Diaphragm Pump Controlled by Thermal Flow Sensor

Xiang Zheng Tu

  

A diaphragm pump controlled by a thermal flow sensor is shown in the above figure.

The pump assembly has a thermal flow sensor, a microcontroller, a NPN switch and a solenoid driven diaphragm pump. The microcontroller has a 10 bits ADC, due to noise and other accuracy diminishing factors, its true accuracy is less than 10 bits. This application provides a software-based oversampling technique, resulting in 16 bits resolution. When the diaphragm pump is in operation a fluid flow is driven to pass over the thermal flow sensor. The sensor measures the flow rate and output an electronic signal to the microcontroller. After ADC conversion a pulse width modulation (PWM) signal is generated by the microcontroller.  It is send to the NPN switch for applying a current to the solenoid driven diaphragm pump. The electromagnetic core of the diaphragm pump moves against a spring to slide a diaphragm into the discharge position. When current is removed, the diaphragm slides back into the suction position.

There are two ways for controlling the flow rates of the diaphragm pumps. When the used PWM frequency is in the range of 25 to 200 Hz the solenoid responds (full stroke) over the duty cycle range of control. At zero duty cycle the solenoid does not move, the pump is not opened and therefore the flow is zero. At 50% duty cycle the solenoid moves through full stroke and opens the pump to full flow. Since the pump is only allowing full flow for 50% of the time, the time averaged flow in theory will be 50% of maximum flow. This type of control is called “digital” because the pump is fully open or fully closed, “on” or “off”. Other way is the PWM frequency limited in the range of 200 to 1000 Hz which produces the time averaged current and does not allow the solenoid to fully respond as in digital control. In this case linear position control is realized and any flow rate between zero and maximum can be chosen by the user.

Theoretically diaphragm pumps can produce the same flow at a given speed (RPM) no matter what the discharge pressure. However, a slight increase in internal leakage as the pressure increases prevents a truly constant flow rate. The following figure shows the measured flow rates in one-hour intervals for an infusion pump. The X-axis reference lines showed the acceptable flow rate (5 mL/h ± 15%). In all experiments, pumps initially infused at a rate faster than their nominal flow, and then returned closer to their set rates up to the complete deflation. The percentage of the flow rate error (deviation from 5 mL/h ± 15%) was 100% in the first and second hours of infusion, 96% in the third hour, 60% in the 20th hour and zero percent in the rest of the infusion time. Flow rate error in the initial hours of infusion was due to fast pump flows, and in the 20th hour due to slow infusion rates.