Synchronicity and pure bending of bimorphs: a new approach to piezoelectric energy harvesting

摘要

Kinetic energy harvesting with piezoelectric bimorphs has attracted considerable research interest in recent years. Many works have been dedicated to the modelling and optimisation of the cantilevered geometry to increase power density, bandwidth, etc. The increased efficiency coming from the use of trapezoidal beams has been recognised, but little has been done to produce the same uniform strain within the most commonly available rectangular beams. This work proposes a new approach via a compliant rotational structure which permits to deform a set of bimorphs in pure bending. When applied to a harvester with multiple bimorphs, since their deflections are synchronous, the power signals produced are in phase and power conditioning is simplified and made more efficient. The kinematic requirements for uniform strain are discussed, the novel structure is proposed and modelled with finite elements, a prototype is presented and characterised to support the modelling. Modelling shows that the proposed structure induces almost perfectly uniform strain in the piezoelectric beams for all useful rotation angles, demonstrating that, compared to a traditional cantilever, twice as many charges can be produced when the same maximum strain is applied to the material. Experiments with a prototype having sectioned electrodes permitted to demonstrate that the tip, almost inactive in the traditional cantilever configuration, can be made to generate even more charges than the root. Experiments with a step in applied torque simulated plucking excitation and confirmed both synchronicity and the benefits of pure bending. Furthermore, excitation by base-vibration at 56.7 Hz and 5g acceleration produced 3.4 mW in the bimorph subjected to pure bending and 1.3 mW for the reference one. The principle of synchronous pure bending via helper structures can be applied in general to increase the performance of piezoelectric energy harvesters.