Towards versatile legged robots through active

Robots with legs and arms have the potential to support human s in dangerous, dull or dirty tasks. A major motivation behind research on such robots is their potentialversatility. However, these robots come at a high price in mechanical and control complexity. Hence, until they can demonstrate a cle r advantage over their simpler counterparts, robots with arms and legs will not ful fill their true potential. In this paper, we discuss the opportunities for versatile ro bots that arise by actively controlling the mechanical impedance of joints and particu larly legs. In contrast to passive elements like springs, active impedance is achieve d by torque-controlled joints allowing real-time adjustment of stiffness and damp ing. Adjustable stiffness and damping in realtime is a fundamental building block towa rds versatility. Experiments with our 80 kg hydraulic quadruped robot HyQ demon strate that active impedance alone (i.e. no springs in the structure) can succe sfully emulate passively compliant elements during highly-dynamic locomotion task s (running, jumping and hopping); and, that no springs are needed to protect the actu ation system. Here we present results of a flying trot, also referred to as running t rot. To the authors’ best knowledge this is the first time a flying trot has been successf ully implemented on a robot without passive elements such as springs. A critical discussion on the pros and cons of active impedance concludes the paper. This artic le is an extension of our previous work (Semini et al. (2013)) presented at the Int rnational Symposium on Robotics Research (ISRR) 2013.