MEMS components for NMR atomic sensors

This paper introduces a batch fabrication method to manufacture micro-electromechanical system (MEMS) components for nuclear magnetic resonance (NMR) atomic sensors, such as NMR gyroscope (NMRG) and NMR magnetometer (NMRM). The components presented are: 1) micro-coils generating the magnetic field with the magnetic field homogeneity of H=354 ppm; 2) spherical micro-fabricated atomic cells confining alkali metal and noble gases; 3) micro-heaters keeping the alkali metal in a vapor state while minimizing residual magnetic fields; and 4) origamilike silicon structures with integrated optical reflectors preserving 90.9% of the light polarization. The introduced design utilized a glassblowing process, origamilike folding, and a more traditional MEMS fabrication. We presented an analytical model of imperfections, including errors associated with micro-fabrication of MEMS components. In light of the developed error model, phenomenological dynamic model describing NMR sensors, and experimental evaluation of components, we predicted the effect of errors on performance of NMRG and NMRM. We concluded that with a realistic design, a 5-mrad angular misalignment between coils and folded mirrors, and a 100- μm linear misalignment between folded coils, it would be feasible to achieve an NMRG with ARW 0.1 deg/rt-hr and an NMRM with sensitivity on the order of 10 fT/rt-Hz. [2018-0169

Adaptive Threshold for Zero-Velocity Detector in ZUPT-Aided Pedestrian Inertial Navigation

We present a study on the adaptive threshold of the zero-velocity detector, which enables the detector to adjust to gait patterns with different speeds, from as low as walking with 80 steps per minute to as high as running with 160 steps per minute, without any tuning of design parameters during the navigation. This approach enables the zero-velocity update (ZUPT)-aided navigation algorithm to work properly with time-varying speed in a single navigation process. A Bayesian-based approach was applied to determine the adaptive threshold in the likelihood ratio test with a uniform prior information and time-varying cost function. The position error in a velocity-changing navigation scenario was demonstrated to be reduced by 12 times after applying the adaptive threshold instead of a fixed threshold

MEMS components for NMR atomic sensors

This paper introduces a batch fabrication method to manufacture micro-electromechanical system (MEMS) components for nuclear magnetic resonance (NMR) atomic sensors, such as NMR gyroscope (NMRG) and NMR magnetometer (NMRM). The components presented are: 1) micro-coils generating the magnetic field with the magnetic field homogeneity of H=354 ppm; 2) spherical micro-fabricated atomic cells confining alkali metal and noble gases; 3) micro-heaters keeping the alkali metal in a vapor state while minimizing residual magnetic fields; and 4) origamilike silicon structures with integrated optical reflectors preserving 90.9% of the light polarization. The introduced design utilized a glassblowing process, origamilike folding, and a more traditional MEMS fabrication. We presented an analytical model of imperfections, including errors associated with micro-fabrication of MEMS components. In light of the developed error model, phenomenological dynamic model describing NMR sensors, and experimental evaluation of components, we predicted the effect of errors on performance of NMRG and NMRM. We concluded that with a realistic design, a 5-mrad angular misalignment between coils and folded mirrors, and a 100- μm linear misalignment between folded coils, it would be feasible to achieve an NMRG with ARW 0.1 deg/rt-hr and an NMRM with sensitivity on the order of 10 fT/rt-Hz. [2018-0169

Design Considerations for Micro-Glassblown Atomic Vapor Cells

This paper presents a design process for miniaturized atomic vapor cells using the micro-glassblowing process. It discusses multiple design considerations, including cell geometry, optical properties, materials, and surface coating. The geometry and the optical properties were studied using experimentally verified analytical and Finite Element Models (FEM). The cell construction material and surface coating were the focus of our experimental study on factors that affect the relaxation time (T2) of nuclear spins. We showed that the wafer-level coating process with Al2O3 increased the 131Xe T2 by 3× and by switching from Borosilicate glass (Pyrex) to Aluminosilicate glass (ASG), T2 was improved by 2.5× , for the same species. The improvement in the T2 is projected to reduce the ARW of an NMR gyro and the sensitivity of an NMR magnetometer by 3× with Al2O3 coated cells and by 2.5× with ASG cells. [2019-0158]

Development of 3D Fused Quartz Hemi-Toroidal Shells for High-Q Resonators and Gyroscopes

In this paper, recent developments in the design and fabrication of micromachined fused quartz hemi-toroidal shells are presented. The fabrication is based on micro glassblowing process, demonstrated to enable the realization of high-Q MEMS resonators and gyroscopes. The design optimization of the shell geometry is performed using parametric finite element analysis. The effect of geometric parameters on the scaling of the resonant frequencies and energy dissipation are discussed. Three variations of the micro-glassblowing process are studied in the paper, concluding that shell resonators with a broad operational frequency range without losing the symmetry and Q-factor are feasible. Finite element models are presented to simulate the presented glassblowing processes, which are used to predict the final geometry of shell resonators accurately. Operational frequency as low as 5 kHz and Q-factor as high as 1.7 million is demonstrated on the fabricated shell resonators. The proposed process modifications demonstrate a low-cost and scalable fabrication of 3D shells for resonators and gyroscopes, which can be used in inertial navigation and timing applications. [2019-0179]

Compensation of frequency split by directional lapping in fused quartz micro wineglass resonators

We present, for the first time, a permanent structural asymmetry compensation method for fused quartz micro wineglass resonators. Using the technique, we demonstrated a near six times reduction of structural asymmetry (n = 2 wineglass mode), culminating in reduction of the frequency split from 41 to 7 Hz. This is an iterative process. In each iteration, the structural asymmetry was first identified by measuring the mode shape of the resonators. Then, directional lapping was performed with specially designed lapping fixtures to accurately control the lapping angle down to 1°. Analytical predictions and numerical simulations were conducted to study the structural asymmetry phenomenon and the effects of compensation on the quality factor of the structure, showing the ability of this process to reduce the structural asymmetry, while not affecting the overall quality factor of the resonators

Modeling the effect of imperfections in glassblown micro-wineglass fused quartz resonators

In this paper, we developed an analytical model, supported by experimental results, on the effect of imperfections in glassblown micro-wineglass fused quartz resonators. The analytical model predicting the frequency mismatch due to imperfections was derived based on a combination of the Rayleigh's energy method and the generalized collocation method. The analytically predicted frequency of the n = 2 wineglass mode shape was within 10% of the finite element modeling results and within 20% of the experimental results for thin shells, showing the fidelity of the predictive model. The postprocessing methods for improvement of the resonator surface quality were also studied. We concluded that the thermal reflow of fused quartz achieves the best result, followed in effectiveness by the RCA-1 surface treatment. All the analytical models developed in this paper are to guide the manufacturing methods to reduce the frequency and damping mismatch, and to increase the mechanical quality factor of the device

Origami-Like 3-D Folded MEMS Approach for Miniature Inertial Measurement Unit

This paper presents a miniature 50 mm3 inertial measurement unit (IMU) implemented using a folded microelectromechanical systems (MEMS) process. The approach is based on wafer-level fabrication of high aspect-ratio single-axis sensors interconnected by flexible hinges and folded into a 3-D configuration, like a silicon Origami [1]. Two different materials for flexible hinges have been explored, including photo-definable polyimide and parylene-C. We report, for the first time, an IMU prototype with seven operational sensors: three accelerometers, three gyroscopes, and a prototype of a reference clock. This paper concludes with the results of experimental characterization of inertial sensors demonstrating the feasibility of the proposed approach for a compact IMU