## 4 Experimental Study
### Analysis of Experimental Results  
In order to verify the theoretical analysis and simulation results, the vertical single-frequency sinusoidal excitation signal was applied to the APIVI platform. The changes in the acceleration response of the six-dimensional orthogonal vibration isolation platform before and after the active control under the sinusoidal excitation of 5 Hz, 40 Hz, 80 Hz, and 150 Hz were tested. The experimental results are shown in Figure 15. The experimental results are similar to the simulation results, which can verify the validity of the simulation analysis. In the ultra-low-frequency region, the controller shows no attenuation effect, but introduces electromagnetic interference and causes burrs. In the low-frequency band, which does not exceed the first natural frequency, it is obvious that the excitation signal is amplified with passive control, and the APIVI platform can attenuate the excitation signal by 33% with hybrid active and passive control. Near the first-order natural frequency of 80 Hz, the experimental results are slightly different from the simulation results. The experimental results show that the excitation signal is weakly attenuated with passive control, and the APIVI platform shows a large attenuation effect on the excitation signal with hybrid active and passive control. The first-order natural frequency of the actual system is lower than the simulation result. In the high-frequency region, similar to the simulation results, the vibration attenuation reached 77%, which was slightly lower than simulation, because the actual driving force could not reach the optimal active control force calculated by the controller.  
In order to test the robustness of the control algorithm, a random excitation signal was applied to the lower platform of the APIVI platform. The experimental test result is shown in Figure 16. The control system was turned on 2 s after the start of data collection. The acceleration response of the load platform was significantly reduced. Compared with the situation without control, the peak value of the response of the load platform is attenuated by 60%.  
Figure 15: Acceleration response of APIVI platform with passive control and hybrid control under vertical sine signal disturbance with different frequencies: (**a**) 5 Hz; (**b**) 40 Hz; (**c**) 80 Hz; (**d**) 150 Hz.  
Vertical sweep frequency signals were applied to the lower platform to compare the system transmission characteristics with and without active control, as shown in Figure 17. The first-order resonance peak appears at 68 Hz. The excitation signal can be attenuated only when the excitation frequency exceeds 80 Hz, and the attenuation effect is better with the increase in the excitation frequency. The hybrid active and passive control effect of the robust control strategy is better than passive control in low- and high-frequency bands, which can completely reduce the first-order resonance peak of the system with a maximum attenuation 18 dB. The effective attenuation frequency range is above 18 Hz, which does not reach the simulation results. That may be due to processing and assembly errors or idealized processing of the model during simulation. The transmission characteristic experiment proves that APIVI platform with robust control strategy can effectively attenuate the vibration at the middle- and low-frequency band and the resonance peak by using active control, and attenuate the high-frequency vibration by using its good passive structure and high-frequency band vibration isolation performance.