What is a motion reference unit (MRU)?

An inertial sensor package that measures a vessel's motion in all six degrees of freedom — surge, sway, heave, roll, pitch and yaw. In offshore lifting it supplies the real-time heave signal that active heave compensation drives against.

What is the difference between an IMU, a VRU and an MRU?

An IMU outputs raw accelerations and angular rates with no processing. A VRU adds attitude computation — roll and pitch against the vertical. An MRU adds processed heave and full motion outputs via digital signal processing, which is what an AHC control loop needs.

Does passive heave compensation need an MRU?

No — a passive compensator responds mechanically through its gas spring and damping, with no sensors required. Advanced systems use sensor feedback differently: the ANTARES adaptive PHC uses it to optimise its own operating parameters, not to drive an actuator.

Where should an MRU be mounted?

At or near the crane pedestal. Mounting it elsewhere introduces lever-arm errors: roll and pitch produce different local heave at different points on the vessel, so the sensor must measure the motion where the crane actually is. Axis alignment matters for the same reason.

Motion reference units

The sensor that tells the crane what the sea just did — six degrees of freedom, in real time.

Motion Reference Units (MRU)

Q: How accurate is an MRU?

Typical heave accuracy is ±5 cm or 5% of amplitude, with update rates of 10–100 Hz and measurable periods up to about 30 seconds. Critical AHC installations often run dual-redundant MRUs.

What is an MRU?

A Motion Reference Unit (MRU) is an inertial sensor that measures a vessel’s motions in all six degrees of freedom: surge, sway, heave (translations) and roll, pitch, yaw (rotations). In offshore lifting, the MRU provides the real-time heave signal that active heave compensation (AHC) systems use to drive the compensating winch.

Practical application: For practical application of this topic, see engineering studies and analysis and ANTARES Adaptive PHC.

Modern MRUs combine accelerometers and gyroscopes — typically MEMS-based or fibre-optic — with digital signal processing to separate true heave motion from sensor noise and long-period drift. Key specifications include heave accuracy (typically ±5 cm or 5% of amplitude), update rate (10–100 Hz), and maximum measurable period (typically up to 30 seconds).

Common MRU manufacturers include Kongsberg (Seatex MRU series), iXblue, and SMC. The MRU is mounted at or near the crane pedestal to measure the motions at the point of interest.

Heave accuracy±5 cm or 5% of amplitude, typical

Update rate10 – 100 Hz

Measurable periodup to ≈ 30 s

MRU in Active vs Passive Systems

Active heave compensation (AHC): The MRU is essential — it provides the feedforward heave signal that the control system uses to drive the winch motor or hydraulic cylinder in anti-phase with the vessel motion. Without an accurate MRU, AHC cannot function. The MRU signal is processed through a Kalman filter or similar algorithm to predict the heave motion one or two samples ahead, giving the control system time to respond.

Passive heave compensation (PHC): A standard PHC does not require an MRU — it responds mechanically to load changes through gas spring dynamics. However, some advanced passive systems use MRU data for monitoring and performance verification. The ANTARES adaptive PHC uses sensor feedback to optimise its operating parameters in real time, achieving near-active performance without the full complexity of an AHC system.

IMU, VRU or MRU — which one is which

Sensor	Measures	Outputs	Enough for AHC?
IMU	Raw accelerations and angular rates	Unprocessed inertial data	No — needs external processing
VRU	IMU + attitude computation	Roll and pitch against the vertical	Not alone — no processed heave
MRU	Full six degrees of freedom with onboard DSP	Real-time heave, roll, pitch (and more), drift-corrected	Yes — this is the AHC input signal

The heave channel is the hard part: separating true wave-period motion from sensor noise and long-period drift is what the MRU’s signal processing (and Kalman filtering downstream) exists for.

Installation and Calibration

Proper MRU installation is critical for AHC performance. Key considerations:

Location: Mount at or near the crane pedestal. Mounting far from the crane introduces lever-arm errors as roll/pitch motions create different heave at different locations on the vessel.
Alignment: The MRU axes must be aligned with the vessel’s coordinate system. Misalignment causes cross-coupling errors where roll motion contaminates the heave signal.
Calibration: Initial calibration establishes sensor offsets and scale factors. Some MRUs self-calibrate using GNSS aiding.
Redundancy: Critical AHC systems often use dual-redundant MRUs to maintain operation if one sensor fails.

For compensator selection, understanding whether the crane system includes an MRU (and its specifications) helps determine whether an active, passive, or adaptive passive solution is the best fit.

Motion reference units — frequently asked

What is an MRU?

An inertial sensor package measuring all six degrees of freedom — surge, sway, heave, roll, pitch, yaw. For lifting, its job is the real-time heave signal that active heave compensation drives against.

IMU vs VRU vs MRU?

IMU = raw accelerations and rates; VRU = adds roll/pitch attitude; MRU = adds processed, drift-corrected heave and full motion outputs. Only the MRU delivers what an AHC control loop consumes directly.

Does a passive compensator need an MRU?

No — passive systems respond mechanically, sensor-free. The ANTARES adaptive PHC uses sensor feedback only to optimise its own settings, not to drive an actuator.

Where is the MRU mounted, and why does it matter?

At or near the crane pedestal. Roll and pitch create different local heave at different points on the hull — the lever-arm effect — so the sensor must sit where the crane is, with axes aligned to the vessel frame.

How accurate is an MRU?

Typically ±5 cm or 5% of heave amplitude at 10–100 Hz, resolving periods up to about 30 s. Critical systems run dual-redundant units.

Related on Norwegian Dynamics

Active Heave Compensation — Inline AHC, MRU input, hydraulic actuation and where AHC fits.
Vessel Motions — How the vessel response RAO drives crane-tip motion.
How Active Heave Compensation Works — Control loop, MRU input and actuation chain.

Working on a lift that needs this?

MRU placement and AHC tuning are tightly coupled. If you're scoping a system tell us the vessel and we'll advise.