1 Introduction to Vibratory MEMS Gyroscopes
All MEMS gyroscopes currently use vibrating proof masses. Those masses typically vibrate at a high frequency. As the sensor housing rotates in inertial space a Coriolis force is induced on the proof mass. The Coriolis force causes a vibration in an orthogonal plane and the amplitude of the orthogonal motion can be measured.
2 Operating Principle of MEMS vibratory gyros
A simplified model of vibratory gyros is shown in Figure 1. The system has 2 orthogonal vibration modes; one mode corresponds to the vibration of the mass in the x-direction, the other the y-direction. The vibration frequency of the x-axis is ωx and the equivalent for the y-axis is ωy. Typically ωx is almost equal to ωy.
When powered on, the mass is driven in the x-direction with a driving frequency ωd which is close to ωy. When the entire MEMS gyro is rotated about the z-axis (out of the plane of the screen), an alternating force in the y-direction is caused by the Coriolis force. The amplitude of this vibration in the y-direction is used as a measure of the angular rate.
All MEMS gyroscopes use the Coriolis effect.
- is the force of the proof mass,
- is the proof mass,
- is the velocity of the mass, and
- is the angular velocity of the reference frame (or sensor housing).
2.1 The role of mechanical resonance in vibratory gyros
The Coriolis force is typically weak. As a result mechanical resonance is used to amplify the motion and thus keep the signal to noise ratio high over the desired bandwidth. The driving frequency, ωd, and the 2 resonance frequencies, ωx and ωy, must be designed carefully.
3 Types of Resonating MEMS Gyroscopes
All MEMS gyros require a resonating mass. The most common types of resonating MEMS gyros are
- Tuning Fork Gyros (TFG)
- Hemispherical Resonating Gryo (HRG) or Wine Glass Resonator Gyro
- Vibrating-Wheel Gyros
- Foucault Pendulum Gyros
- SensorMag site on the basics of MEMS sensors
- Mohite, S., Patil, N., Pratap, R., "Design, modeling and simulation of vibratory micromachined gyroscopes.", Journal of Physics: Conference Series 34 (2006), pp. 757-763.
- Thermoelastic Damping and Engineering for High Q MEMS Resonators
- Boa, M. 2004 Micro Mechanical Transducers. 2nd. Elsevier. ISBN 044450558X
- Gaura, E., and Newman, R 2006 Smart MEMS and Sensor Systems. 1st. Imperial College Press. ISBN 1860944930
- Boa, pp. 16-19