Design Development, and optimization of electromagnetic vibration Energy Harvesting Device

Abstract

In this thesis report, an account of research work carried out on the design, development, and optimization of the power output of a vibration-based electromagnetic energy device (VBEH) is presented. The topic selected is based on the research gaps identified in an exhaustive literature review conducted in the area of energy harvester in general and vibration-based energy harvester, in particular. VBEH transforms ambient vibration energy (KE) into electrical energy. A laboratory model of Single Degree Freedom Vibration-Based Electromagnetic Energy Harvester (SDOF VBEH) has been designed and developed, which consists of a mechanical system of spring-mass-damper connected to an electrical load circuit through a vibration transducer. The vibration transducer comprises of high residual flux density cylindrical permanent magnet which oscillates in a copper coil and produces electro-motive-force (EMF) across the coil terminals to which the electrical load circuit is connected. The design and development of the copper coil has been carried out as per coil design guidelines, and proper copper fill factor and high residual flux density rare earth magnetic material Neodymium Iron Boron (NdFeB) has been selected as an oscillating magnet. The harvester mass of the VBEH is suspended on a helical compression spring, and the other end of the spring is connected to the base of the VBEH device. The vibration transducer is placed parallel to the suspension spring such that the transducer magnet is connected to the harvester mass and the coil is connected to the base facilitating the in-line relative displacement between magnet and coil to convert vibration KE energy into electrical voltage. An experimental test is a setup with the necessary instrumentation for the performance analysis of the developed model of SDOF VBEH. The sensors for i) measurement of harvester mass displacement response and the VBEH base displacement ii) voltage (EMF) across the vibration transducer copper coil and iii) the speed measurement of variable speed electric drive motor have been provided. The base of VBEH is subjected to variable frequency harmonic excitation by a cam-follower mechanism driven by a variable speed DC motor. The cam eccentricity provides the necessary value of the amplitude of excitation. In this experimental test setup, first of all, i) the relative displacement between harvester mass and base, without and with vibration transducer has been measured at various ii frequencies of excitation. Using these results, the values of mechanical damping ratio ζm and electrical damping ratio ζe have been calculated ii) the effect of variation of the values of ζm and ζe on the maximum average power of VBEH is analyzed and iii) the pure resistive load across the transducer coil is varied, and its effect on the maximum average power output is analyzed at various excitation frequencies. From this analysis, it is observed that i) for maximum average power generated from VBEH, the values of ζm and ζe should be equal and ii) the values of ζm should be as less as possible, and for maximum average power harvested from a VBEH, the electrical load resistance should be nearly equal to the internal resistance of the copper coil of the vibration transducer. As such, it is important and necessary to determine ζm and ζe experimentally for the VBEH performance analysis. Hence in the next phase of research, the effect of variation of resistive, inductive, and capacitive load impedances on the maximum average power harvested from VBEH is investigated. In this case, the electrical load circuit parameters viz. resistance (R), inductance (L), and capacitance (C) are so chosen that the natural frequency ωe (𝜔𝑒 = 1 √𝐿𝐶 ) of the electrical R-L-C load circuit is made equal to the natural frequency ωn (𝜔𝑛 = √ 𝐾 𝑚 ) of the mechanical sub-system of VBEH, which in turn, is tuned to the resonant frequencyof the harmonic excitation. The electrical circuit load resistance controls the electrical damping ratio ζe. As such, the effect of combined R-L-C load impedance on the maximum average harvested power is studied using the experimental setup developed for the same. From the experimental results, it is seen that the average harvested power is maximum at the resonant frequency, which is also equal to the natural frequency of the R-L-C load circuit. The maximum values occur when the resistive load is equal to the internal resistance of the transducer coil. This result is significant from the point of view of SDOF VBEH design for the given application. Also, it is observed that the value of electrical damping ratio ζe obtained experimentally is less than that obtained from its analytical expression. As such, it is important to determine electrical damping ratio ζe experimentally for estimation of power harvested from a VBEH when combined resistive, inductive, and capacitive loads are connected to VBEH, especially when the value of electrical damping ratio ζe is very small compared to mechanical damping ratio ζm. These findings are useful for deciding allowable electrical R-L-C load to obtain maximum harvested power from VBEH. iii The power output of an SDOF VBEH is at maximum at or near resonance and over a small frequency band. In order to enhance the power output of traditional SDOF VBEH and to widen its operational frequency band, in the next phase of research, the SDOF VBEH design has been transformed into a Two Degree Freedom Vibration Based Electromagnetic Energy Harvester (2DOF VBEH) to amplify the vibration received by harvester mass by inserting a dynamic magnifier mass in between the base and the harvester mass-springsystem. It is shown that this change in design also helps to widen the effective operational frequency range of SDOF VBEH. For this purpose, an expression for the power generated by 2DOF VBEH has been derived using the approach of Tang and Zuo [28]. The effect of mass ratio µ harvester mass to amplifier mass and frequency tuning ratio f on the effective operational frequency bandwidth be of the developed 2DOF VBEH has been investigated using the expression derived for the effective operational frequency band. Using the experimental test setup developed for 2DOF VBEH, the effect of change in mass ratio µ on the harvested power of the 2DOF VBEH is analyzed. The findings of this investigation will be useful to provide the guidelines for selecting an appropriate value of mass ratio µ and tuning ratio f for the design of a 2DOF VBEH to enhance the power output and widen the effective operational frequency band of a traditional SDOF VBEH. In the final stage of the research, the analysis of the optimization of a power output of a 2DOF VBEH is carried out using the method of surface plots and associated contour diagrams. It is shown that these diagrams will be useful for obtaining the value of the optimal power output from 2DOF VBEH and its effective operational frequency band under a given set of values of mass ratio µ, tuning ratio f, and electrical damping ratio ζe at various excitation frequencies. In the last place, a discussion on the results of research work carried out is taken up, and conclusions are presented