Authors: B.J. Gallacher, J.S. Burdess, A.J. Harris and K.M. Harish
Affilation: Newcastle University, United Kingdom
Pages: 383 - 386
This paper reports on an excitation scheme that enables control of the damping of an electrostatically actuated MEMS sensor through combined external forcing and parametric excitation. System damping is of fundamental importance in vibratory MEMS and limits the response amplitude in conventional externally forced sensors for example magnetometers, gyroscopes and accelerometers. Through the application of an appropriate parametric term, the system damping may be reduced by orders of magnitude thus enabling mechanical amplification. This is directly applicable to the minimisation of electrical "feedthrough" inherent in many MEMS devices and thus results in an improved signal to noise ratio. The application of parametric only excitation schemes for the minimisation of "electrical feedthrough" have been described in [1-3]. However in these schemes the absence of an external forcing term makes the development of a controlled oscillator difficult. A ring gyroscope as shown in fig.1, is used as a vehicle to demonstrate the principle. Only one mode of vibration of the gyroscope is considered and this corresponds to the primary mode of the device, which under conventional operation is externally forced. The equation of motion for the primary mode of an electrostatically actuated device is shown to be in the form of a Hill's equation . A multiple scales perturbation method is used to analyse the response of the device when subjected to combined external forcing and parametric resonance . It is shown that the system damping may be parametrically controlled and thus permits mechanical amplification through the Quality-factor of the mode. This mechanical amplification may be used to reduce the degree of "electrical feedthrough" from the external forcing signal, which would otherwise result in contamination and hence reduced sensor performance. The amount of mechanical amplification is shown to depend upon the frequency and amplitude of the parametric excitation term. In the special case where the parametric excitation frequency and amplitude is located on the stability boundary of parametric resonance illustrated in fig.2, the amount of mechanical amplification tends to infinity. The degree of mechanical amplification in traversing line AD is shown in figure (3).