Motion Reference Units Vertical Reference Sensors
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Motion Reference Units Vertical Reference Sensors.
Motion and vertical reference units and sensors are important in position keeping.
At least one vertical reference sensor must be selected to provide the DP system with roll and pitch information (2 of the six degrees of freedom) . This information is important as the PRS Position reference system requires this data from the VRS/MRU to calculate the vessel position accurately.

A ship with a PRS selected that may be 30 meters above the sea level and rolling in a sea way could form a considerable arc which would move the PRS either side of the vessel center line as we can see illustrated here.

Without VRS/MRU input the system assumes wrongly that the vessel has moved away from the PRS position. By using MRU/VRS the effects/errors are negated by referring the PRS to a fixed horizontal plane.

A VRS may be equipped with a heave sensor, the heave information is used for monitoring

VRU – Vertical Reference Unit. This was a vertical gyro type vertical reference unit that was used in the early DP systems.

VRS – Vertical Reference Sensor is an analogue sensor consisting of sphere half filled with fluid which provides the horizontal plane and enclosed in an outer housing which is fixed to the hull. This method uses induction between a coil floating on the surface of the fluid and coils fixed to the housing unit.

A VRS may be equipped with a heave sensor, the heave information is used for monitoring.

Today MRU – Motion Reference Units are fitted on modern vessels and these units are constructed with inclinometers and accelerometers.

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Motion Reference Units DP Ships