  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 smart compliant structure which permits to
deform a set of bimorphs in pure bending. Furthermore, since the 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.
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. Synchronicity is also
experimentally verified for the prototype, as power signals resulting from
impact excitation are observed to be in phase. The principle of synchronous
pure bending via helper structures can be applied in general to increase the
performance of piezoelectric energy harvesters.
