8
IMU User Guide revision 3.1 SMC Ship Motion Control www.shipmotion.eu
2
SYSTEM DESCRIPTION
The SMC motion sensors have three separate axial measurement component groups converting
signals from actual movements via three accelerometers and three gyroscopes into output data of
angles and attitude. The output parameters are presented in a digital output string via RS422 and
RS232. On hardware versions 8.5 or higher Ethernet communication is available.
The signal from the gyroscopes are combined with the signal from the accelerometers and are
processed in a Kalman filter inside the motion sensor to provide output values for accelerations,
attitudes and angles with limited influence from noise and other inaccuracies.
Heave, surge and sway are calculated by integrating the acceleration in the X, Y and Z axis twice. The
integrated data is filtered with a high pass filter. The calculations of the distances are optimized for
continuous motion and not for static distance measurements, as the high pass filter will filter the
position over time to zero. The dynamic motion filtering is designed to measure motions over a
period between 1 s and 25 s.
Before delivery all motion sensors are calibrated. The readings from the accelerometers and angular
rate gyroscopes are calibrated for alignment, linearity and temperature variations, to ensure they
meet the performance specifications.
The calibration is optimized for angles up to +/-30 degrees of angle. The best motion sensor
performance is achieved within this angle range. If the motion sensor angle exceeds the calibrated
angular range the motion sensor will continue to measure the data, but performance may be
decreased.
