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Abstract

Als intelligente Materialien oder Smart Materials werden Materialien bezeichnet, die aktorische Eigenschaften vorweisen und gleichzeitig als (ihr eigener) Sensor betrieben werden können. Dazu gehören unter anderem piezoelektrische Aktoren, thermische und magnetische Formgedächtnislegierungen (FGL) sowie dielektrische Elastomere (DE), welche eine wichtige Untergruppe der elektroaktiven Polymere (EAP) darstellen. Die speziellen und flexiblen Formfaktoren dieser Aktoren ermöglichen die Umsetzung von bisher nicht möglich gewesenen Antriebskonzepten. Gleichzeitig können ihre sensorischen Eigenschaften zur Generierung zusätzlicher Informationen genutzt werden („Self- Sensing“).
Published on GIT-Labor – Portal für Anwender in Wissenschaft und Industrie (
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24.05.2018
Aktor-Sensor-Systeme
Dielektrische Elastomere für intelligente Materialien
Als intelligente Materialien oder Smart Materials werden Materialien
bezeichnet, die aktorische Eigenschaften vorweisen und gleichzeitig als (ihr
eigener) Sensor betrieben werden können. Dazu gehören unter anderem
piezoelektrische Aktoren, thermische und magnetische
Formgedächtnislegierungen (FGL) sowie dielektrische Elastomere (DE),
welche eine wichtige Untergruppe der elektroaktiven Polymere (EAP)
darstellen. Die speziellen und flexiblen Formfaktoren dieser Aktoren
ermöglichen die Umsetzung von bisher nicht möglich gewesenen
Antriebskonzepten. Gleichzeitig können ihre sensorischen Eigenschaften
zur Generierung zusätzlicher Informationen genutzt werden („Self-
Sensing“).
DE bestehen im einfachsten Fall aus einer dünnen (typischerweise 20 – 100µm)
Elastomerfolie, die beidseitig mit einer hauchdünnen (< 5µm), leitfähigen und
dehnbaren Elektrode beschichtet ist (Abb.1a). Somit bilden sie einen flexiblen und
dehnfähigen Kondensator. Als Materialien für das Elastomer werden je nach
Anwendung Silikone, Acrylate, Polyurethane oder Naturkautschuk verwendet. Die
Elektroden bestehen i.d.R. aus einer Mischung von Industrieruß und dem
verwendeten Basiselastomer. Diese günstigen Materialien bieten, ebenso wie der
Verzicht auf Seltene Erden, bereits einen großen Kosten- und Ressourcenvorteil.
Des Weiteren erlaubt der einfache Aufbau, kombiniert mit industriellen
Massenproduktionsverfahren (wie z. B. Siebdruck) die Herstellung kostengünstiger
Sensoren, Aktoren und Generatoren für die Verwendung in kleinen, leichten und
energieeffizienten Systemen [1, 2].
Im Sensorbetrieb werden DE durch eine äußere Kraft verformt bzw. gedehnt. Dies
sorgt für eine Kapazitätsänderung, welche als Messsignal dient. Solche Sensoren
können problemlos Dehnungen von über 100% mit einer Auflösung kleiner 0,1%
(Sensitivität 0,026%) erreichen, wie es Beispiele der Firma Parker für industrielle
Anwendungen [3] oder der Firma Stretchsense für die Erkennung von
Bewegungsabläufen zeigen [4]. Des Weiteren eignen sich solche Sensoren auch zur
Druckmessung [5]. Bei der Nutzung von DE als Generator müssen diese im
gedehnten Zustand mit einer Hochspannung geladen werden.
Durch eine anschließende Relaxation des DE wird die in der Dehnung gespeicherte
mechanische Energie in elektrische umgewandelt [6-8]. Die Energieerzeugung aus
menschlicher Bewegung, Wind- und Industrievibrationen, aber auch
großtechnische Meereswellenenergiegewinnung sind hierbei Gegenstand der
Forschung. Energiegewinnungskonzepte auf der Basis von DE können aber
genauso gut auf kleinskalige Systeme übertragen werden, um somit die Versorgung
energieautarker Sensorsysteme sicherzustellen. Besonders interessant ist die
Anwendung von DE für leichte, schnelle (kHz-Bereich) und energieeffiziente
Aktoren mit sehr hohen Dehnungsraten, wie der Vergleich mit anderen
Aktorprinzipien in der Tabelle zeigt.
Funktionsprinzip
Legt man ein elektrisches Feld von typischerweise 50-80V/µm an den DE, führt die
elektrostatische Anziehung der entgegengesetzten Ladungen auf den beiden
Elektroden zu einer Dickenkompression des Elastomers, sowie einer Ausdehnung in
der Fläche (Abb.1b). Hierbei bestimmt die Aktorbauform, welche
Geometrieänderung maßgeblich für die Bewegungserzeugung genutzt wird. Daher
unterscheidet man Stapel- und Membranaktoren. Stapelaktoren bestehen
typischerweise aus mehreren Hundert DE-Lagen (ähnlich einem Piezoaktor) und
nutzen die Dickenänderung zur Erzeugung hoher Kräfte im zweistelligen
Newtonbereich bei moderaten Dehnungen von ca. 10% der Ausgangslänge [13-15].
Membranaktoren hingegen nutzen die Flächenausdehnung zur Aktorik. Dies
erlaubt das generieren von Dehnungen >100%, wobei die Kräften eher im
niedrigen einstelligen Newtonbereich liegen. Bei beiden Bauformen können sowohl
Hub als auch Kraft über die Aktorgeometrie skaliert werden [16]. Zudem lassen
sich Membranaktoren auch stapeln, um größere Kräfte zu erzeugen. Mit diesem
Mischkonzept können Kräfte von über 100N erzeugt werden (Abb.2) [17].
Von besonderer Bedeutung für potentielle Industrie 4.0-Anwendungen ist die
kombinierte Ausnutzung von Aktor und Sensoreffekt. Hierbei wird während der
Aktuierung permanent auch die Kapazität des DEA ausgelesen. Somit zeigen DEA
einen Self-Sensing-Effekt, womit sich intelligente und regelbare Aktorsysteme
aufbauen lassen, die keine zusätzliche Sensorik benötigen. Hierbei kann eine
Positionsgenauigkeit <5% für dynamische [18] und <1% für quasistatische
Bewegungen erreicht werden [19]. Des Weiteren eignet sich eine zusätzliche
Messung des Elektrodenwiderstands, um Aussagen über die Alterung des DEA zu
treffen [20]. Die Themen Alterung und Lebensdauer sind auch Gegenstand
aktueller Forschung [21]. Erste Untersuchungen haben bereits gezeigt, dass DEA
mehrere Millionen Betriebszyklen unbeschadet überstehen können [22].
Einsatzgebiete
Die Zahl der kommerziell verfügbaren Produkte ist z.Z. noch relativ gering und
beschränkt sich auf oben genannte Sensoren sowie Aktoranwendungen im Bereich
adaptiver Optik von der Firma Optotune [23]. Dies ist unter anderem dem noch
geringen Alter (erste Forschungsarbeiten in den 1990-iger Jahren) der Technologie
geschuldet. Jedoch hat die industrienahe Forschung in den letzten Jahren einige
vielversprechende Anwendungsfelder hervorgebracht. Besonders geeignet sind
DEAs für die Verwendung als Ventiltrieb. Dies zeigt der Vergleich eines
konventionellen Ventils mit Elektromagnet und einer Version, in der der
Elektromagnet durch einen DEA ersetzt ist (Abb.3) [24]. Beide Ventile sind in der
Lage, 3bar und einen maximalen Volumenstrom von 9L/min mit der gleichen
Dynamik zu schalten. Allerdings ergibt sich in der Anwendung (Ventil 1,5Sekunden
offen, danach 0,5Sekunden geschlossen halten) eine Energieersparnis von über
99%, da der DE zum Öffnen nur einmal geladen werden muss und diese Position
anschließend stromlos hält. Durch diesen geringen Energieverbrauch findet auch
keine Erwärmung des Ventils statt, was bei der Handhabung sensibler Medien eine
wichtige Rolle spielen kann. Weiterhin lässt sich der DE im Gegensatz zum
Elektromagnetventil auch proportional ansteuern, um eine Durchflussregelung zu
realisieren. Weitere Studien müssen zeigen, ob die Self-Sensing-Funktionalität auch
Rückschlüsse auf den anliegenden Druck zulassen.
Eine weitere Anwendung sind Vibrationsförderer (Abb.4). Hier liegen die Vorteile
von DEA vor allem in ihrer hohen Dynamik. Diese erlaubt es, das Bewegungsprofil
der Förderrinne ohne Rüstzeit an ein neues Fördergut anzupassen. Der Self-
Sensing Effekt erlaubt zusätzlich Rückschlüsse auf die Fördermenge oder Art des
Förderguts. Die hohe Dynamik von DEA wird auch in weiteren Anwendungsfeldern
genutzt. Durch ihre hohe Gestaltungsfreiheit erlauben DE den Aufbau
hochkompakter und an die Anwendung angepasster Aktoren für haptisches
Feedback, bspw. in Knöpfen und Touchscreens oder als Lautsprecher. Zudem
lassen sich DEA aufgrund ihrer mechanischen Flexibilität auch in Kleidungsstücke
wie z.B. Handschuhe integrieren.
Neben der kombinierten Nutzung von Aktorik und Sensorik ist die intrinsische
Nachgiebigkeit und Elastizität von DEA eine attraktive Eigenschaft in Hinblick auf
Mensch-Roboter-Kollaboration. In Kombination mit den geringen Aktormassen
können so sehr sichere Robotersysteme entstehen, die im Gegensatz zu steif und
starr wirkenden Systemen einen natürlich weichen Eindruck vermitteln.
Autoren
Steffen Hau1, Paul Motzki2, Stefan Seelecke1
Zugehörigkeiten
1Lehrstuhl für intelligente Materialsysteme, Universität des Saarlandes,
Saarbrücken, Deutschland
2Zentrum für Mechatronik und Automatisierungstechnik gGmbH, Saarbrücken,
Deutschland
Kontakt
Steffen Hau
Arbeitsgruppe Dielektrische Elastomere
Universität des Saarlandes
Saarbrücken, Deutschland
steffen.hau@imsl.uni-saarland.de
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Thesis
Full-text available
Dielectric elastomer actuators (DEAs) feature high energy efficiency, light weight, design flexibility and the use of low cost materials and processes. This holds particularly true for membrane actuators, which, in addition to the dielectric elastomer comprise a separate biasing system. The particular design of the biasing system may tramatically improve the DEA performance, but at the same time, it adds complexity to such a design process. Therefore, in this work, a systematic design approach to adapt DEA systems to specific applications is developed. It allows calculation of all relevant design parameters and incorporates experimentally validated scaling laws to account for actuator geometry effects. Finally, the capability of the design process is illustrated at two examples. In the first one, the force output of circular membrane DEAs, which is typically in the hundreds of millinewton range, is increased by more than two orders of magnitude. For the first time, record-high forces of 100 Newton are generated, while an innovative overall system design maintains compactness. The second system is designed for high reversible actuation strains in the range of >50%. The use of silicone as elastomer additionally results in high-speed actuation. DEA systems with such outstanding performance prove that they are capable of competing with existing technologies such as solenoids, while adding additional functionality and, in the future, smartness through “self-sensing” properties.
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By applying an electric field to a transducer based on dielectric electroactive polymers (DEAP) a relatively high amount of deformation with considerable force generation is achieved. Due to their unique features DEAP-transducers are a promising alternative for conventional actuator systems based on the electromagnetic principle. To maximize the force or absolute deformation of a DEAP-based actuator multilayer technologies are favorable. Although these actuators recently gained a lot of interest, the development of automated manufacturing processes for such transducers are still at a very early stage. Therefore, the authors present the conceptual design and realization of a novel automated process based on pre-fabricated elastomer material for manufacturing DEAP-based multilayer stack-actuators with homogeneous and reproducible properties. For this purpose, the specific design and topology of the conceptualized multilayer stack-actuator from a single layer actuator film towards the encapsulation of the stacked multilayer actuator is explained in a first step. Due to its smart design, advantageous features like safety fuses can be integrated in these multilayer actuators. Furthermore, for its design and optimal integration in various applications a multiphysics FE model is proposed. Afterwards, the manufacturing process consisting of several sub-processes is presented in detail. The quality of the developed process and the proposed FE model is demonstrated by an experimental validation of several manufactured multilayer DEAP stack-actuators made from polyurethane and silicone. Finally, the obtained results are concluded and an outlook concerning an improved actuator characteristic based on a material optimization is given.
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This paper reports the design, fabrication, and testing of a soft dielectric elastomer power generator with a volume of less than 1 cm3. The generator is well suited to harvest energy from ambient and from human body motion as it can harvest from low frequency (sub-Hz) motions, and is compact and lightweight. Dielectric elastomers are highly stretchable variable capacitors. Electrical energy is produced when the deformation of a stretched, charged dielectric elastomer is relaxed; like-charges are compressed together and opposite-charges are pushed apart, resulting in an increased voltage. This technology provides an opportunity to produce soft, high energy density generators with unparalleled robustness. Two major issues block this goal: current configurations require rigid frames that maintain the dielectric elastomer in a prestretched state, and high energy densities have come at the expense of short lifetime. This paper presents a self-supporting stacked generator configuration which does not require rigid frames. The generator consists of 48 generator films stacked on top of each other, resulting in a structure that fits within an 11 mm diameter footprint while containing enough active material to produce useful power. To ensure sustainable power production, we also present a mathematical model for designing the electronic control of the generator which optimizes energy production while limiting the electrical stress on the generator below failure limits. When cyclically compressed at 1.6 Hz, our generator produced 1.8 mW of power, which is sufficient for many low-power wireless sensor nodes. This performance compares favorably with similarly scaled electromagnetic, piezoelectric, and electrostatic generators. The generator’s small form factor and ability to harvest useful energy from low frequency motions such as tree swaying or shoe impact provides an opportunity to deliver power to remote wireless sensor nodes or to distributed points in the human body without the need for costly periodic battery replacement.
Conference Paper
Sensing motion of the human body is a difficult task. From an engineers’ perspective people are soft highly mobile objects that move in and out of complex environments. As well as the technical challenge of sensing, concepts such as comfort, social intrusion, usability, and aesthetics are paramount in determining whether someone will adopt a sensing solution or not. At the same time the demands for human body motion sensing are growing fast. Athletes want feedback on posture and technique, consumers need new ways to interact with augmented reality devices, and healthcare providers wish to track recovery of a patient. Dielectric elastomer stretch sensors are ideal for bridging this gap. They are soft, flexible, and precise. They are low power, lightweight, and can be easily mounted on the body or embedded into clothing. From a commercialisation point of view stretch sensing is easier than actuation or generation - such sensors can be low voltage and integrated with conventional microelectronics. This paper takes a birds-eye view of the use of these sensors to measure human body motion. A holistic description of sensor operation and guidelines for sensor design will be presented to help technologists and developers in the space.