Leg Mass Influences the Jumping Performance of Compliant One-legged Robots.

Compliant leg behavior and the spring-loaded inverted pendulum (SLIP) model help understand the dynamics of legged locomotion while few works could be found discussing the effects of leg mass, which is an intriguing and worthwhile question for designers of compliant legged robots. This work is focused on the influence of leg mass on jumping performance using a compliant one-legged robot designed by the authors. An extended SLIP model with the center of mass of the torso offset from the hip and with the leg mass considered is presented for the designed robot, and the bio-inspired virtual pendulum posture control (VPPC) and the velocity-based leg adjustment method (VBLA) are used to achieve stable jumping of the robot. Afterward, the maximum forward pushed velocity at which the robot can recover its stability is investigated for different leg masses, and this velocity is used as a criterion to analyze the effect of leg mass. Results show that the maximum allowed pushed velocity decreases as the leg mass increases, suggesting that leg mass leads to a reduction in the stability of the robot. Results also show that stability decreases sharply when the ratio of leg mass to the total mass of the robot is higher than 0.35, which is close to the ratio of leg mass to the total mass in humans, according to human anatomical data. It suggests that the leg mass ratio should be kept below 0.35 in the practical design of legged robots so that the leg mass would have less effect on the locomotion performance of the robot.