Ionic Systems for Electromechanical Transducers: Energy Harvesting and Soft Robotics

As technology becomes more integrated into our daily lives, there is an ever-increasing demand for versatile and cost-effective electromechanical transducers which convert mechanical energy into electrical energy or vice versa and are used as [i] actuators to produce movement, [ii] sensors that react to mechanical stimuli, and [iii] generators to produce electricity. Traditional electromechanical transducers are comprised of metals, magnets, and electrical conductors which are highly capable in select applications with controlled and predictable environments, but also ill-suited for applications such as interactions with humans and operation at low frequencies. However, biological systems, through millions of years of evolution, can easily function in these applications and countless others, due to the use of soft and compliant materials and ionic systems, enabling wide adaptability and versatility. This work takes inspiration from nature to develop new electromechanical transducers which incorporate soft and flexible materials with ionic conductors for applications in energy generation and soft robotics. The first part of this work introduces a variable electric double layer (EDL) generator, an energy generation system that harnessed mechanical motion to influence the capacitance of a charged EDL and increase the electrical energy. Traditional electromechanical transducers for energy generation rely on electromagnetic principles that are ill-suited to capture low frequency mechanical motion. Conversely the variable EDL generator introduced in this work provides a model system in which all experimental parameters are easy to access in order to gain a detailed understanding of the energy flows of this conversion process rooted in electrostatic principles. The system uses titanium electrodes in NaCl aqueous electrolyte and operates as a charge pump. Analysis of the voltage-charge work-conjugate plane confirmed net positive electrical energy generation and also enabled exploration of possible avenues to improve the performance of the variable EDL generator through materials and electrical circuit optimization. An outlook for the variable EDL generator is discussed in which external mechanical input such as oscillating ocean waves could drive the generator. The second part of this work continues to take inspiration from nature by again using ionic conductors and combining them with soft and flexible materials to create robotic actuators capable of producing life-like motion. This work combines the benefits and avoids the pitfalls of two prior muscle-mimetic soft actuators through the marriage of electrostatic and hydraulic principles to introduce a new material system termed a hydraulically amplified self-healing electrostatic (HASEL) actuator. A key characteristic of HASEL actuators is the ability to electrically self-healing from dielectric breakdown due to the use of a liquid dielectric. Additional work shows that the capacitive structure of HASEL actuators enables the ability to simultaneously actuate and sense capacitance (and thus position). Another key characteristic of an actuator is the efficiency, and thus the efficiency of HASEL actuators is experimentally measured both for the electromechanical transduction process as well as the full system efficiency of a HASEL actuator driven by a portable driving electronics which will be especially valuable when exploring portable applications. Lastly an outlook for HASEL actuators is discussed with possible enhancements in material selection, self-healing, self-sensing, and efficiency. The third part of this work aims to introduce a new multifunctional material &ndash; a transparent, self‐healing, highly stretchable ionic conductor &ndash; that is used in dielectric elastomer actuators (DEA), another soft robotic actuator. This work demonstrates the ability of a DEA made with the self-healing material to mechanically self-heal from severe mechanical damage and continue to function, which was shown in contrast to a DEA made with another type of ionic conductor that does not have the ability to self-heal. We also performed experimental analysis of mechanical and electrical properties of the self-healing material. Possible follow on work is suggested to enhance the self-healing abilities of both dielectric elastomer actuators as well as HASEL actuators. In closing, this work explores the transition of HASEL actuators from the academic lab into commercial applications. Key considerations toward this goal are discussed, including the technology readiness level of HASEL actuators, remaining technical challenges, and immediate next steps to study fabrication and lifetime.</p

The Development and Control of Soft Robotic Materials Driven by Hydraulically Amplified Self-Healing Electrostatic (HASEL) Actuators

Soft robotics is a growing research area focused on the development of compliant, adaptable, and bio-inspired robotic systems. Compared to traditional robotic solutions, soft robots are better suited for medical devices, wearable electronics, human-robot interaction, and other unique applications. The use of compliant materials enables design simplicity and bio-inspiration as well as entirely new functionalities not present in rigid robotic solutions.Electrostatic actuators are an effective way to drive soft robotic motion because of their low cost, mechanical simplicity, and high actuation bandwidth. A specific class of electrostatic actuator, the Hydraulically Amplified Self-healing Electrostatic (HASEL) actuator, further improves performance. However, system integration of HASEL actuator-driven robots is lacking. One approach to solving this problem is through the use of robotic materials which integrate actuation, sensing, communication, and control through a scalable constituent unit. Developing a HASEL actuator-driven soft robotic material would enable the creation of high degree of freedom robots with increased functionality. To do so, several challenges related to the sensing and feedback control of HASEL actuators must first be addressed.This thesis describes research efforts in the design and control of HASEL actuator-driven systems. Chapter 1 presents a literature review of soft robotics, soft actuators, and introduce the concept of robotic materials. Chapter 2 then presents a system identification and control technique for a single HASEL actuator. The system integrates a soft capacitive sensor onto the actuator. Then the control of a multi-HASEL-actuator robot using a novel magnetic sensing mechanism is presented in Chapter 3. Chapter 4 builds upon these results by introducing sTISSUE, a soft robotic material using HASEL actuators and the magnetic sensing mechanism. Multiple advanced demonstrations highlight the capabilities of this robotic material. Chapter 5 presents a multi-functional artificial potential field control method to enable highly controllable object manipulation with actuator arrays. Finally, Chapter 6 provides concluding statements and suggested next steps

Controller Development, Decoupled Sensing Methods, and Scalable Sensor Fabrication Methods for Hydraulically Amplified Self-Healing Electrostatic (HASEL) Actuators

Robotics is entering a new realm of possibilities. The technology associated with traditional robotics is moving away from their rigid and linear predecessors. Soft materials are being used to design mechanisms and robotic bodies. New avenues of robot dynamics and control theory are being opened by the use of non-linear materials in new fabrication methods.The work presented in this thesis highlights efforts to harness and control the untapped potential contained within soft robotics. First done through controlling a planar HASEL actuator. The methods and controller developed highlight the challenges in soft robotics but show a promising future through countering time dependent non-linear properties of rubber. Those challenges come from the self-sensing methodology used with soft actuators. Motivating an investigation into decoupled sensing methods. A decoupled sensing method that is highly accurate and repeatable is developed in the second section of this work. Within that section a look at scalability is discussed as many of the sensors in soft robotics today are time and labor intensive. The results and conclusions arrived at from this work paint a promising picture for these new and exciting robots and sensors by showing that both the time dependent properties in rubber can be countered and accurate sensing methods can be established.</p

Towards Untethered Soft Robots Driven By Electrohydraulic Artificial Muscles

As humans, we are continually integrating technology into our everyday lives. From wearable smart watches to autonomous vacuum cleaners, modern day machines are steadily moving out of warehouses and factories and into our homes to enhance our lifestyles. In doing so, there is an ever-growing need for machines that can safely operate in extremely diverse or unpredictable environments, which often includes collaborative spaces near humans. This requirement presents challenges for traditional robots that commonly employ rigid architectures driven by heavy motors, gears, and linkages, which rely on precise computation of the state at each degree of freedom to safely function. Moreover, the underlying mechanics of these modern-day robotic architectures are fundamentally different than those which have evolved naturally; biological organisms exploit a host of compliant, robust, and multifunctional structures that tightly integrate actuation, sensing, and control. These biological structures, in animals for instance, enable feats of strength, agility, and autonomy that are currently impossible for human-made robots. A paradigm shift in robotic design and implementation is required for the next generation of machines. This approach will reinvent the idea of a robot, moving from a rigid block design to a soft continuum that integrates lightweight, compliant, and versatile components. While this approach will require multi-disciplinary advances in material science, control theory, and engineering, a fundamental component of these machines will ultimately be the actuators that drive them. Thus, researchers and engineers are developing soft actuators that mimic the strength, speed, and scalability of natural muscle. These bio-inspired components could unlock a multitude of applications for machines, and even blur the lines between science and science fiction. For example, soft wearable robots can provide haptic feedback for an immersive virtual reality experience, ultra-adaptable soft robotic cephalopods could explore marine environments to conduct research and reconnaissance, while highly resilient space robots could explore extraterrestrial environments to uncover the origins of life. This dissertation is focused on a novel type of soft actuator (or artificial muscle) called a Hydraulically Amplified Self-healing ELectrostatic (HASEL) actuator, and its application to soft robotics devices. The first chapter will explore the state-of-the-art in soft robotics technologies, with a focus on soft actuation. The second chapter will elucidate the fundamentals of HASEL actuators and their application to soft robotic technologies. The third chapter will detail a toolkit based on off-the-shelf-materials that can be used to prototype, fabricate, power, and test HASEL actuators. This chapter will detail exemplary designs of HASEL, their modes of actuation and performance, as well as their application to soft-robotic devices such as a continuum robot capable of grasping and manipulating delicate objects. Continuing to the fourth chapter, a novel design for a linearly contractile actuator is presented, characterized on both an experimental and theoretical basis, and then demonstrated as a soft tubular pump. In the fifth chapter, we develop a state-of-the-art 10-channel high voltage power supply to independently control groups of HASEL actuators. This power supply features a compact form-factor that is about the size of a standard smart phone. Next, chapter six will focus on soft robots for space exploration. A feasibility study details a robot design for asteroid mining, and initial prototypes are discussed with a focus on electrohydraulic actuation and electrostatic adhesion mechanisms for robot locomotion and grappling. Finally, chapter seven concludes the dissertation with a summary of the developments presented here, while also laying the framework for future studies.</p

Electromechanical Performance of HASEL Actuators: Fundamentals and Applications

Soft robotic systems are well-suited to unstructured, dynamic tasks and environments, but are currently limited by the soft actuators that power them. Most current soft actuators are based on pneumatics or shape-memory alloys, which have issues with efficiency, response speed, and portability. More recently, dielectric elastomer actuators (DEAs) have shown promise, but they have limited material selection, fabrication methods, and modes of actuation. As such, there remains a need for new types of high performance and well-rounded soft actuators. This dissertation focuses on the development and exploration of a new class of soft electrohydraulic actuators called hydraulically amplified self-healing electrostatic (HASEL) actuators. The first part of this dissertation (Chapter 2) presents of a subclass of HASEL actuators called Peano-HASELs that simultaneously introduce a breakthrough materials system based on thermoplastic films as well as a novel contractile mode of actuation. This approach enables industrially-amenable fabrication techniques, vastly expands the usable materials for actuator construction, and results in high performance actuators with fast linear contraction on activation. The second part of this dissertation (Chapter 3) elucidates the fundamentals of the electromechanical coupling that drives HASEL actuators. An analytical model is developed that accurately describes the quasi-static actuation behavior of Peano-HASEL actuators without relying on fitting parameters. Using this model, we identify a theory-driven approach to actuator design, including a roadmap for actuators with drastically improved specific energies. The final section of this dissertation (Chapter 4 and 5) looks towards more integrated and applied designs of HASEL actuators. First, a new type of articulating actuator is presented that integrates both compliant and rigid components. These spider-inspired electrohydraulic soft-actuated (SES) joints demonstrate high torque and high-speed actuation in an independently-addressable multi-joint limb, a bidirectional actuator, and a versatile gripper. Second, a biodegradable materials system is presented for high performance Peano-HASEL actuators that reduce environmental impact. Finally, a new method of capacitive self-sensing is presented that enables inexpensive and compact circuits that use only off-the-shelf and low voltage components. The results presented in this dissertation provide a framework for the development of high-performance actuators that may one day power the next generation of capable soft robots.</p

Embedded Sensing and Control for High-Speed Electrohydraulic Soft Robots

Soft robotics is a field of robotic system design characterized by materials and structures that exhibit large-scale deformation, high compliance, and rich multifunctionality. The incorporation of soft and deformable structures endows soft robotic systems with the compliance and resiliency that makes them well-adapted for unstructured and dynamic environments. While actuation mechanisms for soft robots vary widely, soft electrostatic transducers such as dielectric elastomer actuators (DEAs) and hydraulically amplified self-healing electrostatic (HASEL) actuators have demonstrated promise due to their muscle-like performance. Despite significant leaps in design and modeling of HASEL actuators thus far, the actuators in and of themselves are limited in terms of estimating their own states and individually lacks useful system-level functionalities. To address the mentioned shortcomings, this body of research has enabled embedded perception and intelligence to electromechanical systems driven by electro-hydraulic actuators, by (i) developing capacitive self-sensing and magnetic sensing techniques to estimate shape changes of electro-hydraulic actuators in real-time; (ii) designing and implementing hardware and software controllers for an array of electrohydraulic actuators to enable complex motions and their collective useful functionalities; and (iii) demonstrating the integration of embedded sensing and control to bring soft robots driven by these actuators closer to practical, real-world applications. While not exhaustive, the introduced system technologies provide the fundamental building blocks for more advanced functionalities and features that can potentially integrate HASEL actuators to augment and enrich our daily lives

Overtwisting and Coiling Highly Enhances Strain Generation of Twisted String Actuators

Twisted string actuators (TSAs) have exhibited great promise in robotic applications by generating high translational force with low input torque. To further facilitate their robotic applications, it is strongly desirable but challenging to enhance their consistent strain generation while maintaining compliance. Existing studies predominantly considered overtwisting and coiling after the regular twisting stage to be undesirable non-uniform and unpredictable knots, entanglements, and coils formed to create an unstable and failure-prone structure. Overtwisting would work well for TSAs when uniform coils can be consistently formed. In this study, we realize uniform and consistent coil formation in overtwisted TSAs, which greatly increases their strain. Furthermore, we investigate methods for enabling uniform coil formation upon overtwisting the strings in a TSA and present a procedure to systematically "train" the strings. To the authors' best knowledge, this is the first study to experimentally investigate overtwisting for TSAs with different stiffnesses and realize consistent uniform coil formation. Ultra-high molecular-weight polyethylene (UHMWPE) strings form the stiff TSAs whereas compliant TSAs are realized with stretchable and conductive supercoiled polymer (SCP) strings. The strain, force, velocity, and torque of each overtwisted TSA was studied. Overtwisting and coiling resulted in approximately 70% strain in stiff TSAs and approximately 60% strain in compliant TSAs. This is more than twice the strain achieved through regular twisting. Lastly, the overtwisted TSA was successfully demonstrated in a robotic bicep

Bioinspired Soft Robotics: state of the art, challenges, and future directions

Purpose of Review: This review provides an overview of the state of the art in bioinspired soft robotics with by examining advancements in actuation, functionality, modeling, and control. Recent Findings: Recent research into actuation methods, such as artificial muscles, have expanded the functionality and potential use of bioinspired soft robots. Additionally, the application of finite dimensional models has improved computational efficiency for modeling soft continuum systems, and garnered interest as a basis for controller formulation. Summary: Bioinspiration in the field of soft robotics has led to diverse approaches to problems in a range of task spaces. In particular, new capabilities in system simplification, miniaturization, and untethering have each contributed to the field's growth. There is still significant room for improvement in the streamlining of design and manufacturing for these systems, as well as in their control

The-state-of-the-art of soft robotics to assist mobility: a review of physiotherapist and patient identified limitations of current lower-limb exoskeletons and the potential soft-robotic solutions

Background: Soft, wearable, powered exoskeletons are novel devices that may assist rehabilitation, allowing users to walk further or carry out activities of daily living. However, soft robotic exoskeletons, and the more commonly used rigid exoskeletons, are not widely adopted clinically. The available evidence highlights a disconnect between the needs of exoskeleton users and the engineers designing devices. This review aimed to explore the literature on physiotherapist and patient perspectives of the longer-standing, and therefore greater evidenced, rigid exoskeleton limitations. It then offered potential solutions to these limitations, including soft robotics, from an engineering standpoint. Methods: A state-of-the-art review was carried out which included both qualitative and quantitative research papers regarding patient and/or physiotherapist perspectives of rigid exoskeletons. Papers were themed and themes formed the review’s framework. Results: Six main themes regarding the limitations of soft exoskeletons were important to physiotherapists and patients: safety; a one-size-fits approach; ease of device use; weight and placement of device; cost of device; and, specific to patients only, appearance of the device. Potential soft-robotics solutions to address these limitations were offered, including compliant actuators, sensors, suit attachments fitting to user’s body, and the use of control algorithms. Conclusions: It is evident that current exoskeletons are not meeting the needs of their users. Solutions to the limitations offered may inform device development. However, the solutions are not infallible and thus further research and development is required