12
IMU User Guide revision 3.1 SMC Ship Motion Control www.shipmotion.eu
2.2
DEFINITIONS
Alignment
The alignment of the motion sensor is the positioning of the IMU onto the structure of the rig or
vessel. The physical alignment must be done as accurately as possible and then it can be fine-tuned
in the system configuration software by entering offsets for roll (X), pitch (Y) and the Z-axis.
Yaw in the SMC IMU units
Without an external aiding input the yaw in the SMC motion sensor will drift over time and so it
cannot be used as an absolute heading output. Positive yaw is a clockwise rotation. The yaw output
from the SMC unit, when it is not aided from an external heading input, is the integration of the yaw
gyroscope or the integrated rotation in the Z axis in the earth coordinate system.
Roll
Roll is the rotation about the roll axis (X) of the vessel. SMC defines the port up as a positive roll.
Pitch
Pitch is the rotation about the pitch axis (Y) of the vessel. SMC defines the bow down as a positive
pitch.
Heave
Heave is the vertical dynamic motion of the vessel. The heave is calculated by a double integration of
the vertical acceleration. The vertical position is filtered with a high pass filter. Heave measures the
relative position dynamically and
cannot
be used for a static height position measurement. An
upwards motion is defined as a positive heave.
Surge and Sway
Surge and Sway are the horizontal dynamic motion of the vessel. Surge is the linear motion along the
roll axis; a positive surge is when the vessel is moving in the bow direction. Sway is the linear motion
along the pitch axis where a positive sway is in the port direction. The surge and sway calculation are
attained by a double integration of the horizontal acceleration. The horizontal position is filtered
with a high pass filter. The dynamic horizontal linear measurement is a relative position and cannot
be used for a static horizontal position measurement.
Center of Gravity
Centre of gravity CG is the mass center of a vessel.
X-axis/Roll axis
The X axis is the bow/stern axis in the vessel. Rotation in the X axis will generate a roll motion where
a positive rotation is port side up.
Y-axis/Pitch axis
The Y axis is the port/starboard axis in the vessel. Rotation in the Y axis will generate a pitch motion
where a positive rotation is bow down.
Z-axis
The Z axis is the vertical axis pointing up and down in the vessel. Rotation in the Z axis will generate a
yaw motion, where positive yaw is a clockwise rotation.
RMS
Root mean square (RMS) is a statistical measure of the magnitude of a varying quantity.
