Soft-Bodied Robots

Flexible biologically inspired machines are made of concatenated rigid modules linked using multi-axis joints. Caterpillars have become one of the main inspirations behind the study of soft-bodied organisms especially as these are successful climbing herbivores. The multi-legged crawling of the caterpillars is different from the peristaltic movements of mollusks or worms and gaits of articulated animals. Caterpillars are capable of twisting and bending to an extent that is impossible with a rigid skeleton. These climbing herbivores vary their body tension using dynamic hydrostatics.

Very few efforts have been taken till date to develop soft-bodied robots with an ability to crawl, twist and deform. Various flexible robots have been developed based on continuously bending elements, conformable wheels and peristalsis. Most of them are operated in a specific environment.

Architectural Concept for Soft Robots

The architecture of the soft robot includes components of actuation, perception and control of a robot system. In addition, the robot is equipped with an actuator interface, actuator pressure source and the fluidic actuator system. The actuation power source is required to be a portable pressure generation device capable of being incorporated into the body of robot. Energy-efficient miniature valves are also present in the support hardware.

The soft robot is actuated with the application of pressure on fluidic elastomer actuators. The deformation of the actuator is accompanied by the expansion of embedded fluidic channels in an elastomeric substrate due to pressure. However, this deformation follows desired motion upon the incorporation of corresponding physical constraints.

Model for Soft-Bodied Robotics - Caterpillar Locomotion

Waves resulting from movement passes from the terminal segment (TS) of caterpillar to the head. In addition, a kinematic transition takes place between the posterior segments and the mid-body segments. As a result, adjacent abdominal segment and TS are raised to stance phase. The segments are then pivoted around an attachment point at a terminal proleg (TP), where the point is in the form of an inverted pendulum.

Vertical displacements take place prior to the change of horizontal velocity by 30°. Height and horizontal velocity are in phase in the mid-body segments, where each body segment is maintained at maximum length at the time of stance phase. The segments are compressed at the first part of the swing phase with the forward movements of waves and extended again before entering into the stance.

SoftBot

SoftBot is designed to be a contoured cylinder made of highly elastic silicone rubber. It is capable of moving using shape memory alloy springs that acts as actuators and are directly connected to the inner side of the body wall. The body is equipped with an inner compartment used for holding the control system components. The space between the body wall and the inner compartment is pressurized for transmitting forces and regulating stiffness. The robot is also capable of getting compressed into a freeform volume upon the release of pressure.

The key benefits of SoftBot are:

• Easy to build
• Extreme mobility
• Fault tolerant capability

In the following video, you see a clear demonstration of fluid movement by the SoftBot:

Model for Soft-Bodied Robotics - Peristaltic Locomotion

The mesh-tube structure is provided with multiple spirals, where each spiral is similar to that of a stretched spring. The tube is deformed in an antagonistic regime with the application of stress to form a rhombus. The system’s movement is simulated using a model of mesh-tube structure. Peristalsis is resulted from radial contractions of the segments in a sequential manner.

Due to radial contractions, the contact point is released from the flat ground and travelled via a swing phase of the initial contact point. A travelling wave is created while considering the trajectories of points along the tube. This results in a generation of waves of leg trajectories instead of deformation of the body. Locomotion without legs using peristaltic wave generated by antagonistic longitudinal and radial actuators can be explained with this simulation.

Conclusion and Future Work

The soft-bodied robots are capable of completing the curling phase of their motion within a short time. Also, the body coordination of these robots is effectively enhanced with the help of mechanical coupling between the flexor units. It is necessary for any rolling locomotion to overcome low-speed instability in order to roll efficiently and so the initial acceleration is significant. However, in soft-bodied robots, forward motion depends on the transformation of rotational energy into translation through the wheeling mechanism.

The soft robots are observed to have the following characteristics:

• They are under-actuated systems that make use of adaptive control
• They undergo passive body deformation in order to adapt to the environment
• Under typical loading conditions, they undergo large deformation.

Future work involves finding out the reason behind the production of mechanical power by the caterpillar within a short time.

References

• Trimmer.B.A, Takesian.A.E, Sweet.B.M, Biology Department and Tufts University Biomimetic Devices Laboratory, Medford, USA, 7^th International Symposium on Technology and the Mine Problem, 2006.
• Onal.C.D, Chen.X, Rus.D, Whitesides.G.M, Computer Science and Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Department of Chemistry and Chemical Biology, Harvard University, Cambridge, USA.
• Lin.H.T, Leisk.G.G, Trimmer.B, Department of Biology, Department of Mechanical Engineering , Tufts University, Medford, IOP Publishing Ltd, 2011, 14pp.