Novel actuator materials are necessary to advance the field of soft robotics. However, since current solutions are limited in terms of strain, strain rate, or robustness, a new actuator type was developed. In its basic configuration, this actuator consisted of four layers and self-coiled into a helix after pre-stretching. The actuator principle was a dielectric polymer actuator. Instead of an elastomer, a thin thermoplastic film, in this case polyethylene, was used as the dielectric and the typically low potential strain was amplified more than 40 times by the helical set-up. In a hot press, the thermoplastic film was joined together with layers of carbon black employed as electrodes and a highly elastic thermoplastic polyurethane film. Once the stack was laser cut into thin strips, they were then stretched over the polyethylene (PE) film’s limit of elasticity and released, thus forming a helix. The manufactured prototype showed a maximum strain of 2% while lifting six times its own weight at actuation frequencies of 3 Hz, which is equivalent to a strain rate of 12%/s. This shows the great potential of the newly developed actuator type. Nevertheless, materials, geometry as well as the manufacturing process are still subject to optimization.
There is an increasing interest to use novel elastomers with inherent or modified advanced dielectric and mechanical properties, as components of dielectric elastomer actuators (DEA). This requires corresponding techniques to assess their electromechanical performance. One performance criterion is the electrically induced deformation of the active electrode area. In this work, a rectangular DEA is used to investigate the influence of the ratio between the active electrode and the passive area on the actuator deformation. For this purpose, a dielectric silicone film is bonded on one surface to a unidirectional carbon fiber fabric. Thereby, highly anisotropic mechanical properties are implemented. When strains are applied perpendicular to the fiber direction, the composite hardly contracts in the fiber direction due to the superior stiffness of the fibers. In addition, the conductive fiber structure also acts as a highly anisotropic compliant electrode. By application of a second paste-like electrode onto the silicone film a DEA is created that operates in a pure shear configuration. This assembly enables the modification of the active-to-passive area ratio and the investigation of its effect on the actuator deformation. Image-based measurements are used to determine the strain of the active electrode area. The experimental results are compared to a lumped-parameter model that considers the electromechanical properties of the fiber-reinforced DEA. In summary, the ratio of the active-to-passive area has a significant influence on the measured deformation. Especially for novel actuator materials that do not exhibit large strains, an active-to-passive ratio of 50 % proves to be particularly advantageous.
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