A Set-Up of 7 Laser Triangulation Sensors and a Draw-Wire Sensor for Measuring Relative Displacement of a Piston Rod Mechanical Lead-Through Transmission in an Offshore Wave Energy Converter on the Ocean Floor
A concept for offshore wave energy conversion is being developed at the Swedish Centre for Renewable Electric Energy Conversion at Uppsala University in Sweden. The wave energy converter (WEC) in focus contains a piston rod mechanical lead-through transmission for transmitting the absorbed mechanical wave energy through the generator capsule wall while preventing seawater from entering the capsule. A set-up of 7 laser triangulation sensors has been installed inside the WEC to measure relative displacement of the piston rod and its corresponding seal housing. A draw-wire sensor has also been set up to measure translator position and the axial displacement of the piston rod. The paper gives a brief introduction to the Lysekil research site, the WEC concept, and the direct drive of WEC prototype L2. A model of operation for the piston rod mechanical lead-through transmission is given. The paper presents sensor choice, configuration, adaptation, mounting, and measurement system calibration along with a description of the data acquisition system. Results from 60？s measurements of nominal operation two months apart with centered moving averages are presented. Uncertainty and error estimations with statistical analyses and signal-to-noise ratios are presented. Conclusions are drawn on the relative motions of the piston rod and the seal housing under normal operating conditions, and an assessment of the applicability of the measurement system is made. 1. Introduction A number of wave energy converter (WEC) technologies are in development around the world [1–6]. The Swedish Centre for Renewable Electric Energy Conversion at Uppsala University, Sweden, has for the past decade been working on a WEC characterized as point absorber with a surface-floating buoy and an encapsulated permanent magnet linear generator bolted to a foundation on the ocean floor. The WEC is suitable for offshore installation, outside of breakwater areas, for absorbing wave power primarily in heave mode at depths ranging from 20 to 100？m. The maintenance and life time of a WEC can be far more important to the economical viability than the conversion efficiency, especially if it is an offshore installation requiring divers and large sea vessels [7, 8]. The WEC design at Uppsala University constitutes a straightforward design based on a direct drive with few mechanical parts. Few mechanical parts promote a longer maintenance-free life time. All parts of the wave energy conversion system are sheltered at the seabed, except for the relatively cheap and robust surface floating buoy. Large surface
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