Weight reduction and maintenance simplification are high in the agenda of companies and researchers active in the aerospace sector. Energy harvesters are being investigated because they enable the installation of wireless sensor nodes, providing structural health monitoring of the aircraft without additional cabling. This paper presents both a weight-optimized composite wing structure and a piezoelectric harvester for the conversion of mechanical strain energy into electrical energy. Finite elements modelling was used for the minimum-weight optimisation within a multi-constraints framework (strength, damage tolerance, flutter speed and gust response). The resulting structure is 29% more compliant than the original one, but is also 45% lighter. A strain map was elaborated, which details the distribution of strain on the wing skin in response to gust loading, indicating the optimal locations for the harvesters. To assess the potential for energy generation, a piezoelectric harvester fixed to a portion of the wing was modelled with a multi-physics finite elements model developed in ANSYS. The time-domain waveforms of the strain expected when the aircraft encounters a gust (gust frequencies of 1, 2, 5 and 10 Hz were considered) are fed into the model. The effects of harvester thickness and size, as well as adhesive thickness, were investigated. Energy generation exceeding 10 J/m<sup>2</sup> in the first few second from the beginning of the gust is predicted for 100μ-thick harvesters. The high energy density, low profile and weight of the piezoelectric film are greatly advantageous for the envisaged application.
μCompact and lightweight energy harvesters are needed to power wireless sensor nodes (WSNs). WSNs can provide health monitoring of aircraft structures, improving safety and reducing costs by enabling predictive maintenance. A simple solution, which meets the requirements for lightness and compactness, is represented by piezoelectric generators fixed to the surface of the wing (i.e. the wing skin). Such piezoelectric patches can harvest the strain energy available when the wing is flexed, as occurs, for example, in the presence of gust loading. For this study, monolithic piezoelectric sheets and macro fibre composite (MFC) generators were fixed to plates made of two materials commonly used for aircraft wing skin: Al-2024 aluminium alloy and an epoxy-carbon fibre composite. The plates then underwent harmonically varying loading in a tensile testing machine. The power generation of the harvesters was measured at a selection of strain levels and excitation frequencies, across a range of electrical loads. The optimal electrical load, yielding maximum power extraction, was identified for each working condition. The generated power increases quadratically with the strain and linearly with the frequency. The optimal electrical load decreases with increasing frequency and is only marginally dependent on strain. Absolute values of generated power were highest with the MFC, reaching 12mW (330μW/cm<sup>2</sup>) under 1170μstrain peak-to-peak excitation at 10Hz with a 66kΩ load. Power generation densities of 600μW/cm<sup>2</sup> were achieved under 940μstrain with the monolithic transducers at 10Hz. It is found that MFCs have a lower power density than monolithic transducers, but, being more resilient, could be a more reliable choice. The power generated and the voltage outputs are appropriate for the intended application.
The modern drive towards mobility and wireless devices is motivating intense research in energy harvesting (EH) technologies. In an effort to reduce the battery burden of people, we are investigating a novel piezoelectric wearable energy harvester. As piezoelectric EH is significantly more effective at high frequencies, in opposition to the characteristically low-frequency human activities, we propose the use of an up-conversion strategy analogous to the pizzicato musical technique. In order to guide the design of such harvester, we have modelled with Finite Elements (FE) the response and power generation of a piezoelectric bimorph while it is "plucked", i.e. deflected, then released and permitted to vibrate freely. An experimental rig has been devised and set up to reproduce the action of the bimorph in the harvester. Measurements of the voltage output and the energy dissipated across a series resistor are reported and compared with the FE predictions. As the novel harvester will feature a number of bimorphs, each plucked tens of times per step, we predict a total power output of several mW, with imperceptible effect on the wearer's gait.
This paper presents, for the first time, a coupled piezoelectric-circuit finite element model (CPC-FEM) to analyze the
power output of vibration-based piezoelectric energy harvesting devices (EHDs) when connected to a resistive load.
Special focus is given to the effect of the resistive load value on the vibrational amplitude of the piezoelectric EHDs, and
thus on the current, voltage, and power generated by the EHDs, which are normally assumed to be independent of the
resistive load in order to reduce the complexity of modelling and simulation. The CPC-FEM presented uses a cantilever
with the sandwich structure and a seismic mass attached to the tip to study the following load characteristics of the EHD
as a result of changing the load resistor value: (1) the electric outputs of the EHD: current and voltage, (2) the power
dissipated by the resistive load, (3) the vibration amplitude of tip displacement, and (4) the shift in resonant frequency of
the cantilever. Significant dependences of the characteristics of the piezoelectric EHDs on the externally connected
resistive load are found, rather than independency, as previously assumed in most literature. The CPC-FEM is capable of
predicting the generated power output with different resistive load values while simultaneously considering the effect of
the resistor value on the vibration amplitude. The CPC-FEM is invaluable for validating the performance of a device
before fabrication and testing, thereby reducing the recurring costs associated with repeat fabrication and trials, and also
for optimizing device design for maximal power-output generation.
A new structure for micro-machined piezoelectric mechanical filters with high-Q is presented. The structure is composed of two silicon cantilevers which have thin film PZT transducers deposited on top of each and are mechanically connected together by a silicon linkage. A model is proposed in which finite element analysis (FEA) is combined with a two-port microwave network representation to show high-Q characteristics. Using the model, key parameters in the design include the substrate thickness, and the position, length and width of the linkage, and the effect of these on filter performance, including the resonant frequencies of the in-phase and out-of-phase vibrational modes, bandwidth, quality factor (Q), insertion loss and ripple are also investigated. The results show that the coupling produces additional resonances compared with ladder type filters composed of uncoupled cantilevers, and a significant increase in Q of about a few hundred times higher with correct design, than uncoupled cantilevers.