# State-of-the-Art Review of Structural Vibration Control: Overview and Research Gaps
## 4 Semi-Active Control Systems  
The introduction of structural control in civil engineering is widely acknowledged on a global scale, attributed to the groundbreaking article authored by [PERSON] [46] and the subsequent promotion of its implementations by [PERSON] [47]. Despite the successful application of numerous structural control strategies, their widespread adoption has been hindered by challenges related to cost, dependence on external power sources, and the complexity of mechanical aspects throughout the structure's lifespan. Although active control effectively reduces structural responses, its requirement for an extensive external power source and its potential to destabilize the structure are significant concerns regarding its applicability. Semi-active control systems offer the advantages of versatility and adaptability of active control systems, without requiring an ample energy supply, while maintaining the reliability of passive control systems [41]. Comprehensive state-of-the-art reviews on active and semi-active control [2; 26; 27; 28] and MR are notable [6; 13; 29; 31; 32]. [PERSON] and [PERSON] [13] conducted an extensive literature review that examined dynamic characteristics and distinctive attributes of various semi-active systems. The review presents the control systems experimentally tested at the component level and within scaled-down structural models, and it considers semi-active systems, including stiffness control devices, electrorheological dampers, magnetorheological (MR) dampers, friction control devices, fluid viscous dampers, tuned mass dampers (TMDs), and tuned liquid dampers. Table 2 lists the different semi-active control devices and their salient features. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Control Systems** & & **Key Features** \\ \hline Stiffness control devices & \(\bullet\) & Used to alter the stiffness and, consequently, the natural vibration properties of the structure to which they are connected. \\ Stiffness control devices & \(\bullet\) & The system mainly regulates the rigidity of a structure to achieve a non-resonant state during earthquakes. \\  & \(\bullet\) & The semi-active stiffness devices are activated or \\  & & deactivated to incorporate or remove, respectively, the stiffness of the structure’s bracing system. \\ \hline Electrorheological dampers & \(\bullet\) & Comprise a hydraulic cylinder that holds micron-sized dielectric particles dispersed in a fluid (typically oil). \\ Magnetorheological (MR) dampers & \(\bullet\) & In a strong electric field, the particles align and polarize, thereby providing greater resistance to flow. \\  & \(\bullet\) & Altering the electric field can adjust the dynamic \\  & & performance of an ER damper. \\ \hline \hline \end{tabular} \
\begin{tabular}{c c} \hline \hline \multirow{4}{*}{
\begin{tabular}{c} Magnetorheological (MR) dampers \\ \end{tabular} } & \(\bullet\) & MR dampers generally comprise a hydraulic cylinder filled with micron-sized, magnetically polarizable particles dispersed in a fluid (commonly oil). \\  & \(\bullet\) & The behavior of MR fluid is managed by applying a magnetic field to the fluid. \\  & \(\bullet\) & Without a magnetic field, the MR fluid moves freely; however, when subjected to a magnetic field, it \\  & & behaves like a semi-solid. \\  & \(\bullet\) & MR fluids can withstand a maximum yield stress in the range of 50–100 kPa [35; 43] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Critical features of semi-active control devices. Semi-active systems offer a strong alternative to both passive and active systems, as they outperform the former in terms of structural performance and the latter in terms of