MRU-B2 motion reference unit

The Inertial Labs MRU-B2 is a MEMS-based strap-down motion reference unit (MRU) for marine and hydrographic applications. Made by Inertial Labs, a VIAVI Solutions company, it measures heave, surge, sway, pitch and roll of any vessel, platform or device on which it is mounted. The B2 belongs to the basic tier of the MRU line and combines heave and attitude measurement in one sealed housing.

Inertial Labs developed its MRU line to meet marine and hydrographic requirements. The unit uses solid-state accelerometers and gyroscopes on three axes, and its low-noise gyroscopes give high-frequency, real-time measurement of rotation about all three rotational axes. Output signals include heave, heave velocity, heave acceleration, surge, sway, pitch and roll, pitch and roll rate, accelerations, angular rates, significant wave height, temperature, barometric data and a pulse-per-second signal. Heading, position and velocity are outputs of other models in the line, such as the MRU-E and MRU-P.

The key figures explain how the unit behaves in service. Heave accuracy is 5% or 5 cm (RMS), whichever is greater, the figure an integrator uses to judge vertical motion measurement for compensation or motion analysis. The gyroscopes measure up to 450 °/s with a bias in-run stability of 1 °/h, so the unit follows fast vessel motion and keeps a stable attitude reference over time. The accelerometers cover ±8 g with a bias in-run stability of 5 µg. The output rate is user-settable up to 200 Hz, which lets the integrator match the data stream to the receiving controller. The unit weighs 220 g, operates from -40 °C to 70 °C and draws about 1 W, with a higher draw when the optional internal data logger is fitted.

For integration, the MRU-B2 accepts a supply of 9–36 V DC. Depending on the ordering code, the interfaces are Ethernet, RS-232, RS-422 and CAN. Output formats are binary, TSS-1, NMEA ASCII, Kongsberg/Seatex, Ship Motion Control SMC and Teledyne TSS, which allows the unit to replace existing motion reference units. The datasheet lists compatibility with single-beam and multibeam echosounders from Teledyne, R2Sonic, WAASP, Kongsberg, EdgeTech, NORBIT and IMAGENEX, and with HYPACK and QINSY software. The connector is a Binder-series type, supplied with a pigtail cable or a Souriau connector, and the flanged housing bolts to the mounting surface. A bracket is available for fitting the unit into existing systems, and an enclosed subsea version, the MRU-B2S, is offered for underwater use.

The sealed enclosure has an IP67 rating. Vibration and EMC immunity and emission are specified to the IEC and EN standard listed under certifications, and Inertial Labs shows an ABS Design Assessed mark on the MRU datasheet.

Typical applications for the MRU-B2 are survey vessel motion analysis and offshore refinery motion control. The algorithms of the MRU line are intended for survey vessels, ships, active heave compensators, cranes, helidecks, ROVs, AUVs, dynamic positioning systems, buoys, echosounders and offshore platforms.