Magnetic Field Mapper Documentation
Contents
1. 13 4 NON DEFAULT CALIBRATION 18 4 1 USE A PREVIOUSLY RECORDED 18 OFF LINE PROCEDURE NRROEERTR 19 4 3 MAGNETIC FIELD MAPPING 20 5 CUSTOMER n 21 Document MT0202P J Xsens Technologies B V ii Magnetic Field Mapper Documentation 1 Introduction An MTi MTw motion tracker MT can be used to easily and accurately record 3D orientation When an MT is mounted to an object that contains ferromagnetic materials the measured Earth magnetic field is distorted warped and causes an error in measured orientation if the magnetometers are used to estimate orientation This is not the case in all filter profiles Please refer to the product User Manual for details However the disturbance of the magnetic field caused by mounting the MT on a ferromagnetic object can be corrected for using a specialized calibration procedure that is described in this document The calibration procedure can be executed in a few minutes and yields a new set of electronic datasheet va2. xXSens 2 Theory of Operation 2 1 Background The direction of the measured earth magnetic field is used as a 3D compass to determine the direction of the north heading or yaw used as an absolute reference in the calculation of 3D orientation A locally disturbed warped magnetic field causes an error in orientation that can be quite substantial The earth magnetic field is altered by ferromagnetic materials permanent magnets or very strong currents several amperes Whether or not an object is ferromagnetic should preferably be checked by using the motion tracker s magnetometers It can also be checked with a small magnet but be careful you can easily magnetize some ferromagnetic materials causing even larger errors If you find that some object is magnetized hard iron effect this is often the case with for example stainless steels that are normally not magnetic it may be possible to degauss the object NOTE Never expose the MT to strong magnetic fields The MT contains the absolute possible minimum of ferromagnetic materials hard and soft magnetic materials Nonetheless some minor components can be magnetized permanently by exposure to strong magnetic fields This will not damage the unit but will render the calibration of the magnetometers useless typically observed as a large deviation in heading For mild magnetization it may be possible to compensate for the magnetization of the device by a re calibration
3. magnetic field mapping Taking care not to expose the MT to strong magnetic fields such as close proximity of permanent magnets speakers electromotor etc will make sure magnetization does not occur In practice the distance to the object and the amount of ferromagnetic material determines the amount of disturbance The disturbance of the earth magnetic field can be divided into two kinds of effects 1 Disturbance caused by objects in the environment near the motion tracker like file cabinets or vehicles that move independently with respect to the MT This type of disturbance is non deterministic and cannot be fully compensated for However the amount of error caused by the disturbance can be reduced by optimally using the available sensor information and valid assumptions about the application This is the task of the Xsens Kalman Filter XKF running in the DSP 2 Disturbance caused by mounting the MT to an object of which the motion is to be recorded the MT moves with the object The error in magnetic field only depends on the orientation and can therefore be predicted i e it is deterministic and taken into account during motion tracking Using a mapping of the disturbance warping of the magnetic field the errors caused by this type of disturbance can in theory be reduced to zero The calculations and methodology required to achieve this is supplied by the MagField Mapper add on and this documentation This type of correctio
4. progress Please wait Restart If the measurement is succeeded you can store the calibration results in the non volatile memory of the MT Check the optional warnings and review the quality indicators see also section 3 2 If you are satisfied with the results click Write to store the new calibration parameters in the MT x memory Magfield Mapper Magfield Mapping done Press Write to write new magnetic field mapping to Motion Tracker memory Document MT0202P J 9 Magnetic Field Mapper Documentation O Xsens Technologies B V xSerns 3 4 Explanation of the reports The Magnetic Field Mapper generates several reports that are discussed below field Kannan Roni Figure 3 In the results this figure shows the total movement of the MTi over all directions of an imaginary sphere In the left image the original magnetic field readings are shown the right shows the magnetic field readings after the MFM procedure The rounder this sphere is the better the MFM results are Spabal of Corrected m m c OMerence imes 50 A EDO zz ki io n 4 Bor Figure 4 In the results this figure shows if the MFM was able to apply the new magnetic field model to all corrected magnetic field measurements of the file or measurement When large spikes are visible the data set used for the MFM had errors which may result in a magnetic field
5. results are unrealistic the MagField Mapper will not output the results This chapter provides several example reports in order to be able to recognize disturbances Correct MagFieldMapping B Magfield Mapper Results Hatogram 00 s 25 Xi 0 5 Detributien of Odlerence Neem before eed after Mapping EDOM sje 7 BB oon Corrected e Normairedm e DP erence times 50 10 7 ino mo Mow Figure 7 This report has 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 Document MT0202P J Xsens Technologies B V 13 Magnetic Field Mapper Documentation xSeaerns Too much magnetic disturbance PR Masfield Mapper Rests we gt o o Hategram Fepduais I RICE iro scaing 3 25 kl Neem before eod after Magheld Mapping 9 z 20 7 wed for ew a Figure 8 The points in the right sphere hover just above the surface of the model the points do not fit to the model and many points even fall outside the graph see the red bars residuals are large and the norm is not straight and or close to 1 Document MT0202P J Magnetic Field Mapper Documentation Xsens Technologies B V 14 xSeaerns Moving too fast PR ay tr
6. B or C C C The MFM SDK has the following components DLL and that return MFM parameters and data for visualizations Examples for C MATLAB C The structure of the MFM is as following Wrapper Conversion VES Xsens MFM SDK Communication Xsens Motion Tracker Note Extensive HTML documentation for the MFM SDK is included in the SDK See the Xsens folder e g C Program Files x86 Xsens MT Software Suite 4 24 Documentation Magfieldmapper Document MT0202P J Xsens Technologies B V 20 Magnetic Field Mapper Documentation xSerns 5 Customer Support Please contact us if you have any questions regarding the use of Magnetic Field Mapper its performance for your specific application or any other questions WWW http www xsens com su Phone 31 0 88 9736700 Document MT0202P J Magnetic Field Mapper Documentation Xsens Technologies B V 21
7. MagField Mapper software For MTw first establish a wireless connection using the MT Manager 100 Hz Document MT0202P J Xsens Technologies B V 6 Magnetic Field Mapper Documentation Write results to MotionTracker non volatile memory Write results to MFM results file Previous 3 Press Scan Magfield Mapper E oris x xXSens following Use Motion Tracker 07700054 Process Previously recorded file 4 Click Next and select the location where the software stores the log file of the calibration measurement Document MT0202P J Xsens Technologies B V 7 Magnetic Field Mapper Documentation amp Magfield Mapper x xsens Logging directory Change C Users marcel AppData Local Temp Please make sure to Slowly rotate motion tracker in all directions 5 The software is now ready to start the calibration measurement Read section 3 2 for more information about how to perform the measurement Click Start to begin the measurement Capturing data Perform the mapping movements now Capturing data to C fusers marcel AppData Local Temp MT 07700054 006 mtb 6 When the sequence of rotations is completed press Stop to end the measurement and to start the analysis of the data Document MT0202P J Xsens Technologies B V 8 Magnetic Field Mapper Documentation amp Magfield Mapper ipe ee x xsens Magnetic field disturbance mapping in
8. Magnetic Field Mapper Documentation xSerns Document MTO202P Revision J 28 October 2013 Hatogram Fendusis i 17 0 25 8 os 24 E 9 2 A E 07 iino mut scaing E ki Neem before eod after Mapping Corrected e Normalkred m n D erence times 50 A ED OD Linn A zz kl gH E LT L0 10 9 T 3 00 ioo Xsens Technologies B V Xsens North America Inc Pantheon 6a phone 31 0 88 973 67 10557 Jefferson Blvd phone 310 481 1800 P Q Box 558 fax 31 0 88 973 67 01 Suite C fax 310 416 9044 7500 AN Enschede e mail info xsens com CA 90232 Culver City e mail info xsens com The Netherlands internet www xsens com USA internet www xsens com XSens Revisions Revision A Date June 29 2005 Sept 26 2012 Dec 18 2012 2 May 2013 28 October 2013 By PSL MHA MHA MHA MHA Changes Initial release Release 4 0 MFM 2 1 Release 4 1 Release 4 1 5 new template Release 4 2 MFM 2 1 without MATLAB Component Runtime 2005 2013 Xsens Technologies B V All rights reserved Information in this document is subject to change without notice Xsens MVN MotionGrid MTi MTi G MTx MTw Awinda and KiC are registered trademarks or trademarks of Xsens Technologies B V and or its parent subsidiaries and or affiliates in The Netherlands the USA and or other countries All other trademarks are the property of their respectiv
9. To reach all points as can be seen in Figure 3 as a rule of thumb a calibration trial of around 3 minutes should suffice provided that the object is rotated over a sufficiently large angle and held sufficiently still If one of these requirements is not met a longer calibration trial may prove to be useful e Fora 2D calibration measurement it is recommended that the object moves through a full 360 circle e It is extremely important to perform the calibration in a magnetic homogeneous field Try to conduct the measurement at least 3 meters from large ferromagnetic objects such as radiators and iron desks e During the calibration trial Xsens filter is used for inclination measurement This means that the inclination accuracy will be less when the sensor is accelerated Especially centripetal accelerations that occur e g during swinging result in a large inclination error 3 3 Performing the calibration measurement If the MT can be directly connected to a PC the MagField Mapper software can be used to record the calibration measurement data If this is not the case read also section 4 2 to setup the off line procedure 1 To start the recording of the calibration measurement start the MagField Mapper plug in from the MT Manager Use the Tools menu 2 MagField Mapper software will now start up Choose the Write results to MotionTracker non volatile memory option if the MotionTracker is directly connected to the PC running the
10. alled or check if it available using the network otart the MagField Mapper software Select Write results to MFM result file and press Next Select Load to browse for the log file and select Next Check the results and if you are satisfied click Write to generate the MFM results file This file is stored in the same folder as the MagField Mapper executable in most cases the following folder C Program Files 86 Xsens Magnetic Field Mapper The filename of the results file is mfm Results XXXXXXXX bin and X is the devicelD serial number Write calibration results to MT The MFM results file contains a 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 Follow the next instructions to update the calibration parameters Copy the MFM results file to the system that has a direct connection to the MT Ensure that the MT is powered Make sure that the MT is in Config state Write the binary data message in the MFM results file to the sensor using binary communication A 1 Document MT0202P J Xsens Technologies B V 19 Magnetic Field Mapper Documentation xSeaerns 4 3 Magnetic Field Mapping SDK The Magnetic Field Mapping algorithm is also available in the Software Development Kit This makes it possible to embed the Magnetic Field Mapping in 39 party programs or to use MATLA
11. ble it may help to think about scanning the surface of a sphere with the MT x axis It is important to cover as many orientations as possible at least that many that will cover the envelope of motion of your application In case of a 2D calibration measurement the object has to be rotated through at least a full 360 circle It is recommended to do this with constant and low 15 km h speed Note The MFM algorithm will always try to find a solution even if there is only a partial capture This means that when the MFM has been performed in the orientations of the intended application only the heading will be accurate in those orientations but possibly not outside that captured envelope The Magnetic Field Mapper will tell you if the measurement was OK Limited angles Document MT0202P J Xsens Technologies B V 5 Magnetic Field Mapper Documentation xXSens The calibration procedure works best if the MT is rotated through a large amount of possible orientations If this is difficult for the object for which the MT has to be calibrated the procedure might still give accurate results with mild magnetic disturbances It should be noted that orientations that are recorded with a calibration file that is generated with a limited set of orientations will only be accurate for that particular range of orientations rotation range Calibration remarks e It is required to hold the in as many different orientations as possible
12. e MagField Mapper is that mapping can be done with a small envelope of motion Though there are only a few points the distribution of these points on the sphere is adequate for a good observability of the MFM calibration parameters The calibration parameters determined is valid for MFM motion envelope but may be incorrect beyond the mapped area Document MT0202P J Xsens Technologies B V 17 Magnetic Field Mapper Documentation xXSens 4 Non default calibration procedures This section describes other than the default procedure to calibrate the MT for magnetic disturbances 4 1 Usea previously recorded measurement For every calibration the MagField Mapper software process a log file which contains the sensor data logged during measurement The location of these log files is set prior to the measurement in one of the MagField Mapper dialog These log files have the following name data XXXXXXXX SSS bin where the X stands for the device ID or serial number of the MT and S is a sequence simple file counter If itis necessary to use the results of a previously recorded measurement the MagField Mapper can process a previously recorded log file Use the following procedure to accomplish this 1 Start the MagField Mapper software 2 Select Write results to MotionTracker non volatile memory choose the correct baud rate and press Scan 3 Inthe Change Input type dialog choose Process previously recorded file and
13. e owners Document 202 Xsens Technologies B V Magnetic Field Mapper Documentation xXSens Table of Contents 1 INTRODUCTION Meu es oes assess sie answers nates 1 2 THEORY OF OPERATION IEEE DUNCEIU IEEE n 2 21 BACKOROUND 2 225 3 MAGNETIC FIELD MAPPING PROCEDURE 5 31 MOUNTING OF WIT 5 3 2 HOW TO PERFORM A CALIBRATION MEASUREMENT RR nnne 5 3 3 PERFORMING THE CALIBRATION 6 EXPLANATION OF THE REPORTS 10 5 S a 12 3 5 1 EXAMPLE REPORTS OF ERRONEOUS
14. lues extended Motion Tracker Specification eMTS data that can be written to the MT non volatile memory Once written to the eMTS data the orientation data calculated by the MTi will be accurate even when mounted on the ferromagnetic object The calibration procedure is suitable for both 3D applications where the object is rotating through a substantial range of orientations e g a camera and 2D applications where the object moves more or less in one plane e g a car or boat An accurate calibration is obtained by recording the MT signals while rotating the object with the MT mounted on it in a space without other nearby ferromagnetic materials Once the object is rotated over a sufficiently large amount of orientations the MagField Mapper can then calculate new calibration parameters that can immediately be used in the MT The calibration has only to be performed once during the period in which the MT is mounted on the same location on the object If properly carried out the resulting accuracy will be comparable to the accuracy experienced with the MT without any ferromagnetic materials nearby NOTE This manual discusses the Magnetic Field Mapper version 2 1 for MTi 10 series MTi 100 series and MTw For the Magnetic Field Mapper for Magnetic Field Mapper versions that support legacy devices please refer to version F of this document Document 202 Xsens Technologies B V 1 Magnetic Field Mapper Documentation
15. model of lesser quality Document MT0202P J Xsens Technologies B V 10 Magnetic Field Mapper Documentation Revdushi Magne tc Peace 13 DU eire soto cine mii 9 5 2 4 p gt 5 Q t O aeirosuto sg n 25 X Figure 5 the results this figure shows the residuals of the corrected magnetic field vector of the file or measurement with respect to the new magnetic field model When measurements are visible outside Gaussian distribution the data set used for the MFM had errors which may result in a magnetic field model of lesser quality Neem before eod after Mapping FED OD oi zs kl 7 wed t M m Lp L0 Figure 6 In the results 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 The more flat the red line is and the closer to 1 0 the better the Magfield Mapping Document MT0202P J 11 Magnetic Field Mapper Documentation O Xsens Technologies B V xXSens 3 5 Error causes If the calibration procedure is not giving the desired results it may be caused by one of the following error Sources Cause Explanation Non M homogeneous magnetic The effect of a non homogeneous measurement field shows field in measurement volume large residuals even though you follow procedure To remedy this problem t
16. n is commonly known as compensation for hard and soft iron effects Degaussing is a procedure to apply strong alternating magnetic fields with decreasing magnitude in random direction to an object that has been magnetized The effect of the strong alternating fields is to remove any magnetized aligned domains in the object If you degauss please make sure the MT is not anymore mounted on the object Document MT0202P J Xsens Technologies B V 2 Magnetic Field Mapper Documentation xXSens 2 2 Method In a non disturbed magnetic field the 3D measured magnetic field vector has a magnitude of one and therefore all measured points lay on the circumference of a sphere with the centre at zero In the case of a disturbed magnetic field this sohere is both shifted and warped The calibration procedure described in this documents aims to derive a function that maps the measured magnetic field vector to a sphere This function is then implemented in new eMTS data stored in non volatile memory in your MT The calibration procedure requires the inclination Accuracy of inclination is influenced by linear accelerations Large accelerations will have a large effect on the accuracy of the calibration results Since the MagField Mapper algorithm cannot distinguish between an external disturbed magnetic field and a disturbance caused by the object on which the motion tracker is attached it is extremely important that the measurement is carried o
17. o tot d Emm Hatogram Renduais LENS Puer Neem before eod after Mapping Corrected 4 m Ditference times 50 t EDO h 22 kl 7 on n Xx Figure 9 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 huge in all directions and the norm after MFM is not 1 Document MT0202P J Xsens Technologies B V 15 Magnetic Field Mapper Documentation xSeaerns Not enough points Mogfeid Rests 0o o og 8 Hatogram evduals Magnet IP 22 ESA Normakre Corrected e Nermatned gt DAMerence imes 50 A t EDM Z2 ki HH t Figure 10 The spheres show hardly any points and are poorly distributed resulting in large residuals The norm after the MFM procedure is not close to 1 Document MT0202P J Xsens Technologies B V 16 Magnetic Field Mapper Documentation xSeaerns Points too concentrate Mapper Rents ee Hategram Fepduais 17 Pur 2 fe os e ll auto scaing E kl Neem before eod after Magheld Mapping Corrected Normale m 9 Diference tmes 50 A t ED OD zs kl zz ki 7 Bos pmo v Mor Figure 11 One of the advantages of th
18. press Next 4 The rest of the procedure is identical to the default one see section 3 3 Document 202 Xsens Technologies B V 18 Magnetic Field Mapper Documentation xXSens 4 2 Off line procedure If the Motion Tracker cannot be directly connected to a PC for an on line measurement it is possible to manually generate a log file with measurement data This log file can be processed off line by the MagField Mapper software As the calibration results cannot be directly stored in the MT the MagField Mapper generates a binary result file This file contains a custom MT message that updates the calibration data of the MT if the message if transmitted to the MT Generate a log file To generate a valid MagField Mapper log file follow these instructions 1 Set the MT in un calibrated raw data output mode and set the output settings to enable the sample counter Status byte should be disabled 2 Senda Reset message or power cycle the MT 3 Start recording all the data received after sending the Reset message or at power up 4 While recording the incoming data into a file perform the measurement See section 3 2 for more information about how to perform the measurement Process the log file After generating the log file the MagField Mapper software can process this and generate a result file Follow these steps to generate a MFM result file 1 Copy the log file to a system on which the MagField Mapper is inst
19. ry to perform the calibration measurement in a different place or remove nearby metal objects Saturation The disturbance of the magnetic field can be so extreme that the magnetometers are saturated In this case a warning will be given Reposition the MT on the object away from the ferromagnetic material and not close to sharp edges to remedy this problem 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 large amount of possible orientations even though measurements with a limited range of motion will most likely give good results as well Extreme disturbance of magnetic It can be that the disturbance of the magnetic field is so extreme field that the program cannot find any function to correct the disturbance This could occur more easily when one of the other error causes play a significant role The result of such an error will become apparent in very high residuals or an error message Document 202 Xsens Technologies B V 12 Magnetic Field Mapper Documentation xSeaerns 3 5 1 Example reports of erroneous results The MagField Mapper algorithm will always try to produce a result Only when the MagField Mapper algorithm internal
20. tware or by transmitting a specialized message to the Motion Tracker which is generated by the MagField Mapper software In the case that after mounting the MT cannot be directly connected to a PC that runs the MagField Mapper software it is possible to perform magnetic field mapping off line This is described in section 4 2 The standard procedure is described in the following sections 3 1 Mounting of MT The MT is attached to the object for which the magnetic field mapping procedure is carried out Make sure that wherever the MT is placed it cannot move with respect to the object There are no specific requirements in terms of mounting orientation of the MT on the object Remarks Every time the sensor is temporarily removed from the object it is advised to repeat the calibration procedure gt f the geometry of the object is significantly altered e g the geometry is changed components are added or removed it is advised to repeat the calibration procedure The calibration procedure is more accurate for smaller disturbances If possible try to position the sensor one to a few centimetres inches away from ferromagnetic materials The MagField Mapper will automatically warn you if the magnetic field sensors in the MT become saturated 3 2 How to perform a calibration measurement During a 3D calibration measurement the object to which the sensor is attached has to be rotated through as many different orientations as possi
21. ut in a homogeneous magnetic field As a rule of thumb there should be no ferromagnetic objects within at least three meters from the place in which the measurement is carried out Keep in mind that the structure of the building you are in floor and ceiling is likely to contain magnetic materials ee Neem belote eod after Macfekd Macping Y 25 kl y weed for MPM y Barve uu Figure 1 3D representation of the norm of the magnetic field vector before red and after blue compensation using magnetic field mapping The black dots represent the measurements used by the MFM algorithm Document 202 Xsens Technologies B V 3 Magnetic Field Mapper Documentation oroyi Magne oc Resdos Ai O M eio nto scing E 80 9 5 9 4 1 23 9 2 9 1 ver T OG Q i C aero esto scing s 25 Figure 2 Part of calibration report showing the distribution of the residuals after an MFM procedure Document MT0202P J Xsens Technologies B V 4 Magnetic Field Mapper Documentation xXSens 3 Magnetic field mapping Procedure The magnetic field mapping procedure consists of the following steps 1 Mounting of MT 2 Recording of calibration measurement using MagField Mapper software or other logging software 3 Processing of measurement data by MagField Mapper software 4 Writing the results to Motion Tracker using MagField Mapper sof
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