## 4 Experimental Results and Discussions
### Double-Beam with Dual-Connected SSDI  
As pointed out by the previous calculation (Figure 6), the effect of the dual-connected SSDI varies with the amplitude of the relative displacements between two components. To verify these results, the phase of exciting force to Beam 1 is fixed to 0 and the phase of Beam 2 is adjustable from 0 to 360 degree. Three circuit connection modes are considered in the test: open circuit, PP mode and PN mode. The displacement response of Beam 1 in three cases are measured when phase difference is 180 degree. Results are compared in Figure 14, where one can find that PN mode has the best damping effect. Setting the amplitude of open circuit mode as 100%, the vibration reduction of PP mode is about 16% and that of PN mode is about 44%. Figure 13: Displacement comparison curves (indicated by the voltage of PZT2) in time domain with and without SSDI (50 Hz). We also excite the two beams by white noise, and measure the displacement response curves of Beam 1 in three cases are, as presented in Figure 15a. The corresponding frequency response curves are shown in Figure 15b. The damping effect is summarized in Table 3 in terms of the reduction of displacement amplitude of Beam 1. It also proves that the vibration at first two resonant frequencies can be suppressed when shared SSDI shunt is applied especially in PP connection. Eventually, we fix the amplitudes of excitation applied to two beams and let the exciting phase of Beam 2 vary from 0 to 360 degree with equal intervals (45 degree). The maximum of displacement amplitude of Beam 1 with PP and PN modes are shown in Figure 16. The curves verify that the displacement difference (realized by setting phase difference of the exciting signals of two beams)  
Figure 14: Comparison of displacements in three cases (50 Hz, phase difference 180 degree). Figure 15: Comparison of damping-effect in three cases (white noise, phase difference: 180 degree): (**a**) displacement response in time domain; (**b**) frequency response. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Electric** & **1 st-Order** & **Maximum** & **2 nd-Order** & **Maximum** \\
**Circuits** & **Frequency (Hz)** & **Displacement (mV)** & **Frequency (Hz)** & **Displacement * (mV)** \\ \hline Open-circuit & 50.05 & 164.6 & 280 & 3.891 \\ SSDI-PN & 50.1 & 158 & 280.4 & 3.558 \\ SSDI-PP & 50.1 & 118.5 & 280.4 & 2.801 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Damping comparison of the double-beam system in three cases (white noise, 180 degree). will affect the damping effect directly, and the greater the difference the better the damping effect. The vibration reduction of PP is larger than that of PN connection nearly at each case. It should be noted that the displacement amplitude of Beam 2 is suppressed simultaneously due to the performance of the shared SSDI shunt. As mentioned, in both the PP and PN connections, the switching is controlled by the same way, i.e., by the relative displacement of two structures. In this regard, both PP and PN connections have poor performance for the in-phase vibration because the relative displacement is always zero and the switching cannot be triggered. The PP connection is expected to work best with out-of-phase vibration, and when the switching is closed, the voltage difference between PZT1 and PZT2 also reaches the maximum point. This generates the best damping. However, the PN connection still works poorly with out-of-phase vibration, because when the switching is closed (the maximum displacement difference occurs), the voltage difference between PZT1 and PZT2 is zero. These theoretical analyses are validated by results in Figure 16. In the experiments it is impossible to precisely control the amplitude and phase of two beams, and there will be some error. Therefore, in the out-of-phase case the voltage difference between PZT1 and PZT2 are not precisely zero. This can somehow generate some damping. That's why we can still observe some damping from the PN connection.