Liquid Viscosity Sensing Using Nonlinear Higher Order Harmonic Excitation in Fiberoptic Vibrating Sensor

Abstract

In practice, the vibration amplitudes are always kept sufficiently small in order to avoid nonlinear vibration effects. In this paper, however, nonlinear vibration is intentionally excited to improve the sensitivity of viscosity measurements, in particular nonlinear higher order harmonic excitation. The results show that a nonlinear effect drastically improves sensitivity to viscous damping. Theory and experimental results are presented. Three different experiments were conducted: (1) observe the frequency responses of the third harmonic in a linear system; (2) excite the system into a nonlinear region using different driving (~2 kHz) and response frequencies (~6 kHz); (3) excite the system into a nonlinear region using the same frequency for both driving and response (~6 kHz). For liquid viscosity measurement, the gain per unit of viscosity for the superharmonic case is highest among all cases at 3 dB/cp.