Motion reference units (MRU) determine the orientation of an object relative to an inertial frame of reference or another body. To control this data output, referred to as attitude, MRUs integrate sensors for orientation measurements, as well as actuators for applying torque to position an object to the desired attitude. Algorithms supply actuators with commands based on current and desired attitudes.

Operation

MRUs utilize motion sensors with a single-axis or multi-axis configuration. They incorporate gyroscopes capable of detecting motion in space without the need to track external objects. Gyroscopes come in a variety of types. Select models feature a spinning mass or ring lasers that reflect light to detect rotation. Other types include the hemispherical resonator gyro (HRG), also known as the wine-glass gyroscope. An HRG calculates the orientation of oscillation, created by a resonating shell in the inertial space, in relation to the position of a moving object.

Vibrating gyroscopes operate based on Coriolis forces that generate vibration patterns around the rotation axis and apply the acceleration effect to sense rotation.

A miniature gyroscope manufactured using MEMS (microelectromechanical system) technology provides an inertial measurement unit (IMU). Optional units integrate multiple gyros and accelerometers.

Motion reference units are employed in large vessels operating at sea. MRUs capture roll, pitch, yaw and heave measurements with high accuracy to monitor vessel performance. These measurements apply to observing the wave height or determining dynamic positioning.

Features

MRUs support a vast range of features, including:

• Axis accelerometers
• Axis gyro meters
• Axis magnetic sensors
• Graphic display software
• Internal GPS receiver
• True north magnetic map
• Titanium enclosure
• Extended Kalman Filter (EKF)
• MEMs  (Microelectromechanical systems)
• Pre-calibration
• Automatic heave adjustment

• Built-in web interface

Applications

Motion reference units support a multitude of uses across several industries including:

• Subsea
• Marine
• Aviation
• Oil and Gas
• Transportation
• Surveying
• Telecommunications 

Below is a detailed look at some specific uses of MRUs:

Stabilization

Vessels featuring telecommunication antennae require reliable stabilization systems to compensate for the ship's roll and pitch and inspect the radio beams. MRUs deliver measurements to achieve system stability and ensure smooth transmission. Maritime MRUs also come with advanced inertial sensors, such as linear accelerometers, as well as MEMS gyros for maritime operation. MEMS gyros combine low noise operation with superior bias stability and exceptional gain accuracy. Solid-state sensors, having no moving parts, provide robust performance with lifelong reliability.

Active Heave Compensation (AHC)

Information related to the heave amplitude of a crane tip is essential for AHC systems integrated into the equipment, such as offshore cranes. Adequate heave compensation requires access to real-time information from vessel heave motion, velocity, and crane tip acceleration. Roll and pitch data captured by the MRU, integrated into the AHC system, is applied in controlling vessel motion by transferring the heave motion from the tip of the crane to the mounting point of the MRU. The system ensures efficient operation and compensation by adapting to varying conditions and maintaining load stability.

Positioning Using Hydro Acoustics

Vessels, used for subsea operations, determine their position using hydro acoustics. Measuring the distance and bearing from hull-mounted transducers to the seabed transponders under noise-free and stable conditions achieve the position. MRUs attain optimal compensation of the vessel motion to achieve this.

High-speed motion damping

Systems providing high-speed motion damping or ride control utilize roll and pitch angle and angular rate data for the foil control system. An MRU delivers the required data at a rapid output rate and with minimal latency.

MRUs combine a wide range of applications, including:

• Compensation of motion for single and multi-beam echo sounders
• High-speed craft motion control and damping systems
• Heave compensation for offshore cranes
• Hydro acoustic positioning
• Monitor ship motion
• Measurement of ocean waves
• Motion compensation and stabilization for antennas
• Remotely Operated Vehicles (ROV) attitude and orientation measurement
• Autonomous Underwater Vehicles (AUV) attitude and orientation measurement
• Diver tracking systems
• Underwater construction
• Directional drilling
• Dredging operations
• Dynamic positioning
• Offshore structure motion monitoring
• Ship motion monitoring
• GPS-aided course changes
• USBL navigation
• Single beam surveying
• Multi-beam surveying
• Bathymetric survey systems

Selection

Most units comply with IHO (International Hydrographic Organization) standards. In case of undersea applications, determine the required depth level of the unit to ensure optimal performance. For vessel motion stabilization, verify the MRU captures pitch and yaw output data.

References

Image credits:

Kongsberg Maritime, Inc. | Ship Motion Control