Shape optimization for underactuated appendages in resistive media

Locomotion within resistive media (RM) is often non-inertial and so is heavily dependent on the geometric configuration of the body to produce motion. In this work, we are inspired by appendage-based swimming through RM and seek to understand how limb-shape and actuation properties influence swimming performance. We performed experiments with an underactuated linkage and measured the force-torque profiles as the arm moved through granular media. We vary the limb geometry by defining two design parameters: the ratio of the lengths of successive links, and the ratio of the limiting angle of successive joints. We conducted rotational experiments having varying design parameters to determine which values optimize the propulsion efficiency. For the experiments, we attached an underactuated appendage to a 6-axis force/torque sensor on the robot arm's end effector. The appendage was then inserted into the RM and rotated along the vertical axis from 0 to 180 degrees, then rotated back. To quantify the effectiveness of different link geometries, we define efficiency coefficients for each stroke. We observed that both design parameters influence the force profile by affecting the timing at which the links fully stretch. To further analyze this, we applied a resistive force model to simulate the observed force profiles. This work expands the understanding of efficient appendage-based propulsion in various RM.