Modeling the Locomotion of Articulated Soft Robots in Granular Medium

We introduce a numerical tool for modeling articu- lated soft robots that couples discrete differential geometry-based simulation of elastic rods, our model for the articulated structure, and other external forces. Parallel to simulations, we build an untethered robot testbed, in the granular medium, comprised of multiple flexible flagella that are rotated about an axis by a motor. Drag from the granules causes the flagella to deform and the deformed shape generates a net forward propulsion. External drag depends on the flagellar shape, while the change in flagellar shape is the result of the competition between the external loading and elastic forces. We find reasonable quantitative agreement between experiments and simulations. Owing to a rod-based kinematic representation of the robot, the simulation can run faster than real-time in some cases, and, therefore, we can use it as a design tool for this class of soft robots. We find that there is an optimal rotational speed at which maximum efficiency is achieved. Moreover, both experiments and simulations show that increasing the number of flagella from two to three decreases the speed of the robot. This indicates that our simulator is potentially applicable for unknown physics exploration. We also gain insight into the mechanics of granular medium - while resistive force theory can successfully describe the propulsion at low number of flagella, it fails when more flagella are added to the robot.