Thermal flow sensor and method of generating flow rate detection signal by the thermal flow sensor

摘要

Provided is a thermal flow sensor capable of obtaining a flow rate detection signal that differs depending on a flow direction of a fluid, with a simple configuration and at low cost. The thermal flow sensor includes: a bridge circuit (1) for outputting a flow rate detection signal (VM); a fluid direction detection circuit (2) for outputting a fluid direction detection signal (VD); and an arithmetic circuit (3, 4, 6) configured to: generate a first output signal (VQF) and a second output signal (VQR) based on the flow rate detection signal (VM) and the fluid direction detection signal (VD); and select the first output signal (VQF) when the fluid direction detection signal (VD) shows normal flow and select the second output signal (VQR) when the fluid direction detection signal (VD) shows reverse flow, to thereby output a flow rate detection signal (VOUT).

主权项

1. A thermal flow sensor, comprising:
a bridge circuit including a heating element, a fluid temperature detector, and a resistor, the heating element being supplied with a heating current so that temperature of the heating element becomes higher than temperature of a fluid detected by the fluid temperature detector by a predetermined temperature, the bridge circuit being configured to output a flow rate detection signal corresponding to the heating current; a fluid direction detection circuit for outputting a flow direction of the fluid as a fluid direction detection signal; and an arithmetic circuit being configured to:
generate a first output signal and a second output signal based on the flow rate detection signal output from the bridge circuit and the fluid direction detection signal output from the fluid direction detection circuit; andselect the first output signal when the fluid direction detection signal shows normal flow and select the second output signal when the fluid direction detection signal shows reverse flow, to thereby output a flow rate detection signal that takes the flow direction into account so that the flow rate detection signal takes different values between the normal flow and the reverse flow even at the same flow rate, wherein the arithmetic circuit comprises:
a first arithmetic circuit for generating the first output signal so as to have a value lower than a value of VQ when the fluid direction detection signal shows the reverse flow and generating the first output signal so as to have a value equal to the value of VQ when the fluid direction detection signal shows the normal flow, where VQ represents the flow rate detection signal output from the bridge circuit;a second arithmetic circuit for generating the second output signal in accordance with the following expression:
VQ0+A×(VQ0−VQ), where VQ represents the flow rate detection signal output from the bridge circuit, VQ0 represents a value of the flow rate detection signal in a flat calm, and A represents an arbitrary gain; and a third arithmetic circuit for performing OR operation of the first output signal generated by the first arithmetic circuit and the second output signal generated by the second arithmetic circuit, to thereby output the flow rate detection signal that takes the flow direction into account.