Time Series-, Time-Frequency- and Spectral Analyses of Sensor Measurements in an Offshore Wave Energy Converter Based on Linear Generator Technology
(English)In: Energy and Power Engineering, ISSN 1949-243XArticle in journal (Other academic) Submitted
The second wave energy converter (WEC) prototype launched at the Lysekil research site on the Swedish west coast in March 2009 contained a number of sensor systems for measuring the mechanical performance of the WEC and mechanical subsystems within the WEC. Measurements were carried out during the first experimental period starting from May 15th and ending on September 23rd. One of the measurement systems was a set-up of 7 laser triangulation sensors for measuring relative displacement of the piston rod mechanical lead-through transmission in the direct drive. Investigation into the measurements in the time domain with close-ups, in the frequency domain with Fast Fourier transform (FFT) and with time-frequency analysis with short time Fourier transform (STFT) is carried out to map the spectral content in the measurements. End stop impact is clearly visible in the time-frequency analysis. The FFT magnitude spectra are investigated for identifying the cogging bandwidth among other vibrations. Two measurement periods separated by 2.5 month are presented to be able to draw conclusion on what happens with time. Comparisons and correlations are made between different sensors measuring simultaneously, especially for the laser triangulation sensor measurements. Noise levels are investigated. Filtering is discussed for further refinement of the laser triangulation sensor signals in order to separate noise from actual physical displacement and vibration. Measurements are presented from the relative displacement of the piston rod mechanical lead-through, from magnetic flux in the air gap, mechanical strain in the WEC structure, translator position and piston rod axial displacement and active AC power. Generator cogging, fluctuations in the damping force and in the Lorenz forces in the stator are distinguished and varies depending on translator speed. Vibrations from cogging seem to be present in the early measurement period while not so prominent in the late measurement period in August. Vibrations frequencies due to wear is possible from comparing with the noise at generator standstill and the vibration sources in the generator. It is concluded that a moving average is sufficient filter in the time domain for further analysis of the relative displacement of the piston rod mechanical lead-through transmission.
wave energy converter, linear generator, sensor measurements, spectral analysis, cogging, filtering, laser triangulation sensor, draw-wire sensor, force transducer, strain gauges, search coils, power generation
Ocean and River Engineering
Research subject Engineering Science with specialization in Science of Electricity
IdentifiersURN: urn:nbn:se:uu:diva-179733OAI: oai:DiVA.org:uu-179733DiVA: diva2:545951
ProjectsThe Lysekil Wave Power Project