Springy paw pads let robot dogs run farther and longer

Engineers at Seoul National University of Science and Technology have designed porous feet for quadruped robots that can store the energy of the impact with the ground and return it on the next step. Conventional springs and shock absorbers were ineffective because they only work at fast gaits, so for a slow-moving inspection robot dog other energy-recovery mechanisms were needed. The proposed feet are hemispherical and 3D-printed from a flexible polymer in the form of a triply periodic minimal surface (TPMS), a continuous three-dimensional lattice of repeating pores and struts that combines low weight, tunable stiffness, and the ability to absorb and release stored mechanical energy. The researchers tested three lattice structures — simple, gyroid, and diamond — and found the diamond lattice with a relative density of 60% to perform best: it deformed sufficiently, stored a lot of energy, and lost relatively little energy to internal friction. Since elastic feet alone were insufficient — with improper gait the stored energy dissipates or causes the robot's body to sway, forcing the electric motors to correct its posture — the engineers trained a controller via deep reinforcement learning. The model incorporated the deformation and recovery of the TPMS structure, and the reward function encouraged movements that aligned the actuators' work with the storage and release of elastic energy at ground contact. The system was tested on a quadruped robot RBQ-10, comparing the new feet with conventional solid hemispherical supports. The robot maintained stability across the entire speed range while reducing the average battery power consumption by 1.4–6.2% at speeds from 0.4 to 1 m/s. The development is intended for robots used in inspections, cargo transport, and search-and-rescue operations, where extended autonomous operation is critical.