What is an MRU vs VRU?

Motion Reference Unit (MRU)

Definition: A motion Reference Unit (MRU) is a specialized device and self-contained sensor that measures motion in all six degrees of freedom (DoF): Roll, Pitch, Yaw, Surge, Sway and Heave. The six DoF positions, velocities and accelerations are measured by the MRU using high-end gyroscopes and accelerometers (and sometimes magnetometers) together with advanced fusion algorithms. An MRU has high accuracy Roll and Pitch measurements, and measures oscillatory Heave, Surge and Sway motions for wave periods at sea. Linear motions with very long periods, or steps, cannot be measured by an MRU as it assumes a 0 mean heave position.

Vertical Reference Unit (VRU)

Definition: A VRU is an advanced device that measures the attitude (roll and pitch) of an object using high-end accelerometers and gyroscopes, combined with advanced sensor fusion algorithms. VRUs provide very accurate roll and pitch measurements, even in dynamic environments.

Comparison to MRU: A VRU shares similarities with an MRU, incorporating real-time, high-precision accelerometers and gyroscopes along with advanced sensor fusion algorithms in a single self-contained unit. Both devices calculate coupled motions across all six degrees of freedom (DoF): Roll, Pitch, Yaw, Surge, Sway, and Heave. However, while the MRU outputs data for all six DoF, the VRU is limited to providing roll and pitch data only. This restricted output makes the VRU generally more cost-effective than the MRU.

Norwegian Subsea offers high-performance MRUs (MRU Compact, MRU Marine, MRU Subsea, MRU Ex) designed for demanding marine and subsea environments. Importantly, all our MRU models are also available in an Inclinometer / VRU version. These versions provide the exact same high-accuracy Roll and Pitch performance as the full MRU, even in irregular coupled motions, but without the Heave, Surge, and Sway outputs. This allows customers to select the precise functionality they need, ensuring optimal performance and cost-effectiveness.