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This section discusses the Xsens Avior series signal processing and algorithm description.
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.
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.
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.
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 MTi, 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 MTi 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 MTi 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 MTi 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 MTi does not change. This is in contrast to the NorthReference heading behaviour, which forces the MTi 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 MTi 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. |
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 |
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 MTi 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 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.
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.
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.
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 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.
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.
.png?Policy=eyJTdGF0ZW1lbnQiOlt7IlJlc291cmNlIjoiaHR0cHM6Ly9kemY4dnF2MjRlcWhnLmNsb3VkZnJvbnQubmV0L3VzZXJmaWxlcy8yNzc1NC8zNTEzNS9ja2ZpbmRlci9pbWFnZXMvcXUvMjAyNS9pbWFnZSg0KS5wbmciLCJDb25kaXRpb24iOnsiRGF0ZUxlc3NUaGFuIjp7IkFXUzpFcG9jaFRpbWUiOjE3NjU4Mzg2MjJ9fX1dfQ__&Signature=maCbvFugoop9QsbtNyc9S9TLMxGPRE17yFGqV7lkZGXSP8AXqWTbn4GB1SwRasqkEN7AX9s9ZpN7A73qAlpBHcS9GLg9aI6saAhERFyTfFcaG-IVfkh5pGJSCSisiQ9E13ZgTitMOGMwP9d4dG7zthFaj8LZSJ7liiEYSyQmM3-9krGYALFDY7HD9aXzOEp3u6SRZV3xZz4mVKoD6Wc6zUVGXVUoCq4AgmZRqICK57YdczjYos-ErE18jbtWGUye0S74j4Ls13IZYenUljggMdb6A7NAcejMmtRDCQgwjxrOh1SgMsHlJJWi-hH-vpzwGaiDC-gi~a3K-7L3Ul6L1Q__&Key-Pair-Id=K2TK3EG287XSFC)
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 MTi 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.