Chapter 3 Innovative Piezoelectric Cantilever Beam Shape for Improved Energy Harvesting Iman Mehdipour and Francesco Braghin Abstract Piezoelectric cantilevered beams have been used as a MEMS energy harvester for the last decade because of their less natural frequencies in comparison with other types of boundary conditions. Defining a new shape of cantilever beam to reduce the natural frequency in compared with conventional one is so worthwhile because it causes more flexible bending stiffness and a larger bonding area of piezoelectric layer. So, higher efficiency of the PZT energy harvester can be expected. In order to achieve this goal, a new S-shape PZT cantilever energy harvester is proposed. In this study, the software COMSOL Multiphysics is used to analysis and investigate the characteristics of the suggested model. Preliminary results from modal analysis confirm that in the same volume of mass and effective length of the straight and S-shape beam, the proposed model is more flexible and experiences several natural frequencies which are less than second natural frequency of the conventional cantilevered energy harvester. It is predicted, by mechanical and electrical analyzing, the proposed model produces much higher output voltage than the conventional flat vibration energy harvester, mainly because of lower resonance frequencies. In better word, the proposed model needs less amplitude of excitation force for its maximum efficiency. Keywords S-shaped cantilevered beam • Piezoelectric energy harvester • Low frequency vibration • Maximum harvested voltage • COMSOL software 3.1 Introduction Harvesting energy from wasted ambient vibration has received great interest during last decades because of high power density of vibrational sources [1]. This harvested energy, which is converted from mechanical energy to electrical one by means of electrostatic, electromagnetic, and piezoelectric techniques, can provide appropriate electrical energy for low power consumption electronic devices [2–4]. The conventional single-degree-of-freedom (SDOF) has the maximum efficiency in a very narrow bandwidth around its natural frequency. The ambient vibration frequency of structures like machine and civil structure are much lower than the natural frequency of traditional energy harvesters. One way to tune the single-degree-freedom energy harvester to achieve maximum efficiency is reducing the natural frequency of the energy harvester. For instance, for a straight beam energy harvester, adding mass at the tip of the harvester or increasing the length of the beam can help to reduce the natural frequency of the energy harvester. Therefore, for very low frequency of ambient vibration, the length of the harvester should be impractically long or excessively heavy [5]. To overcome these limitations, several researchers proposed different structures. Increasing bandwidth frequency for vibration energy harvester by means of (2 DOF) vibrating body is a way without changing the proof mass [6]. Tunable rotational energy harvester which consists of a rotational spring and a suspended weight is another way to tune for low ambient frequency [7]. A multi-resonant energy harvester [8] and the spiral geometry of the beam [9] are also two methods to reduce the natural frequency of the energy harvester. A new zigzag shape vibration energy harvester is proposed by Karami and Inman [10, 11] and established an analytical model of energy harvesting structure. A new horizontal S-shaped PZT cantilevered model of energy harvester is provided by Huicong Liu et al. [12]. This kind of energy harvesting model is appropriate to scavenge energy from vibrations at frequency less than 30 Hz. Recently, a corrugated cantilevered beam I. Mehdipour • F. Braghin ( ) Department of Mechanical Engineering, Politecnico Di Milano, Via La Masa 1, 20156 Milan, Italy e-mail: francesco.braghin@polimi.it © The Society for Experimental Mechanics, Inc. 2015 A. Wicks (ed.), Shock & Vibration, Aircraft/Aerospace, and Energy Harvesting, Volume 9, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-15233-2_3 19
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