# Research on a Piezoelectric Pump with Flexible Valves
## 4 Experimental Results and Discussion
### Flow Rate and Output Pressure Measurement  
The output performance experiments of the PPFV were implemented. The experimental schematic is illustrated in Figure 8. The accuracy of the electronic balance is 0.01 g, and the accuracy of the pressure sensor is 0.01 Pa. To reduce siphon effects, we selected large diameter tanks and tried to keep the liquid level in both tanks at the same height, and at the same time, reduce the test time to improve the accuracy of the experiments.  
Figure 6: Curves of frequency and amplitude.  
Figure 7: The maximum displacement of each point and the fitted curve.  
The flow rate-frequency relationships of the prototype at different voltages are shown in Figure 9. It shows that when the voltages were 60 V, 80 V, and 100 V, the maximum flow rates were 55.78 mL/min, 89.02 mL/min, and 119.61 mL/min, respectively. The frequency points corresponding to these maximum flow rates were all located at 25 Hz. Under different voltages, the curves showed similar rules. Within the range of 10-25 Hz, the frequency was positively correlated with the flow rate, and with the increase in frequency, the flow rate also increased. What is more, within the range of 25-40 Hz, the flow rate decreased with the increase in frequency. In the range of 10-40 Hz, the main reason why the flow rate first rises and then falls is that the amplitude of the PZT vibrator varies with the frequency. In the selected input signals, the flow rate reached its maximum while the frequency was 25 Hz and the voltage was 100 V. When the voltage was lower than 80 V and the frequency was lower than 15 Hz, the flow rates were almost less than 5 mL/min. The curves of the flow rate with the voltage are shown in Figure 9. Obviously, in the voltage range of 40 V to 100 V, the voltage is almost positively correlated with the flow rate; the flow rate increases when the voltage increases. What is more, the overall trend of flow rate variation is basically consistent with the trend of amplitude variation.  
Figure 10 presents the results of the output pressure test at a voltage of 100 V. The curve of the output pressure is approximately a quadratic curve. When comparing it with Figure 9, it can be noted that the trend of the flow rate curve is basically the same. When the frequency was 25 Hz, the output pressure reached the maximum, 6.16 kPa. The results show that the PPFV has a higher flow rate and higher output pressure when the frequency is about 25 Hz.  
Figure 8: Traffic and output pressure test platform.  
Figure 9: Curves of flow rate with frequency.  
The performance of this pump was compared with that of the same type of pump. In these two pumps, only the structure of the valves was different. Pump A was equipped with the flexible valves in this study, and Pump B was installed with the compliant structure proposed in the literature [19]. Both pumps were tested at a voltage of 100 V, changing only the driving frequency of the PZT vibrator. The comparison results are presented in Figures 11 and 12. From the results, they work at the same optimal frequency point, 25 Hz. The maximum flow rate of Pump A is 119.61 mL/min, and the maximum output pressure is 6.16 kPa. Pump B has a maximum flow rate of 96.04 mL/min and a maximum output pressure of 5.61 kPa. As a result, the flow rate of Pump A is 24.5% higher than that of Pump B, and the output pressure is 13% higher because the flexible valves of Pump A have a hinged structure, which enhances the flexibility of the valves and makes the valves open and close more flexibly. Therefore, Pump A performs better than Pump B under the same driving conditions.