Virtual Constraints and Hybrid Zero Dynamics for Realizing Underactuated Bipedal Locomotion

Underactuation is ubiquitous in human locomotion and should be ubiquitous in bipedal robotic locomotion as well. This chapter presents a coherent theory for the design of feedback controllers that achieve stable walking gaits in underactuated bipedal robots. Two fundamental tools are introduced, virtual constraints and hybrid zero dynamics. Virtual constraints are relations on the state variables of a mechanical model that are imposed through a time-invariant feedback controller. One of their roles is to synchronize the robot's joints to an internal gait phasing variable. A second role is to induce a low dimensional system, the zero dynamics, that captures the underactuated aspects of a robot's model, without any approximations. To enhance intuition, the relation between physical constraints and virtual constraints is first established. From here, the hybrid zero dynamics of an underactuated bipedal model is developed, and its fundamental role in the design of asymptotically stable walking motions is established. The chapter includes numerous references to robots on which the highlighted techniques have been implemented.Comment: 17 pages, 4 figures, bookchapte

Planning and Control Strategies for Motion and Interaction of the Humanoid Robot COMAN+

Despite the majority of robotic platforms are still confined in controlled environments such as factories, thanks to the ever-increasing level of autonomy and the progress on human-robot interaction, robots are starting to be employed for different operations, expanding their focus from uniquely industrial to more diversified scenarios. Humanoid research seeks to obtain the versatility and dexterity of robots capable of mimicking human motion in any environment. With the aim of operating side-to-side with humans, they should be able to carry out complex tasks without posing a threat during operations. In this regard, locomotion, physical interaction with the environment and safety are three essential skills to develop for a biped. Concerning the higher behavioural level of a humanoid, this thesis addresses both ad-hoc movements generated for specific physical interaction tasks and cyclic movements for locomotion. While belonging to the same category and sharing some of the theoretical obstacles, these actions require different approaches: a general high-level task is composed of specific movements that depend on the environment and the nature of the task itself, while regular locomotion involves the generation of periodic trajectories of the limbs. Separate planning and control architectures targeting these aspects of biped motion are designed and developed both from a theoretical and a practical standpoint, demonstrating their efficacy on the new humanoid robot COMAN+, built at Istituto Italiano di Tecnologia. The problem of interaction has been tackled by mimicking the intrinsic elasticity of human muscles, integrating active compliant controllers. However, while state-of-the-art robots may be endowed with compliant architectures, not many can withstand potential system failures that could compromise the safety of a human interacting with the robot. This thesis proposes an implementation of such low-level controller that guarantees a fail-safe behaviour, removing the threat that a humanoid robot could pose if a system failure occurred

Exploiting inherent robustness and natural dynamics in the control of bipedal walking robots

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2000.Includes bibliographical references (p. 115-120).Walking is an easy task for most humans and animals. Two characteristics which make it easy are the inherent robustness (tolerance to variation) of the walking problem and the natural dynamics of the walking mechanism. In this thesis we show how understanding and exploiting these two characteristics can aid in the control of bipedal robots. Inherent robustness allows for the use of simple, low impedance controllers. Natural dynamics reduces the requirements of the controller. We present a series of simple physical models of bipedal walking. The insight gained from these models is used in the development of three planar (motion only in the sagittal plane) control algorithms. The first uses simple strategies to control the robot to walk. The second exploits the natural dynamics of a kneecap, compliant ankle, and passive swing-leg. The third achieves fast swing of the swing-leg in order to enable the robot to walk quickly (1.25m). These algorithms are implemented on Spring Flamingo, a planar bipedal walking robot, which was designed and built for this thesis. Using these algorithms, the robot can stand and balance, start and stop walking, walk at a range of speeds, and traverse slopes and rolling terrain. Three-dimensional walking on flat ground is implemented and tested in simulation. The dynamics of the sagittal plane are sufficiently decoupled from the dynamics of the frontal and transverse planes such that control.-of each can be treated separately. We achieve three-dimensional walking by adding lateral balance to the planar algorithms. Tests of this approach on a real three-dimensional robot will lead to a more complete understanding of the control of bipedal walking in robots and humans.by Jerry E. Pratt.Ph.D

Running synthesis and control for monopods and bipeds with articulated

Bibliography: p. 179-20

Control Implementation of Dynamic Locomotion on Compliant, Underactuated, Force-Controlled Legged Robots with Non-Anthropomorphic Design

The control of locomotion on legged robots traditionally involves a robot that takes a standard legged form, such as the anthropomorphic humanoid, the dog-like quadruped, or the bird-like biped. Additionally, these systems will often be actuated with position-controlled servos or series-elastic actuators that are connected through rigid links. This work investigates the control implementation of dynamic, force-controlled locomotion on a family of legged systems that significantly deviate from these classic paradigms by incorporating modern, state-of-the-art proprioceptive actuators on uniquely configured compliant legs that do not closely resemble those found in nature. The results of this work can be used to better inform how to implement controllers on legged systems without stiff, position-controlled actuators, and also provide insight on how intelligently designed mechanical features can potentially simplify the control of complex, nonlinear dynamical systems like legged robots. To this end, this work presents the approach to control for a family of non-anthropomorphic bipedal robotic systems which are developed both in simulation and with physical hardware. The first is the Non-Anthropomorphic Biped, Version 1 (NABi-1) that features position-controlled joints along with a compliant foot element on a minimally actuated leg, and is controlled using simple open-loop trajectories based on the Zero Moment Point. The second system is the second version of the non-anthropomorphic biped (NABi-2) which utilizes the proprioceptive Back-drivable Electromagnetic Actuator for Robotics (BEAR) modules for actuation and fully realizes feedback-based force controlled locomotion. These systems are used to highlight both the strengths and weaknesses of utilizing proprioceptive actuation in systems, and suggest the tradeoffs that are made when using force control for dynamic locomotion. These systems also present case studies for different approaches to system design when it comes to bipedal legged robots

Bio-inspired vertebral column, compliance and semi-passive dynamics in a lightweight robot

International audienceThis paper presents the humanoid robot Acroban. We study two main issues: 1) Compliance and semi-passive dynamics for locomotion of humanoid robots regarding robustness against unknown external perturbations; 2) The advantages of a bio-inspired multi-articulated vertebral column. We combine mechatronic compliance with structural compliance due to the use of flexible materials. And we explore how these capabilities allow to enforce morphological computation in the design of robust dynamic locomotion. We also investigate the use of compliance to design semi-passive motor primitives using the torso and the arms as a system of accumulation/release of potential/kinetic energy

Design and Control of a Knee Exoskeleton for Assistance and Power Augmentation

Thanks to the technological advancements, assistive lower limb exoskeletons are moving from laboratory settings to daily life scenarios. This dissertation makes a contribution toward the development of assistive/power augmentation knee exoskeletons with an improved wearability, ergonomics and intuitive use. In particular, the design and the control of a novel knee exoskeleton system, the iT-Knee Bipedal System, is presented. It is composed by: a novel mechanism to transmit the assistance generated by the exoskeleton to the knee joint in a more ergonomic manner; a novel method that requires limited information to estimate online the torques experienced by the ankles, knees and hips of a person wearing the exoskeleton; a novel sensor system for shoes able to track the feet orientation and monitor their full contact wrench with the ground. In particular, the iT-Knee exoskeleton, the main component of the aforementioned system, is introduced. It is a novel six degree of freedom knee exoskeleton module with under-actuated kinematics, able to assist the flexion/extension motion of the knee while all the other joint\u2019s movements are accommodated. Thanks to its mechanism, the system: solves the problem of the alignment between the joint of the user and the exoskeleton; it automatically adjusts to different users\u2019 size; reduces the undesired forces and torques exchanged between the attachment points of its structure and the user\u2019s skin. From a control point of view, a novel approach to address difficulties arising in real life scenarios (i.e. noncyclic locomotion activity, unexpected terrain or unpredicted interactions with the surroundings) is presented. It is based on a method that estimates online the torques experienced by a person at his ankles, knees and hips with the major advantage that does not rely on any information of the user\u2019s upper body (i.e. pose, weight and center of mass location) or on any interaction of the user\u2019s upper body with the environment (i.e. payload handling or pushing and pulling task). This is achieved v by monitoring the full contact wrench of the subject with the ground and applying an inverse dynamic approach to the lower body segments. To track the full contact wrench between the subject\u2019s feet and the ground, a novel add on system for shoes has been developed. The iT-Shoe is adjustable to different user\u2019s size and accommodates the plantar flexion of the foot. It tracks the interactions and the orientation of the foot thanks to two 6axis Force/Torque sensors, developed in-house, with dedicated embedded MEMS IMUs placed at the toe and heel area. Different tasks and ground conditions were tested to validate and highlight the potentiality of the proposed knee exoskeleton system. The experimental results obtained and the feedback collected confirm the validity of the research conducted toward the design of more ergonomic and intuitive to use exoskeletons

Feedback Control of a Bipedal Walker and Runner with Compliance.

This dissertation contributes to the theoretical foundations of robotic bipedal locomotion and advances the experimental state of the art as well. On the theoretical side, a mathematical formalism for designing provably stable, walking and running gaits in bipedal robots with compliance is presented. A key contribution is a novel method of force control in robots with compliance. The theoretical work is validated experimentally on MABEL, a planar bipedal testbed that contains springs in its drivetrain for the purpose of enhancing both energy efficiency and agility of dynamic locomotion. While the potential energetic benefits of springs are well documented in the literature, feedback control designs that effectively realize this potential are lacking. The methods of virtual constraints and hybrid zero dynamics, originally developed for rigid robots with a single degree of underactuation, are extended and applied to MABEL, which has a novel compliant transmission and multiple degrees of underactuation. A time-invariant feedback controller is designed such that the closed-loop system respects the natural compliance of the open-loop system and realizes exponentially stable walking gaits. A second time-invariant feedback controller is designed such that the closed-loop system not only respects the natural compliance of the open-loop system, but also enables active force control within the compliant hybrid zero dynamics and results in exponentially stable running gaits. Several experiments are presented that highlight different aspects of MABEL and the feedback design method, ranging from basic elements such as stable walking, robustness under perturbations, energy efficient walking to a bipedal robot walking speed record of 1.5 m/s (3.4 mph), stable running with passive feet and with point feet. On MABEL, the full hybrid zero dynamics controller is implemented and was instrumental in achieving rapid walking and running, leading upto a kneed bipedal running speed record of 3.06 m/s (6.8 mph).Ph.D.Electrical Engineering: SystemsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/89801/1/koushils_1.pd