Modelling of Broadband Piezoelectric Energy Harvesters

A nonlinear magnetic coupling model for piezoelectric energy harvesting systems is proposed in this paper. For the purpose of enhancing harvesting efficiency from wide-spectrum vibrations, a magnetic coupling structure of piezoelectric cantilevers is presented. However, the nonlinear dynamic of broadband piezoelectric energy harvesters could not be adequately described due to complex nonlinear magnetic force. Furthermore, the broken frequency can not be predicted using the designed dimensionless model. In order to solve those issues, the nonlinear magnetic force is established using polynomial equation. Based on Hamilton principle and finite element theory, a nonlinear model of the standard piezoelectric cantilever with magnetic coupling is established. Frequency sweeping experiments with various excitation are carried out. The results show that the output characteristic of the proposed model is approximate to that of experimental system under the same condition, and also their broken frequency is very close.

Model Reduction of Piezoelectric Energy Harvesters Subject to Band-Limited, Stochastic Base Excitation

In many scenarios where vibration energy harvesting can be utilized — particularly those involving bio-motions or environmental disturbances — energy sources are broadband and non-stationary. On the other hand, design procedures have been predominantly developed for harmonic or white noise excitation, specifically for single degree of freedom approximations of the transducer. In this paper, a general approach for design optimization of cantilevered, piezoelectric energy harvesters in the presence of band-limited, white-noise excitation is outlined. For this study, human and vehicular motions are considered; these complex waveforms are distilled into a small set of dominant features with regard to their impact on the power output of the device. Criteria based on modal participation factors, including pre-filtering of the disturbance, are used in guiding the reduction of the input and plant degrees of freedom in order to make the design optimization problem tractable. This process determines the error in assuming a low-order model for the transducer in the presence of broadband noise that may excite multiple modes of vibration. Furthermore, this study considers the quantitative impact of charge cancellation in higher modes and the benefits of inserting multiple electrodes along the length. To illustrate these methods, energy harvesters are designed for acceleration data collected from walking and car idling. It is shown that a simple method that is a generalization of naïve approaches that assume harmonic or white noise excitation and a single degree of freedom can determine which simplifications are appropriate and the inaccuracies that can be expected from them.

Novel Dielectric Stack Actuators for Dynamic Applications

In order to realize dielectric elastomer stack actuators suitable for dynamic applications a new actuator design with rigid, perforated electrodes is developed. The low surface resistance of the metal electrodes predestines this concept for dynamic applications where higher currents are present. Detailed numerical analyses are performed to show the potential of this approach, to study the complex material deformation and to optimize the aperture geometry. A multilayer stack actuator is then manufactured and characterized experimentally under various load conditions to gain suitable parameters for a parametrized model. It is subsequently used to attenuate vibrations of a truss structure. By careful adjusting the parameters it functions both as passive absober and as actuator. A comparison of experimental and simulation results proves the high quality of the simulation model. The work shows the great potential of the new design concept for future applications especially in the field of smart structures.

Theoretical and Experimental Evaluation of System Energy Balance and Power Generation Efficiency for Piezocomposite Vibration Energy Harvester Under Low Intensity Excitation Condition

In the authors’ previous study, the vibration energy harvester of the piezoelectric bimorph cantilever type was proposed for vibration condition monitoring applications of industrial rotating machinery. According to an ISO standard, vibration level of newly commissioned class I rotating machinery is under 0.71mm/sec rms in all frequency range. Authors assumed that the typical casing or pedestal vibration amplitude of the rotating machinery was 0.71 mm/sec rms and this low intensity excitation condition was the input for experimental evaluation of the voltage generation performance of the piezocomposit vibration energy harvester. The vibration energy harvester consists of the surface bonded two Macro-Fiber Composites (MFCs). In this study, energy transfer efficiency was derived from the system energy balance during the natural period of the proposed vibration energy harvester. Energy balance equations were successfully obtained from the governing equations of the piezoelectrically coupled electromechanical system. The maximum AC power through 114.3 Kilo-Ohm resistor which includes instrument internal resistances experimentally obtained 242.07 microwatt when subjected to vibration source input magnitude of 0.71 mm/s rms at the resonant frequency of the harvester (29.42 Hz). The impedance matching between MFCs and the electrical resistive load was effective for maximizing AC power transfer of the vibration energy harvester. Estimated energy transfer from mechanical system to electrical system shows the agreement with the experimentally evaluated generating power during the natural period of the vibration energy harvester with about 3% difference. Estimated energy transfer efficiency was about 30% for different excitation magnitudes: 0.71, 0.568 and 0.355 mm/sec rms.

Thermomechanical Behavior of Low CTE Metal-Matrix Composites Fabricated Through Ultrasonic Additive Manufacturing

Shape memory and superelastic NiTi are often utilized for their large strain recovery and actuation properties. The objective of this research is to utilize the stresses generated by pre-strained NiTi as it is heated in order to tailor the CTE of metal-matrix composites. The composites studied consist of an Al 3003-H18 matrix with embedded NiTi ribbons fabricated through an emerging rapid prototyping process called Ultrasonic Additive Manufacturing (UAM). The thermally-induced strain of the composites is characterized and results show that the two key parameters in adjusting the effective CTE are the NiTi volume fraction and prestrain of the embedded NiTi. From the observed behavior, a constitutive composite model is developed based constitutive SMA models and strain matching composite models. Additional composites were fabricated to characterize the NiTi-Al interface through EDS and DSC. These methods were used to investigate the possibility of metallurgical bonding between the ribbon and matrix and determine interface shear strength. Interface investigation indicates that mechanical coupling is accomplished primarily through friction and the shear strength of the interface is 7.28 MPa. Finally, using the developed model, a composite was designed and fabricated to achieve a near zero CTE. The model suggests that the finished composite will have a zero CTE at a temperature of 135°C.

The Effect of the Configuration of the Amplification Device on the Power Output of a Piezoelectric Strip

We investigated the behaviour of a polyvinylidene-fluoride piezoelectric strip (‘stalk’) clamped at the leading edge, and hinged to an amplification device (‘leaf’) at the trailing edge. Flutter of this cantilevered system was induced within smooth, parallel flow, and an AC voltage was generated from the PVDF strip. A polypropylene, triangle comprised the leaf. Two leaf parameters were varied so as to quantify their effect on the power output of the system: 1) the area, and 2) the aspect ratio. It was found that the highest power output was realised with the 2nd-largest leaf across a range of wind speeds, but the variation in power measurements was large. Thus, the 3rd-largest leaf was found to give the highest power output with the lowest power variation. This leaf area was then fixed and the aspect ratio varied. It was found that the largest aspect ratio-leaf rendered the highest power output, but had a relatively high start-up wind speed.

Thermodynamic Modeling of Ionic Polymer-Metal Composite Beams

A thermodynamically consistent model to simulate the electromechanical response of ionic polymer-metal composite (IPMC) beams has been developed based on Euler-Bernoulli beam theory. Appropriate assumptions have been made and suitable forms for the Helmholtz free energy and the rate of dissipation have been chosen. The governing equations, describing the actuation and sensing behavior of IPMC strips in air, have been formulated using a set of kinematic assumptions, the power theorem, and the maximum rate of dissipation hypothesis, neglecting inertial effects. The model has been extended to solve for large deformations in IPMC cantilevers with certain loading conditions. The model has been shown to simulate the electromechanical responses of both Nafion and Flemion based IPMC strips. This includes the initial overshoot followed by a gradual back-relaxation observed in the tip deflection measurements of Nafion based IPMC strips under the application of a step voltage. It has been shown that a coupled convective heating term in the rate of dissipation function is crucial for simulating this overshoot and the back relaxation.

Wing Rotation and Lift Modeling and Measurement in SUEX Flapping Wing Mechanisms

This paper presents detailed modeling and experimental testing of wing rotation and lift in the LionFly, a flapping wing mechanism powered by piezoelectric bimorph actuators fabricated using SUEX dry film. The goal of this paper is to understand the flapping and rotation dynamics and the lift-producing mechanisms in this device. A linear vibrational model is developed and augmented with nonlinear aerodynamic forces using the blade element method. Experimental testing using a laser vibrometer in air and in vacuum characterizes small amplitude flapping and rotation. Strobe photography and high definition image processing measures high amplitude wing trajectories. A lift measurement system using a force transducer is designed and used to measure average lift in the LionFly. The LionFly produces 46° peak flapping and 44° peak rotation resulting in lift of 71 μN at 37 Hz.

Cooling Strategies for a SMA Wire Actuator in a Feed Axis

Shape memory alloys (SMA) are smart materials which can be activated thermally. They are suitable for the use as actuators due to their ability to remember an imprinted shape through thermal activation. In addition, actuators based on shape memory alloys offer a higher work output in relation to their volume compared to other actuator concepts. Other advantages of using SMA in actuation applications include the ability to design lightweight systems and the comparatively low material costs. On the other hand, designing an SMA actuator poses a challenge in case a specific rate of feed has to be achieved. These difficulties become especially apparent if the actuator is used to create a defined displacement not only in its activation direction, but in the returning (deactivation) direction as well. This might occur, for example, while devising an SMA-driven feed axis. During the activation of the SMA, the speed of the actuator and therefore the speed of the axis can be influenced by choosing a specific thermal energy transfer method. For instance, when using the intrinsic resistance for heating purposes, the speed can be controlled by changing the electrical current running through the SMA. However, after the deactivation (end of the heating phase) of the shape memory alloy, the transformation needs a considerably longer time. For an exemplary SMA wire actuator, the transformation time in room temperature can be five times higher than the activation time. For usage in a feed axis, the actuator should produce similar speeds in both the activation and deactivation direction. To achieve this, different strategies for cooling the SMA after cutting off the current are investigated. These strategies include an active air cooling system with different flow characteristics and the operation of the actuator in a cooling fluid. In a nutshell, the paper compares different ways of cooling an SMA wire actuator to increase the transformation speed after deactivation. The aim is to make the deactivation speed as manageable as the activation speed.

Equilibrium Conditions and Evolution of Needle Domain Arrays in Ferroelectric Single Crystals

Experimental imaging of ferroelectric single crystals often reveals the formation of periodic arrays of needle-shaped domains of a compatible polarization variant coexisting with a homogeneous single domain parent variant. This study examines the far field electromechanical loading conditions that favor the formation, existence and evolution of compatible and stable needle domain array patterns using a phase-field modeling approach. The infinite arrays of needles are modeled via a representative unit cell and the appropriate electrical and mechanical periodic boundary conditions. A theoretical investigation of the generalized loading conditions is carried out to determine the sets of averaged loading states that lead to stationary needle tip locations. The resulting boundary value problems are solved using a non-linear finite element method to determine the details of the needle shape as well as the field distributions around the needle tips.