Design of fast walking with one- versus two-at-a-time swing leg motions for RoboSimian

This paper presents two prototype fast walking gaits for the quadruped robot RoboSimian, along with experimental results for each. The first gait uses a statically stable one-at-a-time swing-leg crawl. The second gait uses a two-at-a-time swingleg motion, which requires deliberate planning of zero-moment point (ZMP) to balance the robot on a narrow support base. Of particular focus are the development of practical means to exploit the fact that RoboSimian has high-dimensionality, with seven actuators per limb, as a means of partially overcoming low joint velocity limits at each joint. For both gaits, we use an inverse kinematics (IK) table that has been designed to maximize the reachable workspace of each limb while minimizing joint velocities during end effector motions. Even with the simplification provided by use of IK solutions, there are still a wide range of variables left open in the design of each gait. We discuss these and present practical methodologies for parameterizing and subsequently deriving approximate time-optimal solutions for each gait type, subject to joint velocity limits of the robot and to real-world requirements for safety margins in maintaining adequate balance. Results show that careful choice of parameters for each of the gaits improves their respective walking speeds significantly. Finally, we compare the fastest achievable walking speeds of each gait and find they are nearly equivalent, given current performance limits of the robot.