Datawell BV Hippy 40-MkII, Manual

Page: 6 / 35

6
2 Principle of operation
Acceleration measurement 2.1
The deflection of the tip of a clamped cantilever is a measure for the acceleration. (Vertical
acceleration since the accelerometer is mounted on a gravity stabilised platform).
The cantilever is placed in a fluid in which an electric field is present.
So the potential of the cantilever is a measure for this deflection.
No static friction is involved; the acceleration measurement is without hysteresis.
High pass filter for heave measurement 2.2
The heave is obtained by double integrating the acceleration.
Temporary offset of the platform caused by horizontal acceleration (varying ship's speed) leads
to false acceleration outputs; in order to minimise the resulting false heave outputs a high pass
filter is used.
The remaining false output caused by a 180° reversal of ship's direction is proportional to the
square of the ship's speed, and amounts to approx.0.15 m at ship's speed of 1 m/sec.
The resulting transfer of high pass filter is:
 
 
 
ib a a i
  
1 . 2 1 1
2
8 . 30 T a

170 T b

time period T

Resulting amplitude transfer and phase shift is given under the specifications for some
frequencies.