Granular Jamming Feet Enable Improved Foot-Ground Interactions for Robot Mobility on Deformable Ground

Recent studies on dynamic legged locomotion have focused on incorporating passive compliant elements into robot legs which can help with energy efficiency and stability, enabling them to work in wide range of environments. In this work, we present the design and testing of a soft robotic foot capable of active stiffness control using granular jamming. This foot is designed and tested to be used on soft, flowable ground such as sand. Granular jamming feet enable passive foot shape change when in contact with the ground for adaptability to uneven surfaces, and can also actively change stiffness for the ability to apply sufficient propulsion forces. We seek to study the role of shape change and stiffness change in foot-ground interactions during foot-fall impact and shear. We have measured the acceleration during impact, surface traction force, and the force to pull the foot out of the medium for different states of the foot. We have demonstrated that the control of foot stiffness and shape using the proposed foot design leads to improved locomotion, specifically a <inline-formula><tex-math notation="LaTeX">$\approx\!\text{52}\%$</tex-math></inline-formula> reduced foot deceleration at the joints after impact, <inline-formula><tex-math notation="LaTeX">$\approx\!\text{63}\%$</tex-math></inline-formula> reduced depth of penetration in the sand on impact, higher shear force capabilities for a constant depth above the ground, and <inline-formula><tex-math notation="LaTeX">$\approx\!\text{98}\%$</tex-math></inline-formula> reduced pullout force compared to a rigid foot.