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1IntroductionThis master thesis aims at investigating magnetic sensors to see which isthe most suitable for the application of tracking of a dipole magnet. Whena suitable sensor is found, a sensor platform will be developed using theselected magnetic sensor together with an appropriate MCU.1.1BackgroundThis project is carried out together with a company which develops technology for the next generation human-computer interaction. According toa non-disclosure agreement (NDA) the company name is not given in thismaster thesis report and they will be referred to as Company A.Company A has previously developed a sensor platform which measuresthe magnetic field of an external magnet and is able to track and determinethe position of the magnet in three dimensions with five degrees of freedom.The application of the platform is mainly to work as a new type of interactiontool between humans and computers.During the development of this technology, Company A has focusedmainly on the software and no effort was put to determine which magneticsensors and other hardware that are most suitable for this application ofdipole magnet tracking.The previous version of the platform was developed during the development of the algorithm and had the main purpose of delivering test data toevaluate the tracking algorithm. This version has hardware that is too expensive in both price and size of the components to make a cost efficientproduct. Some of the components are also no longer manufactured. Company A has a plan to launch a home user version and therefore there is aneed for a cost efficient solution of the platform.Since the previous development of the platform there has been a majordevelopment of the magnetic sensor technology and therefore there is a needfor an investigation of the sensors currently available on the market.1.2PurposeThe purpose of the master thesis is to investigate and determine which magnetic sensors are most suitable for tracking position and orientation of anexternal magnet. With the magnetic sensor that, together with company A,is determined to be most suitable, a new sensor platform is developed.1

1.3Question formulationThe questions that will be covered in this thesis are: What are the main criteria for the magnetic sensors to fulfill to besuitable for tracking of a dipole magnet? Which magnetic sensor technology is most suitable for magnet tracking?1.4DelimitationsThe investigation will only cover 3-axis magnetic sensors, as that is a requirement from Company A. The complete design of the sensor platform willnot be presented due to NDA-restrictions. Instead, the individual parts likeprocessor-, sensor-, PSU- and antenna design are presented individual. Thetesting of the final hardware design do not include any magnet tracking, dueto the lack of time to program the MCU. The master thesis does not includefunctionality tests of parts that include programming. This includes testingof the magnetic sensor placed on the sensor platform. The antenna is testedin an EMC chamber with a tool from Texas Instruments, SmartRF studio.1.5Project structureThe project work is carried out by the author together with Company A.The work process is divided into these phases:1. Investigation of magnetic sensors - theoretical background study andpractical testing.2. Main sensor platform hardware design - Component selection, schematics, PCB-design and prototype manufacturing.3. Test and verification - Function test, antenna-performance test andproposals for future development.1.6MaterialIn this section the materials used in the master thesis are presented.1.6.1Computer softwaresTo execute the project several computer softwares are used to ease the designprocess. This includes softwares for electronics circuit design, circuit simulation, embedded software development and mathematical computation.2

Advanced Design System Advanced Design System (ADS) is an electronic design automation software for RF, microwave, and high speed digitalapplications [1]. ADS can be used as a tool for frequency-domain and timedomain simulation and electromagnetic field simulation. In this project, ADSis used to simulate and tune the 2.4 GHz PCB-antenna.Altium Designer Altium designer is a complete electrical design automation (EDA) software for PCB, FPGA and embedded software design. Altium designer has a wide design support including component library andthe schematic design environment is closely paired to the PCB-layout toolmaking the design process easier. Altium Designer includes tools for fabrication file extraction and verification directly in the software.Arduino IDE Arduino IDE is an open source software for embedded software development of the Arduino MCU platforms. It is simple and easy touse and have many examples and references in the online community. Inthis project the Arduino IDE is used to program the Arduino MCU:s usedto collect data samples from the sensors during the sensor tests.MATLAB MATLAB is a computation tool from MathWorks. MATLABis short for ”Matrix Laboratory” because it only uses matrices as variables.Matlab is used for many things including data acquisition, algorithm development, signal processing and advanced computations. In this project,MATLAB is used to collect data form the sensors during the tests and thenvisualize the results to make it easier to draw conclusion.3

2Theoretical backgroundThis section contains theory about magnetic sensors and the inverted-F antenna.2.1Magnetic SensorsA magnetometer is an instrument that provides information about the magnetic field that is applied to it. There are various technologies used in magnetic sensors to measure the magnetic field. In this thesis, three technologieshave been chosen to have a closer look at, Magnetoresistive (MR)-sensors,Hall effect sensors and search-coil magnetometer sensors.2.1.1Magnetoresistive sensorsThe MR sensor is build out of a thin film of anisotropic magnetoresistance(AMR) materials. AMR materials change their electrical resistance as aresponse of the change in angel and strength between the magnetizationvector of the external magnetic field and the direction of the electrical currentflowing through the AMR material [31]. The most common AMR-materialused is permalloy, which is a ferromagnetic alloy made out of nickel andiron. The sensor structure of the MR-sensors is shown in figure 1a. Thethin film has a magnetization vector in the longitudinal direction of thethin film. When no external magnetic field is applied to the thin film, theresistance is at the minimum rate. When an external magnetic field is appliedperpendicular to the direction of the current the magnetization vector of thethin film rotates (in the plane of the thin film) and the resistance of thepermalloy increases. One big drawback of this structure is that the responseis the same for both polarities of the magnetization. This issue is solved byapplying a so-called barber poles of a highly conductive material on top ofthe thin film of permalloy. This material is usually aluminum [34]. Thesebars are places in a 45 angle with respect to the longitudinal axis of thethin film, as shown in figure 1b. Due to that the aluminum bars are moreconductive than the permalloy the current will be forced to take the shortestpath between the bars. This path has a direction that is perpendicular tothe direction of the bars. Now the current and the magnetization vector aremaking a 45 angle with each other.4

(b) Barber pole electrode structure(a) Original electrode structureFigure 1: Thin film structure of MR-sensorsThe introduction of this angle between the magnetization vector and thecurrent changes the sensitivity and linearity of the sensor. As seen in figure 2,with barber poles a linear region is introduced around the zero and the sensoris now able to sense the difference between positive and negative magneticfields.Figure 2: Resistive response of the MR-sensorThis phenomenon is explained analytically in equation 1.5

R(ϕ) r (rk r ) cos2 (ϕ)(1)Where R is the resistance, dependent of the angle(ϕ) between the externalmagnetic field and the longitudinal axis. r and rk are the resistance in thematerial along the axis perpendicular and parallel with the longitudinal axis.This is an even function and is therefore symmetrical across the null point ofthe X-axis, as seen in figure 2. The introduction of the barber poles, whichmakes the current path diverse by 45 in respect to the longitudinal axis,changes the equation toR(ϕ) r (rk r ) cos2 (ϕ 45 )(2)and in other words adds a 45 phase shift to the resistive response curve ofthe thin film.The AMR-material have different magnetic domains in the material. Thedirection of the magnetization in each of these domains can be changed byan external magnetic field, if it is stronger than the operational range ofthe material. The direction of the magnetization of these domains need tobe aligned in the same direction, in order to get the correct properties ofthe material. To restore this phenomena and align the magnetic domains,a Set/reset circuit is used. This circuit generates a strong magnetic fieldclose to the thin film during a short pulse which align the direction of themagnetization of the domains in the same direction. If the domains are notaligned the response of the magnetoresistive material to a magnetic field arereduced. Figure 3 illustrates the set/reset circuit.6

Figure 3: Effect of the Set/reset circuit in a MagnetometerThe MR-sensor is the most suitable sensor technology for magnet tracking. The sensor has good sensitivity and can sense far field magnetic fieldfrom an external magnet as long as the strength of the magnet is strongerthan the earths magnetic field.2.1.2Hall effect sensorsThe hall effect sensor is built out of a thin film of p-type semiconductor, calledHall element. The principle of the hall effect sensor is based on Lorentz force,which is a physical law that describes the force applied on a charged particle(electron) in an electro-magnetic field.In a hall effect sensor a current is sent through the hall element. When noexternal magnetic field is applied to the sensor, the electrons flows straightthrough the material and takes the shortest path between the two poles.When a magnetic field is applied over the hall element, the electrons diversefrom the shortest path over the material. The positive charged particleswill accumulate on one side and the negative charged will accumulate onthe other side. The separation of the differently charged particles generatesa potential difference between the two sides of the hall element, the hallvoltage. By measuring the hall voltage, the strength of the magnetic fieldcan be determined. The principle of Hall effect is illustrated in figure 4.7

Figure 4: Principle of the hall effect sensor[35][36]The hall voltage is often in magnitude of a few µV and therefore amplification circuitry is needed to be able to measure the voltage with an ADC.8

2.1.3Search-coil Magnetometer sensorThe search coil, or induction coil, is a sensor technology measuring the changeof magnetic flux through a coil of conductive material with a ferromagneticcore. The principle is based on Faraday law of induction and Lenz’s law.The basic principle of the sensor is that an inductive element together with acapacitance and an amplifier forms a LC-oscillator. This is a self-oscillatingcircuit. The oscillating frequency and amplitude are dependent on the valueof the inductance. Through the Faraday and Lenz relationship, it is shownthat the inductance of the coil can be changed if an external magnetic fieldis applied to the coil. This change in inductance will change the frequencyand amplitude of the oscillating signal of the circuit. Through observingthese parameters the changes in the magnetic field can be measured. In theapplication of 3-axis sensor this sensor has a big drawback. Due to the factthat the coil itself generates a magnetic field, simultaneous measurements arenot possible. Therefore, the measurements for each axis have to be executedsequentially and the output data rate of the magnetic sensor will be threetimes slower.2.2Antenna Theory - Inverted F antennaAn antenna is, according to Balanis[29], ”a means for radiating or receivingradio waves”. What the antenna does is to convert electromagnetic energyform the transmitter to electromagnetic waves which can travel in free-space.The antenna can therefore be seen as a transitional structure between freespace and a guiding device. The guiding device is referred to as the transportmedium which transports the electromagnetic energy from the transmitterto the antenna or from the antenna to the reviver, usually a transmissionline.Antennas have three types of radiation patterns, isotropic, directional oromnidirectional. An isotropic radiator is defined as ”a hypothetical losslessantenna having equal radiation in all direction”[29]. A directional radiatoris more efficient on transmitting and reviving electromagnetic waves in aspecific direction. An omnidirectional radiator radiates in all directions in agiven plane.An inverted-F antenna (IFA) is an evolution of the quarter-wavelengthmonopole antenna and is used in many wireless communication applications.The IFA has two big advantages over traditional monopole antennas. Theantenna is more compact and the impedance matching is accomplished bythe design of the antenna without the need of external components.The IFA is, as mentioned a type of monopole antenna. The monopole9

antenna is usually a straight wire which forms the antenna. By bending it 90 to make a part of the trace parallel with the ground plane, a capacitance isintroduced to the antenna. This ”L” shaped antenna is called an inverted Lantenna and is illustrated in figure 5 if part A is removed. When the shortingpath(part A in figure 5) is introduced, it contributes with an inductive partto the design.Figure 5: Basic design of an IFAIn figure 6 the equivalent circuit of the IFA is shown. L1 is the inductancecreated by part A in figure 5, C2 is the capacitance created by the openstub marked as part B in figure 5 and R1 is the radiation resistance of theantenna. To accomplish impedance matching at the resonance frequency, L1and C2 should cancel out leaving only the radiation resistance, R1.Figure 6: Equivilant circuit of an IFAThe design typology of the IFA lacks a complete analytical solution. Theguidelines that are available to use as a starting points of the antenna designare:λ L S H(3)0.05λ W 0.1λ(4)10

where λ is the wavelength of the resonance frequency, W is the width ofthe antenna trace and L,S and H are as shown in figure 5.Equation 3 and 4 works only as a starting point of the design and mustbe optimized for the specific application. Depending on the amount of surrounding ground plane, dielectric constant of the substrate and other factorsthe capacitance, C1, and inductance, L1, may not have the expected valueand the resonance frequency may differ from the calculated value of equation3 and 4. The easiest method of tuning is by trial and error. With modernssimulation tools the resonance frequency can be observed and corrected byusing the guidelines in the list below. Increase L to decease the resonance frequency. Decrease L to increase the resonance frequency. Increase W to to increase the resonance frequency. Decrease W to to decrease the resonance frequency.11

3Analysis and evaluation of magnetic sensorsIn this section the analysis of the magnetic sensors will be presented. Theinvestigation of the magnetic sensors aimed at determine which sensors arethe most suitable for magnet tracking. The analysis is split into two parts,first a market survey is performed to see which sensors are available. In thesecond part the most interesting sensors from the market survey are tested.Some sensors are available to buy as evaluation modules (EVM) that areready to connect and evaluate right away. If the sensor does not have anEVM to buy, a PCB is designed to work as an EVM in order to evaluatethe sensor. After the second part the most suitable and most cost efficientmagnetic sensor is chosen to be designed into the sensor platform.3.1Market surveyThe aim of the market survey is to find all available sensors on the marketthat could be suitable for the application of the sensor platform covered inthis thesis. The basic requirements that the sensors have to fulfill are: The magnetic sensor should have three measurement axes. The magnetic sensor should have at least 6 Gauss full scale measurement range.The market survey is limited to involve sensors available from three of theleading semiconductor distributors in the world. These distributors areDigiKey Electronics, Mouser Electronics and Farnell Element 14.The sensors found in the market survey are shown in table 1, togetherwith the parameters that are most interesting to compare.12

Table 1: Summary of market GMV[21]RM3100[22]MMC5883MA[23]ODR 1000534600ProtocolAnalogI2CI2C, SPII2CI2CI2C, SPII2CI2C, SPII2C, SPII2C, SPII2CI2CI2C, SPII2C, SPII2CAnalogI2CI2C, SPII2CRange [G] 6 8 16 10 10 50 8,1 8 13 13 1300 24 20 24 16 20 12 8 8Resolution [mG]N/A20.311Not foundNot found233980Not found836N/A0.420.250.05Noise level [mG]4.355.344334.356610003.55061.50.1Not found0.031.2Price: 1k units1 [ 521.520.61.940.80.61.75Not found315.51.5The first column, ODR, represents the maximum output sample rate thatthe magnetic sensor is capable to deliver complete samples of X, Y and Z axisto the MCU. The second column, Protocol, is the supported communicationprotocols in the sensor. Analog means that the magnetic sensor output isan analog voltage that changes according to the change in the magnetic fieldthat is applied. The third column, Range, is the maximum supported rangeof magnetic field strength of the sensor, measured in Gauss. One Gauss isequal to 100 µT. The fourth column, Resolution, is the resolution of thesensor. It is calculated through milli-gauss per LSB. The fifth column, Noiselevel, is the RMS noise level of the magnetic sensor measured in milli-Gauss.All the sensors are compared according to the parameters in the list below. Resolution of the magnetic field Measurement range Output Data Rate (ODR) Noise level Communication protocol used Unit price1Price per unit if you order one thousand(1000) units13

From the summary in table 1 the magnetic sensors with the highest potential to fit the application are chosen to be tested in part two. The requirements of choosing the sensors for part two are determined to be; themagnetic sensor shall have as low noise level as possible. The communicationprotocol sh

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