Design and experimental verification of magneto-mechanical energy harvesting concept based on construction steel.

Abstract

The development of self-powered system for powering small scale power electronic devices such as wireless networks and nodes, radio frequency based tags or readers and wireless sensors for applications like structural condition monitoring (SCM) and wireless data recording are getting very popular. The integration of vibration based energy harvesters with the above mentioned devices is a promising approach towards self-powered systems. The techniques of vibration based energy harvesting involve utilization of either piezo-electric or magnetostrictive materials. However, the active materials mostly employed in energy harvesters are either too expensive or are not commonly available. The objective of the study is to utilize construction material more specifically structural steel as an active material because of its abundant availability and practical applications in bridges buildings and rail tracks etc. The literature study regarding various energy harvesting techniques and their applications are presented first to emphasize the importance of vibration based energy harvesting. The prototype design of the proposed energy harvester including the design of mechanical grips and magnetic circuit are discussed in detail. Three different test samples are utilized in which two samples are constructed in the form of a stack using 1 mm and 1.5 mm thick steel sheets and the third sample is a solid steel bar with the dimensions of 20 mm x 20 mm. The free length and cross-sectional area of each sample are 100 mm and 400 mm2 respectively. The measurement method developed for single steel tester is utilized and a new method for obtaining magnetization curves is proposed in the study. In order to determine the effect of stress on magnetization curves, the test sample is first stressed statically using AC magnetization to obtain the stress dependent magnetization curves. It is observed that the permeability of the test material changes under tensile and compressive stress showing the stress dependent magnetic characteristic of the material. To experimentally verify the validity of measurement method and the proposed method, the test sample is stressed dynamically using DC magnetization inducing voltage in the pickup coil. The induced voltage is because of the inverse magnetostriction also known as Villari effect. The results from the solid steel sample and the sample made up of steel sheets are compared during cyclic loading. The steel sheet sample does not go into saturation because of the changing magnetic circuit length as well as the air gap caused by the buckling of individual sheets. Whereas, the induced voltage from the pickup coil starts dropping in case of solid sample which shows that the material is reaching saturation. To validate the magnetization curves obtained from the proposed method, the magnetizing current (I) for maximum ΔB (change in flux density) is calculated which is compared with the I at peak amplitude of the induced voltage curve. The results from the calculations do not take into the account the eddy current losses or hysteresis and therefore the measured results deviate slightly from the calculated results. The maximum power is measured at the point of maximum ΔB value by varying the load resistance for two different cases of cyclic loading. The average output power is measured 13.3 μW for cyclic loading from zero to -20 MPa and 8.76 μW for cyclic loading from 2.5 to 25 MPa at 11 Hz of mechanical vibration using 2.62 Ω load resistance.