Draft:Soft robotics

Soft Robotics is a subfield of robotics that deals with robots built out of soft and deformable material like silicone, plastic, fabric, rubber, or compliant mechanical parts like springs. Soft robots can actively interact with the environment and can undergo “large” deformations relying on inherent or structural compliance [citation needed] respectively due to the softness or the morphological features of its body.

File:Octopus 1

soft artificial octopus arm

Aspects of Soft Robots

Soft robots are often, but not necessarily, bio-inspired and are known to have a number of advantages over classical robotic devices based on traditional robotic technologies. Soft and deformable structures are crucial in systems that deal with uncertain and dynamic tasks and environments, e.g. grasping and manipulation of unknown objects, locomotion in rough terrains, and physical contacts with living cells and human bodies.

“Soft” may refer to the structural compliance of a robot, that means that the softness comes from ad hoc geometrical arrangements and morphology of hard materials – so that structural strains are magnified compared with local material deformation (e.g. compliant mechanisms). Several industrial robots use compliant mechanisms, such as the KUKA Lightweight Robot (LWR) that is characterized by a low mass-payload ratio and a programmable, active compliance.

“Soft” may also refer to an inherent material compliance that involves bulk material properties – including soft matter (e.g. elastomers, polymers, gels, etc.), which guarantee a safe interaction with the environment. As examples of soft robots, the soft silicone-based caterpillar robot inspired by the manduca sexta, the GoQBot, exploits SMA actuators and the incompressibility of fluids to deliver performance resembling those of the hydrostatic skeletons (Trimmer et al. 2006; Lin et al., 2011, figure 1a). The octopus-inspired robots (OCTOPUS, OctoProp and PoseiDRONE) combine the use of soft materials, SMA actuators and cable driven transmission to accomplish dexterous manipulation by artificial muscular hydrostats (Cianchetti et al., 2011 Laschi et al., 2012; figure 1b, c), legged locomotion (Calisti et al., 2011) and swimming by jet propulsion (Giorgio Serchi et al., 2013, figure 1d), and use the same principles of Octopus vulgaris with a biomimetic design approach (Margheri et al, 2012; Mazzolai et al, 2012). At Harvard University a series of soft robots based on pneumatic actuation has been developed, such as starfish-like (Morin et al., 2012, figure 1e), and tentacle-like robots (Martinez et al., 2013, figure 1f), which show large deformation and camouflage capability. The JamBots (Steltz et al., 2010, Brown et al., 2010) are another example of how soft materials in combination with soft actuation technologies can be used for robot locomotion (figure 1g) and grasping (figure 1g). In JamBots, the material properties can be changed with granular jamming (determining anisotropies) and motion can be generated with pneumatic actuators or with cable-driven systems, as in the case of the MIT jammable manipulator (Cheng et al., 2012). Soft and flexible materials can be also be part of the actuation system itself as in the case of the use of EAP in the starfish-like robot (Otake et al., 2002) or in the tissue-engineered multi-limbed medusoid robot (Nawroth et al., 2012, figure 1i), or in the Meshworm robots where a series of SMA springs arranged in antagonistic manner supported by a flexible braided mesh-tube structure is used to produce a peristaltic motion (Seok et al., 2012).

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Examples of Soft Robots

Here is a list of examples of soft robots.

• Octopus robot developed in the EU project OCTOPUS
• Soft caterpillar inspired robot from Tufts University http://www.sciencedaily.com/releases/2007/01/070128105355.htm
• Soft starfish inspired robot developed by Robert Shepherd (Cornell University), see
• Roboy - a tendon driven robot developed by the Artificial Intelligence Laboratory of the University of Zurich

Components for Soft Robots

In the context of soft robotics a lot of so-called smart material are used, especially for actuation and sensing. The type of materials used ranges from silicone, paper, wood, to metal (e.g. in springs).

Soft Actuator

Shape Memory Alloys (SMAs)

Main article: Shape-memory_alloy

Shape Memory Alloys are metal alloys capable of undergoing a certain strain and subsequently recover their original shape when heated (Funakubo, 1987). SMAs allow creating robotic systems that are drastically small in size, weight, and complexity, when compared with traditional robots. As actuators for mechatronics systems, these alloys present several drawbacks like low efficiency, high hysteresis and non-linearity, but the interest on these alloys is still growing for their flexibility, large force–weight ratio, limited volume, inherent sensing capability, and noise-free operation which enable the employment of this technology in soft robotics (Cianchetti, 2013; Cianchetti 2014).

Memory Polymers (SMPs)

Main article: Shape-memory_polymer

Shape Memory Polymers belong to a class of smart polymers, which have drawn considerable research interest in the last few years because of their applications in micro electromechanical systems and actuators in biomedical devices. Shape memory polymers exploit the same principle as SMAs: by stretching the rubber at a temperature above the crystallisation point and then cooling it in that extended shape will lock‐in the deformed material. On heating to melt the crystals, the original network configuration is recovered. In several fields of applications, SMP materials have been proved to be suitable substitutes to metallic ones because of their flexibility, biocompatibility and wide scope of modifications. A comprehensive review can be found in (Ratna and Karger‐Kocsis, 2008).

Electroactive polymers

Main article: Electroactive_polymers

Electro Active Polymers (EAPs) are a new emerging and promising class of technologies which already demonstrated the possibility to fill the gap between natural muscles and artificial artefacts. Most of them are based on polymeric matrixes activated with different mechanisms, but they are all endowed with the capability of varying their size and shape when an electric stimulus is supplied (Mirfakhrai et al., 2007). They have power densities exceeding those of biological muscle, are readily scalable and free-form fabricable, and are ideally suited to biomimetic and biomedical soft robotic applications. However, currently they have limited applicability because of the high electric fields required (in the case of electronic EAP) or the slow response and low lifetime (for the ionic EAP).

Flexible fluidic actuator

Flexible fluidic actuator is a term used for a wide range of system types, but generically they comprise an expansion chamber defined by an inner wall of an expandable girdle which is connected to at least two anchoring points. The expansion chamber can be pressurized acquiring a minimum or a maximum volume. Thus actuators are able to adapt and transform a fluid pressure force against the inner wall into a traction force or a bending movement. A recent review can be found in (De Greef et al., 2009). The main disadvantages of fluidic systems are the risk of leakages or—even worst—the possibility of burst of the inflated chambers.

Cable-driven actuation

Cable-driven actuation has the benefit of providing a distributed and continuous action and cables can be fitted at spots within a soft robot where it would be hard to place other actuators otherwise, since powerful motors can be embedded outside the robot thus keeping it flexible (Calisti et al., 2010). Since cable transmission is continuous and is subject to negligible backlash issues, control is greatly simplified, but friction losses along the robot due to the cables may reduce the controllability of the system. Compared with the other actuation methods, cable actuation offers low inertia, weight and volume, it guarantees fast response times and long range transmission of force and power.

Semi-active actuators

A special class of materials offers the possibility to change its mechanical properties due to controlled physical stimuli. Thermo- magneto- and electro-rheological materials possess the capability to change the stiffness from values resembling low viscosity fluids to values similar to solid materials by applying thermal (Cheng et al., 2010), magnetic or electric fields (Yalcintas and Dai, 1999), respectively. The main drawbacks are due to control issues and low response time (for thermal activation) or the high fields required (for the magnetic and electric activation). Granular jamming is another phenomenon that is arising a growing interest for the impressive behaviour which enables particles to act like a liquid, solid, or something in between depending on an applied vacuum level (Liu and Nagel, 1998).

Variable Impedance Actuators

Variable Impedance Actuators are also know as variable compliance, adjustable stiffness, and controllable stiffness actuators. Passive compliant actuators contain an elastic element, e.g. a spring which can store energy, which is not the case for actuators with active compliance, where the controller of a stiff actuator mimics the behavior of a spring. The latter has the disadvantage that no energy can be stored in the actuation system and due to the limited bandwidth of the controller, no shocks can be absorbed. An advantage of active compliance is that the controller can make the compliance online adaptable. Online adaptability means that the compliance can be adapted during normal operation. A famous robot that uses active compliance for safety is the Kuka/dlr leightweight arm. The most well know example of a passive compliant actuator is the original Series Elastic Actuator SEA [2]), which is a spring in series with a stiff actuator. The compliance of this actuator is fixed and is determined by the selection of the spring; thus, the physical compliance cannot be changed during operation. To obtain variable stiffness, the virtual stiffness of the actuator is adjusted by dynamically adjusting the equilibrium position of the spring. To combine energy storage and adaptable compliance, an elastic element to store energy is needed, together with a way to adapt the compliance. A substantial number of designs have been developed like AWAS, Maccepa, VSA joint, AMASC, VS-joint, vsaUT. Also parallel springs are investigated for actuation and gravity compensation in order to reduce the torque through the motor, a new concept in this field is the series-parallel elastic actuator (SPEA) where several parallel springs can be engaged or disengaged mimicking the variable recruitment of the muscle fibres in a biological muscle.

Soft Sensors

The kind of system that has been mostly investigated in soft applications is tactile sensing. One of the most widely used methods is to detect structural deformation with embedded strain sensors in an artificial skin. Highly sensitive fibre optic strain sensors have been embedded in a plastic robotic finger for force sensing and contact localization (Park et al., 2009). Embedded strain gages and the detection of capacitance change with embedded capacitive sensor (Yamada et al., 2002) arrays are other approaches for tactile sensing (Kirchner et al., 2008). There have been stretchable skin-like sensors proposed with different methods: a strain sensing fabric composites developed using an electrically conductive elastomer (Ulmen and Cutkosky, 2010); a stretchable tactile sensor proposed also using polymeric composites (Ventrelli et al., 2009); a highly twistable tactile sensing array made with stretchable helical electrodes (Cheng et al., 2009); an artificial skin sensor consisting of multi-layered mirco-channels filled with a conductive liquid capable of detecting multi-axis strains and contact pressure (Park et al., 2011). In addition, many of the compliant soft actuators described above, such as EAPs, can simultaneously act as actuators and electromechanical sensors (Jumg et al., 2008).

Soft Structures

By employing hybrid structural approaches, such as Tensegrity Structures, Robots can also be soft at the "structural" level, even if they use a mix of soft and rigid component elements. A Tensegrity Robot is structurally integrated through its continuous elastic tensile network, and exhibits many of the properties commonly associated with soft robots, such as passive compliance, robust distribution of contact forces, highly deformable shape, and distributed parallel actuation approaches. Likewise, they have the same challenges in sensing, modeling and control due to their passive compliance, oscillatory response, and non-linear structural dynamics. Tensegrity robots have been shown to benefit from many of the same sensing, modeling, and control approaches as other soft robots, such as the use of distributed emergent controllers such as neuroscience inspired Central Pattern Generators. One advantage that tensegrity robots present relative to purely soft robots is the opportunity to integrate existing sensors and actuators into the rods, yet still achieve a structurally compliant system.

Tensegrity Robots have been developed in hardware and simulation by a growing number of research labs over the last decade, and have been proposed for a wide range of applications, including bio-mimicry and space exploration. Recently NASA released the beta version of an open source tensegrity robotics simulation library.

Modeling Soft Robots

In order to model a soft robot, traditional mathematical tools, typically used for rigid robot dynamic modeling, cannot be applied. A continuum approach is required, with distributed parameters and nonlinear partial differential equations need to be considered. Existing models can be divided into two main categories: constant and non-constant curvature approximation. They vary in the degree of precision obtained. The constant curvature approximation can be considered as the simplest approach to modelking soft robots. As specified in various constant curvature works [citation missing], this approximation fails in many practical cases, for example, when the environmental loads (such as gravity) are significant or in the case of nonconstant sections of the robot arm. Non-constant curvature models belong to three categories: continuum approximation of hyperredundant systems [citation missing], the spring–mass model [citation missing], and the Cosserat geometrically exact approach [citation missing]. Continuum approximation of hyperredundant systems was probably the first continuum approach to be proposed in the robotics community, unveiling a variety of applications such as snake locomotion or trunk manipulation. However, the latest robotic platforms developed, which are made of highly deformable continuum bodies, have led quite naturally to using a pure continuum approach to modeling from the beginning. Cosserat geometrically exact models are the closest to the mechanics of a soft robot structure and actuation.

Control Architectures

In general, soft robotic structures are hard to control. The reason is that soft bodies are highly complex and, therefore, hard to model. Typical properties are strong nonlinearities, a high-dimensional (potentially infinite) state space, bifurcation behavior, underactuation, and delayed communication - all of which are difficult to handle individually and state a serious problem when combined. Due to this facts a lot of times the soft robots are not actively controlled or are very imprecisely controlled with a simple feedforward controller.

A concept often mentioned in the context of soft robotics is Morphological Computation, which his based on the observation in Nature that the morphology seems play an important role in biological systems as it seems to contribute to computation aspect of the agent. The concept has repeateadly applied in robot design, especially, with soft robots. The general approach is to cleverly design (soft) body, such that they can take over part of the control of the robot. For example, in locomotion a spring in a leg enables the robot to cope with uneven ground as this external perturbations can be absorbed by the springs.

Despite the number of successful designs there exist very little theoretical work to establish a mathematical framework to describe the computational power of (soft) bodies and to use it to design a control approach for soft robots. Füchslin et al. discussed how morphological computation in the context of control can look like. Some initial work on mathematical frameworks has been done by Hauser et al..

Scientific Community

Although people have been using soft material for robots for a long time, only recently is an international community forming. For example, since October 2012 exists an IEEE RAS Technical Committee on Soft Robotics, which coordinates the international community around this field of research. In 2013 the International Journal on Soft Robotics was funded. It publishes quarterly results from the field. In October 2013 started RoboSoft – A Coordination Action for Soft Robotics funded by the European Commission under the 7th Framework Programme, Future and Emerging Technologies (FET) Open Scheme. RoboSoft aims at creating and consolidating an international scientific community of scientists and roboticists working in the field of soft robotics to combine their efforts and enable the accumulation and sharing of scientific and technological knowledge to maximize the opportunities and materialize the huge potential impact of soft robotics technologies.

International Journals

• Soft Robotics (SoRo)

International Events

• 2014 Workshop on Advances on Soft Robotics, 2014 Robotics Science an Systems (RSS) Conference, Berkeley, CA, July 13, 2014
• 2013 International Workshop on Soft Robotics and Morphological Computation, Monte Verità, July 14-19, 2013
• 2012 Summer School on Soft Robotics, Zurich, June 18-22, 2012

Projects

• EU Project OCTOPUS, coordinated by Prof. Cecilia Laschi, BioRobotics Institute, Scuola Superiore Sant'Anna, Italy
• EU Project LOCOMORPH, coordinated by Prof. Rolf Pfeifer, Artificial Intelligence Laboratory, University of Zurich, Switzerland
• EU Project ECCE, coordinated by Prof Owen Holland, Department of Informatics, University of Sussex, UK
• EU Project Myorobotics, coordinated by Prof. Alois Knoll, Robotics and Embedded Systems, Technical University of Munich, Germany
• IGERT, coordinated by Prof. Barry Trimmer, Neuromechanics and Biomimetic Devices Laboratory, Tufts University, Boston, USA
• EU ERC Grant Project SPEAR, coordinated by Prof. Bram Vanderborght, Robotics and Multibody Mechanics Group, Vrije Universiteit Brussel, Belgium

Educational and Scientific Tools in Soft Robotics

There are a number of educational tools for soft robotics.

• EmbedIT
• LocoKit
• Myorobotics
• Cubebot (link is missing)

Fields of Application

Medicine

STIFF-FLOP

Marine robotics

PodeiDrone

Bioinspired robotics

OCTOPUS

Industry

Applications of the soft robots are in tasks where there is needs to move in physical interaction with an unknown and dynamic environment and the controlled body-actuator system must achieve abilities like: 1) Safety to humans (and resilience to self-damage) in operations where the robot is required positional accuracy and swiftness of motion, while cooperating, physically interacting or even possibly colliding with the humans and their environment, such as e.g. in collaborative robotics. This has been studied using a pneumatic soft arm, the DLR Hand/Arm system, and illustrated through impacts’ response (e.g. by hammering on the fingers and knocking using a baseball on the arm) and the co-worker Baxter. The social robot Probo uses SEAs to have safe and huggable HRI with children. 2) Efficiency (e.g. natural gait generation, adaptation in legged locomotion applications and prosthetics for lower limbs). Like in the bipeds Lucy, Veronica, Dribbel and Tulip where the natural dynamics are exploited to reduce the energy efficiency. Or the AMPfoot, Cyberlegs, BIOM and Sparky prostheses were energy is stored and released during walking. By storing energy in a spring a ball can be thrown or kicked much further as when a stiff actuator is used. 3) Robustness to external perturbations and unpredictable model errors (changes) of the environment, of the robot kinematics and dynamics, or of the dynamics of a human interacting with it. This is often required in tasks like hammering, holding cups, drumming; typical tasks with tools such as screwdriving, cutting, polishing, drawing or stabilizing a humanoid robot like Coman. 4) Adaptability and force accuracy in the interaction with the operator, in applications in which continuous contact and accurate force exchange is necessary, such as in “hands-on” assistive devices, rehabilitation, exoskeletons and haptics. This is achieved for example in the exoskeletons Knexo, altar, Lopes, etc. [LINKS are missing]

A good overview video is provided in https://www.youtube.com/watch?v=Im4ryS4vav4 There are a number of educational tools for soft robotics.