MTi 1-series architecture

• MTi 1-series configurations
  • MTi-1 IMU
  • MTi-2 VRU/AHT
  • MTi-3 AHRS
  • MTi-7 GNSS/INS
  • MTi-8 RTK GNSS/INS
• GNSS input
  • u-blox (UBX) input
  • NMEA input
  • Septentrio (SBF) input
  • Trimble (GSOF) input
• Barometer input
• Signal processing pipeline
  • Strapdown integration
  • Xsens sensor fusion algorithm for VRU and AHRS product types
  • Xsens sensor fusion algorithm for GNSS/INS product types
  • Lever Arm Correction (MTi-8 only)
• Magnetic interference
  • Magnetic Field Mapping
  • In-run Compass Calibration (ICC)
  • Active Heading Stabilization (AHS)
• Frames of reference

This section discusses the MTi 1-s module architecture including the various configurations available and the signal processing pipeline.

MTi 1-series configurations

The MTi 1-s module is a fully tested self-contained module available as an Inertial Measurement Unit (IMU), Vertical Reference Unit (VRU) / Active Heading Tracker (AHT), Attitude and Heading Reference System (AHRS) and GNSS aided Inertial Navigation System (GNSS/INS). It can output 3D orientation data (Euler angles, rotation matrix or quaternions), orientation and velocity increments (∆q and ∆v), position and velocity quantities and calibrated sensor data (acceleration, rate of turn, magnetic field). Depending on the product, output options may be limited to sensor data and/or unreferenced yaw.

The MTi 1-s module features a 3D accelerometer, a 3D gyroscope, a magnetometer, a high-accuracy crystal and a low-power MCU. The MCU coordinates the timing and synchronization of the various sensors, applies calibration models (e.g. temperature models) and runs the sensor fusion algorithm. The MCU also generates output messages according to the proprietary XBus communication protocol. The data output is fully configurable, so that the MTi 1-s limits the load, and thus power consumption, on the user application processor.

MTi-1 IMU

The MTi-1 module is an IMU that outputs calibrated 3D rate of turn, 3D acceleration and 3D magnetic field. The MTi-1 also outputs coning and sculling compensated orientation increments and velocity increments (∆q and ∆v). Advantages over a gyroscope-accelerometer combo-sensor are the inclusion of synchronized magnetic field data, on-board signal processing and the easy-to-use synchronization and communication protocol. Moreover, the testing and calibration over temperature performed by Xsens result in a robust and reliable sensor module that can be integrated within a short time frame. The signal processing pipeline and the suite of output options allow access to the highest possible accuracy at any output data rate, limiting the load on the user application processor. 

MTi-2 VRU/AHT

The MTi-2 is a 3D VRU/AHT. Its algorithm computes 3D orientation data with respect to a gravity referenced frame: drift-free roll, pitch and unreferenced yaw. Although the yaw is unreferenced, it is superior to gyroscope integration. In addition, it outputs calibrated sensor data: 3D acceleration, 3D rate of turn and 3D magnetic field data. All modules of the MTi 1-series output data generated by the strapdown integration algorithm (orientation and velocity increments - ∆q and ∆v). The 3D acceleration is also available as so-called free acceleration, which has the local-gravity subtracted. The drift in unreferenced heading can be limited using the #Active Heading Stabilization feature.

MTi-3 AHRS

The MTi-3 supports all features of the MTi-1 and MTi-2, and, in addition, is a full magnetometer-enhanced AHRS. In addition to the roll and pitch, it outputs a true magnetic North referenced yaw and calibrated sensor data: 3D acceleration, 3D rate of turn, 3D orientation and velocity increments (∆q and ∆v) and 3D earth-magnetic field data. Free acceleration is also computed by the MTi-3 AHRS.

MTi-7 GNSS/INS

The MTi-7 provides a GNSS/INS solution offering a position and velocity output in addition to orientation output. The MTi-7 uses advanced sensor fusion algorithms developed by Xsens to synchronize the inputs from the module’s on-board accelerometer, gyroscope and magnetometer with the data from an external GNSS receiver and/or barometer. The raw sensor signals are combined and processed at a high data rate of 800 Hz to produce a real-time data stream with the device’s 3D position, velocity and orientation (roll, pitch and yaw).

MTi-8 RTK GNSS/INS

The MTi-8 is an improved version of the MTi-7, designed to be used with high precision GNSS receivers. Using RTK (Real-Time Kinematics) functionality, GNSS receivers can determine global position at centimeter-level accuracy. The MTi-8 features more advanced sensor fusion algorithms and signal processing pipelines that allow it to process these data without loss of accuracy/precision. The MTi-8 also takes into account timing errors and the distance between the GNSS antenna and the MTi itself.

GNSS input

The MTi-7 and MTi-8 require data from an external GNSS receiver to provide a full GNSS/INS solution. This can be achieved by connecting a GNSS receiver that communicates with one of the following supported protocols:

• u-blox' UBX protocol
• NMEA sentences (officially supported with firmware version 1.10.0 and up)
• Septentrio's SBF protocol (beta support with firmware version 1.18.0 and up)
• Trimble's GSOF protocol (beta support with firmware version 1.18.0 and up)

The use of each of these supported protocols is discussed in more detail in the paragraphs below.

u-blox (UBX) input

When connecting to a u-blox receiver (e.g. u-blox MAX-M8), the MTi will configure it correctly on start-up. No prior configuration of the u-blox receiver is required. It is, however, recommended to inform the MTi of what type of u-blox receiver is connected. This can be done using the Device Settings window in MT Manager (version 2021.4 and later), or using an Xbus message called SetGnssReceiverSettings, described in the MT Low Level Communication Protocol Documentation. The user can select one of the officially supported u-blox receiver series: MAX-M8 (default), NEO-M8 or ZED-F9.

NMEA input

Almost all GNSS receivers support the output of NMEA messages, which means that this functionality enables the use of virtually any external GNSS receiver. It is important to note that both the GNSS receiver and the MTi must be configured prior to connecting both systems to each other. The NMEA input mode can be enabled using the Device Settings window in MT Manager (version 2021.4 and later), or using an Xbus message called SetGnssReceiverSettings, described in the MT Low Level Communication Protocol Documentation.

The table below summarizes the settings needed to configure the MTi-7/8 to use the NMEA input mode. This will enable the MTi to use the GNSS data and provide the user with a full GNSS/INS solution. The MTi will also synchronize its internal clock with the UTC time that is present in the sentences.

An example of how to setup an external GNSS receiver using the NMEA protocol can be found on BASE.

Septentrio (SBF) input

Please note that both the GNSS receiver and the MTi must be configured prior to connecting both systems to each other. The Septentrio input mode can be enabled using the Device Settings window in MT Manager (version 2021.4 and later), or using an Xbus message called SetGnssReceiverSettings, described in the MT Low Level Communication Protocol Documentation.

The table below summarizes the settings needed to configure the MTi-7/8 to use the Septentrio input mode. This will enable the MTi to use the GNSS data and provide the user with a full GNSS/INS solution. The MTi will also synchronize its internal clock with the UTC time that is present in the sentences.

An example of how to setup an external GNSS receiver using the SBF protocol can be found on BASE.

Trimble (GSOF) input

Please note that both the GNSS receiver and the MTi must be configured prior to connecting both systems to each other. The Trimble input mode  can be enabled using the Device Settings window in MT Manager (version 2021.4 and later), or using an Xbus message called SetGnssReceiverSettings, described in the MT Low Level Communication Protocol Documentation.

The table below summarizes the settings needed to configure the MTi-7/8 to use the Trimble input mode. This will enable the MTi to use the GNSS data and provide the user with a full GNSS/INS solution. The MTi will also synchronize its internal clock with the UTC time that is present in the sentences.

An example of how to setup an external GNSS receiver using the GSOF protocol can be found on BASE.

Barometer input

The MTi-7 and MTi-8 can make use of an external barometer, allowing them to produce more accurate estimates of altitude. The barometer needs to be connected over the auxiliary SPI interface; refer to the Hardware Integration Manual for further details. The MTi-7/8-Development Kits already include a GNSS daughter card which also features a barometer.

The following barometers are supported:

• Bosch BMP280
• Bosch BMP384 (only with firmware version 1.18.0 and up)
• Bosch BMP388 (only with firmware version 1.18.0 and up)
• Bosch BMP390 (only with firmware version 1.18.0 and up)

After powering up the MTi-7/8, it will automatically detect and configure the connected barometer. No user input is required, however it is possible to check whether the connection has been established successfully by sending the RunSelfTest command; refer to the MT Low-Level Communication Protocol Document for further details.

Signal processing pipeline

The MTi 1-series is a self-contained module, so all calculations and processes such as sampling, coning & sculling compensation and the Xsens sensor fusion algorithm run on board.

Strapdown integration

The Xsens optimized strapdown algorithm performs high-rate dead-reckoning calculations at 800 Hz, allowing accurate capture of high frequency motions. This approach ensures a high bandwidth. Orientation and velocity increments are calculated with full coning & sculling compensation. These orientation and velocity increments are suitable for any 3D motion tracking algorithm. Increments are internally time-synchronized with other sensors. The output data rate can be configured with different frequencies up to 100 Hz. The inherent design of the signal pipeline with the computation of orientation and velocity increments ensures there is absolutely no loss of information at any output data rate. This makes the MTi 1-series attractive for systems with limited communication bandwidth. 

Xsens sensor fusion algorithm for VRU and AHRS product types

The Xsens sensor fusion algorithm optimally estimates the orientation with respect to an Earth fixed frame utilizing the 3D inertial sensor data (orientation and velocity increments) and 3D magnetometer.

The user can set the sensor fusion algorithm with different filter profiles in order to get the best performance based on the application scenario (see table below). These filter profiles contain predefined filter parameter settings suitable for different user application scenarios.

In addition, all filter profiles can be used with the #Active Heading Stabilization setting, which significantly reduces heading drift during magnetic disturbances. The #In-run Compass Calibration setting can be used to compensate for magnetic distortions that are caused by every object the MTi is attached to.

Xsens sensor fusion algorithm for GNSS/INS product types

The Xsens sensor fusion algorithm in the MTi-7 and MTi-8 has several advanced features. The algorithm adds robustness to the orientation and position estimates by combining measurements and estimates from the inertial sensors and GNSS receiver in order to compensate for transient accelerations and magnetic disturbances.

When the MTi-7/8 has limited/mediocre GNSS reception or even no GNSS reception at all (outage), the sensor fusion algorithm seamlessly adjusts the filter settings in such a way that the highest possible accuracy is maintained. The GNSS status is continuously monitored and the filter accepts GNSS data when available and sufficiently trustworthy. The sensor will continue to output position, velocity and orientation estimates, although the accuracy is likely to degrade over time as the filters will have to rely on dead-reckoning. If the GNSS outage lasts longer than 45 seconds, the MTi-7/8 stops output of the position and velocity estimates and begins sending these outputs once the GNSS data becomes acceptable again.

The table below reports the different filter profiles the user can set based on the application scenario. Every application is different, and results may vary from setup to setup. It is recommended to reprocess recorded data with different filter profiles in MT Manager to determine the best results in your specific application.

[1] External aiding sensors for the MTi-7/8

[2] This filter profile can be used even when the barometer is not part of the design.

Lever Arm Correction (MTi-8 only)

Due to the increased position accuracy of the MTi-8 when RTK corrections are provided, it is important to define the distance between the MTi and the GNSS antenna, also known as the GNSS lever arm. The figure below highlights the effect of the lever arm on measurements taken with cm-level accuracy.

Lever arm correction for an MTi-8

The lever-arm describes the position of the GNSS antenna with respect to the origin of measurement of the MT device (see Design and Packaging). This information is essential to the sensor fusion algorithm in order to correct its position and velocity measurements accordingly. The lever arm can be set from the Device Settings window in MT Manager, or by using the setGnssLeverArm low-level communication command (refer to the MT Low Level Communication Protocol Document for details). More background information on the GNSS lever arm can be found on BASE.

Example of lever arm measurements for an MTi with RTK functionality

Magnetic interference

Magnetic interference can be a major source of error for the heading accuracy of any AHRS. As an AHRS uses the magnetic field to reference the dead-reckoned orientation on the horizontal plane with respect to the (magnetic) north, a severe and prolonged distortion in that magnetic field will cause the magnetic reference to be inaccurate. The MTi 1-series module has several ways to cope with these distortions to minimize the effect on the estimated orientation.

Magnetic Field Mapping

When the distortion moves with the MTi (i.e. when a ferromagnetic object solidly moves with the MTi module), the MTi can be calibrated for this distortion. Examples are the cases where the MTi is attached to a car, aircraft, ship or other platforms that can distort the magnetic field. It also handles situations in which the sensor has become magnetized. These types of errors are usually referred to as soft and hard iron distortions. The Magnetic Field Mapping procedure compensates for both hard iron and soft iron distortions.

The magnetic field mapping (calibration) is performed by moving the MTi together with the object/platform that is causing the distortion. The results are processed on an external computer (Windows or Linux), and the updated magnetic field calibration values are written to the non-volatile memory of the MTi 1-series module. The magnetic field mapping procedure is extensively documented in the Magnetic Calibration Manual.

In-run Compass Calibration (ICC)

In-run Compass Calibration is a way to calibrate for magnetic distortions present in the sensor operation environment using an onboard algorithm. The ICC is an alternative to the offline MFM (Magnetic Field Mapper). It results in a solution that can run embedded on different industrial platforms (leaving out the need for a host processor like a PC) and relies less on specific user input. The MFM tool, which does require a host processor, is, however, still recommended over or in addition to the ICC. The ICC is aimed at applications for which the MFM solution cannot be used (e.g. MTi 1-s that is not able to be connected to a PC), when MFM is not sufficient (e.g. applications that move outside of the plane of motion used during the calibration), or when the user uses the same MFM result performed for one sensor to calibrate different sensors (typical for large volume applications).

It should be noted that magnetic distortions present in the environment of the motion tracker that move independently or change over time are not compensated by the ICC unless they are changing very slowly. Such distortions do not affect the parameter estimation; they are simply not compensated for. This also means that (ferromagnetic) objects should not be attached to or detached from the sensor while ICC is running.

If the user is able to perform a calibration motion in a homogeneous magnetic field or environment that is representative of the application, then it is possible (and recommended) to use the "Representative Motion" feature (RepMo). RepMo is available in MT Manager, XDA and Low-Level Communication Protocol (Xbus protocol).

Additional details are available on BASE.

Active Heading Stabilization (AHS)

The Active Heading Stabilization (AHS) is not a magnetic calibration procedure, but a software component within the sensor fusion engine designed to give low-drift unreferenced heading solution in a disturbed magnetic environment. AHS is not tuned for nor intended to be used with GNSS/INS devices. Therefore, Xsens discourages the use of this feature for GNSS/INS devices, including the MTi-7/8.

For more information on the activation and use of AHS, refer to BASE.

Frames of reference

The MTi 1-series module uses a right-handed coordinate system and the default sensor frame is defined as shown in the figure below. For a more exact location of the sensor frame origin, refer to the Hardware Integration Manual. Some of the commonly used data outputs with their output reference coordinate system are listed in the table below.

Default sensor fixed coordinate system for the MTi 1-series module

The default local reference coordinate system is East-North-Up (ENU). In addition, the MTi 1-s module has predefined output options for North-East-Down (NED) and North-West-Up (NWU). Orientation resets have an effect on all outputs that are by default output with an ENU reference coordinate system.

For the MTi-7/8, the Local Tangent Plane (LTP) is a local linearization of the Ellipsoidal Coordinates (Latitude, Longitude, Altitude) in the WGS-84 Ellipsoid. Velocity data calculated by the sensor fusion algorithm is provided in the same coordinate system as the orientation data, and thus adopts orientation resets as well. For the MTi-3, the Latitude, Longitude and Altitude can be stored in the non-volatile memory of the MTi. Refer to the MT Low-level Communication Protocol Document for more information. The default location stored in memory is that of the calibration setup in Enschede, the Netherlands.

Triggering and Synchronization

The MTi 1-series supports a limited set of synchronization options. Please refer to BASE for more information: Synchronization with the MTi