Actuation-based microsensors

We have evaluated fluid flow effects induced by off-the-shelf mini-actuators with available microstructure flow, pressure and temperature sensors and demonstrated the feasibility of exciting new sensing approaches. Specifically, we discuss new approaches for experimental and analytical determinations of: sub-millisecond flow sensor response time versus flow velocity and microsensor structure; compact and affordable composition correction, C sub V , for volumetric fluid flow sensors; concentration of binary mixtures, based on measurement of C sub V ; and fluid properties based on actuator-induced flow or compression, such as viscosity or gamma = c sub p /c sub v , respectively. The micromachined thermal flow sensors, i.e. thermal microanemometers, that we used consisted of either: off-the-shelf, front-etched microbridge sensor chips of approx 1.7 x 1.7 mm, with bridges of approx 0.2 x 0.25 mm, or developmental, very rugged, Microbrick sensor chips of equal size but without the etched cavities. For actuators, we used commercially available, 10-12 mm OD, membrane-based, low-cost, earphone speakers, with resonances in the 2 kHz region. We found the useful operating frequency range of both sensors and actuators, with due consideration to resonance effects, to be in the 40-100 Hz range and the one most free of disturbances for the actuators used. The flow sensors themselves showed the capability of operating beyond 500 Hz, especially the rugged version, which showed response times down to approx 0.2 ms. This Microbrick sensor is burst-proof and designed for operation in harsh environments featuring gas or liquid mass fluxes up to 500 g cm exp -2 s exp -1 , with condensible vapors and suspended sand or dust. With the above devices we demonstrated a new approach for on-line fluid flow sensor composition correction, which is needed to correct errors caused by fluid composition changes. Previously developed, time-consuming and costly composition correction for thermal flow sensors relied on either individual calibration or via measurement of thermal conductivity, specific heat and Prandtl number. Those methods can now be replaced by this one-step, on-line, low-cost, actuation-based normalization, which can be adapted as well to other flow sensing technologies, such as orifice flow sensors. Using the same mini-actuators to induce flows in laminar flow restrictors, we also report on the demonstration of a very compact and affordable approach to the measurement of visc