# Monitoring of Liquid Viscosity for Viscous Dampers through a Wireless Impedance Measurement System
## 1 Introduction  
Traditional structures are designed to withstand natural disasters, such as earthquakes, strong winds, and tsunamis, by enhancing the seismic performance of the structure itself [1, 2] (e.g., improving strength, stiffness, and ductility). Due to the uncertainties concerning earthquakes and structures, traditional seismic design methods cannot guarantee the safety of the structures under unpredictable large-intensity earthquakes [3]. To reduce the damage caused by earthquakes, various dampers or other vibration suppression technologies are usually applied in buildings or bridges as supplementary approaches to enhance the seismic performance of structures [4, 5]. [PERSON] studied the application of viscous dampers on highway bridges to reduce the bearing displacements and eliminate the isolator damage [6]. The dampers are often combined with the base isolators to improve the seismic reliability of structures [7, 8]. Except passive vibration control technologies, active and semi-active approaches are also developed in civil engineering to provide effective protection in earthquakes [9]. The dampers can substantially dissipate the seismic energy of the structure and mitigate seismic-induced structural damage [10]. [PERSON] used viscous dampers for the seismic reconstruction of steel arch bridges [11]. [PERSON] used base isolation technology to protect highly vulnerable statues and delicate objects [12]. [PERSON] used the isolation technique to reduce the seismic action of the marble sculptures at the Accademia Gallery in Florence, where the sliders were on a small-size double concave curved surface [13]. Viscous dampers have the following characteristics: good energy dissipation effect [14], conveniently adaptable to implement a small amount of earthquake energy dissipation [15];minimal impact on structural stiffness; flexible arrangement, connection methods, easy integration with the building; low maintenance costs [16]. The use of viscous dampers in civil engineering improves the energy dissipation capacity of the structure, enhances the seismic performance of the structure, and helps better meet safety requirements [17]. Viscous dampers dissipate vibration when the motion of the piston inside the damper forces a viscous fluid to flow from one cavity of the cylinder to another [18]. This flow converts the kinetic energy of the viscous fluid into thermal energy and transfers the energy to the external environment. The energy dissipation and damping effect of viscous dampers depends on the viscosity of the damping fluid in the dampers [19]. Over time, the viscosity of the damping fluid will gradually decrease, affecting the effectiveness of the damper [20]. Further decreases in viscosity can eventually lead to the failure of the damper, and prevent it from performing its protective function in the event of an earthquake or arrival of strong winds. Hence, it is necessary to continuously monitor the viscosity of the damping fluid. In recent years, Structural health monitoring (SHM) methods have been increasingly used in civil engineering [21; 22]. The definition of SHM is to monitor the status of the tested structure by using damage free monitoring technology [23; 24] and data analysis means including a variety of digital signal processing [25; 26]. It mainly aims to identify whether there is damage in the structure [27], determine the specific damage location [28], quantify the degree of structural damage, and evaluate the remaining service life of the structure [29]. In addition, piezoelectric ceramics can be used as both sensors and actuators based on the direct and inverse piezoelectric effects [30; 31]. In the electro-mechanical impedance (EMI) method [32], the piezoelectric ceramic transducer acts as both an actuator and a sensor to evaluate the health state [33] of the monitored structure by comparing impedance measurements during its baseline healthy state [34] and measurements after damage has occurred [35; 36].