Structural Design Trade-offs for Mems Vibratory Rate Gyroscopes with 2-dof Sense Modes

Our previous work demonstrated the advantages of MEMS vibratory gyroscopes with 1 degree of freedom (DOF) drive and 2-DOF sense modes which were shown to be robust to temperature drifts. These devices were designed with frequencies below 1 kHz; many applications, however, require gyroscopes with operational frequencies above 1 kHz for the rejection of ambient vibrations. This paper discusses the design trade-offs associated with increasing the frequency of the 3-DOF gyroscope design concept. Lumped parameter models were used to simulate the effects of frequency increases on the device, focusing on the 2DOF sense mode. The simulations showed that the sense mode peak spacing increases with frequency which ultimately causes a decrease in sensitivity. A series of 3-DOF gyroscope prototypes with different operational frequencies ranging from 0.7 kHz to 5.1 kHz were designed, fabricated, and characterized. INTRODUCTION All vibratory micromachined gyroscopes integrate two high precision subsystems – a self tuned oscillator, called the drive mode, and a micro-g accelerometer, called the sense mode [1,2]. The operation of the device depends on a transfer of energy between these modes which is detected and processed to produce the device output [1]. While these subsystems are generally de∗Author of correspondence, Email: [email protected] signed to have their own independent vibration characteristics, the complete sensor dynamics are sensitive to the relative location of the drive and sense resonant frequencies [3]. Conventional micromachined vibratory gyroscopes generally use a single degree of freedom for both the drive and sense mode, forming a 2-DOF dynamic system [2, 3]. For these types of devices, the sensor gain can be increased by mode matching, where the drive and sense natural frequencies are designed to be equal. The increased gain comes at the cost of robustness as fluctuations in the operational parameters can cause large changes in amplitude due to the instability of resonance [3]. Because of this, conventional devices are generally designed with a few percent mismatch in frequencies. The 3-DOF micromachined gyroscope is a novel design concept introduced in [4]. The difference of this design from conventional devices is the addition of a second sense mass, forming a coupled 2-DOF sense mode. This alters the frequency response so that a region of magnitude and phase stability is formed between the two coupled resonant frequencies. Using low frequency prototypes, it was demonstrated that this design approach yields angular rate sensors robust to fabrication imperfections and environmental conditions [4]. However, the 3-DOF design approach has been shown to have drawbacks associated with increases in operational frequencies, where increases in peak spacings can cause a drop in sensor gain [5]. This paper investigates the effects of increasing the oper1 Copyright c © 2007 by ASME