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1/f noise is present in every aspect of nature. Sensors and read-out electronics have the ultimate detection limit set by the noise floor of the white noise. In order to increase signal-to-noise ratio (SNR) of low frequency signals buried by high 1/f noise, the signal can be up-converted to a high frequency signal that lies in the lower white noise regime of the sensing device. Mechanical modulation can be employed to move low frequency electronic signals to higher frequency region through the use of microresonators. This thesis has two goals: (1) develop and fabricate a hybrid micromechanical-magnetoresistive magnetic field sensor; and (2) design an electrometer to measure currents collected from air streams containing ionized nano-particles. First, we designed magnetoresistive-microelectromechanical systems (MR-MEMS) hybrid devices based on the monolithic integration of magnetic thin films and silicon-on-insulator (SOI) MEMS fabrication techniques. We used MgO-based magnetic tunnel junctions (MTJ) placed on a bulk micromachined silicon MEMS device to form a hybrid sensing device. The MEMS device was used to mechanically modulate the magnetic field signal detected by the MTJ, thereby reducing the effects of 1/f noise on the MTJ's output. Two actuator designs were investigated: cantilever and electrostatic comb-drive. The second component of the thesis presents a MEMS-based electrometer for the detection of small currents from ionized particles in a particle detection system for air-quality monitoring. One method of particle detection ionizes particles and then feeds a stream of charged particles into a Faraday cup electrometer. We replaced the Faraday cup with a filtering porous mesh sensing-electrode coupled to a MEMS electrometer with a noise floor below 1 fA rms. Experiments were conducted with fA level currents produced by 10 nm diameter particles within an airflow of 1.0 L/min. The MEMS electrometer was compared and calibrated using commercial electrometers and particle counters.
Lorentz force magnetic sensors based on micro-electromechanical system (MEMS) resonators, measuring the vector components of the magnetic field, have recently attracted substantial commercial interest in inertial navigation systems (INSs) and compasses for smartphones. Over the last decade, substantial research effort has focused on improving the magnetic field sensitivity and resolution of Lorentz force magnetic sensors relying on either amplitude modulation (AM) or frequency modulation (FM), however, they mostly suffer from narrow bandwidth and low scale factor temperature stability, and their bias instability is poor due to high offset in the sensor output, which precludes their use in INSs and compasses. In this thesis, both AM and FM Lorentz force magnetic sensors are investigated to solve each of the above-mentioned problems, where AM sensors are operated either off-resonance in open-loop or at resonance in closed-loop. The MEMS magnetic sensors studied in this work are based on either a single resonator or dual resonators. The experimental results presented here make the Lorentz force sensor compatible with INSs and navigation-grade compasses. In the first part of this study, a method for improving bandwidth and thermal stability of the scale factor is presented. The method is successfully demonstrated using two nominally-identical, resonator-based AM magnetometers: the first is operated off-resonance in open-loop to measure magnetic field, and the second is operated as a closed-loop oscillator to provide a frequency reference for Lorentz force generation. With the proposed method, the sensor’s temperature sensitivity is reduced by a factor of 24, and a wide bandwidth (38 Hz) that is independent of the sensor’s mechanical bandwidth (3.2 Hz) is achieved. However, it is observed that the open-loop AM sensor operating off-resonance suffers from poor bias instability that is found to be limited by offset-related 1/f (flicker) noise. The root cause of 1/f noise is demonstrated to be 1/f noise on the ac and dc bias voltages applied to the sensor, and the effects of 1/f noise sources on the sensor’s bias instability are explored. To reduce offset-related 1/f noise, an innovative method based on chopping the dc bias voltage applied to the resonator is described. Using the chopping method, the sensor’s bias instability is reduced from 27 nT to 7 nT (the best bias instability reported to date for a resonant MEMS magnetometer). The second part of this study focuses on closed-loop AM operation. A force-rebalanced Lorentz force magnetometer is demonstrated, which is the first demonstration of a three-axis magnetic field sensing oscillator incorporating force-rebalanced operation. The proposed force-rebalanced magnetometer shows significantly superior scale factor stability performance over temperature change and allows larger bandwidth compared to conventional closed-loop magnetometers. However, the force-rebalanced sensor is plagued by offset arising from the electrostatic force used to drive the sensor into resonance. Because the offset is strongly temperature-dependent, the sensor’s bias instability degrades in the presence of temperature variations. This problem is successfully solved by designing a dual-resonator magnetometer, having two identical resonators with opposing axes of field sensitivity. In the last part, sensor operation is demonstrated using quadrature FM (QFM) readout, where the field is measured by monitoring the change in oscillation frequency. It is theoretically and experimentally demonstrated that FM sensors potentially provide wide bandwidth and improved stability over temperature as compared to conventional AM sensors. However, their output stability is still poor due to the temperature dependence of the sensor’s resonant frequency. To solve this problem, a dual-resonator QFM magnetic sensor composed of a matched pair of differentially operated resonators on the same silicon die are developed. Experimental data show that a differential measurement scheme using the dual resonator significantly improves the sensor’s bias instability.
New approaches offer the promise of providing energy efficient, low cost, small, and highly sensitive magnetic sensors. However, the 1/f noise of these new types of sensors is a major obstacle. Many army applications, such as detecting moving targets, require sensitivity at low frequencies. This paper reports development of a device, the MEMS flux concentrator, invented at ARL, that minimizes the effect of 1/f noise in sensors. The device accomplishes this by shifting the operating frequency to higher frequencies where 1/f noise is much lower. This shift is accomplished by modulating the magnetic field before it reaches the sensor. In our device, the magnetic sensor, a GMR sensor, is placed between flux concentrators that have been deposited on MEMS flaps. The motion of the MEMS flaps modulates the field by a factor of 3 at frequencies from 8 to 15 kHz. The MEMS flux concentrator should increase the sensitivity of many magnetic sensors by two to three orders of magnitude. An equally important benefit is that, because it is a modulation technique, it eliminates the problem of dealing with the large DC bias of most magnetoresistive sensors.
How Can We Lower the Power Consumption of Gas Sensors? There is a growing demand for low-power, high-density gas sensor arrays that can overcome problems relative to high power consumption. Low power consumption is a prerequisite for any type of sensor system to operate at optimum efficiency. Focused on fabrication-friendly microelectromechanical systems (MEMS) and other areas of sensor technology, MEMS and Nanotechnology for Gas Sensors explores the distinct advantages of using MEMS in low power consumption, and provides extensive coverage of the MEMS/nanotechnology platform for gas sensor applications. This book outlines the microfabrication technology needed to fabricate a gas sensor on a MEMS platform. It discusses semiconductors, graphene, nanocrystalline ZnO-based microfabricated sensors, and nanostructures for volatile organic compounds. It also includes performance parameters for the state of the art of sensors, and the applications of MEMS and nanotechnology in different areas relevant to the sensor domain. In addition, the book includes: An introduction to MEMS for MEMS materials, and a historical background of MEMS A concept for cleanroom technology The substrate materials used for MEMS Two types of deposition techniques, including chemical vapour deposition (CVD) The properties and types of photoresists, and the photolithographic processes Different micromachining techniques for the gas sensor platform, and bulk and surface micromachining The design issues of a microheater for MEMS-based sensors The synthesis technique of a nanocrystalline metal oxide layer A detailed review about graphene; its different deposition techniques; and its important electronic, electrical, and mechanical properties with its application as a gas sensor Low-cost, low-temperature synthesis techniques An explanation of volatile organic compound (VOC) detection and how relative humidity affects the sensing parameters MEMS and Nanotechnology for Gas Sensors provides a broad overview of current, emerging, and possible future MEMS applications. MEMS technology can be applied in the automotive, consumer, industrial, and biotechnology domains.
Lists citations with abstracts for aerospace related reports obtained from world wide sources and announces documents that have recently been entered into the NASA Scientific and Technical Information Database.
Advances in materials science and engineering have paved the way for the development of new and more capable sensors. Drawing upon case studies from manufacturing and structural monitoring and involving chemical and long wave-length infrared sensors, this book suggests an approach that frames the relevant technical issues in such a way as to expedite the consideration of new and novel sensor materials. It enables a multidisciplinary approach for identifying opportunities and making realistic assessments of technical risk and could be used to guide relevant research and development in sensor technologies.
“Microsystems and Nanotechnology” presents the latest science and engineering research and achievements in the fields of microsystems and nanotechnology, bringing together contributions by authoritative experts from the United States, Germany, Great Britain, Japan and China to discuss the latest advances in microelectromechanical systems (MEMS) technology and micro/nanotechnology. The book is divided into five parts – the fundamentals of microsystems and nanotechnology, microsystems technology, nanotechnology, application issues, and the developments and prospects – and is a valuable reference for students, teachers and engineers working with the involved technologies. Professor Zhaoying Zhou is a professor at the Department of Precision Instruments & Mechanology , Tsinghua University , and the Chairman of the MEMS & NEMS Society of China. Dr. Zhonglin Wang is the Director of the Center for Nanostructure Characterization, Georgia Tech, USA. Dr. Liwei Lin is a Professor at the Department of Mechanical Engineering, University of California at Berkeley, USA.
The transformation of vibrations into electric energy through the use of piezoelectric devices is an exciting and rapidly developing area of research with a widening range of applications constantly materialising. With Piezoelectric Energy Harvesting, world-leading researchers provide a timely and comprehensive coverage of the electromechanical modelling and applications of piezoelectric energy harvesters. They present principal modelling approaches, synthesizing fundamental material related to mechanical, aerospace, civil, electrical and materials engineering disciplines for vibration-based energy harvesting using piezoelectric transduction. Piezoelectric Energy Harvesting provides the first comprehensive treatment of distributed-parameter electromechanical modelling for piezoelectric energy harvesting with extensive case studies including experimental validations, and is the first book to address modelling of various forms of excitation in piezoelectric energy harvesting, ranging from airflow excitation to moving loads, thus ensuring its relevance to engineers in fields as disparate as aerospace engineering and civil engineering. Coverage includes: Analytical and approximate analytical distributed-parameter electromechanical models with illustrative theoretical case studies as well as extensive experimental validations Several problems of piezoelectric energy harvesting ranging from simple harmonic excitation to random vibrations Details of introducing and modelling piezoelectric coupling for various problems Modelling and exploiting nonlinear dynamics for performance enhancement, supported with experimental verifications Applications ranging from moving load excitation of slender bridges to airflow excitation of aeroelastic sections A review of standard nonlinear energy harvesting circuits with modelling aspects.