Recent research on wave generation utilizing absorbing forcecontrolled machines has shown three key factors:a theoretical transfer function between demand signal and surface elevation can be derived;
high absorption efficiency can be achieved over a wide range of frequencies and
little second-order spurious content is introduced when driven with a first-order demand signal.
Thus far, both theoretical and experimental work has been limited to the generation of regular wave trains and hinged wave board geometries. The present study extends this work to the generation of irregular waves considering both flapand piston-type wave machines. Whilst limiting the discussion to the effects arising at first-order, specifically addressing the theoretical transfer function and absorption efficiency, an enhanced understanding of the machine's controller is sought. The implemented absorption strategy is based on controlling the complex relationship between applied force and wave board velocity. The properties of the optimum a-causal controller for this particular strategy are investigated and a causal, hence practical, approximation is derived. A causal controller based on infinite impulse response filters (IIR) and direct optimization in the frequency domain is considered. The relative merits of this approach are compared to methods based on an approximation in the time domain. Experimental evidence is also provided to substantiate the IIR modelling approach. The results are directly relevant to the operation of many installed force-controlled wave machines, provide guidance as to the effective operation of others, contribute to the wave power debate and could be incorporated within advanced numerical wave tanks to provide simultaneous generation and absorption.
INTRODUCTION Over the last three decades much effort has been made to formulate the problem of optimum control for wave energy devices. The techniques developed in this field are also adopted to describe the ideal controller for absorbing wave makers in laboratory flumes.