MEMS-based Tuning Fork microgyroscopes: Dynamical response and functional design

This paper deals with the optimisation of the mechanical behaviour of a tuning fork microgyrometer, aimed at improving its performance. Two different configurations of the instrument have been analysed and compared, the ‘wide gap’ design and ‘narrow gap’ design. In the former case the air gap between the vibrating forks and the flat substrate is so large that the air flow around each fork is not influenced by the substrate presence. This geometrical configuration results in a very low air damping, that allows the instrument to operate at atmospheric pressure. In the ‘narrow gap’ case the distance between the forks and the substrate is instead very small. As a consequence, the instrument needs to operate under very low pressure conditions. Although this requirement represents a drawback of the narrow gap solution, we have found out 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. The thickness of the air gap represents, indeed, an additional parameter that can be adjusted by the designer to optimise the performances of the instrument. We have developed an accurate analytical model of the sensor, focusing, in particular, the attention on the two tines of the drive mode, which are the structural components that more than others influence the performance of the whole instrument. The optimal design of these fundamental elements can be, indeed, carried out neglecting the interaction with the remaining part of the sensor structure. We have more over performed a 3D FEM analysis of the sensor structure to validate some of the assumptions introduced in the formulation. In this paper we show how to utilise the model as a helpful designing tool for this kind of device and, in particular, how to design the instrument to minimise the amplitude error.