Multi-converter piezoelectric harvesters based on mono-axial and bi-axial configurations are proposed. The harvesters exploit two and four piezoelectric converters (PCs) and adopt an impinging spherical steel ball to harvest electrical energy from human motion. When the harvester undergoes a shake, a tilt, or a combination of the two, the ball hits one PC, inducing an impact-based frequency-up conversion. Prototypes of the harvesters have been designed, fabricated, fastened to the wrist of a person by means of a wristband and watchband, and experimentally tested for different motion levels. The PCs of the harvesters have been fed to passive diode-based voltage-doubler rectifiers connected in parallel to a storage capacitor, Cs = 220 nF. By employing the mono-axial harvester, after 8.5 s of consecutive impacts induced by rotations of the wrist, a voltage vcs(t) of 40.2 V across the capacitor was obtained, which corresponded to a stored energy of 178 μJ. By employing the bi-axial harvester, the peak instantaneous power provided by the PCs to an optimal resistive load was 1.58 mW, with an average power of 9.65 μW over 0.7 s. The proposed harvesters are suitable to scavenge electrical energy from low-frequency nonperiodical mechanical movements, such as human motion.
Auxetic structures are attracting increasing interest because of their large variety of applications at both the macro and micro scales. In this work, an innovative auxetic unit cell is presented, numerically studied, fabricated at the microscale and experimentally tested. Its versatility in terms of stiffness and Poisson's ratio and its compatibility with commercial Micro Electro-Mechanical Systems (MEMS) fabrication processes suggest applications such as motion conversion and amplification mechanism in MEMS devices or on-chip biaxial tests. A MEMS auxetic and periodic structure exhibiting an overall equivalent Poisson's ratio lower than −1 is then proposed. The static and dynamic behaviours are studied both numerically and through a simplified analytical model. Experimental results confirm the simulated behaviour of the structure thus verifying the first MEMS auxetic structure with electrostatic actuation and capacitive readout.
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