## 4 Experimental Results  
The controls were carried out using the LabVIEW Real-Time Module, National Instrument, Austin, TX, USA and the NI acquisition system cRIO-9066, National Instrument, Austin, TX, USA equipped with the analog output card NI-9203 and the IPE conditioning cards NI-9234 to measure the structure response. It is important to highlight that saturation blocks are included to reduce stroke and force saturation risks. In Figure 10, the cRIO, amplifiers, and a computer recording the results coming from the second loop of the block diagram can be seen.  
Before defining the perturbation in the experimental results, the actuator stroke saturation, which also limits the maximum damping imparted, must be considered. Note that this stroke saturation could result in dramatic adverse effects on the actuator performance and its hardware. This issue is important to define the maximum perturbation the controller can damp in its linear behavior.  
Figure 10: Experimental setups.  
Figure 9: Magnitude (in dB) plot of the closed-loop simulation with the identified model (blue) and the system with a variation of \(+25\%\) (yellow) and of \(-25\%\) (red).  
Only two APS Electro-Seis Dynamic Shaker 400 are available in the laboratory, which are driven by the power amplifiers APS 144 and 145. Thus, to compare the FRFs obtained in open loop (Figure 7) with the closed-loop results, an additional excitation must be added. This excitation was an impulse hammer (PCB PIEZOTRONICS model 086D50). Figure 11 shows the locations of the two actuators at nodes 16 and 22. Near these nodes (actuators and accelerometers), three impacts to the floor, with an interval of 20 s, were carried out in each node.  
Figure 12 shows the comparison between the OL and CL results. Notably, the damping of the first mode exceeded 30 dB at nodes 16 and 22, with node 22 exhibiting a damping of over 20 dB. Additionally, a minor peak around 2.2 Hz is evident in the closed-loop responses. This peak was attributed to the characteristics of the inertial actuator utilized. Its presence governs instability at lower frequencies, where the poles of the actuator may become unstable if the values of \(\gamma_{1}\) and \(\gamma_{2}\) are significantly increased.  
The results depicted in Figure 12 underscore the effectiveness of the devised MIMO IRC scheme in efficiently damping multiple unmodeled structural modes while maintaining stability. Importantly, the stability conditions established in this study ensure that any MIMO controller adhering to these criteria is inherently robust against spillover issues, thereby making it suitable for practical implementation across various engineering applications.  
It is noteworthy that the methodology employed in this research can serve as a blueprint for designing stability-based AVC schemes capable of addressing similar low-frequency challenges encountered in different control contexts. By leveraging alternative control laws, which may have been previously hindered by comparable low-frequency limitations, engineers can now employ a similar approach to develop robust and effective AVC strategies tailored to their specific system requirements.  
Figure 11: IRC MIMO control of the laboratory floor structure.