  Impact mitigation is crucial to the stable locomotion of legged robots,
especially in high-speed dynamic locomotion. This paper presents a leg
locomotion system including the nonlinear active compliance control and the
active impedance control for the steel wire transmission-based legged robot.
The developed control system enables high-speed dynamic locomotion with
excellent impact mitigation and leg position tracking performance, where three
strategies are applied. a) The feed-forward controller is designed according to
the linear motor-leg model with the information of Coulomb friction and viscous
friction. b) Steel wire transmission model-based compensation guarantees ideal
virtual spring compliance characteristics. c) Nonlinear active compliance
control and active impedance control ensure better impact mitigation
performance than linear scheme and guarantee position tracking performance. The
proposed control system is verified on a real robot named SCIT Dog and the
experiment demonstrates the ideal impact mitigation ability in high-speed
dynamic locomotion without any passive spring mechanism.
