## 6 Experiments  
The identified model combined with the proposed controller was implemented on the experimental setup. The controller was tested in the real time data acquisition system with the same sampling frequency of 200 Hz. The experimental setup was excited using the same excitation signal used in simulation. The open loop test was conducted to view the results in the absence of the controller. The comparisons of the free vibrations (without control) results between simulation and experiment are shown in Figure 14. It is clear that the experimental results are in good agreement with simulation for the open loop test. It should be pointed out that the amplitude of the time response in simulation is a little larger than that in experiment. This is because the identified model \(G(s)\) corrects for the neglected modes. In addition, the tip displacement measured by the laser displacement sensor is plotted in Figure 15. It can be seen that the uncontrolled tip displacement of the manipulator still has an amplitude of 0.25 mm, after a damping period of 60 s. The time taken for the vibration to settle down is so long that vibration control must be carried out to accelerate this process. Figure 14: Comparisons of free vibration results between simulation and experiment. Figure 13: Simulation results with different locations of the closed loop poles: (**a**) time response of the closed loop system; (**b**) control voltage of PZT actuators; (**c**) control voltage of PZT actuators; and (**d**) power spectral density of the control voltage. Next, the controller with the optimal locations of desired poles was implemented. The closed loop time responses with the PPM are shown in Figure 16a. It can be seen that the system vibrations are well suppressed with the proposed controller, and the damping period of the vibration response is shortened significantly. Furthermore, the power spectral density comparisons for 10 seconds(s) to 25 s are shown in Figure 16b. Compared with the responses of the open loop, the measured magnitude at the first resonant frequency drops from 5.6 decibel (dB) to 4.1 dB, and a drop of 2.3 dB is observed for the second resonant frequency. Evidently, substantial drops are observed at the first two resonant frequencies. Moreover, the closed loop time responses for the simulated and experimental results match closely, as shown in Figure 17. It is noted that the control efficiency in experiment is a little better than that in simulation. This is because the amplitude of time response in simulation is a little larger than that in experiment as shown in Figure 14. The measured tip displacement is shown in Figure 18. After a damping period of 13 s, the amplitude of the controlled tip displacement reduces to 0.05 mm. Thus, a quick vibration convergence is achieved with the proposed controller. Figure 16: Comparisons of experimental responses between the open and closed loop: (**a**) time response; and (**b**) power spectral density. Figure 15: Tip displacement measured by the laser displacement sensor. To further evaluate the performance of the controller, the same structure was subjected to an external disturbance by the electromagnetic vibrator (EMV). The controller is switched on when the structure is excited by the vibrator. The open loop (without control) and closed loop (with the controller) time responses are shown in Figure 19a. In the absence of the controller, the uncontrolled response measured by the strain sensors still has an amplitude of 0.14 V after a damping period of 50 s, while now is suppressed to 0.05 V within 9 s. and the power spectral density comparisons for a period of 25 s are presented in Figure 19b. Figure 19: Comparisons of experimental responses excited by the electromagnetic vibrator (EMV): (**a**) time response; and (**b**) power spectral density. Figure 15.0 Comparisons of the closed loop time responses. Figure 15.1 Comparisons of the measured tip displacement. It is noticed that the first two vibration modes are well suppressed.