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Title page for ETD etd-12182013-005718

Type of Document Dissertation
Author Hofacker, Mark Elliot
Author's Email Address mark.e.hofacker@gmail.com
URN etd-12182013-005718
Title Model-Based Design and Experimental Validation of Multi-Domain Dynamic Energy Conversion Devices
Degree PhD
Department Mechanical Engineering
Advisory Committee
Advisor Name Title
Eric Barth Committee Chair
Kenneth Pence Committee Member
Michael Goldfarb Committee Member
Pietro Valdastri Committee Member
Robert Webster Committee Member
  • free piston engine compressor
  • Stirling thermocompressor
  • bridge vibration energy harvester
  • dynamic energy conversion
Date of Defense 2013-12-11
Availability unrestricted
This dissertation presents the unique design and control of three energy conversion devices. A prototype bridge vibration energy harvester, a free-piston engine compressor, and a Stirling thermocompressor were modeled, designed and constructed. Although these projects differ in many important ways, this dissertation describes how to cast widely different energy conversion devices, such as these, into a common impedance matching framework.

This framework is first used to describe the design and control of a bridge vibration energy harvester meant to power bridge health monitoring electronics. Impedance matching considerations were applied to the mechanical design of a low friction 1-DOF mechanism and to a control law derived using the maximum power transfer theorem. The harvester’s dynamics were cast as a Thevenin equivalent circuit and an unstable, canonical controller that harvests the maximum power from every frequency was derived by taking the complex conjugate of the circuit’s multi-domain impedance. An implementable, stable controller was found through constrained optimization and is shown in simulation to improve performance over an equivalent, passively controlled device.

The second application of this framework is a free piston engine compressor intended to serve as an untethered pneumatic power supply for a compact rescue crawler robot. A prototype device is presented that makes improvements over previous iterations including a self-balancing, figure-8 liquid piston configuration, onboard electronics and control, a finite state control scheme, and an improved compressor head. The advantages of the check valve’s dynamics and the figure-8 piston configuration are proven mathematically. These improvements result in a low-vibration, stand-alone device that experimentally demonstrated a 60% increase in pumping pressure over previous iterations.

The third application of this framework is a Stirling thermocompressor intended to serve as a quiet, untethered, pneumatic power supply for an ankle foot orthosis. The goal of high efficiency at the target power density is pursued through the use of novel heat exchangers in combination with high operating temperature and frequency. The motion of the displacer piston is controlled utilizing a brushless DC motor which drives a continuous linear reciprocating screw. A dynamic model of the heat transfer and pressure dynamics portions of the thermocompressor are developed and experimentally validated.

Although the bridge vibration energy harvester, free piston engine compressors, and Stirling thermocompressor are quite dissimilar, this dissertation describes how to cast widely different energy conversion devices into a common impedance matching framework. Each of the devices presented in this document emphasize different aspects of the three major conceptual components: the energetic source, the source impedance and the load impedance. By considering the relevant conceptual components for each device, insights were gained into the fundamental mechanisms needed to transfer energy across energetic domains.

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