Soft Sensors for Curvature Estimation under Water in a Soft Robotic Fish

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, soft bending actuators facilitate shape changes on the body to gain insight into undulatory locomotion, and allow exploration of mechanisms for body stiffness control with 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 at two locations of the undulating soft robotic fish body for curvature estimation in order to close the control feedback loop. Microchannels within the soft sensors are filled with a liquid metal, eutectic Gallium Indium (eGaIn). This marks the first study in which resistive eGaIn soft sensors have been tested under water. Results indicate that the sensor allows for measurement of changes of body curvature. As fin curvature changes, bending of the soft pneumatic actuator causes dimensional changes in the enclosed liquid metal which correspond to changes in electrical conductivity: Resistance increased proportionally with bending. The resistance measurement enabled by the sensor correlates with tail fin curvature. This fin displacement information opens the door to provide feedback for swimming with closed-loop control. The sensing of body-caudal fin shape changes-facilitated by soft sensors injected with liquid metal-could enhance maneuverability in rapid turning responses in future biologically inspired swimming robots.