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Xsens MTi User Manual

Magnetic calibration manual
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Magnetic Calibration Manual
User Manual
Please visit mtidocs.xsens.com for the latest version of this document.
Created: Monday, March 13, 2023
Copyright © Xsens. All Rights Reserved.

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  • Page 1 Magnetic Calibration Manual User Manual Please visit mtidocs.xsens.com for the latest version of this document. Created: Monday, March 13, 2023 Copyright © Xsens. All Rights Reserved.
  • Page 2 The information contained in this document is subject to change without notice. This document contains proprietary information which is protected by copyright. All rights are reserved. No part of this document may be photocopied, reproduced, or translated to another language without the prior written consent of Xsens.

  • Page 3: Table Of Contents

    Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Table of Contents Magnetic Calibration Manual ......................Introduction ..........................Magnetic Distortion Theory ....................Using the Magnetic Field Mapper ................... Non-default calibration procedures ..................Copyright © 2021 Xsens Page 3...

  • Page 4: Magnetic Calibration Manual

    Magnetic Calibration Manual Xsens MTi User Manual Repository Click the icon to download the entire document in PDF. Copyright © 2021 Xsens Page 1...

  • Page 5: Introduction

    (Earth) magnetic field is distorted (warped) and an error in estimated orientation may arise. Whether or not the MTi uses the magnetometers to estimate orientation depends on the device's functionality and selected filter profile.
  • Page 6 field. Depending on the location on Earth, this may result in an offset correction of several degrees. The location of the MTi (in this may result in an offset correction of several degrees. The location of the MTi (in...

  • Page 7: Magnetic Distortion Theory

    Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Magnetic Distortion Theory Background The direction of the measured Earth's magnetic field relative to the orientation of the device is used as a (3D) compass. This direction is used as an absolute reference in the MT's orientation estimation algorithms (heading or yaw).
  • Page 8 Magnetic Calibration Manual Xsens MTi User Manual Repository assumptions about the application. This is the task of the Xsens Kalman Filter (XKF) or Xsens Estimation Engine (XEE) running inside the on-board processor of the MT. Static disturbances. These disturbances are often caused by mounting the MT to an object of which the motion is to be recorded (the MT moves with the object).
  • Page 9 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Flowchart for tackling magnetic distortions. Method In a non-disturbed magnetic field, we define the 3D measured magnetic field vector to have a magnitude (magnetic norm) equal to 1 and, therefore, all measured points would ideally lie on the circumference of a 3-dimensional sphere with its centre at zero.
  • Page 10 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository The norm of the magnetic field vector before (red) and after (blue) compensation using the Magnetic Field Mapper. The black dots represent the samples used by the MFM algorithm.

  • Page 11: Using The Magnetic Field Mapper

    Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Using the Magnetic Field Mapper Using the Magnetic Field Mapper Mounting of the MT How to perform a calibration measurement Performing the calibration measurement Explanation of the Advanced Results...
  • Page 12 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Every time the sensor is temporarily removed from the object, it is advised to repeat Every time the sensor is temporarily removed from the object, it is advised to repeat the calibration procedure.
  • Page 13 During the calibration trial, Xsens orientation estimates are used for inclination During the calibration trial, Xsens orientation estimates are used for inclination measurements. This means that the inclination accuracy will be less when the sensor measurements.
  • Page 14 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Home screen of the Magnetic Field Mapper Press 'Scan' to scan for the connected MT. If the MT is not found, the following dialog is shown. Error message indicating no devices are found Scan the required port and click Next.
  • Page 15 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Click 'Next' and select the location where the software stores the log file of the calibration measurement. Screen where you select the logging directory The software is now ready to start the calibration measurements. Read #How to perform a calibration measurement for more information about how to perform the measurement.
  • Page 16 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository When the sequence of rotations is completed, press 'Stop' to end the measurement and to start the analysis of the data. A window will pop up, visualizing the calibration results.
  • Page 17 The Advanced Results window of the Magnetic Field Mapper generates several reports which are discussed below. In the Advanced Results window, this figure shows the total movement of the MTi over all directions of an imaginary sphere. In the left image, the original magnetic field measurements are shown.
  • Page 18 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository In the Advanced Results window, this figure shows whether the MFM was able to apply the new magnetic field model to all corrected magnetic field measurements of the file or measurement.
  • Page 19 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository In the Advanced Results window, this figure shows the norm of the magnetic field before and after Magnetic Field Mapping. It also shows which data points were used for the Magnetic Field Mapping, indicated by '+'.
  • Page 20 Explanation Large accelerations If the object is accelerated too much during calibration, this will cause an error. If large accelerations cannot be avoided, contact the Xsens support team at http://www.xsens.com/support. Limited rotation The calibration procedure is designed to process measurements in which the MT is rotated through a...
  • Page 21 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository This report shows an even distribution of points on the sphere, has a normal distribution of residuals (fit within the Gaussian model), has small residuals in all directions and a norm after MFM of close to 1.
  • Page 22 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository The points in the sphere do not form solid lines, almost all points are outside the graph in the Gaussian model plot, the residuals are very large in all directions and the norm after MFM is not 1.
  • Page 23 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Example report for a 2D Magnetic Field Mapping. The application is a wheeled robotic ground vehicle consisting of a large metal structure with batteries and motors nearby. During the calibration, the vehicle drove two full circles.

  • Page 24: Non-Default Calibration Procedures

    Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Non-default calibration procedures This section describes non-default procedures to calibrate the MT for magnetic disturbances. Non-default calibration procedures Use a previously recorded measurement Off-line Magnetic Field Mapping Magnetic Field Mapping SDK...
  • Page 25 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Home screen with "File Processing" selected Load a previously recorded file from a previous MFM procedure. By default, you can find these in the temp folder: type %temp% in the Windows Explorer bar.
  • Page 26 "Revert" button in MT Manager, or by sending the RestoreFactoryDefaults command using the Xsens Device API or Xbus communication. If resetting the configuration settings (such as baud rate or filter profile) is undesired, then it is also possible to only revert the magnetometer calibration parameters.
  • Page 27 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository Home screen for the off-line procedure Click 'Load' to browse for the log file. Screen to select the file for the off-line procedure Click Start, check the results and if you are satisfied, click 'Write file for selected devices' to generate the MFM results file.
  • Page 28 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository The filename of the MFM results file is: X_result_DID.bin where X is the original file name and DID the Device ID. Writing calibration results to your MT The MFM results file contains a single Xbus message that contains the calibration results. If this message is sent to the MT, it will update the non-volatile memory which holds the calibration values.
  • Page 29 Low-Level Communication Protocol document (MT0101P). Protocol document (MT0101P). The MTi is unavailable for communication for a short period after the calibration The MTi is unavailable for communication for a short period after the calibration result message has been sent.
  • Page 30 Examples for C#, MATLAB, C++ Structure of the MFM SDK Note: Extensive HTML documentation for the MFM SDK is included in the SDK. See the Xsens folder (e.g. C:/Program Files/Xsens/MT Software Suite x.x/Documentation/MFM SDK) In-Run Compass Calibration (ICC) and Representative Motion (RepMo) Disclaimer: ICC and RepMo are currently still in beta for all MT devices.
  • Page 31 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository features are not available for MTw devices. features are not available for MTw devices. Representative Motion (RepMo) Similar to the Magnetic Field Mapper, Representative Motion (RepMo) provides a solution to calibrate the sensor for magnetic distortions caused by objects that move with the MT.
  • Page 32 MT Manager in the following example. The results of an MTi 1-series attached to a battery are shown in the figure below. It is important to note that the magnetic norm (black line in the middle graph) should be constant if the sensor is properly calibrated and it is moving in a magnetically homogeneous environment.
  • Page 33 MTi. However, in contrast to the (single-run) Magnetic Field Mapper and Representative Motion, ICC is a feature that is continuously running in the background of the onboard filters of the MTi in order to improve magnetometer calibration for hard and soft iron effects.
  • Page 34 Magnetic Calibration Manual Xsens MTi User Manual Repository As a use case example, consider a scenario in which the MTi sensor is rigidly mounted on a car. The user can perform an MFM in a homogeneous environment by performing a circle with the car.
  • Page 35 MT memory using the commands described in the previous paragraphs. As soon as the command for storing the ICC results is sent to the MTi, the ICC/RepMo parameters and magnetometer calibration parameters will be copied from the RAM memory to the non-volatile memory where the previous calibration parameters will be updated.
  • Page 36 The initial calibration parameters are a result of the factory calibration by Xsens. If ICC or the Representative Motion feature is on, filter states and compensation parameters are available in the RAM-memory. The magnetic field data (data output as shown in MT Manager) are not influenced by parameters in the RAM-memory.
  • Page 37 Magnetic Calibration Manual Xsens MTi User Manual Repository Once the command is given by the user to store the ICC calibration parameters, the MTi will write the parameters to the eMTS on the non-volatile memory. This way, the magnetometer sensor readings are calibrated with the ICC parameters before they are fed into the sensor fusion algorithm.
  • Page 38 Magnetic Calibration Manual Magnetic Calibration Manual Xsens MTi User Manual Repository can remain enabled to run in the background and further improve parameter estimation. Copyright © 2021 Xsens Page 35...

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