Broadband energy harvesting using nonlinear techniques

• Main Author: Avvari Panduranga Vittal
• Other Authors: Soh Chee Kiong
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Civil engineering::Structures and design
• Online Access: http://hdl.handle.net/10356/73447
• Institution: Nanyang Technological University

In the quest for long-term service of various low power sensor systems and wireless sensor networks, there has been prominent research in the field of energy harvesting. Vibration based piezoelectric energy harvesting (PEH) has gained popularity since vibrations are ubiquitous and the piezoelectric materials provide a high-power density for conversion of vibrations to electrical energy. Most initial harvesters consisted of a linear PEH device (a cantilever bimorph beam) vibrating at resonance which provided very limited bandwidth of frequency for field operations. Due to such drawbacks of linear PEH devices a gradual shift towards broadband energy harvesters has been observed lately. The broadband energy harvesters can perform efficiently in a much broader range of frequency and such an enhanced design is implemented by the usage of various techniques, the foremost of them being the introduction of nonlinearity into the system. The nonlinearity can be introduced into the system using magnets. The interaction between magnets is nonlinear in nature and in the presence of magnetic interaction forces the PEH system’s response becomes nonlinear. This thesis discusses the importance of enhancing the bandwidth and the various parameters effecting it, along with introduction of innovative designs to enhance the bandwidth of a PEH system. Traditionally the interaction forces between magnets present in a PEH system were defined using a dipole-dipole formulation which assumes the magnets to be point dipoles. Though this formulation provided a decent understanding into the behavior of the PEH system, it’s accuracy is limited when the magnets are placed close to each other, which is a commonplace in many nonlinear PEH systems. This discrepancy was overcome by using an enhanced magnetostatic interaction formulation. The usage of this enhanced integral formulation provides an added advantage in the analytical modeling and minimizes the discrepancies in the interaction forces when the magnets are placed close to each other wherein shape effects come into the picture. Moreover, the conventional analytical models assumed the non-rotation of magnets, and neglected the effect due to magnetostatic forces acting in the axial direction. These fundamental limitations were addressed explicitly in this work and the limiting conditions where the assumptions from conventional models provide a decent degree of accuracy are also stated. Using the formulation developed, a PEH beam with one and two end magnets was analyzed and validated experimentally. The improvement in the bandwidth of operation for a nonlinear PEH system subjected to low level base excitation magnitudes has been dealt in detail. The study on enhanced magnetostatic formulation was followed by a detailed investigation of the dependence of the operating bandwidth on the stiffness and effective strain transfer values of a nonlinear PEH system. An analytical approach was explored augmented with a comprehensive experimental investigation to assess the performance of nonlinear PEH systems using substrates of different materials, thus inducing the variation of stiffness into the system. It was concluded that the bandwidth is considerably higher for a flexible beam but the effective strain transfer rate is much lower, thereby minimizing the peak output response of the system. In addition to the study of enhancing the bandwidth of operation in the presence of end magnets for a nonlinear PEH system. An innovative approach of utilizing a coupled PEH system consisting of two magnetically coupled cantilever PEH beams undergoing transverse and parametric vibrations was also explored. Even though the PEH system was subjected to lower levels of base excitation (0.2g, g = 9.81 m/s2), the response obtained was far superior to that of the linear configuration. The bandwidth at a level of 100 µW was enhanced by 300%. This design provides a promising alternative to the traditional PEH systems. Over the course of experimental investigation of the harvesters, damage due to prolonged exposure to external loading was observed. To address this, a unique fatigue based study to understand the damage progression and the corresponding usability of the PEH beam has been presented with detailed experimental, analytical and finite element based studies. A comprehensive FEA model for the piezoelectric macro fiber composite transducer has been introduced. This not only enhances the accuracy in modeling PEH beams but also provides a detailed overview into the crack propagation of the damaged piezo transducers. The study ended with making provisions for a few limiting criterion and design recommendations of minimizing the number of times the maximum strain value exceeds 500 µε during the normal functioning of the PEH system.