Author: Seyed Parsa Taheri Tehrani

Publisher:

Published: 2017

Total Pages:

ISBN-13: 9780355149593

Micro-machined vibratory gyroscopes are very small devices (up to a few millimeters in dimension) that work based on Coriolis force coupling between two resonance modes. The small size, low power consumption, and cheap price make these sensors popular in automotive, gaming, smart phones, and robotics industries. These sensors referred to as MEMS (microelectromechanical system) gyroscopes are currently not used for navigation applications because due to their miniature size and imperfections in fabrication methods they do not have enough accuracy. In this thesis, we present methods in design and control algorithms for MEMS vibratory gyroscopes to cancel the effect of imperfections in fabrication and improve gyroscopes' performance. First chapter of this thesis is an introduction on MEMS vibratory gyroscopes and their principles and standard operations modes.The second chapter presents the structural design and analysis of a single-structure 3-axis MEMS gyroscope. The gyroscope has four resonant modes of interest and uses a decoupling mechanism whereby auxiliary masses are used to actuate the drive mode of the gyroscope in order to reduce drive-force coupling to sense modes' motion (one of the sources of errors in MEMS gyroscopes). The use of auxiliary masses results in a two degree-of-freedom (DOF) mechanism of the drive mode. To compare the effectiveness of using auxiliary masses two gyroscope types has been design one actuated from auxiliary masses (type A) and one actuated from major masses (type B). The two designs are simulated analytically to study the displacement of each mass in each design while comparing the force required to achieve that displacement for drive mode. Experimental data from fabricated devices show how using auxiliary masses will decrease drive force coupling and as a result improve the gyroscope's performance. Third chapter describes the operation of a high quality factor gyroscope in various regimes where electromechanical nonlinearities introduce different forms of amplitude-frequency (A-f) dependence. Experiments are conducted using an epitaxially-encapsulated 2 x 2 mm2 quad-mass gyroscope (QMG) with a quality factor of 85,000. The device exhibits third-order Duffing nonlinearity at low bias voltages (15 V) due to the mechanical nonlinearity in the flexures and at high bias voltages (35 V) due to third-order electrostatic nonlinearity. At intermediate voltages (26 V), these third-order nonlinearities cancel and the amplitude-frequency dependence is greatly reduced. A model is developed to demonstrate the connection between the electromechanical nonlinearities and the amplitude-frequency dependence, also known as the backbone curve. Gyroscope operation is demonstrated in each nonlinear operating regime and the key performance measures of the gyroscope's performance, angle random walk (ARW) and bias instability, are measured as a function of drive-mode vibration amplitude. While the bias instability is nearly independent of the drive-mode’s nonlinearity, we find that ARW increases when the third-order nonlinearities are minimized, and the decrease in ARW due to increase of amplitude is independent of drive mode's type of nonlinearity.In the fourth chapter we present a direct angle measurement method in gyroscopes. Towards the objective of direct angle measurement using a rate integrating gyroscope (RIG) without a minimum rate threshold and performance limited only by electrical and mechanical thermal noise, in this chapter we present the implementation of a generalized electronic feedback method for the compensation of MEMS gyroscope damping asymmetry (anisodamping) and stiffness asymmetry (anisoelasticity) on a stand-alone digital signal processing (DSP) platform. Using the new method, the precession angle dependent bias error and minimum rate threshold, two issues identified by Lynch for a MEMS RIG due to anisodamping are overcome. To minimize angle dependent bias, we augment the electronic feedback force of the amplitude regulator with a non-unity gain output distribution matrix selected to correct for anisodamping. Using this method, we have decreased the angle dependent bias error by a factor of 30, resulting a minimum rate threshold of 2.5 dps. To further improve RIG performance, an electronically-induced self-precession rate is incorporated and successfully demonstrated to lower the rate threshold.