Tuning fork microgyrometers: Narrow gap vs. wide gap design

We analyse the performances of two different configurations of a tuning fork microgyrometer, the so called 'wide gap' design and 'narrow gap' design. In the former case the air gap between the vibrating forks and the walls of the surrounding frame is so large that the air flow around each fork is not influenced by the presence of the frame itself. This geometrical configuration results in a very low air damping, and, hence, allows the instrument to operate at atmospheric pressure. In the case of 'narrow gap' design the distance between the forks and the frame walls is instead very small. As a consequence, the instrument needs to operate under very low pressure conditions, since, at higher pressures, the presence of a thin layer of air would increase the air damping to very large values, and would not allow the correct operation of the instrument. Although the requirement of low pressure conditions represents a drawback of the narrow gap solution, we show that this instrument configuration, when compared to the wide gap design, allows to achieve a significantly smaller dynamic error and a significantly wider range of linearity. Indeed the thickness of the air gap represents an additional parameter that can be adjusted by the designer to optimise the performances of the instrument. An accurate analytical model of the sensor is presented in the paper, which constitutes a helpful designing tool for this kind of device. In particular we focus the attention on the two tines of the drive mode, which are indeed the structural components that more than others influence the instrument performances. We show that the optimal design of these fundamental elements can be obtained by neglecting the interaction with the remaining part of the sensor structure, and show how to design the instrument to minimise the amplitude error. The influence of air damping, structural damping and geometry on the system response in terms of bandwidth and dynamic error is also investigated.