Designing Optimized Mechanical Structures for Body-Based Piezoelectric Harvesting

Investigators - Xiaokun Ma, Dr. Christopher D. Rahn

Sponsors - NSF

Energy harvesting from human motion has unique challenges. Typical base excitations have low frequency (<10Hz) and low amplitude (<1g). Shock inputs have broad band (not tonal) frequency distribution and can damage structure. This project takes a model-based approach to the design and optimization of mechanical structures for body-based piezoelectric energy harvesters. Our goal is to design small devices (on the order of cm²) which operate at low frequency excitation with an efficient mode shape.

A piezoelectric compliant mechanism (PCM) energy harvester is designed, modeled, and analyzed that consists of a piezoelectric unimorph clamped at the base and attached to a compliant mechanism at the tip. The compliant mechanism has two flexures that amplify the tip displacement to produce large motion of a proof mass and a low frequency first mode with an efficient (nearly quadratic) shape. The compliant mechanism is fabricated as a separate, relatively rigid frame with flexure hinges, simplifying the fabrication process and surrounding and protecting the piezoelectric unimorph. Experiments with a fabricated PCM energy harvester prototype show that the compliant hinge stiffness can be carefully tuned to enforce a quadratic boundary condition, approaching the theoretical high power output and mode shape efficiency.

The bridge structure of the PCM also introduces an axial tensioning nonlinearity that self-limits the response to large amplitude impacts, improving the robustness of the device. A nonlinear model of the PCM energy harvester under large base excitation is derived to determine the maximum power that can be generated by the device. Experiments show that the compliant mechanism introduces a stiffening effect and a much wider bandwidth than the proof mass cantilever design. The PCM outperforms the cantilever in both average power and power-strain sensitivity at high accelerations due to the PCM axial stretching effect and its more uniform strain distribution.