Soft Robotics

inflatable devices for robotic actuation


Introduction
Soft Robotics is a research area that draws inspiration from invertebrates – animals (like squid, starfish or worms) that do not have internal skeletons – to develop alternatives to hard bodied robots, which consist mainly of rigid and heavy metal parts. While conventional systems have a long tradition and various advantages, like reliability, precision, strength and durability, they’re also facing many limitations that seem hard to overcome with predominant technologies. Some of these restraints are based on their mechanical complexity, like the agitation in difficult terrain or the handling of fragile objects, whilst others come from their weight, limited ranges of motion of standard actuators, or the costs involved in producing such systems.

In soft robotics the potential of compliant soft materials that allow for continuous deformation are explored to create structures whose ranges of motion are directly defined by the properties of the materials used in their fabrication. Through this shift the field of robotics becomes more closely related to materials science and chemistry than to mechanical engineering.

Institute
Chair for CAAD, ETH Zürich

Author
Manuel Kretzer


Device Structure and Operation
While this interest bridges various areas, like the development of electroactive polymers, shape memory polymers or pneumatically driven McKibben-type actuators, generally referred to as “air-muscles”, one particular focus is on the creation of pneumatic networks embedded in silicon based elastomers of variable stiffness.

This technology, which was originally developed at the Department of Chemistry and Chemical Biology and the Wyss Institute for Biologically Inspired Engineering at Harvard University, draws continuous inspiration from nature and has investigated a number of different potentials.

Locomotion
Soft organisms, many of which are found in marine environments, are highly versatile locomotors since they don’t have the need for endo- or exoskeletons like land animals, that have to retain form in a gravitational field. Trying to mimic some of the motions and features of such organisms, soft elastomeric robots use embedded pneumatic networks and chambers that inflate during actuation. The combination of layers of differently extensible materials and the resolution, size and arrangement of the internal network can then be used to create bending motion. The researchers have thus constructed a tetrapod that can lift each of its four legs independently. The embedded pneumatic networks were pressurized through computer controlled external sources and, when synchronized, led to crawling and undulating movement . The non-linearity in the motion of the single elements can produce very complex actuation by relying on a rather basic control system. [1].

Gripping
To further prove the potential of active soft structures, researchers from the same group have produced a starfish-like, six legged gripper, consisting of an active compliant layer, a closing layer made from a stiffer silicone and a third textured layer to enhance the gripping effect. The actuated legs of the gripper curled downward and were able to carry various objects without damaging or harming them during manipulation, since the soft interface distributed the force over the complete contact area. The experiment showed that soft robots have high potential for applications that require handling delicate objects and where the implementation of feedback sensors would be too difficult or expensive [2].

Camouflage
Again drawing inspiration from animals, like squid, cuttlefish, chameleons or fireflies, that can dynamically change their body patterns and colors for reasons of disguise, protection, deterrence, display or communication, a network similar to the one used for inflation, can allow the transportation and emergence of colored fluids within the soft machine. Even though this system is obviously much less complex than the processes happening in color-changing organisms, it allows for relatively fast changes and a large variety of color, can cover large areas and transport both liquids and fluids. Furthermore, since the color layers are very lightweight they don’t constrain the movement of the soft robot [3].

Immediacy
As described above, pneumatic expansion of a network of channels embedded in soft elastomers provides a simple method to create complex movements. However the speed of actuation is relatively slow, due to time the air needs to fill and inflate the micro channels. A possible solution and the most recent investigation into soft silicone robotics, is the creation of explosive bursts within the network, using a basic chemical reaction. In particular the explosive combustion of hydrocarbons, triggered by an electrical impulse, a technique that is omnipresent in the actuation of hard systems, allowed a tripedal robot to “jump” 30 times its body height in less than 0.2 seconds. By improving the timing of actuation, the jump height could be further increased, the energy efficiency improved and the direction controlled [4].

Applications
Whilst much of this research is clearly militarily motivated the basic principles and the simplicity of the systems imply various possibilities for architecture and design. Especially the non-linear movement, the organic appearance and soft surface could allow the creation of adaptive environments that would feel much less mechanic and more natural than current explorations do. However what remains to be solved will be the massive infrastructure and pneumatic system, needed to actuate the components, when reaching spatial dimensions.


References
[1] R. Shepherd, F. Ilievski, W. Choi, S. Morin, A. Stokes, A. Mazzeo, X. Chen, M. Wang, and G. Whitesides, “Multigait soft robot”. In: Proceedings of the National Academy of Sciences, vol. 108, no. 51, pp. 20400-20403, 2011.
[2] F. Ilievski, A. Mazzeo, R. Shepherd, X. Chen, and G. Whitesides, “Soft Robotics for Chemists”. In: Angewandte Chemie, Int. Ed., no. 50, pp. 1890-1895, 2011.
[3] S. Morin, R. Shepherd, S. Kwok, A. Stokes, A. Nemiroski, and G. Whitesides, “Camouflage and Display for Soft Machines”. In: Science 337, pp. 828-832, 2012.
[4] R. Shepherd, A. Stokes, J. Freake, J. Barber, P. Snyder, A. Mazzeo, L. Cademartiri, S. Morin, and G. Whitesides, “Using Explosions to Power a Soft Robot”. In: Angewandte Chemie, no. 125, pp.2964-2968, 2013.

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