Frequency modulated magnetometer using a double-ended tuning fork resonator

A Lorentz force MEMS magnetometer based on a double-ended tuning fork (DETF) for out-of-plane sensing is presented here. A novel configuration using a hexagonal-shaped Lorentz force transducer is used, which simplifies the sensor configuration and improves its sensitivity. Frequency modulated devices were fabricated in an in-house process on silicon on insulator wafers (SOI) and then tested in vacuum. The final devices have a differential configuration and experimental characterization shows a sensitivity of 4.59 Hz/mT for a total input current (on the Lorentz bar) of 1.5 mA.The first author is supported by FCT—Fundação para a Ciência e Tecnologia through the grant PDE/BDE/114564/2016. This work is supported by FCT with the reference project UID/EEA/04436/2013, COMPETE 2020 with the code POCI-01-0145-FEDER-006941.info:eu-repo/semantics/publishedVersio

High-resolution seismocardiogram acquisition and analysis system

Several devices and measurement approaches have recently been developed to perform ballistocardiogram (BCG) and seismocardiogram (SCG) measurements. The development of a wireless acquisition system (hardware and software), incorporating a novel high-resolution micro-electro-mechanical system (MEMS) accelerometer for SCG and BCG signals acquisition and data treatment is presented in this paper. A small accelerometer, with a sensitivity of up to 0.164 µs/µg and a noise density below 6.5 µg/ Hz is presented and used in a wireless acquisition system for BCG and SCG measurement applications. The wireless acquisition system also incorporates electrocardiogram (ECG) signals acquisition, and the developed software enables the real-time acquisition and visualization of SCG and ECG signals (sensor positioned on chest). It then calculates metrics related to cardiac performance as well as the correlation of data from previously performed sessions with echocardiogram (ECHO) parameters. A preliminarily clinical study of over 22 subjects (including healthy subjects and cardiovascular patients) was performed to test the capability of the developed system. Data correlation between this measurement system and echocardiogram exams is also performed. The high resolution of the MEMS accelerometer used provides a better signal for SCG wave recognition, enabling a more consistent study of the diagnostic capability of this technique in clinical analysis.This work is supported by FCT with the reference project UID/EEA/04436/2013, COMPETE 2020 with the code POCI-01-0145-FEDER-006941

Influence of mechanical stress in a packaged frequency-modulated MEMS accelerometer

Frequency modulated accelerometers composed of two double-ended tuning fork (DETF) resonators on a differential configuration were characterized for their sensitivity to force applied to their package. Commonly, differential architectures are employed to cancel common mode errors, such as the mechanical stress or temperature dependency. The device dependence to mechanical stress was experimentally measured for forces up to 15 N and a reduction of about 5.6 times was obtained on the differential measurement. Additionally, the silicon dies were glued to chip-carriers using two different glues with distinct properties, and their sensitivity to stress was compared. The effectiveness of a viscoelastic glue over an epoxy-based glue for stress decoupling was tested. Long-term measurements under constant force were experimentally performed and for a time period of approximately 100 min, the stress relaxation and creeping of the viscoelastic glue enabled the recovery to the initial output of the sensor.The first author is supported by FCT - Fundacao para a Ciencia e Tecnologia through the grant PDE/BDE/114564/2016. This work is supported by FCT with the reference project UID/EEA/04436/2019

Control of electrostatic microactuators on the instability zone

Dissertação de mestrado integrado em Engenharia Eletrónica Industrial e ComputadoresNos dias de hoje, os dispositivos baseados em MEMS (Micro-electromechanical systems) são uma constante no mercado, desempenhando as mais diversas tarefas, desde a medição da aceleração ou pressão, até à utilização como microfones. Dispositivos MEMS são também usados como micro-atuadores, sendo que no caso dos micro-atuadores electroestáticos, devido às suas características, foram desenvolvidos sistemas contendo microsondas ou micro-espelhos com resultados muito promissores. O deslocamento entre placas paralelas electrostaticamente atuadas é apenas estável até 1/3 da distância total. Esta dissertação propõe o controlo de micro-atuadores na zona de instabilidade, sendo que a implementação de leis de controlo adequadas a MEMS permite deslocamentos controlados para além da zona de estabilidade. Foram implementadas duas leis distintas, o controlo On-Off e controlo PID com linearização em malha fechada, de forma a controlar bidireccionalmente o deslocamento dos elétrodos. Os resultados experimentais do sistema de controlo demonstram o seguimento de diversas referências a diferentes frequências. Foi conseguido um deslocamento superior a 88.9% da distância entre placas, o que equivale a uma melhoria de 267%. Estes resultados foram obtidos de forma muito similar nas duas teorias implementadas. A utilização de sistemas de controlo com leis adaptadas a MEMS é eficiente para minimizar as limitações dos micro-atuadores electroestáticos. O uso de uma FPGA aumenta o desempenho do sistema de forma significativa.Nowadays, devices based on MEMS (Micro-electromechanical systems) are a reality in the market, being used, for example, to measure acceleration, pressure or to serve as microphones. In the field of electrostatic microactuators, due to their characteristics, some micro-probes and micro-mirrors were developed with very promising results. The displacement between parallel-plates electrostatically actuated is just stable until 1/3 of the total gap. Therefore, this dissertation proposes the control of microactuators in the instability zone, where the implementation of appropriate control laws for MEMS are used to enable controlled displacements in the instability region. Two distinct laws were implemented, the On-Off control and a PID control with feedback linearization, enabling bidirectional control of the electrodes displacement along the full available gap. The experimental results show the tracking of many references for different frequencies. A displacement up to 88.9% was reached, leading to an improvement of 267%. Both implemented theories obtained very similar results. A control system made with appropriate control laws for MEMS is efficient to minimize the electrostatic microactuators limitations. A FPGA was used to increase the performance of the entire setup.Actuellement, les dispositifs basés sur la technologie MEMS (Microelectromechanical systems) se sont imposés sur le marché, réalisant différentes tâches, telle que, mesurer des accélérations, des pressions ou pouvant encore servir de microphone. Cependant, dans le champ des micro-actuateurs électrostatiques, due à leur caractéristique, des microsondes e des micro-miroirs ont été implémentés, obtenant des résultats très prometteurs. Le déplacement, entre plaques parallèles actionnées de forme électrostatique, est juste stable jusqu’à un tiers de la distance totale. Donc, cette dissertation propose le contrôle de micro-actuateurs dans la région d’instabilité, étant l’implémentation de loi de contrôle adéquate au MEMS une solution pour augmenter le déplacement au-delà de la région instable. Deux différentes loi ont été implémentées, le contrôle On-Off et le contrôle PID avec une linéarisation, pour contrôler bi-directionnellement le déplacement des électrodes. Les résultats expérimentaux du système de contrôle démontrent le suivi de plusieurs références à certaines fréquences. Le déplacement atteint a été supérieurs à 88.9% de la distance entre plaques, ce qui équivaut à une amélioration de 267%. Ces résultats ont été obtenus par les deux théories de forme similaires. L’utilisation de système de contrôle contenant des lois adaptées au dispositif MEMS est efficace pour minimiser les limitations inhérentes aux micro-actuateurs électrostatiques. L’emploi d’une FPGA augmente la performance du système significativement

Low-pressure resonant MEMS accelerometers for automotive applications

Tese de Doutoramento (Programa Doutoral em Sistemas Avançados de Engenharia para a Indústria)Os acelerómetros MEMS têm um papel importante no desenvolvimento de novas soluções/funcionalidades para a indústria automóvel, nomeadamente na melhoria das condições de segurança e de condução. Não obstante, dispositivos com melhor performance, dimensões mais reduzidas e menor custo possibilitariam a implementação de mais casos de uso/funcionalidades. A encapsulação em vácuo tem sido alvo de estudo para desenvolver sensores MEMS de dimensões reduzidas e de baixo custo, uma vez que viabiliza a integração monolítica de todos os sensores que compõe uma unidade de medição inercial. Contudo, os acelerómetros MEMS tradicionais não operam corretamente nestas condições. Esta tese foca-se no desenvolvimento de um acelerómetro MEMS ressonante baseado em ressonadores DETF ("double-ended tuning fork"). O sensor proposto é composto por dois ressonadores numa configuração diferencial para minimizar os efeitos de modo comum, ao qual foram adicionadas alavancas para aumentar a sensibilidade. Os dispositivos foram fabricados num processo standard de microfabricação na Bosch e encapsulados em vácuo. Foi desenvolvido um sistema que permite operar os ressonadores em malha aberta e malha fechada. Em malha fechada, a frequência de excitação dos DETF é constantemente atualizada, aumentando a linearidade e gama de medição. Após a caraterização exaustiva realizada é possível destacar uma alta sensibilidade (170.7 Hz/g) e boa não-linearidade (<0.63 %), em dimensões reduzidas (0.25 mm2), uma largura de banda de 63 Hz e uma gama de medida de ±5 g. Adicionalmente, é ainda apresentado um magnetómetro MEMS diferencial e modulado em frequência, que permite provar a deteção de campo magnético com DETFs. O sensor é também composto por um transdutor de força de Lorentz, tendo sido otimizado para aumentar a sua sensibilidade. Os magnetómetros foram fabricados no INL num processo SOI e é também apresentada a sua validação experimental.MEMS accelerometers are paramount in the development of the automotive industry, where many applica-tions improved the security and driving experience. However, better, cheaper, and smaller devices would enable even further use-cases. Encapsulation in vacuum has been studied to develop more affordable and smaller devices since it would allow the monolithic integration of the sensors composing an inertial measurement unit. Nevertheless, traditional accelerometers do not operate properly at low pressures. This work focus on the development of a resonant accelerometer based on double-ended tuning fork (DETF) resonators. The accelerometer proposed is composed of two DETF on a differential configuration to min-imise common-mode effects, and a force amplification mechanism was added to increase sensitivity. The designed devices were fabricated in a Bosch standard surface micromachining process and encapsulated in vacuum. A system was implemented to validate the accelerometers, operating the resonators in open-loop and closed-loop. In closed-loop, the driving frequency of the DETF is constantly updated, increasing the sensor linearity and full-scale. An extensive characterisation was performed and a good non-linearity (<0.63% FS) and high sensitivity (170.7 Hz/g) on a small footprint (0.25 mm2) were highlighted as the main achievements. A bandwidth of 63 Hz and a measurement range of at least ±5 g were also reported. A differential frequency-modulated magnetometer was designed, implemented and characterised to prove the feasibility of magnetic field sensing using DETF resonators. A novel Lorentz force transducer was proposed and optimised to increase the device's sensitivity. The magnetometers were fabricated on an in-house two mask SOI-process, and the experimental validation of the envisioned sensor was performed

Resonant accelerometer based on double-ended tuning fork and a force amplification mechanism

Resonant accelerometers are an alternative to amplitude modulated devices due to their higher integration capabilities, since they are encapsulated in vacuum and are stable at low pressures. Vacuum is required for some sensors (i.e., gyroscopes) but amplitude modulated accelerometers tend to be unstable under such conditions and therefore cannot be integrated in the same package. Herewith, a device composed by double-ended tuning fork resonators (DETF) and a force amplification mechanism for sensitivity enhancement is presented. Characterization of the fabricated devices was performed, and the design was successfully validated. A sensitivity close to 80 Hz/g was experimentally measured and the DETF characterization for different driving (AC) and bias voltages (DC) is also presented.The first author is supported by FCT– Fundação para a Ciência e Tecnologia through the grant PDE/BDE/114564/2016. This work is supported by FCT with the reference project UID/EEA/04436/2013, COMPETE 2020 with the code POCI-01-0145-FEDER-006941.info:eu-repo/semantics/publishedVersio

Full-gap tracking system for parallel plate electrostatic actuators using closed-loop control

Bi-directional full-gap tracking on parallel-plate electrostatic actuators using closed-loop control is demonstrated in this paper. On-Off control is explored, where the actuator position is controlled by actuating or releasing the actuator based on the actual position (weather it is above or below the desired position). Noise and delays introduced by the readout circuit compromise the performance of this control law working at 5 MHz sampling frequency. A Proportional-Integral-Derivative control law with feedback linearization is also implemented. This second control law is more complex than the on-off control and requires extensive data processing. In addition, in order to effectively control the actuator, the structure characteristics need to be available as well as the response of the readout circuit (voltage/distance converter). In order to decrease the steady-state error and the overshoot of the structure position, various proportional-integral-derivative (PID) gains are tested. Both controllers are implemented using a FPGA (Field-programmable gate array), which increases the performance of the controller. The experimental results, for both the controllers, show bi-directional tracking of several wave forms at frequencies up to 1 kHz (limited by the dynamics of the actuators). The extension of the travel range is about 88.9% of the full-gap available (due to the mechanical stoppers present in the structure). This corresponds to a displacement range extension of 267%.info:eu-repo/semantics/publishedVersio

Bi-directional extended range parallel plate electrostatic actuator based on feedback linearization

In this paper, we present a bi-directional extended range parallel-plate electrostatic actuator using feedback linearization control. The actuator can have stable displacements up to 90% of the full-gap (limited by mechanical stoppers) on both directions, i.e., the device can move +/- 2 mu m within a +/- 2.25 mu m gap. The system has successfully tracked references until 1kHz (limited by the dynamics of the device) and it presents a capacitor tuning range of 17, using an actuation voltage from 0 to 10V. The results presented here are a clear advance in respect to the current state-of-the-art in terms of tracking capabilities, total stable displacement and tuning range.info:eu-repo/semantics/publishedVersio

Novel magnetic readout for hybrid spintronic MEMS devices

This digest reports for the first time on the concept, fabrication and characterization of a spintronics magnetic readout for hybrid MEMS. The method uses magnetic sensors (spin valves) and permanent magnets (on both the movable and the fixed parts of the MEMS structure) to detect displacement. The use of such transduction method has several potential advantages: it allows a small form factor, straightforward readout electronics, xyz integrability, and does not require high aspect ratio silicon gaps, allowing the replacement of conventional capacitive transductors by a mechanism that occupies much less chip area. The proof-of-concept is presented for an in-plane movable structure. Both the full-gap linear displacement and the electrostatic pull-in behavior are monitored and displayed.info:eu-repo/semantics/publishedVersio