16 research outputs found
Modelling the electrostatic actuation of MEMS: state of the art 2005.
Most of MEMS devices are actuated using electrostatic forces. Parallel or lateral plate
actuators are the types commonly used. Nevertheless, electrostatic actuation has some
limitations due to its non-linear nature. This work presents a methodic overview of the
existing techniques applied to the Micro-Electro-Mechanical Systems (MEMS) electrostatic
actuation modeling and their implications to the dynamic behavior of the electromechanical
system
Development and experimental analysis of a micromachined Resonant Gyrocope
This thesis is concerned with the development and experimental analysis of a resonant gyroscope. Initially, this involved the development of a fabrication process suitable for the construction of metallic microstructures, employing a combination of nickel electroforming and sacrificial layer techniques to realise free-standing and self-supporting mechanical elements. This was undertaken and achieved. Simple beam elements of typically 2.7mm x 1mm x 40µm dimensions have been constructed and subject to analysis using laser doppler interferometry. This analysis tool was used to implement a fill modal analysis in order to experimentally derive dynamic parameters. The characteristic resonance frequencies of these cantilevers have been measured, with 3.14kHz, 23.79kHz, 37.94kHz and 71.22kHz being the typical frequencies of the first four resonant modes. Q-factors of 912, 532, 1490 and 752 have been measured for these modes respectively at 0.01mbar ambient pressure. Additionally the mode shapes of each resonance was derived experimentally and found to be in excellent agreement with finite element predictions. A 4mm nickel ring gyroscope structure has been constructed and analysed using both optical analysis tools and electrical techniques. Using laser doppler interferometry the first four out-of-plane modes of the ring structure were found to be typically 9.893 kHz, 11.349 kHz, 11.418 kHz and 13.904 kHz with respective Q-factors of 1151, 1659, 1573 and 1407 at 0.01 mbar ambient pressure. Although electrical measurements were found to be obscured through cross coupling between drive and detection circuitry, the in-plane operational modes of the gyroscope were sucessfully determined. The Cos2Ө and Sin2Ө operational modes were measured at 36.141 kHz and 36.346 kHz, highlighting a frequency split of 205kHz. Again all experimentally derived modal parameters were in good agreement with finite element predictions. Furthermore, using the analysis model, the angular resolution of the gyroscope has been predicted to be approximately 4.75º/s
Energy efficient control of electrostatically actuated MEMS
Plenty of Micro-electro-mechanical Systems (MEMS) devices are actuated using electrostatic forces, and specially, parallel-plate actuators are extensively used, due to the simplicity of their design. Nevertheless, parallel-plate actuators have some limitations due to the nonlinearity of the generated force. The dissertation analyzes the dynamics of the lumped electrostatically actuated nonlinear system, in order to obtain insight on its characteristics, define desired performance goals and implement a controller for energy efficient robustly stable actuation of MEMS resonators.
In the first part of the dissertation, the modeling of the electromechanical lumped system is developed. From a complete distributed parameters model for MEMS devices which rely on electrostatic actuation, a concentrated parameters simplification is derived to be used for analysis and control design.
Based on the model, energy analysis of the pull-in instability is performed. The classic approach is revisited to extend the results to models with a nonlinear springs. Analysis of the effect of dynamics is studied as an important factor for the stability of the system. From this study, the Resonant Pull-in Condition for parallel-plate electrostatically actuated MEMS resonators is defined and experimentally validated.
Given the importance of the nonlinear dynamics and its richness in behaviors, Harmonic Balance is chosen as a tool to characterize the steady-state oscillation of the resonators. This characterization leads to the understanding of the key factors for large and stable oscillation of resonators. An important conclusion is reached, Harmonic Balance predicts that any oscillation amplitude is possible for any desired frequency if the appropriate voltage is applied to the resonator. And the energy consumption is dependent on this chosen oscillation frequency.
Based on Harmonic Balance results, four main goals are defined for the control strategy:
Stable oscillation with large amplitudes of motion; Robust oscillation independently of MEMS imperfections; Pure sinus-like oscillation for high-grade sensing; and Low energy consumption.
The second part of the dissertation deals with the controller selection, design and verification.
A survey of prior work on MEMS control confirms that existing control approaches cannot provide the desired performance. Consequently, a new three-stage controller is proposed to obtain the desired oscillation with the expected stability and energy efficiency. The controller has three different control loops. The first control loop includes a Robust controller designed using on µ-synthesis, to deal with MEMS resonators uncertainties. The second control loop includes an Internal-Model-Principle Resonant controller, to generate the desired control action to obtain the desired oscillation. And the third control loop handles the energy consumption minimization through an Extremum Seeking Controller, which selects the most efficient working frequency for the desired oscillation.
The proposed controller is able to automatically generate the needed control voltage, as predicted by the Harmonic Balance analysis, to operate the parallel-plate electrostatically actuated MEMS resonator at the desired oscillation. Performance verification of stability, robustness, sinus-like oscillation and energy efficiency is carried out through simulation.
Finally, the needed steps for a real implementation are analyzed. Independent two-sided actuation for full-range amplitude oscillation is introduced to overcome the limitations of one-sided actuation. And a modification of standard Electromechanical Amplitude Modulation is analyzed and validated for position feedback implementation. With these improvements, a MEMS resonator with the desired specifications for testing the proposed control is designed for fabrication. Based on this design, testing procedure is discussed as future work.Molts microsistemes (MEMS) són actuats amb forces electrostà tiques, i especialment, els actuadors electrostà tics de plaques paral.leles són molt usats, degut a la simplicitat del seu disseny. Tot i això, aquests actuadors tenen limitacions degut a la no-linealitat de les forces generades. La tesi analitza el sistema mecà nic no-lineal actuat electrostà ticament que forma el MEMS, per tal d'entendre'n les caracterÃstiques, definir objectius de control de l'oscil.lació, i implementar un controlador robust, estable i eficient energèticament. A la primera part de la tesi es desenvolupa el modelat del sistema electromecà nic complert. A partir de la formulació de parà metres distribuïts aplicada a dispositius MEMS amb actuació electrostà tica, es deriva una formulació de parà metres concentrats per a l'anà lisi i el disseny del control. Basat en aquest model, s'analitza energèticament la inestabilitat anomenada Pull-in, ampliant els resultats de l'enfocament clà ssic al model amb motlles no-lineals. Dins de l'anà lisi, l'evolució dinà mica s'estudia per ser un factor important per a l'estabilitat. D'aquest estudi, la Resonant Pull-in Condition per a actuadors electrostà tics de plaques paral.leles es defineix i es valida experimentalment. Donada la importà ncia de la dinà mica no-lineal del sistema i la seva riquesa de comportaments, s'utilitza Balanç d'Harmònics per tal de caracteritzar les oscil.lacions en estacionari. Aquesta caracterització permet entendre els factors claus per a obtenir oscil.lacions estables i d'amplitud elevada. El Balanç d'Harmònics dóna una conclusió important: qualsevol amplitud d'oscil.lació és possible per a qualsevol freqüència desitjada si s'aplica el voltatge adequat al ressonador. I el consum energètic associat a aquesta oscil.lació depèn de la freqüència triada. Llavors, basat en aquests resultats, quatre objectius es plantegen per a l'estratègia de control: oscil.lació estable amb amplituds elevades; robustesa de l'oscil.lació independentment de les imperfeccions dels MEMS; oscil.lació sinusoïdal sense harmònics per a aplicacions d'alta precisió; i baix consum energètic. La segona part de la tesi tracta la selecció, disseny i verificació dun controlador adequat per a aquests objectius. La revisió dels treballs existents en control de MEMS confirma que cap dels enfocaments actuals permet obtenir els objectius desitjats. En conseqüència, es proposa el disseny d'un nou controlador amb tres etapes per tal d'obtenir l'oscil.lació desitjada amb estabilitat i eficiència energètica. El controlador té tres llaços de control. Al primer llaç, un controlador robust dissenyat amb µ-sÃntesis gestiona les incertes es dels MEMS. El segon llaç inclou un controlador Ressonant, basat en el Principi del Model Intern, per a generar l'acció de control necessà ria per a obtenir l'oscil.lació desitjada. I el tercer llaç de control gestiona la minimització de l'energia consumida mitjançant un controlador basat en Extremum Seeking, el qual selecciona la freqüència de treball més eficient energèticament per a l'oscil.lació triada. El controlador proposat és capaç de generar automà ticament el voltatge necessari, igual al previst pel Balanç d'Harmònics, per tal d'operar electrostà ticament amb plaques paral.leles els ressonadors MEMS. Mitjançant simulació se'n verifica l'estabilitat, robustesa, inexistència d'harmònics i eficiència energètica de l'oscil.lació. Finalment, la implementació real és analitzada. En primer lloc, un nou esquema d'actuació per dos costats amb voltatges independents es proposa per aconseguir l'oscil.lació del ressonador en tot el rang d'amplituds. I en segon lloc, una modificació del sensat amb Modulació d'Amplitud Electromecà nica s'utilitza per tancar el llaç de control de posició. Amb aquestes millores, un ressonador MEMS es dissenya per a ser fabricat i validar el control. Basat en aquest disseny, es proposa un procediment de test plantejat com a treball futur.Postprint (published version
A process technology for realizing integrated inertial sensors using deep reactive ion etching (DRIE) and aligned wafer bonding
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (p. 89-90).by Chi-Fan Yung.S.M
Degree-per-hour mode-matched micromachined silicon vibratory gyroscopes
The objective of this research dissertation is to design and implement two novel micromachined silicon vibratory gyroscopes, which attempt to incorporate all the necessary attributes of sub-deg/hr noise performance requirements in a single framework: large resonant mass, high drive-mode oscillation amplitudes, large device capacitance (coupled with optimized electronics), and high-Q resonant mode-matched operation. Mode-matching leverages the high-Q (mechanical gain) of the operating modes of the gyroscope and offers significant improvements in mechanical and electronic noise floor, sensitivity, and bias stability. The first micromachined silicon vibratory gyroscope presented in this work is the resonating star gyroscope (RSG): a novel Class-II shell-type structure which utilizes degenerate flexural modes. After an iterative cycle of design optimization, an RSG prototype was implemented using a multiple-shell approach on (111) SOI substrate. Experimental data indicates sub-5 deg/hr Allan deviation bias instability operating under a mode-matched operating Q of 30,000 at 23ºC (in vacuum). The second micromachined silicon vibratory gyroscope presented in this work is the mode-matched tuning fork gyroscope (M2-TFG): a novel Class-I tuning fork structure which utilizes in-plane non-degenerate resonant flexural modes. Operated under vacuum, the M2-TFG represents the first reported high-Q perfectly mode-matched operation in Class-I vibratory microgyroscope. Experimental results of device implemented on (100) SOI substrate demonstrates sub-deg/hr Allan deviation bias instability operating under a mode-matched operating Q of 50,000 at 23ºC. In an effort to increase capacitive aspect ratio, a new fabrication technology was developed that involved the selective deposition of doped-polysilicon inside the capacitive sensing gaps (SPD Process). By preserving the structural composition integrity of the flexural springs, it is possible to accurately predict the operating-mode frequencies while maintaining high-Q operation. Preliminary characterization of vacuum-packaged prototypes was performed. Initial results demonstrated high-Q mode-matched operation, excellent thermal stability, and sub-deg/hr Allan variance bias instability.Ph.D.Committee Chair: Dr. Farrokh Ayazi; Committee Member: Dr. Mark G. Allen; Committee Member: Dr. Oliver Brand; Committee Member: Dr. Paul A. Kohl; Committee Member: Dr. Thomas E. Michael
High performance readout circuits and devices for Lorentz force resonant CMOS-MEMS magnetic sensors
In the last decades, sensing capabilities of martphones have greatly improved since the early mobile phones of the 90’s. Moreover, wearables and the automotive industry require increasing
electronics and sensing sophistication. In such echnological advance, Micro Electro Mechanical Systems (MEMS) have played an important role as accelerometers and gyroscopes were the first
sensors based on MEMS technology massively introduced in the market. In contrast, it still does not exist a commercial MEMS-based compass, even though Lorentz force MEMS magnetometers
were first proposed in the late 90’s.
Currently, Lorentz force MEMS magnetometers have been under the spotlight as they can offer an integrated solution to nowadays sensing power. As a consequence, great advances have been
achieved, but various bottlenecks limit the introduction of Lorentz force MEMS compasses in the market. First, current MEMS magnetometers require high current consumption and high biasing
voltages to achieve good sensitivities. Moreover, even though devices with excellent performance and sophistication are found in the literature, there is still a lack of research on the readout electronic circuits, specially in the digital signal processing, and closed loop control. Second, most research outcomes rely on custom MEMS fabrication rocesses to manufacture the devices. This is the same approach followed in current commercial MEMS, but it requires different fabrication processes for the electronics and the MEMS. As a consequence, manufacturing cost is high and
sensor performance is affected by the MEMS-electronics interface parasitics.
This dissertation presents potential solutions to these issues in order to pave the road to the commercialization of Lorentz force MEMS compasses. First, a complete closed loop, digitally controlled readout system is proposed. The readout circuitry, implemented with off-the-shelf commercial components, and the digital control, on an FPGA, are proposed as a proof of concept of the feasibility, and potential benefits, of such architecture. The proposed system has a measured
noise of 550 nT / vHz while the MEMS is biased with 300 µA rms and V = 1 V . Second, various CMOS-MEMS magnetometers have been designed using the BEOL part of the TSMC and SMIC 180 nm standard CMOS processes, and wet and vapor etched. The devices measurement and characterisation is used to analyse the benefits and drawbacks of each design as well as releasing process. Doing so, a high volume manufacturing viability can be performed. Yield values as high as 86% have been obtained for one device manufactured in a SMIC 180 nm full wafer run, having a sensitivity of 2.82 fA/µT · mA and quality factor Q = 7.29 at ambient pressure. While a device manufactured in TSMC 180 nm has Q = 634.5 and a sensitivity of 20.26 fA/µT ·mA at 1 mbar and V = 1 V. Finally, an integrated circuit has been designed that contains all the critical blocks to perform the MEMS signal readout. The MEMS and the electronics have been manufactured using
the same die area and standard TSMC 180 nm process in order to reduce parasitics and improve noise and current consumption. Simulations show that a resolution of 8.23 µT /mA for V = 1 V and BW = 10 Hz can be achieved with the designed device.En les últimes dècades, tenint en compte els primers telèfons mòbils dels anys 90, les capacitats de sensat dels telèfons intel·ligents han millorat notablement. A més, la indústria automobilÃstica i de wearables necessiten cada cop més sofisticació en el sensat. Els Micro Electro Mechanical Systems (MEMS) han tingut un paper molt important en aquest avenç tecnològic, ja que acceleròmetres i giroscopis varen ser els primers sensors basats en la tecnologia MEMS en ser introduïts massivament al mercat. En canvi, encara no existeix en la indústria una brúixola electrònica basada en la tecnologia MEMS, tot i que els magnetòmetres MEMS varen ser proposats per primera vegada a finals dels anys 90. Actualment, els magnetòmetres MEMS basats en la força de Lorentz són el centre d'atenció donat que poden oferir una solució integrada a les capacitats de sensat actuals. Com a conseqüència, s'han aconseguit grans avenços encara que existeixen diversos colls d'ampolla que encara limiten la introducció al mercat de brúixoles electròniques MEMS basades en la força de Lorentz. Per una banda, els agnetòmetres MEMS actuals necessiten un consum de corrent i un voltatge de polarització elevats per aconseguir una bona sensibilitat. A més, tot i que a la literatura hi podem trobar dispositius amb rendiments i sofisticació excel·lents, encara existeix una manca de recerca en el circuit de condicionament, especialment de processat digital i control del llaç. Per altra banda, moltes publicacions depenen de processos de fabricació de MEMS fets a mida per fabricar els dispositius. Aquesta és la mateixa aproximació que s'utilitza actualment en la indústria dels MEMS, però té l'inconvenient que requereix processos de fabricació diferents pels MEMS i l’electrònica. Per tant, el cost de fabricació és alt i el rendiment del sensor queda afectat pels parà sits en la interfÃcie entre els MEMS i l'electrònica. Aquesta tesi presenta solucions potencials a aquests problemes amb l'objectiu d'aplanar el camà a la comercialització de brúixoles electròniques MEMS basades en la força de Lorentz. En primer lloc, es proposa un circuit de condicionament complet en llaç tancat controlat digitalment. Aquest s'ha implementat amb components comercials, mentre que el control digital del llaç s'ha implementat en una FPGA, tot com una prova de concepte de la viabilitat i beneficis potencials que representa l'arquitectura proposada. El sistema presenta un soroll de 550 nT / vHz quan el MEMS està polaritzat amb 300 µArms i V = 1 V . En segon lloc, s'han dissenyat varis magnetòmetres CMOS-MEMS utilitzant la part BEOL dels processos CMOS està ndard de TSMC i SMIC 180 nm, que després s'han alliberat amb lÃquid i gas. La mesura i caracterització dels dispositius s’ha utilitzat per analitzar els beneficis i inconvenients de cada disseny i procés d’alliberament. D'aquesta manera, s'ha pogut realitzar un anà lisi de la viabilitat de la seva fabricació en massa. S'han obtingut valors de yield de fins al 86% per un dispositiu fabricat amb SMIC 180 nm en una oblia completa, amb una sensibilitat de 2.82 fA/µT · mA i un factor de qualitat Q = 7.29 a pressió ambient. Per altra banda, el dispositiu fabricat amb TSMC 180 nm presenta una Q = 634.5 i una sensibilitat de 20.26 fA/µT · mA a 1 mbar amb V = 1 V. Finalment, s'ha dissenyat un circuit integrat que conté tots els blocs per a realitzar el condicionament de senyal del MEMS. El MEMS i l'electrònica s'han fabricat en el mateix dau amb el procés està ndard de TSMC 180 nm per tal de reduir parà sits i millorar el soroll i el consum de corrent. Les simulacions mostren una resolució de 8.23 µT /mA amb V = 1 V i BW = 10 Hz pel dispositiu dissenyat
Microfabricated liquid density sensors using polyimide-guided surface acoustic waves
The simultaneous measurements of liquid density and refractive index on the same liquid sample are desirable. This thesis investigates the development of a micro- fabricated liquid density sensor that can be integrated into existing refractometers. A discussion of density sensing techniques and review of suitable sensors is given, leading to the choice of a Love mode surface acoustic wave (SAW) device. Love modes are formed by focussing the acoustic energy in a thin waveguide layer on a surface acoustic wave device. The horizontal-shear wave motion reduces attenuation in liquid environments, and the high surface energy density theoretically gives the highest sensitivity of all SAW devices. This study follows the development of a Love mode liquid density sensor using a polyimide waveguide layer. The novel use of polyimide offers simple and cheap fabrication, and theoretically gives a very high sensitivity to surface loading due to its low acoustic velocity. Love mode devices were fabricated with different polyimide waveguide thicknesses. The optimum thickness for a compromise between low loss and high sensitivity was 0.90 - 1.0 μm. These devices exhibited a linear shift in frequency with the liquid density-viscosity product for low viscosities. The response was smaller for high viscosities due to non-Newtonian liquid behaviour. Dual delay-line structures with a smooth 'reference' and corrugated 'sense' delay- lines were used to trap the liquid and separate the density from the density-viscosity product. A sensitivity up to 0.13 μgcm(^-3)Hz(^-1) was obtained. This is the highest density sensitivity obtained from an acoustic mode sensor. Experimental results show a zero temperature coefficient of frequency is possible using polyimide waveguides. These are the first Love mode devices that demonstrate temperature independence, highlighting the importance of polyimide as a new waveguide material
Study of a micro force sensor for colloidal engines applications
The goal of this study is the design, manufacturing and test of micro-force sensors
based on MEMS technology. With a wide range of application it focuses on the characterization
of the propulsive properties of the colloidal engines in the aerospace field.
There is a wide range of applications for microforce sensors but in this project we
have focused on the characteristics of colloidal engines for aerospace field in order to
determine the resolution and the ranges of forces that the sensor should measure
Micro-g MEMS accelerometer based on time measurement
Programa Doutoral em Engenharia Electrónica e de ComputadoresThe MEMS sensor market has experienced an amazing growth on the last decades, with
accelerometers being one of the pioneers pushing the technology into widespread use with its
applications on automotive industry. Since then, accelerometers have been gradually replacing
conventional sensors due mainly to its lower cost. As the performance of MEMS accelerometers
improves, the applications range where they replace conventional accelerometers increases.
Nowadays, there is still a large range of applications for which suitable MEMS accelerometers
are yet to be developed.
This work focuses on the development of a high performance accelerometer taking
advantage of the high sensitivity of a non-linear phenomenon that occurs in electrostatically
actuated movable capacitive microdevices: electrostatic pull-in. Although the pull-in effect has
been known for more than 40 years, it is usually avoided when dealing with movable
microstructures as it leads to a region of instability, where the position of movable parts cannot
be fully controlled. In the last decade, the pull-in displacement profile of 1-DOF parallel-plates
devices has been the subject of research that revealed the presence of a so-called meta-stability.
This meta-stability occurs in specific damping and voltage actuation conditions and translates as
a non-linear displacement profile, rather than simple time-of-flight. This feature makes the pull-in
time duration significantly longer, and it happens to be extremely sensitive to intervenient forces,
such as external acceleration. Basically, measuring the pull-in time of specifically designed
microstructures (while maintaining the other parameters constant) allows the measurement of the
external acceleration that acts on the system. Using a pull-in time measurement rather than direct
capacitance/displacement/acceleration transduction presents several advantages. The most
important is the fact that time can be measured very accurately with technology readily available.
For instance, if one uses a 100MHz clock on the time counting mechanism, which corresponds to
a time measurement resolution of 100 ns, given the 0.26 μs/μg sensitivity of the accelerometer
developed in this work, an acceleration resolution of 0.38 μg could be achieved.
One of the main challenges of the time based accelerometer development is the damper
design, as damping is of outmost importance in defining the accelerometer performance
parameters, namely sensitivity and noise. A new squeeze-film damper geometry design has been
presented and studied. It consists of flow channels implemented on the parallel-plates that relieve the squeeze-film damping pressures generated when the device is moving. This geometry has
proved to be very effective in increasing the capacitance/damping ratio in parallel-plates, which
was up to now a great challenge of in-plane parallel-plates design.
This work reports the development of an open-loop accelerometer with 0.26 μs/μg
sensitivity and 2.7 μg /√Hz noise performance. The MEMS structures used for its experimental
implementation were fabricated using a commercially available SOI micromachining process.
The main drawbacks of this accelerometer were the low system bandwidth and non-linearity.
Closed-loop approaches using electrostatic feedback were explored in this work in order to
overcome these limitations, and the dynamic range was successfully extended to 109 dB along
with improvements on the linearity.
From the thorough damping study performed in this work, a new application for the pullin
time using the same microstructures was developed. It consists of a gas viscosity sensing
application. At the low frequencies operated, damping is directly proportional to the viscosity of
the gas medium. The experimental results obtained with gases with viscosities ranging from 8 μP
to 18 μP have shown a sensitivity of 2 ms/μP, making the pull-in time viscosity sensor a very
promising approach.Nas últimas décadas assistiu-se a um imenso crescimento no mercado de sensors MEMS,
tendo os acelerómetros sido uma das maiores forças impulsionadoras desse crescimento devido
às suas aplicações na indústria automóvel. Desde então, a gama de aplicações destes sensores
expandiu-se multidirecionalmente, novas aplicações emergiram e acelerómetros convencionais
em aplicações já existentes foram substituÃdos por acelerómetros MEMS. Isto deve-se
essencialmente ao seu baixo custo e pequenas dimensões. Há no entanto, aplicações para as quais
o desempenho dos acelerómetros MEMS ainda não é suficiente.
O objectivo deste trabalho é desenvolver um acelerómetro de elevado desempenho tirando
partido da elevada sensibilidade do efeito de pull-in a forças externas tais como a aceleração. O
efeito de pull-in, descrito pela primeira vez há mais de 40 anos, ocorre em dispositivos
capacitivos com partes móveis. Este é um efeito não-linear geralmente evitado/indesejado, uma
vez que se traduz numa instabilidade que dificulta o controlo da posição das partes móveis. Na
última década foi dedicada alguma investigaçao cientÃfica a este fenómeno, tendo sido descoberta
a existência de um perfil de deslocamento particular, denominado meta-estabilidade, em
determinadas condições de amortecimento e de actuação electrostática. Esta caracterÃstica do
pull-in torna a sua duração extremamente sensÃvel a variações nas forças intervenientes, incluindo
aceleração externa. Assim sendo, a medição do tempo de pull-in de micro-estruturas
especificamente concebidas para o efeito pode ser utilizada para medir aceleração. Esta
abordagem apresenta vantagens significativas em comparação com a transdução direta de
capacidade para aceleração (caso da generalidade dos acelerómetros capacitivos).
Nomeadamente, a variável tempo pode ser medida com elevada precisão com relativa facilidade
e sem necessidade de desenvolvimentos tecnológicos (o que não é o caso da medição de
capacidade). Por exemplo, o uso de uma frequência de relógio de 100 MHz no mecanismo de
contagem de tempo permite uma resolução de 100 ns na medição de tempo, o que corresponde,
considerando a sensibilidade de 0.26 μs/μg do acelerómetro desenvolvido neste trabalho, a uma
resolução na medição de acceleração de 0.38μg.
Um dos maiores desafios do desenvolvimento de um acelerómetro baseado no tempo de
pull-in é o desenho do amortecedor, pois a sensibilidade e o ruÃdo/resolução do sensor final
dependem do nÃvel de amortecimento. Uma nova geometria para o amortecedor (estabelecido por um mecanismo de squeeze-film) é apresentada e estudada neste trabalho. Esta consiste em abrir
canais nas placas paralelas facilitando assim o fluxo de ar quando as placas se movem. Ficou
provado que esta geometria é eficaz na redução da razão capacidade/amortecimento, o que
constituÃa um problema recorrente no desenho de dispositivos de placas paralelas in-plane.
Neste trabalho é descrito o desenvolvimento de um acelerómetro em malha aberta com
uma sensibilidade de 0.26 μs/μg e 2.7 μg /√Hz de ruÃdo. As estruturas MEMS utilizadas na sua
implementação foram fabricadas num processo de microfabrico SOI comercial. As principais
desvantagens desta abordagem são pequena gama dinâmica devido à não-linearidade da resposta.
Neste trabalho foram exploradas abordagens em malha fechada, usando feedback electrostático,
de modo a ultrapassar estas limitações, tendo sido alcançado um aumento da gama dinâmica para
109 dB, com grandes melhoria na linearidade.
Uma nova aplicação para o tempo de pull-in foi também desenvolvida: medição de
viscosidade de gases. Uma vez que as microstruturas utilizadas são operadas a baixas
frequências, o amortecimento é proporcional à viscosidade. O estudo efectuado mostra que o
tempo de pull-in é muito sensÃvel ao amortecimento e portanto a variações de viscosidade. Os
resultados experimentais obtidos com gases e misturas de gases com viscosidades entre 8 μP e
18 μP mostraram uma sensibilidade de 2 ms/μP, confirmando o potencial da utilização de tempo
de pull-in na medição de viscosidade.The author, Rosana Maria Alves Dias, was supported by Portuguese Foundation for Science and Technology (SFRH/BD/46030/2008)