Optimal Modulation of Joint Stiffness with Guaranteed Stability for Quadruped Robots

Modulation of joint stiffness is a natural ability of humans, which enables us to interact smoothly and stably with environments. Inspired by this observation, variable impedance control has also been studied for robots in various applications. However, the design of joint impedance profiles that fulfill feasible requirements and optimality is non-trivial. In this paper, we present a novel scheme to modulate the joint stiffness with guaranteed stability for torque-controlled quadruped robots. The joint stiffness and desired contact forces are optimized coordinately in a quadratic programming (QP) formulation, where the constraints of non-slipping contacts and torque limits are also imposed. Moreover, the stability during stiffness modulation is guaranteed by a tank-based passivity constraint. The effectiveness of the proposed stiffness modulation is validated on our quadruped robot CENTAURO, demonstrating that it is capable of generating more compliant contacts while ensuring necessary tracking performances.