Time Domain Control of a Single Mode Wave Energy Device Available to Purchase

This paper presents results of ongoing work on time-domain control of a wave energy converter in small-amplitude irregular waves. Two cases with control are investigated computer simulations" (i) where a control force cancels the time-invariant reactive terms in the device dynamics, and (ii) where an additional control force derived from predictions of device velocity attempts to match the time-varying radiation impedance of the device. An Auto- Regressive (AR) model is used in this case to predict the future oscillations using measurements of past velocity alone. Introduction It has been known for many years that active control of primary converter oscillations can lead to significant gains in the longterm productivity of a wave energy device in real seas (Salter, et al 1979; Budal and Falnes, 1980). One approach to such control is frequency domain "adjustable tuning" to increase the energy absorption in wave spectra other than those with peak frequencies close to the device resonant frequency. However, the full promise of active control cannot be realized without time-domain control. A number of authors have considered time-domain control methods (Hoskin, et al, 1986; Eidsmoen, 1995; Korde, 1999), while the proposed solutions have generally tended to be sub-optimal or near-optimal. This is in part because application of the correct control force on the primary converter at any instant requires an accurate prediction of the converter oscillations of the future (Naito and Nakamura, 1986). Given the irregular nature of the incident waves in a real sea, such prediction is very difficult. This paper examines an Auto-Regressive (AR) scheme (see, for instance, Box, et a11994) for predicting the velocity of a single-mode device over a short duration into the future. The predicted velocity is incorporated into the control force applied on the device at the current instant.