Graduate student Brandon Harkhu will be defending his thesis titled “Additively Manufactured Planar Spiral Springs for Nonlinear Stiffness in Miniature Axial Flux Energy Harvesters.”

Brandon Harkhu thesis defense

• Date: Tuesday, April 21
• Time: 12:30-1:30 p.m.
• Location: IST 1049
• Major: Mechanical engineering
• Thesis committee: Dr. Edwar Romero-Ramirez (advisor), Dr. Gerardo Carbajal and Dr. Chris Kelley

Abstract

Harmonic oscillators and electrical transduction methods as a joint system can be used to form an energy harvester to collect energy from forced mechanical motion. This study investigates a pendulum rotating about a translating axis; where, energy can be harvested from vibration of the rotation axis.

By constraining a magnetic rotor and planar stator along the axis of the pendulum displacement, harvested energy becomes proportional to angular displacement of the pendulum. Harmonic oscillators produce maximum displacement when vibrating at the natural frequency of the system, at resonance.

A nonlinear torsional spring is coupled to the rotating pendulum axis to alter the behavior of the oscillatory system. The nonlinearities present in the spring can alter the sharp, linear resonance range to a wider, nonlinear range, such that more energy is harvested at broadband frequencies.

A mathematical model is developed using the Lagrangian mechanics approach to derive the equations of motion for the vibrating system. A numerical study is conducted in MATLAB to theoretically predict the system’s outputs, such as angular displacement and induced voltage along a sweeping frequency. Experimental testbeds are developed to validate the model; the first investigates the effect of geometry and material on the nonlinear spring response and the second investigates the effect of nonlinear springs on the performance of the energy harvester.

This work finds that using a logarithmic spring path provides the most nonlinear spring response with a constant and tapered cross-sectional area in the static case and varies in softening and hardening behavior depending on the chosen polymer material color. Dynamically, it is found that the spiral springs induce nonlinear behavior on the harvester output as frequency sweeping in different directions yields distinct resonant behavior.

For more information, please contact Brandon Harkhu.