In this paper, a dynamic model for an artificial finger driven by Shape Memory Alloy (SMA) wires is presented. Due to their high energy density, these alloys permit the realization of highly compact actuation solutions with potential applications in many areas of robotics, ranging from industrial to biomedical ones. Despite many advantages, SMAs exhibit a highly nonlinear and hysteretic behavior which complicates system design, modeling, and control. In case SMA wires are used to activate complex robotic systems, the further kinematic nonlinearities and contact problems make the modeling significantly more challenging. In this paper, we present a finite element model for a finger prototype actuated by a bundle of SMA wires. The commercially available software COMSOL is used to couple the finger structure with the SMA material, described via the Müller-Achenbach-Seelecke model. By means of several experiments, it is demonstrated how the model reproduces the finger response for different control inputs and actuator geometries.
Dielectric elastomers represent an attractive technology for smart actuator, sensor, and generator systems. In order to estimate how the performance of a membrane dielectric elastomer actuator (DEA) changes with the available design parameters (e.g., geometry, electrodes), numerous characterization experiments have to be performed. Alternatively, accurate simulations tools capable of predicting the system performance can be used to effectively optimize the design of DEA applications. In particular, Finite Element (FE) simulations allow to map global quantities as well as locally distributed quantities such as stress and strain fields as well as the electric field, and therefore appear as suitable for applications in which complex membrane geometries or electrode patterns are used. In this work, an FE model based on Comsol Multiphysics is introduced. This model is based on an electro-mechanically coupled formulation for large deformations, which also includes viscoelastic effects and electrodes geometry, while neglecting inertial effects. Due to the poor aspect ratio of membrane structures discretized with three-dimensional continuum elements, computation times appear as excessively large. To overcome this issue, the geometry is reduced to a two-dimensional structure. In order to simulate the local electric field distribution, both electrodes are discretized separately. For model identification and validation, specimens with and without imprinted electrodes are tested. Based on the developed model, the influence of the discretized electrodes is then examined, by varying electrode dimensions. Furthermore, fringe fields at the electrode edges are investigated in order to better understand local phenomena, e.g., the electrical breakdown mechanisms.
Bio-inspired hand-like gripper systems based on shape memory alloy (SMA) wire actuation have the potential to enable a number of useful applications in, e.g., the biomedical field or industrial assembly systems. The inherent high energy density makes SMA solutions a natural choice for systems with lightweight, low noise and high force requirements, such as hand prostheses or robotic systems in a human/machine environment. The focus of this research is the development, design and realization of a SMA-actuated prosthetic hand prototype with three fingers. The use of thin wires (100 μm diameter) allows for high cooling rates and therefore fast movement of each finger. Grouping several small wires mechanically in parallel allows for high force actuation. To save space and to allow for a direct transmission of the motion to each finger, the SMA wires are attached directly within each finger, across each phalanx. In this way, the contraction of the wires will allow the movement of the fingers without the use of any additional gears. Within each finger, two different bundles of wires are mounted: protagonist ones that create bending movement and the antagonist ones that enable stretching of each phalanx. The resistance change in the SMA wires is measured during actuation, which allows for monitoring of the wire stroke and potentially the gripping force without the use of additional sensors. The hand is built with modern 3D-printing technologies and its performance while grasping objects of different size and shape is experimentally investigated illustrating the usefulness of the actuator concept.