Specifications

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5.4.8 Magnetic compass
The fluxgate compass measures the components of the earth magnetic field in three
perpendicular directions referenced to the buoy frame: x-, y- and z-axis. The compass consists
of an aluminium cube with three holes in three mutually perpendicular directions. In each hole a
magnetic field sensor is placed. This part requires extremely little service.
Before any checks can be carried out we must make sure that the local magnetic field is
stable and homogeneous. This is not a simple matter in many indoor situations with large DC
currents present or near iron structures. Two compass related outputs may be easily obtained
through the console: the orientation of the buoy and the local inclination of the earth magnetic
field. Use the status request command. By rotating the buoy over 90º or 180º angles the
orientation angle can be checked. A correct (within a few degrees) inclination angle indicates
that: (1) the platform offset is small and (2) the compass is functioning well. For optimum
measurements the stabilized platform should be allowed some 20 min to come to rest. The
inclination angle test should reproduce the same value when rotating or tilting the buoy. Such
behaviour in fact proofs that not only the platform offset is all right but also pitch and roll and
the three compass axes sensors function properly.
5.4.9 Pitch and roll
As described, pitch and roll are measured through magnetic coupling between the pick-up coil
on the platform inside the sphere and the respective pair of pick-up coils outside the sphere.
Also these sensors hardly require service ever.
For verification of pitch and roll calibration the buoy is best put in its packing frame with
Forward Ship direction/hull serial number face along one of the 4 sides. A well-defined tilt
angle of the packing frame in any of the 4 directions can be realized by placing a piece of wood
underneath. Consult the buoy axes subsection for directions and signs. Pitch and roll outputs can
be directly measured at the test box, subsection 5.9.9, using a voltmeter.
5.4.10 Horizontal accelerometers
Both fixed x- and y-accelerometers are contained in a small stainless steel can. The can is filled
with a similar fluid as the plastic sphere incorporating the stabilized platform. However,
evaporation through steel is negligible and checking of the fluid level is superfluous.
As for the pitch and roll, the packing frame comes in handy to check the horizontal
accelerometers as well. Consult the buoy axes subsection once more and connect a voltmeter
over the respective terminals at the test box.
5.4.11 Filtering
The final goal is to measure the waves. Now there are two limitations that will keep the buoy
from accurately measuring the waves. At higher frequencies, the wave wavelength becomes
comparable to the buoy dimensions and the buoy will not be able to follow the particular waves
anymore (geometric attenuation). As higher frequency measurements can only introduce noise,
all analog outputs of the DWR-MkIII sensors are filtered by applying a low-pass filter with a
cut off frequency of 1.5 Hz. The filtered sensor outputs are then sampled and transformed to
north, west and vertical accelerations all at a rate of 3.84 Hz. In case of the WR-SG there is no
analog filtering. By using a high sampling rate of 10.24 Hz all filtering can be done digitally.
The cut off frequency is 2.0 Hz.
Another limitation comes from the sensors themselves. At the low frequency end
accelerations become very small and disappear in the sensor noise. Therefore, for the DWR-
MkIII, a digital high-pass filter with a cut off at 30 s is applied to the 3.84 Hz samples. At the
same time it converts the sample rate to 1.28 Hz. Finally, these accelerations are doubly
integrated to give the three-dimensional buoy motion in the frequency range of 0.033-0.64 Hz.
Again for the WR-SG the high-pass filter is applied to the 10.24 Hz samples and cuts off at 24 s.
Furthermore, the sampling rate is converted to 2.56 Hz. After double integration only vertical
buoy motion in the frequency range 0.042-1.0 Hz results.