NUMERICAL STUDY OF A PIEZOELECTRIC VIBRATION ENERGY HARVESTER WITHOUT AND WITH AN ORTHO-PLANAR SPRING USING A MODIFIED H-SHAPE STRUCTURE

The demand for low-power electronic devices in wireless sensor networks (WSNs) has been continuously increasing. Most WSNs rely on battery power, however, and batteries need replacement regularly, resulting in direct and indirect pollution to the environment. Because of its abundance in many environments, kinetic energy, in the form of ambient vibrations, could be used as alternative to batteries by using vibrational energy harvesting techniques. Piezoelectric materials are one of the most common form of transducers and, traditionally, piezoelectric vibrational energy harvesters (PVEHs) are based on a cantilever beam, with one or more piezoelectric layers. However, the drawbacks of such designs are single vibration mode and narrow frequency bandwidth. In this paper, we propose a novel PVEH design based on a modified H-shape structure without and with an ortho-planar spring to solve the issues of single vibration mode and narrow frequency bandwidth. This structure comprises a double clamped beam, with four arms which act like cantilever beams. The centre of the structure is coupled with a compliant orthoplanar spring to create low natural frequency and multiple peaks in the frequency response. A proof mass is placed on the spring to induce high stresses in the structure. A numerical model using finite element analysis (FEA) was developed using the solid mechanics, electrostatics, and electrical circuit modules in COMSOL Multiphysics to compare the output voltage and power of the modified H-shape structure without (baseline) and with an ortho-planar spring (design 1) in the x, y, z axes. The H-shape structure was made of polylactide (PLA) as a substrate. The configuration used six PZT-5H piezoelectric materials connected in parallel, a 17.78 kΩ load resistor, and 0.4 g (g = 9.81 m/s²) base excitation. The results show that: (i) the first three modes of design 1 are 8.9, 15.6, and 17.2 Hz, while the baseline design's modes are 11.8, 15.3, and 17.6 Hz; (ii) both of designs show a multi-modal response, and that design 1 has three peaks, while the baseline has two peaks in the frequency range (0-50 Hz) in the z axis; (iii) both designs can only harvest energy efficiently in the z axis; and (iv) the baseline design achieves maximum voltage and power values of 34.5 V and 33.5 mW respectively, whereas design 1 produces 31.3 V and 27.6 mW respectively in the z axis.