Tendon‐Driven Auxetic Tubular Springs for Resilient Hopping Robots

Compliance in jumping robots improves gait stability and enables energy‐efficient locomotion. Here, 3D printable auxetic tubular springs from thermoplastic polyurethane (TPU) for rapid and sustainable hopping are developed. Because the springs have negative Poisson's ratios, they become stiffer as compression proceeds and theoretically stores 35.2% more energy than a linear spring with the same stiffness. As the stress concentrates on the hinges, it is revealed through experimental, numerical, and analytical investigations that hinge geometries, for example, the lattice angle and hinge radius, governs the global stiffness and robustness of the springs. The hopping robot leg composed of three auxetic tubular springs in parallel sustains more than 1,000 cycles of repeated, one‐degree‐of‐freedom (1‐DOF) vertical hopping and two‐degree‐of‐freedom (2‐DOF) forward hopping. The 2.5 kg‐robot system requires minimum 420 mJ of elastic energy for repeated hopping. The springs are pre‐compressed by tendon‐driven actuators and stores 1.08 J during jumping and release the springs when touching the ground. The power stroke is calculated as 15–18 W. The average velocity of the hopping robot reaches 0.06 m s −1 with the increase of touchdown angle to 0.125 rad. The cost of transport is calculated as 6.7, similar to those of the living organisms.