I. Introduction
In order to increase the safety and to reduce the operating costs of railway vehicles for goods transportation, it is necessary to develop and optimize monitoring and diagnostics functions for such vehicles. In particular, the need exists to design a suitable system (which is composed by an onboard and a wayside part) that is devoted to the monitoring of both the freight wagons health conditions and the status of the transported goods [1]. The working of the onboard part is based on the adoption of a number of different wireless sensor nodes (WSNs). In this paper, we will discuss how to efficiently make self-sufficient (from the energy point of view) such WSNs, by using suitable vibration energy harvesters (VEHs) [2]– [12]. VEHs may be in fact capable to generate, directly onboard, the electric energy that is needed to supply the WSNs since, on freight wagons, an onboard power supply is not available [1]. This paper is focused on the adoption of resonant electromagnetic vibration energy harvesters (REVEHs) for freight wagons applications. Of course, in order to avoid the waste of precious available energy and the consequent need to oversize the REVEHs, the operating point of each system (composed by a REVEH, a power electronics conditioning stage, a storage device, and a WSN) must be properly controlled. In fact, the speed of freight wagons, and hence the associated vibrations, continuously changes during the rides. As a consequence, also the operating point that is associated with the maximum extraction of energy from the REVEH changes and it must be, therefore, continuously tracked. The main aim of this paper is just to present and discuss some useful guidelines which, with reference to freight wagons applications, allow the optimization of the performances of REVEHs-based supply systems for WSNs. In particular, without any loss of generality, a system composed by a specific REVEH, a diode bridge rectifier based ac/dc conversion stage, a battery, and a given WSN is analyzed. A specific experimental activity aimed at the detection of the vibrations that take place on freight trains has been preliminarily carried out. It is worth noting that the application that is analyzed in this paper is characterized by the occurrence of nonsinusoidal time-varying vibrations. Instead, nearly all the papers, which have been published in the literature so far, refer to purely sinusoidal vibrations [2]– [12]. Therefore, no useful guidelines are available at the moment for the optimization of performances of REVEHs-based supply systems for WSNs in freight wagons application. In Section II, the working principle of the REVEHs is briefly resumed and the architecture of the whole system under study is introduced. In Section III instead, the experimental results concerning the characteristics of the vibrations (that is amplitudes, dominant frequencies, dependence on the time-varying speed of the freight wagons) on a typical freight wagon are presented and discussed. The energy requirements of the considered WSN (that is its operating voltage and the time-domain profile of its operating current) are reported in Section IV. Whereas, in Section V, on the basis of the experimental results that are presented in Sections III and IV, the main characteristics of the REVEH that has been considered for the analysis are identified. Section VI is instead devoted to the discussion of the main guidelines leading to the optimization of the performances of the whole system. In Section VII experimental results that validate the numerical analysis are provided. Conclusions end the paper.