Monolithic 2-axis in-plane PZT lateral bimorph energy harvester with differential output

We report a 2-axis (X-Y) piezoelectric energy harvester, whose sensitive axis in-plane is rotationally invariant, a result achieved by spiral in-place bimorph, which can be modeled as a cascade of lateral bimorphs. This is different than conventional piezoelectric energy harvesters that are sensitive only along one axis or can realize multi-axis sensitivity through package level assembly of multiple devices at different orientations. Rotational invariance of the sensitive axis is useful when deployed in applications where the device orientation with respect to the vibration is stochastic in nature. At 1-g applied acceleration, our device generates maximum differential voltages of ~ ±6.8Vpp across a 1MΩ load, at a resonance frequency of ~163.5Hz. The harvester has a Q-factor of 129.5 and electromechanical coupling coefficient of k_t² ~3% for the fundamental lateral mode and a peak normalized output power density of ~0.89μWatt/mm³g^-2.