Highly sensitive smart flow sensor with frequency and duty cycle output

Abstract A thermal flow sensor with smart electronic interface is presented. The sensor is based on four germanium thermistors embedded in a thin membrane. The thermistors form a Wheatstone bridge supplied with a constant current. Both bridge output voltage and voltage at the bridge supply terminals are functions of the flow offering high initial sensitivity and wide measurement range, respectively. The signal interface is based on a relaxation oscillator. The circuit 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 non-monotonic characteristic over a wide flow range accompanied with a very high sensitivity at low flow velocities. Moreover, as the circuit excites the bridge with a constant current, the connection of remote sensors without accuracy degradation is possible.