Smart Flow Sensor with Combined Frequency, Duty-Cycle, and Amplitude Output

A thermal flow sensor with smart electronic interface is presented. The sensor is based on four germanium thermistors embedded in a thin membrane and connected to form a Wheatstone bridge. Both the bridge unbalance voltage and the voltage at the bridge supply terminals under constant current excitation are functions of the flow offering high initial sensitivity and wide measurement range, respectively. The signal interface is based on a relaxation oscillator which provides a rectangular-wave output whose frequency is related to the bridge unbalance, whereas the duty cycle is a function of the voltage at the bridge supply terminals. Hence, both sensor signals are simultaneously and independently carried on the same output signal, featuring unambiguous characteristic over a wide flow range accompanied with a high relative sensitivity at low flow velocities. The amplitude of the output signal depends linearly on the operating temperature, which enables the correction of the temperature dependence of the sensor readouts.