4D Printing Dielectric Elastomer Actuator Based Soft Robots

4D printing is an emerging technology that prints 3D structural smart materials that can respond to external stimuli and change shape over time. 4D printing represents a major manufacturing paradigm shift from single-function static structures to dynamic structures with highly integrated functionalities. Direct printing of dynamic structures can provide great benefits (e.g., design freedom, low material cost) to a wide variety of applications, such as sensors and actuators, and robotics. Soft robotics is a new direction of robotics in which hard and rigid components are replaced by soft and flexible materials to mimic mechanisms that works in living creatures, which are crucial for dealing with uncertain and dynamic tasks. However, little research on direct printing of soft robotics has been reported. Due to the short history of 4D printing, only a few smart materials have been successfully 4D printed, such as shape memory and thermo-responsive polymers, which have relatively small actuation strains (up to ~8%). In order to produce the large motion, dielectric elastomer actuator (DEA), a sheet of elastomer sandwiched between two compliant electrodes and known as artificial muscle for its high elastic energy density and capability of producing large strains (~200%), is chosen as the actuator for soft robotics. Little research on 3D printing silicone DEA soft robotics has been done in the literature. Thus, this thesis is motivated by applying the advantages in 3D printing fabrication methods to develop DEA soft robotics. The ultimate research goal is to demonstrate fully printed DEA soft robots with large actuation. In Chapter 1, the research background of soft robotics and DEAs are introduced, as well as 3D printing technologies. Chapter 2 reports the rules of selecting potentially good silicone candidates and the printing process with printed material characterizations. Chapter 3 studies the effects of pre-strain condition on silicone material properties and the performance of DEA configurations, in order to obtain large actuation strain. In Chapter 4, two facial soft robots are designed to achieve facial expressions as judged by a smiling lip and expanding pupils based on DEA actuation. Conclusions and future developments are given in chapter 5 and 6, respectively

Monolithic shape-programmable dielectric liquid crystal elastomer actuators

Macroscale robotic systems have demonstrated great capabilities of high speed, precise, and agile functions. However, the ability of soft robots to perform complex tasks, especially in centimeter and millimeter scale, remains limited due to the unavailability of fast, energy-efficient soft actuators that can programmably change shape. Here, we combine desirable characteristics from two distinct active materials: fast and efficient actuation from dielectric elastomers and facile shape programmability from liquid crystal elastomers into a single shape changing electrical actuator. Uniaxially aligned monoliths achieve strain rates over 120%/s with energy conversion efficiency of 20% while moving loads over 700 times the actuator weight. The combined actuator technology offers unprecedented opportunities towards miniaturization with precision, efficiency, and more degrees of freedom for applications in soft robotics and beyond

Finite element modeling of dielectric elastomer actuators for space applications

A special actuator device with passive sensing capability based on dielectric elastomer was studied and specialized to be used in space applications. The work illustrates the research project modeling procedure adopted to simulate the mechanical behavior of this material based on a finite element theory approach. The Mooney-Rivlin’s hyperelastic and Maxwell’s electrostatic models provide the theoretical basis to describe its electro-mechanic behavior. The validation of the procedure is performed through a numerical-experimental correlation between the response of a prototype of actuator developed by the Risø Danish research center and the 3D finite element model simulations. An investigation concerning a possible application in the space environment of dielectric elastomer actuators (DEA) is also presented

Electromechanical Instability in Silicone-and Acrylate-based Dielectric Elastomers

Electromechanical instability (EMI) is regarded as a significant factor in preventing dielectric elastomers (DEs) fromachieving large voltage-induced deformations. In this study, the strain-stiffening effect was used to control the occurrence of EMI in DEs. The results show that the stretching ratio required to provide a feasible strain-stiffening effect in silicone rubber (SR) was smaller than that needed for a commercial DE material, VHB 4910. The experimental data were compared with currently used models for the simulation of EMI in DEs. We found that EMI could be eliminated in the deformation of these elastomers when pre-stretching was used. Through the application of a prestretching ratio of above 2.0, EMI was suppressed in both the VHB 4910 and SR samples. The findings of this research are of great significance in the maximization of the electromechanical performance of DE materials

Finite element modeling and validation of a soft array of spatially coupled dielectric elastomer transducers

Dielectric elastomer (DE) transducers are suitable candidates for the development of compliant mechatronic devices, such as wearable smart skins and soft robots. If many independently-controllable DEs are closely arranged in an array-like configuration, sharing a common elastomer membrane, novel types of cooperative and soft actuator/sensor systems can be obtained. The common elastic substrate, however, introduces strong electro-mechanical coupling effects among neighboring DEs, which highly influence the overall membrane system actuation and sensing characteristics. To effectively design soft cooperative systems based on DEs, these effects need to be systematically understood and modeled first. As a first step towards the development of soft cooperative DE systems, in this paper we present a finite element simulation approach for a 1-by-3 silicone array of DE units. After defining the system constitutive equations and the numerical assumptions, an extensive experimental campaign is conducted to calibrate and validate the model. The simulation results accurately predict the changes in force (actuation behavior) and capacitance (sensing behavior) of the different elements of the array, when their neighbors are subjected to different electro-mechanical loads. Quantitatively, the model reproduces the force and capacitance responses with an average fit higher than 93% and 92%, respectively. Finally, the validated model is used to perform parameter studies, aimed at highlighting how the array performance depends on a relevant set of design parameters, i.e. DE-DE spacing, DE-outer structure spacing, membrane pre-stretch, array scale, and electrode shape. The obtained results will provide important guidelines for the future design of cooperative actuator/sensor systems based on DE transducers

Dynamic Modeling of Soft Robotic Dielectric Elastomer Actuator

Dielectric elastomers actuators (DEAs) are among the preferred materials for developing lightweight, high compliance and energy efficient driven mechanisms for soft robots. Simple DEAs consist mostly of a homogeneous elastomeric materials that transduce electrical energy into mechanical deformation by means of electrostatic attraction forces from coated electrodes. Furthermore, stacking multiple single DEAs can escalate the total mechanical displacement performed by the actuator, such is the case of multilayer DEAs. The presented research proposes a model for the dynamical characterization of multilayer DEAs in the mechanical and electrical domain. The analytical model is derived by using free body diagrams and lumped parameters that recreate an analogous system representing the multiphysics dynamics within the DEA. Hyperelasticity in most elastomeric materials is characterized by a nonlinear spring capable of undergoing large deformation; thus, defining the isostatic nonlinear relationship between stress and stretch. The transient response is added by employing the generalize Kelvin-Maxwell elements model of viscoelasticity in parallel with the hyperplastic spring. The electrostatic pressure applied by the electrodes appears as an external mechanical pressure that compress the material; thus, representing the bridge between the electrical and mechanical domain. Moreover, DEAs can be represented as compliant capacitors that change their capacitance as it keeps deforming; consequently, this feature can be used for purposes of self-sensing since there is always a capacitance value that can be mapped into the actual displacement. Therefore, an analytical model of an equivalent circuit of the actuator is also derived to analyze the changes in the capacitance while the actuator is under duty. The models presented analytically are then cross-validated by finite element methods using COMSOL Multiphysics® as the software tool. The results from both models, the analytical and FEM model, were compared by virtually recreating the dynamics of a multilayer DEA with general circular cross section and material parameters from VHB4905 3M commercially available tape. Furthermore, this research takes the general dynamical framework built for DEAs and expand it to model the dynamical system for helical dielectric elastomer actuators (HDEAs) which is a novel configuration of the classical stack that increases the nonlinearity of the system. Finally, this research present a complementary study on enhancing the dielectric permittivity for DEAs, which is an electrical material property that can be optimized to improve the relationship between voltage applied and deformation of the actuator