Vibratory MEMS Gyroscopes

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Vibratory MEMS Gyroscopes
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1 Introduction to Vibratory MEMS Gyroscopes

Figure 1: Generic 2-axis mass-spring-damper model
See Also



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[1]

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.

LaTeX: F=2M\nu \times \Omega


where:

LaTeX: F is the force of the proof mass,
LaTeX: M is the proof mass,
LaTeX: \nu is the velocity of the mass, and
LaTeX: \Omega is the angular velocity of the reference frame (or sensor housing).

2.1 The role of mechanical resonance in vibratory gyros

Figure 1: Draper Tuning Fork Gyroscopes

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[2]

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


4 Resources

4.1 Notes

  1. Boa, pp. 16-19
  2. Sensormag