Development of a sensitive superconducting gravity gradiometer for geological and navigational applications

A sensitive and stable gravity gradiometer would provide high resolution gravity measurements from space. The instrument could also provide precision tests of fundamental laws of physics and be applied to inertial guidance systems of the future. This report describes research on the superconducting gravity gradiometer program at the University of Maryland from July 1980 to July 1985. The report describes the theoretical and experimental work on a prototype superconducting gravity gradiometer. The design of an advanced three-axis superconducting gravity gradiometer is also discussed

MEMS Accelerometers

Micro-electro-mechanical system (MEMS) devices are widely used for inertia, pressure, and ultrasound sensing applications. Research on integrated MEMS technology has undergone extensive development driven by the requirements of a compact footprint, low cost, and increased functionality. Accelerometers are among the most widely used sensors implemented in MEMS technology. MEMS accelerometers are showing a growing presence in almost all industries ranging from automotive to medical. A traditional MEMS accelerometer employs a proof mass suspended to springs, which displaces in response to an external acceleration. A single proof mass can be used for one- or multi-axis sensing. A variety of transduction mechanisms have been used to detect the displacement. They include capacitive, piezoelectric, thermal, tunneling, and optical mechanisms. Capacitive accelerometers are widely used due to their DC measurement interface, thermal stability, reliability, and low cost. However, they are sensitive to electromagnetic field interferences and have poor performance for high-end applications (e.g., precise attitude control for the satellite). Over the past three decades, steady progress has been made in the area of optical accelerometers for high-performance and high-sensitivity applications but several challenges are still to be tackled by researchers and engineers to fully realize opto-mechanical accelerometers, such as chip-scale integration, scaling, low bandwidth, etc

Silicon bulk micromachined, symmetric, degenerate vibratorygyroscope, accelerometer and sensor and method for using the same

When embodied in a microgyroscope, the invention is comprised of a silicon, four-leaf clover structure with a post attached to the center. The whole structure is suspended by four silicon cantilevers or springs. The device is electrostatically actuated and capacitively detects Coriolis induced motions of the leaves of the leaf clover structure. In the case where the post is not symmetric with the plane of the clover leaves, the device can is usable as an accelerometer. If the post is provided in the shape of a dumb bell or an asymmetric post, the center of gravity is moved out of the plane of clover leaf structure and a hybrid device is provided. When the clover leaf structure is used without a center mass, it performs as a high Q resonator usable as a sensor of any physical phenomena which can be coupled to the resonant performance

The Air Force Institute of Technology (AFIT) Micro Electro-Mechanical Systems (MEMS) Interferometric Gyroscope (MiG)

With the invention of Micro Electro-Mechanical Systems (MEMS) it has become possible to fabricate micro-inertial sensors. These new sensors have application in creating autonomous guided weapons systems. New technologies like Micro Unmanned Aerial Vehicles (UAVs), which cannot use conventional inertial sensors, rely on technologies like micro-inertial sensors to operate. Also, such sensors have the capability to reduce both power and space consumption on conventional aircraft. This technology is not yet mature, and current micro-inertial sensors do not have the accuracy required for highly precise navigation. To try to increase the accuracy of micro-inertial sensors, researchers are turning toward micro-optical gyroscopes. Creating a working micro-optical gyroscope is a difficult proposition as their small size precludes micro-optical gyroscopes from having large enough path lengths to sense useful rotation rates. Techniques need to be developed to create micro-optical gyroscopes with path lengths long enough to sense navigation grade rotation rates. This research proposes a new type of MEMS optical gyroscope. The device, called the AFIT MiG is an open loop Sagnac interferometer on a MEMS die. Mirrors are placed on the die to spiral light inward from the outside to the center of the die thereby increasing the optical path length of the device. When the AFIT MiG was simulated using flight profiles generated in MATLAB™, the optical path length of the device was long enough to measure rotation rates, which were greater in strength than the noise inherent in the measurement. This research also shows the ability to propagate light around an open loop MEMS interferometer with enough signal strength at the detector to measure

Design and initial characterization of a compact, ultra high vacuum compatible, low frequency, tilt accelerometer

A compact tilt accelerometer with high sensitivity at low frequency was designed to provide low frequency corrections for the feedback signal of the Advanced Laser Interferometer Gravitational Wave Observatory active seismic attenuation system. It has been developed using a Tungsten Carbide ceramic knife-edge hinge designed to avoid the mechanical 1/f noise believed to be intrinsic in polycrystalline metallic flexures. Design and construction details are presented; prototype data acquisition and control limitations are discussed. The instrument's characterization reported here shows that the hinge is compatible with being metal-hysteresis-free, and therefore also free of the 1/f noise generated by the dislocation Self-Organized Criticality in the metal. A tiltmeter of this kind will be effective to separate the ground tilt component from the signal of horizontal low frequency seismometers, and to correct the ill effects of microseismic tilt in advanced seismic attenuation systems

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

Identifying loading and response mechanisms from ten years of performance monitoring of a tall building

Author version of article. The final published version is available from the publisher website via: doi:10.1061/(ASCE)0887-3828(2008)22:1(24)© 2008 ASCEIn 1993 Shimizu Corporation provided the opportunity to record manually readings of stress and strain gauges they had embedded at the 18th storey of a 65-storey office tower under construction in Singapore. Static readings continued during construction and long after, and capitalising on access to the building and assistance of both contractor and owner, monitoring systems for tracking wind, acceleration and deflection were installed and progressively upgraded. Further, a comprehensive ambient vibration survey and finite element model updating exercise provided a thoroughly validated analytical model of the structure. This model has been used in parallel with the analog wind and tremor ‘super-sensor’ of the building itself to provide direct evidence and characterization of the seismic and wind loadings on the building. This paper describes the evolution of the monitoring system and its capabilities together with some of the insights the system provided into structural and loading mechanisms during its operational life until early 2005.

Spaceborne Gravity Gradiometers

The current status of gravity gradiometers and technology that could be available in the 1990's for the GRAVSAT-B mission are assessed. Problems associated with sensors, testing, spacecraft, and data processing are explored as well as critical steps, schedule, and cost factors in the development plan

A sensitive horizontal atom interferometer for testing acceleration from an in-vacuum source mass

Light-pulse atom interferometry has been powerful in testing fundamental physics. Because of its precision and sensitivity to inertial forces, the study of gravitational phenomena using an atom interferometer with various configurations has become a new trend in this community. This thesis presents my effort in characterising and improving the acceleration sensitivity of an 87Rb atom interferometer that is sensitive to the horizontal acceleration of atomic motion, which can be used to constrain chameleon dark energy force from an in-vacuum source mass. We achieved ≈ 10^7 atoms for interferometry in 800 ms including loading 87Rb atoms into a magneto-optical trap, vertically launching them upwards, preparing their internal state in |F = 1,mF = 0⟩ state and velocity selection. After minimising AC Stark shifts with a novel scheme using microwave spectroscopy, this centimetre-scale fountain configuration allowed us to interrogate our atom interferometer with three Raman pulses separated by T = 33 ms, which achieved an acceleration sensitivity of 30-40 μm/s^2 per shot. This acceleration sensitivity is more than a factor of two better than 86-156 μm/s^2 per shot reported for the last generation. We quantified the noise contribution to our acceleration sensitivity, which was found to be dominated by phase noise. With this improved interferometer, we measured an acceleration of −2.597 ± 0.186 μm/s^2 in 92 hours. This negative result is not compatible with a chameleon dark energy induced fifth-force, which leads to a discussion of systematics.Open Acces