Review of applications of self‐excited oscillations to highly sensitive vibrational sensors

Vibrational sensors with resonators are suited for online monitoring because of their fast response and ability to measure instantly and continuously. Also, their miniaturization realizes much higher resolution. In this review, we begin by discussing sensor resolution based on the natural frequency shift of the resonator. For mass and stiffness sensing, the detection method for the natural frequency shift by self‐excited oscillation is characterized comparing with that by external excitation. Self‐excited oscillation automatically compensates for the viscous damping effects of the environment on the resonators, thus ensuring direct and accurate detection of the natural frequency shift. Another application of self‐excited oscillation is to viscometers. Instead of detecting the natural frequency shift, this system detects the critical feedback gain to generate self‐excited oscillation. This is important for measuring very high viscosity, where the peak of the frequency response curve is ambiguous or does not exist, meaning the Q value cannot be estimated from such curves. Another method, introduced for ultra‐sensitive mass sensing, is based on the eigenmode shift in multiple weakly coupled resonators. In this system, the self‐excitation compensates for the viscous damping effects to enable direct detection of the eigenmode shift. Vibrational sensors with resonators are suited for online monitoring because of their fast response and ability to measure instantly and continuously. Also, their miniaturization realizes much higher resolution. In this review, we begin by discussing sensor resolution based on the natural frequency shift of the resonator. For mass and stiffness sensing, the detection method for the natural frequency shift by self‐excited oscillation is characterized comparing with that by external excitation….