Showing papers in "Smart Materials and Structures in 2008"

Powering MEMS portable devices—a review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems

Abstract: Power consumption is forecast by the International Technology Roadmap of Semiconductors (ITRS) to pose long-term technical challenges for the semiconductor industry. The purpose of this paper is threefold: (1) to provide an overview of strategies for powering MEMS via non-regenerative and regenerative power supplies; (2) to review the fundamentals of piezoelectric energy harvesting, along with recent advancements, and (3) to discuss future trends and applications for piezoelectric energy harvesting technology. The paper concludes with a discussion of research needs that are critical for the enhancement of piezoelectric energy harvesting devices.

A vibration energy harvesting device with bidirectional resonance frequency tunability

Abstract: Vibration energy harvesting is an attractive technique for potential powering of wireless sensors and low power devices. While the technique can be employed to harvest energy from vibrations and vibrating structures, a general requirement independent of the energy transfer mechanism is that the vibration energy harvesting device operate in resonance at the excitation frequency. Most energy harvesting devices developed to date are single resonance frequency based, and while recent efforts have been made to broaden the frequency range of energy harvesting devices, what is lacking is a robust tunable energy harvesting technique. In this paper, the design and testing of a resonance frequency tunable energy harvesting device using a magnetic force technique is presented. This technique enabled resonance tuning to ±20% of the untuned resonant frequency. In particular, this magnetic-based approach enables either an increase or decrease in the tuned resonant frequency. A piezoelectric cantilever beam with a natural frequency of 26 Hz is used as the energy harvesting cantilever, which is successfully tuned over a frequency range of 22‐32 Hz to enable a continuous power output 240‐280 μW over the entire frequency range tested. A theoretical model using variable damping is presented, whose results agree closely with the experimental results. The magnetic force applied for resonance frequency tuning and its effect on damping and load resistance have been experimentally determined. (Some figures in this article are in colour only in the electronic version)

Issues in mathematical modeling of piezoelectric energy harvesters

Abstract: The idea of vibration-to-electric energy conversion for powering small electronic components by using the ambient vibration energy has been investigated by researchers from different disciplines in the last decade. Among the possible transduction mechanisms, piezoelectric transduction has received the most attention for converting ambient vibrations to useful electrical energy. In the last five years, there have been a considerable number of publications using various models for the electromechanical behavior of piezoelectric energy harvester beams. The models used in the literature range from elementary single-degree-of-freedom (SDOF) models to approximate distributed parameter models as well as analytical distributed parameter solution attempts. Because of the diverse nature of researchers working in energy harvesting (including electrical, mechanical and materials engineers), several oversimplified and incorrect physical assumptions have been propagated in the literature. Issues of the correct formulation for piezoelectric coupling, correct physical modeling, use of low fidelity models, incorrect base motion modeling, and the use of static expressions in a fundamentally dynamic problem are discussed and clarified here. These common indiscretions, which have been repeated in the existing piezoelectric energy harvesting literature, are addressed and clarified with improved models, and examples are provided. This paper aims to provide corrections and necessary clarifications for researchers from different engineering disciplines interested in electromechanical modeling of piezoelectric energy harvesters.

Detection, localization and characterization of damage in plates with an in situ array of spatially distributed ultrasonic sensors

Abstract: Permanently attached piezoelectric sensors arranged in a spatially distributed array are under consideration for structural health monitoring systems incorporating active ultrasonic methods. Most damage detection and localization methods that have been proposed are based upon comparing monitored signals to baselines recorded from the structure prior to initiation of damage. To be effective, this comparison process must take into account any conditions other than damage that have changed the ultrasonic signals. Proposed here is a two-step process whereby damage is first detected and is then localized and characterized. The detection strategy considers the long time behavior of the signals in the diffuse-like regime where distinct echoes can no longer be identified. The localization strategy is to generate images of damage based upon the early time regime when discrete echoes from boundary reflections and scattering sites are meaningful. Results are shown for an aluminum plate with artificial damage introduced in combination with temperature variations. The loss of local temporal coherence combined with an optimal baseline selection procedure is shown to be effective for the detection of damage, and a delay-and-sum imaging method applied to the residual signals both localizes the damage and provides characterization information.

Vibration energy harvesting by magnetostrictive material

Abstract: A new class of vibration energy harvester based on magnetostrictive material (MsM), Metglas 2605SC, is designed, developed and tested. It contains two submodules: an MsM harvesting device and an energy harvesting circuit. Compared to piezoelectric materials, the Metglas 2605SC offers advantages including higher energy conversion efficiency, longer life cycles, lack of depolarization and higher flexibility to survive in strong ambient vibrations. To enhance the energy conversion efficiency and alleviate the need of a bias magnetic field, Metglas ribbons are transversely annealed by a strong magnetic field along their width direction. To analyze the MsM harvesting device a generalized electromechanical circuit model is derived from Hamilton’s principle in conjunction with the normal mode superposition method based on Euler‐Bernoulli beam theory. The MsM harvesting device is equivalent to an electromechanical gyrator in series with an inductor. In addition, the proposed model can be readily extended to a more practical case of a cantilever beam element with a tip mass. The energy harvesting circuit, which interfaces with a wireless sensor and accumulates the harvested energy into an ultracapacitor, is designed on a printed circuit board (PCB) with plane dimension 25 mm × 35 mm. It mainly consists of a voltage quadrupler, a 3 F ultracapacitor and a smart regulator. The output DC voltage from the PCB can be adjusted within 2.0‐5.5 V. In experiments, the maximum output power and power density on the resistor can reach 200 μW and 900 μ Wc m −3 , respectively, at a low frequency of 58 Hz. For a working prototype under a vibration with resonance frequency of 1.1 kHz and peak acceleration of 8.06 m s −2 (0.82 g), the average power and power density during charging the ultracapacitor can achieve 576 μ Wa nd 606 μ Wc m −3 , respectively, which compete favorably with piezoelectric vibration energy harvesters. (Some figures in this article are in colour only in the electronic version)

Smart aggregates: multi-functional sensors for concrete structures—a tutorial and a review

Abstract: This paper summarizes the authors' recent pioneering research work in piezoceramic-based smart aggregates and their innovative applications in concrete civil structures. The basic operating principle of smart aggregates is first introduced. The proposed smart aggregate is formed by embedding a waterproof piezoelectric patch with lead wires into a small concrete block. The proposed smart aggregates are multi-functional and can perform three major tasks: early-age concrete strength monitoring, impact detection and structural health monitoring. The proposed smart aggregates are embedded into the desired location before the casting of the concrete structure. The concrete strength development is monitored by observing the high frequency harmonic wave response of the smart aggregate. Impact on the concrete structure is detected by observing the open-circuit voltage of the piezoceramic patch in the smart aggregate. For structural health monitoring purposes, a smart aggregate-based active sensing system is designed for the concrete structure. Wavelet packet analysis is used as a signal-processing tool to analyze the sensor signal. A damage index based on the wavelet packet analysis is used to determine the structural health status. To better describe the time-history and location information of damage, two types of damage index matrices are proposed: a sensor-history damage index matrix and an actuator–sensor damage index matrix. To demonstrate the multi-functionality of the proposed smart aggregates, different types of concrete structures have been used as test objects, including concrete bridge bent-caps, concrete cylinders and a concrete frame. Experimental results have verified the effectiveness and the multi-functionality of the proposed smart aggregates. The multi-functional smart aggregates have the potential to be applied to the comprehensive monitoring of concrete structures from their earliest stages and throughout their lifetime.

An optimized self-powered switching circuit for non-linear energy harvesting with low voltage output

Abstract: Harvesting energy from environmental sources has been of particular interest these last few years. Microgenerators that can power electronic systems are a solution for the conception of autonomous, wireless devices. They allow the removal of bulky and costly wiring, as well as complex maintenance and environmental issues for battery-powered systems. In particular, using piezoelectric generators for converting vibrational energy to electrical energy is an intensively investigated field. In this domain, it has been shown that the harvested energy can be greatly improved by the use of an original non-linear treatment of the piezoelectric voltage called SSHI (Synchronized Switch Harvesting on Inductor), which consists in intermittently switching the piezoelectric element on a resonant electrical network for a very short time. However, the integration of miniaturized microgenerators with low voltage output (e.g. MEMS microgenerators) has not been widely studied. In the case of low voltage output, the losses introduced by voltage gaps of discrete components such as diodes or transistors can no longer be neglected. Therefore the purpose of this paper is to propose a model that takes into account such losses as well as a new architecture for the SSHI energy harvesting circuit that limits such losses in the harvesting process. While most of the study uses an externally powered microcontroller for the non-linear treatment, this circuit is fully self-powered, thus providing an enhanced autonomous microgenerator. In particular this circuit aims at limiting the effect of non-linear components with a voltage gap such as diodes. It is shown both theoretically and experimentally that the harvested power can be significantly increased using such a circuit. In particular, experimental measurements performed on a cantilever beam show that the circuit allows a 160% increase of the harvested power compared to a standard energy harvesting circuit, while the classical implementation of the SSHI shows an increase of only 100% of the output power in the classical case.

Model of a single mode energy harvester and properties for optimal power generation

Abstract: The process of acquiring the energy surrounding a system and converting it into usable electrical energy is termed power harvesting. In the last few years, the field of power harvesting has experienced significant growth due to the ever increasing desire to produce portable and wireless electronics with extended life. Current portable and wireless devices must be designed to include electrochemical batteries as the power source. The use of batteries can be troublesome due to their finite energy supply, which necessitates their periodic replacement. In the case of wireless sensors that are to be placed in remote locations, the sensor must be easily accessible or of disposable nature to allow the device to function over extended periods of time. Energy scavenging devices are designed to capture the ambient energy surrounding the electronics and covert it into usable electrical energy. The concept of power harvesting works towards developing self-powered devices that do not require replaceable power supplies. The development of energy harvesting systems is greatly facilitated by an accurate model to assist in the design of the system. This paper will describe a theoretical model of a piezoelectric based energy harvesting system that is simple to apply yet provides an accurate prediction of the power generated around a single mode of vibration. Furthermore, this model will allow optimization of system parameters to be studied such that maximal performance can be achieved. Using this model an expression for the optimal resistance and a parameter describing the energy harvesting efficiency will be presented and evaluated through numerical simulations. The second part of this paper will present an experimental validation of the model and optimal parameters.

A piezoelectric fibre composite based energy harvesting device for potential wearable applications

Abstract: Rapid technological advances in nanotechnology, microelectronic sensors and systems are becoming increasingly miniaturized to the point where embedded wearable applications are beginning to emerge. A restriction to the widespread application of these microsystems is the power supply of relatively sizable dimensions, weight, and limited lifespan. Emerging micropower sources exploit self-powered generators utilizing the intrinsic energy conversion characteristics of smart materials. 'Energy harvesting' describes the process by which energy is extracted from the environment, converted and stored. Piezoelectric materials have been used to convert mechanical into electrical energy through their inherent piezoelectric effect. This paper focuses on the development of a micropower generator using microcomposite based piezoelectric materials for energy reclamation in glove structures. Devices consist of piezoelectric fibres, 90–250 µm in diameter, aligned in a unidirectional manner and incorporated into a composite structure. The fibres are laid within a single laminate structure with copper interdigitated electrodes assembled on both sides, forming a thin film device. Performances of devices with different fibre diameters and material thicknesses are investigated. Experiments are outlined that detail the performance characteristics of such piezoelectric fibre laminates. Results presented show voltage outputs up to 6 V which is considered enough for potential applications in powering wearable microsystems.

Active vibration control of beams with optimal placement of piezoelectric sensor/actuator pairs

Abstract: This paper considers the optimal placement of collocated piezoelectric actuator?sensor pairs on flexible beams using a model-based linear quadratic regulator (LQR) controller. A finite element method based on Euler?Bernoulli beam theory is used. The contributions of piezoelectric sensor and actuator patches to the mass and stiffness of the beam are considered. The LQR performance is taken as the objective for finding the optimal location of sensor?actuator pairs. The problem is formulated as a multi-input multi-output (MIMO) model control. The discrete optimal sensor and actuator location problem is formulated in the framework of a zero?one optimization problem which is solved using genetic algorithms (GAs). Classical control strategies like direct proportional feedback, constant gain negative velocity feedback and the LQR optimal control scheme are applied to study the control effectiveness. The study of the optimal location of actuators and sensors is carried out for different boundary conditions of beams like cantilever, simply supported and clamped boundary conditions.

Energy harvesting based on Ericsson pyroelectric cycles in a relaxor ferroelectric ceramic

Abstract: This work deals with energy harvesting from temperature variations. It is shown here that direct pyroelectric energy harvesting (connecting an adapted resistance, for example) is not effective, whereas Ericsson-based cycles give energy 100 times higher. The principle and experimental validation of the Ericsson cycle are shown with the example of 0.90Pb(Mg1/3Nb2/3)O3?0.10PbTiO3 ceramic. Harvested energy reached 186?mJ?cm?3 for 50??C temperature variation and electric field cycle of 3.5?kV?mm?1. A correlation between the electrocaloric effect and pyroelectric energy harvesting is then shown. Harvested electric energy with Ericsson cycles can be simply expressed as electrocaloric heat multiplied by Carnot efficiency. Several examples are then given from materials with the highest known electrocaloric effect. This leads to energies of hundreds of mJ?cm?3 for a limited 10??C temperature variation. Compared to Carnot's efficiency, this is much higher than the best thermoelectric materials based on the Seebeck effect.

The rheology of shear thickening fluid (STF) and the dynamic performance of an STF-filled damper

Abstract: This paper presents a study of the rheological properties of shear thickening fluid (STF) and its application as a damper. The STF samples, with different weight fractions, were prepared by dispersing nanosized silica particles in a solvent. By using a parallel-plate rheometer, both steady-state and dynamic experiments were carried out to investigate the rheological properties of STFs. Experimental results indicated that these suspensions show an abrupt increase in complex viscosity beyond a critical dynamic shear rate, as well as this increase being reversible. Working with the fabricated STF materials, a prototype damper was fabricated and its dynamic performances were experimentally evaluated. An equivalent linear model through effective elastic stiffness and viscous damping was developed to address both the damping and the stiffness capabilities of the damper. Also, a mathematical model was developed to investigate working mechanisms of STF-based devices.

Harvesting raindrop energy: experimental study

Abstract: At present, the energy autonomy of a microsystem is limited by the lifespan of the battery. Hence the development of the energy harvesting concept, whereby the energy needed to power the sensor is taken from the operating environment. However, there is no single solution suitable for all types of environment. In this paper, we look at a still unexploited source of energy: rain. Our system scavenges the vibration energy from a piezoelectric flexible structure impacted by a water drop. We present an experimental device that validates the aforementioned theoretical results.

Practical issues related to the application of the electromechanical impedance technique in the structural health monitoring of civil structures: I. Experiment

Abstract: The advent of smart materials such as the piezo-impedance transducer (lead zirconate titanate, PZT) and optical fiber (FBG) has ushered in a new era in the field of structural health monitoring (SHM) based on non-destructive evaluation (NDE). So far, successful research and investigations conducted on the electromechanical impedance (EMI) technique employing a piezo-impedance transducer are often laboratory based and mainly theoretical. Real-life application of the technique, especially under harsh environments, has frequently been questioned. In this research project, investigative studies were conducted to evaluate the problems involved in real-life applications of the EMI technique, attempting to reduce the gap between theory and application. This two-part paper presents a series of experimentation (part I) and numerical verification (part II) on various issues related to real-life application, including the durability of PZT transducers, and the effects of bonding and temperature under conceivable nominal construction site conditions. The repeatability of electrical admittance signatures acquired from the PZT patches surface bonded on aluminum structures was found to be excellent up to a period of one and a half years. Experimental investigations revealed that the bonding thickness should preferably be thinner than one-third of the patch to avoid any adverse effect caused by the PZT patch's resonance on the admittance signatures which reflect the host structural behavior. On the other hand, the effect of temperature on the admittance signatures was found to be closely related to the thickness of bonding, as an increase in temperature would reduce the stiffness of the bonding layer, thus affecting strain transfer. It was concluded that PZT patches with thick bonding thickness and high frequency of excitation are undesirable, especially at elevated temperatures.

A novel approach for the structural identification and monitoring of a full-scale 17-story building based on ambient vibration measurements

Abstract: For reliable and practical application of structural health monitoring approaches in conjunction with dense sensor arrays deployed on 'smart' systems, there is a need to develop and evaluate alternate strategies for efficient problem decomposition to rapidly and accurately determine the occurrence, location and level of small changes in the underlying structural characteristics of a monitored system based on its vibrational signature. Furthermore, there is also a need to quantify the level of uncertainties in the identified system characteristics so as to have a measurable level of confidence in the parameters to be relied on for the detection of genuine changes (damage) in the monitored system. This study presents the results of two time-domain identification techniques applied to a full-scale 17-story building, based on ambient vibration measurements. The Factor building is a steel frame structure located on the UCLA campus. This building was instrumented permanently with a dense array of 72-channel accelerometers, and the acceleration data are being continuously recorded. The first identification method used in this study is the NExT/ERA, which is regarded as a global (or centralized) approach, since it deals with the global dynamic properties of the structure. The second method is a time-domain identification technique for chain-like MDOF systems. Since in this method the identification of each link of the chain is performed independently, it is regarded as a local (or decentralized) identification methodology. For the same reason, this method can be easily adopted for large-scale sensor network architectures in which the centralized approaches are not feasible due to massive storage, power, bandwidth and computational requirements. To have a statistically meaningful results, 50 days of recorded data are considered in this study. The modal parameter and chain identification procedures are performed over time windows of 2 h each and with 50% overlap. Using the NExT/ERA method, 12 dominant modes of the building were identified. It was observed that variations in the frequency estimation are relatively small; the coefficient of variation is about 1–2% for most of the estimated modal frequencies. Chain system identification was successfully implemented using the output-only data acquired from the Factor building. Probability distributions of the estimated coefficients of displacement and velocity terms in the interstory restoring functions (which are the mass-normalized local stiffness and damping values) that were found based on the chain system identification are presented. The variability of the estimated parameters due to temperature fluctuations is investigated. It is shown that there is a strong correlation between the modal frequency variations and the temperature variations in a 24 h period.

Vibration damping with negative capacitance shunts: theory and experiment

Abstract: This paper analyzes in detail the enhancement of piezoelectric stack transducers by means of the well known 'negative' capacitive shunting. The stability is thoroughly studied: starting from the electrical admittance curve of the transducer, a method is introduced that quantifies the stability margins of the shunted structure. Two different implementations (series vs parallel) are investigated, and the lack of robustness of the parallel one is demonstrated. Next, this technique is experimentally applied on a truss structure. Its performances are compared with those of passive shunt circuits and with those of an active control law, the so-called Integral Force Feedback or IFF. As expected, the damping introduced by the negative capacitance shunt is larger than the damping obtained with the passive shunts; it remains, however, one order of magnitude smaller than that obtained with the IFF.

Piezoelectric sensor based nondestructive active monitoring of strength gain in concrete

Abstract: The recent advent of smart materials, such as piezoelectric materials, shape-memory alloys, and optical fibers, has added a new dimension to present structural health monitoring techniques. In particular, the electro-mechanical impedance (EMI) sensing technique utilizing piezoelectric materials has emerged as a potential tool for the implementation of a built-in monitoring system for damage detection of civil structures. However, there is little effort to apply this technique for concrete monitoring. In this study, an effort to extend the applicability of the EMI sensing technique is made for strength gain monitoring of early age concrete. PZT (piezoelectric lead zirconate titanate) patches are employed to sense the EMI signature of curing concrete. A series of experiments was conducted on concrete specimens to verify the applicability of the EMI sensing technique. The results show the excellent potential of the EMI sensing technique as a practical and reliable nondestructive method for strength gain monitoring.

Development of the rate-dependent Prandtl?Ishlinskii model for smart?actuators

Abstract: A generalized Prandtl–Ishlinskii model is proposed for characterizing the rate-dependent hysteresis behavior of smart actuators. A rate-dependent play operator is formulated and integrated to the Prandtl–Ishlinskii model together with a dynamic density function to predict hysteresis properties as a function of the rate of change of the input. Relaxation functions are further proposed to relax the congruency in the output of the Prandtl–Ishlinskii model. The fundamental properties of the proposed rate-dependent operator are systematically provided, which conform with important effects of the time rate of input on the hysteresis output established from the reported experimental data. Additional laboratory experiments were performed to characterize the rate-dependent hysteresis behavior of a PZT actuator under excitation in the 1–500 Hz frequency range. The measured data were used to demonstrate the validity of the proposed generalized model. The comparisons suggest that the proposed rate-dependent operator and density functions allow for prediction of the rate-dependent hysteresis under dynamically varying inputs. From the simulation results attained under varied dynamic inputs, it is shown that the proposed model can predict both major and minor hysteresis loops, and that the hysteresis increases significantly with increasing frequency.

Magnetorheology of submicron diameter iron microwires dispersed in silicone oil

Abstract: We investigate the magnetorheological (MR) properties of suspensions containing iron microwires with 260 nm diameter and two distinct length distributions of 54 ± 52 µm and 76 ± 51 µm suspended in silicone oil (045 Pa s) The rheological properties of these fluids were determined using a parallel plate rheometer equipped with a variable strength electromagnet The shear stress was measured as a function of shear rate for increasing applied magnetic fields These results were modeled using the Bingham-plastic constitutive model to determine the apparent yield stress and viscosity as a function of increasing volume fraction and length of microwires At a saturated magnetic flux density, the yield stress using the 54 µm microwires was found to be 065, 223, and 476 kPa for the 2, 4, and 6 vol% suspensions, respectively For the 76 µm wires, the yield stress increases to 82 kPa for the 6 vol% suspension Compared with conventional MR fluids employing spherical particles, the degree of settling is markedly decreased in the microwire-based fluids At 6 vol%, conventional fluids display appreciable settling whereas the microwire-based fluids display no discernable settling Moreover, the rod-shaped microwires are shown to increase the yield stress of the fluids and enhance the MR performance

Self-healing woven glass fabric/epoxy composites with the healant consisting of micro-encapsulated epoxy and latent curing agent

Abstract: This paper reports a study of self-healing woven glass fabric reinforced epoxy composites. The healing agent was a two-component one synthesized in the authors' laboratory, which consisted of epoxy-loaded urea-formaldehyde microcapsules as the polymerizable binder and CuBr2(2-methylimidazole)4 (CuBr2(2-MeIm)4) as the latent hardener. Both the microcapsules and the matching catalyst were pre-embedded and pre-dissolved in the composites' matrix, respectively. When the microcapsules are split by propagating cracks, the uncured epoxy can be released into the damaged areas and then consolidated under the catalysis of CuBr2(2-MeIm)4 that was homogeneously distributed in the composites' matrix on a molecular scale. As a result, the cracked faces can be bonded together. The influence of the content of the self-healing agent on the composites' tensile properties, interlaminar fracture toughness and healing efficiency was evaluated. It was found that a healing efficiency over 70% relative to the fracture toughness of virgin composites was obtained in the case of 30?wt% epoxy-loaded microcapsules and 2?wt% latent hardener.

Investigation of the strain transfer for surface-attached optical fiber strain sensors

Abstract: The relation between the component strain and the strain on a surface-attached optical fiber is governed by the effectiveness of shear transfer through the adhesive and the polymeric coating(s) on the optical fiber. A classical shear lag model can predict the strain transfer through a soft layer well. However, experiments showed that the results are unsatisfactory for bare fiber with stiff adhesive case. A 3D-FEM is established to model the strain transfer of a surface-mounted strain sensor and it is verified by experiments. Then, it is used to investigate the influence of four geometric parameters of the adhesive: (1) side width, (2) top thickness, (3) bond length, (4) bottom thickness, on the strain transfer. By sensitivity analysis, it is revealed that the bond length and the bottom thickness are dominant factors. Based on finite element results, the parameter of the analytical model is modified to suit stiff layer cases. Important considerations for practical installation of surface-attached optical interferometric and FBG strain sensors will be discussed.

Characterization of the harvesting capabilities of an ionic polymer metal composite device

Abstract: Harvesting systems capable of transforming dusty environmental energy into electrical energy have aroused considerable interest in the last two decades Several research works have focused on the transformation of mechanical environmental vibrations into electrical energy Most of the research activity refers to classic piezoelectric ceramic materials, but more recently piezoelectric polymer materials have been considered In this paper, a novel point of view regarding harvesting systems is proposed: using ionic polymer metal composites (IPMCs) as generating materials The goal of this paper is the development of a model able to predict the energy harvesting capabilities of an IPMC material working in air The model is developed by using the vibration transmission theory of an Euler?Bernoulli cantilever IPMC beam The IPMC is considered to work in its linear elastic region with a viscous damping contribution ranging from 01 to 100?Hz An identification process based on experimental measurements performed on a Nafion? 117 membrane is used to estimate the material parameters The model validation shows a good agreement between simulated and experimental results The model is used to predict the optimal working region and the optimal geometrical parameters for the maximum power generation capacity of a specific membrane The model takes into account two restrictions The first is due to the beam theory, which imposes a maximum ratio of 05 between the cantilever width and length The second restriction is to force the cantilever to oscillate with a specific strain; in this paper a 03% strain is considered By considering these two assumptions as constraints on the model, it is seen that IPMC materials could be used as low-power generators in a low-frequency region The optimal dimensions for the Nafion? 117 membrane are length = ?12?cm and width = ?62?cm, and the electric power generation is 3?nW at a vibrating frequency of 709?rad?s?1 IPMC materials can sustain big yield strains, so by increasing the strain allowed on the material the power will increase dramatically, the expected values being up to a few microwatts

Skin design studies for variable camber morphing airfoils

Abstract: This paper identifies the desirable attributes of a flexible skin of a morphing wing. The study is conducted using airfoil camber morphing as an example. The ideal flex-skin would be highly anisotropic, having a low in-plane axial stiffness but a high out-of-plane flexural stiffness. Reduced skin axial stiffness allows morphing at low actuation cost. However, for some substructure and actuation designs, a lower limit on the skin's in-plane axial stiffness may be required to prevent unacceptable global camber deformation under aerodynamic loads. High flexural stiffness prevents local deformation of skin sections between supports due to aerodynamic pressure loads, and avoids buckling of skin sections under compression as the airfoil cambers under actuation force. For the camber morphing application the strain levels in the flex-skin are not expected to exceed around 2%. If the axial stiffness of the flex-skin is reduced significantly, it may be necessary to consider aerodynamic stiffness (negligible vis-a-vis structural stiffness for classical airfoils) to accurately calculate deformation under loading. The approach followed in the study can be used to identify specifications for the skin and then reverse engineer and design highly anisotropic composite skins that meet the specifications.

A model of ionic polymer–metal composite actuators in underwater operations

Abstract: Ionic polymer metal composites (IPMCs) are active materials that exhibit a bi-directional electromechanical coupling: a voltage produces membrane bending, while by bending an IPMC membrane a voltage output is obtained. IPMCs are of increasing interest in a number of application fields. More specifically, IPMCs can work in wet environments, even in water, and this represents a valuable capability in a number of applications fields such as underwater robotics, surveillance, and biomedical applications. In this work a totally new model of an active IPMC beam, solicited by a voltage signal and immersed in water, is introduced. The model estimates the moment produced by the applied voltage. Therefore, the classical Euler–Bernoulli cantilever beam theory and the concept of hydrodynamic function are used to describe the interaction between the beam and the water. Knowledge of this interaction allows estimation of the IPMC active beam motion in water.

Optimal design of magnetorheological valves via a finite element method considering control energy and a time constant

Abstract: This study presents an optimal design for magnetorheological (MR) valves for minimizing the control energy to be applied to coils to control the pressure drop of the valves The optimization problem identifies parameters such as applied current, coil wire size and geometric dimensions of the valves which satisfy the specified pressure drop and inductive time constant requirements After describing the configuration of MR valves, their pressure drops are obtained on the basis of the Bingham model of MR fluid Then, the control energy which is an objective function and the inductive time constant are derived Subsequently, an optimization procedure using a golden-section algorithm and a local quadratic fitting technique is constructed via a commercial finite element method parametric design language Using the optimization tool developed in this study, optimal MR valve configurations are identified, which are constrained to a specific cylindrical volume defined by its radius and height In addition, optimization results for MR valves with different required pressure drops and different constrained volumes are obtained and presented

Large scale testing of nitinol shape memory alloy devices for retrofitting of bridges

Abstract: A large scale testing program was conducted to determine the effects of shape memory alloy (SMA) restrainer cables on the seismic performance of in-span hinges of a representative multiple-frame concrete box girder bridge subjected to earthquake excitations. Another objective of the study was to compare the performance of SMA restrainers to that of traditional steel restrainers as restraining devices for reducing hinge displacement and the likelihood of collapse during earthquakes. The results of the tests show that SMA restrainers performed very well as restraining devices. The forces in the SMA and steel restrainers were comparable. However, the SMA restrainer cables had minimal residual strain after repeated loading and exhibited the ability to undergo many cycles with little strength and stiffness degradation. In addition, the hysteretic damping that was observed in the larger ground accelerations demonstrated the ability of the materials to dissipate energy. An analytical study was conducted to assess the anticipated seismic response of the test setup and evaluate the accuracy of the analytical model. The results of the analytical simulation illustrate that the analytical model was able to match the responses from the experimental tests, including peak stresses, strains, forces, and hinge openings.

A mixture theory framework for modeling the mechanical actuation of ionic polymer metal composites

Abstract: An ionic polymer metal composite (IPMC) is a porous charged polymer saturated with an electrolytic solvent and plated by two metallic electrodes. A voltage difference across the electrodes generates structural deformations; similarly, a mechanical deformation yields a voltage difference across the electrodes. The electrolytic solvent comprises a mobile ionic species and an uncharged solvent. Interactions between mobile ions and the solvent and between the solvent and the backbone polymer are responsible for sensing and actuation. We present a mixture theory framework for mechanical modeling of IPMCs and of species interactions occurring therein. The model consists of three coupled linear partial differential equations, and it is applicable to a large variety of IPMC geometries and microstructures. The framework allows for a thorough description of actuation mechanisms, including osmotic pressure, hydraulic pressure, and electrostatic forces. The model describes the presence of boundary layers of mobile ions and solvent concentrations in the vicinity of the electrodes. We particularize the general three-dimensional model to a slender IPMC, and we derive a one-dimensional distributed model using the Euler–Bernoulli beam theory and a parallel-plate approximation. We validate our theoretical findings through a set of experiments conducted on Nafion-based IPMCs.

Embedded SMA wire actuated biomimetic fin: a module for biomimetic underwater propulsion

Abstract: An embedded shape memory alloy (SMA) wire actuated biomimetic fin is presented, and based on this module for biomimetic underwater propulsion, a micro robot fish (146 mm in length, 30 g in weight) and a robot squid (242 mm in length, 360 g in weight) were developed. Fish swim by undulating their body and/or fins. Squid and cuttlefish can also swim by undulating their fins. To simplify engineering modeling, the undulating swimming movement is assumed to be the integration of the movements of many flexible bending segments connected in parallel or in series. According to this idea, a biomimetic fin which can bend flexibly was developed. The musculature of a cuttlefish fin was investigated to aid the design of the biomimetic fin. SMA wires act as 'muscle fibers' to drive the biomimetic fin just like the transverse muscles of the cuttlefish fin. During the bending phase, elastic energy is stored in the elastic substrate and skin, and during the return phase, elastic energy is released to power the return movement. Theorem analysis of the bending angle was performed to estimate the bending performance of the biomimetic fin. Experiments were carried out on single-face fins with latex rubber skin and silicone skin (SF-L and SF-S) to compare the bending angle, return time, elastic energy storage and reliability. Silicone was found to be the better skin. A dual-face fin with silicone skin (DF-S) was tested in water to evaluate the actuating performance and to validate the reliability. Thermal analysis of the SMA temperature was performed to aid the control strategy. The micro robot fish and robot squid employ one and ten DF-S, respectively. Swimming experiments with different actuation frequencies were carried out. The speed and steering radius of the micro robot fish reached 112 mm s−1 and 136 mm, respectively, and the speed and rotary speed of the robot squid reached 40 mm s−1 and 22° s−1, respectively.

Study on magnetorheological shear thickening fluid

Abstract: In this paper, a magnetic-field-controlled and speed-activated magnetorheological shear thickening fluid (MRSTF) is presented. We fabricated a kind of shear thickening fluid (STF) which was composed of nanosize silica particles suspended in a solvent, ethylene glycol, at high concentrations. Then the micron-size carbonyl iron particles with different volume fractions were added to the STF to fabricate the MRSTF. Their dynamic properties in different shear strain rates and magnetic fields were tested by using a rheometer. The suspension shows an abrupt increase in complex viscosity beyond a critical dynamic shear rate and a magnetic-field-controllable characteristic, as well as being reversible.