The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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System and Circuit Design for a Capacitive MEMS Gyroscope
Mikko Saukoski
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Electronics, Communications and Automation for public examination and debate in Auditorium S1 at Helsinki University of Technology (Espoo, Finland) on the 18th of April, 2008, at 12 noon.
Dissertation in PDF format (ISBN 978-951-22-9297-4) [2648 KB]
Dissertation is also available in print (ISBN 978-951-22-9296-7)
Abstract
In this thesis, issues related to the design and implementation of a micro-electro-mechanical angular velocity sensor are studied. The work focuses on a system based on a vibratory microgyroscope which operates in the low-pass mode with a moderate resonance gain and with an open-loop configuration of the secondary (sense) resonator. Both the primary (drive) and the secondary resonators are assumed to have a high quality factor. Furthermore, the gyroscope employs electrostatic excitation and capacitive detection.
The thesis is divided into three parts. The first part provides the background information necessary for the other two parts. The basic properties of a vibratory microgyroscope, together with the most fundamental non-idealities, are described, a short introduction to various manufacturing technologies is given, and a brief review of published microgyroscopes and of commercial microgyroscopes is provided.
The second part concentrates on selected aspects of the system-level design of a micro-electro-mechanical angular velocity sensor. In this part, a detailed analysis is provided of issues related to different non-idealities in the synchronous demodulation, the dynamics of the primary resonator excitation, the compensation of the mechanical quadrature signal, and the zero-rate output. The use of ΣΔ modulation to improve accuracy in both primary resonator excitation and the compensation of the mechanical quadrature signal is studied.
The third part concentrates on the design and implementation of the integrated electronics required by the angular velocity sensor. The focus is primarily on the design of the sensor readout circuitry, comprising: a continuous-time front-end performing the capacitance-to-voltage (C/V) conversion, filtering, and signal level normalization; a bandpass ΣΔ analog-to-digital converter, and the required digital signal processing (DSP). The other fundamental circuit blocks, which are a phase-locked loop required for clock generation, a high-voltage digital-to-analog converter for the compensation of the mechanical quadrature signal, the necessary charge pumps for the generation of high voltages, an analog phase shifter, and the digital-to-analog converter used to generate the primary resonator excitation signals, together with other DSP blocks, are introduced on a more general level. Additionally, alternative ways to perform the C/V conversion, such as continuous-time front ends either with or without the upconversion of the capacitive signal, various switched-capacitor front ends, and electromechanical ΣΔ modulation, are studied.
In the experimental work done for the thesis, a prototype of a micro-electro-mechanical angular velocity sensor is implemented and characterized. The analog parts of the system are implemented with a 0.7-µm high-voltage CMOS (Complimentary Metal-Oxide-Semiconductor) technology. The DSP part is realized with a field-programmable gate array (FPGA) chip. The ±100°/s gyroscope achieves 0.042°/s/√H̅z̅ spot noise and a signal-to-noise ratio of 51.6 dB over the 40 Hz bandwidth, with a 100°/s input signal.
The implemented system demonstrates the use of ΣΔ modulation in both the primary resonator excitation and the quadrature compensation. Additionally, it demonstrates phase error compensation performed using DSP. With phase error compensation, the effect of several phase delays in the analog circuitry can be eliminated, and the additional noise caused by clock jitter can be considerably reduced.
Keywords: angular velocity sensors, capacitive sensor readout, capacitive sensors, electrostatic excitation, gyroscopes, high-voltage design, integrated circuits, micro-electro-mechanical systems (MEMS), microsystems
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© 2008 Helsinki University of Technology
Last update 2011-05-26