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EAP Technology @ Empa

Main research activities

In the last decade the interest for “smart materials”, which respond to external stimuli by changing their shape or size, has increased. The operating principle of electronic EAP is based on Maxwell stresses generated by electrostatic fields. Among the electronic EAPs especially the dielectric elastomers are functional materials that have promising potential as muscle-like actuators due to their inherent compliancy and good overall performance. The combination of large active deformations, high energy densities and fast response is unique to dielectric elastomers. Actuator systems based on dielectric elastomer represent the main research and development activities of the EAP group at the Empa. This includes the synthesis of new dielectric materials, evaluation of fabrication technologies, modelling of the material-system (actuators) and developing new EAP actuator designs.

The basic unit of dielectric elastomer actuators consists of a dielectric elastomer film (e.g. silicone or acrylic elastomer) sandwiched between two compliant electrodes. In this arrangement, the polymer acts as a dielectric in a compliant capacitor.

 

Fig.1:    Basic unit of dielectric elastomer actuators: when applying a voltage the elastomeric film is compressed in thickness and expands thus in area.

When an electrical voltage is applied between the electrodes, an electrostatic field occurs and the electrostatic forces from the charges on the electrodes squeeze uniformly the (incompressible) elastomer film. As a result, the polymer material is enlarged elastically in the plane (Maxwell stress). As soon as the voltage is switched off and the electrodes are short-circuited the capacitor contracts back to its original shape. The observed response of the polymer film is caused primarily by the interaction between the electrostatic charges on the electrodes. Based on the principle of operation of soft DE EAP, mainly two directions to perform work against external loads are possible:

·         Work in planar directions (expanding actuator): Under electrical activation of a DE basic unit the film expands in planar directions and can thus work against external pressure loads in both planar directions.

·         Work in thickness direction (contractive actuator): Under electrical activation the electrodes squeeze the DE film in thickness direction. Thus, the actuator can work against external tensile loads acting in thickness direction.

Both working principles are exploited at the Empa and appropriate actuators have been built in relation to different projects.

 


Materials

Silicone and acrylic based polymers are most widely used as dielectric elastomers in actuator configurations, due to their excellent elastic and electroactive properties.

The acrylic film

While the acrylic based elastomer VHB 4910 (3M) is commercially available and exhibits large electromechanical strains, producing high pressures and extraordinarily high specific elastic energy densities, it is nowadays the most used material to produce muscle-like actuators. The outstanding performance of the VHB 4910 has been obtained when the acrylic films are highly pre-strained. New approach for stress free DE film has been made by the interpenetrating network (IPN) process of pre-strained acrylic film.

The silicone film

Silicone based dielectric elastomer, as the second material-category, reveal fast responses and broad temperature stability and can easily be tailored to the specific application when fabricated. Typically they are fabricated by conventional polymer fabrication techniques such as spin coating or casting of thin films. Empa has developed a novel class of silicones with high dielectric constant as dielectric actuator materials. We studied the influence of different hardener types as well as different fillers and degrees of cross linking on the mechanical and electromechanical properties of the resulting silicone films.

Models

High performance actuators can be achieved by computational optimizations of the actuator designs. Therefore, Empa pursues research in the field of modelling for dielectric elastomer actuators. This includes the mechanical behaviour of the elastomeric film, the electromechanical coupling, numerical models and experimental characterization. In particular the mechanical behavior of the widely used VHB 4910 film is quite complex to describe due to its viscoelasticity (time dependency), anisotropy (due to pre-strain) and large deformations reached under activation (hyperelasticity). Most of the recent works and well known models describe large strains and neglect viscoelasticity. The material parameters of all cited models are derived from uniaxial tensile tests and are not sufficient for the characterization of the three-dimensional material behaviour. Therefore, most of the models are successful to describe the uniaxial behaviour but all models fail to make reasonable predictions for the actuator behaviour. To fill this gap a PhD thesis with the topic “Modeling dielectric elastomer actuators” has been accomplished at Empa Dübendorf and ETH Zürich.


Devices

The actuator technology @ Empa

Up to now a large number and many different kinds of demonstrators of dielectric elastomer actuators exist, but no product using this type of actuator is commercially available on the market yet. Examples of such applications include mobile mini and micro robots, micro air vehicles, disk drives, flat panel loudspeakers, prosthetic devices and electro acoustic transducers and extensive adaptive structures. Different actuator designs have been carried out for experimental purposes, as they are the extender (planar), bimorph, tube, diaphragm, spider and bow-tie (single-layer) actuator.

The main development activities @ Empa are focused on planar (shell-like), linear expanding rolled or linear contractive stacked actuators.

The expanding rolled actuator

Any EAP actuator consisting of only one thin film layer usually cannot reach by far the required activation force. For the up-scaling of the actuators’ forces many dielectric films must be switched in parallel (stacked). A simple way to reach such stacking of films is wrapping the DE film around a supporting core. This very promising novel actuator configuration is denoted as spring roll actuator (Fig 2). Thereby, a biaxially pre-strained and double-side coated dielectric film is wrapped around a compressed elastic coil. As soon as this cylindrical element is activated it elongates axially and contracts back when deactivated. The presently reported elongation potential of spring roll actuators is in the range of 34%, and forces of up to 15N were achieved. The actuator elongates axially when voltage is applied and contracts back when deactivated.


 


Fig.2:     Rolled spring actuators in different sizes (left), rolled core- and stress-free actuator (right).

When stress free dielectric film is wrapped up a core free actuator design can be achieved which shows good long term reliability and a very slim shape. Many design studies have shown that the rolled polymer film configuration represents the best design for the application as antagonist actuator.

The contractive tension force actuator

The contractive actuator in stack configuration made of acrylic film or silicone membrane represents the most advanced actuator design. This actuator consists of a large number of stacked thin DE films (ca. 50µm) pieces which are coated with compliant electrodes. In this configuration the dielectric film (sandwiched between two electrodes) acts as a “spacer” and does not take any tensile stress in planar direction, which is an important beneficial issue concerning strength and fatigue behaviour of the film. The electrostatic field induced actuation force and displacement is normal to the plane of the electrode and dielectric film. The as obtained contraction of the actuator can be used to drive the device under a specific external service load. The design of the stack actuator is based on a series of capacitors electrically connected in parallel with alternating polarities of each layer. Therefore the actuation voltage has to be applied on two separated compliant electrodes addressing the associated conductive layers.

 


 


Fig.3:     Pile-up configuration of planar dielectric film coated with compliant conductive electrodes

The piled up multilayer actuator represents the appropriate design when contraction of the actuator and tensile force exhibition at actuation is required. Adding a large number of equal elastomer and electrode layers an actuator in the macro scale can be produced. The mechanical property of the actuator can be adjusted by the number of applied layers (following absolute actuation length) as well as by the size of the planar area of the DE film (resulting actuation force). The challenge in terms of processing and handling lies in the practical implementation of the dielectric material and the exploitation of its theoretical actuation potential (up to 33% contraction and 0.4N/mm2 actuation pressure). With regard to the actuator configuration efficient manufacturing process of the material system at high precision becomes to a key importance. Furthermore special attention has to be paid to the design of the load introduction parts at both ends of the actuator which represents the interface to the stiff prosthesis structure (determination of the film shear stress deformation by modelling the hyper-elastic DE film).

Video showing the stack actuator in three different modes:

Mode 1

Mode 2

Mode 3

The active shell

Active shell structures with large out-of-plane deformation potential may be used to generate an interaction between the structural shape and the environment. Exemplarily, such shell-like actuators may be utilized for the propulsion of vehicles through air or water.

Many active shell devices were built @ Empa in order to explore the potential of the DE actuator technology for the design of shell-like actuators with the ability to perform complex out-of-plane deflections (one of them is shown in fig.4). These experiments showed that the so-called agonist-antagonist configuration, where pre-strained DE films are attached from both sides to a hinged backbone structure, holds good performance in terms of active out-of-plane deflections and forces.

The shell-like actuators are designed in the macro-scale with continuous surfaces, which is lightweight and can actively exhibit large, quasi- or fully continuous out-of-plane deflections. In addition, the shell has to be capable to withstand external loads acting to its surface. Regarding its structure, highly integrated solutions with low mechanical complexity, which consist of commercially available components, are preferred.

Fig.4:     Experimental implementation of a biaxial bending actuator based on the conventional agonist-antagonist configuration.

In order to adapt the conventional agonist-antagonist configuration to a shell-like actuator, which can exhibit biaxial bending deformations, an appropriate support structure is required. The support structure has to preserve the biaxial pre-strain in the DE film, while allowing for a uniaxial or biaxial bending deformation.

Video showing a bending shell actuator

Video of a 7 segment shell actuator mimicking a swimming mode

Optical devices

Optotune develops, manufactures and markets the next generation of adaptive optical elements.

Their patented technology based on electroactive polymers enables the implementation of a revolutionary phase shifter and a lens with tunable focal length. The working principle of the lens is similar to that of the human eye: Instead of moving lenses back and forth, Optotune bends them by applying a voltage. This additional degree of freedom simplifies the design of focus and zoom systems in many applications, outperforming existing solutions in terms of size, cost, power consumption and robustness.

 

Left: Shaping one side of the lens changes its focal length. Right: By shaping both sides of Optotune’s lens, an adjustable and very compact zoom with one single lens can be built.

For more information, please click here:

Flyer Optotune

Presentation Optotune


Facilities

The main research facility at the Empa consists of a 2700 sq ft EAP research laboratory. Beside of the complete equipment for actuator and EAP material characterizing a fully automated and worldwide unique film pre-stretching machine has been developed and established in the lab (fig.4, left). Large pre-stretched films of the max. size of 0.5x1m can be produced within a few seconds. For the manufacturing of the pile-up contractive actuator a fully automated “stacker” machine was developed and set up at the Empa which will be in operation in the summer 2008 (fig.5, right). Only stress free dielectric elastomer material can be applied for this type of actuator which is represented by the relaxed IPN (Inter-Penetrating Network) acrylic film or silicone membrane. For this reason the infrastructure was upgraded by a large vacuum oven for the curing process (fig.4, right).

The coating process takes place in two different fume hoods provided with a spray coating setup.

Fig. 4 Pre-stretching machine (left), vacuum oven with integrated heat radiation plates (right)

The EAP testing facilities consists of 3 different test cells fully equipped with controllable loading and digital measuring equippement connected to 8 different high voltage supplies:

•        Test cell for large area shell actuator for 2 dimensional characterizing (fig.5, left).

•        Test rig for linear actuator

•        Test station for basic materials

Fig.5   biaxial characterisation rig (left), machine for automated stack actuator fabrication (right)

With all test beds the electro-mechanical characteristic of the EAP actuator such as deformation resp. displacement (none contacting methods: laser beam, video extensometer etc.), forces and electric voltage / current can be digitally measured and evaluated under different conditions.

Additional fabrication facilities (Thin film production and thin coating treatment as plasma coating, spin coating, vapour deposition etc.) and equipment for polymer synthesis (lab for functional polymers) as well as further many measurement-technology testing and material analysis facilities are available in neighbouring labs of the Empa (www.empa.ch).


Selected Internal Research Projects (ongoing and past)

See also the following posters:

Electroactive Polymer (EAP) Actuators

Modeling and Simulation of Dielectric Elastomer Actuators

Spring Roll Dielectric Elastomer Actuators for a Portable Force Feedback Device

Development of a Shell-like Electroactive Polymer (EAP) Actuator

Fish-like propulsion of an airship based on electro-active polymers

Fish-like propulsion of an airship based on electro-active polymers II

Arm-Wrestling Robot:

The first arm wrestling match between a human arm and a robotic arm driven by electroactive polymers (EAP) was held at the EAPAD conference in 2005. The primary objective was to demonstrate the potential of the EAP actuator technology for applications in the field of robotics and bioengineering. The Swiss Federal Laboratories for Materials Testing and Research (Empa) was one of the three organizations participating in this competition. The robot presented by Empa was driven by a system of rolled dielectric elastomer (DE) actuators.

Fig.5a    The structure of the arm wrestling robot is made of carbon fiber composite and contains the bundle of rolled EAP actuators.

Since all actuators are intended to be placed inside the robot body, the available space would be about the size of the upper torso of a human. Despite this limitation, the actuators have to be arranged in such that the needed wrestling force and motion can be produced. This limiting factor determines essentially the design and the arrangement of the driving actuators.

By filling the robot box with DE spring roll actuators, the best utilization of the available space is achieved for “slim” actuators with outer radius of approximately 12 mm. As a result, up to 256 DE spring roll actuators are arranged in two agonist-antagonist groups. Around each actuator, a core of 2 m of pre-stretched acrylic film is wrapped, which results in 35 film wrappings. For practical reasons, both the agonist and the antagonist actuator groups are subdivided into two actuator banks each equipped with up to 64 actuators.

Fig. 6 Working principle of the actuator banks and the robot arm.

 

In general, a human-like agonist-antagonist operating principle enables the reversible rotational movement of the robot arm. When assembling the robot, the pre-stretch force in the two actuator groups Bloc I and Bloc II are in equilibrium. In the neutral position (position 0), all actuators are deactivated and the robot arm is rotated by 350 from horizontal. By activation of the actuator Bloc I, the robot arm rotates to the upright starting position (position 1) and is thus ready for the wrestling action. During the wrestling match, Bloc I is deactivated and Bloc II activated simultaneously. Thus, the robot exhibits its maximal wrestling force Fwrestling at position 1. The resulting motion depends on the reaction force of the human opponent. If the opponent produces less than 200 N, the robot arm performs the intended motion from the starting position to winning position 2.

Airship

The worldwide first EAP propelled airship was made at Empa in collaboration with aeroix GmbH and the Technical University of Berlin. This lighter-than-air vehicle with 8 m in length consists of a slightly pressurized Helium filled body of a biologically inspired form with Dielectric Elastomer (DE) actuators acting as muscles and deforming the body and tail fin in a fish-like manner.

Fig.7      Model airship with EAP actuators in black    

The actuators on the airship work – like biological muscles - in an agonist-antagonist configuration. While the actuators on one side of the airship are activated, the corresponding actuator on the other side contracts. Thus the body and tail fin are excited in an undulating movement which propels the airship like a fish through the air. The EAPs can be actuated with varying frequency, activation voltage and a phase shift between the body and the tail fin movement. In fluid-dynamical similarity to the rainbow trout, the appropriate motion pattern (deflections, frequency and phase shift) were defined and verified by wind tunnel tests. The expected travelling velocity was calculated. The “skeleton” and passive parts of the airship consist of an ultra-light carbon-sandwich structure and a model-airship hull material developed by aeroix GmbH in Berlin. The electrical supply and control system was developed at Empa and everything was optimized for minimum of weight. The flight of this fish-like airship can be controlled with a joy stick connected to a ground-based portable computer. The flight control data are processed by a LabView program and transmitted by WLAN to the receiver system in the gondola on the airship. This version of the fully EAP propelled airship had his maiden flight on 16th of July 2009 in Duebendorf Switzerland. For the first time, actuators of this size could be manufactured, characterized and employed. The functionality of the fish-like propulsion in air could be demonstrated.

Video of the demonstration of the first Blimp purely propelled by EAPs:

Full version

Short version

Contact:

Dr. Gabor Kovacs     gabor.kovacs@empa.ch

Deputy head of laboratory / Head of EAP group

Silvain Michel           silvain.michel@empa.ch

Senior Scientist

Christa Jordi            christa.jordi@empa.ch

PhD Student

Publications

Papers in scientific journals

  1. G. Kovacs, L. Düring, S. Michel, G. Terrasi, 2009, stacked dielectric elastomer actuator for tensile force transmission, Sensors and Actuators: A. Physical 155 (2009): 299-307.
  2. S. Michel, Xuequn. Q. Zhang, M. Wissler, Ch. Loewe, G. Kovacs, A comparison between a silicone and acrylic elastomers as dielectric materials in electroactive polymer actuators, Polymer International, 2009, in press
  3. Lochmatter, P. and G. Kovacs (2008). "Design and characterization of an actively defomable shell structure composed of interlinked active hinge segments driven by soft dielectric EAPs." Sensors and Actuators A: Physical A141(2): 588-597.
  4. Lochmatter, P. and G. Kovacs (2008). "Design and characterization of an active hinge segment based on soft dielectric EAPs." Sensors and Actuators A: Physical A141(2): 577-587.
  5. Lochmatter, P., G. Kovacs, et al. (2007). "Design and characterization of shell-like actuators based on soft dielectric electroactive polymers." Smart Materials and Structures 16(2007), No 4 1265-1276
  6. Lochmatter, P., G. Kovacs, et al. (2007). "Characterization of dielectric elastomer actuators based on a hyperelastic film model." Sensors and Actuators A: Physical 135(2007): 748 - 757.
  7. Lochmatter, P., G. Kovacs, et al. (2007). "Characterization of dielectric elastomer actuators based on a visco-hyperelastic film model." Smart Materials and Structures 16: 477-486.
  8. Kovacs G, Lochmatter P, Wissler M. An arm wrestling robot driven by dielectric elastomer actuators. Smart Materials & Structures. 2007 Apr;16(2):S306-S17.
  9. X. Zhang, Ch. Löwe, M. Wissler, B. Jähne, G. Kovacs, Dielectric Elastomers in Actuator Technology, Advanced Engineering Materials, 2005 May;7(5):361-7
  10. M. Wissler, E. Mazza, Modeling of a pre-strained circular actuator made of dielectric elastomer actuators, Sensors and Actuators A, vol. 120, pp. 184 – 192, 2005
  11. M. Wissler, E. Mazza, Modeling and simulation of dielectric elastomer actuators, Smart Materials and Structures, vol. 14, pp. 1396 – 1402, 2005
  12. M. Wissler, E. Mazza, Mechanical behaviour of an acrylic elastomer used in dielectric elastomer actuators, Sensors and Actuators A, vol. 134, pp. 494-504, 2007
  13. M. Wissler, E. Mazza, Electromechanical coupling in dielectric elastomer actuators, Sensors and Actuators
  14. Wissler, M. (2006). "Graphite and carbon powders for electrochemical applications." Journal of Power Sources.
  15. Zhang, X. Q., M. Wissler, et al. (2005). "A comparison between silicone and acrylic actuators." Sensors and Actuators A: Physical.

 

Book chapters

Dielectric Elastomers as Electromechanical Transducers: Fundamentals, Materials, Devices, Models and Applications of an Emerging Electroactive Polymer Technology, Elsevier science and technology book, in press:

  1. Gabor Kovacs , Patrick Lochmatter , Michael Wissler , Claudio Iseli and Lukas Kessler, Robotic arm, chapter 27
  2. Rui Zhanga, Patrick Lochmatter, Gabor Kovacs, Andreas Kunz, François Contic, Portable force feedback device based on miniature rolled dielectric elastomer actuators, chapter 22
  3. Michael Wissler, Edoardo Mazza, Modeling of prestrained circular actuators, chapter 18

 

Most important conference papers

  1. Kovacs G., Düring L., Contractive tension force stack actuator based on soft dielectric EAP, SPIE Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices (EAPAD), Vol. 7287, San Diego (USA), 2009.
  2. Kovacs G, Düring L. et al., Field Induced Deformation of Active Structures Based on Dielectric Elastomers, ICAST 2008, Ascona, Switzerland
  3. Kovacs G, Soon Mok Ha et al., 2008, Study on core free rolled actuator based on soft dielectric EAP, SPIE: Smart Materials and Structures, San Diego
  4. Lochmatter P., Kovacs G., 2007, Concept study on active shells driven by soft dielectric EAP, SPIE: Smart Materials and Structures, San Diego
  5. Lochmatter, P., G. Kovacs, et al. (2006). Design and characterization of shell-like dielectric elastomer actuators. 17th Int. Conf. on Adaptive Struct. and Techn. (ICAST), Taipei (Taiwan).
  6. Lochmatter, P., S. Michel, et al. (2006). Electromechanical model for static and dynamic activation of elementary dielectric elastomer actuators. Smart Struct. and Mat.: Electroactive Polymer Actuators and Devices (EAPAD), San Diego (USA), SPIE.
  7. Wissler, M., E. Mazza, et al. (2005). "Circular pre-strained dielectric elastomer actuators: modeling, simulation and experimental verification." Proceedings of SPIE 5759.
  8. Pei Q., Ha S.M., Pelrine R., Kovacs G., 2007, Enhanced performance of IPN electroelastomer, SPIE: Smart Materials and Structures, San Diego
  9. Wissler M., Mazza E., Kovacs G., 2007, Electromechanical coupling in cylindrical dielectric elastomer actuator, SPIE: Smart Materials and Structures, San Diego
  10. G. Kovacs, P. Lochmatter, Arm Wrestling Robot Driven by Dielectric Elastomer Actuators, SPIE: Smart Structures and Materials, San Diego, Calif. 2006.
  11. X. Zhang, M. Wissler, G. Kovacs, B. Jähne, R. Brönnimann, Electromechanical Characterisation of Dielectric Silicone Actuators, SPIE: Smart Structures an Materials 2004, San Diego, Calif. 2004.
  12. R. Zhang, A. Kunz, G. Kovacs, S. Michsl, A. Mazzone, Dielectric Elastomer Acutators for a Portable Force Feedback Device, Eurohaptics 2004, Munich, Germany
  13. A. Mazzone, A. Kunz, R. Zhang, G. Kovacs, Novel Actuators for Haptic Displays based on Elektroactive Polymers, Virtual Reality Software and Technology, Osaka, Japan, 2003.
  14. Michel, S. (2006). Elektroaktive Polymere (EAP) als Aktoren. Wissensforum Seminar Kunststofftechnik für Sensoren und Aktoren Fürth, Germany, VDI Wissensforum IWB GmbH.
  15. Wissler, M. and E. Mazza (2005). "Modeling and Finite Element Simulation of Dielectric Elastomer Actuators." NAFEMS Seminar: „Numerical Simulation of Electromechanical Systems“.
  16. Zhang, R., A. Kunz, et al. (2004). Dielectric Elastomer Actuators for A Portable Force Feedback Device. EuroHaptics2004, Munich.
  17. Zhang, R., A. Kunz, et al. (2006). Dielectric Elastomer Spring Roll Actuators for a Portable Force Feedback Device. Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, Alexandria, Virginia, USA, IEEE.
  18. Zhang, R., P. Lochmatter, et al. (2006). Spring Roll Dielectric Elastomer Actuators for a Portable Force Feedback Glove. Smart Structures and Materials, Electroactive Polymer Actuators and Devices (EAPAD), San Diego, SPIE.

 

 

 


PhD Thesis

1.     Lochmatter, P. (2007). Development of a Shell-like Electroactive Polymer (EAP) Actuator. D-MAVT. Zuerich, ETH Zurich. Dr. sc. techn. Nr. 17221.

2.      Wissler, M. (2007). Modeling Dielectric Elastomer Actuators. MAVT. Zurich, ETH Zurich. Dr. sc. techn. Nr 17142

3.      Zhang, R. (2007). Development of Dielectric Elastomer Actuators and their Implementation in a Force Feedback Interface. D-MAVT. Zuerich, ETH Zurich. Dr. sc. techn. Nr. 17584.

 

Patent

  1. PCT/CH2009/000141, PED-0802: “Dielektrischer Zug- Druck Aktor”
  2. PCT/CH2006/000198, PED-0501: “Propulsion Unit for Lighter-Than-Air Aircraft“

 


Outlook

EAP technololgy is enabling various applications where size, shape and / or generating forces can be adapted to the specific circumstances.

Video of a balloon  actuator changing it's shape

 


Kontakt: Dr. G. Kovacs, S. Michel

 

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