This two-part paper discusses the analysis and control of legged locomotion in terms of N-step capturability: the ability of a legged system to come to a stop without falling by taking N or fewer steps. We consider this ability to be crucial to legged locomotion and a useful, yet not overly restrictive criterion for stability. Part 1 introduced the N-step capturability framework and showed how to obtain capture regions and control sequences for simplified gait models. In Part 2, we describe an algorithm that uses these results as approximations to control a humanoid robot. The main contributions of this part are (1) step location adjustment using the 1-step capture region, (2) novel instantaneous capture point control strategies, and 3) an experimental evaluation of the 1-step capturability margin. The presented algorithm was tested using M2V2, a 3D force-controlled bipedal robot with 12 actuated degrees of freedom in the legs, both in simulation and in physical experiments. The physical robot was able to recover from forward and sideways pushes of up to 21 Ns while balancing on one leg and stepping to regain balance. The simulated robot was able to recover from sideways pushes of up to 15 Ns while walking, and walked across randomly placed stepping stones.

Capturability-based analysis and control of legged locomotion, Part 2: Application to M2V2, a lower-body humanoid

This paper presents a momentum-based control framework for floating-base robots and its application to the humanoid robot “Atlas”. At the heart of the control framework lies a quadratic program that reconciles motion tasks expressed as constraints on the joint acceleration vector with the limitations due to unilateral ground contact and force-limited grasping. We elaborate on necessary adaptations required to move from simulation to real hardware and present results for walking across rough terrain, basic manipulation, and multi-contact balancing on sloped surfaces (the latter in simulation only). The presented control framework was used to secure second place in both the DARPA Robotics Challenge Trials in December 2013 and the Finals in June 2015.

Design of a Momentum-Based Control Framework and Application to the Humanoid Robot Atlas

To walk efficiently over complex terrain, humans must use vision to tailor their gait to the upcoming ground surface without interfering with the exploitation of passive mechanical forces. We propose that walkers use visual information to initialize the mechanical state of the body before the beginning of each step so the resulting ballistic trajectory of the walker’s center-of-mass will facilitate stepping on target footholds. Using a precision stepping task and synchronizing target visibility to the gait cycle, we empirically validated two predictions derived from this strategy: (1) Walkers must have information about upcoming footholds during the second half of the preceding step, and (2) foot placement is guided by information about the position of the target foothold relative to the preceding base of support. We conclude that active and passive modes of control work synergistically to allow walkers to negotiate complex terrain with efficiency, stability, and precision.

https://www.pnas.org/content/pnas/114/32/E6720.full.pdf

The critical phase for visual control of human walking over complex terrain

Walking is the most common terrestrial form of locomotion in animals. Its great versatility and flexibility has led to many attempts at building walking machines with similar capabilities. The control of walking is an active research area both in neurobiology and robotics, with a large and growing body of work. This paper gives an overview of the current knowledge on the control of legged locomotion in animals and machines and attempts to give walking control researchers from biology and robotics an overview of the current knowledge in both fields. We try to summarize the knowledge on the neurobiological basis of walking control in animals, emphasizing common principles seen in different species. In a section on walking robots, we review common approaches to walking controller design with a slight emphasis on biped walking control. We show where parallels between robotic and neurobiological walking controllers exist and how robotics and biology may benefit from each other. Finally, we discuss where research in the two fields diverges and suggest ways to bridge these gaps.

http://iopscience.iop.org/article/10.1088/1748-3190/10/4/041001/pdf

Controlling legs for locomotion—insights from robotics and neurobiology

This PhD thesis covers simulation and control of biped walking robots. Simulations and experiments were performed with the robots Johnnie and Lola developed at the Institute of Applied Mechanics, Technische Universitat Munchen, Germany.

The chapter on modeling and simulation describes the approach to simulating the rigid multibody dynamics. It includes specialized models for simulating drive friction, nonlinear drive kinematics and the unilateral ground contact. Component models, contact and differential equation solvers are combined to a family of robot simulations with varying modeling depth and computational cost.

The thesis also presents a hierarchical system for real-time walking control. Novel aspects include a trajectory generator based on spline collocation and a stabilizing controller based on hybrid position/force control. Lola’s redundant toe and pelvis joints are used to reduce joint speeds and avoid joint limits. 

All methods were verified on both robots in walking experiments and simulations. In experiments Lola reached a maximum walking speed of 3.34 km/h. using a computer vision system developed at the Institute of Autonomous Systems Technology at the Universitat der Bundeswehr in Munich, Germany, Lola is also capable of safely exploring previously unknown environments.

https://mediatum.ub.tum.de/doc/997204/file.pdf

Simulation and Control of Biped Walking Robots

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.

/pdf/the-yobotics-ihmc-lower-body-humanoid-robot-4tvt7u2ay9.pdf

The Yobotics-IHMC Lower Body Humanoid Robot

Koolen et al. - 2012 - Capturability-based analysis and control of legged locomotion, Part 1 Theory and application to three simple gait models(2)

Stepping for Balance Maintenance Including Push-Recovery

Future human and robotic planetary expeditions could benefit greatly from expanded Extra-Vehicular Activity (EVA) capabilities supporting a broad range of multiple, concurrent surface operations. Risky, expensive and complex, conventional EVAs are restricted in both duration and scope by consumables and available manpower, creating a resource management problem. A mobile, highly dexterous Extra-Vehicular Robotic (EVR) system called Centaur is proposed to cost-effectively augment human astronauts on surface excursions. The Centaur design combines a highly capable wheeled mobility platform with an anthropomorphic upper body mounted on a three degree-of-freedom waist. Able to use many ordinary handheld tools, the robot could conserve EVA hours by relieving humans of many routine inspection and maintenance chores and assisting them in more complex tasks, such as repairing other robots. As an astronaut surrogate, Centaur could take risks unacceptable to humans, respond more quickly to EVA emergencies and work much longer shifts. Though originally conceived as a system for planetary surface exploration, the Centaur concept could easily be adapted for terrestrial military applications such as de-Gig, surveillance and other hazardous duties.

/pdf/centaur-a-mobile-dexterous-humanoid-for-surface-operations-3cis0zqxqc.pdf

Jerry E. Pratt

Papers

The Yobotics-IHMC Lower Body Humanoid Robot

Koolen et al. - 2012 - Capturability-based analysis and control of legged locomotion, Part 1 Theory and application to three simple gait models(2)

Stepping for Balance Maintenance Including Push-Recovery

Centaur: a mobile dexterous humanoid for surface operations