Advantages of Electrostatic Spring Hardening in Biomimetic Hair Flow Sensors

We report on a fully adaptable flow sensor with adjustable detection limit, responsivity, range, and bandwidth by addition of electrostatic spring hardening (ESH) to our previously developed microelectromechanical systems hair flow sensors. The sensor´s mechanical transfer shows large voltage-controlled electromechanically affected responsivity for frequencies below the sensor´s resonance. Using capacitive readout, a bias voltage-controlled sensory threshold is obtained, giving a threefold tunable ac-airflow detection threshold (down to 0.3 mms^-1). The mechanism of spring control also extends to dc-flows, as shown for the first time; electrostatic spring hardening allows to increase the dc-flow measurement range by almost a factor 2, up to about 5 ms^-1. Furthermore, the application of ESH is demonstrated both theoretically and experimentally for nonresonant parametric amplification (NRPA) by achieving suppression of residual frequency components at the cost of overall gain. In addition, we show that ESH allows to extend selective gain and tunable filtering by NRPA to a larger range of flow frequencies.