Michele Pozzi

Plucked piezoelectric bimorphs for knee-joint energy harvesting: modelling and experimental validation

The modern drive towards mobility and wireless devices is motivating intensive research in energy harvesting technologies. To reduce the battery burden on people, we propose the adoption of a frequency up-conversion strategy for a new piezoelectric wearable energy harvester. Frequency up-conversion increases efficiency because the piezoelectric devices are permitted to vibrate at resonance even if the input excitation occurs at much lower frequency. Mechanical plucking-based frequency up-conversion is obtained by deflecting the piezoelectric bimorph via a plectrum, then rapidly releasing it so that it can vibrate unhindered; during the following oscillatory cycles, part of the mechanical energy is converted into electrical energy. In order to guide the design of such a harvester, we have modelled with finite element methods the response and power generation of a piezoelectric bimorph while it is plucked. The model permits the analysis of the effects of the speed of deflection as well as the prediction of the energy produced and its dependence on the electrical load. An experimental rig has been set up to observe the response of the bimorph in the harvester. A PZT-5H bimorph was used for the experiments. Measurements of tip velocity, voltage output and energy dissipated across a resistor are reported. Comparisons of the experimental results with the model predictions are very successful and prove the validity of the model.

The pizzicato knee-joint energy harvester: characterization with biomechanical data and the effect of backpack load

The reduced power requirements of miniaturized electronics offer the opportunity to create devices which rely on energy harvesters for their power supply. In the case of wearable devices, human-based piezoelectric energy harvesting is particularly difficult due to the mismatch between the low frequency of human activities and the high-frequency requirements of piezoelectric transducers. We propose a piezoelectric energy harvester, to be worn on the knee-joint, that relies on the plucking technique to achieve frequency up-conversion. During a plucking action, a piezoelectric bimorph is deflected by a plectrum; when released due to loss of contact, the bimorph is free to vibrate at its resonant frequency, generating electrical energy with the highest efficiency. A prototype, featuring four PZT-5H bimorphs, was built and is here studied in a knee simulator which reproduces the gait of a human subject. Biomechanical data were collected with a marker-based motion capture system while the subject was carrying a selection of backpack loads. The paper focuses on the energy generation of the harvester and how this is affected by the backpack load. By altering the gait, the backpack load has a measurable effect on performance: at the highest load of 24?kg, a minor reduction in energy generation (7%) was observed and the output power is reduced by 10%. Both are so moderate to be practically unimportant. The average power output of the prototype is 2.06???0.3?mW, which can increase significantly with further optimization.

The electrical transport properties of shape memory alloys

Abstract The effectiveness of shape memory alloys as active elements in thermal actuators is proved by the huge amount of applications developed in the last few decades. In recent years, interest has kept growing in the sensing capabilities that they show when they are heated by an electrical current. The electrical resistance measured thereby gives an easily available feedback of the actuator displacement, provided that a suitable relation is found between the two quantities. It becomes, therefore, compulsory to investigate several alloys and different testing conditions to best fit applications with a suitable material. In this paper, a recently developed apparatus is presented. It allows driving the shape memory element with a finely controlled current waveform while the electrical resistance and the displacement are precisely measured. Besides simulating real operation conditions, this apparatus does not suffer from the high temperature limitations that affect currently used thermostatic-bath-based equipment, it will allow investigating the martensitic transformation and its fundamental properties in the newly developed high-temperature alloys (e.g. NiTiHf). The results of testing, under several constant stress states, two specific materials are also discussed: equiatomic NiTi wires, which show poor sensing features and melt spun TiNiCu ribbons, with remarkably promising sensing/actuating characteristics.

Characterization of a rotary piezoelectric energy harvester based on plucking excitation for knee-joint wearable applications

Wearable medical and electronic devices demand a similarly wearable electrical power supply. Human-based piezoelectric energy harvesters may be the solution, but the mismatch between the typical frequencies of human activities and the optimal operating frequencies of piezoelectric generators calls for the implementation of a frequency up-conversion technique. A rotary piezoelectric energy harvester designed to be attached to the knee-joint is here implemented and characterized. The wearable harvester is based on the plucking method of frequency up-conversion, where a piezoelectric bimorph is deflected by a plectrum and permitted to vibrate unhindered upon release. Experiments were conducted to characterize the energy produced by the rotary piezoelectric energy harvester with different electric loads and different excitation speeds, covering the range between 0.1 and 1 rev?s?1 to simulate human gait speeds. The electrical loads were connected to the generator either directly or through a rectifying bridge, as would be found in most power management circuits. The focus of the paper is to study the capability of energy generation of the harvester for knee-joint wearable applications, and study the effects of the different loads and different excitation speeds. It is found that the energy harvested is around 160?490??J and strongly depends on the angular speed, the connected electric loads and also the manufacturing quality of the harvester. Statistical analysis is used to predict the potential energy production of a harvester manufactured to tighter tolerances than the one presented here.

Piezoelectric modelling for an impact actuator

Amplification techniques for piezoelectric actuation are briefly reviewed and an unlatching impact piezo-actuator then proposed. In such a device, a piezoelectric multilayer has the function of imparting kinetic energy to a mass connected to an unlatching mechanism. In order to gather information necessary for the design phase, a Matlab Simulink model of a piezoelectric multilayer was developed. It focuses on the dynamic response of multilayers excited with a voltage-step in different conditions. The model is put to test in several different situations and shown to give satisfactory results. Then the model is used to predict the performance and working conditions of a multilayer in hypothetical conditions in order to find the best operating parameters. It is found that fine tuning of a few electronic components can lead to considerably improved performance.

Magnetic plucking of piezoelectric bimorphs for a wearable energy harvester

A compact and low-profile energy harvester designed to be worn on the outside of the knee-joint is presented. Frequency up-conversion has been widely adopted in recent times to exploit the high frequency response of piezoelectric transducers within environments where only low frequencies are present. Contactless magnetic plucking is here introduced, in a variable reluctance framework, with the aim of improving the mechanical energy transfer into the transducers, which is sub-optimal with contact plucking. FEA and experiments were used to design an optimal arrangement of ferromagnetic teeth to interact with the magnets fixed to the piezoelectric beams. A prototype was made and extensively tested in a knee-joint simulator controlled with gait data available in the literature. Energy and power produced were measured for walking and running steps. A power management unit was developed using off-the-shelf components, permitting the generation of a stable and regulated supply of 26 mW at 3.3 V during walking. Record levels of rectified (unregulated) electrical power of over 50 and 70 mW per walking and running steps, respectively, were measured.

Impulse excitation of piezoelectric bimorphs for energy harvesting: a dimensionless model

Energy harvesting (EH) is a multidisciplinary research area, involving physics, materials science and engineering, with the objective of providing renewable sources of power sufficient to operate targeted low-power applications. Piezoelectric transducers are often used for inertial vibrational as well as direct excitation EH. However, due to the stiffness of the most common material (PZT), compact and light-weight harvesters have high resonant frequencies, making them inefficient at extracting low-frequency power from the environment. The technique of frequency up-conversion, in the form of either plucking or impulse excitation, aims to bridge this frequency gap. In this paper, the technique is modelled analytically with focus on impulse excitation via impact or shock. An analytical model is developed in a standard way starting from the Euler–Bernoulli beam equations adapted to a piezoelectric bimorph. A set of dimensionless variables and parameters is defined and a system of differential equations derived. Here the system is solved numerically for a wide range of the two group parameters present, covering piezoelectric coupling strength between PVDF and PMN-PT. One major result is that the strength of the coupling strongly affects the timescale of the process, but has only a minor effect on the total energy converted. The model can be readily adapted to different excitation profiles.

Experimental characterisation of macro fibre composites and monolithic piezoelectric transducers for strain energy harvesting

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/cm2) under 1170μstrain peak-to-peak excitation at 10Hz with a 66kΩ load. Power generation densities of 600μW/cm2 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.

Harvesting energy from the dynamic deformation of an aircraft wing under gust loading

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/m2 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.

Pizzicato excitation for wearable energy harvesters

Recent decades have witnessed the miniaturization of electronic devices. The reduction in the feature size of gadgets’ components is beneficial not only because we can pack many transistors in a small space, but also because compact devices consume less power. Currently, a good proportion of the volume and weight of portable gadgets is taken up by batteries, which have a size dictated by a difficult compromise between portability and battery life. However, our dependence on batteries may be close to an end. Since miniaturized electronics do not require much power, it is becoming viable to generate the necessary energy within the device itself or with portable energy harvesters (EHs). These systems supply an inexhaustible amount of energy by converting vibrations or other mechanical movement, thermal gradients, or electromagnetic radiation into electricity. Among the first EHs to be studied were scavengers of the vibrational energy naturally present in many environments. These devices power wireless sensor nodes that constantly monitor important environmental parameters for an extended period of time without maintenance or battery replacement. Most of these vibrational-energy harvesters are based on piezoelectric transducers, devices capable of converting energy between physical domains such as electrical into mechanical or vice versa. When it comes to wearable EHs, that is, devices that can harvest the energy generated naturally by the human body (through walking, for example) we are faced with a mismatch. Human movements have characteristically low frequencies (up to a few hertz) while piezoelectric technology can produce sizable power at high frequencies only. A viable solution is to employ frequency up-conversion techniques. These methods are used to convert a low input frequency into a high output one through mechanisms such as gears and cams or through impact or plucking. In pianos, up-conversion is achieved when hammers strike the wires that are then free to vibrate at resonance. For Figure 1. The pizzicato up-frequency conversion. A plectrum approaches a bimorph (1) and mechanical energy is stored elastically after contact (2). Upon release, the bimorph vibrates at resonance (3) and voltage is produced across an electrical load (lower plot). PZT5H: A ‘flavor’ of PZT (lead zirconate titanate) characterized by a specific chemical composition.