###### Abstract  
The power generation capability of an electromagnetic vibration energy harvester augmented with an additional coil was investigated and compared with that of a standard single coil electromagnetic energy harvester. A single degree of freedom model and the corresponding equivalent electric circuit were employed for the analysis of the standard and of the augmented harvesters. The harvester model was validated by means of an accurate experimental characterization of a commercial electromagnetic harvester, i.e., the model-D by ReVibe. The electric circuits for the standard and for the augmented harvesters were implemented by electronic components and experimentally tested to determine the maximum power they are able to generate in four test conditions. Results from simulations and from experiments showed significant improvement of the power extraction performance exhibited by the double coil energy harvester, particularly at frequencies lower than the harvester mechanical resonance frequency.  
energy harvester; electronic optimization circuit; micro power generators; Internet of Things device's supply +
Footnote †: journal: Applied sciences  
non-linear harvesters [15; 16], and various electrical or mechanical tuning techniques [17]. Electrical tuning techniques aim to match the harvester with its optimal load impedance, frequency by frequency, by using properly controlled active power electronic circuits according to the maximum power transfer theorem [18; 19]. Mechanical tuning techniques attempt to regulate some mechanical parameters to adapt the resonance frequency to the vibration frequency [20]. In particular, the resonance frequency of the device can be adjusted by rotating a spring [21], or by exploiting a magnetorheological elastomer [22], a magnetic force [23; 24] or a suitable actuator [25; 26]. Among the tuning techniques, the idea presented in [27] and based on a double coil electromagnetic harvester seems very attractive for its potential.  
Thus, in this paper the performance of a resonant electromagnetic energy harvester characterized by a double coil is investigated and the power extraction capabilities of this harvester are compared with those of a standard single coil resonant electromagnetic harvester. The latter device exploits the time-varying relative displacement between a magnet and a coil caused by the ambient vibrations to generate electrical energy. On the other hand, a double coil electromagnetic energy harvester is equipped with an additional coil, which is electromagnetically coupled with the magnet and is in relative motion with respect to both the magnet and the primary coil.  
Here, the double coil electromagnetic harvester was modeled as a single degree of freedom system and its equivalent electric circuit was exploited to predict by simulations the maximum power that the harvester was able to generate. Moreover, experimental tests were performed on a commercial single coil electromagnetic harvester, the model-D by ReVibe, in order to characterize its behavior and to extract the parameters of its single degree of freedom model. Such a model was used to implement the equivalent electric circuits of both a single coil and of a double coil harvester, by using electronic components mounted on a solder-less breadboard. Then, experimental tests were performed in order to compare the power delivered to their optimal loads by the two types of harvester. Test results showed that the double coil harvester was able to significantly increase the power efficiency at frequencies lower than the mechanical resonance frequency.