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External anatomy of the caterpillar, Manduca sexta, which is an excellent model system for understanding how soft animals control their movements. The abdominal segments are labelled A1-A7 with the most posterior segment designated the TS. The main gripping appendages are the prolegs found on A3-A6 and a specialised pair on the TS.
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The development of truly biomimetic robots requires that soft materials be incorporated into the mechanical design and also used as an integral part of the motor control system. One approach to this challenge is to identify how soft animals control their movements and then apply the found principles in robotic applications. Here I show an example o...
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... caterpillars are an excellent model system for studying the neuromechanics of soft-bodied movements. Their movements are achieved through the coordination of concatenated segments (Figure 1) each containing ≈70 distinct muscles. Because Manduca larvae metamorphose into adult moths having jointed limbs, their muscles are organised into individual, discrete units each with distinct orientations and attachments analogous to the muscular organisation of vertebrates ( Figure 2A). ...
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... three-dimensional kinematic study of straight-line crawling shows that caterpillars do not move by wormlike peristalsis (Trimmer and Issberner 2007). Waves of movement pass from the terminal segment (TS; Figure 1) towards the head, and there is a transition in the kinematics between posterior segments and those in the midbody. The TS and adjacent abdominal segment (A7) are lifted and pulled forward into stance phase; the segments then pivot around the terminal proleg (TP) attachment point in a motion that resembles an inverted pendulum. ...
Citations
... Soft metamaterials can integrate energy harvesting functionality, which sheds light on the broad application potential of soft robotic technology [10,11,12,13]. However, to truly take advantage of soft materials and hyper-redundancy in soft robots, controllability is required [14], which in turn requires novel sensors and design techniques. Salient examples of highly flexible structures for locomotion in nature are found in the water [15]. ...
Soft robotics can be used not only as a means of achieving novel, more lifelike forms of locomotion, but also as a tool to understand complex biomechanics through the use of robotic model animals. Herein, the control of the undulation mechanics of an entirely soft robotic subcarangiform fish is presented, using antagonistic fast-PneuNet actuators and hyperelastic eutectic gallium–indium (eGaIn) embedded in silicone channels for strain sensing. To design a controller, a simple, data-driven lumped parameter approach is developed, which allows accurate but lightweight simulation, tuned using experimental data and a genetic algorithm. The model accurately predicts the robot’s behavior over a range of driving frequencies and a range of pressure amplitudes, including the effect
of antagonistic co-contraction of the soft actuators. An amplitude controller is prototyped using the model and deployed to the robot to reach the setpoint of a tail-beat amplitude using fully soft and real-time strain sensing.
... Unfortunately, soft-materials introduce considerable elasticity and deformability, resulting in robots with nearly infinite degrees of freedom, and significant dynamical complexities. As a result, it can often be difficult to find effective controllers for soft robots, particularly mobile soft robots [2], [4], [6]. Consequently, most soft robotic locomotive behaviors are therefore developed by hand through empirical trial-anderror [6], and their actions tend to be limited to a single gait. ...
... The process of discovering locomotive behaviors for soft robots is often accomplished through empirical trial-and-error on physical robots [6], a method that can be both challenging and time consuming. As a consequence, the ensuing behaviors tend to either be slow, and quasi-static, or else largely uncontrolled [4]- [6], [13]. The use of ad hoc and hard-coded controllers also means that these soft robots are generally unable to autonomously adapt to internal or external changes, for instance when they are physically damaged, or when they encounter a unknown terrain. ...
Mobile soft robots offer compelling applications in fields ranging from urban search and rescue to planetary exploration. A critical challenge of soft robotic control is that the nonlinear dynamics imposed by soft materials often result in complex behaviors that are counterintuitive and hard to model or predict. As a consequence, most behaviors for mobile soft robots are discovered through empirical trial and error and hand-tuning. A second challenge is that soft materials are difficult to simulate with high fidelity -- leading to a significant reality gap when trying to discover or optimize new behaviors. In this work we employ a Quality Diversity Algorithm running model-free on a physical soft tensegrity robot that autonomously generates a behavioral repertoire with no a priori knowledge of the robot dynamics, and minimal human intervention. The resulting behavior repertoire displays a diversity of unique locomotive gaits useful for a variety of tasks. These results help provide a road map for increasing the behavioral capabilities of mobile soft robots through real-world automation.
... 3 Taking inspiration from the natural world, the field of soft robotics seeks to address some of the constraints of conventional rigid robots through the use of compliant, flexible, and elastic materials. 4,5 Trimmer et al., for instance, construct soft robots from silicone rubber, using shape memory alloy microcoil actuation, which can slowly crawl in a controlled manner 6 or roll in an uncontrolled ballistic manner. 7 Similarly, research by Whitesides et al. uses pneumatic inflation to produce slow, dynamically stable crawling motions 8 as well as fast, but less controlled tentacle-like grippers, 9 combustiondriven jumpers 10 and a self-contained microfluidic ''octobot.'' ...
... 4,8 They are by their very nature high dimensional dynamic systems with an essentially infinite number of degrees of freedom. The elasticity and deformability that provide their appeal come at the cost of resonances and tight dynamic coupling between components, 6 properties that are often avoided, or at least suppressed, in conventional engineering approaches to robotic design. This complexity precludes the use of many of the traditional kinematic and inverse-dynamics approaches to robotic control. ...
Living organisms intertwine soft (e.g., muscle) and hard (e.g., bones) materials, giving them an intrinsic flexibility and resiliency often lacking in conventional rigid robots. The emerging field of soft robotics seeks to harness these same properties to create resilient machines. The nature of soft materials, however, presents considerable challenges to aspects of design, construction, and control-and up until now, the vast majority of gaits for soft robots have been hand-designed through empirical trial-and-error. This article describes an easy-to-assemble tensegrity-based soft robot capable of highly dynamic locomotive gaits and demonstrating structural and behavioral resilience in the face of physical damage. Enabling this is the use of a machine learning algorithm able to discover effective gaits with a minimal number of physical trials. These results lend further credence to soft-robotic approaches that seek to harness the interaction of complex material dynamics to generate a wealth of dynamical behaviors.
... The softness of the body introduces compliance into the system, enabling the ability to reproduce animal-like motion. This compliant nature adds a sense of 'self-correction' into the system which enables environmental adaptation that hard robots do not have [19]. This body softness also reduces the damage caused by collision with the human body which enables safe human-robot interaction [20]. ...
... This is a major challenge for modelling the physical system. Trimmer had attempted to use this non-linearity as an advantage to generate adaptive and robust control for the soft robot [19]. Generally, soft robots are light weighted due to the absence of the hard skeleton and supporting structures seen in hard robots. ...
... These materials can be classified into two categories: soft materials used for the robot body and soft materials used as soft actuators. Silicone elastomer or silicone rubber is the most common soft material used for the body of soft robots [7,8,12,15,16,19,[26][27][28]. This material can be fabricated into different shapes and structures with mould and casting processes. ...
Soft robotics is an expanding new research field. This article presents a state-of-the-art review on soft robotics including its research directions, key characteristics, materials, design and fabrication techniques. Although many biomimetic soft robots have been developed, a few of these have industrial applications. This article proposes a soft XY machine table for the purpose of object manipulation. The proposed table combines the concepts from three areas: soft robotics, object manipulation and industrial application. The surface of proposed table is entirely soft and embedded with inflatable air chambers. Surface deformation is generated by inflating these chambers. One object manipulation approach is to generate travelling waves on the deformable surface.
... Soft-bodies offer exciting potential for new types of gaits that exploit the properties of new materials such as compressibility, ability to propagate elastic waves and create resonances. That however requires new methods of control that make use of these phenomena rather than strive to avoid them, as is the case in rigid robotics [6,7]. It is thus no surprise that several authors have already proposed to use evolutionary algorithms to either evolve controllers for soft-bodied animats (see, e.g., [7]) or even morphologies with controllers as a way of automatically discovering original designs [8,9,10,11]. ...
We present a system that combines human creativity and imagination with artificial evolution in order to produce efficient gaits for prespecified morphologies of soft-bodied robots. A designer is free to propose a shape for the robot for which a control mechanism is then found automatically. We assume that the animats are made of a soft material capable of locally contracting and expanding and represent them as 2-D triangular meshes. Animat motion is simulated in a physics engine as a spring-mass system with pressurized body regions. Movement is possible owing to evolved patterns of contraction amplitudes, frequencies and phase shifts, with each body region capable of maintaining its own, independent rhythm of contractions. We analyze evolved gait controllers for terrestrial and aquatic environments for different hand-drawn morphologies and investigate a scenario in which evolution has to combine active material with a passively elastic one to produce energy and resource efficient robots. Our results show how evolving distributed actuation mechanism is a powerful method for producing gaits for elastic bodies that could find its applications in the emerging field of soft-robotics or as a method of automatic animation of video game characters.
... This research may also provide insights important for the design of burrowing soft robots (e.g. Trimmer, 2008;Trivedi et al., 2008;Daltorio et al., 2013). ...
Many soft-bodied invertebrates use a flexible, fluid-filled hydrostatic skeleton for burrowing. The aim of our study was to compare the scaling and morphology between surface-dwelling and burrowing earthworm ecotypes to explore the specializations of non-rigid musculoskeletal systems for burrowing locomotion. We compared the scaling of adult lumbricid earthworms across species and ecotypes to determine if linear dimensions were significantly associated with ecotype. We also compared the ontogenetic scaling of a burrowing species, Lumbricus terrestris, and a surface-dwelling species, Eisenia fetida, using glycol methacrylate histology. We found that burrowing species were longer, thinner, and had higher length-to-diameter ratios than non-burrowers, and that L. terrestris was thinner for any given body mass compared to E. fetida. We also found differences in the size of the musculature between the two species that may correlate with surface crawling or burrowing. Our results suggest that adaptations to burrowing for soft-bodied animals include a disproportionately thin body and strong longitudinal muscles.
© 2015. Published by The Company of Biologists Ltd.
... Unfortunately, the very properties that make soft robots so appealing also introduce significant obstacles, especially in the domains of design and control. Elasticity and deformability come at the cost of resonances and tight dynamic coupling between components [24]-properties that are often assiduously avoided in conventional engineering approaches to robotic design. Small changes to the elasticity of a soft robot can cause unexpectedly large changes in performance. ...
... One of the more fundamental choices in designing soft robots is in picking the placement of linear actuators. In our early design of soft robots [24] there were two assumptions that we took for granted, both influenced by biomimicry. The first is that muscle placement should be bilaterally symmetric, that is, that the left and right sides of the robot should be identical. ...
Abstract Completely soft and flexible robots offer to revolutionize fields ranging from search and rescue to endoscopic surgery. One of the outstanding challenges in this burgeoning field is the chicken-and-egg problem of body-brain design: Development of locomotion requires the preexistence of a locomotion-capable body, and development of a location-capable body requires the preexistence of a locomotive gait. This problem is compounded by the high degree of coupling between the material properties of a soft body (such as stiffness or damping coefficients) and the effectiveness of a gait. This article synthesizes four years of research into soft robotics, in particular describing three approaches to the co-discovery of soft robot morphology and control. In the first, muscle placement and firing patterns are coevolved for a fixed body shape with fixed material properties. In the second, the material properties of a simulated soft body coevolve alongside locomotive gaits, with body shape and muscle placement fixed. In the third, a developmental encoding is used to scalably grow elaborate soft body shapes from a small seed structure. Considerations of the simulation time and the challenges of physically implementing soft robots in the real world are discussed.
... One very tractable system that has been exploited to study these phenomena is the caterpillar, Manduca sexta, which is the larval stage of a large moth (Fig. 1). Some of the unique technical advantages of these animals have already been discussed [31]. However, they also possess characteristics that are of particular interest in spacebased applications. ...
... Fig. 1(b) shows the two phases and relative timing of proleg movement during one step cycle. Different muscles, acting like motors, are phase-delayed in different segments [9]. ...
This paper presents a novel mechanism to implement caterpillar-like locomotion. First, the caterpillar-like locomotive pattern in nature is investigated and analyzed systematically. From a biological point of view, caterpillar locomotion can be abstracted as a body wave, called half wave. It is simple, but efficient. In this paper, a novel control mechanism, maintaining the half wave property, is integrated into an improved central pattern generator (CPG) model. For the first time, an asymmetric oscillation is employed on the model for gait generation. The movement is proved stable according to a kinematic analysis. Modulation is able to use to change the shape of the half wave during locomotion. A series of simulation shows the feasibility of using asymmetric oscillators for locomotion. Furthermore, the latest results obtained demonstrate that the proposed asymmetric locomotion mechanism is easy to implement while offering a satisfactory motion performance in on-site experiments.
... We predict that this mechanism is applicable to caterpillars with different body morphologies [32][33][34] and that similar internal tissue movements may be present in other soft-bodied organisms with an open coelem, such as the leech and some oligochaetes [35], but not in highly muscularized structures, such as the elephant trunk or human tongue [36]. These findings also have significance beyond animal locomotion, because they are already contributing to the design and development of deployable, maneuverable, and orientation-independent soft material robots [37]. Given the ubiquity of studies on the biomechanics of the skeleton and of soft tissues anchored to it in research on mammalian (and, more specifically, human) gait and posture, these findings may prompt a reexamination of the potential role of soft tissues in the biomechanical performance of animals with stiff skeletons. ...
Animals with an open coelom do not fully constrain internal tissues, and changes in tissue or organ position during body movements cannot be readily discerned from outside of the body. This complicates modeling of soft-bodied locomotion, because it obscures potentially important changes in the center of mass as a result of internal tissue movements. We used phase-contrast synchrotron X-ray imaging and transmission light microscopy to directly visualize internal soft-tissue movements in freely crawling caterpillars. Here we report a novel visceral-locomotory piston in crawling Manduca sexta larvae, in which the gut slides forward in advance of surrounding tissues. The initiation of gut sliding is synchronous with the start of the terminal prolegs' swing phase, suggesting that the animal's center of mass advances forward during the midabdominal prolegs' stance phase and is therefore decoupled from visible translations of the body. Based on synchrotron X-ray data and transmission light microscopy results, we present evidence for a two-body mechanical system with a nonlinear elastic gut that changes size and translates between the anterior and posterior of the animal. The proposed two-body system--the container and the contained--is unlike any form of legged locomotion previously reported and represents a new feature in our emerging understanding of crawling.
