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Tuned mass damper (TMD) has been one of the most commonly used passive vibration control devices over the past few decades. While an optimally designed TMD can significantly suppress the structural vibration, detuning often occurs due to various reasons such as change in operating conditions or variation in primary structure properties, resulting in degradation of TMD’s performance. In order to restore its performance, it is necessary to estimate the modal properties of the primary structure, and perform the re-tuning process. Such an exercise requires powerful signal processing methods to successfully extract the structural modes in the presence of closely-spaced modes. This study focuses on the identification of modal frequencies and damping of the structure installed with a TMD. In view of the advantages and limitations of existing modal identification methods, this paper provides a new technique that combines the second-order blind identification (SOBI) method with the empirical wavelet transform (EWT) to delineate closely-spaced frequencies. While the SOBI method does not guarantee the separation of closely-spaced modes and suffers from the limitation of generating mode-mixed modal responses, the EWT operates on the modal responses estimated by the SOBI and yields the closely-spaced natural frequencies. The proposed method is illustrated using a six-story simulation model with a wide range of detuning cases. An experiment on a three-story bench-scale model equipped with a TMD is also conducted to validate the applicability of the proposed method.

A novel active electromagnetic vibration absorber (AEMVA) is developed. It is shown that for suppression of steady-state response to a harmonic excitation, a proportional and derivative feedback control is equivalent to a time-delayed control. The study of the stability issue focuses on a conflicting goal of achieving the marginal stability of the time-delayed vibration absorber and the stability of the combined system. The achievability of the ideal performance of the time-delayed vibration absorber is investigated. An experimental study is conducted to test the proposed AEMVA. An online tuning algorithm is proposed. The experimental results show that with the proposed tuning strategy, the AEMVA can maximize the vibration suppression while maintaining the system stability.both passive and active systems. On the one hand, they preserve the reliability of passive control systems in the event of power loss; on the other hand, they require less power to achieve the adaptability of active control systems.Usually, semi-active control devices achieve the adaptability by adjusting their mechanical properties, such as the stiffness and/or the damping value, through a feedback system. Several stiffness-variable vibration absorbers have been proposed in the literature. Stiffness variation of the device reported in [11] was achieved by varying the effective number of coils in a helical spring used as the absorber stiffness. The semi-active control device proposed in [12] consists of a flexible cantilever beam attached to a mass at its free end. By varying the length of the beam, the absorber frequency can be varied. The device reported in [13] is formed by four coil springs arranged in a planar rhombus configuration whose aspect ratio can be varied by a linear electromechanical actuator. It is noted that all the semi-active devices mentioned above involve an electromechanical-driven system. As a result, a slow action may become the main concern for this type of devices. Alternatively, damping is sometimes added to the absorber systems to prevent resonance and/or to improve the effective bandwidth of operation. Damping value can be made online adjustable by utilizing a variable-orifice valve to alter the resistance to the flow of a conventional hydraulic fluid damper [14]. Most noticeably, electro-rheological or magnetorheological dampers feature a broad-bandwidth modulation of the damping characteristics through the variation in the electrical or magnetic field [15][16][17][18].Compared with mechanical or hydraulic devices, electromagnetic devices possess some unique advantages such as cleanness, faster response time, no mechanical contact, easy integration with control system, compactness, etc. Therefore, the use of electromagnetic means in vibration control has been receiving a considerable attention. The active mass damper used in [19] was formed by a coil and a pair of permanent magnets (PMs). In [20], a voice coil motor was used to obtain an active vibration absorber. In [21], a shunted electromagnetic transducer was c...

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