KONGSBERG Seatex MRU 5 practical combat

KONGSBERG Seatex MRU 5 High Precision Motion Sensor: Comprehensive Configuration, Integration, and Troubleshooting Guide

In modern maritime operations, from dynamic positioning (DP) for deep-sea mapping to helicopter deck monitoring, from ROV (remotely operated vehicle) navigation to high-speed ship roll reduction systems, every link relies on accurate and real-time ship attitude data - including roll, pitch, heave, yaw, and three linear acceleration components. Any posture measurement error can directly lead to distortion of multibeam depth measurement data, DP positioning drift, and even misjudgment of structural monitoring.

The Seatex MRU 5, owned by Kongsberg Maritime, is the third generation solid-state motion sensor designed to meet such demanding requirements. It adopts MEMS technology without mechanical rotating parts, which can provide dynamic roll/pitch accuracy of 0.03 ° RMS level, heave accuracy of 5cm or 5% (whichever is larger), and support data output rate up to 100Hz. This article is based on the official technical specifications of MRU 5, combined with years of shipborne integration experience, to systematically explain the installation specifications, communication interface configuration, output variable selection, connection methods with third-party devices (such as multibeam sonar and DP systems), as well as the most commonly encountered fault diagnosis and troubleshooting processes of the sensor. Whether you are a ship electronics engineer, hydrological surveyor, or DP operator, this article will provide you with technical knowledge that can be directly used for on-site operations.

Overview of Seatex MRU 5 Technology

2.1 Product positioning and intergenerational advantages

MRU 5 is the third generation product of the Seatex MRU series. Compared to its predecessor, it has significantly improved in the following aspects:

Dynamic accuracy improvement: Under ship turning and acceleration/deceleration conditions, by integrating external speed and heading inputs, significant suppression of heave drift is achieved.

All solid state design: no gyroscope moving parts, stronger shock and vibration resistance (can withstand 1000m/s ² non working impact).

Flexible installation: no longer limited by the constraint of "must be installed at the center of gravity of the ship", supports lever arm compensation, and can be fixed through software after installation at any position.

High output rate: The internal attitude update rate reaches 400Hz, and the external output is 100Hz, meeting the requirements of fast dynamic response.

2.2 Typical Application Scenarios

The role of MRU 5 in the core data required for application fields

Multi beam depth measurement sonar real-time motion compensation for heave, roll, pitch, and yaw, correcting the position of beam footprints

Dynamic positioning (DP) integrates three-axis acceleration, angular velocity, and attitude into the Kalman filter to improve position estimation accuracy

ROV/towed fish positioning mother ship attitude relative change calculation of underwater equipment motion relative to mother ship

Helicopter deck monitoring for heave, roll, and pitch to evaluate whether deck movement is within the safe takeoff and landing envelope

Feed forward control algorithm for fin stabilizer angular rate and roll angle of high-speed ship, and adjust fin angle in advance

Long term acceleration integral displacement fatigue analysis and extreme sea state response record of offshore platform structure monitoring

2.3 Output Variables

MRU 5 supports up to 189 output variables, covering:

Attitude angle: roll, pitch, heading (true north or relative heading)

Angular velocity: rotational speed around three axes (°/s)

Acceleration: three-axis acceleration (m/s ²)

Speed and position increment: can be used to calculate navigation trajectory

Rise and fall: vertical displacement relative to the static water surface (adjustable output range ± 50m)

Status words: sensor health status, self-test results, GPS lock flag, etc

Installation guide: Physical installation and lever arm compensation

3.1 Installation location selection

Although the internal algorithm of MRU 5 allows for non center of gravity installation, the following principles still need to be followed:

Rigid and Horizontal: The installation base must be sturdy (steel plate thickness ≥ 8mm) to avoid flexible deformation. Use a spirit level to adjust the reference plane of MRU 5 to be parallel to the ship's horizontal plane (error ≤ 0.5 °), otherwise it will introduce fixed roll/pitch deviation.

Avoid magnetic interference: Due to the sensitivity of the internal magnetometer (used for bow reference) to environmental magnetic fields, MRU 5 should be at least 3 meters away from the magnetic compass and kept away from high current cables (such as the main distribution board), transformers, and winch motors.

Temperature controlled environment: The working temperature range is -5 ℃ to+55 ℃. It is prohibited to install it in the engine room exhaust outlet, next to the boiler, or directly exposed to sunlight on the deck. Suggest leaving a 50mm gap around the sensor to facilitate heat dissipation.

3.2 Mechanical fixation and vibration reduction

The MRU 5 shell is made of aluminum alloy and weighs approximately 1kg. It is fixed with 4 M5 stainless steel bolts. Due to the continuous vibration of the vessel (0.5 m/s ², 10-2000Hz), no additional shock absorbers are required - its internal sensors have anti vibration design. But it is necessary to avoid resonance with the ship structure, which can be confirmed by measuring the vibration spectrum during trial operation.

3.3 Leverage arm compensation setting

When MRU 5 is not installed at the center of gravity (CG) of the vessel, three offsets need to be entered in the Seatex MRC configuration software:

Dx: Longitudinal distance from the center of the sensor to CG (positive front and negative back, meters)

Dy: Horizontal distance (right positive left negative)

Dz: Vertical distance (positive top and negative bottom)

For example, if the sensor is installed on the bridge deck and the CG is 3 meters below the main deck, then dz=-3.0. These parameters are used to convert the acceleration and angular velocity measured by the sensors into real motion at the CG. Incorrect lever arm values are the primary cause of heave errors.

Electrical interface and communication configuration

4.1 Power Connection

MRU 5 accepts 12-30V DC (maximum power consumption of 11W). It is recommended to use a dedicated branch of the 24V emergency distribution board for ships, connected in series with a 1A fast melting fuse. It is recommended to use 0.75mm ² twisted pair shielded wire for the power cord, with positive and negative terminals of PWR+and PWR - respectively. After power on, the panel LED indicator lights will light up in the following order:

Green constantly on: power supply is normal

Orange flashing: starting self-test (about 10 seconds)

Green flashing: Data output is valid

4.2 Overview of Communication Ports

MRU 5 provides multiple serial interfaces:

Main serial port (COM1): RS-232/RS-422 switchable, used for bidirectional binary protocol or ASCII output.

Auxiliary serial port (COM2): RS-232, used for external speed input (VG/VWW, etc.).

Heading input port (COM3): CMOS level, used for external heading data (HDT/HDM) or synchronous triggering.

All baud rates are configurable (4800 to 115200 bps). Recommend using 115200 bps to ensure no packet loss during 100Hz output.

4.3 Protocol Selection and Configuration (MRC Software)

The accompanying Seatex MRC configuration software (Windows compatible) is connected to MRU 5 via a USB to RS-232 cable. Main configuration steps:

Select output variables: Check the required items from 189 variables. Typical multibeam applications require: Roll, Pitch, Heave, Heading, HeaveRate.

Set output format:

Binary (IEEE 32-bit floating-point): The most efficient and recommended for DP systems.

NMEA 0183 proprietary statements, such as $PSXROT and $PSHEV, are used for compatibility with older devices.

Third party sonar format: MRU 5 has over 20 common protocols built-in, including Simrad EM 1000, Atlas Fansweep, Reson Seabat, and more. Simply select the corresponding model from the drop-down menu without writing string parsing code.

Set output rate: 1Hz, 5Hz, 10Hz, 20Hz, 50Hz, 100Hz. It is recommended to use binary format at 100Hz, otherwise ASCII will occupy a lot of bandwidth.

Configure external inputs: Enable speed input (to improve heave accuracy during steering), select NMEA statement type (VTG, VHW, or VBW) and input baud rate.

Save configuration to MRU 5 non-volatile memory: no loss upon power failure.

Accuracy characteristics and performance optimization

5.1 Interpretation of Official Accuracy Indicators

Parameter numerical conditions

Static roll/pitch accuracy 0.025 ° RMS, stationary for 30 minutes

Dynamic roll/pitch accuracy 0.030 ° RMS ± 5 ° amplitude, 5-minute biaxial sinusoidal motion

The dynamic accuracy of lifting and sinking is 5cm or 5% (whichever is greater) with a period of 0-25s, and the effective wave height can reach 50m

Angular velocity noise 0.025 °/s RMS for all axes

Acceleration noise 0.0020 m/s ² RMS-

The heave drift can be ignored and the short-term recovery after turning can be ignored

Practical application precautions:

When the heave value is less than 5cm, the absolute error may reach 5cm, which should be considered as the effective resolution limit under micro wave conditions.

The dynamic roll accuracy of 0.03 ° requires support from external velocity and heading inputs. If these inputs are not connected, the error may increase to 0.1 °~0.2 ° during sharp turns.

5.2 Engineering Techniques for Improving Accuracy

External speed input: MRU 5 defaults to using its own acceleration quadratic integral to calculate heave, but centripetal acceleration during turning can contaminate the vertical channel. After connecting the velocity (VTG/VHW) from GPS or log, the algorithm can separate the centripetal acceleration component and reduce the heave drift by more than 90%.

Polling Mode: For multi beam sonar, motion data is required to be synchronized with sonar ping. Connect the sonar trigger pulse to the CMOS input line (COM3) of MRU 5 and configure it as "External Sync". MRU 5 will only output the attitude once when receiving the pulse, ensuring time alignment.

Regular calibration: Although a calibration certificate is provided at the factory, it is recommended to retest the zero bias and scale factor every two years. It can be automatically completed in calm sea conditions through the "Static Calibration" function of MRC software.

Practical integration with mainstream devices

6.1 Connection with multibeam depth sonar (taking Simrad EM series as an example)

Connect the COM1 (RS-422) of MRU 5 to the motion sensor input port of the sonar processing unit.

Set the output protocol in MRC to "Simrad EM 1000" (or EM 3000/EM 2040 corresponding item), and set the output rate to 100Hz.

On the motion sensor configuration page of sonar software (such as SIS), select the "Seatex MRU" protocol and set the same baud rate as MRU 5.

Test: Start sonar acquisition and observe whether the real-time roll/heave curve is consistent with the actual motion of the ship. If the data jumps, check the shielding grounding.

6.2 Integration with DP system

DP systems such as Kongsberg K-Pos and Siemens typically require raw acceleration and angular velocity data. Recommended configuration:

Output format: Binary (including Ax, Ay, Az, RollRate, PitchRate, YawRate, Roll, Pitch, Heading)

Output rate: 20Hz (DP filter is sufficient, higher will waste CPU)

Communication protocol: Transfer to a serial server via serial port or Ethernet, and forward to a DP controller using UDP.

The compensation value of the lever arm must be rechecked in the DP system to prevent duplication with the compensation in MRU 5.

6.3 Data Extraction of Helicopter Deck Monitoring System

The system requires real-time heave and sway angles, and triggers alarms. NMEA 0183 proprietary statements can be used for output, such as:

PSHEV, 0.52, M, 0.03, D, 1.20, R * 4C (analysis: heave 0.52 meters, rise; Roll 0.03 degrees; Pitch 1.20 degrees)

Send this statement to the alarm unit on the deck and set a threshold (such as alarm when heave>3m or roll>2 °).

Common troubleshooting and diagnosis

7.1 Fault 1: MRU 5 has no output after power on (no data on serial port)

Phenomenon: The power LED is on, but the data receiving software does not display any characters.

Possible reasons:

Serial port line sequence incorrect (TX/RX crossover)

Baud rate mismatch

The output protocol was accidentally set to 'Polling mode' and requires an external trigger to output

Solution steps:

Measure the ground voltage of TX pin through oscilloscope or voltmeter: the idle state should be -5V~-12V (RS-232). If it is 0V, the port is damaged.

Try the "Auto detect" function using MRC software, which can automatically scan baud rates and protocols and reconstruct communication.

Enter the "Output Configuration" page of MRC and confirm that "Output Mode" is not "Polling". If real-time continuous output is required, select 'Continuous'.

Check if binary output is enabled, but the receiving end parses it in ASCII format - the two cannot recognize each other.

7.2 Fault 2: Periodic drift of lift and sink values, even with slow changes when stationary

Phenomenon: The ship is anchored at the dock, and the MRU 5 output heave value increases linearly or fluctuates sinusoidally with time.

Possible reasons:

External speed input is not connected, and the ship experiences slow drift due to slow water flow.

Vertical acceleration zero drift (possibly caused by temperature changes).

The compensation value input for the lever arm is incorrect.

Solution steps:

Temporary access to GPS speed (even if the speed is 0, zero speed reference can be provided). Observe whether the heave is stable. If stable, the problem is that there is no speed assistance.

Perform zero speed calibration: Select "Sensor Health" ->"Zero Calibration" in MRC, follow the prompts to place the sensor on a stationary horizontal plane, and automatically correct the acceleration zero point.

Recheck the value of the lever arm dz: If the symbol is reversed, it will lead to the accumulation of heave errors as the ship sways.

7.3 Fault 3: The dynamic accuracy of roll/pitch is significantly lower than 0.03 °

Phenomenon: Under known regular wave conditions (such as a wave simulation platform), the difference between the output of MRU 5 and the reference attitude gauge exceeds 0.1 °.

Possible reasons:

The installation base of the sensor is not level, and there is a fixed installation angle error.

The missing or incorrect bow input leads to the failure of attitude calculation under coupled motion.

Solution steps:

Check the installation surface with a high-precision digital level. If the offset is 0.2 °, the input offset angle can be corrected through MRC's "Mounting Alignment" without the need for physical reinstallation.

Ensure that MRU 5 receives reliable heading data (from either the gyrocompass or fiber optic compass). The NMEA statement should be HDT (True North) or HDM (Magnetic North), avoiding the use of HDG (including magnetic interference). Suggest inputting through COM3 with an update rate of at least 10Hz.

Check if there is a 'Heading Loss' warning in the' Internal Log 'in MRC.

7.4 Fault 4: There are a large number of garbled characters or verification errors in the output data

Phenomenon: Intermittent packet corruption, manifested as unresolved hexadecimal characters.

Possible reasons:

The cable shielding layer is not grounded at one end, which introduces electromagnetic interference.

Signal attenuation caused by high baud rate (115200 is not recommended when RS-232 exceeds 15 meters).

No terminal resistor is connected when using RS-422.

Solution steps:

Control the communication distance within 10 meters (RS-232). If a longer distance is required, switch to RS-422 mode and check the differential signal voltage (should be>200mV).

Install a 120 Ω terminal resistor at the RS-422 receiving end.

Add a ferrite magnetic ring (model 31 Material) at each end of the cable to suppress common mode interference.

Regular maintenance and lifespan management

8.1 Monthly Inspection

Clean the connector contacts and spray contact preservatives.

Record the temperature of the sensor (read the internal temperature through MRC). If the difference from the ambient temperature is>8 ° C, there may be an abnormal hot spot inside.

Perform a 'Loop Test': Short circuit TX to RX, send a test command, and verify that the serial hardware is functioning properly.

8.2 Annual Calibration

Although MRU 5 has a nominal drift free design, it is strongly recommended to perform on-site zero bias calibration once a year

When the ship is docked or in very calm sea conditions (heave<0.1m), turn off the power of MRU 5 and let it stand for 30 minutes to stabilize the temperature.

Connect MRC, run "Calibration" ->"gyro bias", and follow the wizard to complete the static gyro bias compensation.

Run the 'Accelerometer Bias' calibration. During the calibration process, do not touch the sensors or move the vessel in any way.

8.3 Suggestions for spare parts

Due to the fact that MRU 5 is a solid-state device with a high MTBF calculation value (over 50000 hours), there is generally no need to keep spare parts for the entire machine. But the following consumables should be prepared:

Special data cable (9-pin D-sub to terminal block, 2 pieces)

Configure cable (USB to RS-232, with FTDI chip)

1A 5 × 20mm fuses (5 pieces)

Firmware upgrade and configuration backup

Kongsberg periodically releases MRU 5 firmware updates, mainly to fix algorithm bugs or add new output protocols. Upgrade steps:

Download the latest firmware (. hex file) and upgrade tool from the official website.

Place MRU 5 in "Bootloader mode": Power off, short circuit the BOOT pin of the debugging interface, and power on again.

Use FlashMagic or Kongsberger's "Seatex Firmware Updating" to flash firmware.

Important: After the upgrade, all configurations will be reset. Be sure to save the backup through MRC's' Save Configuration 'before upgrading. Upgrade and then restore through 'Load Configuration'.