Wireless Fluid Flow Sensing Circuit Using Zero offset Thermal Flow Sensor

Xiang Zheng Tu

It should be pointed that the above circuit comprises a negative feed-back loop consisting of a heater, one or two thermopiles, both are integrated on a thermal insulating bridge, an amplifier, and a microcontroller. The heater and the thermopiles are the elements of a thermal flow sensor and they work in conjunction for sensing fluid flows. The amplifier is built in the microcontroller. A reference voltage provided by the microcontroller is sent to the amplifier. When the heater is heated by a starting PWM voltage provided by the microcontroller the thermopiles convert the temperature difference between the bridge of and the environment into a voltage sending to the amplifier for suppressing the reference voltage.

If the flow is zero, the output voltage of the thermopiles and the reference voltage should be equal. This can be realized in the following way. The different voltage between the reference voltage and the output voltage of the thermopiles is first amplified. Then the output voltage of the amplifier sends to the microcontroller for A/D conversion and digital processing. As a result, the starting PWM voltage is modified so as to make the output voltage of the thermopiles eventually reach the reference voltage. The digital number used to produce new PWM voltage represents a chosen temperature for the operation of the thermal flow sensor. It also indicates the flow velocity being zero.

When a flow is applied, the output voltage of the thermopiles may be higher or lower than the reference voltage. Another new digital number is obtained by amplification of the amplifier and digital processing of the microcontroller. So the heater is heated by another new PWM voltage driving the thermopiles to generate an output voltage equaling to the reference voltage. At the same time the sensor is operated back to the chosen temperature. The new output digital number of the microcontroller expresses the applied flow velocity.

With the negative feed-back loop, the operation of the sensor can be maintained at a constant temperature above that of the flow. So zero-offset can be realized without need for low-offset amplification. It also enables the sensor to have faster response since the temperature of the sensor is no longer modified by the flow. A further advantage is that small thermal asymmetries introduced during the sensor fabrication process can be automatically compensated. 

As shown in the above circuit, CC2540 combines a RF transceiver with an industry-standard enhanced 8051 MCU. It is suitable for wireless sensor modules where very low power consumption is required. A main problem with its MCU is lack of PWM output. Fortunately, it has two general-purpose timers, which can be used for creating a PWM interrupt generator.