Motion Reference Units (MRU)
Define the signal, its point and its timing.
State where the motion is measured and where it is needed.
A vessel rotates as well as translating. The sensor location, configured output point and actual crane tip are not automatically the same point.
Concept geometry only. The offsets, axes, signs and motion transformation come from the declared vessel and sensor setup.
Locate the measurement.
Record the physical sensor position and orientation. Check whether the exported signal already refers to another configured point.
Resolve the required point.
Use the relevant translational and angular motion with the surveyed offsets. Pedestal heave alone is not automatically crane-tip heave.
Validate the complete signal.
Check units, signs, timing, output mode and any additional crane flexibility required by the model or control system.
An accuracy number needs its conditions.
The model name and output mode determine what is available. Compare specifications on the same reference, timing and test basis.
What does the channel represent?
Check available heave, attitude, velocity, acceleration and angular-rate outputs, their units and reference point. A six-motion vessel model does not guarantee six processed displacement outputs from every sensor.
When is the estimate valid?
Distinguish real-time and delayed heave. Check timestamp convention, latency, output rate, clock alignment, filtering and validity flags.
Under which conditions?
Read accuracy together with its statistic, period range, motion conditions and aiding requirements. A generic ± figure is not interchangeable with an RMS specification.
For example, Kongsberg’s MRU 3 datasheet lists heave, roll and pitch outputs and distinguishes real-time from delayed heave specifications. Read the conditions in the selected document rather than carrying one headline figure across modes or units.
Manufacturer example: Kongsberg MRU 3 datasheet, May 2024. This illustrates how to read a specification; it is not a product recommendation.
Validate the chain, not just the sensor.
Active compensation needs a defined motion input and a controller that uses it within the equipment limits. The implementation determines the filtering and prediction method.
Measure
Use the selected motion outputs and their declared units, mode, timestamp and validity.
Transform
Resolve axes, alignment and the required response point using the surveyed geometry.
Control
Apply the controller’s configured filtering, timing and prediction, with verified sign conventions.
Actuate
Check residual response against force, stroke, speed and control limits through the intended duty.
Output rate is not end-to-end latency. Account for sensing, processing, transmission and actuation. Residual motion depends on the complete control and mechanical system; an MRU alone does not hold the hook still.
What is an MRU?
A Motion Reference Unit (MRU) combines inertial sensing and processing to provide marine-motion outputs such as heave and attitude. Available channels depend on the model and configuration. In offshore lifting, a defined motion signal can support active compensation, monitoring or verification.
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 selection parameters include the heave-accuracy figure and the conditions it is defined for, latency and output rate, the usable period range, aiding and interfaces — all from the selected unit’s current datasheet.
Choose the unit and mounting arrangement from the application requirements and supplier guidance. Survey the sensor location and alignment, then verify how the reported output is transferred to the point needed by the crane or lift model.
Six rigid-body vessel motions
Surge, sway and heave are translations; roll, pitch and yaw are rotations. They describe rigid-body vessel motion. Check which measured or processed channels the selected unit actually exports.
Passive compensation responds mechanically. Sensor data may support monitoring or adaptive adjustment; active compensation uses a defined motion input for powered control. See the architecture comparison.
MRU in Active vs Passive Systems
Active heave compensation (AHC): A defined motion input informs powered control. Sensor selection, output-point transformation, latency and the controller’s processing must be checked together. Filter type and prediction horizon are implementation-specific; the residual response is limited by sensing, control and actuation capability.
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 for automatic damping control while the load support stays passive — a different mechanism from MRU-driven active control, without the complexity of an AHC system.
IMU, VRU or MRU — which one is which
| Label | Common function | Check before use |
|---|---|---|
| IMU | Inertial acceleration and angular-rate sensing. | What processing, aiding and motion estimation are supplied externally or internally? |
| VRU | Vertical-reference or attitude estimation, commonly roll and pitch. | Which heave or other outputs, if any, are provided by the selected model? |
| MRU | Processed marine-motion outputs, commonly heave and attitude. | Which channels, reference points, modes and timing meet the intended application? |
The label alone does not establish suitability for AHC. Confirm the required processed motion, output point, timing and interface with the control-system design.
Installation and Calibration
Proper MRU installation is critical for AHC performance. Key considerations:
- Location: Follow the supplier’s mounting requirements and the approved installation. Survey the sensor and required output-point coordinates; transform motion rather than assuming pedestal and crane-tip motion are identical.
- Alignment: Record mounting orientation, axes, positive rotations and alignment offsets. Verify the transformation into the vessel and control-system frames.
- Calibration: Apply the selected unit’s calibration and aiding procedure. Check offsets, output modes, status and the acceptance tests required for the application.
- Redundancy: Define the required fault detection, sensor validation, failover and degraded operating behavior. Two sensors alone do not establish continued safe operation.
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 a motion reference unit (MRU)?
What is the difference between an IMU, a VRU and an MRU?
Does passive heave compensation need an MRU?
Where should an MRU be mounted?
How accurate is an MRU?
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.
Turning motion data into a lift decision?
Send the selected sensor documentation, configured outputs, sensor and lifting-point coordinates, alignment conventions, timing/interface details and intended use. We can identify gaps before scoping motion processing or the coupled lift assessment.
