Signal processing and algorithms
This section discusses the Xsens Avior series signal processing and algorithm description.
- Signal processing pipeline
- Xsens sensor fusion algorithm for VRU and AHRS product types
- Xsens sensor fusion algorithm for the GNSS/INS product types
- GNSS input
- Magnetic interference
- Frames of reference
Signal processing pipeline
The Xsens Avior OEM series is a self-contained product. 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 up to 10 kHz, 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 for different frequencies. See #Output data rates. 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 Xsens Avior OEM series also attractive for systems with limited communication bandwidth.
Xsens sensor fusion algorithm for VRU and AHRS product types
The Xsens Avior VRU and AHRS run the newest Xsens sensor fusion algorithm implementing the latest Xsens insights. It optimally estimates the orientation with respect to an Earth fixed frame utilizing the 3D inertial sensor data (orientation and velocity increments) and 3D magnetometer data.
The Xsens sensor fusion algorithm uses assumptions to obtain the orientation estimations. Since the assumptions may be more or less valid based on the characteristics of the typical dynamics of the application, and since the magnetic field differs per application, the Xsens algorithm makes use of a set of filter profiles to be able to use the correct assumptions given the application. This way, the algorithm can be optimized for different types of movements and conditions.
With the Xsens Avior VRU and AHRS, the user can configure different algorithm behaviours by selecting a “base” filter profile and, next to that, a heading behaviour. See image below.
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Filter profile and heading behaviour selection: a tiered approach
The “base” filter profile selection affects the general behaviour of the device, mainly based on the nature of the typical expected dynamics of the application. The heading behaviour, as the name suggests, affects the heading/yaw output of the Avior, and determines how the magnetometer measurements are interpreted. This tiered approach gives more freedom to select the desired behaviour for different user application scenarios. The tables below summarize the filter profile and heading behaviour options.
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 filter profile for your specific application.#1
|
Name |
Product |
Description |
Typical applications |
|---|---|---|---|
|
Responsive |
Xsens Avior VRU and AHRS |
This filter profile is designed for indoor applications as well as applications that experience high dynamics and jerky movements. When the Avior is static, an automatic gyro bias estimation is performed in the background. |
|
|
Robust |
Xsens Avior VRU and AHRS |
This filter profile is suitable for most applications. Compared to the other filter profiles, it has a more robust tuning. When the Avior is static, an automatic gyro bias estimation is performed in the background. |
|
|
General#2 |
Xsens Avior VRU and AHRS |
This filter profile behaves like the General filter profile implemented for the previous generation of Xsens Products (e.g. MTi-30). It is more sensitive to magnetic field changes. It does not perform an automatic gyro bias estimation in the background. This filter profile cannot be combined with the FixedMagRef heading behaviour. |
|
|
Name |
Product |
Description |
Typical applications |
|---|---|---|---|
|
NorthReference |
Xsens Avior AHRS |
This heading behaviour assumes a homogeneous magnetic environment that can be used to estimate a stable north-referenced#3 heading.
|
All applications that require a north-referenced heading and are used in a homogeneous magnetic field. |
|
FixedMagRef |
Xsens Avior AHRS |
This heading behaviour is based on the idea that the heading is not necessarily referenced to the local magnetic north. Instead, it maintains a fixed heading reference frame based on what is defined when the Avior is powered up (based on the initially observed magnetic field). This means that there is no drift with respect to the starting frame when the local magnetic field changes. For example, when moving from room A to room B, where room B has a different local magnetic field direction than room A, the heading output of the Avior does not change. This is in contrast to the NorthReference heading behaviour, which forces the Avior to estimate the heading based on the local magnetic field. |
All applications are used in environments where different magnetic fields are present (e.g. mixed indoor/outdoor applications). |
|
VRU |
Xsens Avior VRU and AHRS |
The yaw is unreferenced. This means that it is initialized at 0° when the Avior is powered up and the yaw will be computed relative to this initial orientation. The magnetic field is not used to estimate the yaw. Because of small inaccuracies that originate when integrating gyroscope data, the Yaw output will contain an error that builds up over time, also known as “drift”. Note however, that because of the working principle of the sensor fusion algorithm, the drift in yaw will be much lower than when gyroscope signals are simply integrated. |
Applications where only roll and pitch is of interest and/or applications that are used in environments where the magnetic field cannot be trusted (e.g. stabilized antenna platforms or pipeline inspection tools). |
|
VRUAHS |
Xsens Avior VRU and AHRS |
This heading behaviour activates the Active Heading Stabilization (AHS) on top of the above-described VRU behaviour. AHS is a software component within the sensor fusion engine designed to give a low-drift unreferenced heading solution, even in a disturbed magnetic environment. The yaw remains unreferenced, but the drift is limited.#4 |
Scenarios where the magnetic field cannot be trusted completely, but a stable yaw is needed. |
Xsens sensor fusion algorithm for the GNSS/INS product types
The Xsens sensor fusion algorithm in the GNSS/INS products has several advanced features. The algorithm adds robustness to the orientation and position estimates by combining measurements and estimates from the inertial sensors, magnetometer and a GNSS receiver in order to compensate for transient accelerations and magnetic disturbances.
The GNSS status is continuously monitored and the filter accepts GNSS data when available and sufficiently trustworthy. When the product has limited/mediocre GNSS reception or even no GNSS reception at all (e.g. during outages), the fusion algorithm seamlessly adjusts the filter settings in such a way that the highest possible accuracy output is maintained. The Avior will continue to output position, velocity and orientation estimates, although the accuracy is likely to degrade over time as the filters can only rely on dead-reckoning. If the GNSS outage lasts longer than 45 seconds, the Avior stops the output of the position and velocity estimates, and resumes sending these outputs once the GNSS data becomes acceptable again.
Lever Arm Correction
For Avior (RTK) GNSS/INS (Single antenna)
Due to the improvement in position accuracy on Avior (RTK) GNSS/INS devices with and without RTK support, it is important to define the distance between the Avior 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 the Avior (RTK) GNSS/INS
The lever-arm describes the position of the GNSS antenna with respect to the origin of measurement of the Avior 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 Avior Dual Antenna RTK GNSS/INS
The primary GNSS antenna lever arm is configured in exactly the same way as for single-antenna devices: it describes the position of the primary antenna with respect to the origin of measurement of the MT device (see Design and Packaging).
In dual-antenna mode, the second antenna provides an additional heading reference, which is independent of magnetic field conditions. To use it, the sensor fusion algorithm needs to know where the secondary antenna is mounted relative to the device. There are three ways to provide this information:
| Option | Configuration | Resulting mode |
|---|---|---|
| 1. No secondary lever arm (default) | Nothing is provided for the secondary antenna | The device operates in single-antenna mode |
| 2. Secondary antenna lever arm | The lever arm from the motion tracker origin to the secondary antenna is provided (in the same way as the primary lever arm) | Dual-antenna mode |
| 3. Antenna baseline angle | The angle between the motion tracker's X-axis and the vector pointing from antenna 1 to antenna 2 is provided, in the range [-180°, +180°] | Dual-antenna mode |
Option 1: Single-antenna mode (default)
If no information is provided for the secondary antenna, the device ignores it and operates as a single-antenna GNSS/INS.
Option 2: Secondary antenna lever arm (recommended for best accuracy)
Provide the lever arm of the secondary antenna, measured from the origin of measurement of the motion tracker to the phase center of the secondary antenna, expressed in the sensor coordinate frame (X, Y, Z), using the same convention as the primary lever arm. This is the most complete description of the installation and allows the algorithm to correct both antenna measurements for their offsets from the device.
Option 3: Antenna baseline angle (quick method)
If the exact position of the secondary antenna is not known, you can instead provide the angle between the motion tracker's X-axis and the vector pointing from antenna 1 to antenna 2. The angle is defined in the range [-180°, +180°]. This method is quick to set up, but is less precise than Option 2, as it does not describe the full antenna position relative to the device.
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Example of Option 3: Antenna baseline angle
Smoothers
The Avior (Dual Antenna) (RTK) GNSS/INS products have optional sensor fusion functions called smoothers for reducing sudden jumps in the output data that may arise from fusing low-rate GNSS receiver messages with high-rate inertial sensor data. There is a smoother for orientation data for both the Avior GNSS/INS and Avior RTK GNSS/INS, and a position/velocity smoother for the Avior RTK GNSS/INS. The smoothers can be enabled from the Device Settings window in MT Manager, or by using the setOptionFlags low-level communication command (see the MT Low Level Communication Protocol Document for details).
Note: This feature is available from Firmware v1.6.0 onwards.
Filter profiles and heading behaviours
The table below reports the different filter profiles and heading behaviours the user can set based on the application scenario. See the previous section for an explanation of this tiered approach. 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
|
Name |
Description |
Typical applications |
|---|---|---|
|
General-INS |
This filter profile is the default setting. Heading behaviour is determined by the heading profile. Altitude (height) is determined by GNSS altitude and accelerometers. When the sensor is static, an automatic gyro bias estimation is performed in the background. |
Currently the only filter profile available. |
|
Name |
Product |
Description |
Typical applications |
|---|---|---|---|
|
NoMag |
Xsens Avior AHRS |
The yaw is initialized at 0° when the Avior is powered up. Once a GNSS fix is obtained, the yaw will converge to a North-referenced heading. The magnetic field is not used to estimate the yaw.
|
Environments where the magnetic field cannot be trusted (e.g. stabilized antenna platforms or pipeline inspection tools). |
|
MagAided |
Xsens Avior AHRS |
Yaw output is mainly based on magnetic heading, together with comparison of GNSS acceleration and the accelerometers. Although this combination makes the yaw more robust than magnetic field alone, a homogeneous or calibrated-for magnetic field is essential for good-performance yaw. |
Environments with a homogeneous magnetic field, and when the absolute heading observability based on GNSS data is too low—especially if your application is often standing still or moving very slowly. |
GNSS Platform
U-blox GNSS receivers support different dynamic platform models in order to adjust the navigation engine to the expected application environment. The GNSS/INS products can be configured to communicate a desired platform model upon start-up. This enables the user to adjust the u-blox receiver platform to match the dynamics of the application. The setting influences the estimates of Position and Velocity and therefore, it affects the behavior of the Xsens filter output.
The platform model can be configured through the Device Settings window in MT Manager (version 2021.4 and later) or using low-level communication by providing the GNSS Platform ID. For more details on the low-level commands used to set the GNSS Platform (SetGnssReceiverSettings), refer to the MT Low Level Communication Protocol Document. For more details on GNSS platform settings, refer to the u-blox Receiver Description Manual.
Note: This feature is available from Firmware v1.6.0 onwards.
Real-Time Kinematic (RTK)
The Avior RTK GNSS/INS supports centimeter-level position accuracy through RTK (Real-Time Kinematic), which uses correction messages from a base station with a known position. The RTK correction data must be supplied as RTCM3 messages, either via the dedicated RTCM input connector (see Functional description) or via the host interface as RTCM messages embedded in xbus (see XMID_ForwardGnssData in the MT Low-Level Communication Protocol Documentation). In the latter case, the NTRIP client in MT Manager can be setup to provide the correction data.
Data output
The Xsens Avior OEM series product variants can output many different data types at many different frequencies. Below is a summary of the most relevant data and maximum output data rates. A full overview is available in the MT Low Level Communication Protocol Documentation.
|
Data Type |
Max Output Data Rate |
|---|---|
|
Orientation data (Euler angles, Rotation Matrix, Quaternions) |
400 Hz |
| DeltaQ, DeltaV | 400 Hz |
|
Acceleration, Rate of Turn, Free Acceleration |
400 Hz |
|
Acceleration HR (High Rate) |
2000 Hz |
|
Rate of Turn HR (High Rate) |
2000 Hz |
Xbus output
The Xbus protocol is Xsens’ standard output protocol utilizing the MTDATA2 data message structure. This output provides a lot of flexibility and enables users to access all functionality of the Avior product range. The Xbus output format is shared with all other MTi products in the Xsens portfolio, so switching between hardware platforms is very easy. More information is available in the MT Low Level Communication Protocol Documentation.
NMEA output
NMEA output is a string output mode which outputs data in the commonly used NMEA 0183 format. More information is available in the MT Low Level Communication Protocol Documentation.
CAN output
The CAN output is an industrial standard interface over which the Xsens Avior OEM series can output its data. CAN-FD functionality is also available with firmware version 1.3.0 or higher. More information on the CAN output can be found in the MT CAN Protocol Documentation.
GNSS input
The Avior GNSS/INS and Avior RTK GNSS/INS 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
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, the Avior will configure it correctly on start-up. No prior configuration of the u-blox receiver is required. It is, however, recommended to inform the Avior of what type of u-blox receiver is connected. This can be done using the Device Settings window in MT Manager, 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: currently supported: ZED-X20P (default for RTK devices) and MAX-F10S (default for non-RTK devices).
NMEA input
Almost all GNSS receivers support the output of NMEA messages, which means that this functionality enables the use of virtually any external single-antenna 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 Avior to use the NMEA input mode. This will enable the Avior to use the GNSS data and provide the user with a full GNSS/INS solution. The Avior will also synchronize its internal clock with the UTC time that is present in the sentences.
|
Setting |
Description |
|---|---|
|
Baudrate |
Minimum 115200 bps |
|
GNSS Message frequency |
4 Hz recommended/default, 10 Hz maximum |
|
Talker ID |
Does not need to be configured (different from 600-series) |
|
Required messages |
GGA, GSA, GST and RMC High precision coordinate formats such as GGALONG are also supported. |
An example of how to setup an external GNSS receiver using the NMEA protocol can be found on BASE.
Note: For the Avior Dual Antenna RTK GNSS/INS, the Generic NMEA input mode is currently only supported in single-antenna mode.
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 estimated orientation on the horizontal plane with respect to the (magnetic) North#3. A severe and prolonged distortion in that magnetic field will cause the magnetic reference to be inaccurate. The Xsens Avior OEM series has several ways to cope with these distortions to minimize the effect on the estimated orientation, which are discussed in the sections below.
Magnetic Field Mapping (MFM)
When the distortion moves with the Avior (i.e. when a ferromagnetic object solidly moves with the Avior module), the Avior can be calibrated for this distortion. Examples are the cases where the Avior 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 Avior mounted on 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 Xsens Avior OEM series. The magnetic field mapping procedure is extensively documented in the Magnetic Calibration Manual.
Frames of reference
The Xsens Avior OEM series uses a right-handed coordinate system. The default sensor-fixed frame (Sxyz) is defined as shown in the figures below. The frame is also printed on the side of the module. For a more exact location of the sensor frame origin, refer to Design and Packaging. When the sensor is rigidly attached to another object or vehicle, it is possible to rotate the sensor-fixed frame Sxyz to an object coordinate frame (Oxyz).#5 The default local earth-fixed frame (LXYZ) is East-North-Up (ENU). In addition, the Xsens Avior OEM series have predefined output options for North-East-Down (NED) and North-West-Up (NWU). Since the Xsens Avior VRU and AHRS cannot receive real time positioning from a GNSS receiver, the user must set correct positional coordinates to allow the Xsens Avior VRU and AHRS to construct the reference frame, magnetic and gravity models.
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Default sensor fixed coordinate system (Sxyz) for the Xsens Avior OEM series
[1] Refer to the BASE article: Recording a data file to be reprocessed in MT Manager.
[2] The General filter profile is only recommended for users who are looking for similar behaviour to the previous generation Xsens products in the typical applications suggested in the table. Using the General filter profile is not recommended for new designed applications.
[3] Note: Under default settings, Yaw (heading) equals 90 degrees when the X-axis of the Avior points north.
[4] For more information on the capabilities of AHS, refer to the BASE article: AHS. Note that in the previous Xsens products, AHS was activated by means of a separate setting.
[5] How to define a new object coordinate system can be found in the Family Reference Manual.