Abstract—This video highlights work to date on the Yobotics-IHMC Lower Body Humanoid Robot. The robot is a twelve degree-of-freedom robot with force controllable Series Elastic Actuators at each degree of freedom. Control algorithms utilize Virtual Model Control, and foot placement is determined using Capture Regions. The robot can recover from moderate disturbances and walk on flat ground. Ongoing work is focused on improving robustness to disturbances, walking more quickly and efficiently, and walking over rough terrain. I. INTRODUCTION HE Yobotics-IHMC biped (Figure 1) is a twelve degree of freedom lower-body humanoid robot. It has 3 degrees of freedom in each hip, one in each knee, and two in each ankle [1]. The robot was built during 2007 and 2008 and control algorithms were first implemented on the robot in late 2008. Linear Series Elastic Actuators [2] with brushless DC motors are used at each joint. These actuators allow for high fidelity force control, which enables low impedance control methods. All 12 of the actuators used in the robot (Figure 2) are identical, except for their plunger end. Virtual model control [3] is used to control the orientation and height of the body. Virtual springs and dampers are connected to the robot and their resulting forces are distributed between the support legs. The location of the Center of Pressure on the feet is used to control the forward and lateral center of mass position and velocity. Since we do not directly servo the joint angles of the robot, our control method is very compliant and robust to disturbances. While the robot is standing, we can move the knee and ankle joints and even the foot location without affecting the control of the body. During balancing and walking, we continuously update an estimation of the Capture Region [4][5]. The Capture Region is the region on the ground in which the robot must Manuscript received March 1, 2009. This work was supported in part by the U.S. Army Tank-Automotive Research, Development and Engineering Center and the U.S. Office of Naval Research. Authors are with the Institute for Human and Machine Cognition, Yobotics, Inc., University of Michigan, Delft University, and Bucknell University. Updated project and contact information is available at http://www.ihmc.us/research/projects/HumanoidRobots/ place its center of pressure in order to stop. As long as the Capture Region intersects the support polygon, the robot can stay balanced. Once the Capture Region no longer intersects the support polygon, the robot must take a step in order to regain balance. To compute the Capture Region, we use a simplified model based on the Linear Inverted Pendulum Model [6] with feet. With this model, the Capture Point dynamics are linear and first order and the Capture Region can be geometrically determined. Figure 1: Front view of assembled robot body and legs. In order to walk, the robot moves its Center of Pressure on its foot to guide the estimated Capture Region towards the The Yobotics-IHMC Lower Body Humanoid Robot Jerry E. Pratt, Ben Krupp, Victor Ragusila, John Rebula, Twan Koolen, Niels van Nieuwenhausen, Chris Shake, Travis Craig, John Taylor, Greg Watkins, Peter Neuhaus, Matthew Johnson, Steve Shooter, Keith Buffinton, Fabian Canas, John Carff, William Howell T CONFIDENTIAL. Limited circulation. For review only. Preprint submitted to 2009 IEEE/RSJ International Conference on Intelligent Robots and Systems. Received March 1, 2009.