Dielectric elastomer actuators consist of an elastomer film sandwiched between compliant electrodes. They work as electrostatic actuators: when a large electric field is applied over the electrodes, the rubber is compressed and the elastomer film elongates in the film plane. The performance of dielectric elastomer actuators (DEA), when a constant potential is applied, is expressed in a universal equation where a combination of the elastomers materials properties enters through a single parameter - a figure of merit. The expansion of the actuator is related to the applied potential for a particular actuator geometry: an actuator that expands under constant width. The derivation takes finite elasticity of the elastomers into account. The figure of merit can be used as guide to optimizing elastomer properties for dielectric elastomer actuators. For very highly pre-strained elastomers, the equations no longer hold. Elastomers with optimal properties are not commercially available. Typical elastomers for electric applications, encapsulation of electronics take an example, show at least one materials property that diminish their performance in DEA. Elastomers are mapped in a diagram expressing the property space for DEA.
Dynamical properties of dielectric elastomer actuators depend upon both electric and mechanical properties of the elastomer. The viscoelastic mechanical properties are intimately connected to network structure of the elastomer. The connection between network structure and the various relaxation times for the rubber that determines its viscoelastic properties are described.
When using polymeric networks to EAP material, certain
requirements need to be fulfilled or at least partly fulfilled.
The networks need to be strong since the driving voltage goes as
the thickness of the film to the second power, but on the other
hand the networks need to be soft and flexible in order to provide
sufficiently motion. Several network parameters can be altered in
order to alter the network properties but it turns out that the
most obvious parameter - the chain length of the network reactants
- has moderately influence at molecular lengths above the
entanglement length only. The inter-chains entanglements dominate
the properties rather than the actual crosslinks. Since chain
lengths below the entanglement length result in hard networks, the
chain length alone does not constitute a tunable parameter.
Therefore, it is obvious to focus on controlling the
entanglements. One way to do this is to make the network in
solution and afterwards remove the solvent. This way the
entanglement contribution is lowered because network chains will
be surrounded by solvent molecules and therefore the number of
trapped entanglements between network chains will be lowered. The
chain length can then be used as an easily tunable parameter.
Dielectric elastomer actuator technology is based on electric field induced deformation. From the viewpoint of materials technology, many points must be addressed, among which are material dielectric properties, breakdown voltage, viscoelastic losses and elastomer spring mechanical properties. From the viewpoint of actuator manufacturing, we will mention elastomer thin film and fiber processing as well as compliant electrode design. However, among all the previously mentioned key-points, compliant electrode design remains the major problem to solve, as electrodes required to distribute the electric field in the material need to be at least as compliant as the active elastomer material. In this paper, we present the analysis of dielectric elastomer-based actuators made with metallic compliant electrodes that show a relatively good overall mechanical performance. Large displacements, force densities and low creep, as well as fast response and million cycles are achieved using micro-structuring and thin-film techniques. We have succeeded in making smart anisotropic compliant metallic electrodes that can maintain conductivity up to 33% expansion before breaking and loosing electrical connectivity. Actuators are made with a silicone elastomer as the active material and silver as coating electrodes. The spring constant of a 3-layer actuator consisting of silver electrodes with a thickness up to 1100 A and elastomer film with a thickness up to 50 micrometers is typically 2 times larger than that of the elastomer film taken alone. Force-displacement and constant load measurements are used as a basis to analyze the mechanical properties of the artificial muscle. Capacity measurements at high frequency in the kilohertz range are carried out to study the built-in sensor properties for feedback control of the actuator.
Dielectric elastomer actuators performance depends on their construction and the way they are driven. We describe the governing equations for the dynamic performance of actuators and show examples of their use. Both the properties of the base elastomer material and the compliant electrodes influence the actuators performance. The mechanical and electrical properties of elastomers are discussed with a focus on an acrylate pressure sensitive adhesive from 3M, which is used by a number of groups. The influence of these properties on the actuator properties is analyzed.
Dielectric elastomer actuators, based on the field-induced deformation of elastomeric polymers with compliant electrodes, can produce a large strain response, combined with a fast response time and high electromechanical efficiency. This unique performance, combined with other factors such as low cost, suggests many potential applications, a wide range of which are under investigation. Applications that effectively exploit the properties of dielectric elastomers include artificial muscle actuators for robots; low-cost, lightweight linear actuators; solid- state optical devices; diaphragm actuators for pumps and smart skins; acoustic actuators; and rotary motors. Issues that may ultimately determine the success or failure of the actuation technology for specific applications include the durability of the actuator, the performance of the actuator under load, operating voltage and power requirements, and electronic driving circuitry, to name a few.
Elastomer films sandwiched between compliant electrodes work as electrostatic actuators when a large electric field is applied over the electrodes. We have analyzed the mechanical and electrical response of actuators to a sinusoidal varying driving voltage. The actuator acts as a capacitor in the electric circuit, but due to very high strains, the capacitance changes during a work cycle. The extension of the actuator is electrostrictive in response, hence it depends on the square of the applied field and oscillates with twice the driving frequency. The response is non-linear. This change in dimension is coupled back into the electric circuit through the capacitance of the film and the current oscillates with the first, third and odd higher-order harmonics. Due to this coupling, measurements of the current allows one to determine the expansion of the actuator, and hence to control the actuator.
Polyacrylate dielectric elastomers have yielded extremely large strain and elastic energy density suggesting that they are useful for many actuator applications. A thorough understanding of the physics underlying the mechanism of the observed response to an electric field can help develop improved actuators. The response is believed to be due to Maxwell stress, a second order dependence of the stress upon applied electric field. Based on this supposition, an equation relating the applied voltage to the measured force from an actuator was derived. Experimental data fit with the expected behavior, though there are discrepancies. Further analysis suggests that these arise mostly from imperfect manufacture of the actuators, though there is a small contribution from an explicitly electrostrictive behavior of the acrylic adhesive. Measurements of the dielectric constant of stretched polymer reveal that the dielectric constant drops, when the polymer is strained, indicating the existence of a small electrostrictive effect. Finally, measurements of the electric breakdown field were made. These also show a dependence upon the strain. In the unstrained state the breakdown field is 20 MV/m, which grows to 218MV/m at 500% x 500% strain. This large increase could prove to be of importance in actuator design.
Conducting polymers show volume changes during electrochemical doping. Their high strength make them potential candidates for being used as artificial muscles. We consider actuators based on three-layer structures consisting of a passive polymer substrate sheet, a thin metal film electrode and a thin film of conducting polymer. In this paper we describe our Three- layer model to study the performance of an actuator based on the transduction of bending to linear movements. We show calculated results for an undulator and C-block characterized, respectively, by a flat and semi-circular shape in the relaxed state. Knowing the mechanical parameters of the considered materials, we evaluate the efficiency of the composite structure in terms of the performed stroke and work. The model shows that the undulator contracts in a nonlinear way with respect to the relative expansion of the materials, whereas the C-block is approximately linear. Contractions as large as 80% and 45% are obtained with the undulator and C-block, respectively. Although the C-block performs better than the undulator in terms of linearity, the undulator is easier to design and manufacture due to its flat shape in the relaxed state.