Design, manufacture and test of a magnetic encoder

An new eddy current based magnetic position encoder structure is proposed and studied in this thesis. The encoder is composed of one read head and one scale with metal plates placed periodically on a substrate. The read head contains one emitter and two receiver pairs which are all rectangular planar coils. The electromagnetic coupling between the emitter and receivers were affected by the relative position of the scale. A system level analytical model of the proposed encoder structure has been derived, from which three different encoder signals forms were generated. An amplification and synchronous demodulation circuit has been designed and fabricated. The circuit board was used successfully to process the encoder output signals in the measurement. Four PCB encoder prototypes were fabricated. These encoder structures were studied using the ANSYS MaxwellTM software package. The simulated and measured results were compared. The best accuracy performance of the PCB encoder is -15 μm to 15 μm from the simulation results and -35 μm to 25 μm from the corresponding measurement. An alternative manufacturing process of the magnetic encoder based on multilayer Low Temperature Co-fired Ceramic (LTCC) technology has also been presented. The fabrication process of the LTCC encoder and equipment used were described. Two different methods were used to characterise the LTCC encoder with good agreement between all approaches attempted. The best accuracy performance of the LTCC encoder was -30 μm to 25 μm and after lookup table correction the improved accuracy ranged from -10 μm to 10 μm

A Differential Monolithically Integrated Inductive Linear Displacement Measurement Microsystem

An inductive linear displacement measurement microsystem realized as a monolithic Application-Specific Integrated Circuit (ASIC) is presented. The system comprises integrated microtransformers as sensing elements, and analog front-end electronics for signal processing and demodulation, both jointly fabricated in a conventional commercially available four-metal 350-nm CMOS process. The key novelty of the presented system is its full integration, straightforward fabrication, and ease of application, requiring no external light or magnetic field source. Such systems therefore have the possibility of substituting certain conventional position encoder types. The microtransformers are excited by an AC signal in MHz range. The displacement information is modulated into the AC signal by a metal grating scale placed over the microsystem, employing a differential measurement principle. Homodyne mixing is used for the demodulation of the scale displacement information, returned by the ASIC as a DC signal in two quadrature channels allowing the determination of linear position of the target scale. The microsystem design, simulations, and characterization are presented. Various system operating conditions such as frequency, phase, target scale material and distance have been experimentally evaluated. The best results have been achieved at 4 MHz, demonstrating a linear resolution of 20 µm with steel and copper scale, having respective sensitivities of 0.71 V/mm and 0.99 V/mm

INTEGRATED MICROSENSOR SYSTEMS WITH MICROCOILS

Doktorska disertacija obravnava magnetni mikrosenzorski sistem za merjenje pomika, izdelan z mikrotuljavicami, ki so izvedene kot mikrotransformatorji. Primarna navitja mikrotransformatorjev so napajana z izmeničnim virom frekvence nekaj MHz in tokom v razredu mAtorej gre za induktivni sistem. Najprej so predstavljene sorodne induktivne rešitve za merjenje linearnega pomika, s poudarkom na integriranih izvedbah. Pregled razpoložljive literature je pokazal, da so integrirane mikrotuljavice v takšnih sistemih običajno izdelane z dodatnim postprocesiranjem integriranega vezja. Osredotočili smo se na prikaz izvedljivosti izdelave monolitnega integriranega mikrosistema za merjenje linearnega pomika z mikrotransformatorji, izvedenimi v notranjih metalnih plasteh integriranega vezja, izdelanega v popolnoma konvencionalnem 350-nanometrskem komercialnem CMOS mikrotehnološkem procesu hkrati s pripadajočimi vezji za obdelavo izhodnih signalov mikrotransformatorjev. Glavni prednosti takega sistema sta njegova cenovna učinkovitost zaradi enostavne izdelave ter odsotnosti potrebe po zunanjem generatorju polja, kot so denimo trajni magneti pri magnetnih (Hallovih) enkoderjih in svetlobni vir pri optičnih enkoderjih. Pripravili smo električno modelno vezje mikrotransformatorja. Za vključitev vpliva merilne letve, ki je uporabljena pri inkrementalnem merjenju pomika, tak model ne zadostuje. Vpliv merilne letve, postavljene nad mikrotransformator, smo modelirali z metodo končnih elementov. Pokazali smo vplive materialnih in geometrijskih lastnosti merilne letve na sekundarno inducirano napetost in ugotovili, da gre za kombinacijo amplitudne in fazne modulacije. Diferencialno izhodno napetost para mikrotransformatorjev kot funkcijo položaja merilne letve smo nadalje obravnavali v matematičnem oziroma visokonivojskem modelu v okolju Matlab/Simulink. S tem smo preučevali demodulacijske metode izmeničnega signala, moduliranega s položajem letve. Kot najenostavnejšo demodulacijsko metodo z ozirom na nadaljnjo integrirano realizacijo merilnega kanala smo izbrali sinhronsko demodulacijo. Ker tarče različnih lastnosti vnašajo v izhodno napetost mikrotransformatorja drugačne amplitudno-fazne karakteristike, sta minimalno popačenje izhodnega signala in z njim povezan optimalni fazni kot demodulacijskega mešalnega signala odvisna od specifičnih lastnosti tarče. Z opisano metodo lahko preučujemo ta odnos, s predstavljenimi modeli pa lahko tudi analiziramo vplive tarče in elementov elektronike merilnega kanala na nelinearnost pozicijskega signala. Predstavljeno metodologijo načrtovanja sistemov z mikrotransformatorji smo v praksi preizkusili na več mikrosistemihv disertaciji je podrobneje predstavljen primer, ki uporablja popolnoma diferencialen merilni kanal in mešalnik z Gilbertovo celico. Izdelani prototip mikrosistema smo karakterizirali: dosežena je bila občutljivost 0,99 V/mm pri bakreni tarči in približni oddaljenosti 200-250 μm med mikrosistemom in tarčo. Na podlagi ugotovitev, pridobljenih z modeliranjem in meritvami, nazadnje predlagamo izboljšan merilni kanal z boljšo ločljivostjo.The dissertation discusses a magnetic microsensor system for displacement measurement comprising microcoils, which are realized as microtransformers. The primary windings of the microtransformers are excited with an AC source with a frequency of several MHz and a current of several mA, thus making the microsystem inductive. First, relevant inductive linear displacement measurement solutions found in the literature are introduced, focusing on integrated devices. The available literature has shown that the integrated microinductors in displacement measurement applications are commonly fabricated during additional post processing steps of the integrated circuit fabrication. We focus on the demonstration of the feasibility of a monolithic integrated microsystem for linear displacement measurement with microtransformers, produced in internal metal layers of an integrated circuit, and fabricated using a completely conventional 350-nanometer commercial microtechnological process, along with corresponding circuits for the processing of the microtransformers’ output signals. The major advantages of such system are its cost-effectiveness due to its straightforward fabrication and the absence of the need for an external field generator, such as permanent magnets at Hall Effect encoders and a light source at optical encoders. A model electric circuit of a microtransformer is presented. Such model is not sufficient to account for the effect of a measurement scale, which is used for the incremental displacement measurement. Therefore, the finite element method is used to model the effect of the measurement scale, placed over the microtransformer. The effects of material and geometric properties of the scale on the secondary induced voltage are demonstrated. The differential output voltage of a microtransformer pair was further analyzed as a mathematical or highlevel model in Matlab/Simulink environment, where the demodulation methods of the AC signal, modulated with the target position, were studied. It is shown that the signal is modulated by a combination of the amplitude and phase modulation. As the most straightforward demodulation method, synchronous demodulation was selected. Since the targets with different properties introduce different amplitude-phase characteristics into the microtransformers’ output voltage, the minimal distortion and the related optimal phase of the mixing signal used for the demodulation are dependent on the specific properties of the target. The described method allows for the investigation of this dependence, and the presented models enable the investigation of the effects of the measurement scale and the electronics comprising the measurement channel on the nonlinearity of the position signal. The described design methodology for microsystems comprising microtransformers was practically evaluated on multiple microsystems. The dissertation describes one microsystem in more detail, employing a fully-differential measurement channel and a mixer with a Gilbert cell. A prototype microsystem was fabricated, demonstrating the sensitivity of 0.99 V/mm with a copper target and approximate microsystem-target distance of 200-250 μm. Finally, based on findings provided by simulations and measurements, an improved measurement channel with better resolution is proposed