Inspired by eel locomotion, we present a preliminary design of a soft robot which swims by undulation movement. The robot body comprises a series of soft pneumatic actuators controlled by Pulse-Width Modulation method to generate the desired motion. A soft actuator was designed and simulated to be of suitable shape to mimic an eel. We investigated midline kinematics in model anguilliform swimmer. Typical kinematic parameters of the robot body during swimming were estimated to examine their change with variation of frequency. A kinematic model of the robot body was built to estimate the thrust force component. The results revealed good potential to develop our design with midline robot body kinematic properties similar to those in natural long body fish, with movement in the robot body able to create thrust force.

The aquatic environment is, in many ways, ideal for soft robots. Water balances much of gravity’s load, reducing the need for a skeleton-like structure, and may be used as the working fluid for soft actuators. Many soft robots take inspiration from aquatic animals, particularly the arms and tentacles of cephalopods (like octopuses and squids). Cephalopod arms are highly complex, evolved structures, and mimicking their diverse abilities with human-made materials and structures is a challenge that integrates mechanical design, manufacturing and modeling. A modular soft bending arm is presented and examined holistically. The design uses rigid radial plates to define the arm’s cross section independently from the actuator type. The design uses connecting keys to non-permanently join actuators to each arm segment. The modular arm design has been prototyped and used extensively as a test bed for rapid design iteration. The actuator manufacturing method and the inherent limitations imposed on design by manufacturing techniques are discussed. Future work will formulate a bending model in terms of design variables, to identify fundamental capability limits and inform on structural changes that improve capability.

Aquatic animals utilize undulatory motion of the body as their predominant mode of locomotion. The most common swimming mode among vertebrates such as fishes, amphibians, and reptiles, is horizontal plane body bending. Geckos can use back and tail undulation to run quadrupedally over the surface of the water, utilizing multiple mechanisms, including semi-planing, and impulse forces. Dead trout can generate passive propulsion in van Karman vortices. Active changes in stiffness during body caudal find swimming could enhance swimming performance. This platform provides a biorobotic model for exploring body stiffness modulation and swimming performance experimentally. Fishes, amphibians and reptiles utilize undulatory motion of the body in the lateral plane as their predominant mode of locomotion. Here, we utilize soft bending actuators for shape changes on the body to gain insight into undulatory locomotion and explore mechanisms for body stiffness control by utilizing soft sensors. Soft pneumatic actuators were attached on each side of a flexible panel with stiffness comparable to that of a fish body. Hyper-elastic soft sensors were embedded for curvature estimation of the undulating soft robotic fish body to close the loop. Soft sensors contained microchannels filled with liquid metal eutectic Gallium Indium (eGaIn). This marks the first study in which a soft sensor utilizing eGaIn has been tested under water. Despite the hydrodynamic pressure, results suggest that the sensor allows for measurement of changes of body curvature. During bending of the soft pneumatic actuator the greater fin curvature and associated length changes correlated with changes in electrical resistance in the liquid metal enclosed within micro channels. Resistance increased proportionally with bending. The resistance measurement enabled by the sensor correlates with the position of the tail fin curvature, thus providing fin displacement information and opens the door for swimming with feedback. Body-caudal fin shape changes facilitated by soft sensors could enhance maneuverability in rapid turning responses in future biologically inspired swimming robots.