Sigma-Delta Conversion Liquid Flow Switch Circuit

 Xiang Zheng Tu

The present circuit comprises all sigma-delta modulator necessary elements. Among them are an integrator, a comparator, a 1-bit DAC, and a summing junction. The integrator function is acted by a capacitor. The comparator plays as a 1-bite quantizer, which is built in a microcontroller. The 1-bite DAC is a PWM outputted from the microcontroller, which is resulted by filtering and emerging of the output of the comparator. The summing junction combines the input signal and the PWM output, which is carried out by a thermal flow sensor.

The thermal flow sensor is micromachined to have a resistor heater and one or two thermopiles on a suspended bridge created in a silicon substrate. The heater is located at the central region of the bridge, where the temperature is raised above the environment temperature by heating the heater. The hot and cold junctions of the thermopile are located near the heater and the surrounding silicon frame, respectively. The temperature difference between the central region and the sounding frame is measured by the thermopile utilizing the Seebeck effect in which a thermal electromotive force is generated in proportion to the temperature different.

It should be noted that the thermal flow sensor is an inherent low pass filter. When the heater is heated by a frequency change of the PWM output, the thermopile will take some time to respond. A model of the response of the sensor can be based on a simple heat transfer analysis. The rate at which the sensor exchanges heat with its environment must equal the rate of change of the internal energy of the sensor. Since in a fluid, the dominant mechanism of heat exchange is convection (neglecting conduction and radiation), the energy balance is

hA(T∞ − T) = mc(dT/dt) ,    (1)

where h is the convection coefficient, A is the surface area of the sensor, T is the temperature, m is the bridge mass, and c is the bridge heat capacity.

Writing equation (1) in the form

τ (dT/dt) + T = T∞ ,         (2)

where the time constant is τ = mc/hA, which is the response of the sensor reaching 63.2% of its final value. It has been obtained that the time constant of the sensor is about 1ms or the cut-off frequency of the sensor is about 1kHz.

In operation of the present circuit, the capacitor integrates an input signal, so its output passes a threshold voltage established with the comparator and the voltage reference. The input signal is provided by the thermopile of the thermal flow sensor with its heater by applying the PWM outputted from the microcontroller, which is added a negative feedback signal from the output of the comparator. The added signal is converted by the capacitor to a voltage that is presented to one of the two inputs of a comparator. When the voltage passes the reference voltage of the comparator the output of the comparator toggles between high and low. The output is fed back to the input of the sensor via the PWM. Additionally, the output of the comparator is fed forward to the digital filter of the microcontroller. With time, the output of the digital filter provides a bit stream result.

With the present circuit, the thermal flow sensor can be operated in a constant temperature mode. The reference voltage expresses the constant temperature that the heater is expected to be heated. In order to do this, the PWM should be adjusted carefully so that the thermopile produces the output exactly equal to the reference voltage. Since the performance of the sensors may have dispensability, zero offset is needed before fluid flow measurement. A bit stream produced in the zero offset process represents no fluid flow. When a fluid flow is conducted the temperature of the heater is reduced due to the fluid flow cooling effect. The PWM needs to be adjusted again as to get the temperature of the heater back to the original value.

The present circuit is an inexpensive and high resolution solution to the thermal flow sensors for countless applications. In the circuit, the conversion from analog to digital is performed with the internal-voltage reference, comparator, and two counters in the microcontroller. These internal-microcontroller analog peripherals, along with a thermal flow sensor, are used to complete the implementation of a first-order modulator. This modulator is then combined with an output-digital filter, which also is implemented in the microcontroller unit, to complete the circuit. Consequently, the only components external to the microcontroller are a thermal flow sensor, several resistors, and a capacitor.