/Legless Locomotion for Legged Robots

Legless Locomotion for Legged Robots

Ravi Balasubramanian, Alfred Rizzi and Matthew T. Mason
Tech. Report, CMU-RI-TR-04-05, Robotics Institute, Carnegie Mellon University, January, 2004

Download Publication (PDF)

Copyright notice: This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author’s copyright. These works may not be reposted without the explicit permission of the copyright holder.


Legged robots offer rough terrain mobility, but sometimes conventional legged locomotion may fail, for example, when the robot gets stuck on a rock, and no leg has contact with the environment (the robot is high-centered). We propose a novel locomotion technique for a high-centered legged robot. We hypothesize that by using its legs as reaction masses, the robot can rock and roll on its stomach and move incrementally. In this report, we use simple models to study how leg motions affect body attitude configuration for a high-centered robot. Our interest in this problem arose from our experience with a highly mobile, robust hexapod robot getting high-centered in challenging terrain. One way of freeing a high-centered legged robot is to locomote the robot till it topples over. To simplify the analysis, we used Rocking and Rolling Robot (RRRobot). RRRobot is a hemisphere that has two short legs with small masses at their distal ends. Assuming RRRobot rolls on the flat ground without slipping, the goal is to locomote the always-high-centered RRRobot on the plane by finding suitable leg trajectories. Informal experiments with a teleoperated RRRobot suggest that a properly designed leg motion can produce attitude oscillations that result in incremental translation of the robot. The result is a form of legless, whole-body locomotion using the legs as reaction masses. We use simulation and dynamics analysis to understand how leg motions can be used to create small body attitude changes. We study two simplified versions of RRRobot, one in which body motion is restricted to yaw and the other in which body motion is restricted to roll and yaw. We present some observations from control theory and show that simple controls exist to locally modify body attitude configuration in the simplified models. We hypothesize that such attitude oscillations when coupled with the contact constraints will produce RRRobot translation. More experiments are necessary to verify this. Interesting future directions of work are to study the influence of body shape on the proposed locomotion technique and to control the direction of locomotion. A shorter version of this report is published in Proceedings of the International Conference on Intelligent Robots and Systems, 2003.

BibTeX Reference
author = {Ravi Balasubramanian and Alfred Rizzi and Matthew T. Mason},
title = {Legless Locomotion for Legged Robots},
year = {2004},
month = {January},
institution = {Carnegie Mellon University},
address = {Pittsburgh, PA},
number = {CMU-RI-TR-04-05},
keywords = {Dynamics, Legless Locomotion, Kinematics, Halteres},