Chapter 6 Torque Arm Actuated Bi-Stable Buckled Energy Harvester Characterization D.A. Porter and T.A. Berfield Abstract A bi-stable energy harvester utilizing PVDF strips driven via two torque arms with end masses and pseudo pinned in the middle is evaluated. A sinusoidal acceleration is applied to the base of the device with varying frequencies and magnitudes while the compression of the center beam is achieved by applying a small displacement to the center beam. Frequency sweeps will be done forwards as well as backwards to evaluate hysteresis performance. Peak voltages, natural frequencies, snap-through acceleration values, static actuation displacement values, and material properties for unknowns are derived experimentally. While many parametric values such as beam length, compliance arm length, and proof mass can be varied, the focus of this study is on the effects of the compliance arm width on bi-stability switching and energy harvesting potential. For vibration-based energy harvesting, performance parameters such as power generated, power density, frequency broadening, frequency shifting, and optimal load impedance will be quantified. Results show that wider compliance arms decrease buckling amplitude, but increase the bi-stability switching regime and the overall power production. Current data also indicates that an optimal compression load exists for a given acceleration value. Keywords Bi-stable structures • Buckling • Energy harvesting • Piezoelectric materials • Vibrations 6.1 Introduction Vibration-based energy harvesting devices represent an intriguing class of power supplies that may serve as potential replacements for batteries in specific operating environments. In particular, vibration driven energy scavengers offer an alternative power source solution for inaccessible or remote wireless sensors, such as for structural health monitoring applications. One of the primary challenges with these devices is designing them for maximized productivity under low frequency chaotic vibration cases representative of realistic driving conditions. Structures that demonstrate non-linear behavior have been shown to respond quite efficiently over a broad range of driving frequencies [1–3]. Pairing these types of nonlinear structures with a piezoelectric material or other energy production mechanism (electrostatics, etc.) allows for energy production that is based on strains induced within the composite system under transient deformations. Nonlinear behavior can be produced through a dual energy well arrangement using magnetic fields [2–5], or mechanically by using a bi-stable buckled structure [6–11]. For the mechanically bi-stable buckled structures, the transition between buckled states, or “snap-through”, can induce very large deformations which generate a spike in electrical output. Thus, the mechanical stability state switching conditions are critical for optimizing power production. In this work, the influence of geometrical parameters is explored for a recently development bi-stable energy harvesting structure. Unlike most bi-stable structures which use a Mode 1 buckled state, this device utilizes a quasi-pinned beam that symmetrically displaces the supports to induce a Mode 2, “S” shaped buckled beam. As shown schematically in Fig. 6.1, this design also employs two torque arms which transfer a bending moment to the center beam when transverse accelerations are applied (i.e., vibrations of the base). This torque is used to induce switching between the two stable buckled configurations of D.A. Porter (*) • T.A. Berfield Department of Mechanical Engineering, University of Louisville, 200 Sackett Hall, Louisville, KY 40292, USA e-mail: daport02@louisville.edu N. Sottos et al. (eds.), Experimental and Applied Mechanics, Volume 6: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06989-0_6, #The Society for Experimental Mechanics, Inc. 2015 49
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