I. Introduction
Electromagnetic resonant Vibration energy Harvesters (EVHs) [1] - [3] are designed to convert into electricity the mechanical energy of ambient vibrations. Due to the wide availability of vibrations into the environment [4] –[6], EVHs are particularly attractive for supplying sensors of Wireless Sensor Networks in IoT applications for the monitoring of transportation vehicles, of industrial plants (Industry 4.0), of the environment and of smart cities [7] –[9]. Since EVHs are AC sources, while sensors are DC, properly controlled AC/DC electronic interfaces are needed between EVHs and loads [10] –[12]. Therefore, an EVH based complete system is composed by a mechanical subsystem (the EVH itself) and an electrical/electronic subsystem (the power conversion and control circuitries). Due to the low power density of EVH systems, a key aspect in their practical application is represented by the efficiency of the conversion process from the mechanical to the electric energy [13] –[15]. Such an efficiency depends, apart from the characteristics of the mechanical subsystem, also on the Maximum Power Point Tracking (MPPT) efficiency [18] and on the efficiency of the conversion stage, that is on the efficiency of the electrical subsystem [19] - [21]. is the efficiency of the adopted MPPT control algorithm in tracking the time varying MPP and is given by the ratio between the energy Eact that is actually extracted and the maximum energy Etheor that could be theoretically extracted by the EVH at its coil terminals. is instead the ratio between the energy Eload that is provided to the DC load and the energy depends on the adopted power stage topology, the chosen switching frequency, the characteristics of the used switching and linear components, the shape and values of the current and voltage waveforms etc. Without going into details, it can be easily guessed that both the above efficiencies can be only very roughly estimated by means of numerical simulations, but to get reliable information, experimental characterizations are mandatorily needed. This means that the availability of the EVHs to test together with the chosen AC/DC controlled interfaces and the availability of suitable shaker devices is mandatory in the laboratory, to carry out the experimental activity that is necessary to get the desired efficiency information on the electrical subsystem. Moreover, due to the resonant nature of EVHs, it is nearly always necessary to adapt the electromechanical characteristics of EVHs to those ones of the vibration spectra that are encountered in the multitude of the possible practical applications of interest [22] –[24]. This means in practice that, since it is very difficult to adjust the electromechanical parameters of a given EVH after its manufacturing process, a great number of EVHs must be available or must be built to test the efficiency of electrical subsystems in different practical applications of interest.