Investigation of Variable Stiffness Based Polymer Morphing Structures

Morphing structures have a multitude of potential applications in many engineering fields. Control surfaces for aircraft, pumps and valves for artificial organs or any application where a complex motion is required and can benefit from incorporating a morphing structure. The ability of a morphing structure to change its shape or configuration can potentially allow designs and functionality that would be impossible to realize without morphing. The focus of this thesis is to investigate the feasibility and operation of a structural shape morphing structure. Structural shape morphing is defined as shape change through material modulus changes. This allows fewer actuators, fewer moving parts and lower energy consumption to effect the same change in configuration as a traditional shape change structure. The morphing structure examined was designed and created specifically for this work and is composed almost entirely of polymer materials. The design utilizes electrical voltage to control the structural stiffness and motion of the structure independently. A review of literature covering fabrication and modeling in morphing structures, polymer actuators, variable modulus materials and variable stiffness structures is presented. A conceptual design for a shape morphing structure is fabricated and refined and an analytical model is developed for the structure to predict its response to applied voltages. The morphing structure’s capabilities are measured through experimental testing and the predictions of the analytical model are compared to the results. We demonstrate shape morphing by deforming the structure with an actuator, increasing the effective structural modulus and then removing the actuation force. Our tests show a fixed shape change in the structure of up to 20% of the total deformation. Feasibility is also discussed for real-world applications and suggested areas for further exploration on the topic are presented

Performance-Optimized Dielectric Elastomer Actuator System with Scalable Scissor Linkage Transmission

Thanks to their outstanding properties, in the last few years Dielectric Elastomer Actuators (DEAs) have increasingly attracted the interest of the scientific community and generated a surge in the effort devoted to their industrialization. Compared to conventional actuator systems, DEAs are based on inexpensive and widely available polymeric materials, which make them potentially attractive from a market perspective. However, DEA systems with a given layout and dimensions have a fixed force-stroke response that is only suitable for a specific load profile. This leads to a wide variety of designs combined with small production volumes and high costs, limiting the competitive advantage. This work addresses this issue by proposing a combination of DEA systems with compliant scissor linkage transmission mechanisms, which provide linear stroke and force scaling and simultaneously maintain performance optimization by leaving the convertible energy density of the DEA unaffected. For this purpose, three systems are designed, based on a same strip-shaped DEA combined with inclined buckled beam biasing mechanisms. Two of the systems are coupled with scissor linkages that offer transmission ratios of 3:1 and 1:3, respectively, to adapt the system to different load profiles. The system design is explained in detail, and the functional principle is validated through experiments

Experiments and multi-field modeling of inelastic soft materials

Soft dielectrics are electrically-insulating elastomeric materials, which are capable of large deformation and electrical polarization, and are used as smart transducers for converting between mechanical and electrical energy. While much theoretical and computational modeling effort has gone into describing the ideal, time-independent behavior of these materials, viscoelasticity is a crucial component of the observed mechanical response and hence has a significant effect on electromechanical actuation. This thesis reports on a constitutive theory and numerical modeling capability for dielectric viscoelastomers, able to describe electromechanical coupling, large- deformations, large-stretch chain-locking, and a time-dependent mechanical response. This approach is calibrated to the widely-used soft dielectric VHB 4910, and the finite-element implementation of the model is used to study the role of viscoelasticity in instabilities in soft dielectrics, namely (1) the pull-in instability, (2) electrocreasing, (3) electrocavitation, and (4) wrinkling of a pretensioned three dimensional diaphragm actuator. Results show that viscoelastic effects delay the onset of instability under monotonic electrical loading and can even suppress instabilities under cyclic loading. Furthermore, quantitative agreement is obtained between experimentally measured and numerically simulated instability thresholds. Filled rubber-like materials are important engineering materials, and they are widely used in aerospace, automotive, and other industries. However, their nonlinear, inelastic, and rate-dependent constitutive behavior is not fully understood and modeled with varying degrees of success. Much of the previous literature has focused on either capturing quasi-static stress-softening behavior or rate-dependent viscous effects, but generally not both concurrently. This thesis develops a thermody- namically consistent constitutive model which accounts for both of those phenomena concurrently. A set of comprehensive mechanical tensile tests are conducted on the filled rubber Viton. The constitutive model is then calibrated to the experimental data, and numerically implemented into the finite element package Abaqus by writing a user material subroutine UMAT. The constitutive model is validated by comparing a numerical simulation prediction with an inhomogeneous deformation experiment. As an extension to the study of Viton, this thesis also develops a constitutive model to quantitatively capture thermal recovery of the Mullins effect. The model is then calibrated to experiments in the literature, and numerically implemented by writing a user material subroutine for the finite element program Abaqus/Standard. Lastly, simulation results suggest that the unanticipated behaviors due to recovery of Mullins effect are possible

Functional Soft Robotic Actuators Based on Dielectric Elastomers

Dielectric elastomer actuators (DEAs) are a promising soft actuator technology for robotics. Adding robotic functionalities--folding, variable stiffness, and adhesion--into their actuator design is a novel method to create functionalized robots with simplified actuator configurations. We first propose a foldable actuator that has a simple antagonistic DEA configuration allowing bidirectional actuation and passive folding. To prove the concept, a foldable elevon actuator with outline size of 70 mm × 130 mm is developed with a performance specification matched to a 400 mm wingspan micro air vehicle (MAV) of mass 130 g. The developed actuator exhibits actuation angles up to ± 26 ° and a torque of 2720 mN·mm in good agreement with a prediction model. During a flight, two of these integrated elevon actuators well controlled the MAV, as proven by a strong correlation of 0.7 between the control signal and the MAV motion. We next propose a variable stiffness actuator consisting of a pre-stretched DEA bonded on a low-melting-point alloy (LMPA) embedded silicone substrate. The phase of the LMPA changes between liquid and solid enabling variable stiffness of the structure, between soft and rigid states, while the DEA generates a bending actuation. A proof-of-concept actuator with dimension 40 mm length × 10mm width × 1mm thickness and a mass of 1 g is fabricated and characterized. Actuation is observed up to 47.5 ° angle and yielding up to 2.4 mN of force in the soft state. The stiffness in the rigid state is ~90 × larger than an actuator without LMPA. We develop a two-finger gripper in which the actuators act as the fingers. The rigid state allows picking up an object mass of 11 g (108 mN), to be picked up even though the actuated grasping force is only 2.4 mN. We finally propose an electroadhesion actuator that has a DEA design simultaneously maximizing electroadhesion and electrostatic actuation, while allowing self-sensing by employing an interdigitated electrode geometry. The concept is validated through development of a two-finger soft gripper, and experimental samples are characterized to address an optimal design. We observe that the proposed DEA design generates 10 × larger electroadhesion force compared to a conventional DEA design, equating to a gripper with a high holding force (3.5 N shear force for 1 cm^2) yet a low grasping force (1 mN). These features make the developed simple gripper to handle a wide range of challenging objects such as highly-deformable water balloons (35.6 g), flat paper (0.8 g), and a raw chicken egg (60.9 g), with its lightweight (1.5 g) and fast movement (100 ms to close fingers). The results in this thesis address the creation of the functionalized robots and expanding the use of DEAs in robotics

Modeling and Design Optimization of a Rotational Soft Robotic System Driven by Double Cone Dielectric Elastomer Actuators

Dielectric elastomers (DEs) consist of highly compliant electrostatic transducers which can be operated as actuators, by converting an applied high voltage into motion, and as sensors, since capacitive changes can be related to displacement information. Due to large achievable deformation (on the order of 100%) and high flexibility, DEs appear as highly suitable for the design of soft robotic systems. An important requirement for robotic systems is the possibility of generating a multi degree-of-freedom (MDOF) actuation. By means of DE technology, a controllable motion along several directions can be made possible by combining different membrane actuators in protagonist-antagonist configurations, as well as by designing electrode patterns which allow independent activation of different sections of a single membrane. However, despite several concepts of DE soft robots have been presented in the recent literature, up to date there is still a lack of systematic studies targeted at optimizing the design of the system. To properly understand how different parameters influence the complex motion of DE soft robots, this paper presents an experimental study on how geometry scaling affects the performance of a specific MDOF actuator configuration. The system under investigation consists of two cone DE membranes rigidly connected along the outer diameter, and pre-compressed out-of-plane against each other via a rigid spacer. The electrodes of both membranes are partitioned in four sections that can be activated separately, thus allowing the desired MDOF actuation feature. Different prototypes are assembled and tested to study the influence of the inner radius as well as the length of the rigid spacer on the achievable motion range. For the first experimental study presented here, we focus our analysis on a single actuation variable, i.e., the rotation of the rigid spacer about a fixed axis. A physics-based model is then developed and validated based on the collected experimental measurements. A model-based investigation is subsequently performed, with the aim of studying the influence of the regarded parameters on the rotation angle. Finally, based on the results of the performed study, a model-based optimization of the prototype geometry is performed

Effect of the nonlinear material viscosity on the performance of dielectric elastomer transducers

As a typical type of soft electroactive materials, dielectric elastomers (DEs) are capable of producing large voltage-induced deformation, which makes them desirable materials for a variety of applications in transduction technology, including tunable oscillators, resonators, biomimetics and energy harvesters. The dynamic and energy harvesting performance of such DE-based devices is strongly affected not only by multiple failure modes such as electrical breakdown, electromechanical instability, loss-of-tension and fatigue, but also by their material viscoelasticity. Moreover, as suggested by experiments and theoretical studies, DEs possess nonlinear relaxation processes, which makes modeling of the performance of DE-based devices more challenging. In this thesis, by adopting the state-of-art modeling framework of finite-deformation viscoelasticity, the effects of nonlinear viscosity of the polymer chains on the oscillation and frequency tuning of DE membrane oscillators are firstly investigated. From the simulation results, it is found that the nonlinear viscosity only affects the transient state of the frequency tuning process of DE oscillators. Secondly, with both finite-deformation viscoelasticity and deformation-dependent viscosity of polymer chains considered, the energy conversion efficiency and harvested energy of dielectric elastomer generators under equi-biaxial loading are also examined. It is found that when a nonlinear viscosity model is used, DE generators appear to reach an equilibrium state faster and the nonlinear viscosity significantly influences the energy harvesting performance. The modeling framework developed in this work is expected to provide useful guidelines for predicting the performance of DE-based oscillators and energy harvesters as well as their optimal design