CMOS systems and circuits for sub-degree per hour MEMS gyroscopes

The objective of our research is to develop system architectures and CMOS circuits that interface with high-Q silicon microgyroscopes to implement navigation-grade angular rate sensors. The MEMS sensor used in this work is an in-plane bulk-micromachined mode-matched tuning fork gyroscope (M² – TFG ), fabricated on silicon-on-insulator substrate. The use of CMOS transimpedance amplifiers (TIA) as front-ends in high-Q MEMS resonant sensors is explored. A T-network TIA is proposed as the front-end for resonant capacitive detection. The T-TIA provides on-chip transimpedance gains of 25MΩ, has a measured capacitive resolution of 0.02aF /√Hz at 15kHz, a dynamic range of 104dB in a bandwidth of 10Hz and consumes 400μW of power. A second contribution is the development of an automated scheme to adaptively bias the mechanical structure, such that the sensor is operated in the mode-matched condition. Mode-matching leverages the inherently high quality factors of the microgyroscope, resulting in significant improvement in the Brownian noise floor, electronic noise, sensitivity and bias drift of the microsensor. We developed a novel architecture that utilizes the often ignored residual quadrature error in a gyroscope to achieve and maintain perfect mode-matching (i.e.0Hz split between the drive and sense mode frequencies), as well as electronically control the sensor bandwidth. A CMOS implementation is developed that allows mode-matching of the drive and sense frequencies of a gyroscope at a fraction of the time taken by current state of-the-art techniques. Further, this mode-matching technique allows for maintaining a controlled separation between the drive and sense resonant frequencies, providing a means of increasing sensor bandwidth and dynamic range. The mode-matching CMOS IC, implemented in a 0.5μm 2P3M process, and control algorithm have been interfaced with a 60μm thick M2−TFG to implement an angular rate sensor with bias drift as low as 0.1°/hr ℃ the lowest recorded to date for a silicon MEMS gyro.Ph.D.Committee Chair: Farrokh Ayazi; Committee Member: Jennifer Michaels; Committee Member: Levent Degertekin; Committee Member: Paul Hasler; Committee Member: W. Marshall Leac

Design of a MEMS-based 52 MHz oscillator

Mechanical resonators are widely applied in time-keeping and frequency reference applications. Mechanical resonators are preferred over electrical resonators because of their high Q. In the $4.1 billion (2008) timing market, quartz crystals are still ubiquitous in electronic equipment. Quartz crystals show excellent performance in terms of stability (shortterm and long-term), power handling, and temperature drift. MEMS resonators are investigated as a potential alternative to the bulky quartz crystals, which cannot be integrated with IC technology. MEMS offer advantages in terms of size, cost price, and system integration. Efforts over recent years have shown that MEMS resonators are able to meet the high performance standards set by quartz. Critical success factors are high Q-factor, low temperature drift, low phase noise, and low power. This PhD thesis addresses the feasibility of scaling MEMS resonators/oscillators to frequencies above 10 MHz. The main deliverable is a 52 MHz MEMS-based oscillator. The MEMS resonators at NXP are processed on 8-inch silicon-on-insulator (SOI) wafers, with a SOI layer thickness of 1.5 µm and a buried oxide layer thickness of 1 µm. The strategic choice for thin SOI substrates has been made for two reasons. First, MEMS processing in thin silicon layers can be done with standard CMOS processing tools. The silicon dioxide layer serves as a sacrificial layer. Second, identical substrates are used for the Advanced Bipolar CMOS DMOS (ABCD) IC-processes. This class of processes can handle high voltages (ABCD2 up to 120V). The high voltage capability is suitable for the transduction of the mechanical resonator. Both MEMS and IC are processed on a similar substrate, since the strategic aim is to integrate the MEMS structure with the IC-process in the long run. Frequency scaling is investigated for both the capacitive and the piezoresistive MEMS resonator. MEMS resonators have been successfully tested from 13 MHz to over 400 MHz. This is achieved by decreasing the size of the resonator with a factor 32. We show that the thin SOI layer and the decreasing size of the resonator increase the effective impedance of the capacitive resonator at higher frequencies. For the piezoresistive resonator, we show that this readout principle is insensitive to geometrical scaling and layer thickness. Therefore, the piezoresistive readout is preferred at high frequencies. The effective impedance can be kept low, at the expense of higher power consumption. Frequency accuracy can be improved by decreasing the initial frequency spread and the temperature drift of the MEMS resonator. The main source of initial frequency spread is geometrical offset, due to the non-perfect pattern transfer from mask layout to SOI. A FEM tool has been developed in Comsol Multiphysics to obtain compensated layouts. The resonance frequency of these designs is first-order compensated for geometric offset. The FEM tool is used to obtain compensated resonators of various designs. We show empirically that the compensation by design is effective on a 52 MHz square plate design. For the compensated design, frequency spread measurements over a complete wafer show that there are other systematic sources of frequency spread. The resonance frequency of the silicon MEMS resonator drifts about –30 ppm/K. This is due to the Young’s modulus of silicon that depends on temperature. We have investigated two compensation methods. The first is passive compensation by coating the silicon resonator with a silicon dioxide skin. The Young’s modulus of silicon dioxide has a positive temperature drift. Measurements on globally oxidized structures show that the right oxide thickness reduces the linear temperature drift of the resonator to zero. A second method uses an oven-control principle. The temperature of the resonator is fixed, independent of the ambient temperature. A demo of this principle has been designed with a piezoresistive resonator in which the dc readout current through the resonator is used to control the temperature of the resonator. With both concepts, more than a factor 10 reduction in temperature drift is achieved. To demonstrate the feasibility of high-frequency oscillators, a MEMS-based 56 MHz oscillator has been designed for which a piezoresistive dogbone resonator is used. The amplifier has been designed in the ABCD2 IC-process. The MEMS oscillator consumes 6.1 mW and exhibits a phase noise of –102 dBc/Hz at 1 kHz offset from the carrier and a floor of –113 dBc/Hz. This demonstrates feasibility of the piezoresistive MEMS oscillator for lowpower, low-noise applications. Summarizing, this PhD thesis work as part of the MEMSXO project at NXP demonstrates a MEMS oscillator concept based on the piezoresistive resonator in thin SOI. It shows that by compensated designs for geometric offset and oven-control to reduce temperature drift, a frequency accuracy can be achieved that can compete with the performance of crystal oscillators. In a benchmark with MEMS competitors the concept shows the lowest phase noise, making it the most suited concept for wireless applications

Low-noise preamplifier for capacitive sensors

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (p. 129).by Shida Iep.M.S

Contributions to Positioning Methods on Low-Cost Devices

Global Navigation Satellite System (GNSS) receivers are common in modern consumer devices that make use of position information, e.g., smartphones and personal navigation assistants. With a GNSS receiver, a position solution with an accuracy in the order of five meters is usually available if the reception conditions are benign, but the performance degrades rapidly in less favorable environments and, on the other hand, a better accuracy would be beneficial in some applications. This thesis studies advanced methods for processing the measurements of low-cost devices that can be used for improving the positioning performance. The focus is on GNSS receivers and microelectromechanical (MEMS) inertial sensors which have become common in mobile devices such as smartphones. First, methods to compensate for the additive bias of a MEMS gyroscope are investigated. Both physical slewing of the sensor and mathematical modeling of the bias instability process are considered. The use of MEMS inertial sensors for pedestrian navigation indoors is studied in the context of map matching using a particle filter. A high-sensitivity GNSS receiver is used to produce coarse initialization information for the filter to decrease the computational burden without the need to exploit local building infrastructure. Finally, a cycle slip detection scheme for stand-alone single-frequency GNSS receivers is proposed. Experimental results show that even a MEMS gyroscope can reach an accuracy suitable for North seeking if the measurement errors are carefully modeled and eliminated. Furthermore, it is seen that even a relatively coarse initialization can be adequate for long-term indoor navigation without an excessive computational burden if a detailed map is available. The cycle slip detection results suggest that even small cycle slips can be detected with mass-market GNSS receivers, but the detection rate needs to be improved

Latest research trends in gait analysis using wearable sensors and machine learning: a systematic review

Gait is the locomotion attained through the movement of limbs and gait analysis examines the patterns (normal/abnormal) depending on the gait cycle. It contributes to the development of various applications in the medical, security, sports, and fitness domains to improve the overall outcome. Among many available technologies, two emerging technologies that play a central role in modern day gait analysis are: A) wearable sensors which provide a convenient, efficient, and inexpensive way to collect data and B) Machine Learning Methods (MLMs) which enable high accuracy gait feature extraction for analysis. Given their prominent roles, this paper presents a review of the latest trends in gait analysis using wearable sensors and Machine Learning (ML). It explores the recent papers along with the publication details and key parameters such as sampling rates, MLMs, wearable sensors, number of sensors, and their locations. Furthermore, the paper provides recommendations for selecting a MLM, wearable sensor and its location for a specific application. Finally, it suggests some future directions for gait analysis and its applications

Silicon microaccelerometer fabrication technologies

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1997.Includes bibliographical references (leaves 275-282).by Charles Heng-Yuan Hsu.Ph.D

System design of a low-power three-axis underdamped MEMS accelerometer with simultaneous electrostatic damping control

Recently, consumer electronics industry has known a spectacular growth that would have not been possible without pushing the integration barrier further and further. Micro Electro Mechanical Systems (MEMS) inertial sensors (e.g. accelerometers, gyroscopes) provide high performance, low power, low die cost solutions and are, nowadays, embedded in most consumer applications. In addition, the sensors fusion has become a new trend and combo sensors are gaining growing popularity since the co-integration of a three-axis MEMS accelerometer and a three-axis MEMS gyroscope provides complete navigation information. The resulting device is an Inertial measurement unit (IMU) able to sense multiple Degrees of Freedom (DoF). Nevertheless, the performances of the accelerometers and the gyroscopes are conditioned by the MEMS cavity pressure: the accelerometer is usually a damped system functioning under an atmospheric pressure while the gyroscope is a highly resonant system. Thus, to conceive a combo sensor, aunique low cavity pressure is required. The integration of both transducers within the same low pressure cavity necessitates a method to control and reduce the ringing phenomena by increasing the damping factor of the MEMS accelerometer. Consequently, the aim of the thesis is the design of an analog front-end interface able to sense and control an underdamped three-axis MEMSaccelerometer. This work proposes a novel closed-loop accelerometer interface achieving low power consumption The design challenge consists in finding a trade-off between the sampling frequency, the settling time and the circuit complexity since the sensor excitation plates are multiplexed between the measurement and the damping phases. In this context, a patenteddamping sequence (simultaneous damping) has been conceived to improve the damping efficiency over the state of the art approach performances (successive damping). To investigate the feasibility of the novel electrostatic damping control architecture, several mathematical models have been developed and the settling time method is used to assess the damping efficiency. Moreover, a new method that uses the multirate signal processing theory and allows the system stability study has been developed. This very method is used to conclude on the loop stability for a certain sampling frequency and loop gain value. Next, a 0.18μm CMOS implementation of the entire accelerometer signal chain is designed and validated

Second International Symposium on Magnetic Suspension Technology, part 2

In order to examine the state of technology of all areas of magnetic suspension and to review related recent developments in sensors and controls approaches, superconducting magnet technology, and design/implementation practices, the 2nd International Symposium on Magnetic Suspension Technology was held at the Westin Hotel in Seattle, WA, on 11-13 Aug. 1993. The symposium included 18 technical sessions in which 44 papers were presented. The technical sessions covered the areas of bearings, bearing modelling, controls, vibration isolation, micromachines, superconductivity, wind tunnel magnetic suspension systems, magnetically levitated trains (MAGLEV), rotating machinery and energy storage, and applications. A list of attendees appears at the end of the document

Integrated reference circuits for low-power capacitive sensor interfaces

This thesis consists of nine publications and an overview of the research topic, which also summarizes the work. The research described in this thesis concentrates on the design of low-power sensor interfaces for capacitive 3-axis micro-accelerometers. The primary goal throughout the thesis is to optimize power dissipation. Because the author made the main contribution to the design of the reference and power management circuits required, the overview part is dominated by the following research topics: current, voltage, and temperature references, frequency references, and voltage regulators. After an introduction to capacitive micro-accelerometers, the work describes the typical integrated readout electronics of a capacitive sensor on the functional level. The readout electronics can be divided into four different functional parts, namely the sensor readout itself, signal post-processing, references, and power management. Before the focus is shifted to the references and further to power management, different ways to realize the sensor readout are briefly discussed. Both current and voltage references are required in most analog and mixed-signal systems. A bandgap voltage reference, which inherently uses at least one current reference, is practical for the generation of an accurate reference voltage. Very similar circuit techniques can be exploited when implementing a temperature reference, the need for which in the sensor readout may be justified by the temperature compensation, for example. The work introduces non-linear frequency references, namely ring and relaxation oscillators, which are very suitable for the generation of the relatively low-frequency clock signals typically needed in the sensor interfaces. Such oscillators suffer from poor jitter and phase noise performance, the quantities of which also deserve discussion in this thesis. Finally, the regulation of the supply voltage using linear regulators is considered. In addition to extending the battery life by providing a low quiescent current, the regulator must be able to supply very low load currents and operate without off-chip capacitors