Design of Thermal Flow Sensor Circuit

Being Offset Free, Temperature Drift Free, Noise Free and Interchangeable

Xiang Zheng Tu

1.    Schematic diagram of POSIFA thermal flow sensor

Reference to figure 1, a thermal flow sensor compromises a heater and two thermopiles 1 and 2. Suppose there is no fluid flow passing over the sensor.  The heater is heated by a filtered PWM output converted voltage generated by a microcontroller such as a PIC16 (L) F1704/8. The thermopiles will produce static outputsV_T1 and V_T2, respectively. If the filtered PWM converted voltage is set at 3V, the value of the static outputs is ranging from 30mv to 50mv and the V_T2 is always higher than V_T2 about 1.5mv. It should be noted that the filtered PWM output converted voltage contains a noise signal that is in the form of ringing with a periodic repetition. Since this noise signal is common mode to both the thermopiles, they can be canceled each other by sending to a differential amplifier for signal processing.

 

   

2.  Selecting a PWM output converted voltage for fixing the output V_T2 of the thermopile 2 at 30mv

Reference to figure 2, a comparator of the microcontroller is used to compare the output V_T2 of the thermopile 2 with a 30mv reference voltage V_Ref  provided by the microcontroller for sending out a digital output signal for adjusting PWM output converted voltage. When the thermopile 2 produces the output V_T2 equal to the reference voltage V_Ref, the PWM output converted voltage is fixed and the selection is finished.

According to our experience the thermal flow sensors with same static output have a similar sensitivity. They can be interchangeable for most of applications. 

  

3.    Creating a DAC output replacing the output of thermopile 2

Reference to figure 3, there is still no fluid flow passing over the sensor. A digital-to-analog converter (DAC) of the microcontroller is used to convert the output V_T2 of the thermopile 2 into a variable voltage V_DAC. The variable voltage V_DAC is derived by a resistor ladder and can be ratiometric with the input source. Moreover its all electrical properties maintain the same as the input source, which include the environment temperature influence and electromagnetic interference. As shown in the figure 3, the output V_T2 of the thermopile 2 is send to the DAC through an amplifier TP5552. TP5552 is used as a buffer for impedance transformation, because the recommended maximum source impedance of the input source is limited to be 10kΩ for the microcontroller, which is much lower than the impedance of the thermopiles.

4.    Adjusting the output V_DACT2 of a DAC for best match to the output V_T1 of the thermopile 1.

Reference to figure 4, the output V_DAC of the DAC is sent to the positive input of another comparator of the microcontroller and compares with the output V_T1 of the thermopile 1. The digital output of the comparator is used to change the output V_DAC of the DAC and eventually become V_DACT2 that is equal to the output V_T1 of the thermopile 1. Since them the output V_T2 is replaced by the output V_DACT2.

It should be understand that all these steps are performed without fluid flow passing over the sensor and can be done in the calibration process of the sensor and in the initiate stage of each sensor operation. 

After finishing these steps the output V_T2 of the thermopile 2 is replaced by the new reference voltage V_DACT1. The outputs of the differential output of the two thermopiles can be zero. This means the offset, temperature drift and noise of the two thermopiles can be canceled each other so that the sensor becomes the offset, temperature drift and noise free.

5.    Offset free, temperature drift free and noise free thermal flow sensor is realized by a differential amplifier

Reference to the figure 5, when a fluid flow passes over the sensor the thermopiles 1 and 2 will produce voltages -ΔV_T1 and -ΔV_T2 superimposed to the original output V_T2 and V_T1. Please note that -ΔV_T2 will be ratiomatric as V_T2 does and superimposed to the output V_DACT2, which is expressed as ΔV_DACT2. When a differential amplifier is used to multiply the difference between V_T1 - ΔV_T1 and V_T2 - ΔV_T2DAC, the common mode signals will be rejected. These common mode signals include temperature drift and electromagnetic noise. This means the thermal flow sensor becomes offset free, temperature drift free and noise free.

It can be seen in the figure that the used differential amplifier is another TP5554.  It was tolled that TP5552 is a dual chopper stabilized zero-drift operational amplifier, which features very low input offset voltage and low noise and may be operated with a relative high gain.