Compliant, Large-Strain, and Self-Sensing Twisted String Actuators with Applications to Soft Robots

The twisted string actuator (TSA) is a rotary-to-linear transmission system that has been implemented in robots for high force output and efficiency. The basic components of a TSA are a motor, strings, and a load (to keep the strings in tension). The twisting of the strings shortens their length to generate linear contraction. Due to their high force output, energy efficiency, and compact form factor, TSAs hold the potential to improve the performance of soft robots. Currently, it is challenging to realize high-performance soft robots because many existing soft or compliant actuators exhibit limitations such as fabrication complexity, high power consumption, slow actuation, or low force generation. The applications of TSAs in soft robots have hitherto been limited, mainly for two reasons. Firstly, the conventional strings of TSAs are stiff and strong, but not compliant. Secondly, precise control of TSAs predominantly relies on external position or force sensors. For these reasons, TSA-driven robots are often rigid or bulky.To make TSAs more suitable for actuating soft robots, compliant, large-strain, and self-sensing TSAs are developed and applied to various soft robots in this work. The design was realized by replacing conventional inelastic strings with compliant, thermally-activated, and conductive supercoiled polymer (SCP) strings. Self-sensing was realized by correlating the electrical resistance of the strings with their length. Large strains are realized by heating the strings in addition to twisting them. The quasi-static actuation and self-sensing properties are accurately captured by Preisach hysteresis operators. Next, a data-driven mathematical model was proposed and experimentally validated to capture the transient decay, creep, and hysteretic effects in the electrical resistance. This model was then used to predict the length of the TSA, given its resistance. Furthermore, three TSA-driven soft robots were designed and fabricated: a three-fingered gripper, a soft manipulator, and an anthropomorphic gripper. For the three-fingered gripper, its fingers were compliant and designed to exploit the Fin Ray Effect for improved grasping. The soft manipulator was driven by three TSAs that allowed it to bend with arbitrary magnitude and direction. A physics-based modeling strategy was developed to predict this multi-degree-of-freedom motion. The proposed modeling approaches were experimentally verified to be effective. For example, the proposed model predicted bending angle and bending velocity with mean errors of 1.58 degrees (2.63%) and 0.405 degrees/sec (4.31%), respectively. The anthropomorphic gripper contained 11 TSAs; two TSAs were embedded in each of the four fingers and three TSAs were embedded in the thumb. Furthermore, the anthropomorphic gripper achieved tunable stiffness and a wide range of grasps

Soft pneumatic actuators: a review of design, fabrication, modeling, sensing, control and applications

Soft robotics is a rapidly evolving field where robots are fabricated using highly deformable materials and usually follow a bioinspired design. Their high dexterity and safety make them ideal for applications such as gripping, locomotion, and biomedical devices, where the environment is highly dynamic and sensitive to physical interaction. Pneumatic actuation remains the dominant technology in soft robotics due to its low cost and mass, fast response time, and easy implementation. Given the significant number of publications in soft robotics over recent years, newcomers and even established researchers may have difficulty assessing the state of the art. To address this issue, this article summarizes the development of soft pneumatic actuators and robots up until the date of publication. The scope of this article includes the design, modeling, fabrication, actuation, characterization, sensing, control, and applications of soft robotic devices. In addition to a historical overview, there is a special emphasis on recent advances such as novel designs, differential simulators, analytical and numerical modeling methods, topology optimization, data-driven modeling and control methods, hardware control boards, and nonlinear estimation and control techniques. Finally, the capabilities and limitations of soft pneumatic actuators and robots are discussed and directions for future research are identified

Theoretical analysis and simulations applied to rational design strategies of nanostructured materials

Orientador: Douglas Soares GalvãoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb WataghinResumo: Esse documento apresenta uma coleção de trabalhos realizados dentro do amplo campo de materiais nanoestruturados, focando-se em descrições teóricas analíticas e simulações computacionais de diversos novos materias desse tipo. Uma nova fibra supereslástica e condutora é reportada. Essa fibra altamente esticável (até 1320%) é criada envolvendo-se um núcleo cilíndrico de borracha com uma camada de folha de nanotubos de carbono. O material resultante exibe uma interessante estrutura de enrugamentos hierárquicos na sua superfície, o que lhe garante propriedades elétricas úteis como conservar a sua resistencia constante enquanto esticada. Adicionando-se mais camadas de borracha ou nanotubos podemos obter aplicações como sensores de movimento ou deformação, atuadores/músculos artificiais ativados por corrente ou temperatura e operados reversivelmente por um mecanismo de acoplamento entre tensão e torção. Nós explicamos suas propriedades de condução elétrica e os fenômenos físicos envolvidos em cada uma dessas aplicações. Também desenvolvemos um novo método para o desenho racional de polímeros molecularmente impressos usando dinâmica molecular para simular o processo de impressão molecular e a análise subsequente utilizando experimentos de cromatografia simulada. Obtivemos com sucesso a primeira evidência teórica do mecanismo de impressão exibindo afinidade e seletividade para a substância alvo 17-beta-estradiol. Desenhamos e simulamos uma nova estrutura com formato de piramide em kirigami de grafeno, composta de uma folha de grafeno cortada em um padrão específico a fim de formar uma pirâmide quando sofre tensão na direção normal ao plano. Nós calculamos a resposta dessa estrutura a uma carga estática, quando ela age como uma mola de proporções nanométriacs. Também, utilizando simulações de dinâmica molecular de colisões balísticas, constatamos que a resistência desse material a impactos é ainda maior que de uma folha de grafeno puro, sendo ainda mais leve. Um novo método de reforçar fios de nanotubos de carbono, chamado ITAP, também é reportado. Esse método foi capaz de melhorar a resistencia mecanica do fio em até 1,5 vezes e torná-lo muito mais resistente ao ataque de ácido quando comparado com um fio não tratado. Utilizamos simulações de dinâmica molecular para testar a hipótese de que esse tratamento é suficiente para gerar ligações covalentes entre as paredes externas de nanotubos diferentes, o que seria responsável pelas propriedades do material. Aplicamos um algoritmo genético modificado ao problema do folding de proteínas em um modelo de rede 3D HP. Testamos o algoritmo utilizando um conjunto de sequencias de teste que têm estado em uso pelos últimos 20 anos na literatura. Fomos capazes de melhorar um dos resultados e demonstramos a aplicação e utilidade de operadores não canônicos que evitam a convergência prematura do algoritmo, sendo eles o operador de compartilhamento e efeito maternalAbstract: This document presents a colection of works done within the broad subject of nano-structured materials, focusing on analytical theoretical descriptions and computational simulations of new kinds of this class of materials. A new superelastic conducting fiber is reported, with improved properties and functionalities. They are highly stretchable (up to 1320%) conducting fibers created by wrapping carbon nanotube sheets on stretched rubber fiber cores. The resulting structure exhibited an interesting hierarchical buckled structure on its surface. By including more rubber and carbon nanotube layers, we created strain sensors, and electrically or thermally powered tensile and torsional muscles/actuators operating reversibly by a coupled tension-to-torsion actuation mechanism. We explain its electronic properties and quantitatively explain the compounded physical effects involved in each of these applications. We also developed a new method for the rational design of molecularly imprinted polymers using molecular dynamics to simulate the imprinting process and subsequent chromatography studies. We successfully obtained the first theoretical evidence of actual imprinting happening under unconstrained simulations showing affinity and selectivity to the target substance 17-beta estradiol. We designed and simulated a new graphene kirigami pyramid structure, composed of a cut graphene sheet in a specific pattern in order to form a pyramid when under stress perpendicular to the plane. We calculated the response to static loading of this structure that acts like a nano-sized spring. Also, with simulated ballistic collisions we obtained increased resistance to impact in comparison with a pure graphene sheet. A new method of strengthening carbon nanotube yarns, called ITAP, consisting of annealing at high temperature in vacuum is reported. This method is shown to increase the mechanical resistance of the wire up to 1.5 times and make it much more resistant to acid corrosion when compared to pristine non-treated wires. We applied a modified genetic algorithm to the protein folding problem using an 3D HP lattice model using known test sequences that have been in use for the last 20 years and obtained an improvement for the best solution found for one of these proteins. Also, the importance of new non-canonical operators that prevent rapid convergence of the algorithm was demonstrated, namely the Sharing and Maternal Effect operatorsDoutoradoFísicaDoutor em Ciências141198/2012-5CNP

Soft actuator and agile soft robot

Robots play an important part in many aspects of our society by doing repetitive, dangerous, or precision tasks. Most existing robots are made of rigid components, which lack passive compliance and pose a challenge in adapting to the environment and safe human-robot interaction. Rigid robots may be equipped with sensors and programmed with proprioceptive feedback control to achieve active compliance, but this may fail in the event of unforeseen situations or sensor failure. In contrast, animals have evolved flexible or soft body parts to help them adapt to changing environments. Soft robotics is an emerging field in robotics, drawing inspiration from nature by integrating soft material into the actuator and mechanical design. With the inclusion of soft material, soft actuators and robots can deform actively/passively, making it possible to sense, absorb impact, and adapt to its environment with deformation. However, while soft actuators/robots have superior properties to rigid ones, they are often challenging to manufacture and control precisely. In addition, they may suffer from slow speed and material degradation. Thus, in this thesis, we aim to address the issues in developing high-performance soft actuators and soft robots. The thesis is divided into two parts. In the first part, we focus on improving the manufacturability and performance of a self-contained soft actuator originated in the Creative Machines Lab. The soft actuator is composed of a cured silicone-ethanol mixture embedded with heating coils. When the coils are electrically actuated, ethanol trapped inside undergoes liquid-vapor transitions, and thus the actuator undergoes extreme volume change. While this actuator exhibits high strain and high stress, it is very slow to actuate, has limited life cycles, and requires molds to manufacture. The first part of the thesis will address these issues. Specifically, in chapter 2, we discuss using multi-material 3D printing to automate the manufacturing of silicone-ethanol composite. In chapter 3, we discuss using laser-cut flexible Kirigami patterns to improve the manufacturability of its heating element. Chapter 4 characterizes its actuation profile and addresses improvements to the thermal conductivity by infusing thermally conductive fillers. Soft actuation is an actively researched area; however, many high-performance soft actuators are challenging to manufacture and thus are less accessible to the general robotics community. Conventional actuators such as electric motors are widely available but lack flexibility. Therefore, the second part of the thesis aims at combining rigid motors with soft materials to design and control high-performance hybrid soft robots. Simulation is a good way to evaluate and optimize robot design and control. However, existing simulators that support motor-driven soft robots have limited features. Chapter 5 discusses this issue and presents a physically based real-time soft robot simulator capable of simulating motor-driven soft robots. In addition, chapter 5 presents the design and control of a 3D printed hybrid soft quadruped robot. Chapter 6 presents the design and control of a 3D printed hybrid soft humanoid robot. The two parts of the thesis aim to improve aspects in soft actuators and soft robots. In conclusion, we summarize the lessons learned in developing soft actuators/robots and new possibilities and challenges for advancing soft robotics research

Modeling and Programming Shape-Morphing Structured Media

Shape-morphing and self-propelled locomotion are examples of mechanical behaviors that can be "programmed" in structured media by designing geometric features at micro- and mesostructural length scales. This programmability is possible because the small-scale geometry often imposes local kinematic modes that are strongly favored over other deformations. In turn, global behaviors are influenced by local kinematic preferences over the extent of the structured medium and by the kinematic compatibility (or incompatibility) between neighboring regions of the domain. This considerably expands the design space for effective mechanical properties, since objects made of the same bulk material but with different internal geometry will generally display very different behaviors. This motivates pursuing a mechanistic understanding of the connection between small-scale geometry and global kinematic behaviors. This thesis addresses challenges pertaining to the modeling and design of structured media that undergo large deformations. The first part of the thesis focuses on the relation between micro- or mesoscale patterning and energetically favored modes of deformation. This is first discussed within the context of twisted bulk metallic glass ribbons whose edges display periodic undulations. The undulations cause twist concentrations in the narrower regions of the structural element, delaying the onset of material failure and permitting the design of structures whose deployment and compaction emerge from the ribbons' chirality. Following this discussion of a periodic system, we study sheets with non-uniform cut patterns that buckle out-of-plane. Motivated by computational challenges associated with the presence of geometric features at disparate length scales, we construct an effective continuum model for these non-periodic systems, allowing us to simulate their post-buckling behavior efficiently and with good accuracy. The second part of the thesis discusses ways to leverage the connection between micro/mesoscale geometry and energetically favorable local kinematics to create "programmable matter" that undergo prescribed shape changes or self-propelled locomotion when exposed to an environmental stimulus. We first demonstrate the capabilities of an inverse design method that automates the design of structured plates that morph into target 3D geometries over time-dependent actuation paths. Finally, we present devices made of 3D-printed liquid crystal elastomer (LCE) hinges that change shape and self-propel when heated.</p

Modeling, simulation, and control of soft robots

2019 Fall.Includes bibliographical references.Soft robots are a new type of robot with deformable bodies and muscle-like actuations, which are fundamentally different from traditional robots with rigid links and motor-based actuators. Owing to their elasticity, soft robots outperform rigid ones in safety, maneuverability, and adaptability. With their advantages, many soft robots have been developed for manipulation and locomotion in recent years. However, the current state of soft robotics has significant design and development work, but lags behind in modeling and control due to the complex dynamic behavior of the soft bodies. This complexity prevents a unified dynamics model that captures the dynamic behavior, computationally-efficient algorithms to simulate the dynamics in real-time, and closed-loop control algorithms to accomplish desired dynamic responses. In this thesis, we address the three problems of modeling, simulation, and control of soft robots. For the modeling, we establish a general modeling framework for the dynamics by integrating Cosserat theory with Hamilton's principle. Such a framework can accommodate different actuation methods (e.g., pneumatic, cable-driven, artificial muscles, etc.). To simulate the proposed models, we develop efficient numerical algorithms and implement them in C++ to simulate the dynamics of soft robots in real-time. These algorithms consider qualities of the dynamics that are typically neglected (e.g., numerical damping, group structure). Using the developed numerical algorithms, we investigate the control of soft robots with the goal of achieving real-time and closed-loop control policies. Several control approaches are tested (e.g., model predictive control, reinforcement learning) for a few key tasks: reaching various points in a soft manipulator's workspace and tracking a given trajectory. The results show that model predictive control is possible but is computationally demanding, while reinforcement learning techniques are more computationally effective but require a substantial number of training samples. The modeling, simulation, and control framework developed in this thesis will lay a solid foundation to unleash the potential of soft robots for various applications, such as manipulation and locomotion

Model Based Control of Soft Robots: A Survey of the State of the Art and Open Challenges

Continuum soft robots are mechanical systems entirely made of continuously deformable elements. This design solution aims to bring robots closer to invertebrate animals and soft appendices of vertebrate animals (e.g., an elephant's trunk, a monkey's tail). This work aims to introduce the control theorist perspective to this novel development in robotics. We aim to remove the barriers to entry into this field by presenting existing results and future challenges using a unified language and within a coherent framework. Indeed, the main difficulty in entering this field is the wide variability of terminology and scientific backgrounds, making it quite hard to acquire a comprehensive view on the topic. Another limiting factor is that it is not obvious where to draw a clear line between the limitations imposed by the technology not being mature yet and the challenges intrinsic to this class of robots. In this work, we argue that the intrinsic effects are the continuum or multi-body dynamics, the presence of a non-negligible elastic potential field, and the variability in sensing and actuation strategies.Comment: 69 pages, 13 figure