The MTi-3, MTi-30, MTi-630 and MTi-300 (AHRS) as well as the MTi-7, MTi-670 and MTi-G-7xx (GNSS/INS) are able to output a North referenced Yaw, also referred to as Heading. By default the orientation output is in the East-North-Up (ENU) frame, meaning that Roll, Pitch and Yaw will equal 0 degrees when the x-axis of the MTi is aligned with East, the y-axis with North and the z-axis upward. When the x-axis of the MTi is pointing North, Yaw will equal 90 degrees.
The MTi-2, MTi-20, MTi-620 and MTi-200 (VRU) are able to output an unreferenced Yaw. The VRU does not use the magnetic field for referencing, but can use it for stabilization. This means that the orientation upon powering up will be used as a reference (Yaw = 0).
There can be several reasons for unstable or incorrect heading (Yaw). These reasons have been listed below.
- The magnetic environment is not homogeneous. You can use the magnetic norm to identify a non-homogenous magnetic environment. The magnetometer requires a homogeneous magnetic field to reach the accuracy in the datasheet (under static and dynamic conditions). Especially when using the magnetic North as a reference (e.g. filter profiles GeneralMag on the MTi-7, MTi-670 or MTi-G-710 and NorthReference in the MTi-3 or MTi-630), a deviation in the magnetic field will mean a deviation in the heading. Please refer to the Magnetic Calibration Manual and the MTi User Manual for more information about the influence of magnetic materials and magnets.
- You have not performed a magnetic calibration a.k.a. Magnetic Field Mapping (MFM). Please perform a MFM. How to perform a correct MFM is described in our Magnetic Calibration Manual.
- You have previously performed a Magnetic Field Mapping (MFM). If you have performed a MFM in a certain environment and you change the structure the Motion Tracker is attached to, the MFM may become invalid. For example, if you have performed an MFM with the MT integrated in a car, then the magnetic calibration parameters compensate for the (static) ferromagnetic effects of the body of the car. If then the MT is taken out of the car, the magnetic calibration parameters will still try to compensate for the ferromagnetic effects. This may cause inaccurate orientation readings. Please perform another MFM whenever the (static) environment changes. For more information also see the Magnetic Calibration Manual.
- You have changed the orientation output frame of the MTi. The easiest way to bring the MTi back to its default orientation output frame (ENU) is by clicking "Revert" in the MT Settings window of MT Manager.
- It can also be that the gyroscope biases are too high. This usually results in a constant drift of the Yaw output. Distinguishing the gyroscope bias from the magnetic field disturbance is only possible when the change in magnetic field is significantly higher than the gyroscope offset. Performing a No Rotation Update at the start of the measurement can solve this, as the gyro bias will be low enough to continuously estimate it during the measurement.
- When the magnetometer is clipping, the magnetic field will not be used in the sensor fusion algorithm. When there is a desire to continue to use the magnetic field, place the MTi in another location where the magnetic field is less disturbed.
MTi-2, MTi-20, MTi-620 and MTi-200 (VRU) specific
- You are using an MTi-2, MTi-20 or MTi-200 without Active Heading Stabilization (AHS). The heading of these units is not stabilized by default. You can use one of the following:
MTi-3, MTi-30, MTi-630 and MTi-300 (AHRS) specific
- The AHRS constantly adapts or "converges" to the local magnetic field. Adopting the new magnetic field however occurs tens of seconds after the magnetic field has changed. This is intended behavior of the Motion Tracker to prevent short lasting magnetic distortions influencing the heading. See this article for more details and an example video.
- The Earth's magnetic field intensity and declination vary across the Earth and over the years. Because of this, the MTi is equipped with an up-to-date World Magnetic Model (WMM), stored in its on-board memory. The location of the MTi (in latitude/longitude/altitude format) can also be stored manually in its on-board memory, such that the MTi can accurately reference its Yaw with respect to the local magnetic field. Depending on the location on Earth, this may result in an offset correction of several degrees. The location of the MTi can be set in the Device Settings window of MT Manager, via the Xsens Device API or using low-level communication. The default location in memory is the test and calibration environment in Enschede, the Netherlands. GNSS-enabled devices will automatically retrieve the location from the GNSS receiver.
- You have set the filter profile to a VRU filter profile. This filter profile does not have a magnetically referenced heading. You can do one of the following:
MTi-7, MTi-670 and MTi-G-7xx (GNSS/INS) specific
- The heading observability is too low. This can occur when
- the application is moving in a straight line
- the application is not moving or very slowly moving (less than 7 m/s)
- GNSS reception is lost