This paper reviews currently recognized needs for advances in precision navigation and timing technology, summarizes
ongoing efforts, and discusses future technological developments being pursued under the aggregated
DARPA/MTO Microtechnology for Positioning, Navigation, and Timing (micro-PNT) program.
The Fabry-Perot Interferometer based accelerometer is proposed for use in fiber optic sensor networks. Despite
the potential for high performance in the detection of vibrations, previously such sensors had limited use in these
networks. Since the sensor operates as an optical transmission loss filter, simple serial network implementations
are difficult. However, by forming the optical resonance cavity of the sensor with wavelength dependent reflective
surfaces, a simply serialized network of sensor can be demonstrated through the wavelength division multiplexing
of the interferometric signal fringes. This paper summarizes the concept and experimentally demonstrates and
evaluated the serialization of two acceleration sensors.
We introduce a technology for robust and low maintenance sensor networks capable of detecting micro-g accelerations in a wide frequency bandwidth (above 1,000 Hz). Sensor networks with such performance are critical for navigation, seismology, acoustic sensing, and for the health monitoring of civil structures. The approach is based on the fabrication of an array of highly sensitive accelerometers, each using a Fabry-Perot cavity with transparent passbands at specific wavelengths that allows for embedded optical detection and serialization. A unique feature of this approach is that no local power source is required for each individual sensor. Instead one global light source is used, providing an optical input signal which propagates through an optical fiber network from sensor to sensor. The information from each sensor is embedded into the transmitted light as a wavelength division multiplexed signal. We present for the first time the preliminary demonstration of a system of two linear serialized wavelength division multiplexed Fabry-Perot sensors of less then 1.5dB loss per device. The sensors are formed using an optical thin film multilayer structure that takes advantage of the natural non-uniformity in deposited thin films to allow serialization.
This paper studies a design approach that yields robust vibratory MEMS gyroscopes. The design is based on multiple drive-mode resonators with incrementally spaced frequencies, distributed symmetrically around the center of a supporting frame. These resonators are structurally constrained in the tangential direction with respect to the supporting frame. In the presence of an angular rotation rate about the z-axis, a harmonic Coriolis force is induced on each proof mass. These force vectors lie in the tangential direction, generating a resultant torque on the supporting frame. The net Coriolis torque excites the supporting frame into torsional oscillation about the z-axis, which is capacitively detected to generate angular rate measurement. Two batches of prototypes have been fabricated using in-house single crystal silicon on insulator (SCS-SOI) bulk-micromachining and EFABTM process commercially available from Microfabrica. Wideband drive operation was demonstrated in SOI devices. EFAB process yielded 850 Hz devices with quality factor 250 in air (bandwidth 3 Hz) and 850 in vacuum. Increase of temperature from 25o to 125oC shifts the resonant frequency down by roughly bandwidth, while quality factor drops by 8%. Parasitics model associated with EFAB consists of only a lumped capacitor and is simpler than two-parametric parasitics circuit in SOI devices. Nonlinear parametric excitation of motion at resonant frequency by super-harmonic AC voltage was experimentally characterized. This actuation method provides high amplitude of motion and separates motion from parasitics in frequency domain. The actuation method can potentially further improve the bandwidth and gain characteristics of distributed mass gyroscope.
This paper reports our progress toward development of a unilateral vestibular prosthesis. The sensing element of the prosthesis is a custom designed one-axis MEMS gyroscope. Similarly to the natural semicircular canal, the microscopic gyroscope senses angular motion of the head and generates voltages proportional to the corresponding angular accelerations. Then, voltages are sent to the pulse generating unit where angular motion is translated into voltage pulses. The voltage pulses are converted into current pulses and are delivered through specially designed electrodes, conditioned to stimulate the corresponding vestibular nerve branch. Our preliminary experimental evaluations of the prosthesis on a rate table indicate that the device's output matches the average firing rate of vestibular neurons to those in animal models reported in the literature. The proposed design is scalable; the sensing unit, pulse generator, and the current source can be potentially implemented on a single chip using integrated MEMS technology.
This paper describes a technique for detection of anisoelasticities in rate integrating gyroscopes as part of a self-calibrative control architecture. In contrast to laser trimming typically done in post processing to compensate for structural imperfections, the on-chip control architecture uses feedforward voltage control to tune the non-linear negative spring effects inherent in parallel plate electrodes in order to electrostatically 'trim' the structural non-idealities. As the first steps toward the feedforward control realization, we present three different algorithms that can be implemented on-chip for identification of structural anisoelasticities. The first technique utilizes the results of measured static displacements, requiring precise knowledge of displacements and applied forces. The second technique involves solving for the non-ideal stiffness parameters using Principal Component Analysis and Fourier transforms of the dynamic system response. The last technique embellishes on the second by the addition of an energy compensation control to overcome damping effects in low Q systems. Finally, the implementation of this algorithm in the electrostatic trimming' of structural imperfections is discussed.
This paper reports a design concept for MEMS gyroscopes that shifts the complexity of the design from control architecture to system dynamics, utilizing the passive disturbance rejection capability of the 4-DOF dynamical system. Specifically, a novel wide-bandwidth micromachined gyroscope design approach based on increasing the degrees-of-freedom of the oscillatory system by the use of two independently oscillating interconnected proof masses is presented along with preliminary experimental demonstration of implementation feasibility. With the concept of using a 4-DOF system, inherent disturbance rejection is achieved due to the wide operation frequency range of the dynamic system, providing reduced sensitivity to structural and thermal parameter fluctuations. Thus, less demanding active control strategies are required for operation under presence of perturbations. The fabricated prototype dual-mass gyroscopes successfully demonstrated a dramatically wide driving frequency range within where the drive direction oscillation amplitude varies insignificantly without any active control, in contrast to the conventional gyroscopes where the mass has to be sustained in constant amplitude oscillation in a very narrow frequency band. Mechanical amplification of driven mass oscillation by the sensing element was also experimentally demonstrated, providing large oscillation amplitudes, which is crucial for sensor performance.
Micromachined gyroscopes are probably the most challenging type of transducers ever attempted to be designed in micro-world. A nail-size dynamic system integrated with control electronics on the same silicon chip is designed to be a very sensitive sensor which is potentially able to detect maneuvers and motions beyond human perception. Along with exciting opportunities which MEMS gyroscopes could bring to everyday life, the miniaturization introduces many new technical challenges. Multi-degree of freedom dynamics, sensitivity to fabrication imperfections, dynamic instability, limited control resources - all these raise a number of fundamentally challenging issues in the design, analysis, and control of micromachined gyroscopes. In this paper, we summarize principles of operation, review recent research and development efforts, and discuss potential applications and the future market of silicon based micromachined gyroscopes.
An alternative to a classical wavelength interferometer (an array of hand-assembled etalons consisting of two semi-transparent mirrors separated by a fixed-cavity) is the implementation of wide band tunable filter using Micro-Electro-Mechanical Systems (MEMS) technology. This approach will allow a single tunable device to replace an array of fixed-cavity filters reducing cost and parts. MEMS technology offers many advantages, including scalability for wide tuning range, sensitivity for precision sensing, and batch fabrication capability for cost reduction While at the same time, MEMS technology introduces many new challenges, which include fabrication yield, device reproducibility, and fabrication imperfections - all are factors seriously limiting performance of MEMS interferometers. Without defects, reflectance must be greater than 99.69% in order to achieve finesse of 1000 to be useful for DWDM applications. Though, the presence of defects limits performance and becomes more pronounced at higher reflectance values. For example, component misregistration while fabricating MEMS interferometer with 95% and 98% reflectance values, result in the reduction of effective finesse of 10% and 42%, respectively. This paper discusses several models for analyzing imperfections in MEMS tunable-cavity interferometers. Based on thermal expansion and component misregistration analysis, we conclude that a passive MEMS-based filter cannot achieve performance required for DWDM applications.
KEYWORDS: Gyroscopes, Microelectromechanical systems, Finite element methods, Temperature metrology, Thermal analysis, Thermal effects, 3D modeling, Systems modeling, Thermal modeling, Chemical elements
This paper describes the structural and thermal modeling of a Micro Electro Mechanical System (MEMS) z-axis angular gyroscope. The gyroscope consists of a oscillating proof mass supported by a suspension made up of six concentric interconnected rings rigidly attached to an anchored frame. The device is capable of measuring angular displacement through precession of the proof mass line of oscillation in the presence of rotation induced Coriolis force. Using a strain energy method, a closed form solution for the effective stiffness of the suspension system is developed, which is confirmed using finite element modeling. A comparative study of the suspension with a commonly used serpentine spring suspension demonstrates that the studied device is robust to thermal fluctuations and residual stresses. A parametric analysis is used to identify an appropriate micromachining technology suitable for the fabrication of the angular gyroscope.
Many industry experts predict that the future of fiber optic telecommunications depends on the development of all-optical components for switching of photonic signals from fiber to fiber throughout the networks. MEMS is a promising technology for providing all-optical switching at high speeds with significant cost reductions. This paper reports on the the analysis of two designs for 2-DOF electrostatically actuated MEMS micromirrors for precision controllable large optical switching arrays. The behavior of the micromirror designs is predicted by coupled-field electrostatic and modal analysis using a finite element analysis (FEA) multi-physics modeling software. The analysis indicates that the commonly used gimbal type mirror design experiences electrostatic interference and would therefore be difficult to precisely control for 2-DOF motion. We propose a new design approach which preserves 2-DOF actuation while minimizing electrostatic interference between the drive electrodes and the mirror. Instead of using two torsional axes, we use one actuator which combines torsional and flexural DOFs. A comparative analysis of the conventional gimbal design and the one proposed in this paper is performed.
This paper reports a novel micromachined gyroscope design with inherent disturbance-rejection capabilities. The proposed approach is based on increasing the degrees-of-freedom (DOF) of the oscillatory system by the use of two independently oscillating proof masses. Utilizing dynamical amplification in the 4-DOF system, inherent disturbance rejection is achieved, providing reduced sensitivity to structural and thermal parameter fluctuations and damping changes over the operating time of the device. In the proposed system, the first mass is forced to oscillate in the drive direction, and the response of the second mass in the orthogonal direction is sensed. The response has two resonant peaks and a flat region between peaks. Operation is in the flat region, where the gain is insensitive to frequency fluctuations. Simulations indicate over 15 times increase in the bandwidth of the system due to the use of the proposed architecture. In addition, the gain in the operation region has low sensitivity to damping changes. Consequently, by utilizing the disturbance-rejection capability of the dynamical system, improved robustness is achieved, which can relax tight fabrication tolerances and packaging requirements and thus result in reducing production cost of micromachined gyroscopes.
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