Experimental investigation on a Fe-Ga close yoke vibrational harvester by matching magnetic and mechanical biases

M Zucca,P Rasilo,S Palumbo

doi:10.1016/j.jmmm.2018.08.085

Abstract

The output power generated by a vibrational magnetostrictive energy harvester depends on several parameters, some of them linked to the mechanical source, as vibration amplitude and frequency, others related to design quantities, like mechanical preload, magnetic bias, coil turns and load impedance. Complex models have been developed in literature to reproduce the behavior of these devices. However, for output variables such as power and voltage, one moves in a space of many variables and it is not trivial to reconstruct an overall behavior of the device.The aim of this paper is to provide a wide picture concerning the device behavior investigating experimentally the output power and voltage as a function of the mechanical and especially magnetic bias, varying the amplitude and frequency of the driving vibration. A galfenol rod (Fe81Ga19) sample inserted in a three-legged magnetizer is utilized to vary the magnetic bias and to provide the flux closure to the sample, while a dynamic test machine provides both the mechanical bias and the driving vibration at different frequencies up to 100 Hz. The paper analysis has highlighted that the output power and voltage depend on the magnetic bias according to an exponentially modified Gaussian distribution. Keeping constant the other parameters and varying the mechanical bias, a family of modified Gaussian distributions is obtained. Moreover, fixing the electric load, the amplitude and frequency of the vibration, the couple of values “magnetic bias – mechanical preload” corresponding to the maximum output power of the device depicts a linear behavior.The results here obtained point out that it is possible to simplify the design of magnetostrictive energy harvesters and to obtain high output power even with permanent magnets providing a relatively small coercive field. The results have been confirmed by using two yokes equipped with permanent magnets on the external columns. The maximum output average power obtained with permanent magnets has been 796 mW equal to 6.5 mW/cm3 with a sinusoidal vibration amplitude of 40 MPa at 100 Hz.

Highlights

Energy harvesters (EHs) represent an ideal energy supply for wireless sensors, and especially for microelectromechanical systems, since they are able to generate electric energy using sources generally untapped [1]
A first step was achieved by fixing the mechanical preload and by analyzing the output power versus the magnetic bias
This paper aims at deepening the analysis of the effects of magnetic field bias correlated with mechanical prestress on the performances of a direct-force galfenol harvester fitted with a close magnetic circuit, an aspect up to now less discussed in literature

Summary

Introduction

Energy harvesters (EHs) represent an ideal energy supply for wireless sensors, and especially for microelectromechanical systems, since they are able to generate electric energy using sources generally untapped (i.e. the exhaust heat or the vibrations generated by an engine) [1]. A comparison between galfenol and Terfenol-D [5] shows the better performances of the first one in vibrational energy harvesting applications where the mechanical excitation vibrational frequency is lower than 100 Hz. In addition, Fe-Ga provides a good compromise between magnetoelastic properties and workability. The magnetic bias is provided by a permanent magnet (PM), but few papers discuss the role of this parameter and none do it extensively. In [18], the output voltage of a PM unimorph energy harvester is experimentally analyzed as a function of a variable magnetic bias given using 0, 1, 2 and 3 PMs. In [19] the PM of a cantilever transducer is chosen by analyzing the magnetic bias effect through a finite element approach, based on experimental fieldmagnetization characteristics drawn as a function of stress

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