device velocities. The control law was tested
in irregular sea-states that evolved over hours (a rapid, but
realistic time-scale) and that changed instantly (an unrealistic
scenario to evaluate controller response). For both cases, the
controller converges to gains that closely approximate the postcalculated
optimal gains for all degrees of freedom in a sufficiently
short-time for realistic sea states. In addition, electrical
power was found to be relatively insensitive to gain tuning over
a broad range of gains, implying that an imperfectly tuned
controller does not result in a large penalty to electrical power
capture. Because the controller relies on an identified model of
device intrinsic impedance, the sensitivity of power capture was
evaluated with respect to uncertainty in the constituent terms
of intrinsic impedance. Power capture is found to be relatively
insensitive to uncertainty of 20% in constituent terms of the
identified intrinsic impedance model. An extension of this control
law that allows for adaptation to a changing device impedance
model over time is proposed for long-term deployments, as
well as an approach to explicitly handle constraints within this
architecture.