Design of Soft Outer Shells for Control of Large Spherical Robots

While soft-shelled spherical robots offer promising mobility and robustness for planetary exploration, large-scale, mission-capable robots present several design challenges. This letter presents the design, manufacturing, and performance evaluation of multiple soft outer shell iterations for RoboBall III, a large, pendulum-driven spherical robot. Shell geometry, material properties, and fabrication processes are systematically examined to understand their effects on mobility and control authority. Empirical results, including sphericity analysis, contact patch behavior, passive response, steer compensation fingerprinting, and agility demonstrate that minimizing shell asymmetry significantly improves steering performance. The final shell design achieves near-ideal sphericity and passive angle retention, enabling more predictable control with reduced input effort.