GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 659 device. It is well proven that non-linear VEHs present a larger and flatten frequency response than systems based on linear oscillators (Cottone et al. , 2009; Zhu et al. , 2010). In addition, these transducers, can be used both as generators or totally passive sensors. In this way we can reduce the size, weight and increase the energy autonomy. In terms of size reduction, as an alternative to inductive-based vibration-to-electricity converters, electrostatic and piezoelectric systems are now widely employed both for sensing and energy harvesting. The use of different transduction scheme allows freeing the magnetic interaction for applying further control of the response function of the device. Such devices, as we scale down size and increase the stiffness of the elastic elements, could have RFrs too high to detect seismic vibrations. We believe that such drawbacks could be effectively overcome by using softening techniques to implement a nonlinear bistable oscillator. Cottone et al. (2009), for example, using magnets in opposition has driven the potential of oscillating beam from a classic oscillator behavior, to a softened one, up to a bistable potential as a function of the magnetic interaction strength (Fig. 1a). In order to reduce the RFr, the best condition is the one where the potential is approximately flat around the equilibrium position, as the effective spring constant is softened. Fig. 1 - View of the a) schematic of the working principle and b) experimental setup with piezoelectric cantilever. We aim at applying this concept in a device based on a cantilever beam with a probing mass on the tip. As the beam is anchored to the sensor case, the seismic vibration induces an inertial force acting on the probing mass. Its movement, which can be measured by piezoelectric transduction scheme, provides an effective detection of the seismic waves. The dynamical response of the oscillating system can be changed from linear to nonlinear bistable by varying the distance between opposing magnets, one on the tip of the beam and one connected to the case. As illustrated in Fig. 1b, our first sensor is characterized by a piezoelectric beam from Midé Volture (V21BL) with size of 83 x 14 x 0.78 mm 3 . Two cylindrical Neodymium (NdFeB) magnets of 3 mm of diameter are mounted in repulsive configuration on the tip of the cantilever and on micrometric linear stage. The system is fixed onto a shaker fromTIRAGMBH controlled by a function generator and an accelerometer anchored at the vibrating base. The output voltage generated by the piezo beam is then acquired by a National Instrument DAQ card. In order to test the working principle of this system, we have first reproduced numerical simulations of the concept in order to evaluate the frequency response of the nonlinear piezoelectric oscillator. The output voltage has been simulated by performing numerical integration of the governing differential equations of the piezo system under vibrating white noise at various distance of the magnets (Fig. 2). It results that for large distance (Fig. 2a) the system behaves as linear oscillator, whereas for close distance (Fig. 2c) the system is constrained to vibrates in one of the minima of the double-well potential. As a particular optimal distance