I. Introduction
The last few years, with the fast increase of Internet of Things applications in the Industry 4.0 revolution, have seen an increasing focus on Wireless Sensor Networks (WSNs) [1]. WSNs are usually fed by batteries that have a number of well-known drawbacks. Energy harvesting, that is the conversion of ambient energy into electrical energy, offers an interesting alternative to batteries or, at least, it allows the increase of their life-time, avoiding their frequent replacement. Among the different energy sources studied in the literature of the last years, vibration energy is one of the most attractive for powering WSNs [2]–[4] because it can be found almost anywhere. Although vibration energy harvesters are generally characterized by low power levels, they are able to make WSNs self-sufficient from the energy point of view thanks to the duty cycle based operation [2].