"There have been significant developments in the field of robotics. Significant development consists of new configurations, control mechanisms, and actuators based upon its applications. Despite significant improvements in modern robotics, the biologically inspired robots has taken the center stage. Inspired by nature, biologically inspired robots are called ‘soft robots’. Within these robots lies a secret ingredient: the actuator. Soft robotic development has been driven by the idea of developing actuators that are like human muscle and are known as ‘artificial muscle’. Among different materials suitable for the development of artificial muscle, the dielectric elastomer actuator (DEA) is capable of large deformation by applying an electric field. Theoretical formulation for DEA was performed based upon the constitutive hyperelastic models and was validated by using finite element method (FEM) using ABAQUS. For FEM, multistep analysis was performed to apply pre-stretch to the membrane before applying actuation voltage. Based on the validation of DEA, different configurations of DEA were investigated. Helical dielectric elastomer actuator and origami dielectric elastomer actuator were investigated using theoretical modeling. Comparisons were made with FEM to validate the model. This study focus on the theoretical and FEM analysis of strain within the different configuration of DEA and how the actuation strain of the dielectric elastomer can be translated into contraction and/or bending of the actuator."--Abstract.
Inspired by the biology, soft robots have drawn tremendous attention due to its large and continuous deformation, friendly human-machine interaction, large number of degrees of freedom (DOFs), capability of absorbing energy. They have been explored in broad applications ranging from dexterous soft gripper to the novel assistive devices. In the recent decade, numerous soft actuating materials and deformable structures have been developed to construct soft robots, including hydrogels, shape memory polymers (SMPs), dielectric elastomer actuators (DEAs), fluid elastomer actuators (FEAs) and magnetic actuators. However, those materials and structures have well-known limitations such as slow actuation speed, irreversibility, high voltage input and bulky controlling systems. Liquid crystal elastomers (LCEs), as newly emerging soft actuating materials, exhibit large and reversible deformation and versatile actuation modes. Based on the molecular structure, LCE can be viewed as a combination of liquid crystal molecules and polymer networks. When the LCE is heated above the critical temperature, it can generate large deformation because of the nematic-isotropic phase transition. However, in terms of the practical use of LCE, a few challenges exist such as lack of programmable operation and slow responsive speed for LCEs, which need to be addressed. In this dissertation, we first integrate flexible heating wire into LCE tube, forming electrically controlled soft tubular actuator. By selectively applying low electrical voltage, this soft tubular actuator can exhibit multiple actuation modes, such as different directional bending and homogeneous contraction. The LCE soft tubular actuator can also be integrated to construct untethered robot that can execute multiple functionalities. To address the slow responsive speed of LCE based soft actuator, we embed microfluidic channel into LCE, forming vascular LCE soft actuator. Through alternatively injecting hot and cold fluid into its internal fluidic channel, the vascular LCE soft actuator can generate fast actuation as well as recovery. In addition, by introducing the disulfide bonds into the LCE materials, the newly obtained vascular LCE based soft actuator has shown repairability and recyclability. Finally, we use electrospinning technique to fabricate LCE microfiber that can be actuated by NIR light. We demonstrate that the electrospun LCE fiber can be easily integrated to micro-robotic system and machine as artificial muscle fiber.
The subject of this book is the current comprehensive research and development of soft actuators, and encompasses interdisciplinary studies of materials science, mechanics, electronics, robotics and bioscience. As an example, the book includes current research on actuators based on biomaterials to provide future perspectives for artificial muscle technology. Readers can obtain detailed, useful information about materials, methods of synthesis, fabrication and measurements. The topics covered here not only promote further research and development of soft actuators but also lead the way to their utilization and industrialization. One outstanding feature of the book is that it contains many color figures, diagrams and photographs clearly describing the mechanism, apparatus and motion of soft actuators. The chapter on modeling is conducive to more extensive design work in materials and devices and is especially useful in the development of practical applications. Readers can acquire the newest technology and information about the basic science and practical applications of flexible, lightweight and noiseless soft actuators, which are quite unlike conventional mechanical engines and electric motors. The new ideas offered in this volume will provide inspiration and encouragement to researchers and developers as they explore new fields of applications for soft actuators.
Over 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.
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.
"Advancements in software engineering have enabled the robotics industry to transition from the use of giant industrial robots to more friendly humanoid robots. Soft robotics is one of the key elements needed to advance the transition process by providing a safer way for robots to interact with the environment. Electroactive polymers (EAPs) are one of the best candidate materials for the next generation of soft robotic actuators and artificial muscles. Lightweight dielectric elastomer actuators (DEAs) provide optimal properties such as high elasticity, rapid response rates, mechanical robustness and compliance. However, for DEAs to become widely used as artificial muscles or soft actuators, there are current limitations, such as high actuation voltage requirements, control of actuation direction, and scaling, that need to be addressed. This study presents a novel approach inspired by the natural skeletal muscles to overcome the drawbacks of conventional DEAs. Instead of fabricating a large DEA device, smaller sub-units can be fabricated and bundled together to form larger actuators, similar to the way myofibrils form myocytes in skeletal muscles. Soft lithography and other microfabrication techniques were utilized to allow fabrication of silicone based multilayer stacked DEA structures, composed of hundreds of micro-sized DEA units with mechanically compliant electrodes. Experiments show that free-standing multilayer DEA structures can be fabricated using existing microfabrication tools. Three fabrication approaches, using spin coating, film casting and injection molding were evaluated to improve the repeatability of the fabrication process. Multi-layer DEA fibers can be actuated in sub-kV range while maintaining actuation ratio above 5%."--Abstract.
Dielectric elastomer actuator (DEA) is a key element for the soft robots, which has received increasing attention. However, the main difficulties in modeling soft actuators such as dielectric elastomer actuators are time-dependent viscoelasticity and their material nonlinearity. It is important to consider the viscoelasticity of the dielectric elastomer (DE) to fully understand its mechanical behavior. However, so far only a few works have been presented considering the viscoelasticity of the DE material together with the effect of temperature and deformation. In this thesis, a dynamic electromechanical-coupled model for a rectangle dielectric elastomer a commonly used material (the acrylic elastomer VHB 4910) has been proposed, with taking into consideration of the influence of temperature, voltage, and frequency on the DE. The proposed model is based on the free energy physical-based principle, where the general Kelvin-Voigt model is applied to describe the viscoelasticity of the DE, and the Maxwell force together with the Electrostrictive force are considered. The influence of temperature and deformation on the DE is included in this model. The model in this study is a dynamic electromechanical model of a DE actuator, and can effectively describe the dynamic characteristics of the DE. By using the Differential Evolution, the model parameters were identified. The model was implemented and simulated in MATLAB, and the simulation and the actual experiment agrees to a great extent. The experimental test conducted in this study matches with the simulations results, which means that the proposed model can be practical to predict and describe DEAs electromechanical and viscoelastic behavior. Predicting the electromechanical and viscoelastic behavior of the DE is extremely useful for controlling a viscoelastic DEA and paving the way to improve the control performance, and also develops applications in soft robotics.
Soft robotic devices require highly stretchable and flexible materials to ensure that they can conform to delicate objects and work safely alongside humans. As a result, new flexible actuation technologies must be developed so that soft robotic platforms can achieve their desired function. Dielectric elastomer actuators are an emerging actuator technology that possesses high energy density in a lightweight package. However, since these actuators utilize a thin, soft elastomer film, they are particularly delicate and depending on their configuration, do not efficiently outcouple their outputted mechanical energy. Rigid frames, metallic springs, and other mechanisms have been used in the past to improve the performance of these actuators in various configurations, but these highly stiff components are not ideal for soft robotic applications. Novel strategies can be implemented to mitigate these issues. In this work, we highlight new strategies to create novel dielectric elastomer actuator configurations with high power density, large strokes, and high force output. First, we discuss the development of a core-free rolled dielectric elastomer. We then discuss a flexible DEA-based fluidic pump that can output high flowrates and withstand large pressures for its small size. Finally, we detail a novel mechanism to control the deformation direction of DEAs, and leverage this mechanism to create a patch-like haptic interface for virtual reality applications.
