Empirical Contact Models for Soft Spherical Robots in Drake

Accurate dynamic modeling of soft-shelled spherical robots is challenging due to coupled rigid–soft body interactions and pressure-dependent contact behavior. This letter presents a modeling strategy for an empirically tuned pendulum-driven inflatable spherical robot. The approach combines a rigid-body dynamics engine in Drake with non-conservative effects. The robot's rigid-body model is generated from a custom URDF and augmented with interchangeable joint friction modules. Three alternative outer shell contact models are also considered: Drake's native hydroelastic contact, a pressure-dependent injected stiffness–damping model derived from isolated shell experiments, and a rigid point-contact baseline. Shell dynamics are characterized in the steering direction using a custom locking fixture, yielding empirical pressure-related frequency and damping relationships to parameterize the models. Ramp descent experiments across multiple inflation pressures validate the framework, showing that an appropriate model reduces drive velocity prediction error compared to a rigid point-contact case. The approach enables modular integration of additional dynamic effects, supports data-driven parameter tuning, and provides a reproducible pathway for accurate simulation of soft spherical robots.