Motion Reference Unit – MRU 3

The MRU 3 is a fifth generation roll, pitch and heave motion sensor is specially designed for heave compensation applications.


Typical applications

Typical applications for the MRU 3 model are real-time heave compensation of single beam echosounders and offshore cranes. The unit provides heave measurements to meet IHO standards. This unit has to be mounted in a fixed mounting direction with the connector pointing up or down.


This cost-effective MRU 3 model incorporates 3-axis Micro-Electro-Mechanical-Structures (MEMS ) sensors for both linear acceleration and angular rate. This unit achieves high reliability by using solid state sensors with no rotational or mechanical wear-out parts.

The unit is delivered with a Windows based configuration and data presentation software. In this software vector arms from where the MRU is mounted to centre of gravity (CG) and two individually configurable monitoring points (MP s) can be defined. The heave measurement can be output in four different locations (the MR U itself, CG, MP 1 and MP 2) simultanously on the same serial line or Ethernet port. Typical monitoring point is the transducer head or the crane tip.

Variables output

The MRU 3 outputs roll, pitch and heave together with linear acceleration in 3-axes. The MR U 3 outputs heave position and velocity. In addition roll and pitch angles and corresponding angular rate vectors are output.

External inputs

The MRU 3 accepts input of external speed and heading information on separate serial lines or Ethernet for improved accuracy in heave, roll and pitch during turns and accelerations. For time synchronization the MR U accepts 1-second time pulse (1PPS ) input on a TTL line (XIN ) or as RS -232/422 signal.

Digital I/O protocols

For this fifth generation MR U data is available through both Ethernet interface and serial lines enabling easy distribution of MRU data to multiple users on board the vessel. Output protocols for commonly used survey equipment are available on two individually configurable serial lines and Ethernet/UDP .

Additional information