Optimisation of a Hybrid Energy Scavenger with Piezoelectric/Magnetic Coupling for Sensor-Bearing Units

Abstract

Vibration monitoring based on wireless distributed sensors is currently used in steelmaking plants to early detect structural damage occurring in the rolling mill components. This approach allows overcoming some severe limitations of access to those industrial equipments, but sensors need a local power supply. Vibration energy harvesting based on piezoelectric materials is therefore proposed for this purpose. Nevertheless, very often it happens that dimensions of the energy scavenger are incompatible with the size of the system, thus not allowing a perfect tuning of its resonance upon the frequency of the dynamic excitation. Moreover, sometimes the amplitude of vibration is too low to induce a sufficient amount of energy conversion. Those problems motivated a previous work of the author, about the feasibility of plucking the flexible structure through either a relative motion or rotation of the harvested system and the energy scavenger, respectively. To avoid the drawbacks due to the wear in plucking the material, a contactless electromechanical coupling was proposed. The interaction between two permanent magnets, being one applied to the scavenger tip and the other fixed, was used to excite the vibration and the electromechanical conversion through the piezoelectric layer. The effectiveness of such hybrid system composed by the structure with surface bonded piezoelectric layers and the couple of magnets was investigated and compared to the power requirements of some sensors currently used to measure the dynamic response of the backup roll bearings located at the outer crown of the rolling mill. An optimisation of the whole device to increase the overall performance is proposed by following some approaches assessed in the literature and tested on some specimens of energy scavenger. The optimisation activity was based on a suitable selection of the piezoelectric material aimed at reaching the highest electromechanical coupling with a good mechanical strength and on a suitable shaping of the electrode surface aimed at assuring the largest efficiency in the energy conversion.