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Significant deformations of bodies made from compliant magnetoactive elastomers (MAE) in magnetic fields make these materials promising for applications in magnetically controlled actuators for soft robotics. Reported experimental research in this context was devoted to the behaviour in the quasi-static magnetic field, but the transient dynamics are of great practical importance. This paper presents an experimental study of the transient response of apparent longitudinal and transverse strains of a family of isotropic and anisotropic MAE cylinders with six different aspect ratios in time-varying uniform magnetic fields. The time dependence of the magnetic field has a trapezoidal form, where the rate of both legs is varied between 52 and 757 kA/(s·m) and the maximum magnetic field takes three values between 153 and 505 kA/m. It is proposed to introduce four characteristic times: two for the delay of the transient response during increasing and decreasing magnetic field, as well as two for rise and fall times. To facilitate the comparison between different magnetic field rates, these characteristic times are further normalized on the rise time of the magnetic field ramp. The dependence of the normalized characteristic times on the aspect ratio, the magnetic field slew rate, maximum magnetic field values, initial internal structure (isotropic versus anisotropic specimens) and weight fraction of the soft-magnetic filler are obtained and discussed in detail. The normalized magnetostrictive hysteresis loop is introduced, and used to explain why the normalized delay times vary with changing experimental parameters.
A novel method for controlling the rebound behavior of small balls made of Al2O3 with a radius of 2.381 mm is presented. It uses different types of micro-structured surfaces of soft magnetoactive elastomers. These surfaces were fabricated via laser micromachining and include fully ablated surfaces as well as micrometer-sized lamellas with a fixed width of 90 µm, height of 250 µm and three different gap sizes (15, 60 and 105 µm). The lamellas can change their orientation from edge-on to face-on configuration according to the direction of the external magnetic field from a permanent magnet. The orientation of the external magnetic field significantly influences the rebound behavior of the balls, from a coefficient of restitution e of to < 0.1. The highest relative change in the coefficient of restitution between zero field and face-on configuration of is observed for lamellas with a gap of 60 µm. Other characteristics of the ball rebound such as the penetration depth into an Magnetoactive elastomer and the maximum deceleration are investigated as well. The proposed method does not require a constant power supply due to the use of permanent magnets. It may find novel applications in the field of impact engineering.
An effective method for on-demand control over the impact dynamics of droplets on a magnetoresponsive surface is reported. The surface is comprised of micrometer-sized lamellas from a magnetoactive elastomer on a copper substrate. The surface itself is fabricated using laser micromachining. The orientation of the lamellae is switched from edge-on (orthogonal to the surface) to face-on (parallel to the surface) by changing the direction of a moderate (<250 mT) magnetic field. This simple actuation technique can significantly change the critical velocities of droplet rebound, deposition, and splashing. Rebound and deposition regimes can be switched up to Weber number We < 13 ± 3, while deposition and splashing can be switched in the range of 32 < We < 52. Because a permanent magnet is used, no permanent power supply is required for maintaining the particular regime of droplet impact. The presented technology is highly flexible and enables selective fabrication and actuation of microstructures on complex devices. It has great potential for applications in soft robotics, microfluidics, and advanced thermal management.
It is shown that the advancing (ACA) and receding (RCA) contact angles of water on extremely soft (shear modulus of the order of 10 kPa) magnetoactive elastomer (MAE) films significantly depend on the applied magnetic field. The difference between these angles, known as the contact angle hysteresis, is examined. The roles of the filler concentration and material softness are elaborated. The highest change in the contact angle hysteresis (CAH) from 34° in the absence of magnetic field to 76° in a magnetic field of 0.4 T is achieved for the softest sample with the lowest mass fraction of iron particles (70 wt%). The dependence of the CAH on magnetization history (“magnetic hysteresis”) is observed. This magnetic hysteresis is clearly pronounced for the ACA and has little effect on the RCA. Magnetic field-induced changes of the surface roughness exhibit qualitatively the same hysteresis behavior with regard to the external magnetic field as the ACA. The results are promising for the development of smart surfaces for applications where the dynamic wetting has to be controlled.
Magnetoactive elastomers (MAEs) are promising materials for realization of magnetic field-controlled soft actuators. Herein, a systematic investigation of magnetic field-induced macroscopic deformations of soft MAE cylinders with a diameter of 15 mm in uniform quasi-static magnetic fields directed parallel to the cylinder’s axis is reported. The measurements were based on image processing. Thirty-six MAE samples differing in the weight fraction of the iron filler (70 wt%, 75 wt% and 80 wt%), alignment of filling particles, and the aspect ratio (0.2, 0.4, 0.6, 0.8, 1.0 and 1.2) were fabricated. MAE cylinders exhibited high relative change in height (up to 35% in the field of 485 kA/m) and lateral contraction. The dependence of the maximum extensional strain on the aspect ratio was obtained and compared with theoretical considerations. A concave dent was formed on the free circular base in magnetic fields. This concavity was characterized experimentally. A significant volumetric strain of the order of magnitude of 10% was calculated in MAEs for the first time. In consequently repeated magnetization cycles, the remanent extensional strain significantly increased after each cycle. The results are qualitatively discussed in the framework of the modern views on the magnetically induced macroscopic deformations of MAEs. The directions of further research are outlined.
Commercially available contact angle (CA) measuring devices usually do not allow for the application of magnetic fields to the sample under test. A setup for measuring the CA of liquids on magnetosensitive surfaces has been developed specifically for investigating the surfaces of magnetoactive elastomers (MAEs). The addition of a programmable linear stage, which moves a permanent magnet, allows for fine control of the magnetic field applied to the MAE without the need for large and power-consuming electromagnets. Paired with a custom control and evaluation software, this measurement setup operates semiautomatically, limiting operator error and increasing precision, speed, as well as repeatability of static and dynamic CA measurements for different magnetoactive materials. The software is equipped with robust droplet fitting algorithms to avoid experimental challenges arising with soft magnetoactive materials, such as the curling of sample edges or diffuse non-reflective surfaces. Several application examples on MAE surfaces, both processed and unprocessed, are presented.
Efficient transportation of droplets (∽10 ¹ ̶̶̶̶ 10 ² µl) and small solid objects (∽10 ¹ ̶ 10 ² mm ³ ) have important applications in many fields, such as microfluidics, lab‐on‐a‐chip devices, drug delivery, etc. A novel multifunctional surface consisting of a periodic array of micro‐lamellae from a soft magnetoactive elastomer (MAE) on a plastic substrate is reported for these purposes. The physical origin of the propulsion is the bending of soft magnetic lamellae in non‐uniform magnetic fields, which is also observed in uniform magnetic fields. The magnetoactive surface is fabricated using a facile and rapid method of laser ablation. The propulsion of items is realized using a four‐pole rotating magnet. This results in a cyclic lamellar fringe motion over the micro‐structured surface and brings an advantage of easy reciprocation of transport by rotation reversal. Two modes of object transportation are identified: “pushing” mode for precise control of droplet and solid positioning and “bouncing” mode for heavier solid objects transportation. A water droplet of 5 μl or a glass sphere with a 2.1 mm diameter can be moved at a maximum speed of 60 mm s ⁻¹ . The multifunctionality of the proposed mechatronic platform is demonstrated on the examples of selective solid‐liquid separation and droplet merging.
Stimuli responsive materials are key ingredients for any application that requires dynamically tunable or on-demand responses. In this work we report experimental and theoretical investigation of magnetic-field driven modifications of soft-magnetic elastomers whose surface was processed by laser ablation into lamellar microstructures that can be manipulated by a uniform magnetic field. We present a minimal hybrid model that elucidates the associated deflection process of the lamellae and explains the lamellar structure frustration in terms of dipolar magnetic forces arising from the neighbouring lamellae. We experimentally determine the magnitude of the deflection as a function of magnetic flux density and explore the dynamic response of lamellae to fast changes in a magnetic field. A relationship between the deflection of lamellae and modifications of the optical reflectance of the lamellar structures is resolved.
Anhand zweier Beispiele wird ein interaktives System vorgestellt, mit welchem Live-Messungen an elektronischen Bauteilen und Schaltungen im Hörsaal durchgeführt werden können. Das System basiert auf dem USB-Messgerät „Analog Discovery 2“ der Firma Digilent, erweitert um eine Adapterplatine mit Steckbrett und eine Webcam.
Zunächst werden drei Aspekte erläutert, in welchen ein solches System positive Auswirkungen auf den Kompetenzerwerb haben kann: Praxisbezug, Aktivierung und Vernetzung. Anschließend werden die zwei Beispiele ausgeführt. In Beispiel 1 wird die persönliche Alltagserfahrung mit Akkus über interaktive Clicker-Fragen und begleitende Messungen mit dem Modell der linearen Quelle verknüpft. Die Studierenden lernen, oberflächliche Lösungen wie die Messung der Leerlaufspannung zu hinterfragen und das Verhalten realer Quellen anhand der gemessenen I-U-Kennlinien zu verstehen. Beispiel 2 zeigt kurz die Möglichkeiten auf, den abstrakten Formalismus der komplexen Wechselstromrechnung am Beispiel des Serienschwingkreises im Zeit- und Frequenzbereich
messtechnisch erlebbar zu machen.
Soft robotics
(2022)
Theoretical Modeling of Magnetoactive Elastomers on Different Scales: A State-of-the-Art Review
(2022)
A review of the latest theoretical advances in the description of magnetomechanical effects and phenomena observed in magnetoactive elastomers (MAEs), i.e., polymer networks filled with magnetic micro- and/or nanoparticles, under the action of external magnetic fields is presented. Theoretical modeling of magnetomechanical coupling is considered on various spatial scales: from the behavior of individual magnetic particles constrained in an elastic medium to the mechanical properties of an MAE sample as a whole. It is demonstrated how theoretical models enable qualitative and quantitative interpretation of experimental results. The limitations and challenges of current approaches are discussed and some information about the most promising lines of research in this area is provided. The review is aimed at specialists involved in the study of not only the magnetomechanical properties of MAEs, but also a wide range of other physical phenomena occurring in magnetic polymer composites in external magnetic fields.
We demonstrate the control of wettability of non-structured and microstructured magnetoactive elastomers (MAEs) by magnetic field. The synthesized composite materials have a concentration of carbonyl iron particles of 75 wt.% (≈27 vol.%) and three different stiffnesses of the elastomer matrix. A new method of fabrication of MAE coatings on plastic substrates is presented, which allows one to enhance the response of the apparent contact angle to the magnetic field by exposing the particle-enriched side of MAEs to water. A magnetic field is not applied during crosslinking. The highest variation of the contact angle from (113 ± 1)° in zero field up to (156 ± 2)° at about 400 mT is achieved in the MAE sample with the softest matrix. Several lamellar and pillared MAE structures are fabricated by laser micromachining. The lateral dimension of surface structures is about 50 µm and the depth varies between 3 µm and 60 µm. A systematic investigation of the effects of parameters of laser processing (laser power and the number of passages of the laser beam) on the wetting behavior of these structures in the absence and presence of a magnetic field is performed. In particular, strong anisotropy of the wetting behavior of lamellar structures is observed. The results are qualitatively discussed in the framework of the Wenzel and Cassie-Baxter models. Finally, directions of further research on magnetically controlled wettability of microstructured MAE surfaces are outlined. The obtained results may be useful for the development of magnetically controlled smart surfaces for droplet-based microfluidics.
In this work, the resonance enhancement of magnetoelectric (ME) coupling at the two lowest bending resonance frequencies was investigated in layered cantilever structures comprising a magnetoactive elastomer (MAE) slab and a commercially available piezoelectric polymer multilayer. A cantilever was fixed at one end in the horizontal plane and the magnetic field was applied horizontally. Five composite structures, each containing an MAE layer of different thicknesses from 0.85 to 4 mm, were fabricated. The fundamental bending resonance frequency in the absence of a magnetic field varied between roughly 23 and 55 Hz. It decreased with the increasing thickness of the MAE layer, which was explained by a simple theory. The largest ME voltage coefficient of about 7.85 V/A was measured in a sample where the thickness of the MAE layer was ≈2 mm. A significant increase in the bending resonance frequencies in the applied DC magnetic field of 240 kA/m up to 200% was observed. The results were compared with alternative designs for layered multiferroic structures. Directions for future research were also discussed.
In this work, multiferroic cantilevers comprise a layer of a magnetoactive elastomer (MAE) and a commercially available piezoelectric polymer-based vibration sensor. The structures are fixed at one end in the horizontal plane and the magnetic field is applied vertically. First, the magnetoelectric (ME) response to uniform, triangle-wave magnetic fields with five different slew rates is investigated experimentally. Time and field dependences of the generated voltage, electric charge, and observed mechanical deflection are obtained and compared for four different thicknesses of the MAE layer. The ME responses to triangular and sinusoidal wave excitations are examined in contrast. Second, the ME response at low frequencies (≤3 Hz) is studied by the standard method of harmonic magnetic field modulation. The highest ME coupling coefficient is observed in the bias magnetic field strength of ≈73 kA/m and it is estimated to be about 3.3 ns/m (ME voltage coefficient ≈ 25 V/A) at theoretically vanishing modulation frequency (f→0 Hz). Presented results demonstrate that the investigated heterostructures are promising for applications as magnetic-field sensors and energy harvesting devices.
In this paper, the results of Thermally Stimulated Depolarization Currents (TSDC)-measurements of a complex system of stacked dielectric layers, like typically appearing in semiconductor packages, is presented. Characteristics of the individual isolation layers are presented solely and the results are compared to the stack. The investigated layers are comprising polyimide film, alkali-free glass, polyolefin film and epoxy based mold compound. Measurements have been carried out in the temperature range from 19 - 195°C at varying polarization fields for every material. Various relaxation peaks, originated by different polarization mechanisms within the materials could be observed. A comparison between the TSDC-peaks of the layer stack and the individual materials is shown. Our results demonstrate, that the super positioned spectrum is getting dominated by a single portion, the mold compound.
Herein, the effect of an applied moderate (~240 mT) magnetic field on the work of adhesion (WoA) of mechanically soft (the shear modulus ~10 kPa) magnetoactive elastomer (MAE) samples with two different mass fractions (70 and 80 wt%) of carbonyl iron powder (CIP) is concerned. The unfilled elastomer sample is used for comparison. Due to some sedimentation of filling particles, the concentration of inclusions in thin (~10 μm) subsurface layers is different. It is shown that the WoA increases (up to 1.8-fold) on the particle-enriched side (PES) in the magnetic field and its value is higher for higher filler concentration. On the particle-depleted side (PDS), WoA does not depend on particle concentration and on the magnetic field. Adhesion and friction are coupled in MAEs. No statistically significant difference in the friction coefficient, determined from the extended Amontons´ law, depending on sample side, CIP concentration, or presence of magnetic field is found. However, the PDS in the magnetic field demonstrates significantly higher critical shear stress compared to that for the PES or PDS in the absence of magnetic field. Correlations between different surface properties are discussed. Obtained results are useful for the development of magnetically controllable soft robots.
Adaptive surface microstructures are used extensively in nature to control various surface properties such as wettability, adhesion, self-cleaning, drag reduction, etc. Regulation of these properties can be achieved with the appropriate employment of a multitude of smart materials, whose characteristics/response can be controlled by noncontact stimuli, e.g., light, heat, or magnetic field. One of the very promising magneto-regulable smart materials are magnetoactive elastomers (MAEs). They are comprised of a compliant polymer matrix with embedded micrometer-sized ferromagnetic particles. The particles interact with each other and a magnetic field. This results in remarkable tunability of the physical properties of MAEs. This paper reports a fast, resilient, and tailored method for direct surface micromachining of MAEs that enables micro-structuring without mechanical contact between the tool and the material, bypassing the usual constraints of conventional fabrication methods. It is shown that it is possible to fabricate a large variety of different microstructure geometries whose precision is limited predominantly by the size of magnetic particles. Lamellar structures with a high aspect ratio (up to 6:1) oriented either perpendicularly to the surface, can be strongly bent by applying magnetic fields in the range of 0–250 mT.
Conventional calculations of the inertia tensor in undergraduate physics course are usually done for highly symmetrical bodies. Students might therefore get the impression that the moment of inertia about any axis through the center of mass is the same only for bodies with the highest degree of symmetry relative to this point, e.g., for spheres. A simple, seemingly counterintuitive example is presented, showing that the moment of inertia of a non-regular body, here an assembly of material points, can be the same about any axis passing through its center of mass.
The concept of Industrial Revolution 4.0 (IR4.0) has sparked the urgency of many manufacturers to revisit their manufacturing processes and search for opportunities to further improved their production output. Unfortunately, it is difficult to improve a process with inadequate data. Many of the SMEs in developing countries are still using manufacturing machines from the yesteryears which do not have computational and connectivity capabilities. Therefore, we developed an end-to-end Internet-of-Things (IoT) solution, which reliably tracks the production performance of manufacturing machines. This paper elaborates on the designs and the rationale behind it. As of the writing of this paper, our IoT system has been deployed in real manufacturing environment and has been running for approximately 90 days on a 24/7 basis without data lost.
A surface relief grating with a period of 30 mu m is embossed onto the surface of magnetoactive elastomer (MAE) samples in the presence of a moderate magnetic field of about 180 mT. The grating, which is represented as a set of parallel stripes with two different amplitude reflectivity coefficients, is detected via diffraction of a laser beam in the reflection configuration. Due to the magnetic-field-induced plasticity effect, the grating persists on the MAE surface for at least 90 h if the magnetic field remains present. When the magnetic field is removed, the diffraction efficiency vanishes in a few minutes. The described effect is much more pronounced in MAE samples with larger content of iron filler (80 wt%) than in the samples with lower content of iron filler (70 wt%). A simple theoretical model is proposed to describe the observed dependence of the diffraction efficiency on the applied magnetic field. Possible applications of MAEs as magnetically reconfigurable diffractive optical elements are discussed. It is proposed that the described experimental method can be used as a convenient tool for investigations of the dynamics of magnetically induced plasticity of MAEs on the micrometer scale.
Effect of magnetic-field-induced restructuring on the elastic properties of magnetoactive elastomers
(2021)
Composite materials where magnetic micrometer-sized particles are embedded into a compliant polymer matrix are known as magnetorheological (or magnetoactive) elastomers (MAEs). They are distinguished by huge variations in their physical properties, when in a magnetic field, which is commonly attributed to the restructuring of the filler. The process of the magnetic-field-induced restructuring in a magnetorheological elastomer is interpreted as progression towards percolation. Such a physical model was previously used to explain the dependence of the magnetic permeability and dielectric permittivity of MAEs on the magnetic field strength. Based on this hypothesis, the magnetorheological effect in MAEs is considered theoretically. The theoretical approach is built upon a self-consistent effective-medium theory for the elastic properties, extended to the variable (field dependent) percolation threshold. The proposed model allows one to describe the large variations (over several orders of magnitude) of the effective elastic moduli of these composite materials, known as the giant magnetorheological (MR) and field-stiffening effects. The existence of a giant magnetic Poisson effect is predicted. The relation of the proposed model to the existing theories of the MR effect in MAEs is discussed. The results can be useful for applications of MAEs in magnetic-field-controlled vibration dampers and isolators.
A simple method for structuring of the surface of a magnetoactive elastomer (MAE) on the tens of micrometers scale, which capabilities extend beyond conventional mold-based polymer casting, is reported. The method relies on the ablation of the material by absorption of nanosecond infrared pulses from a commercial laser. It is shown that it is possible to fabricate parallel lamellar structures with a high aspect ratio (up to 6:1) as well as structures with complex scanning trajectories. The method is fast (fabrication time for the 7 × 7 mm2 is about 60 s), and the results are highly reproducible. To illustrate the capabilities of the fabrication method, both orthogonal to the MAE surface and tilted lamellar structures are fabricated. These magnetosensitive lamellae can be easily bent by ±45° using an external magnetic field of about 230 mT. It is demonstrated that this bending allows one to control the sliding angle of water droplets in a great range between a sticky (>90°) and a sliding state (<20°). Perspectives on employing this fabrication technology for magnetosensitive smart surfaces in microfluidic devices and soft robotics are discussed.
The significant effect of an external dc magnetic field on the splashing behavior of ethanol drops impacting on the unstructured (flat) surface of soft magnetoactive elastomers (MAEs) is reported. The Weber number corresponding to the transition between the deposition and the splashing regime is reduced by ≈20% in a moderate magnetic field of ≈300 mT. Alongside this effect, a two-fold increase of the initial deceleration of the ejection sheet is observed for the softest sample. The main underlying mechanism for the observed phenomena is believed to be the magnetic-field-induced stiffening of the MAEs. Further possible mechanisms are magnetically induced changes in the surface roughness and magnetic-field-induced plasticity (magnetic shape memory effect). The potential application areas are magnetically regulable wetting and magneto-responsive surfaces for controlling the drop splashing.
A voltage transformer employing the magnetoelectric effect in a composite ceramic heterostructure with layers of a magnetostrictive nickel–cobalt ferrite and a piezoelectric lead zirconate–titanate is described. In contrast to electromagnetic and piezoelectric transformers, a unique feature of the presented transformer is the possibility of tuning the voltage transformation ratio K using a dc magnetic field. The dependences of the transformer characteristics on the frequency and the amplitude of the input voltage, the strength of the control magnetic field and the load resistance are investigated. The transformer operates in the voltage range between 0 and 112 V, and the voltage transformation ratio K is tuned between 0 and 14.1 when the control field H changes between 0 and 6.4 kA/m. The power at the transformer output reached 63 mW, and the power conversion efficiency was 34%. The methods for calculation of the frequency response, and the field and load characteristics of the transformer are proposed. The ways to improve performance characteristics of magnetoelectric transformers and their possible application areas are discussed.
The ability to synchronize expectations among human-robot teams and understand discrepancies between expectations and reality is essential for human-robot collaboration scenarios. To ensure this, human activities and intentions must be interpreted quickly and reliably by the robot using various modalities. In this paper we propose a multimodal recognition system designed to detect physical interactions as well as nonverbal gestures. Existing approaches feature high post-transfer recognition rates which, however, can only be achieved based on well-prepared and large datasets. Unfortunately, the acquisition and preparation of domain-specific samples especially in industrial context is time consuming and expensive. To reduce this effort we introduce a weakly-supervised classification approach. Therefore, we learn a latent representation of the human activities with a variational autoencoder network. Additional modalities and unlabeled samples are incorporated by a scalable product-of-expert sampling approach. The applicability in industrial context is evaluated by two domain-specific collaborative robot datasets. Our results demonstrate, that we can keep the number of labeled samples constant while increasing the network performance by providing additional unprocessed information.
Thermomechanical shape memory materials have certain disadvantages when it comes to 3D volumetric reproduction intended for rapid prototyping or robotic prehension. The need to constantly supply energy to counteract elastic retraction forces in order to maintain the required geometry, together with the inability to achieve conformal stability at elevated temperatures, limits the application of thermal shape memory polymers. Form removal also presents problems as most viscoelastic materials do not ensure demolding stability. This work demonstrates how magnetoactive boron−organo−silicon oxide polymers under the influence of an applied magnetic field can be used to achieve energy free sustainable volumetric shape memory effects over extended periods. The rheopectic properties of boron−organo−silicon oxide materials sustain form removal without mold distortion.
The Applied Research Conference which is held every year at another University of Applied Sciences in Bavaria is the main event for all students in the Master of Applied Research program. They come together to present their work in oral presentations and full papers, which are published in the proceedings, as well as poster presentations of the students after their 1st semester. For sure it is interesting for Professors and interested people to see the results of upcoming scientists and researchers. Due to the Corona pandemic, this year it is not possible to organize the conference as usual in presence. We have to refrain from face-to-face discussions, having together a cup of coffee. We as Professors at the OTH Regensburg wanted to give our students the chance to finish this semester successfully – despite all limitations due to the pandemic situation. Therefore we decided to organize the RARC 2020 – Regensburg Applied Research Conference 2020 – for the Master of Applied Research students of OTH Regensburg as an online conference. However, for a Technical University the situation is rather a challenge than a problem. Using a variety of online tools for teaching during this semester, we have enough experience to find a setup for RARC2020. We received 28 submissions for full papers, which were peer reviewed and 26 of them were accepted – you will find them in this proceedings, and they will be presented orally on July 31st, 2020. Additionally, there are 22 posters which will be presented on the same day. During the Plenary Opening Session, after a welcome by our President Prof. Dr. Wolfgang Baier we will have 3 Keynote speakers:
Prof. Dr. phil. habil. Karsten Weber: Erkenntnistheorie für Ingenieure
Prof. em. Georg Scharfenberg: 11 Years Master of Applied Research Alumnis
Veronika Fetzer: Entrepreneurship
We propose a theoretical approach for calculating effective electric and magnetic properties of composites, with field dependent restructuring of the filler. The theory combines the effective medium approximation, extended to a field-dependent (variable) percolation threshold, with an approximate treatment of the nonlinearity of material properties. Theoretical results are compared with experiments on magnetorheological elastomers, which in the context of investigated phenomena are often called magnetoactive elastomers (MAEs). In MAEs with soft polymer matrices, the mutual arrangement of inclusions changes in an applied magnetic field. This reorganization of the microstructure leads to unconventionally large changes of electrical and magnetic properties. The obtained theoretical results describe observed phenomena in MAEs well. For the magnetodielectric effect, qualitative agreement between theory and experiment is demonstrated. In the case of magnetic permeability, quantitative agreement is achieved. The theoretical approach presented can be useful for the development of field-controlled smart materials and design of intelligent structures on their basis, because the field dependence of physical properties can be predicted. (C) 2019 Elsevier B.V. All rights reserved.
The magnetodielectric response of magnetoactive elastomers (MAEs) in its dependence on filler concentration, magnetic field, and test frequency is studied experimentally. MAEs are synthesized on the basis of a silicone matrix filled with spherical carbonyl iron particles characterized by a mean diameter of 4.5 mu m. The concentration of the magnetic filler within composite materials is equal to 70, 75, and 80 mass%. The effective lossless permittivity epsilon as well as the dielectric loss tan grow significantly when the magnetic field increases. The permittivity increases and the dielectric loss decreases with increasing filler concentration. In the measurement frequency range between 1 kHz and 200 kHz, the frequency hardly affects the values of epsilon and tan in the absence of a magnetic field. However, both parameters decrease considerably with the growing frequency in a constant magnetic field. The more strongly the magnetic field is applied, the larger the change in permittivity and loss tangent at the same test frequency is observed. An equivalent circuit formulation qualitatively describes the main tendencies of the magnetodielectric response.
This paper concerns recent research into the autonomous formation of micro-structures in low carbonyl iron powder (CIP) concentration magneto active polymers (MAP). Higher concentrations of CIP show an isotropic distribution of magnetic filler throughout the entire sample, while autonomous structure formation is possible at mass concentrations lower than 3%. The formation of micro-toroids commences as CIP concentration approaches 1% wt. Further development of coherent rings with a defined order follow as CIP concentrations increase toward 2% wt, whilst exceeding 3% wt leads to the same isotropic distribution found in higher concentration MAP. Structured samples containing between 1% wt and 3% wt CIP were investigated using X-Ray tomography where solitary structures could clearly be observed. The ring structures represent microinductivities whose geometries can be manipulated during fabrication. In addition, these structures are magnetic field sensitive. This is not only relevant to applications in the GHz and THz areas but recent research has revealed implications for optical, thermal, acoustic and even chemical MAP sensors. (C) 2019 Author(s).
The magnetization of a magnetoactive elastomer (MAE) with microparticles of soft magnetic carbonyl iron embedded in a highly elastic matrix has been studied. It is shown that at high temperatures its magnetization curve has the form of a specific hysteresis loop. This hysteresis is attributed to the influence of displacement of magnetized particles in the elastically soft elastomer matrix under the effect of magnetic forces, leading to the change of magnetic interaction between the particles. In this case, there is a maximum in the field dependence of the magnetic susceptibility, the occurrence of which has been associated with the competition between rearrangement of particles, when they are displaced in a magnetic field, and saturation of particles' magnetization. When the MAE is cooled below approximately 225 K, both the magnetic hysteresis and the maximum in the field dependence of the magnetic susceptibility disappear. When the MAE material is cooled below the solidification temperature of the elastomer matrix, the displacements of the magnetic particles during magnetization are blocked by the rigid matrix. The magnetization reversal of the MAE is reversible. This means that the shape of subsequent magnetization loops remains constant and the sample returns into the initial non-magnetized state after the magnetic field is turned off.
The direct magnetoelectric (ME) effect is investigated in a planar structure comprising mechanically coupled layers of a magnetostrictive fibrous composite (MFC) and a piezoelectric ceramics (lead zirconate titanate, PZT). The MFC is an array of Ni-wires with a diameter of 200 mu m that are aligned parallel to each other in a single layer. The wires are separated by a distance of 250 or 500 mu m and fixed in a polyamide matrix. The structure was placed in a tangential constant field H and was excited by an alternating magnetic field h parallel to H, while the voltage generated by the PZT layer was measured. The resulting field dependences of the magnetization M(H) and the magnetostriction lambda(H) were determined by the orientation of the field H in the plane of the structure and the distance between the Ni-wires. The ME coupling coefficient of the structure decreased from 4.8 to 0.25 V/A when the orientation of H was changed from parallel to perpendicular to Ni-wires. With an increase in the excitation field amplitude h, a nonlinear ME effect in the output voltage, namely frequency doubling, was observed. The frequency and field dependences of the efficiency of the ME transduction in the MFC-piezoelectric heterostructure are well described by the existing theory.
This work describes the development of a new miniature extruder, essential to cavity-free 3D printing of silicone-based smart materials. This makes the 3D printing of magnetoactive and electroactive polymer soft robotic components and devices directly from CAD data possible. The special feature of such an extruder is that it is designed for use with addition-crosslinking RTV-2 silicones, including solid particulate additives. The extruder merges the respective components automatically during extrusion which obviates the need for premixing and vacuum evacuation. Problems associated with inhomogeneities and unwanted cavity production are consequently eliminated. Rheological details necessary to the design, together with some preliminary performance results, are presented.
Polydimethylsiloxane based magnetoactive elastomers demonstrate above the melting transition range (e.g. at room temperature) an induced uniaxial magnetic anisotropy, which grows with increasing magnetic field. By freezing a material down to 150 K, displaced iron microparticles are immobilized, so that the magnetic anisotropy can be measured. Magnetic anisotropy “constant” is a consequence of particle displacements and a characteristic of the energy of internal deformations in the polymer matrix. The maximum anisotropy constant of the filling is at least one order of magnitude larger than the shear modulus of the pure elastomer (matrix). In a magnetic field, the gain in the rigidity of the composite material is attributed to the magnetomechanical coupling, which is in turn a source of anisotropy. The concept of effective magnetic field felt by the magnetization allows one to explain the magnetization curve at room temperature from low-temperature measurements. The results can be useful for developing vibration absorbers and isolators.
Collaboration between robots and humans requires communicative skills on both sides. The robot has to understand the conscious and unconscious activities of human workers. Many state-of-the-art activity recognition algorithms with high performance rates on existing benchmark datasets are available for this task. This paper re-evaluates appropriate architectures in light of human work activity recognition for working cells in industrial production contexts. The specific constraints of such a domain is elaborated and used as prior knowledge. We utilize state-of-the-art algorithms as spatiotemporal feature encoders and search for appropriate classification and fusion strategies. Furthermore, we combine keypoint-based with appearance-based approaches to a multi-stream recognition system. Due to data protection rules and the high effort of data annotation within industrial domains only small datasets are available that reflect production aspects. Therefore, we use transfer learning approaches to reduce the dependency on data volume and variance in the target domain. The resulting recognition system achieves high performance for both singular person action and human-object interaction.
Bio-Inspired Shape-Adaptive Soft Robotic Grippers Augmented with Electroadhesion Functionality
(2019)
Soft robotic grippers (SRGs) have been extensively employed in robotic grasping and manipulation applications due to the fact that they are beneficial for pick-and-place of difficult-to-handle and delicate objects with various geometries and stiffness in a comfortable and safer way. This article presents a bio-inspired and shape-adaptive SRG augmented with electroadhesion (EA) functionality, FinEA, by a cost-effective combination of a Fin Ray structured two-fingered SRG with two soft-stretchable EA pads. The EA pads were manufactured by screen printing a layer of electrically conductive and elastomeric carbon black powder mixed with polydimethylsiloxane onto a dielectric substrate. The compliant Fin Ray fingers, composed of soft longitudinal beams and rigid cross beams, were structurally optimized by the finite element method using ABAQUS/CAE based on three key parameters: the open angle of the longitudinal beams, the spacing between the cross beams, and the incline angle of the cross beams. The soft beams were produced by a traditional soft lithography method, whereas the rigid cross beams were three-dimensionally printed. The resultant FinEA grippers were capable of lifting not only flat/thin materials without distorting them due to the employment of EA, but also concave and convex objects due to the passively shape-adaptive Fin Ray structure and the EA functionality. In addition, the proposed FinEA grippers were able to grasp delicate materials and objects whose diameters are larger than the overall gripper length. Furthermore, 65% more weight in shear were picked up by the FinEA gripper when 4 kV was applied compared with 0 V. The FinEA concept provides useful and alternative solution for controllable adhesion-based SRGs and may facilitate the inspiration and development of future SRGs with added functionality and enhanced versatility.
We investigated magnetic-field-induced modifications of the surface roughness of magnetoactive elastomers (MAEs) with four material compositions incorporating two concentrations of ferromagnetic microparticles (70 wt% and 80 wt%) and exhibiting two shear storage moduli of the resulting composite material (about 10 kPa and 30 kPa). The analysis was primarily based on spread optical reflection measurements. The surfaces of all four materials were found to be very smooth in the absence of magnetic field (RMS roughness below 50 nm). A maximal field-induced roughness modification (approximately 1 m/T) was observed for the softer material with the lower filler concentration, and a minimal modification (less than 50 nm/T) was observed for the harder material with the higher filler concentration. All four materials showed a significant decrease in the total optical reflectivity with an increasing magnetic field as well. This effect is attributed to the existence of a distinct surface layer that is depleted of microparticles in the absence of a magnetic field but becomes filled with particles in the presence of the field. We analyzed the temporal response of the reflective properties to the switching on and off of the magnetic field and found switching-on response times of around 0.1 s and switching-off response times in the range of 0.3-0.6 s. These observations provide new insight into the magnetic-field-induced surface restructuring of MAEs and may be useful for the development of magnetically reconfigurable elastomeric optical surfaces.
This chapter introduces composite smart materials known as magnetoactive (MAEs) or magnetorheological elastomers. It starts by defining these materials and distinguishing them from relevant magnetorheological fluids and ferrofluids. It then gives the overview of constitutive materials for the polymer matrix and filler particles. Next, the influence of external magnetic field on physical properties of MAEs is discussed. The emphasis is made on mechanical properties, which are the most important for real-world applications. In particular, magnetomechanical effects such as magnetostriction, magnetodeformation, and magnetorheological effect are discussed. The magnetic Payne effect is presented as an example of a nonlinear behavior. Electromagnetic and acoustic properties are also considered. The chapter finishes with the brief discussion of the future prospects in research and development of MAEs.
The magnetic properties of a magnetoactive elastomer (MAE) filled with mu m-sized soft-magnetic iron particles have been experimentally studied in the temperature range between 150 K and 310 K. By changing the temperature, the elastic modulus of the elastomer matrix was modified, and it was possible to obtain magnetization curves for an invariable arrangement of particles in the sample and in the case when the particles were able to change their position within the MAE under the influence of magnetic forces. At low (less than 220 K) temperatures, when the matrix becomes rigid, the magnetization of the MAE does not show a hysteresis behavior, and it is characterized by a negative value of the Rayleigh constant. At room temperature, when the polymer matrix is compliant, a magnetic hysteresis exists where the dependence of the differential magnetic susceptibility on the magnetic field exhibits local maxima. The appearance of these maxima is explained by the elastic resistance of the matrix to the displacement of particles under the action of magnetic forces.
We present a method to extract monopole and dipole polarizability from experimental measurements of two-dimensional acoustic meta-atoms. In contrast to extraction from numerical results, this enables all second-order effects and uncertainties in material properties to be accounted for. We apply the technique to 3D-printed labyrinthine meta-atoms of a variety of geometries. We show that the polarizability of structures with a shorter acoustic path length agrees well with numerical results. However, those with longer path lengths suffer strong additional damping, which we attribute to the strong viscous and thermal losses in narrow channels.
It is shown that the critical exponent for the effective shear modulus of a composite medium where a compliant polymer matrix is filled with ferromagnetic particles may significantly depend on the external magnetic field. The physical consequence of this dependence is the critical behavior of the relative magnetorheological effect.
Surface topographical modifications of a soft magnetoactive elastomer (MAE) in response to variable applied magnetic field are investigated. The analysis is performed in situ and is based on optical microscopy, spread optical reflection and optical profilometry measurements. Optical profilometry analysis shows that the responsivity of magnetic field-induced surface roughness with respect to external magnetic field is in the range of 1 mu m/T. A significant hysteresis of surface modifications takes place for increasing and decreasing fields. Investigations of shape of sessile water droplets deposited on the MAE surface reveal that field-induced topographical modifications affect the contact angle of water at the surface. This effect is reversible and the responsivity to magnetic field is in the range of 20 degrees/T. Despite the increased surface roughness, the apparent contact angle decreases with increasing field, which is attributed to the field-induced protrusion of hydrophilic microparticles from the surface layer.
Field induced plasticity of magneto-sensitive elastomers for gripping technology applications
(2018)
Most of the mechanical simulations for industrial usage are done by finite element (FE-) analysis. Milling machines are mechatronic systems, combining electrical, mechanical and control components for machining certain materials. Modelica provides a powerful and strong tool to simulate different physical ares in one model. For this usage a mechanical model of a 5-Axis-Milling Machine is implemented with rigid and flexible bodies. Specific attention will be paid to which components can be modelled rigid without significant deviation in accordance to the real behaviour of the machine. Two classes of implementing flexible bodies in multi body systems are given by the flexible bodies Library, advantages and disadvantages of both classes will be evaluated. At the end a comparision of the static and dynamic behaviour of the passive structure of the model in contrast to a FE-analysis is given.
The magnetoactive electret
(2017)
A magnet which adheres to every surface, not only those of ferromagnetic materials, has hitherto been the domain of science fiction. Now for the first time such a novel device exists. The fusion of a permanently magnetized magnetoactive polymer containing hard magnetic particles and an electret enhanced with ferroelectric particles has resulted in the development of a new smart device-the magnetoactive electret. Magnetoactive electrets can be made to exhibit the usual magnetic properties of permanent magnetism together with the electrostatic properties of electrets. This results in simultaneous magnetoadhesion and electroadhesion forces from the same elastomeric element. The biasing field, needed to avoid discontinuities concerned with transition through the zero point in operating curves, is normally provided by means of either a magnetic or an electric field. This novel technology provides both bias options in a single device.
The dependence of the resonant direct magnetoelectric effect on temperature is studied experimentally in planar composite structures. Samples of rectangular shapes with dimensions of 5 mm x 20 mm employed ferromagnetic layers of either an amorphous (metallic glass) alloy or nickel with a thickness of 20-200 m and piezoelectric layers of single crystalline langatate material or lead zirconate titanate piezoelectric ceramics with a thickness of 500 m. The temperature of the samples was varied in a range between 120 and 390 K by blowing a gaseous nitrogen stream around them. It is shown that the effective characteristics of the magnetoelectric effectsuch as the mechanical resonance frequency f(r), the quality factor Q and the magnitude of the magnetoelectric coefficient (E) at the resonance frequencyare contingent on temperature. The interrelations between the temperature changes of the characteristics of the magnetoelectric effect and the temperature variations of the following material parametersYoung's modulus Y, the acoustic quality factor of individual layers, the dielectric constant epsilon, the piezoelectric modulus d of the piezoelectric layer as well as the piezomagnetic coefficients ((n)) of the ferromagnetic layerare established. The effect of temperature on the characteristics of the nonlinear magnetoelectric effect is observed for the first time. The results can be useful for designing magnetoelectric heterostructures with specified temperature characteristics, in particular, for the development of thermally stabilized magnetoelectric devices.
The surface dilational modulus is a crucial parameter for describing the rheological properties of aqueous surfactant solutions. These properties are important for many technological processes. The present paper describes a fully automated instrument based on the oscillating bubble technique. It works in the frequency range from 1 Hz to 500 Hz, where surfactant exchange dynamics governs the relaxation process. The originality of instrument design is the consistent combination of modern measurement technologies with advanced imaging and signal processing algorithms. Key steps on the way to reliable and precise measurements are the excitation of harmonic oscillation of the bubble, phase sensitive evaluation of the pressure response, adjustment and maintenance of the bubble shape to half sphere geometry for compensation of thermal drifts, contour tracing of the bubbles video images, removal of noise and artefacts within the image for improving the reliability of the measurement, and, in particular, a complex trigger scheme for the measurement of the oscillation amplitude, which may vary with frequency as a result of resonances. The corresponding automation and programming tasks are described in detail. Various programming strategies, such as the use of MATLAB (R) software and native C++ code are discussed. An advance in the measurement technique is demonstrated by a fully automated measurement. The instrument has the potential to mature into a standard technique in the fields of colloid and interface chemistry and provides a significant extension of the frequency range to established competing techniques and state-of-the-art devices based on the same measurement principle.
Transient magnetorheological response of magnetoactive elastomers to step and pyramid excitations
(2016)
Transient rheological response of magnetoactive elastomers is experimentally studied using dynamic torsion at a fixed oscillation frequency in temporally stepwise changing magnetic fields and oscillation amplitudes. For step magnetic-field excitations, at least three exponential functions are required to reasonably describe the time behavior of the storage shear modulus over long time scales (>10(3) s). The deduced characteristic time constants of the corresponding rearrangement processes of the filler network differ approximately by one order of magnitude: tau(1) less than or similar to 10(1) s, tau(2) similar to 10(2) s, and tau(3) similar to 10(3) s. The sudden imposition of the external magnetic field activates a very fast rearrangement process with the characteristic time under 10 s, which cannot be determined more precisely due to the measurement conditions. Even more peculiar transient behavior has been observed during pyramid excitations, when either the external magnetic field was first stepwise increased and then decreased in a staircase manner at a fixed strain amplitude gamma or the strain amplitude gamma was first stepwise increased and then decreased in a staircase manner at a fixed magnetic field. In particular, the so-called "cross-over effect'' has been identified in both dynamical loading programs. This cross-over effect seems to be promoted by the application of the external magnetic field. The experimental results are discussed in the context of the specific rearrangement of the magnetic filler network under the simultaneous action of the external magnetic field and shear deformation. Striking similarities of the observed phenomena to the structural relaxation processes in glassy materials and to the jamming transition of granular materials are pointed out. The obtained results are important for fundamental understanding of material behavior in magnetic fields as well as for the development of devices on the basis of magnetoactive elastomeric materials.
Magnetoactive elastomers (MAEs) are composite materials comprised of micrometer-sized ferromagnetic particles in a nonmagnetic elastomermatrix. Asingle-particle mechanism ofmagnetostriction in MAEs, assuming the rotation of a soft magnetic, mechanically rigid particle with uniaxial magnetic anisotropy in magnetic fields is identified and considered theoretically within the framework of an alternative model. In this mechanism, the total magnetic anisotropy energy of the filling particles in the matrix is the sum over single particles. Matrix displacements in the vicinity of the particle and the resulting direction of the magnetization vector are calculated. The effect of matrix deformation is pronounced well if the magnetic anisotropy coefficient K is much larger than the shear modulus mu of the elastic matrix. The feasibility of the proposed magnetostriction mechanism in soft magnetoactive elastomers and gels is elucidated. The magnetic-field-induced internal stresses in the matrix lead to effects of magnetodeformation and may increase the elastic moduli of these composite materials.
Magneto-active Polymers (MAP) are closely related to magneto-rheological fluids in that they contain magnetic micro-particles. However, instead of being freely suspended in a carrier fluid they are physically bound within a highly elastic polymer-matrix. The application of an external magnetic field allows changes in the mechanical properties, such as elastic and shear moduli, of the composite material to be achieved. The colossal magneto-rheological effect has been shown to demonstrate changes in Young's modulus by over 1E6 %. Current research concentrates on controllable compliance and the resulting shape memory properties of MAP. This paper deals with recent advancements in this respect, including tubular magneto-active devices with potential applications in medical and other technology.
The onset of turbulent flow around an oscillating sphere in superfluid 4He is known to occur at a critical velocity where is the circulation quantum and is the oscillation frequency. But it is also well known that initially in a first up-sweep of the oscillation amplitude, can be considerably exceeded before the transition occurs, thus leading to a strong hysteresis in the velocity sweeps. The velocity amplitude where the transition finally occurs is related to the density of the remanent vortices in the superfluid. Moreover, at temperatures below ca. 0.5 K and in a small interval of velocity amplitudes between and a velocity that is about 2 % larger, the flow pattern is found to be unstable, switching intermittently between potential flow and turbulence. From time series recorded at constant temperature and driving force, the distribution of the excess velocities is obtained and from that the failure rate. Below 0.1 K we also can determine the distribution of the lifetimes of the phases of potential flow. Finally, the frequency dependence of these results is discussed.
Hysteresis in dynamic modulus, loss factor and normal forces of magnetoactive elastomers (MAEs) comprising various proportions of small (3-5 mu m) and large (50-60 mu m) ferromagnetic particles are experimentally studied using dynamic torsion performed at a fixed oscillation frequency in varying DC magnetic fields. It is shown that hysteresis is a characteristic feature of MAEs observed both under increasing/decreasing magnetic field strength and increasing/decreasing strain amplitude. This hysteresis is attributed to the specific rearrangement of the magnetic filler network under simultaneously applied magnetic field and shear deformation. Rheological properties of the magnetic filler network formed in the magnetic field and, therefore, the rheological properties of MAEs depend strongly on the filler composition and the magnetic field magnitude. Larger magnetic particles and higher magnetic fields provide stronger magnetic networks. Both factors result in the extension of the linear viscoelastic regime to larger strain amplitudes and lead to higher values of shear storage and loss moduli. It is found that the hysteresis width maximises at an intermediate magnetic field where it is attributed to the balance between elastic and magnetic particle interactions. This is apparently where the most significant restructuring of the magnetic network occurs. The hysteresis width decreases with increasing fraction of large particles in the magnetic filler. The loss factor grows significantly when the magnetic network is physically broken by large strains gamma > 1%. A huge (more than one order of magnitude) increase of normal force at maximum magnetic field strengths is observed. It is predicted that any physical quantity depending on the internal structuring of the magnetic filler should demonstrate hysteresis either with a changing magnetic field and constant deformation amplitude or under variable deformation in a constant magnetic field. (C) 2015 Elsevier Ltd. All rights reserved.
By using more powerful LEDs a good thermal management is becoming more and more important. Here usually the thermal interconnects have to be in focus. Therefore a simple and rapid measurement method for the thermal check of the solder joint in a high volume production is necessary. A standard thermal resistance measurement is very time consuming due to the calibration of each device. By using thermal characterized reference devices it is possible to skip the time consuming part. Furthermore, a simple methodology for evaluating the thermal performance of the various metal core PCB (MCPCB) materials and construction is required during development. In this paper we will present how we successfully demonstrated that this new method offers an opportunity to determine the thermal performance of a high power QFN LED on different types of isolated metal substrate (IMS) and with a standard SMU [1] (source measurement units).
Evaluation of highly compliant magneto-active elastomers with colossal magnetorheological response
(2014)
Highly compliant elastomers with a shear storage modulus as low as 25 Pa are prepared using commercially available silicone, plasticizer, and tactile mutator silicone additive. They are used as matrix material for magneto-active elastomers (MAEs) with carbonyl iron contents between 0 and 85 wt %. In the absence of an external magnetic field, the storage modulus of MAEs based on two selected mixtures ranges between ~100 Pa and ~2000 Pa. Addition of a mutator to the matrix mixture results in a long post-cure period depending on the curing temperature and the initial mixture. In the presence of a magnetic field, the presented MAEs exhibit a strong magneto-induced change in storage modulus resulting in a colossal magnetorheological effect of >106 % which is ~30 times higher than previously reported values. The results are of interest in applications using such elastomers as cell substrates with magnetically tunable rigidity.
Experimental study of the magnetic field enhanced Payne effect in magnetorheological elastomers
(2014)
The dynamic modulus and the loss factor of magnetorheological elastomers (MREs) of various compositions and anisotropies are studied by dynamic torsion oscillations performed in the absence and in the presence of an external magnetic field. The emphasis is on the Payne effect, i.e. the dependence of the elastomer magnetorheological characteristics on the strain amplitude and their evolution with cyclically increasing and decreasing strain amplitudes. MREs are based on two silicone matrices differing in storage modulus (soft, G' similar to 10(3) Pa, and hard, G' similar to 10(4) Pa, matrices). For each matrix, the concentration of carbonyl iron particles with diameters of 3-5 mu m was equal to 70 and 82 mass% (22 and 35 vol%, respectively) in the composite material. Samples for each filler content, isotropic and aligned-particles, are investigated. It is found that the Payne effect significantly increases in the presence of an external magnetic field and varies with the cyclical loading which reaches saturation after several cycles. The results are interpreted as the processes of formation-destruction-reformation of the internal filler structure under the simultaneously applied mechanical force and magnetic field. Impacts of matrix elasticity and magnetic interactions on the filler alignment are elucidated.
A low-cost wax-cast molding technique for structuring ultrasoft (Young’s modulus ≤ 40 kPa), agglutinative magnetorheological elastomer (MRE) material is presented. MRE structures ranging from a few millimeters down to the micrometer range with highly reproducible results are possible. Semitransparent MREs are also fabricated and their surfaces modified accordingly. This method opens new possibilities for MREs in biomedical engineering and microfluidic applications. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013
Mechanical cues such as extracellular matrix stiffness and movement have a major impact on cell differentiation and function. To replicate these biological features in vitro, soft substrata with tunable elasticity and the possibility for controlled surface translocation are desirable. Here we report on the use of ultra-soft (Young’s modulus <100 kPa) PDMS-based magnetoactive elastomers (MAE) as suitable cell culture substrata. Soft non-viscous PDMS (<18 kPa) is produced using a modified extended crosslinker. MAEs are generated by embedding magnetic microparticles into a soft PDMS matrix. Both substrata yield an elasticity-dependent (14 vs. 100 kPa) modulation of α-smooth muscle actin expression in primary human fibroblasts. To allow for static or dynamic control of MAE material properties, we devise low magnetic field (≈40 mT) stimulation systems compatible with cell-culture environments. Magnetic field-instigated stiffening (14 to 200 kPa) of soft MAE enhances the spreading of primary human fibroblasts and decreases PAX-7 transcription in human mesenchymal stem cells. Pulsatile MAE movements are generated using oscillating magnetic fields and are well tolerated by adherent human fibroblasts. This MAE system provides spatial and temporal control of substratum material characteristics and permits novel designs when used as dynamic cell culture substrata or cell culture-coated actuator in tissue engineering applications or biomedical devices.
A method of referencing in an extrinsic optical fiber sensor system utilizing temperature dependence of the absorption edge in a semiconductor crystal (semi-insulating iron-doped indium phosphide) is demonstrated. The intensity reference is provided by controlling the temperature of an LED source and transmission measurements with different emission spectra. A transient operation regime is introduced. The entire process is controlled by a microprocessor unit. The performance of the sensor system is investigated and it is shown that the connector losses may be compensated for. Contrary to the published works performed with GaAs crystals it was not observed that the absorption coefficient of the semiconductor follows the law for idealized direct-gap semiconductor but can be described by the so-called Urbach tail. Since the proposed sensor system comprises a single LED source, simple electronics and no optical fiber couplers it is promising for realization of low-cost fiber-optic temperature sensors, e.g. for power transformer monitoring or magnetic resonance imaging applications.
The magnetoelectric response of bi- and symmetric trilayer composite structures to pulsed magnetic fields is experimentally investigated in detail. The structures comprise layers of commercially available piezoelectric (lead zirconate titanate) and magnetostrictive (permendur or nickel) materials. The magnetic-field pulses have the form of a half-wave sine function with duration of 450 µs and amplitudes ranging from 500 Oe to 38 kOe. The time dependence of the resulting voltage is presented and explained by theoretical estimations. Appearance of voltage oscillations with frequencies much larger than the reciprocal pulse length is observed for sufficiently large amplitudes (~1-10 kOe) of the magnetic-field pulse. The origin of these oscillations is the excitation of bending and planar acoustic oscillations in the structures. Dependencies of the magnetoelectric voltage coefficient on the excitation frequency and the applied magnetic field are calculated by digital signal processing and compared with those obtained by the method of harmonic field modulation. The results are of interest for developing magnetoelectric sensors of pulsed magnetic fields as well as for rapid characterization of magnetoelectric composite structures.
A detailed experimental study of the magnetoelectric response of planar bi- and symmetric trilayer composite structures to magnetic field pulses is reported. The structures consist of layers of commercially available piezoelectric (lead zirconate titanate) and magnetostrictive (permendur or nickel) materials. The magnetic-field pulses have the form of a half-wave sine function with duration of 450 μs and amplitude from 500 Oe up to 38 kOe. The measurement method is explained and the measured time dependence of the resulting voltage is presented. The most interesting case, when pulse amplitudes are sufficiently large (~ 1-10 kOe) and various types of acoustic oscillation with frequencies much larger than the reciprocal pulse length are excited in the structures, is considered. The dependencies of the magnetoelectric voltage coefficient on the excitation frequency and the applied magnetic field are calculated. By digital signal processing the results are compared with those obtained by the method of harmonic field modulation (HFM). The findings are of interest for developing magnetoelectric sensors for pulsed magnetic fields as well as for rapid characterization of magnetoelectric composite structures.
Transmission spectra of wet human teeth and dentin slices, together with blood of different flow rates were investigated. The measurements carried out over a wide spectral range, from visible light down to terahertz radiation. The results make it possible to find the optimum light frequency for an all-optical determination of pulpal blood flow and, consequently, for clinically diagnosis of tooth vitality.
The Trilopede
(2012)
The characteristics of both direct and converse magnetoelectric (ME) interactions in bilayer composite structures comprising layers of a commercially available magnetostrictive permendur CoFe alloy and different piezoelectric PZT ceramics are investigated in detail by the method of harmonic field modulation. It is shown that, given the optimum DC magnetic bias field, a high piezomagnetic coefficient in the alloy leads to increased efficiency of ME interactions. The resonant behavior of frequency dependencies of the magnetoelectric voltage is explained by excitation of bending and planar mechanical oscillations in the structures. The results are of interest for developing highly sensitive AC magnetic field sensors and magnetoelectric transducers for energy harvesting from mechanical vibrations.
Transmission spectra of wet human teeth and dentin slices, together with blood of different flow rates were investigated over a wide spectral range, from visible to terahertz (THz) light. The results make it possible to find the optimum light frequency for an all-optical determination of pulpal blood flow and, consequently, for clinically diagnosis of tooth vitality.
PPG signal acquisition and analysis on in vitro tooth model for dental pulp vitality assessmen
(2012)
Smart materials change their properties with external energy supply. Besides the known ferro-fluids and Magneto Rheological Fluid (MRF) also the Electro Active Polymer (EAP) and Magneto Rheological Elastomer (MRE) belong to these intelligent materials. The latest generation of magnetic elastomers represents a new class of composite materials. This consists of small magnetized particles which are sized in the micron or even nanometer range that in turn is bounded in a highly elastic rubber matrix. These materials are very often called MRE. Only recently, it has managed to develop these materials even further, so that very soft composite materials with a young?s modulus up to 10 kPa are possible. These soft polymers could be named magneto-active polymers. The combination of polymers with magnetic materials show novel and often enhanced properties. A precisely controllable young?s modulus and hardness, giant and non-homogeneous deformation behavior and rapid response to the magnetic field opens up new possibilities for various applications. Since MAP represent a very new technology, the behavior of these materials as a function of their composition and external conditions so far are not yet sufficiently understood. Therefore, some fundamental studies are necessary. In this paper, the mechanical surface properties are studied using a micro hardness meter. This work shows the possibility to control mechanical properties at the surface of MAP with new developed magnetic systems.
Transmission spectra of wet human teeth and dentin slices, together with blood of different flow rates were investigated. The measurements carried out over a wide spectral range, from visible light down to terahertz radiation. The results make it possible to find the optimum light frequency for an all-optical determination of pulpal blood flow and, consequently, for clinically diagnosis of tooth vitality.
Optical fiber sensors based on intensity measurement require some form of intensity referencing to avoid errors arising from parasitic losses. Known techniques of referencing such as balanced bridge, divided beam systems or two-wavelength referencing are not suitable for low-cost applications since they use relatively complicated optical components such as multiple LED sources, couplers, filters etc. In this work a novel method of referencing in an extrinsic optical fiber sensor system utilizing temperature dependence of absorption edge in a semiconductor crystal is described. The sensor system comprises a single LED source and no optical fiber junctions. The emission spectrum of an LED depends on its temperature. The reference is provided by controlling the temperature of an LED source and transmission measurements with different emission spectra. The entire process is controlled by a microprocessor unit. Performance of a sensor system is investigated and it is shown that the losses in connectors may be compensated for.
This paper describes a very-high-speed integrated-circuit hardware description language (VHDL)-based analog-node model, an associated driver component for the mixed-signal event-driven (MixED) simulation technique, and some primitive device models applied to radio-frequency integrated circuits. With the presented MixED method, analog circuits are modeled as a composition of controlled sources. Unlike other VHDL-based analog simulation methods, these MixED sources compute not only a real number representing an output voltage but also an output impedance. This allows the outputs of several MiXED sources to be connected in order to drive the same node signal n . The voltage of this record-type signal is automatically computed at its element n . u by resolution functions in compliance with Kirchhoff's current law. The data structure of the node signal n , its self-defined resolution functions, and an associated driver component are presented and discussed to meet different simulation requirements, such as speed, versatility, current accuracy, and adaptive time stepping. Several examples demonstrate how to behaviorally model mixed-signal components with this method with an emphasis on the simulation of a heterodyne receiver. Simulation speeds are compared to VHDL-AMS tools.
Es werden Herstellung und Charakterisierung eines faseroptischen Elementarsensors zur Temperaturmessung vorgestellt. Der Sensorkopf in Form eines Prismas aus Indiumphosphid nutzt die Temperaturabhängigkeit der Absorptionskante des Halbleitermaterials. Aufgrund der Abmessungen des Prismas im Grenzbereich von Feinmechanik zu Mikrosystemtechnik ist ein angepasstes Herstellungsverfahren notwendig.
We study experimentally and theoretically coupling mechanisms between metamaterial elements of the split ring resonator (SRR) type. We show that, depending on the orientation of the elements relative to each other, the coupling may be either of magnetic or electric type or a combination of both. Experimental results on SRRs with resonances around 1.7–1.9 GHz agree quantitatively with results of simulations (CST Microwave Studio). Further simulations provide analysis for a variety of SRRs both in the GHz and in the 20 THz frequency regions. The variety of coupling mechanisms can be employed in designing near field manipulating devices based on propagation of slow waves.
Mechanism of subwavelength imaging with bilayered magnetic metamaterials: Theory and experiment
(2007)
We present a theoretical and experimental study of a bilayered metamaterial structure for subwavelength imaging of magnetic field. The simplest version of such a structure consists of one or two linear arrays of capacitively loaded split pipe resonators. Its subwavelength physics is governed by strongly anisotropic magnetic coupling between individual resonators and by propagation of magnetoinductive waves with wavelength much shorter than the wavelength of the electromagnetic radiation in free space. It is shown that magnetoinductive waves propagating in the lateral direction are undesirable because they spread the image. Good subwavelength imaging is achieved when, due to the strong interlayer coupling, a stop band in the vicinity of the resonant frequency appears in the dispersion characteristics. The imaging properties of the single and double lens are compared and it is shown that the double lens has a superior performance. Excellent agreement is obtained between experimental and theoretical results for the magnetic field in the image plane in the operation frequency range of 30–60 MHz. It is shown that the same mechanism is responsible for image formation using bilayered planar metamaterial structures and a design of such a lens comprising two planar layers with a total of 542 elements is provided. The conclusions are not restricted to the radio frequency region because the elements can be scaled down.
Magnetoinductive waves propagating along a line consisting of two kinds of metamaterial elements are studied. Both elements are made up by the same metallic loop but are loaded by different capacitors resulting in different resonant frequencies. The dispersion characteristics are derived from phase and amplitude measurements for the cases when the line consists of (i) identical elements and (ii) of alternating elements. Both planar (elements in the same plane as the axis of the line) and axial (elements perpendicular to the axis of the line) configurations are investigated. It is shown that in the bi-periodic arrangement of the elements, the dispersion curves have a forward wave in the lower frequency branch and a backward wave in the upper frequency branch independent of the configuration whether it is planar or axial. Comparisons between theoretical and experimental results show good agreement.
Transmission properties of magnetoinductive waves propagating in two coupled one-dimensional metamaterial arrays are studied both experimentally and theoretically for the case when one of the arrays is shifted relative to the other one. Two different kinds of resonant metamaterial elements, split-pipe and spiral resonators, are investigated in the frequency bands centred at 46.2 and 586 MHz, respectively. It is shown that within a certain frequency range close to the resonant frequencies the transmission is strongly dependent on the shift. Theoretical calculations based on the impedance matrix show good agreement with the experimental results
A theoretical and experimental study of magnetic metamaterials with unit cells containing two resonant elements is presented. The properties of these structures, consisting of split rings, are governed by strongly anisotropic magnetic coupling between individual elements. This coupling leads to propagation of slow magnetoinductive waves in the vicinity of the resonant frequency. The wavelength of magnetoinductive waves is much smaller than the free-space wavelength of the electromagnetic radiation. This opens up the possibility of manipulating the near field on a subwavelength scale. We develop a theoretical formulation for coupled chains of metamaterial elements allowing the tailoring of their guiding properties in the near field. In a comprehensive analysis modes of coupled waveguides supporting forward and/or backward waves are identified and the corresponding hybridization mechanisms for dispersion equations of magnetoinductive waves are determined. Analytical predictions are verified both experimentally and numerically on a variety of coupled waveguides. The approach can be employed for the design of near-field manipulating devices.
An experimental study of the properties of magnetoinductive waves in the presence of retardation
(2006)
Magnetoinductive (MI) waves owe their existence to the magnetic coupling between metamaterial elements. First experiments confirming the existence of MI waves were carried out on capacitively loaded loops and Swiss Rolls about three orders of magnitude smaller than the operating wavelengths (5–15 m) so that the radiation effects did not play any significant role. In the present paper MI waves are studied experimentally on various types of split ring resonators of about 1 cm diameter operating in the microwave region between 1 and 2 GHz. Our results prove that retardation has a significant effect upon the propagation of MI waves.
Robot Grippers
(2006)
Since robotic prehension is widely used in all sectors of manufacturing industry, this book fills the need for a comprehensive, up-to-date treatment of the topic. As such, this is the first text to address both developers and users, dealing as it does with the function, design and use of industrial robot grippers. The book includes both traditional methods and many more recent developments such as micro grippers for the optolectronics industry. Written by authors from academia, industry and consulting, it begins by covering the four basic categories of robotic prehension before expanding into sections dealing with endeffector design and control, robotic manipulation and kinematics. Later chapters go on to describe how these various gripping techniques can be used for a common industrial aim, with details of related topics such as: kinematics, part separation, sensors, tool excahnge and compliance. The whole is rounded off with specific examples and case studies. With more than 570 figures, this practical book is all set to become the standard for advanced students, researchers and manufacturing engineers, as well as designers and project managers seeking practical descriptions of robot endeffectors and their applications.
Der Roboter als Hilfsmittel in der Chirurgie // International Mechatronik Forum, Linz, October 2006
(2006)
Resonant frequencies of a combination of split rings: Experimental, analytical and numerical study
(2005)
The resonant frequencies of five different ring resonators are measured with the aid of a network analyser within the frequency range of about 1.5 to 2.8 GHz. The resonant frequencies for those configurations are also determined from numerical calculations using the commercially available MICRO-STRIPES package. The experimental and numerical results are shown to be very close to each other. Analytical results from various authors, available for three of the configurations, are also compared with the experimental results; one of them leads to a large discrepancy, but the other analytical approximations are shown to be not too far off.
The concept and some steps in the development of a new actuator system which enables the haptic perception of mechanically inhomogeneous virtual objects are introduced. The system consists of a two-dimensional planar array of actuator elements containing an electrorheological (ER) fluid. When a user presses his fingers onto the surface of the actuator array, he perceives locally variable resistance forces generated by vertical pistons which slide in the ER fluid through the gaps between electrode pairs. The voltage in each actuator element can be individually controlled by a novel sophisticated switching technology based on optoelectric gallium arsenide elements. The haptic information which is represented at the actuator array can be transferred from a corresponding sensor system based on ultrasonic elastography. The combined sensor-actuator system may serve as a technology platform for various applications in virtual reality, like telemedicine where the information on the consistency of tissue of a real patient is detected by the sensor part and recorded by the actuator part at a remote location.
The use of ultrasound systems for medical examination and diagnosis is nothing new. The extension of ultrasound techniques for real time elastographic analysis purposes represents a more recent development. Nevertheless, as they stand all such techniques rely on the interpretation of two-dimensional visual data displayed on a video screen. In reality however, a medical doctor will make as much use of exploratory touch as he or she does vision, making the simultaneous portrayal of both video and tactile information most desirable [Böse H, Monkman GJ, Freimuth H, Ermert H. Haptisches Sensor-Aktor-System uaf der grundlage der Echtzeitelastographie sowie von elktro- und magnetorheologischen Materialien “HASASEM” [3]. BMBF Antrag 01 IR A14D, Oktober 2000].
This paper discusses the preliminary tests and basic design parameters for single tactels using electrorheological fluids. The final aim is to produce a prototype three-dimensional tactile display comprising electrically switchable micro-machined cells whose mechanical moduli are governed by phase changes experienced by electrorheological fluids. This will be integrated with the latest elastographic ultrasonic sensor systems in order to present the human fingers with controllable surfaces capable of emulating biological tissue, muscle and bone.
Effect of disorder on the conductivity of two-phase strongly inhomogeneous highly filled composites
(2005)
The effect of the “stir” of a structure (small deviations from strict periodicity) on effective conductivity is considered. For determinate and random deviations, concentration and field dependences of the effective conductivity are found. Numerical experiments with determinate deviations are carried out for the cases of linear (with respect to the field) inclusions embedded in both a linear and nonlinear matrix. The numerical results are compared with the effective conductivity calculated analytically.
Resonant frequencies of a split-ring resonator: Analytical solutions and numerical simulations
(2004)
A set of differential equations describing the current andvoltage distribution in a split-ring resonator is derived and solved ana-lytically. The resonant frequencies may be obtained from the solution of a characteristic equation. An approximate solution for the lowest reso-nant frequency agrees with that obtained by heuristic arguments. The analytical results are supported by numerical simulations
An equivalent circuit, consisting of bulk and distributed elements, is derived for describing the properties of a potential metamaterial element capable of providing negative effective permeability. It is the singly split double ring (SSDR), a special case of the split ring resonator (J. B. Pendry et al., IEEE Trans. Microwave Theory Tech. 47, 2075 (1999)), obtained when the gap capacitance in the inner ring is infinitely large. The variables are the inter-ring voltage and the currents flowing in the inner and outer rings. The excitation is assumed in the form of a spatially constant temporally varying magnetic field. The functions, showing the angular variation of the variables, are found by solving a set of differential equations with boundary conditions imposed at the position of the split. It is shown from the analytical solution that the SSDR can have resonant frequencies in the full spectrum from very low to very high frequencies. It is pointed out in particular that whenever the mean diameter of the ring is equal to an odd multiple of the half wavelength it is always possible to find a set of parameters which will give rise to resonance. As examples the resonant frequencies are determined for eight sets of parameters. Results are also derived by replacing the distributed circuit with a number of discrete circuits. It is finally shown that the results obtained from the equivalent circuit model are in excellent agreement with those derived from the MICRO-STRIPES numerical package which solves Maxwell’s equations in the time domain.
Mechanical properties of biological tissue represent important diagnostic information and are of histological and pathological relevance. Malignant tumors are significantly stiffer and more immobile than surrounding healthy tissue. Hard calcifications in vessels occur due to arteriosclerosis. The problem is, that such information is usually not available or can only be obtained by manual palpation, which is subjective and limited in sensitivity. It requires intuitive assessment and does not allow quantitative documentation. Unfortunately, none of the established medical imaging equipment such as magnetic resonance imaging (MRI) or X-ray computed tomography (CT) can provide direct measure of tissue elasticity. On the one hand a suitable sensor is required for quantitative measurement of mechanical tissue properties. On the other hand there is also some need for a realistic haptic display of such tissue properties. Suitable actuator arrays with high spatial resolution acting in real time are required. A haptic sensor actuator system is presented in this paper including a sensitive sensor part and an actuator array for different applications. The mechanical consistency of an object is to be locally specified using a sensor system and represented perceptibly in a remote position on an actuator system for the user. The sensor system uses ultrasound (US) elastography, whereas the actuator array is based on electrorheological (ER) fluids.