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Filter updates for MTi 1-series in MT Software Suite 4.5.4 beta

Filter updates for MTi 1-series in MT Software Suite 4.5.4 beta

The MT Software Suite 4.5.4 beta and firmware 1.0.12 includes significant improvements for the MTi 1-series. Since its introduction in Q3-2015, the MTi-3 AHRS Development Kit has been delivered to many customers and they have been providing feedback as they experience in their specific applications. Tight cooperation with these customers has allowed Xsens to further improve the MTi 1-series. This application note describes the changes made in the sensor fusion algorithm for the MTi 1-series (XKF3i).

Roll/pitch performance

The gyroscopes in the MTi 1-series are different from the gyroscopes in the MTi 10-series and MTi 100-series. The IMU sensor in MTi 1-series is susceptible to performance degradation due to package stress caused by temperature changes, mounting aspects and vibrations.  This is not the case for MTi 10-series and the MTi 100-series which make use of the robust industrial grade gyroscopes and accelerometers that come in a ceramic package.

With data from customers in field trials, Xsens was able to improve the performance further by optimizing the algorithm parameters in the XKF3i sensor fusion core. One of the focus areas was the performance during dynamics. During a trial where the MTi 1-series was mounted on a hand-held application, the orientation error of the MTi-3 with respect to the MTi-300 rose to over 25º.Figure 1 - The graph shows the roll and pitch behavior of the MTi 1-series in a dynamic trial. The errors in roll and pitch (inclination) were sometimes several 10's of degrees.

 

With the new algorithm parameters applied the performance improvement is considerable (see Figure 2).  The MTi-3 with the FW v1.0.12 closely follows the reference motion. 
Figure 2 - From the same trial as in Figure 1, the new tuning parameters show a significant improvement. Especially when the movement is more dynamic, the new filter profiles offer a far better performance.

Next to applying the new filter tuning to this trial, it has also been tested on other applications.

Figure 3 - The performance of the MTi 1-series has improved significantly, offering better or similar performance under all conditions.

 

New filter profiles

The MTi 1-series is used in applications different to the ones seen for the MTi-10 series and MTi 100-series. The filter profiles for the MTi 1-series are so replaced by filter profiles that are more suitable for the typical MTi 1-series applications.

The filter profiles low_mag_dep slowly converges to a new magnetic field after the magnetic field changed. The slow convergence is not required in the applications of the MTi 1-series. This filter profile is therefore replaced by the north_reference filter profile. This filter profile combines the magnetic field with gyroscope integration. When in a homogeneous magnetic field, the magnetic field is used as reference for heading. However, when the magnetic field changes, the gyroscopes will be used to determine the heading. When there has been warm-up period of the MTi 1-series, the gyroscope bias is estimated accurately, so heading without the magnetic field reference will be maintained for a relatively long period after which the heading will slowly converge to the  local magnetic field. To work properly, the filter profile should only be used in a homogeneous magnetic field with short magnetic distortions. This filter profile is recommended for applications with a homogeneous magnetic field where only short periods of magnetic distortions are expected.

The other filter profiles remain unchanged except from the gyroscope tuning. As they are now different from the filter profiles in the MTi 10-series and MTi 100-series, they have been renumbered:

Nr

Filter profile name

Assumptions and behavior

 Note

50

General

Standard filter profile with conservative tuning. Several 10s of seconds resistance to magnetic field changes; then slowly converges to new magnetic field

Can be used for most applications.

51

High_mag_dep

Filter profile relies heavily on magnetic field. Around 10 seconds resistance to magnetic field changes; then converges to new magnetic field. Assumes homogeneous magnetic field.

Applications that can use the magnetometer and can be magnetic field mapped.

52

Dynamic

Filter profile assumes fast changes in magnetic field, but also periods where it doesn’t change. Around 10 seconds resistance to magnetic field changes; then quickly converges to new magnetic field.

Applications that experience accelerations and dynamic movements, e.g. handheld applications

53

North_reference

Filter profiles assumes a homogeneous magnetic field with short magnetic changes allowed. When there is a long-lasting magnetic disturbance, the heading will very slowly converges to the new magnetic field. 

Applications that have low magnetic distortions and where fast heading changes are not expected, e.g. Satellite on the Move, buoys.

54

VRU_general

Behavior as in general for roll and pitch (inclination). The heading is not referenced by the magnetic field. The gyro bias however is estimated continuously, even in in the z-axis. Magnetic distortions may have an effect on the gyro bias estimation accuracy.

Applications where the magnetic field cannot be trusted, e.g. ground robotics in industrial environments.

 

Heading behavior of the various filter profiles

As can be seen in the graph below, the heading behavior is different per filter profile. The descriptions in the above table are clearly shown in the graph below.

From 560 seconds, the heading of the MTi changes from ~85º to ~140º. However, the magnetic field changes only from ~85º to ~130º. In this case the discrepancy was caused by a magnetic distortion from an iron object.

The different filter profiles react differently on the magnetic distortion. The filter profiles will all perform dead-reckoning for a shorter or longer period, except for the VRU_General filter profile that continues to use the gyroscope integration only.

Figure 4 - the filter profiles of the MTi 1-series behave differently on a magnetic distortion. 

 

Download Latest Software

Please visit https://www.xsens.com/mt-software-suite/ for the latest available software

 

Beta Firmware Update Instruction

For beta Firmware Update instructions please see the article Using the Firmware Updater with a beta firmware (FWU 4.0.6)

 

 

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