Enhanced piezoelectric energy harvesting powered wireless sensor nodes using passive interfaces and power management approach

Low-frequency vibrations typically occur in many practical structures and systems when in use, for example, in aerospaces and industrial machines. Piezoelectric materials feature compactness, lightweight, high integration potential, and permit to transduce mechanical energy from vibrations into electrical energy. Because of their properties, piezoelectric materials have been receiving growing interest during the last decades as potential vibration- harvested energy generators for the proliferating number of embeddable wireless sensor systems in applications such as structural health monitoring (SHM). The basic idea behind piezoelectric energy harvesting (PEH) powered architectures, or energy harvesting (EH) more in general, is to develop truly “fit and forget” solutions that allow reducing physical installations and burdens to maintenance over battery-powered systems. However, due to the low mechanical energy available under low-frequency conditions and the relatively high power consumption of wireless sensor nodes, PEH from low-frequency vibrations is a challenge that needs to be addressed for the majority of the practical cases. Simply saying, the energy harvested from low-frequency vibrations is not high enough to power wireless sensor nodes or the power consumption of the wireless sensor nodes is higher than the harvested energy. This represents a main barrier to the widespread use of PEH technology at the current state of the development, despite the advantages it may offer. The main contribution of this research work concerns the proposal of a novel EH circuitry, which is based on a whole-system approach, in order to develop enhanced PEH powered wireless sensor nodes, hence to compensate the existing mismatch between harvested and demanded energy. By whole-system approach, it is meant that this work develops an integrated system-of-systems rather than a single EH unit, thus getting closer to the industrial need of a ready- to-use energy-autonomous solution for wireless sensor applications such as SHM. To achieve so, this work introduces: Novel passive interfaces in connection with the piezoelectric harvester that permit to extract more energy from it (i.e., a complex conjugate impedance matching (CCIM) interface, which uses a PC permalloy toroidal coil to achieve a large inductive reactance with a centimetre- scaled size at low frequency; and interfaces for resonant PEH applications, which exploit the harvester‟s displacement to achieve a mechanical amplification of the input force, a magnetic and a mechanical activation of a synchronised switching harvesting on inductor (SSHI) mechanism). A novel power management approach, which permits to minimise the power consumption for conditioning the transduced signal and optimises the flow of the harvested energy towards a custom-developed wireless sensor communication node (WSCN) through a dedicated energy-aware interface (EAI); where the EAI is based on a voltage sensing device across a capacitive energy storage. Theoretical and experimental analyses of the developed systems are carried in connection with resistive loads and the WSCN under excitations of low frequency and strain/acceleration levels typical of two potential energy- autonomous applications, that are: 1) wireless condition monitoring of commercial aircraft wings through non-resonant PEH based on Macro-Fibre Composite (MFC) material bonded to aluminium and composite substrates; and wireless condition monitoring of large industrial machinery through resonant PEH based on a cantilever structure. shown that under similar testing conditions the developed systems feature a performance in comparison with other architectures reported in the literature or currently available on the market. Power levels up to 12.16 mW and 116.6 µW were respectively measured across an optimal resistive load of 66 277 kΩ for an implemented non-resonant MFC energy harvester on aluminium substrate and a resonant cantilever-based structure when no interfaces were added into the circuits. When the WSCN was connected to the harvesters in place of the resistive loads, data transmissions as fast as 0.4 and s were also respectively measured. By use of the implemented passive interfaces, a maximum power enhancement of around 95% and 452% was achieved in the two tested cases and faster data transmissions obtained with a maximum percentage improvement around 36% and 73%, respectively. By the use of the EAI in connection with the WSCN, results have also shown that the overall system‟s power consumption is as low as a few microwatts during non- active modes of operation (i.e., before the WSCN starts data acquisition and transmission to a base station). Through the introduction of the developed interfaces, this research work takes a whole-system approach and brings about the capability to continuously power wireless sensor nodes entirely from vibration-harvested energy in time intervals of a few seconds or fractions of a second once they have been firstly activated. Therefore, such an approach has potential to be used for real-world energy- autonomous applications of SHM.