Constrained sliding mode control for oscillating water column wave energy converters

Maintaining a setpoint to maximize energy harvest of wave energy converters (WECs) with the uncertainties induced by the ocean waves irregular excitation can be ensured by using sliding mode control. In practice, the infinite switching frequency in the sliding mode is limited by the actuators bandwidth. This work compares multiple second-order sliding mode controllers (SMC) designed for an array of floating oscillating water column (OWC) WECs with varying order of discontinuity. All algorithms enable bounding the control input to respect the physical constraints, e.g. maximal torque introduced by the generators attached to the bi-radial turbines that are driven by the oscillating motion of the air trapped inside the OWC chamber. Practical implementation of the algorithms is facilitated by a smooth approximation of the signum function and a smooth switching between different cases or hysteresis and compared to the ideal switching. The performance of the presented control laws is evaluated while maintaining a constant turbine rotational speed inside the floating OWC WEC array, and the generated power is compared to an ideal control law for different irregular sea states. Copyright (C) 2020 The Authors.