## 1 Introduction  
Vibration control is commonly used in civil engineering [1; 2; 3; 4; 5], and includes passive [6; 7], semi-active [8; 9; 10] and active approaches [11; 12; 13]. In the passive approach, dampers, such as magneto-rheological dampers, fluid viscous dampers, particle dampers and tuned mass dampers (TMDs), have been utilized to suppress structural vibration and dissipate vibration energy of the controlled structures [14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. A typical TMD device mainly consists of a spring-mass system and a damper, with mainly the spring-mass-damper model usually being used to investigate the characteristics of TMDs [25; 26; 27]. When the frequency of the TMD is tuned to the same as that of the controlled structure, the vibration energy of the controlled structure is transferred to the TMD through the spring-mass system and is dissipated by the damper. Owing to their simplicity and effectiveness, TMDs are widely employed in vibration control for bridges [28; 29; 30; 31; 32], high-rise buildings [33; 34; 35], and so on. However, TMDs are sensitive to frequency detuning, i.e., when the natural frequency of the primary structure is changed, the control performance of TMD will be degraded.  
Recently, eddy current dampers (ECDs) have been proposed to suppress structural vibration, and the control effectiveness of ECDs has been demonstrated both experimentally and numerically [36; 37]. ECDs can generate an electromagnetic force to hinder structural movement and dissipate energy through eddy currents, which are induced when an electrical conductor moves through a stationary magnetic field or vice versa. The damping force provided by ECDs mainly depends on the relative moving speed between the conductor and the magnetic field, the strength of the magnetic field, and the gap between the magnet and conductor [38]. Various applications for the use of ECDs for controlling structural vibrations have been studied. [PERSON] et al. [39] investigated the performance of an ECD on vibration control of a cantilever beam. A theoretical model of the ECD was established based on electromagnetic theory, and this model was validated by experiments. [PERSON] et al. [40] proposed a novel planar ECD that can provide damping forces in two different directions, and the analytical model for simulation of the proposed planar ECD was validated by experiments. [PERSON] et al. [41] applied an ECD to provide additional damping for a tuned mass damper system on the vibration control of a large-scale beam structure, and the feasibility of applying the ECD in the vibration control of large structures was verified. [PERSON] and [PERSON] [42] used an ECD to control the vibration of a footbridge and the analytic results demonstrated that the damping provided by the ECD was enhanced under both harmonic and random inputs.  
More recently, eddy current tuned mass dampers (ECTMDs), which combine a traditional TMD and an ECD, have been proposed to control undesirable vibrations. Based on the theories of mechanics and electromagnetism, the behaviors of ECTMDs have been studied by many researchers [37; 38; 39; 40; 41], [PERSON] et al. [43] applied an ECTMD on a bridge to suppress the wind-induced vibrations that occur in the bridge hangers. [PERSON] et al. [44] investigated the vibration suppression performance of an ECTMD in Shanghai Center Tower, and the experimental results showed that the acceleration resulting from wind-induced vibration was reduced by 45-60%.  
The feasibility and effectiveness of ECTMDs in vibration control has been demonstrated by the aforementioned studies. However, the robustness of ECTMDs has rarely been reported, especially in cases of experimental studies. In this paper, the robustness of an ECTMD against frequency detuning was studied experimentally in cases of both free vibration and forced vibration. Furthermore, the robustness of a traditional TMD was also studied to make a comparison with the ECTMD.