In this PhD thesis, a novel working principle for 3-axis frequency-modulated (FM) MEMS accelerometers is proposed. Indeed, the differential frequency readout is performed through the innovative time-switched approach: this methodology is based on a double sampling of the oscillation frequency of a single resonator, consecutively biased in two different configurations in time. The technique enables to avoid offset thermal drift contributions typical of differential resonant accelerometers based on two distinct resonators with unavoidable mismatch in the temperature coefficient of frequency. After a complete behavioral modeling of the conceived system, a compact tri-axis MEMS structure is designed and fabricated using an industrial process. The encouraging results obtained from the characterization of the sensor motivated the design of an integrated analog oscillator, in order to prove the possibility to keep the same key performance coupling the device with a low-power, small-footprint ASIC. To this aim, two different feedback oscillator loops are analyzed and fabricated, theoretically and experimentally identifying the optimum topology in terms of power-noise trade-off. Finally, the tri-axis MEMS-ASIC combo is coupled to an integrated frequency-to-digital converter, demonstrating the feasibility of a fully integrated, digital output, consumer grade tri-axis FM time-switched MEMS accelerometer. The designed system solves the trade-off between offset thermal drift and full-scale-range experienced by state-of-the-art capacitive AM accelerometers. The ratio between these two critical parameter results improved by more than an order of magnitude, without any post-acquisition digital temperature compensation. At the same time, the other key parameters (as resolution, power consumption and bandwidth) remain in line with consumer devices currently on the market. Thus, the presented approach represents a promising strategy to face tight requirements of next-generation applications as mixed-reality and pedestrian inertial navigation.
In questa Tesi di Dottorato, viene propsoto un innovativo principio di funzionamento per la realizzazione di accelerometri MEMS triassiali a modulazione di frequenza. L'innovazione risiede nel fatto che la lettura differenziale di frequenza è basata sul principio time-switched: la metodologia è basata su un doppio campionamento nel tempo della frequenza di risonanza di un singolo risonatore, polarizzato consecutivamente in due diverse configurazioni. La tecnica permette di evitare derive termiche dell'offset presenti negli accelerometri risonanti basati su due risonatori distinti, con inevitabili mismatch dei coefficienti di temperatura della frequenza.Dopo uno studio comportamentale del sistema ideato, una struttura MEMS triassiale compatta è stata progettata e fabbricata utilizzando un processo industriale. I risultati incoraggianti ottenuti dalla caratterizzazione del sensore hanno motivato la progettazione di un oscillatore integrato, in modo da dimostrare la possibilità di mantenere le stesse prestazioni accoppiando il sensore con un'elettronica integrata a basso consumo e bassa area. A questo scopo, sono state considerate due diverse topologie di oscillatore integrato, studiandone analiticamente e sperimentalmente i vantaggi e i punti deboli e indentificando l'architettura ideale dal punto di vista del compromesso consumo-rumore. Infine, l'ASIC e il MEMS sono stati accoppiati con un convertitore frequenza-digitale, dimostrando la fattibilità di un accelerometro triassiale totalmente integrato con specifiche consumer. Il sistema ideato risolve il trade-off tra deriva termica dell'offset e full-scale, dal quale sono affetti tutti gli accelerometri MEMS presenti sul mercato al giorno d'oggi, aumentando la relativa figura di merito di oltre un ordine di grandezza. Allo stesso tempo, tutti gli altri parametri chiave di un accelerometro consumer sono perfettamente in linea con le tipiche specifiche delle applicazioni target. Quindi, l'approccio proposto rappresenta una valida alternativa per fronteggiare le richieste stringenti in termini di stabilità presentate dalle applicazioni di nuova generazione, dalla mixed reality alla navigazione inerziale in assenza di segnale GPS.
TIME-SWITCHED FREQUENCY-MODULATION FOR LOW-OFFSET-DRIFT, WIDE RANGE, FULLY INTEGRATED 3-AXIS MEMS ACCELEROMETERS
MARRA, CRISTIANO ROCCO
Abstract
In this PhD thesis, a novel working principle for 3-axis frequency-modulated (FM) MEMS accelerometers is proposed. Indeed, the differential frequency readout is performed through the innovative time-switched approach: this methodology is based on a double sampling of the oscillation frequency of a single resonator, consecutively biased in two different configurations in time. The technique enables to avoid offset thermal drift contributions typical of differential resonant accelerometers based on two distinct resonators with unavoidable mismatch in the temperature coefficient of frequency. After a complete behavioral modeling of the conceived system, a compact tri-axis MEMS structure is designed and fabricated using an industrial process. The encouraging results obtained from the characterization of the sensor motivated the design of an integrated analog oscillator, in order to prove the possibility to keep the same key performance coupling the device with a low-power, small-footprint ASIC. To this aim, two different feedback oscillator loops are analyzed and fabricated, theoretically and experimentally identifying the optimum topology in terms of power-noise trade-off. Finally, the tri-axis MEMS-ASIC combo is coupled to an integrated frequency-to-digital converter, demonstrating the feasibility of a fully integrated, digital output, consumer grade tri-axis FM time-switched MEMS accelerometer. The designed system solves the trade-off between offset thermal drift and full-scale-range experienced by state-of-the-art capacitive AM accelerometers. The ratio between these two critical parameter results improved by more than an order of magnitude, without any post-acquisition digital temperature compensation. At the same time, the other key parameters (as resolution, power consumption and bandwidth) remain in line with consumer devices currently on the market. Thus, the presented approach represents a promising strategy to face tight requirements of next-generation applications as mixed-reality and pedestrian inertial navigation.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/145763