In this paper, the characteristics of a hybrid regenerative electromagnetic (EM) damper are first determined and experimentally examined. The main idea is to have two modes of operation for the EM damper, namely passive energy harvesting and semi-active modes. In the passive mode, the vibrational energy of an underlying structure is harvested and stored in a rechargeable battery. The harvested energy can then be employed in the semi-active control mode to supply the power demand for the required sensors and microcontroller. This hybrid damper would thus be capable of realizing the characteristics of a selfpowered EM damper. A prototype of the damper was designed and tested under different harmonic excitations. The mechanical and electrical characteristics of both passive and semi-active modes were investigated and verified. The average harvested power and current were measured, and the efficiency of the different elements of the damper is determined. Next, for tuning the semi-active mode, a sliding mode control algorithm was proposed which considers the inherent nonlinear parasitic force of the EM damper. The proposed algorithm aims to track the response of an optimally controlled structure, by having knowledge of the bound of the nonlinear parasitic force. Finally, the effects of the proposed damper and sliding mode controller for vibration mitigation of a small-scale structure is demonstrated through a series of shake table tests, under harmonic and random excitations.

Introduction

The physical principle behind electromagnetic (EM) transducers is in converting mechanical into electrical energy and vice versa. Depending on the conversion interface, they can be used as actuators, dampers, sensors, or energy harvesters. When employed in a vibration control system, EM dampers have the advantage of recovering the vibrational energy which needs to be removed from the structure. By applying harvesting techniques, this energy can be harvested and stored in the form of electrical energy. In comparison, most of the other types of dampers, such as viscous, magnetorheological (MR), and frictional dampers, would eventually dissipate the absorbed energy as heat. In the past few decades, there has been a surge of interest in energy harvesting from the vibrational source induced by an environmental disturbance. In civil engineering, these harvesters can be small-scale oscillators attached to a host structure which have a negligible damping on the underlying structure [1e4], or large-scale regenerative dampers that can accommodate both damping and harvesting features [5,6]. Either employed in a tuned mass damper [7e11] or as a single device [12e15], these harvesters aim to harness the damped energy and to power the sensing and processing units of a structural monitoring and control system. It also seems feasible to engage regenerative dampers with a semi-active control action as their power demand is lower [8,9,11,16]. For an EM damper, no additional power is necessary to generate the damping force and its equivalent damping can be adjusted using a simple switched circuit [8]. However, an MR damper requires an external power supply to the current driver in order to generate the magnetic field. Researchers proposed smart regenerative dampers by attaching an MR damper to an EM harvester [17e19]. In those cases, damping is mainly produced by the MR damper, rather than the EM harvester. Alternatively, incorporation of the damper and harvester in a single EM device could be more efficient. Moreover, using an EM damper has this advantage that the vibrational energy can be recovered outside the machine. This reduces the problem associated with self-heating of the damper [14].