A5.4 - Micromachined Ultra Low-Power Sensor for Angular Resolved Flow Measurement

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
A5 - Flow Sensors II
S. Cerimovic, F. Keplinger - Vienna University of Technology (Austria), A. Talic, R. Beigelbeck, F. Kohl - Austrian Academy of Sciences, Wiener Neustadt (Austria), B. Jakoby, H. Antlinger - Johannes Kepler University Linz (Austria)
129 - 134


Miniaturized sensors based on the calorimetric principle consist typically of a resistive thin-film heater surrounded by a conveniently arranged thermistor array, all embedded in a thin membrane. They rely on flow dependent heat transfer altering the temperature distribution near the heater. The sensors offer high spatial and directional resolution at the expense of high power consumption (typically in the mW range) due to resistive heating.
In order to reduce the dissipated power, we developed a flow direction sensor based on a combined calorimetric-anemometric transduction principle. The sensor utilizes the power efficient selfheating of the thermally coupled thermistors to induce the temperature field needed for flow detection. Two orthogonally arranged ensembles, each consisting of four thermistors, are connected to Wheatstone bridges supplied with a constant current. Due to the convective heat transfer by the passing fluid, the bridges become unbalanced. The directional characteristics of the bridge outputs are approximately sinusoidal with a 90 degree phase shift between each other. Therefore, the two velocity vectors vx and vy and hence, the angle of the flow direction as well as the flow velocity magnitude can easily be calculated.
The device characterization was performed with filtered nitrogen gas as a test fluid at a constant flow velocity of 2 m/s. In order to establish a well defined flow direction, the sensor chip was flush-mounted with the bottom of a rotatable rectangular flow channel. This setup allows 360° rotation of the flow channel with an accuracy of ±1°. The maximum measured angle error over the entire range is below 8°, which is comparable with the results obtained from devices utilizing the calorimetric transduction principle. Owing to a very low supply current (8 μA) and the extremely sensitive thermistors, the total dissipated power amounts to only 35 μW, corresponding to an over-temperature less than 0.5 K. Thus, the power requirement was reduced by more than one order of magnitude with respect to comparable calorimetric flow direction sensors.