SPARC: Spine with Prismatic and Revolute Compliance for Quadruped Robots

Abstract

Quadruped mammals coordinate spinal bending and axial compression to enhance locomotion agility and efficiency. However, existing robotic spines typically lack the active compliance required to support such dynamic behaviours. We present SPARC, a compact 3-DoF sagittal-plane spine module that enables simultaneous revolute and prismatic motions within a 1.26 kg package. Using a floating-base impedance controller, we facilitate independent, task-space tuning of spinal stiffness and damping to mimic biological load-bearing strategies. Benchtop experiments confirm high-fidelity rendering of commanded impedance, with linear force-displacement error within 1.5%. Systematic locomotion simulations reveal a critical speed-dependency: while low-speed efficiency is insensitive to spinal properties, precise impedance tuning becomes indispensable for high-speed performance. Our results demonstrate that an optimally compliant spine reduces power consumption by 21% at 0.9 m/s compared to a rigid-spine baseline. This efficiency gain is mechanistically attributed to the spine's role in augmenting stride length and acting as a mechanical low-pass filter to attenuate high-frequency torque fluctuations. SPARC provides an open-source platform for systematic studies of spine compliance in legged locomotion. Available at: github.com/YueWang996/sparc