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

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

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

Silicon micromachined resonant accelerometer with CMOS interface circuits

Ph.DDOCTOR OF PHILOSOPH

Development and implementation of a deflection amplification mechanism for capacitive accelerometers

Micro-Electro-Mechanical-Systems (MEMS) and especially physical sensors are part of a flourishing market ranging from consumer electronics to space applications. They have seen a great evolution throughout the last decades, and there is still considerable research effort for further improving their performance. This is reflected by the plethora of commercial applications using them but also by the demand from industry for better specifications. This demand together with the needs of novel applications fuels the research for better physical sensors.Applications such as inertial, seismic, and precision tilt sensing demand very high sensitivity and low noise. Bulk micromachined capacitive inertial sensors seem to be the most viable solution as they offer a large inertial mass, high sensitivity, good noise performance, they are easy to interface with, and of low cost. The aim of this thesis is to improve the performance of bulk micromachined capacitive sensors by enhancing their sensitivity and noise floor.MEMS physical sensors, most commonly, rely on force coupling and a resulting deflection of a proof mass or membrane to produce an output proportional to a stimulus of the physical quantity to be measured. Therefore, the sensitivity to a physical quantity may be improved by increasing the resulting deflection of a sensor. The work presented in this thesis introduces an approach based on a mechanical motion amplifier with the potential to improve the performance of mechanical MEMS sensors that rely on deflection to produce an output signal.The mechanical amplifier is integrated with the suspension system of a sensor. It comprises a system of micromachined levers (microlevers) to enhance the deflection of a proof mass caused by an inertial force. The mechanism can be used in capacitive accelerometers and gyroscopes to improve their performance by increasing their output signal. As the noise contribution of the electronic read-out circuit of a MEMS sensor is, to first order, independent of the amplitude of its input signal, the overall signal-to-noise ratio (SNR) of the sensor is improved.There is a rather limited number of reports in the literature for mechanical amplification in MEMS devices, especially when applied to amplify the deflection of inertial sensors. In this study, after a literature review, mathematical and computational methods to analyse the behaviour of microlevers were considered. By using these methods the mechanical and geometrical characteristics of microlevers components were evaluated. In order to prove the concept, a system of microlevers was implemented as a mechanical amplifier in capacitive accelerometers.All the mechanical structures were simulated using Finite Element Analysis (FEA) and system level simulations. This led to first order optimised devices that were used to design appropriate masks for fabrication. Two main fabrication processes were used; a Silicon on Insulator (SOI) process and a Silicon on Glass (SoG) process. The SOI process carried out at the University of Southampton evolved from a one mask to a two mask dicing free process with a yield of over 95%, in its third generation. The SoG is a well-established process at the University of Peking that uses three masks.The sensors were evaluated using both optical and electrical means. The results from the first prototype sensor design (1HAN) revealed an amplification factor of 40 and a mechanically amplified sensitivity of 2.39V/g. The measured natural frequency of the first mode of the sensor was at 734Hz and the full-scale measurement range was up to 7g with a maximum nonlinearity of 2%. The measurements for all the prototype sensor designs were very close to the predicted values with the highest discrepancy being 22%. The results of this research show that mechanical amplification is a very promising concept that can offer increased sensitivity in inertial sensors without increasing the noise. Experimental results show that there is plenty of room for improvement and that viable solutions may be produced by using the presented approach. The applications of this scheme are not restricted only to inertial sensors but as the results show it can be used in a broader range of micromachined devices

Development of adaptive transducer based on biological sensory mechanism

textAn adaptive sensor concept and prototype has been developed based on a sensing element which is analogous to and inspired by the arrangement of outer hair cells and inner hair cells between the basilar membrane and tectorial membrane which form the organ of corti in mammalian cochlea. The bio-inspired design was supported by development of a bond graph model of the electromotility (active response) of outer hair cells. Outer hair cells perform like actuators and simulation results using this model are compared with physiological data found in the literature to verify its characteristic response. Insight gained from the model is used to develop a sensor structure analogous to the organ of corti and designed to measure acceleration. A piezoelectric bimorph was selected as the transducer basis, and a bond graph model of the bimorph in an accelerometer configuration was formulated to aid control design and simulation. There is no published data regarding the type of information transmitted among the inner hair cells, outer hair cells, and brain. Consequently, a controller intended to adjust the adaptation process similar to what might exist in the cochlear system has been developed for the sensor and based on a model referenced adaptive control algorithm. Simulations verify that the algorithm can successfully control and enhance performance of the sensor. Practicability of the design is evaluated by a series of experiments on a prototype. This study focused on using a controller structure that was programmed, implemented, and tested using programmable logic based on FPGA technology. The experiments evaluated how well the adaptive sensor could meet a specified performance requirement. Implementation issues that arise, such as the need for differentiators in the adaptive controller or internal propagation of vibration within the sensor structure, hinder the tuning ability. Nevertheless, the trends indicate that the algorithm can meet the desired performance if certain limitations can be overcome. Finally, recommendations have been made for expansion of the research in such fields as an alternative structure for tuning, sensor networking, and reference sensor configuration.Mechanical Engineerin

Inertial measurement unit modelling

Inerciální měřící jednotka (IMU) patří mezi základní senzorické vybavení současných mobilních robotů, kde se používá především pro odhadování ohamžité orientace robotu v prostoru. V této práci se komplexně zabývám simulací IMU v robotickém simulátoru Gazebo za účelem co nejvěrnějšího modelování odhadovaného úhlu natočení robotu, který je jedním z přímých výstupů IMU senzoru Bosch BNO055. Pro podporu vyhodnocení kvality IMU modelu vzhledem k reálným datům z BNO055 jsem navrhl a implementoval simulační prostřední v rámci Robotického Operačního Systému (ROS), které aproximuje zadanou trajektorii, uloží data ze simulovaného IMU a vygeneruje podklady pro vyhodnocení kvality IMU modelu vzhledem k reálným datům z BNO055. Na základě nejlepších dostupných IMU pluginů v Gazebu jsem implementoval dva URDF/SDF modely IMU sensoru, jejichž funkčnost byla následně ověřena řadou experimentů v simulátoru. Provedené simulace potvrdily funkčnost modelů a zároveň poukázaly na limity realističnosti současných pluginů v Gazebu a nastínily možnosti dalšího vývoje pro zvýšení věrnosti simulací IMU.The inertial measurement unit (IMU) sensors are massively used in mobile service robots to provide orientation estimation. This thesis is concerned with modeling and simulation of IMU sensor in the robotics simulator Gazebo. The main goal of this thesis is to simulate the heading angle output of a real IMU sensor Bosch BNO055 with high fidelity. To enable the IMU model evaluation I designed and implmented a custom IMU simulation framework as a ROS package. This framework approximates the given trajectory with the help of a Gazebo simulation of a robot model with attached IMU sensor model, captures the simulated IMU output and generates data for the comparison concerning provided dataset measured by real BNO055. I used the best currently available IMU plugins to implement two different URDF/SDF IMU models. The simulations demonstrated the functionality of implemented IMU models, but also revealed the fidelity limitations of current IMU plugins in Gazebo, and led to a discussion about possible future improvements

FLEXIBLE LOW-COST HW/SW ARCHITECTURES FOR TEST, CALIBRATION AND CONDITIONING OF MEMS SENSOR SYSTEMS

During the last years smart sensors based on Micro-Electro-Mechanical systems (MEMS) are widely spreading over various fields as automotive, biomedical, optical and consumer, and nowadays they represent the outstanding state of the art. The reasons of their diffusion is related to the capability to measure physical and chemical information using miniaturized components. The developing of this kind of architectures, due to the heterogeneities of their components, requires a very complex design flow, due to the utilization of both mechanical parts typical of the MEMS sensor and electronic components for the interfacing and the conditioning. In these kind of systems testing activities gain a considerable importance, and they concern various phases of the life-cycle of a MEMS based system. Indeed, since the design phase of the sensor, the validation of the design by the extraction of characteristic parameters is important, because they are necessary to design the sensor interface circuit. Moreover, this kind of architecture requires techniques for the calibration and the evaluation of the whole system in addition to the traditional methods for the testing of the control circuitry. The first part of this research work addresses the testing optimization by the developing of different hardware/software architecture for the different testing stages of the developing flow of a MEMS based system. A flexible and low-cost platform for the characterization and the prototyping of MEMS sensors has been developed in order to provide an environment that allows also to support the design of the sensor interface. To reduce the reengineering time requested during the verification testing a universal client-server architecture has been designed to provide a unique framework to test different kind of devices, using different development environment and programming languages. Because the use of ATE during the engineering phase of the calibration algorithm is expensive in terms of ATE’s occupation time, since it requires the interruption of the production process, a flexible and easily adaptable low-cost hardware/software architecture for the calibration and the evaluation of the performance has been developed in order to allow the developing of the calibration algorithm in a user-friendly environment that permits also to realize a small and medium volume production. The second part of the research work deals with a topic that is becoming ever more important in the field of applications for MEMS sensors, and concerns the capability to combine information extracted from different typologies of sensors (typically accelerometers, gyroscopes and magnetometers) to obtain more complex information. In this context two different algorithm for the sensor fusion has been analyzed and developed: the first one is a fully software algorithm that has been used as a means to estimate how much the errors in MEMS sensor data affect the estimation of the parameter computed using a sensor fusion algorithm; the second one, instead, is a sensor fusion algorithm based on a simplified Kalman filter. Starting from this algorithm, a bit-true model in Mathworks Simulink(TM) has been created as a system study for the implementation of the algorithm on chip

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

A Low-Power Interface for Capacitive Sensors With PWM Output and Intrinsic Low Pass Characteristic

A compact, low power interface for capacitive sensors, is described. The output signal is a pulse width modulated (PWM) signal, where the pulse duration is linearly proportional to the sensor differential capacitance. The original conversion approach consists in stimulating the sensor capacitor with a triangular-like voltage waveform in order to obtain a square-like current waveform, which is subsequently demodulated and integrated over a clock period. The charge obtained in this way is then converted into the output pulse duration by an approach that includes an intrinsic tunable low pass function. The main non idealities are thoroughly investigated in order to provide useful design indications and evaluate the actual potentialities of the proposed circuit. The theoretical predictions are compared with experimental results obtained with a prototype, designed and fabricated using 0.32 mu M CMOS devices from the BCD6s process of STMicroelectroncs. The prototype occupies a total area of 1025 x 515 mm(2) and is marked by a power consuption of 84 mu W. The input capacitance range is 0-256 fF, with a resolution of 0.8 fF and a temperature sensitivity of 300 ppm/degrees C