TITLE:
Time Series-, Time-Frequency- and Spectral Analyses of Sensor Measurements in an Offshore Wave Energy Converter Based on Linear Generator Technology
AUTHORS:
Erland Strömstedt, Andrej Savin, Olle Svensson, Mats Leijon
KEYWORDS:
Wave Power; Wave Energy Converter; Linear Generator; Sensor Measurements; Spectral Analysis; Cogging; Filtering; Laser Triangulation Sensor; Draw-Wire Sensor; Force Transducer; Strain Gauges; Search Coil; Power Generation
JOURNAL NAME:
Energy and Power Engineering,
Vol.5 No.1,
January
9,
2013
ABSTRACT: Inside the second experimental wave
energy converter (WEC) launched at the Lysekil research site on the Swedish
west coast in March 2009 a
number of sensor systems were installed for measuring the mechanical
performance of the WEC and its mechanical subsystems. 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. Two measurement periods, separated by 2.5 month, are presented in
this paper. One measurement is made two weeks after launch and another 3 months
after launch. Comparisons and correlations are made between different sensors
measuring simultaneously. 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. 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. 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. Vibration frequencies due to wear are recognized
by comparing with the noise at generator standstill and the vibration sources
in the generator. It is concluded that a moving average is a sufficient filter
in the time domain for further analysis of the relative displacement of the
piston rod mechanical lead-through transmission.