A high-sensitivity resonant sensor realised through the exploitation of nonlinear dynamic behaviour

Abstract

Measurements of viscosity and density allow for the monitoring of fluid quality and processes involving a fluid environment. There are various fields in which such measurements may be required, including oil exploration and production, environmental monitoring, process control, medicine, and the automotive industry. Existing MEMS viscometers and density meters typically measure vibrational characteristics such as resonant frequency, bandwidth and quality factor. This thesis reports on the development of a high-sensitivity resonant sensor. In order to significantly improve sensitivity to changes in viscosity and/or density the proposed sensor will exploit nonlinear dynamic behaviour and measure the frequency separation between singular jump points in the frequency response function. By using a one-mode approximation when excited near resonance, the dynamics of a clamped-clamped slender beam immersed in fluid is that of a standard Duffing oscillator. With harmonic forcing of sufficient magnitude, a bistable region, bounded by amplitude jump points, is seen to occur. The width of this bistable region, δF , is dependent on the damping ratio of the system, which is shown to be a function of the dynamic viscosity and density. Experiments with clamped-clamped silicon <100> beams in a range of Newtonian gases demonstrate that the measurand δF can uniquely identify a fluid, and may be amplified to magnitudes greatly exceeding bandwidth measurements for the same device. In addition, the sensitivity of the proposed nonlinear sensor to changes in fluid properties at low viscosity can be at least an order of magnitude better than that of conventional devices. Forcing magnitude and control is identified as being critical to the measured width of the bistable region. Beam dimensions can be chosen to optimise measurements for the desired application.