Advanced Beam Configurations and Optimisation Strategies for Human Motion-Based Piezoelectric Energy Harvesters
Creators: name: Rathnayaka Mudiyanselage, Iresha
Publisher: name: Southern Cross University
Year: 2024
Subjects: Piezoelectric Energy Harvesters
Advanced Beam Configurations
Human motion
Macro fibre composite (MFC)
Macro fibre composite
MFC

description: Producing electrical energy from mechanical vibrations using piezoelectric energy harvesters (PEHs) has recently become an increasingly intriguing focus for researchers. The harvested power from PEHs can be utilised in various small-scale electronic systems, wireless sensor networks, and medical equipment, with the ability to fully or partially replacing chemical batteries.
description: Among several vibration energy sources, human motion vibrations are abundant and ubiquitous and can be utilised for small-scale energy generation. Generally, human motion occurs in an ultra-low frequency spectrum (&lt; 10 Hz) within a wider operating bandwidth. Integrating human motion into PEH systems is comparatively challenging due to the high-magnitude resonance frequencies, narrow operating bandwidth, and lack of directional sensitivity of the conventional PEH systems. Also, many existing PEH designs operating under ultra-low frequency vibrations struggle to address these drawbacks simultaneously. Several designs employ techniques like frequency up-conversion methods or nonlinear approaches, such as bi-stable or tri-stable mechanisms that use additional attachments (i.e., magnets/springs/impact stoppers and etc.), primarily to further enhance the system’s performance for ultra-low frequency environments. Nevertheless, these approaches bring forth new challenges, given their substantial dependence on the additional accessories. Such as significant decrease in power density, an increase in mechanical damping and friction losses while making these bulky systems less feasible to scale down for practical applications, especially in the context of human motion. Consequently, there is an increasing demand for improved PEH systems designed to efficiently harness ultra-low frequency vibrations, with a primary focus on human motion.
description: Optimising PEH designs solely based on geometry or shape proves to be a viable solution for addressing the above-mentioned issues. However, research specifically focusing on this aspect for ultra-low frequency vibrations is relatively scarce in the existing literature. Therefore, the overarching aim of this thesis was to develop and investigate (three) different geometrically/shape optimised directional harvesters that can operate efficiently within a broader operating bandwidth and generate substantial power output in the ultra-low frequency spectrum. To achieve this aim, the objectives of this thesis were to propose optimised novel uni-directional, bi-directional, and tri-directional PEH designs. Each design was meticulously optimised, analysed using FEA software, and experimentally tested under controlled vibrations using a mechanical shaker. A novel statistical optimisation method was introduced for uni-directional harvesters. Furthermore, bi-directional and tri-directional harvesters underwent additional experimental testing with common aerobic human motions. The uni-directional harvester will be tested with human motion as part of a future study and has not been included in the scope of this thesis.
description: The outcome of this research developed (1) a stress-concentrated uni-directional PEH that can effectively work in the ultra-low frequency spectrum, producing a maximum of 445 μW average power while introducing a convenient method to optimise an uni-directional PEH system; (2) a bi-directional PEH design efficiently operating within the ultralow-frequency range, incorporating three resonance frequencies and thereby broadening the operating bandwidth, resulting in considerably higher output voltages and power, coupled with lower idle time compared to conventional PEH system; moreover, the power density of the proposed bi-directional harvester has doubled compared to an existing PEH design operating under the frequency up-conversion method, as tested with human motion under similar ground conditions; (3) a tri-directional PEH design that yielded a significantly higher average power output of 427 μW at its first resonance, remarkably expanding the operating bandwidth with six natural frequencies within an ultra-low frequency spectrum; the tri-directional harvester demonstrated a notable increase in average power density, showing a 4-fold improvement during walking and a 1.5-fold improvement during running motions compared to the bi-directional harvester proposed in objective 3.
description: Improvements to the developed designs were achieved through geometrical/shape optimisation without any additional attachments (i.e., magnets/springs/impact stoppers and etc.), considering the previously mentioned drawbacks of existing devices and negative impacts on the end user's comfort. These achievements have the potential to enhance the effectiveness of PEH systems in future practical applications, particularly under ultra-low frequency excitation. Finally, it is recommended to focus on introducing active tunable methods for all three proposed designs in future research. This approach would allow them to adapt to random vibrations and enhance efficiency even further.