Constant power operation of a two-dimensional flow sensor

To compensate for changes in ambient temperature, thermal flow sensors are usually heated to a constant temperature above ambient temperature. This operating mode, however, requires an extra ambient-temperature sensor. Using a simple model, it is shown that for a two-dimensional thermal flow sensor, similar compensation may be obtained by using a constant heating power, thus eliminating the need for an ambient-temperature sensor. The flow sensor's interface circuitry consists of two thermal sigma-delta modulators that heat the sensor with constant power and dynamically cancel the flow-induced temperature gradient. The modulator's bit-stream outputs are then a digital representation of the heat distribution in the sensor. Wind tunnel tests confirm the validity of the sensor model used and show that both flow speed and direction may be accurately determined from the bit-streams, with errors less than +/-5% and +/-3(degrees), respectively, for flow speeds between 1 and 25 m/s.