Specifications

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5.4 Wave motion sensors: Accelerometers, inclinometers and compass
5.4.1 Wave height, principle of measurement
The WR-SG and DWR-MkIII measure wave height by means of a single accelerometer. The
sensitive axis of this accelerometer points in the vertical direction. After filtering and double
integration of the acceleration signal the motion of the buoy, hence the wave motion, is
obtained. The strength of the Datawell principle is its gravity-stabilized platform. This patented
principle is unique and we will come to the advantages below. Essentially, the platform is
formed by a suspended disk in a fluid of equal density. By means of a very small metal weight
the disk is made gravity sensitive. The large mass of the fluid in combination with the small
force of the metal makes a pendulum with a natural period of 40 s, corresponding to a pendulum
length of 400 m. This platform remains almost horizontal under any movement which can be
expected at sea. Mounting the accelerometer on this stabilized platform makes the measurement
of wave height through vertical acceleration straightforward.
5.4.2 Wave direction, principle of measurement
Wave direction is determined by measurement of the horizontal motion of the buoy and
correlating this motion with the vertical motion of the buoy. Two mutually perpendicular
accelerometers are mounted in the DWR-MkIII which measure the horizontal buoy motion in
case the buoy is in the upright position. In case of tilt, the pitch and roll angles are determined
by coils around the sensor sensing the electromagnetic coupling with a coil on the stabilized
platform. With the help of the pitch and roll sensors the measurements of the above mentioned
acceleration sensors are transferred to real horizontal acceleration. With the help of a fluxgate
compass the acceleration in buoy-coordinates is transferred to north-west-coordinates.
The beauty of the Datawell principle is that it has kept the vertical acceleration out of all
the transformations, thus ensuring that you get the best wave heights possible.
5.4.3 Buoy axes and references
The DWR-MkIII motion sensor package measures 8 observables: 3 accelerations A
x
, A
y
, A
v
, 3
magnetic field strengths H
x
, H
y
H
z
, and pitch and roll. Figure 5.4.1 defines the directions of x, y,
z and vertical axes. All directions are referenced to the hull serial number plane (y) and normal
(x), the axis of rotation (z) fixed to the buoy and the vertical axis (v) determined by the force of
gravity.
Suppose you were facing the hull serial number. Tilting the buoy towards you would
result a positive pitch and a negative x-acceleration or A
x
output. Note that an accelerometer
sensor actually is a force sensor and that with a tilted buoy the force of gravity will act as an
inertial force. If you add that the direction of acceleration is opposite to the direction of the
inertial force or gravity force, you will understand why A
x
is negative. Similarly, tilting the buoy
towards the left would result a positive roll and a positive y-acceleration or A
y
output.
Considering an upright buoy, if the positive A
x
direction would point towards the north then the
positive A
y
direction would point westward and the positive z-axis would be directed upward.
Double integration would yield north, west and vertical motion.
The signs of the compass outputs correspond to the positive x-, y-, but negative z-
direction. Directing the serial number towards the north will yield a positive H
x
and zero
orientation. Looking from above a right angle clockwise rotation yields +90º orientation and a
positive H
y
. Note that the H
z
-axis is fixed to the hull whereas the A
v
-axis always points up and is
fixed to the stabilized platform.
For a tilted buoy the orientation is the same as for an upright buoy, which may be verified
by tilting the buoy.