On the internal structure of piezo-electric devices: Closed and Open Loop Optimal Strategies

Piezo-electric devices possess the ability to produce high voltages without load. However, to harvest this energy, the complex and capacitive internal structure must be deal with. This capacitive behavior makes the AC-DC conversion too involved. If a buck-boost topology is to be designed to regulate the DC output, the discontinuous mode provides constant input impedance for small amount of power. However, the input impedance of a buck-boost is a function of the switching frequency and inductance, bounding the maximum values. This open-loop strategy is adequate for small size PCB applications, for instance in energy harvesting applied to sensors, whereas, closed-loop and optimal control algorithms will improve the output power. In this paper a closed-loop optimal control algorithm to work with any source of electrical input voltage in a buckboost converter is considered. This optimal control yields a much bigger output power when compared to the case of non-optimal control. With piezo-electric devices, the output power harvested shows a significant improvement, thus mitigating their natural drop in open loop. Some simulations as well as comparisons with real measurements using two different kind of commercial piezo-electric devices are presented. On the other hand, the internal structure of piezo-electric generators is explored using an open loop optimal control strategy along with an intermediate active circuit. Following the reading of the measurements and the theory developed in this paper, conclusions and future work are provided. Key-Words: Closed-Loop, Optimal Control, Piezo-Electric, Internal Structure, Buck-Boost, Discontinuous Mode.