Piezoelectric energy harvesting technologies have received a great attention during the last decade to design self-powered microelectronic devices such as wireless sensor nodes (WSN). Piezoelectric energy harvester (PEH) produces maximum power when its natural frequency closely matches the excitation vibration frequency. In most of real-time applications, frequency of the vibration source is not constant but it is time-varying. Hence, the piezoelectric energy harvester has to be "self tunable" in order to guarantee that its natural frequency closely matches with the excitation frequency....