ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V002T00A001. doi:10.1115/SMASIS2016-NS2.

This online compilation of papers from the ASME 2016 Conference on Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2016) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Modeling, Simulation and Control

2016;():V002T03A001. doi:10.1115/SMASIS2016-9016.

We designed and developed a novel smart structural system for pointing control of large-scale support structures, such as trusses. The system consists of a pointing control mechanism, an internal displacement-sensor, and a controller. Remarkable points of our system are (1) artificial thermal expansion of truss members is utilized as linear actuators, (2) elastic hinges are applied instead of boll joints, and (3) the internal displacement-sensor which does not need external jigs and has high measuring accuracy is applied. In this paper, we conducted the feasibility study and the experimental demonstration. As a result of the feasibility study, the proposed pointing control mechanism can produce several hundred arcseconds of the rotational displacements within three minutes, therefore it has potential for using in practical operations. As a result of the experimental demonstration, we confirmed that the hysteresis of the pointing control mechanism can be kept sufficiently small due to the absence of the sliding parts, and has high control accuracy and followability (the error RMS value for a circle of the radius of 500 μm is 3.6 %).

Commentary by Dr. Valentin Fuster
2016;():V002T03A002. doi:10.1115/SMASIS2016-9025.

A two-dimensional phononic crystal (PC) can exhibit longitudinal-mode negative energy refraction on its lowest (acoustical) frequency pass band. The effective elastodynamic properties of a typical PC are calculated and it is observed that the components of the effective density tensor can achieve negative values at certain low frequencies on the acoustical branches for the longitudinal-mode pass-band, and that negative refraction may be accompanied by either positive or negative effective density. Furthermore, such a PC has a high anisotropy ratio at certain low frequencies, offering potential for application to acoustic cloaking where effective material anisotropy is essential.

Commentary by Dr. Valentin Fuster
2016;():V002T03A003. doi:10.1115/SMASIS2016-9051.

Accurately predicting the onset of large behavioral deviations associated with saddle-node bifurcations is imperative in a broad range of sciences and for a wide variety of purposes, including ecological assessment, signal amplification, and adaptive material/structure applications such as structural health monitoring and piezoelectric energy harvesting. In many such practices, noise and non-stationarity are unavoidable and ever-present influences. As a result, it is critical to simultaneously account for these two factors towards the estimation of parameters that may induce sudden bifurcations. Here, a new analytical formulation is presented to accurately determine the probable time at which a system undergoes an escape event as governing parameters are swept towards a saddle-node bifurcation point in the presence of noise. The double-well Duffing oscillator serves as the archetype system of interest since it possesses a dynamic saddle-node bifurcation. Using this archetype example, the stochastic normal form of the saddle-node bifurcation is derived from which expressions of the escape statistics are formulated. Non-stationarity is accounted for using a time dependent bifurcation parameter in the stochastic normal form. Then, the mean escape time is approximated from the probability density function to yield a straightforward means to estimate the point of bifurcation. Experiments conducted using a double-well Duffing analog circuit verify that the analytical approximations provide faithful estimation of the critical parameters that lead to the non-stationary and noise-activated saddle-node bifurcation.

Topics: Bifurcation
Commentary by Dr. Valentin Fuster
2016;():V002T03A004. doi:10.1115/SMASIS2016-9058.

While adaptive tuning of vibration absorbers (ATVA) have been widely studied for vibration control applications, limited studies have been done to explore their potential for noise control applications. This study aims to utilize magnetorheological elastomer (MRE)-based ATVA to control the radiated sound from an elastic plate excited by a plane wave especially at low frequencies. Radiated sound from a clamped circular plate integrated with MRE-based ATVA is analytically studied using classical plate theory. Rayleigh integral approach is, then, used to express the transmitted sound pressure in terms of the plate’s displacement modal amplitude. A MRE-based ATVA under shear mode is investigated. The semi-active Skyhook controller is proposed to attenuate the transverse displacement of the plate and subsequently reduce the radiated sound. The controller determines the current input to the electromagnet and tunes the MRE-based ATVA with the desired stiffness.

Commentary by Dr. Valentin Fuster
2016;():V002T03A005. doi:10.1115/SMASIS2016-9059.

This study aims to investigate the sound transmission loss (STL) capability of sandwich panels treated with Magnetorheological (MR) fluids at low frequencies. An experimental setup has been designed to investigate the effect of the intensity of the applied magnetic field on the natural frequencies and STL of a clamped circular plate. A multilayered uniform circular panel comprising two elastic face sheets and MR fluid core layer is fabricated. It is shown that as the applied magnetic field increases, the fundamental natural frequency of the MR sandwich panel increases. Moreover, the STL of the panel at the resonance frequency considerably increases under applied magnetic field. Furthermore, an analytical model for the STL of the finite multilayered panels with MR core layer is developed and compared with the experimental measurements. The MR core layer is treated as a viscoelastic material with complex shear modulus. It is shown that good agreement exists between the analytical and experimental results. Parametric study has also been conducted to investigate the effect of face sheets and core layers’ thickness.

Topics: Fluids , Sound
Commentary by Dr. Valentin Fuster
2016;():V002T03A006. doi:10.1115/SMASIS2016-9070.

Nickel Titanium (NiTi) shape memory alloys (SMAs) exhibit shape memory and/or superelastic properties, enabling them to demonstrate multifunctionality by engineering microstructural and compositional gradients at selected locations. This paper focuses on the design optimization of NiTi compliant mechanisms resulting in single-piece structures with functionally graded properties, based on user-defined target shape matching approach. The compositionally graded zones within the structures will exhibit an on demand superelastic effect (SE) response, exploiting the tailored mechanical behavior of the structure. The functional grading has been approximated by allowing the geometry and the superelastic properties of each zone to vary. The superelastic phenomenon has been taken into consideration using a standard nonlinear SMA material model, focusing only on 2 regions of interest: the linear region of higher Young’s modulus of elasticity and the superelastic region with significantly lower Young’s modulus of elasticity. Due to an outside load, the graded zones reach the critical stress at different stages based on their composition, position and geometry, allowing the structure morphing. This concept has been used to optimize the structures’ geometry and mechanical properties to match a user-defined target shape structure. A multi-objective evolutionary algorithm (NSGA II - Non-dominated Sorting Genetic Algorithm) for constrained optimization of the structure’s mechanical properties and geometry has been developed and implemented.

Commentary by Dr. Valentin Fuster
2016;():V002T03A007. doi:10.1115/SMASIS2016-9075.

This paper presents a new approach for the parametric identification of nonlinear systems. The approach is based on subjecting a nonlinear system to a strong high-frequency excitation and monitoring its influence on the slow modulation of the system’s response which occurs near its natural frequency. The identification procedure is outlined and numerically implemented on a Duffing-type system with unknown quadratic and cubic nonlinearities. The proposed technique is then implemented to identify the nonlinear parameters of three different experimental systems. Results demonstrate that the proposed approach predicts the nonlinear parameters with good accuracy.

Topics: Excitation
Commentary by Dr. Valentin Fuster
2016;():V002T03A008. doi:10.1115/SMASIS2016-9084.

The magnetorheological Brake (MRB) is an electromechanical brake in which smart magnetorheological (MR) fluids have been utilized to generate the required braking torque. The purpose of this study is to design optimize a real-size MRB for automobile applications considering geometrical, material and magnetic circuit parameters. The mathematical equations governing the system’s braking torques are derived. The dynamic range of a disk-type MRB expressing the ratio of generated toque at on and off states has been formulated as a function of the rotational speed, geometrical and material properties, and applied electrical current. The magnetic circuit analysis of the proposed MRB is performed to find the relation between magnetic field intensity and the applied electrical current as a function of the MRB geometrical and material properties. Finally, a multidisciplinary design optimization problem has been formulated to identify the optimal brake geometrical parameters to maximize the dynamic range of the MRB under weight, size and magnetic flux density constraints. The optimization problem has been solved using combined Genetic Algorithm and Sequential Quadratic Programming techniques. The optimal design is then compared with those available in the literature.

Commentary by Dr. Valentin Fuster
2016;():V002T03A009. doi:10.1115/SMASIS2016-9098.

Tracking an ensemble of basic signals is often required of control systems in general. Here we are given a linear continuous-time infinite-dimensional plant on a Hilbert space and a space of tracking signals generated by a finite basis, and we show that there exists a stabilizing direct adaptive control law that will stabilize the plant and cause it to asymptotically track any member of this collection of signals. The plant is described by a closed, densely defined linear operator that generates a continuous semigroup of bounded operators on the Hilbert space of states. There is no state or parameter estimation used in this adaptive approach. Our results are illustrated by adaptive control of general linear diffusion systems.

Commentary by Dr. Valentin Fuster
2016;():V002T03A010. doi:10.1115/SMASIS2016-9113.

This paper aims to analyze snap-through behavior of two-layer cross-ply bistable composite laminate square plates. The analyses consider the factors of laminate thickness, temperature and external applied force. In this study, the model was performed on the basis of the classical thin plate theory, the Von Kármán large deformation theory and Principle of Virtual Work. Afterwards the statics equilibrium equation was available. Subsequently the analysis was presented by adjusting the laminate thickness for these prior factors. Through the numerical simulations with Matlab® software, the curvatures in x-direction and y-direction were calculated to investigate the snap-through behavior. Two stable cylindrical configurations and an unstable saddle shape were given with different curvatures to show the equilibrium positions. Then the figures prove the external applied distributed force plays a vital role to the snap-through behavior. The results show that under macroscopic view, the ratio of side-length to thickness is three hundred or less, as the plates are thinner, the snap-through will appear more frequently, and the external forces will be less needed.

Commentary by Dr. Valentin Fuster
2016;():V002T03A011. doi:10.1115/SMASIS2016-9118.

A new equivalent circuit is presented, which describes the transport of a chemical solution with a certain concentration in a fluidic channel. Channels are basic parts of a microfluidic systems and the concentration of the chemical solution can control the opening and closing of valves based on smart hydrogels. This type of microfluidic systems facilitates the autonomous control of fluid flow, e.g. in chemo-fluidic oscillators. Through this channel, the solution is transported at a velocity determined by the flow rate through the channel and its cross section. While the volumetric flow is not delayed in an ideal channel, the channel acts as delay line for the particles and thus for a certain concentration transport through the channel.

In this setting, the transport of the dissolved chemical by water traveling along the delay channel can be described by the one-dimensional transport equation. In order to derive the equivalent circuit, the transport equation is numerically approximated based on the well-known Method of Lines. This method consists in approximating the original PDE via a large system of ODEs. The ODEs are obtained by discretizing the PDE in space, in such a way that each component of the resulting system of ODEs approximates the solution of the PDE at some grid point along the spatial interval. Once the system of ODEs has been constructed, a flow and a difference quantity can be defined and the ODEs interpreted as finite network elements. Since the equations are isomorphic to electrical ODEs of electrical network elements, the fluidic channel can be expressed by an equivalent circuit. Thus the transient behavior of the transport mechanism can be calculated using a circuit simulator as part of a design automation. Simulation results are presented.

Topics: Circuits
Commentary by Dr. Valentin Fuster
2016;():V002T03A012. doi:10.1115/SMASIS2016-9137.

This paper presents the design and control of a two link lightweight robotic arm using a couple of antagonistic Shape Memory Alloy (SMA) wires as actuators. A nonlinear robust control law for accurate positioning of the end effector of the two-link SMA based robotic arm is developed to handle the hysteresis behavior present in the system. The model presented consists of two subsystems: firstly the SMA wires model and secondly the dynamics of the robotic arm itself. The control objective is to position the robotic arm’s end effector in a given operational plane position. For this regulation problem a sliding mode control law is applied to the hysteretic system. Finally a Lyapunov analysis is applied to the closed-loop system demonstrating the stability of the system under given conditions. The simulation results demonstrate the accurate and fast response of the control law for position regulation. In addition, the stability of the closed-loop system can be corroborated.

Commentary by Dr. Valentin Fuster
2016;():V002T03A013. doi:10.1115/SMASIS2016-9146.

This paper focuses on the Synchronized Switching Damping (SSD) control, a semi-active vibration suppression technique based on the coupling between a mechanical structure and piezoelectric actuators. This control family is optimized for structures subjected to mono-harmonic excitation. Anyway, in the last years, different solutions to address multi-harmonic and multi-modal excitation have been developed. One of them is the so-called Modal SSD, where the switches of the shunting circuit are governed by the behavior of the modal coordinates of the structure. Although the control performance is good, this paper shows that there are still some issues to be solved. For this reason, the paper proposes a modified version of this control strategy able to improve the control performance without increasing the circuit complexity. The proposed solution is firstly described in detail and then tested and compared with the existing ones on the numerical model of a cantilever beam.

Commentary by Dr. Valentin Fuster
2016;():V002T03A014. doi:10.1115/SMASIS2016-9162.

This paper presents a structural concept that exploits elastic instabilities in novel periodic lattice structures for shape adaptation purposes. The nonlinear behaviour resulting from the occurrence of local buckling is utilised to achieve significant variations in the global structural response of the lattice. For the proposed structural concept, a unit cell is identified and utilised to investigate the mechanical characteristics for the load cases of uniaxial compression, shear, and rotation, conducting nonlinear finite element simulations. The results of the unit cell characterization are compared to the mechanical response of lattice structures under equivalent loading and convergence is achieved for all considered load cases. This paper therefore introduces a novel design concept to achieve selective compliance, especially beneficial for shape adaptation of wing structures.

Commentary by Dr. Valentin Fuster
2016;():V002T03A015. doi:10.1115/SMASIS2016-9165.

Based on a recently developed shakedown theory for non-smooth nonlinear materials, we derive a criterion for high-cycle fatigue in shape memory alloys (SMAs). The fatigue criterion takes into account phase transformation as well as reorientation of martensite variants as the source of fatigue damage. The mathematical derivation of the criterion is based on the requirement of elastic shakedown for a given structure to achieve unlimited fatigue endurance. Elastic shakedown is defined as an asymptotic state in which damage due to time-varying load becomes confined at the mesoscopic scale, or the scale of the grain, with no discernable inelasticity at the macroscopic scale. From an energy standpoint, elastic shakedown corresponds to a situation where energy dissipation becomes bounded and the response elastic after a certain number of loading cycles. A sufficient condition to achieve this state was established by Melan (1936) [1] and Koiter (1960) [2] for elastoplastic materials and later generalized to hardening plasticity by Nguyen (2003) and to non-smooth non-linear materials by Peigney (2014). The latter formulation is applicable to SMAs obeying the ZM constitutive model (Zaki & Moumni, 2007) and is shown here to allow the derivation of a high-cycle fatigue criterion analogous to the one proposed by Dang Van (1973) for elastoplastic materials. The criterion allows establishing a safe domain in stress deviator space at the mesoscopic scale consisting of a hypercylinder with axis parallel to the direction of martensite orientation. The hypercylinder is delimited along its axis by two transverse hyperplanes representing bounds on admissible stress states consistent with the loading conditions for phase transformation. Safety with regard to high-cycle fatigue, upon elastic shakedown, is conditioned by the persistence of the macroscopic stress path, as the load varies and at every material point, strictly within the hypercylinder. The size of the hypercylinder is shown to strongly depend on the relative amount of martensite present in the SMA.

Commentary by Dr. Valentin Fuster
2016;():V002T03A016. doi:10.1115/SMASIS2016-9181.

Micro-valves play an important role in controlling and operating microfluidic systems. Utilizing stimuli-sensitive hydrogels facilitates the construction of smart micro-valves controlled by temperature, concentration (salt, organic solvent) or pH level.

We propose a finite element model which uses the thermal domain as an auxiliary domain for the volume change response of hydrogels. Behaviors like local displacements within the hydrogel are difficult to measure, but can be reproduced with finite elements. For the application of the micro-valve, the hydrogel model is connected to the fluid domain. The hydrogel is placed directly into the fluid flow and opens or closes the flow path. For this, a full iterative cycle with material properties and remeshing in each simulation step is implemented in ANSYS®.

This model concept and the results will help to better understand, predict and visualize the behavior of hydrogels and support the development of highly integrated hydrogel-based microfluidic circuits.

Commentary by Dr. Valentin Fuster
2016;():V002T03A017. doi:10.1115/SMASIS2016-9186.

Wave propagation inside a host media with periodically distributed inclusions can exhibit bandgaps. While controlling acoustic wave propagation has large impact on many engineering applications, studies on broadband acoustic bandgap (ABG) adaptation is still outstanding. One of the important properties of periodic structure in ABG design is the lattice-type. It is possible that by reconfiguring the periodic architectures between different lattice-types with fundamentally distinct dispersion relations, we may achieve broadband wave propagation tuning. In this spirit, this research pioneers a new class of reconfigurable periodic structures called origami metastructures (OM) that can achieve ABG adaption via topology reconfiguration by rigid-folding. It is found that origami folding, which can enable significant and precise topology reconfigurations between distinct Bravais lattice-types in underlying periodic architecture, can bring about drastic changes in wave propagation behavior. Such versatile wave transmission control is demonstrated via numerical studies that couple wave propagation theory with origami folding kinematics. Further, we also exploit the novel ABG adaptation feature of OM to design structures that can exhibit unique tunable non-reciprocal behavior. Overall the broadband adaptable wave characteristics of the OM coupled with scale independent rigid-folding mechanism can bring on-demand wave tailoring to a new level.

Commentary by Dr. Valentin Fuster
2016;():V002T03A018. doi:10.1115/SMASIS2016-9195.

Origami provides inspiration and solutions to the fabrication and functionality of various structures. Origami design methods in the literature are limited to the idealization of the folds as creases of zeroth-order geometric continuity. This idealization is not proper for origami structures having non-negligible fold thickness or maximum curvature at the folds restricted by material or structural limitations. For these structures, the folds are not accurately represented as creases but instead as bent regions of higher-order geometric continuity. These fold regions of arbitrary order of continuity are denoted in this work as smooth folds. A method for the design of a single planar sheet and its associated pattern of smooth folds that morphs into a given three-dimensional goal shape represented as a polygonal mesh is proposed. The parameterization of the planar sheet and the constraints allowing for a valid smooth fold pattern and matching of the goal shape in a folded configuration are presented. The folding deformation of the determined sheet designs is simulated using a previously derived kinematic model for origami with smooth folds. Various testing examples considering diverse goal shapes are presented. The results demonstrate that each considered sheet design matches its corresponding goal shape in a known folded configuration having fold angles determined from the geometry of the goal mesh. The proposed method can be used for the design of origami structures having folds of arbitrary order of geometric continuity such as origami-inspired active structures.

Topics: Design
Commentary by Dr. Valentin Fuster
2016;():V002T03A019. doi:10.1115/SMASIS2016-9212.

The availability of low-cost, readily programmable digital hardware offers numerous opportunities for novel modeling and control approaches. One such opportunity is the realization of hardware modeling of distributed dynamic systems. Such models could be useful for control algorithms that require high-fidelity models operating in real-time. The ultimate goal is to utilize digital systems with programmable hardware. As a proof-of-concept, multiple discrete microcontrollers have been used to emulate how programmable hardware devices may be used to simulate a distributed vibrating system. Specifically, each microcontroller is treated as a single vibrating mass with stiffness and damping coupling between the masses. Each microcontroller has associated position and velocity variables. The only additional knowledge required to compute the acceleration of each “mass” is thus the position and velocity of each immediate neighboring mass/microcontroller. The computation time is independent of the number of nodes; adding nodes results in no reduction in processing speed. Consequently, the computational approach will be applicable to very high order models. Practical implementation of such models will require digitally programmable hardware such as field-programmable gate arrays (FPGA), however an added benefit will be a still greater reduction in cost, as multiple microcontrollers are replaced by a single FPGA. It is expected that the hardware modeling approach described in this work will have application not only in the field of vibration modeling and control, but also in other fields where control of distributed dynamic systems is desired.

Commentary by Dr. Valentin Fuster
2016;():V002T03A020. doi:10.1115/SMASIS2016-9218.

Spiral Torsion Springs (STS) are generally manufactured employing medium/high-carbon steel alloys shaped as thin rods with rectangular cross section. Meanwhile, plastic materials (e.g. ABS or PLA), currently used in freeform manufacturing processes, may not be suited for several applications, owing to the low material yield strength and the rather poor fatigue life. Despite the above-mentioned limitations, the main advantages of a 3D printing process, as compared to more traditional manufacturing techniques, are the design flexibility and the possibility to directly integrate elastic components within a joint mechanism produced as a single (monolithic) part. In particular, provided that the external forces acting on the spring coils are maintained within a certain threshold and that the spring geometry is suitably optimized, a reliable 3D-printed STS alternative to traditional steel springs is actually feasible. Given these premises, the main purpose of the present paper is to propose a model-based optimization algorithm that allows to optimally size STS for user-specified torque-deflection characteristics. Optimal STS geometries are then realized in ABS via Fused Deposition Manufacturing, and subsequently tested with a purposely-designed experimental set-up. Furthermore, the behavior of each STS sample (in terms of stiffness and equivalent Von Mises stress) is evaluated by means of non-linear finite elements analysis, in order to check the correspondence with the expected behavior. Finally, numerical and experimental results are provided, which demonstrate the prediction capabilities of the proposed modeling/optimization techniques, and confirm that well-behaved STS can be conceived and produced. Envisaged applications concern the development of smart structures for robot design, such as multi-articulated compliant robotic chains that can be used as low-cost manipulators (i.e. arm) or as mini-manipulators (i.e. fingers). The proposed approach effectively simplifies the production and the assembly of the mechanism, also allowing for an easier integration of embedded sensory-actuation systems.

Topics: Torsion , Design , Springs
Commentary by Dr. Valentin Fuster
2016;():V002T03A021. doi:10.1115/SMASIS2016-9228.

High resolution imaging in scanning probe microscopes is conducted by rastering a sharp probe over a sample surface. The rastering is done using piezoelectric elements, converting applied voltage into mechanical motion. For example, imaging of a rectangular field of view is done by applying triangular waveforms with different frequencies to X and Y piezoelectric stage, respectively. A disadvantage of piezoelectric stages is their non-linear response to applied voltage. In addition to that, they show creep, i.e. moving even though the applied voltage is constant. This results in distortions of the acquired image. Furthermore, it can result in not precisely imaging the requested area. A common solution is to add position sensors to the piezoelectric stages and measure actual movements. By using a feedback it would be almost guaranteed that the piezoelectric stage moves as requested. The disadvantage of this approach is that it reduces the bandwidth and increases the noise. The aim of this paper is to study advanced piezoelectric stage models to better control the actual stage movement in an open-loop scan.

Commentary by Dr. Valentin Fuster
2016;():V002T03A022. doi:10.1115/SMASIS2016-9242.

Using a Shape Memory Alloy (SMA) actuator as both an actuator and a sensor provides huge benefits in cost reduction and miniaturization of robotic devices. Despite much effort, reliable and robust self-sensing (using the actuator as a position sensor) has not been achieved for general temperature, loading, hysteresis path, and fatigue conditions. Prior research has sought to model the intricacies of the electrical resistivity changes within the NiTi material. However, for the models to be solvable, nearly every previous technique only models the actuator within very specific boundary conditions. Here, we measure both the voltage across the entire NiTi wire and of a fixed-length segment of it; these dual measurements allow direct calculation of the actuator length without a material model. We review previous self-sensing literature, illustrate the mechanism design that makes the new technique possible, and use the dual measurement technique to determine the length of a single straight wire actuator under controlled conditions. This robust measurement can be used for feedback control in unknown ambient and loading conditions.

Commentary by Dr. Valentin Fuster
2016;():V002T03A023. doi:10.1115/SMASIS2016-9259.

In the conventional implementation of synchronized switch damping (SSD), the switches are intended to occur at every displacement extrema. However, recent work reveals that switching at the vibration peaks is only optimal for displacement reduction under resonance excitation. In general, the optimal switch timing is dependent on the excitation frequency along with the electromechanical coupling and modal damping of the structure. This work seeks to develop a control framework that searches through the possible switch times to find the optimal switch time for synchronized switch damping on an inductor (SSDI) under steady state excitation. The control law does not need any knowledge of the system, only requiring the voltage of the piezo actuator to develop a displacement estimate that is minimized by adjusting the switch timing. Furthermore, the controller naturally senses changes in the excitation and adapts the switch timing to best reduce displacement under the new excitation.

Commentary by Dr. Valentin Fuster
2016;():V002T03A024. doi:10.1115/SMASIS2016-9263.

This paper presents Kinematics and Dynamics of a Shape-Shifting Surface, a robotic system able to take on the shape of arbitrary connected 3D surfaces. Such a surface, which we introduced and described in previous work, consists of piecewise controllable chains in turn composed of serially connected foldable “robotic particles”. Aiming at a high resolution rendering, where tiny particles need to be combined in a large number, a tendon-driven design is a lightweight and scalable solution.

However, improper actuation strategies might expose the system to undesired forces, which can compromise its integrity and stability. To tackle this problem, optimal actuation and planning strategies are required to anticipate unacceptable situations. To this end, a dynamic model is derived to predict the reaction of the system subject to control actions. Being the system both tendon-driven and under-actuated, we have to overcome a number of challenges in deriving this model.

Commentary by Dr. Valentin Fuster
2016;():V002T03A025. doi:10.1115/SMASIS2016-9266.

In the paper, we discuss the development of a model for PZT bimorph actuators used to power micro-air vehicles including Robobee. Due to highly dynamic drive regimes required for the actuators, models must quantify the nonlinear, hysteretic, and rate-dependent behavior inherent to PZT in these regimes. We employ the homogenized energy model (HEM) framework to model the actuator dynamics and numerically we illustrate the capability of the model to characterize the inherent hysteresis. This provides a comprehensive model, which can be inverted and implemented for certain control regimes.

Commentary by Dr. Valentin Fuster
2016;():V002T03A026. doi:10.1115/SMASIS2016-9267.

Foot drop usually happens due to neurological and muscular diseases. It limits individuals’ abilities in ankle and toe represented in dorsiflexion during swing phase, and plantar flexion during heel strike. A non-surgical solution to such weakness is the use of ankle foot orthoses (AFOs) which can assist in such abnormal ambulation. The purpose of this work is to develop a new ankle foot orthosis that helps patients to have more normal ankle joint behavior. The proposed AFO device takes advantage of the superelastic behavior of Ni-rich NiTi alloys. In order to evaluate the performance of the Ni-rich NiTi hinged ankle foot orthoses, several motion analysis tests for a normal walking of a healthy subject were conducted. Also, a finite element model were developed to evaluate the performance of superelastic versus stainless steel springs.

A Ni-rich NiTi wire was wrapped around a designed rod and the two heads were fixed to the rod (to get the shape of a spring). Then a heat treatment process was performed in a furnace to shape set the NiTi wires and to provide them with the needed superelastic behavior. The produced springs were connected to a designed hinged ankle foot orthoses. Motion analysis was performed on a healthy subject during normal walking in the case of using conventional stainless steel springs, and with using the produced NiTi springs. Joint kinematics and kinetics data of left lower limb (which was equipped with the AFO brace) were collected and calculated to compare normal walking patterns to the resultant walking patterns with the proposed ankle foot orthosis.

The CAD file of the AFO, hinge structure and the springs were developed. Each component was meshed and the convergence study were conducted. A finite element model was developed after assembling and introducing all the interactions between parts in Abaqus. The boundary conditions were applied to the system in a way simulating normal walking conditions. Different material properties (stainless steel and superelastic NiTi) were assigned to the springs in the model to evaluate the performance of the system under the aforementioned loading scenario.

The results of the motion analysis on a healthy subject during walking indicate that the use of the superelastic NiTi springs causes more normal walk compare to the use of the conventional stainless steel springs, especially during swing phase and heel strike. Moreover, the ankle has closer stiffness profile to the normal walking in the case of using NiTi springs. The results of the finite element analysis show that the super elastic behavior of NiTi results in more hinge rotation while the stress concentration developed on the springs is within the safe levels and cannot cause failure of the NiTi springs.

Motion analysis and finite element models were conducted for the proposed hinged AFO and the results were compared with conventional AFO. By taking advantage of the super elastic characteristic of NiTi, more normal walking behavior was observed in the case of using the proposed AFO with Ni-rich NiTi springs.

Commentary by Dr. Valentin Fuster
2016;():V002T03A027. doi:10.1115/SMASIS2016-9274.

Equivalent electromechanical circuit analysis models provide efficient tools for analysis of power flow across multiple physical domains. In this work, we use this tool to develop a model of the power flow through mechanical, magnetic and electrical domains for analysis of magnetostrictive material-based devices. The magnetostrictive unimorph system in this study consists of a magnetostrictive galfenol (Fe-Ga alloy) layer bonded to a non-magnetic flexible metal layer, a pickup coil wound around the bimetallic strip and an electrical load. Permanent magnets are used to set a magnetic bias field and to provide a tip mass load at the free end of the cantilevered unimorph. The electrical load is connected to the pickup coil, such that vibration in the magnetostrictive alloy layer caused by the vibrating structure generates electrical energy that is dissipated by the electrical load, thereby damping the vibrations in the structure. The pickup coil output voltage varies with fluctuation in the magnetic flux density due to vibration of the beam.

The electrical load discussed in this paper includes an inductance and a resistor (work that also includes capacitance is on-going). An electromechanical circuit model of the electrically loaded magnetostrictive unimorph system is used to study system dynamics. For this purpose the circuit description is simplified by a transformation of the electrical and magnetic elements into the mechanical domain. The network model of this system and simulations of its dynamic behavior are presented.

Commentary by Dr. Valentin Fuster
2016;():V002T03A028. doi:10.1115/SMASIS2016-9277.

Periodic elastic structures consisting of self-repeating geometric or material arrangements exhibit unique wave propagation characteristics culminating in frequency stop bands, i.e. ranges of frequency where elastic waves can propagate the periodic medium. Such features make periodic structures appealing for a wide range of vibration suppression and noise control applications. Stop bands in periodic media are achieved via Bragg scattering of elastic which is attributed to impedance mismatches between the different constituents of the self-repeating cells. Stop band frequencies can be numerically predicted using mathematical models which generally utilize the Bloch wave theorem and a transfer matrix method to track the spatial and temporal parameters of the propagating waves from one cell to the next. Such analysis generates what is referred to as the band structure (or the dispersion curves) of the periodic medium which can be used to predict the location of the pass and stop bands. Although capable, these models become significantly more involved when analyzing structures with dissipative constituents and/or material damping and need further adjustments to account for complex elastic moduli and frequency dependent loss factors. A new approach is presented which relies on evaluating structural intensity parameters, such as the active vibrational power and energy transmission paths. It is shown that the steady-state spatial propagation of vibrational power caused by an external disturbance accurately reflects the wave propagation pattern in the periodic medium, and can thus be reverse engineered to numerically predict the stop band frequencies for different degrees of damping via a stop band index (SBI). The developed framework is mathematically applied to a one-dimensional periodic rod to validate the proposed method.

Commentary by Dr. Valentin Fuster
2016;():V002T03A029. doi:10.1115/SMASIS2016-9282.

Metamaterials made from flexible structures with piezoelectric laminates connected to resonant shunt circuits can exhibit vibration attenuation properties similar to those of their purely mechanical locally resonant counterparts. Thus, in analogy to purely mechanical metamaterials, electroelastic metamaterials with piezoelectric resonators can exhibit vibration attenuation bandgaps. To enable the effective design of these locally resonant electroelastic metamaterials, the electromechanical behavior of the piezoelectric patches must be reconciled with the modal behavior of the electroelastic structure. To this end, we develop a novel argument for the formation of bandgaps in bimorph piezoelectric beams, relying on modal analysis and the assumption of infinitely many segmented shunted electrodes (unit cells) on continuous piezoelectric laminates bracketing a substrate. As a case study, the frequency limits of the locally resonant bandgap that forms from resonant shunting is derived, and a design guideline is presented to place the bandgap in a desired frequency range. This method can be easily extended to more general circuit impedances, and can be used to design shunt circuits to obtain a desired frequency response in the main structure.

Commentary by Dr. Valentin Fuster
2016;():V002T03A030. doi:10.1115/SMASIS2016-9287.

Additive manufacturing (i.e. 3D printing) has only recently be shown as a well-established technology to create complex shapes and porous structures from different biocompatible metal powder such as titanium, nitinol, and stainless steel alloys. This allows for manufacturing bone fixation hardware with patient-specific geometry and properties (e.g. density and mechanical properties) directly from CAD files. Superelastic NiTi is one of the most biocompatible alloys with high shock absorption and biomimetic hysteresis behavior. More importantly, NiTi has the lowest stiffness (36–68 GPa) among all biocompatible alloys [1]. The stiffness of NiTi can further be reduced, to the level of the cortical bone (10–31.2 GPa), by introducing engineered porosity using additive manufacturing [2–4]. The low level of fixation stiffness allows for bone to receive a stress profile close to that of healthy bone during the healing period. This enhances the bone remodeling process (Wolf’s Law) which primarily driven by the pattern of stress. Also, this match in the stiffness of bone and fixation mitigates the problem of stress shielding and detrimental stress concentrations.

Stress shielding is a known problem for the currently in-use Ti-6Al-4V fixation hardware. The high stiffness of Ti-6Al-4V (112 GPa) compared to bone results in the absence of mechanical loading on the adjacent bone that causes loss of bone mass and density and subsequently bone/implant failure. We have proposed additively manufactured porous NiTi fixation hardware with a patient-specific stiffness to be used for the mandibular reconstructive surgery (MRS). In MRS, the use of metallic fixation hardware and double barrel fibula graft is the standard methodology to restore the mandible functionality and aesthetic.

A validated finite element model was developed from a dried cadaveric mandible using CT scan data. The model simulated a patient’s mandible after mandibular reconstructive surgery to compare the performance of the conventional Ti-6Al-4V fixation hardware with the proposed one (porous superelastic NiTi fixation plates). An optimized level of porosity was determined to match the NiTi equivalent stiffness to that of a resected bone, then it was imposed to the simulated fixation plates. Moreover, the material property of superelastic NiTi was simulated by using a validated customized code. The code was calibrated by using DSC analysis and mechanical tests on several prepared bulk samples of Ni-rich NiTi. The model was run under common activities such as chewing by considering different levels of the applied fastening torques on screws.

The results show a higher level of stress distribution on mandible cortical bone in the case of using NiTi fixation plates. Based on wolf’s law it can lead to a lower level of stress shielding on the grafted bone and over time bone can remodel itself. Moreover, the results suggest an optimum fastening torque for fastening the screws for the superelastic fixations causes more normal distribution of stress on the bone similar to that for the healthy mandible. Finally, we successfully fabricated the stiffness-matched porous NiTi fixation plates using selective laser melting technique, and they were mounted on the dried cadaveric mandible used to create the finite element model.

Commentary by Dr. Valentin Fuster
2016;():V002T03A031. doi:10.1115/SMASIS2016-9290.

This study investigates the dynamic properties of Magneto-Rheological Elastomers (MRE) with hard magnetic particles used as bending actuators under an alternating magnetic field. As earlier studies demonstrated that a dispersion of hard magnetic particles in polymeric materials, aligned in a preferred orientation, cause rotational motion in the sample when a magnetic field is applied perpendicularly to the magnetization direction of the particles. They focused on static responses of MREs with hard magnetic particles. The primary goal of this study is to characterize the dynamic behavior of a flexible bending actuator based on MREs under alternating magnetic fields. In this study, samples from a previous study, consisting of barium hexaferrite particles at 30% concentrations by volume, were tested. A C-shaped electromagnet was constructed to apply alternating magnetic fields along the length of the sample. By securing only one end of the sample to the electromagnet, the sample is free to bend similar to a cantilever beam. Using this setup, the tip displacement of the sample was recorded using a precision load cell and a laser displacement sensor under various input magnetic field strengths and frequencies. The results show that increasing the voltage output or the magnetic field strength increases the displacement of the sample. The results also show that, as the frequency of the sinusoidal voltage input increases, the amplitude of the tip displacement of the sample decrease.

Topics: Actuators , Polymers
Commentary by Dr. Valentin Fuster
2016;():V002T03A032. doi:10.1115/SMASIS2016-9329.

Magnetorheological fluids (MRF) are suspensions of fine, magnetically polarizable particles in a non-magnetic carrier fluid. Under the influence of a magnetic field the particles form chains in the direction of the field lines, whereby the resulting shear stress can be changed high dynamical, largely linear with a good reproducibility by several orders of magnitude. Furthermore, the MRF technology provides a drag torque-free operation by a magnetically induced MR-fluid movement control, resulting in new perspectives regarding the increase of energy-efficiency with improved switching dynamics and comfort for applications e.g. in the powertrain of vehicles. A certain drawback of the MR-fluid control is a hysteresis behavior of the torque generation with respect to the controlling current caused by the partially filled shear gaps. In addition, the hysteresis depends on the rotational speed due to the resulting centrifugal forces, so that the influence of this external disturbance must be compensated, too. In order to obtain a linearized force characteristic of the actuator, a modified Prandtl-Ishlinskii approach for the hysteresis compensation is designed, parametrized and validated by experiments within this paper.

Commentary by Dr. Valentin Fuster

Bio-Inspired Smart Materials and Systems

2016;():V002T06A001. doi:10.1115/SMASIS2016-9014.

The paper begins with a brief overview of historical building coverings. Thermadapt™ thermally adaptive buildings are introduced as a completely new class of shingles, siding and roofing. These elements physically change shape in response to thermal loading. In hot weather with high solar loading, the panels curl up and away from the building. As the temperature cools and the sun sets, the Thermadapt™ elements lie close to the building. In cool temperatures, the elements lie flat agains the building transferring solar energy. In extremely cold temperatures, high convexity inherently forms in the elements, forming a pocket of trapped dead air which forms a highly effective layer of insulation. Thermadapt™ elements are analytically modeled using Classical Laminated Plate Theory (CLPT). Although Thermadapt™ elements may use materials like shape memory alloys, cost concerns drive the use of coefficient of thermal expansion mismatch as the basic driving mechanism. A series of experiments were performed on a variety of Thermadapt™ elements using high CTE mismatch pairs of structural materials including graphite-epoxy and aluminum and Invar and aluminum pairings. Analytical estimates are shown to predict the performance of the Thermadapt™ panels with great accuracy with curvature levels measured and predicted in excess of 5 deg/m/°C. Analytical predictions using CLPT employed a lateral constraint, driving lateral curvature, κy, to zero by the use of stiff lateral constraint mechanisms like edge rolls and lateral corrugations. This constraint was shown to increase deflections by roughly 33% over the unconstrained elements which were simply allowed to encounter equal curvatures in x and y directions, or “doming.”

Commentary by Dr. Valentin Fuster
2016;():V002T06A002. doi:10.1115/SMASIS2016-9034.

Our research focuses on creating smart materials that utilize synthetic cell membranes assembled at liquid interfaces for autonomic sensing, actuation, and energy conversion. Unlike single membrane assemblies, systems featuring many membranes have the potential to offer multi-functionality, greater transduction sensitivity, and even emergent behaviors in response to environmental stimuli, similar to living tissue, which utilizes networks of highly packed cells to accomplish tasks. Here, we present for the first time a novel microfluidic platform capable of generating a stream of alternating droplet compositions, i.e. A-B-A-B, and sequentially capturing these droplets in precise locations to enable the spontaneous formation of synthetic lipid bilayers between droplets of different compositions (i.e. A and B) in an enclosed substrate. This platform preserves a key feature of the droplet interface bilayer (DIB) method, which allows asymmetric conditions within and across the membrane to be prescribed by simply using droplets containing different species. In this work, we demonstrate the ability to assemble bilayers consisting of asymmetric lipid compositions and, separately, show that alternating droplets containing the same lipid type can also be used to control the direction of ion channel insertion. In the first study, A and B droplet types contain liposomes comprised of different lipid types, which are used to establish an asymmetric composition of the leaflets that make up the lipid bilayer. This asymmetry results in a dc, non-zero membrane potential, which we measure via membrane capacitance versus bias voltage. In the second study, alamethicin peptides are included in only one of the droplet types, which enable voltage-dependent insertion to occur only at one polarity. Cyclic voltammetry measurements are performed to confirm the direction of insertion of alamethicin channels in bilayers. Also, these results show the ability to perform simultaneously electrical measurements on multiple DIB, which increases the experimental capacity and efficiency of a microfluidic approach. The ability to produce alternating droplets in a high throughput manner with electrical access provides a system to investigate the effects of lipid asymmetry on the function of membrane proteins in a controlled model system.

Topics: Drops , Microfluidics
Commentary by Dr. Valentin Fuster
2016;():V002T06A003. doi:10.1115/SMASIS2016-9044.

Fluidic artificial muscles have the potential for a wide range of uses; from injury rehabilitation to high-powered hydraulic systems. Their modeling to date has largely been quasi-static and relied on the operator to adjust pressure so as to control force output and utilization while little work has been done to analyze the kinematics of the driving-systems involved in their operation. This paper utilizes an established electro-hydraulic model to perform a study of the components of a fluidic artificial muscle actuated climbing robot. Its purpose is to determine the effect of the robotic subsystems on function and efficiency for a small-scale system in order to extrapolate more general design and analysis schemes for future use. Its results indicate that important aspects to consider in design of the hydraulic system are system payload, operating pressure, pump selection, and FAM construction.

Topics: Muscle
Commentary by Dr. Valentin Fuster
2016;():V002T06A004. doi:10.1115/SMASIS2016-9046.

The goal of our research is to develop new understanding regarding the design and fabrication of mechanically activated liquid-infused porous films. Our unique approach is to consider a thin, elastic material that features well-defined pores, which are plugged with an infusing liquid that preferentially wets to the walls of the pores. By tuning the geometry of the pores, liquid-filled pores can be rearranged into a configuration that creates an open pore by applying stretch to the solid material, and they close (i.e. heal) again when the stretch is removed. Impregnating the pores with liquid seeks to avoid limitations that prevent complete pore closure and allows for tailoring of the pore geometry to drive liquid redistribution in the pore. The specific objective of this research is to study the effects of pore geometry and liquid wetting for creating fully reversible, stretch-activated pores. Our approach is both computational and experimental: Surface Evolver software is utilized to predict minimal energy wetting states of liquid in various pore shapes, and experiments on porous elastomers infused with either water or mineral oil allow measurements of stretch-induced changes in wetting properties and porosity. Both modeling and experiments demonstrate that a tear-shaped pore, which consists of a circular pore that features a taper extending in a radial direction, can enable reversible opening and closing of the pore via liquid redistribution. Our results indicate that infusing liquids with lower surface tensions and lower contact angles on walls of the pore exhibit better reversibility during the application of stretch.

Commentary by Dr. Valentin Fuster
2016;():V002T06A005. doi:10.1115/SMASIS2016-9071.

This paper presents the design of a bio-inspired crawling robot comprised of bi-stable origami building blocks. This origami structure, which is based on Kresling origami pattern, expands and contracts through coupled longitudinal and rotational motion similar to a screw. Controlled snapping, facilitated by buckling instability, allows for rapid actuation as seen in the mechanism of the hummingbird beaks or the Venus flytrap plant, which enables them to capture insects by fast closing actions. On a much smaller scale, a similar buckling instability actuates the fast turning motion of uni-flagellated bacteria. Origami provides a versatile and scale-free framework for the design and fabrication of smart actuators and structures based on this bi-stable actuation scheme. This paper demonstrates how a bi-stable origami structure, having the geometry of a polygonal base prism, can be used to actuate crawling gait locomotion. Bi-stable origami structures exhibit buckling instabilities associated with local bending and buckling of their flat panels. Traditional kinematic analysis of these structures based on rigid-plates and hinges at fold lines precludes the shape transformation readily observed in physical models. To capture this behavior, the model presented utilizes principles of virtual folding to analyze and predict the kinematics of the bistable origami building blocks. Virtual fold approximates panel bending by hinged, rigid panels, which facilitates the development of a kinematic solution via traditional rigid-plate analysis. As such, the kinetics and stability of the structures are investigated by assigning suitable torsional springs’ constants to the fold lines. The results presented demonstrate the effect of fold-pattern geometries on the chirality (i.e. the rotational direction that results in expansion of the structure), and snapping behavior of the bi-stable origami structure. The crawling robot is presented as a case study for the use of this origami structure in various locomotion applications. The robot is comprised of two nested origami ‘building blocks’ with opposite chirality, such that their actuations are coupled rotationally. A servo motor is used to rotationally actuate the expansion and contraction of both the internal and external origami structures to achieve locomotion. Inclined barbs that extrude from the edges of the polygonal base engage with the ground surface, thus constraining the expansion or contraction to forward locomotion, as desired. The robot fabrication methods are presented and results from experiments performed on various surfaces are also discussed.

Commentary by Dr. Valentin Fuster
2016;():V002T06A006. doi:10.1115/SMASIS2016-9074.

A dynamic spar numerical model for passive shape change is validated for a single degree of freedom contact-aided compliant mechanism (CCM) in a flapping spar. CCMs are modeled as compliant joints: spherical joints with distributed mass and three axis nonlinear torsional spring-dampers. Several assumptions were made in the original formulation of the model, such as assuming the spars were rigid and a simple damping model for the compliant joints. An experiment was performed to validate the assumptions and tune the model. Four configurations of the leading edge spar were tested: a solid spar, a previously designed CCM at two spatial locations, and a modified version of the CCM. Reflective markers were placed on each configuration, then the spars were inserted into the wing roots of a clamped ornithopter. An array of computer vision cameras was used to track the spar and CCM kinematics as they were flapped. First, a flapping angle function was extracted using a moving average of the flapping cycles. Then, a genetic algorithm was implemented to tune the stiffness and damping parameters for each of the configuration, minimizing the root mean square error between the model and experimental marker kinematics. The model was able to capture the deflection amplitude and harmonics of the CCMs with very good agreement and minimal to no phase shift.

Commentary by Dr. Valentin Fuster
2016;():V002T06A007. doi:10.1115/SMASIS2016-9096.

This work aims to investigate how bio-inspired morphing wings built with state-of-the-art materials affect the aerodynamics and extend the range of flight conditions. In particular, this study investigates the aerodynamic effects of coupled airfoil and planform sweep morphing. The morphed geometries were chosen to resemble a current morphing design that uses Macro Fiber Composites (MFCs) and Shape Memory Alloy (SMA) wires. The primary mode of camber actuation is achieved using the MFCs which are supplemented using antagonistic SMA wires, forming a hinge ahead of the MFCs. The SMA hinge also allows for bi-directional actuation, resulting in a reflexed airfoil. Numerical simulations were conducted using a Reynolds-averaged-Navier-Stokes (RANS) turbulence model for low-Reynolds-number flow, in addition to wind tunnel experiments. Nine different wing configurations were considered consisting of combinations of 3 sweep angles and 3 airfoil profiles, including unactuated (baseline), monotonic camber actuation, and reflex actuation. These geometries were 3D printed on a high resolution printer. Tests were conducted in a 2 ft. × 2 ft. wind tunnel at the University of Michigan at a flow speed of 10 m/s, consistent with the flow regime expected for this scale of aircraft. The preliminary results suggest a definite improvement in flight performance associated with the proposed coupling.

Topics: Biomimetics , Wings
Commentary by Dr. Valentin Fuster
2016;():V002T06A008. doi:10.1115/SMASIS2016-9105.

This paper covers the new field of thermally adaptive building coverings, their inspiration, basic operational characteristics, analytical modeling and coupon testing. Inspiration for thermally adaptive building coverings come quite notably from various families of thermotropic plant structures. Certain plant cellular structures like those in Mimosa Pudica (Sensitive Plant), Rhododendron leaves or Albizia Julibrissin (Mimosa Tree), exhibit actuation physiology which depends on physical manipulation and/or thermal loading as a function of solar radiation. The paper draws parallels between the differential actuation via cellular turgor pressure manipulation. A parallel with these structures can be seen in the new field of thermally adaptive building coverings which use various forms of cellular foam to aid or enable actuation much in the same way that plant cells are used to move leaves. When exposed to high solar loading, the structures curve upwards and outwards. When cold, these same structures curve back towards the building forming convex pockets of dead air to insulate the building. The paper shows the basic classical laminated plate theory models comparing theory and experiment of such coupons. The study concludes with a basic description of the effectiveness of thermally adaptive building coverings.

Commentary by Dr. Valentin Fuster
2016;():V002T06A009. doi:10.1115/SMASIS2016-9122.

This research discusses preliminary design of a Flapping Wing Micro Aerial Vehicle (FWMAV). One approach is to develop a biologically-inspired flapping wing MAV that can maneuver into confined areas and possess hover capabilities. This platform can potentially be equipped with microphones, cameras, and gas detectors, but one major challenge is the low Reynolds number aerodynamics. The critical components for successful flight include size, weight, and energy efficiency. Preliminary efforts include mechanical designs for payload, biologically inspired chassis, wing and tail.

Commentary by Dr. Valentin Fuster
2016;():V002T06A010. doi:10.1115/SMASIS2016-9143.

Multiple lipid encased water droplets may be linked together in oil to form large networks of droplet interface bilayers thus creating a new class of stimuli-responsive materials for applications in sensing, actuation, drug delivery, and tissue engineering. While single droplet interface bilayers have been extensively studied, comparatively little is known about their interaction in large networks. One particular problem of interest is understanding the impact of the coalescence of two neighboring droplets on the overall structural integrity of the network. Here, we propose a computational modeling scheme that predicts and characterizes the mechanical properties of the multiple lipid bilayer interfaces within the droplet network upon intentional coalescence of adjacent droplets. Droplet networks with tailored architectures are synthesized with the aid of magnetic motor droplets containing a biocompatible ferrofluid. The equilibrium configuration of the droplet networks is compared to computational prediction which defines the overall stability by summing the interfacial energies. Once the networks are completed, failure in selected membranes is induced. As the targeted droplets coalesce together, the equilibrium structure of the network is altered and the remaining droplets may shift to new configurations dictated by their minimized mechanical energies.

Commentary by Dr. Valentin Fuster
2016;():V002T06A011. doi:10.1115/SMASIS2016-9147.

In this paper, we describe the fabrication and testing of a tunably-compliant tendon-driven finger implemented through the geometric design of a skeleton made of the low-melting point Field’s metal encased in a silicone rubber. The initial prototype consists of a skeleton comprised of two rods of the metal, with heating elements in thermal contact with the metal at various points along its length, embedded in an elastomer. The inputs to the systems are both the force exerted on the tendon to bend the finger and the heat introduced to liquefy the metal locally or globally along the length of the finger. Selective localized heating allows multiple joints to be created along the length of the finger.

Fabrication was accomplished via a multiple step process of elastomer casting and liquid metal casting. Heating elements such as power resistors or Ni-Cr wire with electric connections were added as an intermediate step before the final elastomer casting. The addition of a tradition tendon actuation was inserted after all casting steps had been completed. While preliminary, this combination of selective heating and engineered geometry of the low-melting point skeletal structure will allow for further investigation into the skeletal geometry and its effects on local and global changes in device stiffness.

Commentary by Dr. Valentin Fuster
2016;():V002T06A012. doi:10.1115/SMASIS2016-9150.

The ability to functionalize droplet interface bilayers (DIBs) with the MscL channel and its mutants has been demonstrated. In previous work, the V23T gain of function mutant of MscL produced consistent activation when harmonic axial compressions were applied to the aqueous droplets supporting the lipid bilayer, where the channels settle. The deformation of the droplets results, at maximum compression, in an increase in surface area, and thus an increase in tension at the water-lipid-oil interface. This increase in monolayer tension was found to be the product of the relative change in surface area of each of the droplets and the compressibility modulus of the DPhPC monolayer (∼120 mN/m). The tension increase at the water-lipid-oil interface almost doubles to make up the increase in tension in the bilayer interface, resulting in activation of the incorporated MscL channels. However, it was found that the application of a relatively high transmembrane potential (∼100 mV), from an external power source, is a requirement for the activation of the V23T-MscL channels. Here, we investigate and analyze the impact of transmembrane potential on the activity of MscL channels in both a droplet interface bilayer system and E. coli spheroplast via patch-clamp. We demonstrate that the channels became more susceptible to gating upon the application of a negative potential, compared to when a positive potential is applied, proving their sensitivity to voltage polarity.

Topics: Drops
Commentary by Dr. Valentin Fuster
2016;():V002T06A013. doi:10.1115/SMASIS2016-9155.

The development of novel characterization techniques is critical for understanding the fundamentals of material systems. Bioinspired systems are regularly implemented but poorly defined through quantitative measurement. In an effort to specify the coupling between multiple domains seen in biologically inspired systems, high resolution measurement systems capable of simultaneously measuring various phenomena such as electrical, chemical, mechanical, or optical signals is required. Scanning electrochemical microscopy (SECM) and shear-force (SF) imaging are nanoscale measurement techniques which examine the electrochemical behavior at a liquid-solid or liquid-liquid interface and simultaneously probe morphological features. It is therefore a suitable measurement technique for understanding biological phenomena.

SF imaging is a high resolution technique, allowing for nanoscale measurement of extensional actuation in materials with high signal to noise ratio. The sensing capabilities of SECM-SF techniques are dependent on the characteristics of the micro-scale electrodes (ultramicroelectrodes or UMEs) used to investigate surfaces. Current limitations to this technique are due to the fabrication process which introduces structural damping, reducing the signal produced. Additionally, despite the high cost of materials and processing, contemporary processes only produce a 10% yield. This article demonstrates a UME fabrication process with a 60% yield as well as improved amplitude (250% increase) and sensitivity (210% increase) during SF imaging. This process is expected to improve the signal to noise ratio of SF-based measurement systems. With these improvements, SECM-SF could become a more suitable technique for measuring cell or tissue activity, corrosion of materials, or coupled mechanics of synthetic faradaic materials.

Commentary by Dr. Valentin Fuster
2016;():V002T06A014. doi:10.1115/SMASIS2016-9191.

Scanning electrochemical microscopy (SECM) is an electrochemical technique used to measure faradaic current changes local to the surface of a sample. The incorporation of shear force (SF) feedback in SECM enables the concurrent acquisition of topographical data of substrates along with electrochemical measurements. Contemporary SECM measurements require a redox mediator such as ferrocene methanol (FcMeOH) for electrochemical measurements; however, this could prove detrimental in the imaging of biological cells. In this article, nanoscale polypyrrole membranes doped with dodecylbenzene sulfonate (PPy(DBS)) are deposited at the tip of an ultra-microelectrode (UME) to demonstrate a novel modification of the contemporary SECM-SF imaging technique that operates in the absence of a redox mediator. The effect of distance from an insulating substrate and bulk electrolyte concentration on sensor response are examined to validate this technique as a tool for correlated topographical imaging and cation flux mapping. Varying the distance of the PPy(DBS) tipped probe from the substrate in a solution containing NaCl causes a localized change in cation concentration within the vicinity of the membrane due to hindered diffusion of ions from the bulk solution to the diffusion field. The cation transport into the membrane in close proximity to the substrate is low as compared to that in the electrolyte bulk and asymptotically approaches the bulk value at the sense length. At a constant height from the base, changing the bulk NaCl concentration from 5 mM to 10 mM increases the filling efficiency from 35% to 70%. Further, the sense length of this modified electrode in NaCl is about 440 nm which is significantly lower as compared to that of a bare electrode in ferrocene methanol (5–20 μm). It is postulated that this novel technique will be capable of producing high resolution maps of surface cation concentrations, thus having a significant impact in the field of biological imaging.

Topics: Microscopy
Commentary by Dr. Valentin Fuster
2016;():V002T06A015. doi:10.1115/SMASIS2016-9193.

The transport of monovalent cations across a suspended PPy(DBS) polymer membrane in an aqueous solution as a function of its redox state is investigated. Maximum ion transport is found to occur when PPy(DBS) is in the reduced state, and minimum transport in the oxidized state. No deviation in the dynamics of ion transport based on the direction of the applied electrical field is observed. Additionally, it is found that ion transport rates linearly increased proportional to the state of reduction until a steady state is reached when the polymer is fully reduced. Therefore controlled, bidirectional ion transport is for the first time demonstrated. The effect of aqueous Li+ concentration on ion transport in the fully reduced state of the polymer is studied. It is found that ion transport concentration dependence follows Michaelis-Menten kinetics (which models protein reaction rates, such as those forming ion channels in a cell membrane) with an r2 value of 0.99. For the given PPy(DBS) polymer charge density and applied potential across the membrane, the maximum possible ion transport rate per channel is found to be 738 ions per second and the Michaelis constant, representing the concentration at which half the maximum ion transport rate occurs, is 619.5mM.

Topics: Membranes
Commentary by Dr. Valentin Fuster
2016;():V002T06A016. doi:10.1115/SMASIS2016-9210.

Robust and predictable aerodynamic performance of unmanned aerial vehicles at the limits of their design envelope is critical for safety and mission adaptability. In order for a fixed wing aircraft to maintain the lift necessary for sustained flight at very low speeds and large angles of attack (AoA), the wing shape has to change. This is often achieved by using deployable aerodynamic surfaces, such as flaps or slats, from the wing leading or trailing edges. In nature, one such device is a feathered structure on birds’ wings called the alula. The span of the alula is 5% to 20% of the wing and is attached to the first digit of the wing. The goal of the current study is to understand the aerodynamic effects of the alula on wing performance. A series of wind tunnel experiments are performed to quantify the effect of various alula deployment parameters on the aerodynamic performance of a cambered airfoil (S1223). A full wind tunnel span wing, with a single alula located at the wing mid-span is tested under uniform low-turbulence flow at three Reynolds numbers, Re = 85,000, 106,00 and 146,000. An experimental matrix is developed to find the range of effectiveness of an alula-type device. The alula relative angle of attack measured measured from the mean chord of the airfoil is varied to modulate tip-vortex strength, while the alula deflection is varied to modulate the distance of the tip vortex to the wing surface. Lift and drag forces were measured using a six axis force transducer. The lift and drag coefficients showed the greatest sensitivity to the the alula relative angle of attack, increasing the normalized lift coefficient by as much as 80%. Improvements in lift are strongly correlated to higher alula angle, with β = 0° – 5°, while reduction in the drag coefficient is observed with higher alula tip deflection ratios and lower β angles. Results show that, as the wing angle of attack and Reynolds number are increased, the overall lift co-efficient improvement is diminished while the reduction in drag coefficient is higher.

Commentary by Dr. Valentin Fuster
2016;():V002T06A017. doi:10.1115/SMASIS2016-9292.

Artificial muscle systems have the potential to impact many technologies ranging from advanced prosthesis to miniature robotics. Recently, it has been shown that twisting drawn polymer fibers such as nylon can result in torsional or tensile actuators depending on the final fiber configuration. The actuation phenomenon relies on the anisotropic nature of the fibers moduli and thermal expansion. They have high axial stiffness, low shear stiffness, and expand more radially when heated than axially. If a polymer fiber is twisted but not coiled, these characteristics result in a torsional actuator that will untwist when heated. During the fabrication process, these twisted polymers can be configured helically before annealing. In this configuration, the untwisting that occurs in a straight twisted fiber results in a contraction or extension depending on relative directions of twist and coiling. In these ways, these materials can be used to create both torsional or axial actuators with extremely high specific work capabilities. To date, the focus of research on twisted polymer actuators (TPAs) and twisted-coiled polymer actuators (TCPAs) has been actuator characterization that demonstrates the technologies capabilities. Our work focuses here on applying a 2D analysis of individual layers of the TPAs to predict thermally induced twisting angle and fiber length based on virgin (untwisted) material properties and actuator parameters like fiber length and inserted twist. A multi-axis rheometer with a controlled thermal environmental chamber was used to twist, anneal, and test thermally induced actuation. Experimentally measured angle of untwist and axial contraction after heating are compare the the model. In comparing the experimental results with the two dimensional model, it appears that the difference between the 2D model and experimental results can be explained by the longitudinal stresses that develop inside the material. Future work will aim to include these effects in the model in order to be able to use this model in the design of TPAs.

Commentary by Dr. Valentin Fuster
2016;():V002T06A018. doi:10.1115/SMASIS2016-9293.

NiTi has been shown to be of great interest for bone implant applications. Introducing porosity to NiTi bone implants is an effective technique to tune their equivalent modulus of elasticity in order to acquire similar value to that of cortical bone. Moreover, such porous implants allow for better tissue ingrowth due to the interconnecting open pore structure. The effect of porosity percentage on the NiTi equivalent modulus of elasticity is well understood. However, the effect of porosity type on NiTi bone implant’s performance, in terms of the geometrical structure and other mechanical properties, has not yet been investigated. To this end, we simulated three porous structures made of shape memory Ti-rich Ni50.09Ti alloy. The effect of porosity type on the NiTi implant’s geometrical structure and mechanical properties was studied using numerical tests. The purpose is to compare three NiTi implants with different kinds of porosities, at a similar level of porosity (i.e., 69 %). The assigned porosity types in this study are Schwartz-type, Gyroid-type, and Diamond-type.

Three triply periodic minimal surface (TPMS) models (9mm×9mm×9mm) with the assigned fixed level of porosity (69 %) were designed as CAD files using Solidworks. Each model was meshed, and the convergence study was conducted. The three models were then imported into a finite element package (ABAQUS). A UMAT code developed by IUT (Isfahan University of Technology) group was used to simulate the mechanical behavior of the shape memory NiTi alloy. All boundary conditions and loading conditions were applied to the models. Compressive mechanical tests were simulated in the finite element, and the resultant equivalent modulus of elasticity, elongations, stress, and strain was estimated.

The results show anisotropic behavior within the three different porous structures. With the same level of porosity (i.e., 69 %), equivalent modulus of elasticity was observed to be 48.9, 34.8, and 30.2 GPa for Schwartz-type, Gyroid-type, and Diamond-type, respectively. Moreover, the Schwartz-type scaffold was seen to offer the highest stress at plateau start and the lowest residual strain after unloading, in comparison with the other two types of structure.

Commentary by Dr. Valentin Fuster
2016;():V002T06A019. doi:10.1115/SMASIS2016-9321.

Inspired by the fibrillar network in plant cell walls and the helical fibers found in soft bodied hydrostats (e.g. worms, squid, elephant trunks, and octopus arms), fluidic flexible matrix composites (F2MCs) are composite tubes that consist of multiple layers of oriented, high performance fibers, such as carbon, precisely placed in a flexible matrix resin to form high-mechanical advantage actuators and variable stiffness materials. Unique to the F2MC tube is its ability to generate high pressures and volume change with a small external load as a result of the stiff reinforcement fiber orientation in the wall of the tube and the soft supporting elastomer. When a load is applied to the tubes, the volume of the composite pump is reduced and fluid is forced out of the tube by the reinforcing fibers. The objective of this research is to design, fabricate and characterize F2MCs for use in wave energy conversion where ocean waves provide the axial load to drive fluid through the pumps. F2MCs pumps are tested in a water basin and mechanically cycled between 0 Hz and 2 Hz at up to 17 percent strain. Instantaneous input power is found by measuring the displacement and applied force to the actuators, while output power values are derived from pressure and flow rate measurements at the tube outlet. From these measurements the actuator efficiency is subsequently determined.

Commentary by Dr. Valentin Fuster

Energy Harvesting

2016;():V002T07A001. doi:10.1115/SMASIS2016-9004.

Human motions are good energy sources for energy harvesters to support wearable devices. Among them, walking motions have received considerable attention as energy sources due to their large kinetic energy. Most of the studies about energy harvesting from human walking have been tested in real human wearing energy harvesters. In this paper, we use a humanoid robot to study energy harvesting from walking motion. We quantitatively analyze the energy harvesting from walking through the repeatable motion of the humanoid robot. A knee pad is attached on the leg of the humanoid robot. We make a pocket on the knee pad and put a piezoelectric composite as an energy transducer into the pocket. We refer to a trajectory of knee angle during one walking cycle of human from literature. The knee motion is formulated by performing Fourier series fitting for programming the movement of the humanoid robot. Additionally, an electromechanical model is used to explain the electrical responses from the piezoelectric composite in the pocket during the motion of the humanoid robot. We estimate average power transferred from the piezoelectric composite to the load resistances during the knee motion by using the model and validate the theoretical predictions by comparing with experimental results.

Commentary by Dr. Valentin Fuster
2016;():V002T07A002. doi:10.1115/SMASIS2016-9015.

In this work, the effects of joint characteristics on the performance of a nonlinear piezoelectric energy harvester are investigated numerically. Large amplitude deflection unimorph beam with a tip mass and a nonlinear piezoelectric layer is considered as an energy harvester. By applying Euler-Lagrange equation and the Gauss’s law, mechanical and electrical equations of motion are obtained respectively, under two scenarios, i.e. with an ideal (rigid) joint and with a flexible one. A numerical approach is followed to investigate the effects of each nonlinear parameter of the harvester (stiffness, damping and piezoelectric coefficient) on harvested power. Results show that considering ideal joint between harvester and base structure leads to overestimating the maximum output power and the range of effective excitation frequency.

Commentary by Dr. Valentin Fuster
2016;():V002T07A003. doi:10.1115/SMASIS2016-9040.

The article studies the behavior of a mixed parallel-series connection of piezoelectric oscillators attached to the standard interface for wideband energy harvesting. The estimate of power output is obtained analytically considering the formulations of balance of charge, energy and system dynamics. It can be presented based on the generalized matrix formulation of charging on capacitance in terms of equivalent load impedance. The proposed model is subsequently validated numerically through circuit simulations. Finally, a design with a careful choice of parallel-series mixed connection of oscillators is proposed for illustration. With a proper circuit layout triggering the switching of connection, the result shows that the peak power of each array configuration is roughly uniform within the frequency range of interest. Hence, the bandwidth is enlarged without the loss of peak power.

Commentary by Dr. Valentin Fuster
2016;():V002T07A004. doi:10.1115/SMASIS2016-9043.

A magnetic-spring electromagnetic energy harvester which consists of a hollow tube with two magnets fixed to the end and an attracting magnetic stack moving inside it is presented in this paper to harvest energy from human motion. The dynamic model of the electromagnetic generator is derived according to Newton’s law and Ansoft Maxwell software is used to calculate the repulsive force between middle and end magnets. Experimental results under frequency-sweep excitation and constant frequency excitation with different acceleration levels show that the generator has the potential to generate electricity for a broadband frequency range and ability to light up a LED. In the experiments considering human motion, the voltage response caused by the impact between shoes and ground are investigated. Under that condition, the influence of equivalent mass of moving magnetic stack and speeds of motion on the energy harvesting efficiency is analyzed. Results show that larger equivalent mass could obviously improve the performance of the generator and the obtained maximum output power reaches 2.11 mW.

Commentary by Dr. Valentin Fuster
2016;():V002T07A005. doi:10.1115/SMASIS2016-9116.

Vibration energy harvesting extracts electrical energy from vibrating structures. The past studies of vibration energy harvesting suggest that the efficiency can be improved by switch regulation in the harvesting circuit. The switch-regulation is carried out depending on the motion of the target structure with the use of vibration sensors such as displacement sensor or accelerometer. This paper proposes a new vibration self-sensing method for switching energy harvesters that do not use those vibration sensors. In this method, the voltage of the piezoelectric transducer is measured, and the structural vibrational status is estimated from the measured voltage. The transducer voltage is not smooth and does not maintain the sinusoidal wave even when the structure vibrates in a sinusoidal wave because the switch energy harvesting method inverses the transducer voltage at every period. Thus, we establish a state observer based on a Kalman filter to estimate three state values of the target harvesting system: modal displacement, modal velocity, and electric charge in the transducer.

This paper describes the construction processes for the observer. The observed value is the transducer voltage. We also show an electric circuit for measuring the transducer voltage. Finally, we confirm the efficiency of the proposed state observer for switch harvesting with numerical simulations.

Commentary by Dr. Valentin Fuster
2016;():V002T07A006. doi:10.1115/SMASIS2016-9120.

During the last decade, significant research effort has been spent on simple Cantilever Piezoelectric Vibration Energy Harvesters (PVEHs). These types of harvesters have their maximum efficiency in a very narrow bandwidth around their natural frequency and are therefore unfit for being used in presence of random or wide spectrum excitation. In this study, a periodic substructure for multi-frequency energy harvesting with single piezoelectric patch is proposed. This system is able to harvest energy from several modes (the lower ones) and, through proper design, these modes may be obtained in a narrow frequency band. The substructure is designed through an analytical model that allows to place modes exactly where required and tests are carried out to validate the proposed analytical model. The results confirm the validity of the proposed model that could be exploited to determine solutions for further improving both the bandwidth and overall efficiency of the harvester by helping of nonlinearity phenomena.

Commentary by Dr. Valentin Fuster
2016;():V002T07A007. doi:10.1115/SMASIS2016-9157.

Energy harvesting using a triboelectric nanogenerator (TENG) has been a major area of research in the recent years in order to harvest mechanical energy in different scales. High energy conversion efficiency, broad range of application in different systems and relatively easy fabrication process are among the factors demonstrating essential needs for TENG technology development. Performance of a TENG could be affected by many factors such as the frequency of vibration and the surface charge density. As a key factor in improving the power output of TENGs, surface charge density could be modified by the selection of proper charging materials and by increasing the contact area between the tribo-pairs. Although there have been numerous studies analyzing the performance of different tribo-pairs and different interfacial structures for a TENG, a systematical analysis of the contact phenomena between the interfacial structures in order to investigate the effects of different surface properties and structures such as, surface roughness, dielectric properties or the presence of nanostructures is still not available. In the current study, systematical numerical simulations have been performed on the adhesive contact behavior of the macro/nanostructures at the TENG interface. An interaction potential has been used to represent the adhesive interactions while surface deformations are coupled using half-space Green’s function. Furthermore, effects of the deformation of the interfacial structure on the performance and output of the TENG has been investigated by developing a theoretical model for a vertical-contact-mode TENG using a mass-spring system to represent the motion of the moving electrode. Coupling the theoretical model to the instantaneous deformation of the interfacial structure, real-time output of the TENG in terms of short-circuit voltage and open-circuit current has been studied in response to a predefined pressure input. The results of the current study demonstrate the effects of the deformation of the interfacial structure on the output characteristics of TENGs during the transition between partial-contact to full-contact modes. Numerical simulation results represent acceptable correlations with previously reported experimental data. The simulation package developed in this study is capable of simulating the contact behavior of the interfacial structure and predicting the deformed geometry.

Commentary by Dr. Valentin Fuster
2016;():V002T07A008. doi:10.1115/SMASIS2016-9192.

Embedded piezoelectric energy harvesting (PEH) systems in medical pacemakers have been an attractive and well visited research area. These systems typically utilize different configurations of beam structures with forcing originating from heart beat oscillations. The goal of these systems is to remove the pacemaker battery, which makes up 60–80% of the device volume, and replace it with a self-reliant power option. With emerging technologies encouraging a push towards leadless pacemakers typical energy harvesting beam structures are becoming inherently coupled with the heart system. The introduction of the nonlinearity resulting from the bistable magnetic interaction of two magnets is known to enhance energy harvesting performance due to its double-well potential behavior. Introducing the elastic magnifier enables large tip oscillations and high energy orbits for the bistable system. A continuous nonlinear model is derived for the bistable system (BPEH) and a one-degree-of-freedom linear mass-spring-damper model is derived for the elastic magnifier. The elastic magnifier (EM) will not consider the damping negligible due to the viscous nature of the heart, unlike most models. For experimental testing a physical model was created for the bistable structure and fashioned to an elastic magnifier. A hydrogel was chosen as the physical model for the EM. Experimental results have shown that the bistable piezoelectric energy harvester coupled with a linear elastic magnifier (BPEH+LEM) produces more power at certain input frequencies and operates a larger bandwidth than a PEH, BPEH, and a standard piezoelectric energy harvester with the elastic magnifier (PEH+LEM). Numerical simulations were validated by these results showing that this system enters high-energy and high orbit oscillations. It has been shown that BPEH systems implemented in medical pacemakers can have enhanced performance if positioned over the myocardial heart wall.

Commentary by Dr. Valentin Fuster
2016;():V002T07A009. doi:10.1115/SMASIS2016-9203.

When there is a two to one internal resonance ratio between the natural frequencies of the pitch motion and the roll motion of a ship, a nonlinear energy transfer occurs between the modes. If the ship is excited near the pitch natural frequency and at a large enough excitation amplitude, the pitch mode transfers energy to the roll mode. We use this interesting phenomenon to develop a wave power device for off-shore purposes. In this paper, we experimentally show that we can use a horizontal pendulum and use the quadratic nonlinear coupling between the pitch and the roll mode to get full rotation of the pendulum inside the ship. A rotating pendulum will generate orders of magnitude more power than a locally oscillating one when connected to a DC generator. This article measures the angle of the pendulum at the pitch frequency excitation of the ship to experimentally confirm the expected theoretical results on this phenomenon.

Commentary by Dr. Valentin Fuster
2016;():V002T07A010. doi:10.1115/SMASIS2016-9216.

Growing demands for Total Knee Arthroplasty (TKA) and also total knee revision surgery combined with the cost, risk, and complication of the surgery have led to numerus attempts to improve surgical techniques, implant design, and postoperative orthopedic therapies. Although abundant information about knee function and reaction forces and moments have been provided by researchers through biomechanical models, cadaver testing, in-vitro testing, and limited in-vivo measurements, rigorous real-time in-vivo data from knee implants is still required to improve the performance of TKAs. Instrumented knee implants using piezoelectric (PZT) transducers have promising potential to satisfy clinical needs in terms of continuous in-vivo data acquisition, self-powered operation, and retention of prevalent implant design, which can ultimately lead to improved patient satisfaction. In this study, a simplified Ultra High Molecular Weight (UHMW) Polyethylene TKA bearing geometry with an embedded PZT on the bottom surface is proposed and investigated to analyze sensing and power harvesting, and longevity of the conceptual design. As a result, this work is separated into two distinct sections. The first part provides an evaluation study on the performance of the design in terms of output voltage and power using both simulations and experimental tests. Finite element analysis (FEA) is employed to model the stress-strain behavior of the system and to develop effective force reaction on the PZT transducer. An analytical model is used to describe the electromechanical behavior of the PZT transducer under the effective force predicted by FEA, and the output voltage and power of the system are simulated. Furthermore, results obtained from modeling are validated through experimental compression testing using simulated gait conditions. Embedding a PZT element in the knee bearing may cause changes in stress distribution in UHMW and as a result the variation in the fatigue life of the bearings with encapsulated PZTs is considered as a remarkable factor to investigate. Therefore, in the second part of the work, a parametric study on the effect of dimensional parameters on the longevity and electromechanical performance of the design is performed. High cycle stress life of the polyethylene component with embedded PZT transducer as well as transferred force to the PZT and generated voltage under periodic knee load are studied. . The diameter and depth of the pocket machined in the UHMW bearing, the thickness ratio of the PZT element to the UHMW component, and modification of the contact edges inside the PZT pocket and PZT are considered as effective geometrical parameters on the fatigue life of the UHMW bearing and are studied individually. Two designs are investigated; the initial design with sharp corners and a revised design with filleted corners. The results show a significant fatigue life improvement by adding a fillet radius modification on the sharp corners of the UHMW and PZT components accompanied by a slight reduction in output voltage. The effect of pocket diameter is dependent on the geometry and for the initial design the fatigue life and output voltage increase when diameter increases. For the revised design, fatigue life decreases for large fillet radii and increases for small fillet radii and converge as diameter is increased, whereas the output voltage slightly increases with large pocket diameters. Pocket depth has a significant reverse effect on fatigue life and output voltage of the PZT, such that a 0.05 mm deeper pocket results in no force transfer and no voltage but improved fatigue life.

Commentary by Dr. Valentin Fuster
2016;():V002T07A011. doi:10.1115/SMASIS2016-9232.

Harvesting of acoustoelastic wave energy in thin plates and other structures has recently gained attention from the energy harvesting community. To enhance the wave power generated, researchers have investigated metamaterial-inspired concepts to include funnels, mirrors, and defect-based resonators introduced in the metamaterial’s bandgap. Many of these concepts have been demonstrated experimentally using arrangements of cylindrical stubs mounted on the surface of a thin plate, where such stubs scatter plane and cylindrical waves in such a way as to focus mechanical energy. To support these studies, the authors have recently introduced an experimentally-verified analytical framework for investigating the coupled electromechanical response of a single circular piezoelectric harvester adhered to an infinite plate and excited by a distant harmonic point source. This paper extends these ideas to consider a similar physical system with the addition of multiple cylindrical inclusions. These additions require development of an electromechanically-coupled, multiple scattering formulation of significantly increased complexity. The formulation also includes an electrical circuit model for generating electrical current from incident waves interacting with the piezoelectric domain. Following development, the formulation is applied to the determination of optimal arrangements of scatterers which maximize the electrical power generated. Specifically, an optimization study is carried-out in which twenty-five scatterers are first placed in a semi-elliptical arrangement with the aim of focusing wave energy from one elliptical focus (i.e., source location) onto the other. It is known from past studies that additional side lobes are generated due to truncation of the ellipse, and thus not all of the energy can be focused at single point, as desired. To improve upon this situation, an optimization study is performed in which the aspect ratio of the ellipse is varied, with the goal of optimizing the power harvested from the focal point. Results from the optimization studies show conclusively that the side lobes can in fact be minimized, and that harvested power can be significantly improved.

Commentary by Dr. Valentin Fuster
2016;():V002T07A012. doi:10.1115/SMASIS2016-9254.

We present a distributed-parameter electromechanical model and its modal analysis for flexoelectric energy harvesting using centrosymmetric dielectrics by accounting for both the direct and converse effects as well as size dependence of the coupling coefficient. Flexoelectricity is the generation of electric polarization in elastic dielectrics by the application of a non-uniform mechanical strain field, i.e. a strain gradient. In order to accompany atomistic simulations and experimental efforts at small scales, there is a growing need for high-fidelity device models that can also provide an analytical insight into size-dependent electro-elastodynamics of small structures that exhibit and exploit flexoelectricity. Particularly, although the conversion of mechanical energy into electrical energy (i.e. energy harvesting) is more related to the direct effect, it is necessary to accurately model the converse effect for thermodynamic consistency and completeness. To this end, we present a flexoelectric monolayer centrosymmetric energy harvester model (that yields no piezoelectric effect) for converting ambient vibration into electricity. The flexoelectric energy harvester model based on the Euler-Bernoulli beam theory is focused on strain gradient-induced polarization resulting from the bending (transverse) vibration modes in response to mechanical base excitation. Following recent efforts on the converse flexoelectric effect in finite samples, the proposed model accounts for two-way coupling, i.e. the direct and converse effects, and it also captures the effect of geometric scaling on the coupling coefficient. In addition to closed-form solutions of the electromechanical frequency response functions, various case studies are presented for a broad range of material and geometric parameters. Thickness dependence of the electromechanical coupling is analytically shown and is observed in simulations of the electromechanical frequency response functions as well.

Commentary by Dr. Valentin Fuster
2016;():V002T07A013. doi:10.1115/SMASIS2016-9264.

In this paper, we explore structure-borne elastic wave energy harvesting, both numerically and experimentally, by exploiting a Gradient-Index Phononic Crystal Lens (GRIN-PCL) structure. The proposed GRIN-PCL is formed by an array of blind holes with different diameters on an aluminum plate where the orientation and size of the blind holes are tailored to obtain a hyperbolic secant gradient distribution of refractive index guided by finite-element simulations of the lowest asymmetric mode Lamb wave band diagrams. Under plane wave excitation from a line source, experimentally measured wave field successfully validates the numerical simulation of wave focusing within the GRIN-PCL domain. A piezoelectric energy harvester disk located at the first focus of the GRIN-PCL yields an order of magnitude larger power output as compared to the baseline case of energy harvesting without the GRIN-PCL on the uniform plate counterpart for the same incident plane wave excitation. The power output is further improved by a factor of five using complex electrical load impedance matching through resistive-inductive loading as compared to purely resistive loading case.

Commentary by Dr. Valentin Fuster
2016;():V002T07A014. doi:10.1115/SMASIS2016-9288.

This paper describes an innovative method for enhancing the power output of a piezoelectric energy harvester. The proposed approach is adopting inductance to reduce the effect of the internal capacitance of the piezoelectric harvester to boost the power output. Four electrical circuits for a piezoelectric beam harvester are studied; Simple Resistive Load (SRL), Inductive Load (IL), Standard AC-DC, and Inductive AC-DC circuits. An inductor is added to the SRL and standard AC-DC circuits to build the new IL and Inductive AC-DC circuits respectively. The power outputs of the four circuits are then studied. The results show that the adaptation of inductor enhances the power output. The IL circuit enhances the power output comparing to the SRL circuit. The Inductive AC-DC circuit also avails the standard AC-DC circuit.

Commentary by Dr. Valentin Fuster
2016;():V002T07A015. doi:10.1115/SMASIS2016-9291.

Energy harvesting technology can provide a renewable, portable power source for soldiers who rely solely on battery power in the field. Electromagnetic energy harvesters scavenge energy from wasted kinematic and vibration energy in human motion. The motion of interest in this paper is vertical hip displacement during human gait that acts as a base excitation. The placement of a permanent magnet based linear generator mounted in a backpack can make use of this excitation that results in relative motion of the magnet to the coil of copper wire, which induces an electric current. This current can be used to charge a battery or capacitor bank installed on the backpack to power portable electronic devices, thereby reducing the need for extra batteries and overall battery weight. The purpose of this research is to use a multi-variable optimization algorithm to identify an optimal coil and magnet layout for power maximization. Results from this study will pave the way for a more efficient energy harvesting backpack while providing better insight into the efficiency of magnet and coil layout for various applications for electromagnetic power generation from vibration.

Commentary by Dr. Valentin Fuster
2016;():V002T07A016. doi:10.1115/SMASIS2016-9297.

In this paper, a self-supported power conditioning circuit is developed for a footstep energy harvester, which consists of a monolithic multilayer piezoelectric stack with a force amplification frame to extract electricity from human walking locomotion. Based on a synchronized switch energy harvesting on inductance (SSHI) interface and a peak detector topology, the power conditioning circuit is designed to optimize the power flow from the piezoelectric stack to the energy storage device under real-time human walking excitation instead of a simple sine waveform input, as reported in most literatures. The unique properties of human walking locomotion and multilayer piezoelectric stack both impose complications for circuit design. Three common interface circuits, e.g. standard energy harvesting (SEH) circuit, series-SSHI and parallel-SSHI are compared in experiments to find which one is the best suit for the real-time-footstep energy harvester. Experimental results show that the use of parallel-SSHI circuit interface produces 85% more power than the SEH counterpart, while the use of series-SSHI circuit demonstrates the similar performance in comparison to the SEH interface. The reasons for such a huge efficiency improvement by using the parallel-SSHI interface are detailed in this paper.

Commentary by Dr. Valentin Fuster
2016;():V002T07A017. doi:10.1115/SMASIS2016-9299.

The recent literature of energy harvesting has shown growing interest in nonlinear aeroelastic systems for wind energy harvesting. Among other configurations, electrical power extraction from limit cycle oscillations of nonlinear beams and plates combined with a transduction mechanism in axial flow (inspired by flapping flags) have been pointed out as an alternative to traditional horizontal axis wind turbines if implemented on a large scale. Although the literature presents several works on the modeling and experimental verification of cantilevered plates in axial flow using piezoelectric transduction mechanism, the investigation of piezoelectric material distribution along the body length for proper electrode configuration to avoid cancelation of electrical outputs has not been covered. To address this problem, in this work, a fluid-structure interaction model that couples nonlinear beam equation with a lumped vortex-lattice potential flow model is implemented. The nonlinear model of the beam in axial flow is verified against wind tunnel experimental results and a numerical energy flow analysis is employed to determine the distribution of piezoelectric material along the body length. Dynamic strain distribution analysis is then performed to determine the electrode configuration to avoid cancelation of the electrical output.

Commentary by Dr. Valentin Fuster
2016;():V002T07A018. doi:10.1115/SMASIS2016-9301.

This paper presents design optimization of an electromagnetic vibration energy harvester to improve durability and increase life cycle. The energy harvester discussed in this paper has been developed by the Korean Railroad Research Institute, as a maintenance-free power supply for a wireless sensor module that monitors rail bogie axles and bearings. Our research team realized a durability issue in the leaf spring in this harvester because of high stress concentration and the corresponding fatigue failure when the harvester experiences high impact loading in rail operation. The topology optimization method has been applied in this research to design the shape of the leaf spring with a lower stress concentration magnitude while satisfying the multiple functional requirements on vibration amplitude and natural frequency. For fast and effective design search, we firstly identified several initial models from literature. These models have been carefully chosen to minimize any unnecessary parasitic motions. The topology optimization is then applied to produce the new leaf spring structure. The results of this research showed that topology optimization method could reduce the magnitude of stress concentration while satisfying required vibration amplitude and natural frequency of the spring structure.

Commentary by Dr. Valentin Fuster
2016;():V002T07A019. doi:10.1115/SMASIS2016-9304.

This research introduces an integrated vibration energy harvester and electrochemical energy storage device that can effectively convert ambient vibrations directly into stored electrochemical energy. The electrochemical energy storage device is an electrical double layer capacitor (EDLC) with an ionic redox transistor as its membrane separator. This ‘smart’ membrane separator directly rectifies the electrical energy generated by the transduction from the nonlinear energy harvester, creating an ionic polarization across the membrane separator for storage. This electrochemical gradient can be subsequently used for powering sensor electronics as required in various applications, including structural condition monitoring. The alternating voltage developed by the energy harvester (+/−5V around 100 Hz) is connected to an aqueous supercapacitor fabricated from nanofibrous carbon paper electrodes and a polypyrrole-based (PPy(DBS)) smart membrane separator. A potential below −400mV from the energy harvester applied to the supercapacitor turns the smart membrane separator ‘ON’ and results in a unidirectional ionic current of Li+ ions. As the potential developed by the harvester cycles above ∼50 mV, the membrane separator switches ‘OFF’ and prevents the discharge of the rectified current. This leads to a continuous polarization of ions towards electrical fields relevant for powering electronics. This article is the first description and demonstration of an energy harvesting and storage system that can directly convert the electrical energy from a vibration energy harvester into electrochemical energy without the use of passive circuit components for power rectification.

Commentary by Dr. Valentin Fuster

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