Thomas A. Berfield

Dynamic Analysis of a Bi-stable Buckled Structure for Vibration Energy Harvester

Vibration energy harvesting offers a viable alternative to batteries for powering sensors in remote locations. In the past decade, the energy harvesting community has turned to nonlinear structures as an effective means for creating high-performance devices. In particular, researchers have used buckled structures to improve vibration scavenging power production at low frequencies (<100 Hz) and to broaden device operational bandwidths. To achieve these ends, accurate structural models are needed. These models are critical for carrying out a systematic and quantitative device design process. Specifically, the models enable the user to optimize device geometries, arrive at meaningful estimates of power production, and estimate device lifetimes, etc. This work focuses on the dynamic behavior of a bi-stable switching energy harvester made from a buckled beam structure, coupled to two cantilever beams with tip masses via a torsional rod. Results from experimental testing of the energy harvesting structure under different forced vibration conditions are compared with a nonlinear model created of the structure. For the model, linear equations of motion for free vibration of each component have been derived using Hamilton’s principle, and shape functions for each individual component are determined by applying boundary conditions for the linear vibration. Nonlinear dynamic behavior effects are integrated through consideration of large deformation of the main beam. The effects of different parameters on the vibrational system, including the geometry of the structure, buckling load and natural frequency of the cantilever arms, have been investigated. These parameters can play an important role in the optimization process of energy harvesters. Finally, parametric results obtained from the presented method are compared with the experimental data in different aspects.

In-Situ Characterization of Strain in Lithium Ion Battery Anodes

Anode material expansion and cracking is a well-known issue with high-capacity, rechargeable lithium ion battery systems. Substantial strains develop within the anode during both the lithium ion infusion and removal processes. In this work, a custom configuration of the standard CR2032 coin cell battery is used to allow in-situ monitoring of in-plane strain development within the anode via digital image correlation. An anode thin films consisting of amorphous silicon deposited on a metal substrate is tested to determine the influence of film adhesion and battery cycling parameters on the strain-to-failure behavior.

Mechanics of Buckled Structure MEMS for Actuation and Energy Harvesting Applications

Buckled structures offer many great benefits to microelectromechanical systems (MEMS), using naturally occurring residual stresses to provide structures with switchable stable states capable of large transverse deflections. In this work a simple, circular bi-layer diaphragm style of buckled MEMS devices is discussed. The buckling behavior of the system, including buckling height and switching criteria, is modeled and analyzed in ANSYS, then compared with theoretical equations and experimental measurements. Results of this work will help to yield optimal design parameters for both energy harvesting and actuation MEMS applications.Copyright

Mechanical and Thermal Characterization of Fused Filament Fabrication Polyvinylidene Fluoride (PVDF) Printed Composites

Polyvinylidene fluoride (PVDF) is a polymer that offers a variety of desirable material properties. Its high resistance to corrosive acids and its capability to show piezoelectric behavior are some of these properties that are attractive to many industrial applications. Three-dimensional printing of PVDF is extremely difficult using fused filament fabrication processes due to the large coefficient of thermal expansion of homopolymer PVDF, which results in substantial component warping. In the present work, the effect of zirconium tungstate microparticles as a secondary phase within a PVDF matrix is experimentally studied. Viable printing parameters and the corresponding mechanical and thermal behavior of the PVDF composite structures based on digital image correlation tests are presented.

Study on the Fabrication Process of a MEMS Bistable Energy Harvester Based on Coupled Component Structures

Due to the rapid growth in demand for power for sensing devices located in remote locations, scientists’ attention has been drawn to vibration energy harvesting as an alternative to batteries. As a result of over two decades of micro-scale vibration energy harvester research, the use of mechanical nonlinearity in the dynamic behavior of the piezoelectric power generating structures had been recognized as one of the promising solutions to the challenges presented by chaotic, low-frequency vibration sources found in common application environments. In this study, the design and performance of a unique MEMS-scale nonlinear vibration energy harvester based on coupled component structures and bi-stable states are investigated. The coupled-components within the device consist of a main buckled beam bonded with piezoelectric layers, a torsional rod, and two cantilever arms with tip masses at their ends. These arms are connected to the main beam through the torsional rod and are designed to help the main beam snap between its buckled stable states when subjected to sufficient vibration loading. The fabrication of the device will be discussed, including use of plasma-enhanced chemical vapor deposition (PECVD) of silicon nitride under an alternating power field to control compressive stress development within the main buckled beam. After completing the fabrication process, the next step would be testing the device under a variety of vibration loading conditions for its potential use as a vibration energy harvester.

Constraint Effects on Torque-Actuated Bistable Energy Harvesters

Abstract The effect of boundary conditions on a bistable device that buckles into an “S” shape and utilizes polyvinylidene fluoride is evaluated. Four permutations with different center constraints are the perfectly pinned circular steel, rigid glued circular steel, 3×1 and 5×1 3D printed rectangle polylactic acid prototypes. Using a load of 30 MΩ, which was close to the optimal load resistance, frequency sweeps in the forward and reverse directions indicated different nonlinearities depending on if the device is buckled or not. Peak resonant frequencies for the devices are around 18 to 30 Hz with bistable actuation occurring as low as 0.3 grms. Damping was measured using logarithmic decrement, linear and nonlinear half-power methods. Results showed that the devices have an average damping ratio of 4.1%. The buckled 3×1 mm device with short compliance arms generated 12.6 μW at 21.1 Hz when swept forward at 0.4 grms and had the highest figure-of-merit (FoMBW) metric of all devices tested. Unbuckled devices tended to exhibit a spring stiffening nonlinearity while buckled devices obtained higher power outputs in the forward direction but could have their operating frequencies significantly lowered if swept backward. All buckled devices tested during a chirp input could be promoted to high-energy orbits for enhanced performance.