Robust Micromachined Gyroscopes for Automotive Applications Cenk Acar*, Chris C. Painter**, Adam R. Schofield**, and Andrei M. Shkel** * BEI Systron Donner Automotive Division, Concord, USA, cacar@systron.com ** University of California Irvine, CA, USA, cpainter@uci.edu, aschofie@uci.edu, ashkel@uci.edu ABSTRACT This paper reports a micromachined gyroscope with a 2-DoF sense-mode oscillator that provides a flat region in the sense-mode frequency response curve, where the amplitude and phase of the response are insensitive to parameter fluctuations. The sensitivity is also improved by utilizing dynamical amplification of oscillations in the 2-DoF sense-mode oscillator. Thus, improved long-term stability and robustness to fabrication variations, struc- tural and thermal parameter fluctuations and vacuum degradations are achieved, solely by the mechanical sys- tem design. Bulk micromachined prototype gyroscopes exhibited a measured noise-floor of 0.64 0 /s/ √ Hz over a 50Hz bandwidth at atmospheric pressure. The sense- mode response in the flat operating region was also ex- perimentally demonstrated to be inherently insensitive to pressure, temperature and DC bias variations. 1 INTRODUCTION There is currently a drive within the automotive in- dustry to offer and standardize improved safety and comfort capabilities, including electronic stability con- trol, rollover detection and prevention, and intelligent brake systems. Many of these systems have or are in the process of being realized thanks to low-cost and low- power budget based micro inertial sensors. One of the current automotive efforts is in the integration of gy- roscopes in rollover detection systems. These systems have strict requirements for cost, robustness and oper- ation under high g acceleration. To meet these require- ments, the gyroscope structure must be robust to fabri- cation variations and fluctuations in temperature, pres- sure, and external accelerations. This is a tremendous challenge in single degree of freedom gyroscope systems where frequency and amplitude shifts due to environ- mental changes result in a large variation in output sen- sitivity. Towards a robust sensor design for automotive systems, we introduce a novel multi-degree of freedom gyroscope dynamical system. The two degree-of-freedom (2-DoF) sense-mode dy- namical system allows the device to operate in a fre- quency region that is robust to structural, thermal and pressure fluctuations, where amplitude and phase re- main largely unchanged compared to operating close to Figure 1: Scanning electron micrograph of the prototype bulk-micromachined 3-DOF gyroscope with 2-DOF sense- mode. the frequency peaks. Thus, the disturbance-rejection capability is achieved by the mechanical system instead of active control and compensation strategies. In this paper, we present fundamentals of the device design, electronics design for drive, sensing, and control, and optimization of structural parameters in order to maxi- mize sensitivity while retaining a large bandwidth. 2 THE 3-DoF MICROMACHINED GYROSCOPE STRUCTURE The presented design concept addresses the follow- ing major MEMS gyroscope design challenges: 1) The requirement of precisely controlling the relative location of the drive and sense resonance modes from die to die, from wafer to wafer, and within the required temper- ature range; 2) Variation in the Coriolis signal phase due to the shift in natural frequencies; 3) Long-term variation in sensitivity due to packaging pressure degra- dation over time; 4) Minimizing the quadrature error due to mechanical coupling between the drive and sense modes. The proposed gyroscope dynamical system consists of a 2-DoF sense-mode oscillator and a 1-DoF drive- mode oscillator, formed by two interconnected proof masses (Figure 1). The first mass, m 1 , is free to os- cillate both in the drive and sense directions, and is NSTI-Nanotech 2005, www.nsti.org, ISBN 0-9767985-2-2 Vol. 3, 2005 375