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Tracking Position and Orientation of Magnetic Objects Using Magnetometer Networks
Linköping University, Department of Electrical Engineering, Automatic Control. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Electrical Engineering, Automatic Control. Linköping University, Faculty of Science & Engineering.
2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

A framework for estimation and filtering of magnetic dipoles in anetwork of magnetometers is presented. The application in mind istracking of objects consisting of permanent magnets for controllingcomputer applications, though the framework can also be applied totracking larger objects such as vehicles. A general sensor model forthe network is presented for tracking objects consisting of (i) asingle dipole, (ii) a structure of dipoles and (iii) several freely moving(structures of) dipoles, respectively. A single dipole generates amagnetic field with rotation symmetry, so at best five degrees offreedom (5D) tracking can be achieved, where the SNR decays cubicallywith distance. One contribution is the use of structures ofdipoles, which allows for full 6D tracking if the dipole structure is largeenough. An observability analysis shows that the sixth degree of freedom is weaklyobservable, where the SNR decays to the power of four withdistance, and that there is a 180 degree ambiguity around a specificsymmetry axis. Experimental results are presented and compared to areference tracking system, and four public demonstrators based on thisframework are briefly described.

Place, publisher, year, edition, pages
2015.
Keyword [en]
Magnetometers, Tracking, Kalman filtering, Magnetic dipole
National Category
Signal Processing
Identifiers
URN: urn:nbn:se:liu:diva-122395OAI: oai:DiVA.org:liu-122395DiVA: diva2:866121
Projects
COOPLOC
Funder
Swedish Foundation for Strategic Research , COOPLOC
Available from: 2015-10-31 Created: 2015-10-31 Last updated: 2015-11-04Bibliographically approved
In thesis
1. Modeling of Magnetic Fields and Extended Objects for Localization Applications
Open this publication in new window or tab >>Modeling of Magnetic Fields and Extended Objects for Localization Applications
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The level of automation in our society is ever increasing. Technologies like self-driving cars, virtual reality, and fully autonomous robots, which all were unimaginable a few decades ago, are realizable today, and will become standard consumer products in the future. These technologies depend upon autonomous localization and situation awareness where careful processing of sensory data is required. To increase efficiency, robustness and reliability, appropriate models for these data are needed.In this thesis, such models are analyzed within three different application areas, namely (1) magnetic localization, (2) extended target tracking, and (3) autonomous learning from raw pixel information.

Magnetic localization is based on one or more magnetometers measuring the induced magnetic field from magnetic objects. In this thesis we present a model for determining the position and the orientation of small magnets with an accuracy of a few millimeters. This enables three-dimensional interaction with computer programs that cannot be handled with other localization techniques. Further, an additional model is proposed for detecting wrong-way drivers on highways based on sensor data from magnetometers deployed in the vicinity of traffic lanes. Models for mapping complex magnetic environments are also analyzed. Such magnetic maps can be used for indoor localization where other systems, such as GPS, do not work.

In the second application area, models for tracking objects from laser range sensor data are analyzed. The target shape is modeled with a Gaussian process and is estimated jointly with target position and orientation. The resulting algorithm is capable of tracking various objects with different shapes within the same surveillance region.

In the third application area, autonomous learning based on high-dimensional sensor data is considered. In this thesis, we consider one instance of this challenge, the so-called pixels to torques problem, where an agent must learn a closed-loop control policy from pixel information only. To solve this problem, high-dimensional time series are described using a low-dimensional dynamical model. Techniques from machine learning together with standard tools from control theory are used to autonomously design a controller for the system without any prior knowledge.

System models used in the applications above are often provided in continuous time. However, a major part of the applied theory is developed for discrete-time systems. Discretization of continuous-time models is hence fundamental. Therefore, this thesis ends with a method for performing such discretization using Lyapunov equations together with analytical solutions, enabling efficient implementation in software.

Abstract [sv]

Hur kan man få en dator att följa pucken i bordshockey för att sammanställa match-statistik, en pensel att måla virtuella vattenfärger, en skalpell för att digitalisera patologi, eller ett multi-verktyg för att skulptera i 3D?  Detta är fyra applikationer som bygger på den patentsökta algoritm som utvecklats i avhandlingen. Metoden bygger på att man gömmer en liten magnet i verktyget, och placerar ut ett antal tre-axliga magnetometrar - av samma slag som vi har i våra smarta telefoner - i ett nätverk kring vår arbetsyta. Magnetens magnetfält ger upphov till en unik signatur i sensorerna som gör att man kan beräkna magnetens position i tre frihetsgrader, samt två av dess vinklar. Avhandlingen tar fram ett komplett ramverk för dessa beräkningar och tillhörande analys.

En annan tillämpning som studerats baserat på denna princip är detektion och klassificering av fordon. I ett samarbete med Luleå tekniska högskola med projektpartners har en algoritm tagits fram för att klassificera i vilken riktning fordonen passerar enbart med hjälp av mätningar från en två-axlig magnetometer. Tester utanför Luleå visar på i princip 100% korrekt klassificering.

Att se ett fordon som en struktur av magnetiska dipoler i stället för en enda stor, är ett exempel på ett så kallat utsträckt mål. I klassisk teori för att följa flygplan, båtar mm, beskrivs målen som en punkt, men många av dagens allt noggrannare sensorer genererar flera mätningar från samma mål. Genom att ge målen en geometrisk utsträckning eller andra attribut (som dipols-strukturer) kan man inte enbart förbättra målföljnings-algoritmerna och använda sensordata effektivare, utan också klassificera målen effektivare. I avhandlingen föreslås en modell som beskriver den geometriska formen på ett mer flexibelt sätt och med en högre detaljnivå än tidigare modeller i litteraturen.

En helt annan tillämpning som studerats är att använda maskininlärning för att lära en dator att styra en plan pendel till önskad position enbart genom att analysera pixlarna i video-bilder. Metodiken går ut på att låta datorn få studera mängder av bilder på en pendel, i det här fallet 1000-tals, för att förstå dynamiken av hur en känd styrsignal påverkar pendeln, för att sedan kunna agera autonomt när inlärningsfasen är klar. Tekniken skulle i förlängningen kunna användas för att utveckla autonoma robotar.

Place, publisher, year, edition, pages
Linköping University Electronic Press, 2015. 236 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1723
Keyword
Localization, magnetic tracking, extended target tracking, signal processing, machine learning, Gaussian processes, deep dynamical model, discretization
National Category
Signal Processing
Identifiers
urn:nbn:se:liu:diva-122396 (URN)10.3384/diss.diva-122396 (DOI)978-91-7685-903-2 (ISBN)
Public defence
2015-12-04, Visionen, House B, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Projects
COOPLOC
Funder
Swedish Foundation for Strategic Research , COOP-LOC
Note

In the electronic version figure 2.2a is corrected.

Available from: 2015-11-03 Created: 2015-10-31 Last updated: 2015-11-30Bibliographically approved

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Wahlström, NiklasGustafsson, Fredrik

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