Uncertainty of motion tracking system used in a floating wave energy converter model study.

Abstract

Blue Economy CRC deployed a four-float M4 (Moored Multi-Mode Multibody) device to demonstrate the potential of wave energy converters (WECs) for the Australian coast. An experimental study on a 1:15 scale model of the same device was performed in the Model Test Basin of the Australian Maritime College, investigating performance and behaviour, to aid in minimising the risks associated with open sea deployment. In model testing, the uncertainties of instruments and measurements directly affect the reliability of the results. Uncertainty analysis (UA) of WEC model studies is particularly important as it involves complex geometries, large motions and non-linear interactions. During the M4 experiment, the Qualisys motion tracking system was used to evaluate the hinge motion of the device, which is directly associated with the power capture. This paper discusses the details of an experimental investigation to precisely evaluate the uncertainties of this Qualisys motion tracking system. A calibration rig was designed to give known accurate motions to Qualisys marker arrangements, same as in the M4 model. The deviation in motion measured by Qualisys system was evaluated to better estimate the uncertainties of the system. It was noticed that there was a phase lag in other instruments with respect to Qualisys, which was unexpected. This phase lag can cause uncertainties, especially for WEC studies when measurements from different instruments are used in calculating power and this phase difference needs to be considered in the UA of the M4 model study. The conclusions from this study can be used in UA and future WEC model studies, improving the quality of the experiments.