Frequency-tuning for control of parametrically resonant mass sensors

Abstract

Parametric resonance based mass sensing leads to increased sensitivity over other resonant methods. In this work, we present a frequency-tuning approach to measure mass change in an ultra-sensitive mass sensor. This scheme drives the oscillator using an electrical signal with fixed frequency and tunes the parametric resonance frequency to match the driving frequency by feeding back a dc offset to the sensor. Instead of monitoring frequency shift of the oscillation in a micro-oscillator, mass change in the sensor can be detected by measuring the shift in dc offset, making the sensor amenable to closed-loop control. This scheme and the prototype mass sensor with a mass of ∼30ng ∼ 30 ng and a first mode harmonic resonance frequency of ∼83kHz ∼ 83 kHz has been built and experimentally characterized. Good correlation between parametric resonance frequency and dc offset has been shown and the noise floor is less than 1 pg. The sensor is used to track water content variation in a humidity-testing chamber and shows increased efficiency over open loop operation.