INTEGRIRANI MIKROSENZORSKI SISTEMI Z MIKROTULJAVICAMI

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 mA torej 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 mikrosistemih v 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.