Journal of Intelligent Material Systems and Structures

Heinonen, Esa; Juuti, Jari; Moilanen, Veli-Pekka; Palosaari, Jaakko; Jantunen, Heli

doi: 10.1177/1045389X08097384pmid: N/A

In this work, novel structurally graded piezoelectric disc actuators are proposed. The monolithic gradient actuators had nonuniform thickness profiles, which caused distribution of the electric field and resulted in bending of the discs. The geometry and clamping conditions of the actuators were varied and the displacement properties were optimized using ATILA and Comsol Multiphysics finite element modeling (FEM) softwares. The material parameters of commercial PZT-5H were used in the modeling of the actuators — 25 mm in diameter with an original thickness of 0.5mm. Additional steel layers with different thicknesses were introduced under the actuators in `unimorph-fashion' in order to study their effect on the displacements and stresses of the actuators. The modeling results showed that the bending of the monolithic actuators could be realized without any additional passive layers and also that the utilization of clamping further improved the displacement capabilities. For example, ~53 μm axial displacements were enabled with a 1 V/μm electric field (corresponding to the thinnest part of the actuator) without any passive layers. The modeled displacement results were comparable to displacements obtained by pre-stressed PRESTO (~50 μm), THUNDER (~60 μm) and RAINBOW (~70 μm) actuators of 25 mm in diameter with a 1 V/μm electric field.

Das, S.; Kyriakides, I.; Chattopadhyay, A.; Papandreou-Suppappola, A.

doi: 10.1177/1045389X08097386pmid: N/A

In wave-based approach, the presence of damage is visualized in terms of the changes in the signature of the resultant wave that propagates through the structure. In structural health monitoring, the fundamental goal is to detect, localize, and quantify these damage signatures. The current approach uses matching pursuit decomposition (MPD) to compare signals from healthy and damaged structures. However, the major drawback of the MPD is that, in the decomposition process, it performs an exhaustive search over a large dictionary of elementary functions. Therefore, this method of decomposition is associated with a large computational expense. In this research, the Monte Carlo matching pursuit decomposition (MCMPD) is proposed, that adapts a smaller dictionary to the signal structure, thus avoiding the exhaustive search over the time-frequency plane. The proposed algorithm, sequentially estimates a dictionary that contains only those components that match the waveform structure, uses the matching pursuits for the decomposition of the signal and if necessary, adapts the dictionary to the structure of the residues for further decomposition. Finally, we demonstrate using real life data that the MCMPD retains the ability of the matching pursuit to decompose waveforms and quantify them accurately while reducing computational expense.

Rubio, Wilfredo M.; Silva, Emilio C.N.; Bordatchev, Evgueni V.; Zeman, Marco J.F.

doi: 10.1177/1045389X08093548pmid: N/A

This article presents a systematic and logical study of the topology optimized design, microfabrication, and static/dynamic performance characterization of an electro-thermo-mechanical microgripper. The microgripper is designed using a topology optimization algorithm based on a spatial filtering technique and considering different penalization coefficients for different material properties during the optimization cycle. The microgripper design has a symmetric monolithic 2D structure which consists of a complex combination of rigid links integrating both the actuating and gripping mechanisms. The numerical simulation is performed by studying the effects of convective heat transfer, thermal boundary conditions at the fixed anchors, and microgripper performance considering temperature-dependent and independent material properties. The microgripper is fabricated from a 25 μm thick nickel foil using laser microfabrication technology and its static/dynamic performance is experimentally evaluated. The static and dynamic electro-mechanical characteristics are analyzed as step response functions with respect to tweezing/actuating displacements, applied current/power, and actual electric resistance. A microgripper prototype having overall dimensions of 1 mm (L) × 2.5mm (W) is able to deliver the maximum tweezing and actuating displacements of 25.5 μm and 33.2 μm along X and Y axes, respectively, under an applied power of 2.32 W. Experimental performance is compared with finite element modeling simulation results.

Zalachas, Nicolas; Laskewitz, Bernd; Kamlah, Marc; Prume, Klaus; Lapusta, Yuri; Tiedke, Stephan

doi: 10.1177/1045389X08096164pmid: N/A

In micro-electromechanical systems consisting of a piezoelectric thin film on a substrate, due to the clamping by the substrate, the effective piezoelectric film properties are different from the bulk material behavior. However, it is of particular difficulty to determine the transverse piezoelectric parameters for such a system. A simple theoretical model by Muralt et al. (1996) allows calculation of the transverse piezoelectric coefficients in terms of the bulk parameters of the piezoelectric material. Relying on the assumption of a rigid wafer this model is a reasonable first approximation, but on the other hand, for the high accuracy needed in technical applications, it may not always be sufficient. Therefore, a more complex theoretical model for a piezoelectric thin film on an elastic substrate was derived, which delivers more realistic results for the transverse piezoelectric coefficients. This model takes the elasticity of the substrate into account, while the PZT layer is fully covered by an electrode and the vertical displacements are suppressed at the bottom of the substrate. In this way, the model represents a system of infinite lateral extent with no overall bending. As the next step, finite element simulations were carried out to verify the simple theoretical model and the new developed model, and there was a good agreement between the theoretical models and the numerical results. Furthermore, a parametric study was performed considering the influence of various bulk material parameters. Finally, the newly proposed theoretical model was compared to more realistic models where the PZT layer possesses an isolated electrode spot instead of being covered fully by an electrode. Different options were used for the boundary conditions at the bottom of the substrate. First, the same boundary conditions as for the new theoretical model were chosen (suppression of the vertical displacements at the bottom of the substrate). Second, the bottom of the substrate was free to move such that overall bending was no longer prevented. As the main result, the comparison to the new theoretical model taking into account the elastic substrate showed only negligible differences, and, thus, it is suggested for the determination of the effective piezoelectric parameters of piezoelectric layers on elastic substrates.

Korde, Umesh A.

doi: 10.1177/1045389X08096258pmid: N/A

Membrane mirrors may be attractive in some space applications where light weight, deployability, and conformability are desired. This article investigates large-displacement closed-loop control of electrostatically driven mirrors being used in focusing and steering of laser beams. The required transient response characteristics for the membrane response are achieved using a variable area, constant voltage actuation method, which may be advantageous in some applications. Our previous work on variable area control was restricted to membrane deflections not exceeding 1/3 (the available gap size), and assumed that a continuous area variation was available. In this article, the controller design is extended to enable deflections approaching the entire available gap size, which would help to bring down the voltage requirement for a prescribed deflection range. A Lyapunov function-based approach is used here along with a fully nonlinear dynamic model. Although a single-mode lumped parameter model is used, a feedforward technique to compensate for a class of residual modes is investigated in this paper. Also studied in this article is the use of a Lyapunov function-based framework to guide the design of a controller to handle step-wise/discrete area changes. A dynamic observer based on quad-cell beam-position detector measurements and a fully nonlinear plant model is also studied. The article discusses numerical simulation results, which show that the controllers under study are fairly successful at providing closed-loop deflections approaching the full gap size at the required bandwidth.

Belouadah, R.; Kendil, D.; Bousbiat, E.; Ducharne, B.; Guyomar, D.; Guiffard, B.

doi: 10.1177/1045389X08096257pmid: N/A

It is important to model ferroelectric behavior of polymers to understand the physical phenomena exhibited by this kind of material. In this article, a new method based on dry-friction movement is proposed. It is already known that this modeling gives good results for ferroelectric ceramics. However, in the special case of ferroelectric polymers, the resistive current that exists in a low frequency electric field cannot be neglected. In order to adapt the ceramic model to ferroelectric polymers, a new formulation containing a special resistive term is proposed. The viability of the new model has been tested under different amplitudes of low frequency (5 mHz, 10 mHz, 1 Hz) applied electric field. Comparison between simulations and experimental data on polyvinylidene fluoride (β phase) are shown as validation of the model accuracy.

Wiguna, Tedy; Heo, Seok; Hoon Cheol Park, ; Nam Seo Goo,

doi: 10.1177/1045389X08096359pmid: N/A

This paper presents the design and experimental parametric study of a biomimetic fish robot actuated by two lightweight piezo-composite actuators. The biomimetic aspects in this work are the mimicking of an oscillating tail-beat motion and the shape of an artificial caudal fin. Artificial caudal fins that resemble the fins of fish propelled by the body-and-caudal fin mode are fabricated for a parametric study of the way caudal fin characteristics affect the production of thrust at an operating frequency range. The observed caudal fin characteristics are the shape, area, and aspect ratio. A caudal fin with a high aspect ratio contributes to the high swimming speed of the present fish platform. A fish robot propelled by an artificial caudal fin shaped like the fin of a thunniform fish, which has a relatively high aspect ratio, produces a swimming speed as high as 2.364 cm/s for a 300 Vpp input voltage excited at 0.9Hz. The thrust performance of the biomimetic fish robot is examined in terms of the Strouhal number, the Froude number, the Reynolds number, and power consumption.

Browne, Alan L.; Mccleary, Joseph D.; Namuduri, Chandra S.; Webb, Scott R.

doi: 10.1177/1045389X08096358pmid: N/A

As part of an emerging effort in what is now termed the area of mechamatronics (Browne et al., 2004), an effort was begun to assess the suitability of magnetorheological (MR) material-based devices for impact energy management applications. A fundamental property of MR materials is that their yield stress alters almost instantaneously (and proportionally) to changes in the strength of an applied magnetic field. Based on this property, MR-based devices, if found suitable, would be desirable for impact energy management applications because of attendant response tailorability. However, it was identified that prior to adopting MR-based devices for impact energy management applications several key issues needed to be addressed. The present study focuses on one of the most significant of these, the verification of the tunability of the response of such devices at stroking velocities representative of vehicular crashes. Impact tests using a free-flight drop tower facility were conducted on an MR-based energy absorber (shock absorber) for a range of impact velocities and magnetic field strengths. Results demonstrated that over the range of impact velocities tested — 1.0—10 m/s — the stroking force/energy absorption exhibited by the device remained dependent on, and thus could be modified by, changes in the strength of the applied magnetic field.

Jonsdottir, F.; Thorarinsson, E.T.; Palsson, H.; Gudmundsson, K.H.

doi: 10.1177/1045389X08094303pmid: N/A

Microprocessor-controlled prosthetic knees, which rely on magnetorheological (MR) technology, have the potential to increase the comfort and quality of life of amputees. The focus of this study is on a prosthetic knee which is currently on the market and manufactured by the company Ossur Inc. The knee uses magnetic fields to vary the viscosity of the MR fluid, and thereby its flexion resistance. The torque transmissibility of the knee greatly depends on the magnetic field intensity in the MR fluid. The objective of this study is to investigate the strength of the magnetic field and the braking torque in the knee, for a few selected design parameters, and to determine which changes can be made to the existing design in order to maximize the torque output. The magnetic field in the fluid is evaluated by finite element analysis and the torque is calculated by using a Bingham visco-plastic model. A parametric study is carried out for several design parameters where the effect of variation in each parameter on the braking torque is observed. The results of this study give a valuable insight into which parameters should be prioritized for future changes of the knee, with regard to strength and comfortability.

Wallmersperger, Thomas; Horstmann, Antonia; Kroplin, Bernd; Leo, Donald J.

doi: 10.1177/1045389X08096356pmid: N/A

Ionomeric polymer transducers are a class of smart materials which exhibit electromechanical coupling when subjected to low voltage (<5 V) excitation. Generally these materials are soft actuators exhibiting large bending strains (>5%) but correspondingly low force output. The mechanisms producing electromechanical coupling have so far not been completely understood. It is clear from experimental and theoretical investigations that diffusion and migration of ionic species within the polymer are the main cause for electromechanical coupling. For this reason we have developed a thermodynamically based mechanical model — using chemo-electrical inputs — which is able to predict the mechanical output i.e., deformation, bending, etc. for a given applied voltage to the IPMC strip. The chemo-electrical transport model is capable of computing the charge density profile in space and time as well as the current flux for applied electric fields. Based upon thermodynamic laws, the mechanical model has been developed to describe the strain within the material. The mechanical stress in this model is accomplished by two terms of the charge density, a linear and a quadratic one. The linear term represents the volume displacement caused by the charge migration while the quadratic term stands for the electrostatic forces caused by charge imbalances in the material. In this paper, numerical investigations of the electromechanical model as well as displacement measurements have been performed. A comparison of numerical and experimental investigations shows a very good correlation. This confirms the quality and the validity of the developed model.