|Sensors Related Ph.D. and D.Sc. Theses|
Integrated Micromechanical Resonant Sensors for Inertial Measurement Systems
Author: Ashwin Arunkumar Seshia
Chair: Roger T. Howe, University of California, Berkeley
Thumbnail-sized inertial measurement systems based on Micro Electro Mechanical Systems (MEMS) technology have been perceived as a breakthrough in the field of inertial navigation. However, even as micromechanical accelerometers have seen widespread commercialization, vibratory micromechanical gyroscopes have not enjoyed similar success. Previous approaches to high-performance micromechanical gyroscope design have been complicated by two factors: first, the requirement for precise control of the trajectory of a multi-degree of freedom vibrating micromechanical structure, and second, the development of calibration schemes that must precisely identify system parameters that are often sensitive functions of ambient environmental variables. This thesis describes a new angular rate sensor, termed the “Resonant Output Gyroscope”, that considerably simplifies both the control system and the calibration procedure. In its simplest form the device comprises of three coupled micromechanical oscillators. Two of these oscillators sense the Coriolis force acting upon a third vibrating mass (in response to an input rotation rate) as a shift in their operating frequency. Detection of this frequency shift results in an estimate of the input angular motion. A prototype device fabricated at the Sandia National Laboratories has a measured (electronics-limited) noise floor of 0.3 deg/sec/ and a scale factor of 42 mHz/deg/sec. A second-generation device fabricated at Analog Devices Inc. has an estimated noise floor of 18 deg/hr/.
Characterization of micromechanical resonator oscillators and micromechanical resonant accelerometers are detailed as well. Analytical results on noise and distortion performance of micromechanical resonator oscillators are backed up by measurements for a Pierce oscillator topology. Micromechanical resonator oscillators fabricated at the Sandia National Laboratories demonstrated a far-carrier noise floor of less than -105 dBc/Hz for center frequencies ranging from 115 kHz to 265 kHz. The noise performance of micromechanical oscillators is the basis for an analytical result of the degradation of the signalto-noise ratio for resonant accelerometers beyond a certain noise corner frequency. This noise corner frequency is measured to be approximately 300 Hz for a surface-micromachined resonant accelerometer that has a measured scale factor of 17 Hz/g and a minimum detectable acceleration resolution of 40 mg/ at an input frequency of 300 Hz.
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