Modeling, processing, and characterization of dielectric elastomer actuators and sensors

Thesis (Ph.D.)--University of Washington, 2017-06Over the past two decades, electroactive polymers (EAP) have been studied as a material for soft actuator and sensor systems. Dielectric elastomers (DE) are an EAP material which relies on the electrostatic force produced on compliant electrodes to produce deformation. In the converse sense, DE sensors can be used by measuring the electrical energy or impedance change produced under deformation. The two key limitations barring DE from commercial use are high driving voltage, and low output force. The scope of this work is as follows: to improve upon these two limitations by processing of actuators by a pneumatic dispenser, by adding tactile sensing and variable stiffness properties to the actuators, and developing a mechanical model to predict the actuator behavior. This work focuses specifically on the unimorph dielectric elastomer actuator (DEA), which consists of a DE laminate which contracts in the thickness direction and expands in-plane under applied voltage, and is constrained on one face by a passive material, resulting in bending of the structure. The first part of the work is devoted to fabrication, modeling, and characterization of multilayer unimorph DEA. Fabrication is done using two schemes – the first is a conventional one, using commercially available DE films, and the second is a novel method using a robotic dispenser system. The latter technique has two objectives. The first is to reduce the thickness of the DE layers to reduce driving voltage, since the DE deformation is proportional to the square of the applied electric field which itself is inversely proportional to electrode separation. The second is to deposit higher-performance DE materials, in this case, PVDF terpolymer, which exhibits large actuation stresses because of its high dielectric constant and relatively high Young’s modulus. Using the dispenser, DE layers with 10 µm thick layers are repeatably produced, requiring actuation voltages one order of magnitude less than conventional thick DE films. Standard deviation of displacement and blocking force do not exceed 10% and 15% of the mean after 2 minutes of deformation, respectively. Elastic and viscoelastic models are developed for multilayer unimorph DEA consisting of flat and curved geometries. Both models were validated in comparison with experimental data with the latter shown to agree with the experimental data to within one standard deviation of the mean for majority of the deformation. The second section demonstrates the novel use of electrolaminates to create variable stiffness DEA (VSDEA). Variable stiffness structures are of particular interest for soft actuators, because they allow switching between a low stiffness, high displacement mode and a high stiffness mode with large holding force. One device is demonstrated by simply utilizing the passive layer of a DEA as part of an electrolaminate, allowing for four-fold increase in bending rigidity. Another device is demonstrated consisting of a bundle of parallel DEA with electrostatic chucking features to modulate shear strength of the interfaces. This device exhibits a 39-fold increase in stiffness, and a claw actuator using these actuators is capable of lifting an object 17 times its own weight. The final part of this work investigates two novel tactile sensors based on dielectric elastomers (DES). The first uses a dome-shaped protrusion to redistribute tactile forces onto an array of four capacitive sensors. The change in capacitance of the four sensors is used to measure and discriminate the force components of the impinging force. An array of these dome DES are fabricated using the dispenser system, and the ability to differentiate between normal and shear forces was demonstrated, as well as its proximity sensing ability. The tactile sensor array is also shown integrated as the passive layer of a DEA, providing tactile and proximity sensing capability to the actuator. The second tactile sensor features high resolution and scalability, and is built in to a medical assistive device coined the “artery mapper” and is used to determine the location of a target artery for arterial line placement. It is demonstrated locating an artery on a test subject, possible due to its force resolution on the order of 2.8 kPa