Integrated Surface Micromachined Frame Microgyroscopes

In this work, two different integrated surface-micromachined z-axis vibratory rate gyroscopes are evaluated. Frame microgyroscopes using 'Inside Sense Outside Drive' suspension design were proposed and compared with 'Inside Drive Outside Sense' design. In the 'Inside Sense Outside Drive' frame design, the outer mass (frame) is driven at resonance while the inner mass is used to sense the Coriolis motion and vice versa for the 'Inside Drive Outside Sense' frame design.
These frame gyros were integrated with CMOS electronics in Analog Device's MOD-MEMS process using 6um thick polysilicon structural material and a 5V 0.8um CMOS process. On-chip closed-loop drive circuit and differential sense circuit were implemented. Automatic gain control was implemented in the drive circuit to set the amplitude of the drive oscillation. The prototype 'Inside Sense Outside Drive' frame gyroscope exhibited a noise floor of 0.0ldeg/s/Hz (limited by electronic noise) with quadrature and Coriolis offset being less than 100deg/s. The 'Inside Drive Outside Sense' frame gyro exhibited a noise floor of 0.05deg/s/Hz (limited by electronic noise) with quadrature and Coriolis offset being around 300deg/s. All results were taken at an ambient pressure of 70 milli-Torr. The drive resonant frequencies for the 'Inside Drive Outside Sense' and 'Inside Sense Outside Drive' frame gyros were measured to be 18.7 kHz and 16.4 kHz respectively. Drive and sense mode matching was achieved using electrostatic tuning combs.
Tests on each of these vibratory gyros indicate 'Inside Sense Outside Drive' frame gyro has lower quadrature and Coriolis offset by a factor of three and lower noise floor (limited by electronic noise) by a factor of five in comparison wit'h 'Inside Drive Outside Sense' frame gyro. The 'Inside Sense Outside Drive' frame gyro's lower error performance can be attributed to less error force coupled from the drive combs to the sense structure. In the 'Inside Drive Outside Sense' frame gyro, the error forces from the drive combs are directly coupled to the sense combs. This is reduced in the design of 'Inside Sense Outside Drive' frame suspension. The 'Inside Sense Outside Drive' frame gyro achieved lower noise performance because drive and sense modes were closely matched (deltaf < 100Hz) which was not possible in the case of 'Inside Drive Outside Sense' frame gyro.
Publication date: 
October 31, 2002
Publication type: 
Ph.D. Dissertation
Palaniapan, M. (2002). Integrated Surface Micromachined Frame Microgyroscopes. United States: University of California, Berkeley.

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