# Development of a Self-Powered Magnetorheological Damper System for Cable Vibration Control
## 1 Introduction  
Due to high flexibility, low inherent damping, and relative small mass, long stay cables are often susceptible to excessive vibrations under various environmental excitations. Large oscillations may result in undue stresses or fatigue failure in cables or connections, which is detrimental to the serviceability and safety of the entire cable-stayed bridge. Hence, it is of great importance to mitigate cable vibrations. Transversely attached dampers near the anchorage of the cable are one of the most common solutions to this problem. However, the performance of passive viscous dampers in mitigating cable vibration is greatly restricted by the small ratio of the distance from the cable anchorage to the damper over the length of the cable [1]. This may cause the supplemental damping induced by a passive damper to be insufficient to eliminate the problematic vibrations of long stay cables without significantly detracting from the aesthetics of the bridge. As a more promising solution, semi-active control has been proposed to enhance performance since it offers the capability of active control devices without the requirement of large power resources [2]. In particular, magnetorheological (MR) dampers have attracted extensive attention fromthe community because of their excellent performance in both lab tests and engineering practice [3; 4; 5; 6; 7; 8; 9; 10]. To date, MR dampers have been implemented for full-scale applications, including the stay cables on the Dongting Lake Bridge [11], Binzhou Bridge [12], and Sutong Bridge [13]. Nevertheless, these control systems need external power supplies and/or sensors/controllers, which seem to be too costly and complex. Energy harvesting from structural vibrations is a potential solution to the power supply problem of MR dampers [14; 15; 16]. In civil engineering, electromagnetic energy harvesting shows superiority over other mechanisms [17; 18], such as piezoelectricity [19], electrostatic generation [20], and dielectric elastomers [21]. Accordingly, a smart damping system, consisting of an MR damper and a linear electromagnetic generator, has been well developed and investigated by a number of researchers [22; 23; 24; 25]. However, the energy harvesting capability of a linear generator is quite limited [26]. In view of energy harvesting efficiency, self-powered MR dampers based on rotary generators have also been proposed to enhance the energy harvesting efficiency [27; 28]. In this paper, a self-powered MR damper system based on a rotary DC generator for cable vibration control is developed and experimentally investigated. It has been shown that two control devices (i.e., an MR damper and a rotary DC generator in this paper) at the same location of the stay cable will deteriorate the vibration mitigation performance [29; 30]. Therefore, unlike previous compact self-powered MR dampers [23; 27; 28], the proposed system focuses on improving the vibration mitigation efficiency of the stay cable with little consideration to device compactness, where the MR damper and the rotary DC generator are separated rather than integrated into one compact device. In addition, a flexible chain-sprocket is selected as the linear-to-rotation conversion mechanism instead of rigid transmission mechanisms, such as the ball-screw [27] or the rack-pinion [31]. Consequently, the rotary DC generator can be installed at a higher location of the cable to provide more power to the MR damper. The paper is organized as follows. First, a new self-powered MR damper system is proposed and constructed to suppress cable vibrations, and the energy harvesting performance of the rotary DC generator is highlighted. Next, vibration mitigation tests of a model cable attached with different kinds of dampers are conducted, including a passively operated MR damper with constant voltage, the electromagnetic damping alone due to the generator, and a self-powered MR damper system. The corresponding identified modal damping ratios in the first four modes of the cable are then compared and analyzed.