Energy efficiency, lightweight and scalability are key features for actuators in applications such as valves, pumps or any portable system. Dielectric electroactive Polymer (DEAP) technology is able to fulfill these requirements1 better than commonly used technology e.g. solenoids, but has limitations concerning force and stroke. However, the circular DEAP membrane actuator shows a potential increase in stroke in the mm range, when combined with an appropriate biasing mechanism2. Although, thus far, their force range is limited to the single-digit Newton range, or less3,4. This work describes how this force limit of DEAP membrane actuators can be pushed to the high double-digit Newton range and beyond. The concept for such an actuator consists of a stack of double-layered DEAPs membrane actuator combined with a biasing mechanism. These two components are combined in a novel way, which allows a compact design by integrating the biasing mechanism into the DEAP membrane actuator stack. Subsequently, the single components are manufactured, tested, and their force-displacement characteristic is documented. Utilizing this data allows assembling them into actuator systems for different applications. Two different actuators are assembled and tested (dimensions: 85x85x30mm3 (LxWxH)). The first one is able to lift 7.5kg. The second one can generate a force of 66N while acting against a spring load.
Shape Memory Alloys (SMA’s) are known as actuators with very high energy density. This fact allows for the construction of very light weight and energy-efficient systems. In the field of material handling and automated assembly process, the avoidance of big moments of inertia in robots and kinematic units is essential. High inertial forces require bigger and stronger robot actuators and thus higher energy consumption and costs. For material handling in assembly processes, many different individual grippers for various work piece geometries are used. If one robot has to handle different work pieces, the gripper has to be exchanged and the assembly process is interrupted, which results in higher costs. In this paper, the advantages of using high energy density Shape Memory Alloy actuators in applications of material-handling and gripping-technology are explored. In particular, light-weight SMA actuated prototypes of an adaptive end-effector and a vacuum-gripper are constructed via rapid-prototyping and evaluated. The adaptive end-effector can change its configuration according to the work piece geometry and allows the handling of multiple different shaped objects without exchanging gripper tooling. SMA wires are used to move four independent arms, each arm adds one degree of freedom to the kinematic unit. At the tips of these end-effector arms, SMA-activated suction cups can be installed. The suction cup prototypes are developed separately. The flexible membranes of these suction cups are pulled up by SMA wires and thus a vacuum is created between the membrane and the work piece surface. The self-sensing ability of the SMA wires are used in both prototypes for monitoring their actuation.
Circular dielectric electro-active polymer (DEAP) membrane actuators are easy to manufacture and therefore can be uniquely designed to perform optimally for specific applications. The performance of these actuators is naturally dependent on the materials used, and also dictated by the specific geometry of the circular design. For a given overall actuator size, changing their internal geometry will directly change the force and stroke output. In addition the DEAP technology itself is a promising technology for constructing lightweight, cost and energy efficient sensor and actuator systems. Thus, several potential applications like pressure sensors, pumps, valves, micro-positioners and loudspeakers were already proposed. The circular DEAP membrane actuators used in this study consist of a silicone based elastomer, carbon ink based electrodes, and are held together with a stiff frame. Experimentally collected force-displacement curves for these actuators can be used to determine force and stroke output of the actuators as described by Hodgins et al. in. This work presents an efficient method to predict these force-displacement plots and thus stroke and force output for different actuator geometries. These results than can be used to adapt the actuator geometry to the needs of a specific application with its particular force and stroke requirements. The prediction method is based on an average stress-stretch calculation for training samples. The calculated stress-stretch data is then geometry independent and can be used to predict desired geometry dependent force-displacement data for stroke and force output analysis.
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.
Dielectric Electroactive Polymers (DEAP) will undergo large deformations when subject to an electric field making
them an attractive material for use in novel actuator systems. There are many challenges with successful application and
design of DEAP actuators resulting from their inherent electromechanical coupling and non-linear material behavior. FE
modeling of the material behavior is a useful tool to better understand such systems and aid in the optimal design of
prototypes. These modeling efforts must account for the electromechanical coupling in order to accurately predict their
response to multiple loading conditions expected during real operating conditions.
This paper presents a Finite Element model of a dielectric elastomer undergoing out-of-plane, axisymmetric deformation.
The response of the elastomer was investigated while it was subjected to mechanical and electric fields and combined
electro-mechanical actuation. The compliant electrodes have a large effect on the mechanical behavior of the EAP which
needs to be taken into consideration while modeling the EAP as a system. The model is adapted to include the effect of
electrode stiffness on the mechanical response of the actuator. The model was developed using the commercial Finite
Element Modeling software, COMSOL. The results from the mechanical simulations are presented in the form of forcedisplacement
curves and are validated with comparisons to experimental results. Electromechanical simulations are
carried out and the stroke of the actuator for different electrode stiffness values is compared with experimental values
when the EAP is biased with a constant force.
Piezoelectric actuators used in nano-positioning devices exhibit highly non-linear behavior and strong hysteresis, which limits the efficiency of conventional non-model-based controllers. This paper presents a free energy model based on the theory of thermal activation for single crystal piezoceramics that couples mechanical stress and electric field. It is capable of predicting the hysteretic behavior along with the frequency-dependence present in these materials. The model is then coupled with a spring as a first step toward a 1-D model of a commercial nano-positioning stage and is the basis for future control applications. Quasi-static simulations are conducted to illustrate the effects of spring loading on the actuator behavior. A first step towards adapting the model for polycrystalline material is also presented. Simulations are shown to predict the rate-dependent strain response of a spring loaded polycrystalline stack actuator for various pre-stresses.
Dielectric Electro-Active Polymers (EAP's) can achieve substantial deformation (>300% strain) while, compared to
their ionic counterparts, sustaining large forces. This makes them attractive for various actuation and sensing
applications such as light weight and energy efficient valve and pumping systems.. This paper provides a systematic
experimental investigation of the quasi-static and dynamic electro-mechanical properties of a commercially available
dielectric EAP actuator in the frequency range up to 20Hz. In order to completely characterize the fully coupled behavior
force vs. displacement measurements at various constant voltages and force vs. voltage measurements at various fixed
displacements are conducted. The experiments are conducted with a particular focus on the hysteretic and ratedependent
Piezoelectric actuators used in nano-positioning devices exhibit highly non-linear behavior and strong hysteresis, which limits the efficiency of conventional non-model-based controllers. This paper presents the first results of a free energy model based on the theory of thermal activation for single crystal piezoceramics that couples mechanical stress and electric field. It is capable of predicting the hysteretic behavior along with the frequency-dependence present in these materials. The model is then coupled with a spring as a first step toward a SDOF model of a commercial nano-positioning stage and is the basis for future control applications. Quasi-static simulations are presented to illustrate the effects of spring loading on the actuator behavior.
The rate-dependence of piezoelectric materials resulting from the kinetics of domain switching is an important factor that needs to be included in realistic modeling attempts. This paper provides a systematic study of the rate-dependent hysteresis behavior of a commercially available PZT stack actuator. The stack actuator is coupled to a flexure system which provides mechanical loading via a spring. Experiments covering full as well as minor loops are conducted at different loading rates with polarization and strain recorded. In addition, the creep behavior at different constant levels of the electric field is observed. These experiments provide evidence of kinetics being characterized by strongly varying relaxation times that can be associated with different switching mechanisms.