**2.5 Influence of excitation signal asymmetry on micropump performance**

It is generally accepted, that the shape of the driving signal affects the operation of micropumps. Typically, sine- or square-wave micropump driving signals with symmetric positive and negative amplitudes are most often applied [9–12].

In order to investigate the effect of amplitude asymmetry on micropump performance, driving signals of various amplitude asymmetries had to be synthesized. Since the synthesized signal amplitude depends not only on the output load impedance but also on the duration of each half-cycle by extending the time of first and shortening the time of the second half-cycle, the synthesized signal amplitudes increase and decrease, respectively. In other words, if one boost converter has more time available than the other, it will build up comparatively higher voltage on piezoelectric actuator. The time of both half-cycles is defined by a duty cycle setting. This parameter can be arbitrarily set due to the flexibility of the built-in microcontroller.

We introduced a unique approach, which employs an adjustment of the duty cycle ranging from 30−70%, enabled synthetization of excitation signals with various degrees of amplitude asymmetry, needed for elastomeric micropumps flowrate and backpressure performance characterization. The degree of signal asymmetry is defined as *S*ASIM *= V*<sup>+</sup> *-V*<sup>−</sup> . Synthesized signal for four representative duty cycle settings (DC+ = 30%, 40%, 50% and 60%) and for the duration of 22 ms when driving piezoelectric actuator P-5H (Sunnytec Suzhou Electronics Co., Ltd. [13]) is shown in **Figure 13**. When the duty cycle was set to DC+ = 30%, the positive amplitude V+ was lower than the negative amplitude in spite of higher signal slew rate in the first halfcycle. At duty cycle, DC+ = 40% amplitude symmetry of driving signal is reached. The maximal value of the applied electric field is 600 V/mm.

*Influence of Piezoelectric Actuator Properties on Design of Micropump Driving Modules DOI: http://dx.doi.org/10.5772/intechopen.103789*

**Figure 13.** *Synthesized excitation signal for four representative values of duty cycle for the duration of 22 ms.*

**Figure 14** shows micropump [14] flowrate and backpressure performance characteristics vs. excitation signal duty cycle ranging from 30 to 70% at a constant excitation frequency of 100 Hz. Module power supply voltage was set to 9 V.

Excitation signal amplitudes V+ and V− , flowrate and backpressure performance for DI water medium were measured for each of the duty cycle setting using the measurement setup, described in **Figure 10**. By extending the DC<sup>+</sup> duty cycle, the amplitude of the positive half-cycle increases while the negative one decreases. The characteristics are similar to linear functions with opposite slopes +0.43 V%−1 and − 0.43 V%−1. Measured characteristics show that flowrate and backpressure performance values are the highest at the duty cycle of equalization DCeq+ , which is 40%. We assume that in this setting, the efficiency of the suction stroke most probably equals the efficiency of the compression stroke.

By extending the DC<sup>+</sup> duty cycle beyond 40%, the excitation signal amplitude V+ during micropump compression stroke increases and the amplitude V− during the suction stroke decreases.

#### **Figure 14.**

*Micropump flowrate and backpressure performance characteristics vs. excitation signal duty cycle ranging from 30 to 70% at a constant excitation frequency of 100 Hz.*

Remarkably, the comparatively higher excitation signal amplitude V+ in the compression stroke fails to compensate for the decrease in the amplitude V− in the suction stroke and the pumping performance declines. The same trend of performance decline is observed when duty cycle DC+ is set below 40%. The higher excitation signal amplitude V− in the micropump suction stroke cannot compensate for the decrease in the amplitude V+ in the compression stroke and the pumping performance declines again.

Flowrate and backpressure performance of micropump for DI water decreases rather exponentially with increasing degree of signal asymmetry. Based on these characteristics results, it is presumed that the suction and compression stroke performance must be balanced for the highest micropump flowrate and backpressure performance. Hence, the total performance of the pump might only be as high as is dictated by the performance of less efficient stroke. This exponential decrease of pumping performance vs. increasing degree of signal asymmetry coincides with the exponential increase of the pumping performance vs. increasing excitation signal amplitude [15]. Above mentioned exponential trend results from employed active rectifying elements (sequential expansion and throttling of rectifying elements are performed by actuated membrane deformation). If the excitation signal amplitude is reduced, micropump membrane deformation decreases. On the other hand, micropump displacement volume and active rectifying elements efficiency are reduced also.

For micropumps employing passive check valves (sequential opening and closing of check valves are performed by fluidic flow), flowrate and backpressure performance vs. excitation signal amplitude are linear [15]. As the excitation signal amplitude decreases, the displacement volume of micropump decreases, while the efficiency of the passive check valves remains constant. It is speculated that the flowrate and backpressure performance of micropumps employing passive check valves would decrease proportionally with the degree of excitation signal asymmetry *S*ASIM. It was shown, that micropump flowrate and backpressure performance is limited by a lower amplitude when driven by amplitude-asymmetric signal and that for maximum micropump performance the amplitude symmetric signal is required. Indeed, the performance degradation due to excitation signal asymmetry might be compensated by increasing both the negative and positive half-cycle voltage V<sup>−</sup> and V+ e.g. by increasing the controller's power supply voltage. However, with such an approach, the maximum permissible electric field in the piezoelectric actuator would be exceeded, which equals only 490 V/mm for the PZT-5H standard composition [13]. This would result in permanent actuator performance degradation due to depolarization effect. From measured results (**Figure 14**), it can be concluded that micropumps should be driven by an amplitude-symmetric excitation signal. Therefore, it is mandatory to introduce a solution for providing amplitude symmetry to ensure stable long-term micropump operation and high backpressure and flowrate performance characteristics.
