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Events ■ ICAR 2015 Workshop on Compliance Control in Legged Locomotion

ICAR 2015 Workshop on Compliance Control in Legged Locomotion: Active vs. Passive

International Conference on Advanced Robotics (ICAR), Istanbul, Turkey

Workshop on Monday, 27th July 2015, Cibali Room, 9:00 - 17:30


As noted in the seminal work of Zheng & Hemami (1985), a legged robotic system needs to exhibit compliant locomotion behavior while interacting with the environment, regardless of the given terrain type. Many experimental studies substantiated this hypothesis; the incorporation of compliance in legged locomotion is observed to be a key factor in managing the position/force trade-off to address environmental adaptation, disturbance rejection, and dynamically equilibrated gait generation.

With this in mind, this workshop aims to bring together researchers who developed active and/or passive compliance control schemes in various forms to improve legged locomotion behavior. A special emphasis will be given to the comparison between active and passive compliance control schemes. Questions, such as, “Up to what extent can software-controlled active compliance controllers emulate their physical counterparts” and “What are the unique advantages of passive compliance and it comes at what cost” will be chiefly discussed.

Motivation and Objectives:

Contemporary robotics technology allowed the use of highly improved hardware and control architectures, which led to exciting improvements in compliance robotics. For instance, various mechanical design topologies are successfully designed and implemented to address adjustable physical stiffness control, in an attempt to emulate certain behaviors of biological muscles for agile locomotion. That being said, some research groups adopted highly effective torque control schemes that enabled real-time adjustment of stiffness and damping without any physical elasticity. At this point, we have several solutions towards the same goal, thus, the time is right to discuss performance metrics, trade-offs and implementation difficulties of these solutions.

This workshop aims to bring together practitioners to discuss the key aspects of compliance control in legged locomotion. Our main motivation is to create a forum between researchers, so they can convey their own experiences to the community with presentation and discussions. A particular emphasis is given to the real-life implementation issues; the role of active/passive compliance will be discussed in different legged locomotion applications: humanoids, quadrupeds, and exoskeletons.

The following questions will be discussed.

  • It is argued that active compliance schemes can adjust the stiffness and damping properties, thus, enabling highly compliant robot behaviors. Up to what extent can they replace their physical counterparts?
  • Can active compliance controllers handle impulsive disturbances? How to handle delays caused by controllers, sensors etc? Long-term effects?
  • It is argued that passively compliant systems can emulate biological muscles in terms of efficient locomotion generation. Recent studies indicate that they cannot store/release high amounts of energy, therefore, is it true that they can lead to energy-efficient locomotion?
  • At what cost can passive compliance be implemented? What are the design and implementation difficulties? What are recent design trends?
  • In generating locomotion with adjustable impedance (active or passive), how to generate suitable impedance trajectories? What should be the cost function to minimize/maximize in defining target impedance characteristics?.

Schedule (scroll down for the talk abstracts)

9:00 Welcome
9:10 Barkan Ugurlu, Ozyegin University, Istanbul, Turkey
9:50 Claudio Semini, IIT Genova, Italy
10:30 coffee break
10:50 Nikos Tsagarakis, IIT Genova, Italy
11:30 Alexander Herzog, MPI Tuebingen, Germany
12:10 Marco Hutter, ETH Zurich, Switzerland
12:50 lunch break
14:30 Ramazan Unal, Vrije University, Belgium
15:10 Thiago Boaventura, ETH Zurich, Switzerland
15:50  coffee break
16:10 Erhan Oztop, Ozyegin University, Istanbul, Turkey
16:50 discussion
17:20 final remarks
17:30 end of workshop

Invited Speakers

NikosTsagarakis Nikos Tsagarakis, Humanoids and Human-Centered Mechatronics (Head), Dept. of Advanced Robotics, Istituto Italiano di Tecnologia (IIT), Italy

Talk: Efficient and Robust Compliant Actuation Arrangements for WALK-MAN humanoid
Abstract: The mechatronic development of humanoid systems has considerably progressed during the past three decades with various designs based on different actuation technologies, from motorized based systems to hydraulic and soft actuation technologies. Despite the advancements in the design of motorized humanoids/bipeds significant barriers remain, preventing robot hardware (physical structure and actuation) from equalling the performance of human in locomotion and full body motion in terms of physical robustness and efficiency. The talk will introduce ongoing work and results towards the development of our most recent humanoid platform WALK-MAN. Details on the actuation design approaches to achieve the necessary performance in terms of physical resilience, energy efficiency and high power and fast motion capabilities will be presented.


RamazanUnal Ramazan Unal, Dept. of Mechanical Engineering, Vrije University, Brussels, Belgium

Talk: Compliant actuators for lower limb exoskeletons and prostheses
Abstract: An increasing number of robotics researchers have realized that in animals and humans not only the brain creates the intelligence of the body, but that the morphology and biomechanics have a great impact on the way animals and humans think and move. A critical role in this respect is played by the neuro-mechanics of muscles, which have functional performance and control capabilities far in excess reached by artificial actuators. In traditional robotics, stiff actuators controlled as servomotor governed by the principle of “the stiffer the better”, produce high bandwidth control adapted to tasks requiring tracking of a desired trajectory with high accuracy. Many -mostly novel- applications requiring interaction with an unknown and dynamic environment including humans require dynamics that are not well suited to servomotors. Therefore, compliant actuators are being developed, which are inspired by biological motor control, where the influence of muscle spring-like properties and their control are of great importance. In the presentation we will highlight the role of compliant elements to design improved lower limb exoskeletons for assistance and rehabilitation and prostheses.

ErhanOztop Erhan Öztop, Dept. of Mechanical Engineering, Ozyegin University, Istanbul, Turkey

Talk: Central Nervous System Actively Modulates Muscle Impedance: Can we transfer skills from Humans to Impedance Controlled Robots?
Abstract: The sensorimotor learning capacity allows humans to efficiently learn to use new tools and control tasks. To realize this ability in complex systems such as robots is still elusive. Never the less, it is possible to use this human ability to transfer nontrivial sensorimotor skills to robots using a human-in-the-control-loop setup. The idea is to consider the target robotic platform as a tool that can be controlled by a human. Provided with an intuitive interface for controlling the robot, the human learns to perform a given task using the robot. After sufficient learning, the skilled control of the robot by the human provides learning data points that can be used to obtain an autonomous controller so that the robot can perform the task without human guidance. The feasibility of this framework is supported by the neuroscientific findings on body schema and has been shown to work for several robot skill synthesis scenarios. From an engineering point of view, the approach relies on techniques from teleoperation and machine learning, and has the same goals with robot learning by demonstration. The key difference is that the proposed framework includes the human in the control loop and employs the human brain as the adaptive controller to accomplish a given task. Once the control proficiency has been attained, the data generated by the human performance allows the human policy to be transferred to the robot. In this talk, I will introduce the human-in-the-loop framework and outline the current challenges for facilitating a wide impact for the development of adaptive systems for complex environments.


AlexanderHerzog Alexander Herzog, Autonomous Motion Department, Max-Planck-Institute for Intelligent Systems, Tuebingen, Germany

Talk: Active Compliance with LQR-based Momentum Control and Hierarchical Inverse Dynamics
Abstract: Hierarchical inverse dynamics based on cascades of quadratic programs have been proposed for the compliant control of legged robots. They have important benefits but to the best of our knowledge have never been implemented on a torque controlled humanoid where model inaccuracies, sensor noise and real-time computation requirements can be problematic. Using a reformulation of existing algorithms, we propose a simplification of the problem that allows to achieve real-time control. Momentum-based control is integrated in the task hierarchy and a LQR design approach is used to compute the desired associated closed-loop behavior and improve performance. Extensive experiments on various balancing and tracking tasks show very robust performance in the face of unknown disturbances, even when the humanoid is standing on one foot. Our results demonstrate that hierarchical inverse dynamics together with momentum control can be efficiently used for feedback control under real robot conditions. Although, easy to design in our balance experiments, desired momentum trajectories are non-trivial to chose when we tackle more sophisticated tasks. In the second part of my talk I will discuss preliminary simulation experiments that address this problem with a trajectory optimization approach.

MarcoHutter Marco Hutter, Inst. f. Robotik u. Intell. Syst., ETH, Zurich, Switzerland

Talk: Compliant actuation to enhance dynamic locomotion performance of legged robots
Abstract: Including mechanical compliance in the actuation system of legged robotic systems is known to provide several benefits such as precise torque controllability, inherent robustness, increase of peak power, and temporary storage of energy.  However, this comes at a price of more complex system dynamics and, due to the mechanical lowpass behavior, a decreasing control bandwidth – a problem that often scares people away from using highly compliant actuators in versatile robots. In this talk, I want to provide an insight into our experience with the application of highly compliant SEAs at the example of the (autonomous) quadrupedal robot StarlETH. The goal is to foster the understanding how to choose the compliance, how to exploit it using motion optimization and learning tools, and how to control it to achieve versatile behaviors.

ThiagoBoaventura Thiago Boaventura, Agile and Dexterous Robotics Lab., ETH, Zurich, Switzerland

Talk: Interaction force control in wearable robots
Abstract: To improve human quality of life is the main goal in the field of wearable robotics, which includes devices such as exoskeletons and limb-prostheses. These wearable robots are closely attached to human body in a symbiotic way, moving and working in tandem with the human. In several applications, it is paramount to control the robot forces applied to the user. In this talk I will give an overview of the commonly used approaches for interaction force control in wearable robots, and then propose a novel method which is currently being patented by ETH Zurich. This new control methods aims at improving the intrinsic limitations of traditional impedance control for masking inertia. Such approach is particularly suitable to provide an enhanced transparent (i.e. imperceptible) behavior for wearable robots.


BarkanUgurlu Barkan Ugurlu, Legged Locomotion Group (Head), Dept. of Mechanical Engineering, Ozyegin University, Istanbul, Turkey

Talk: Variable Ankle Stiffness Outperforms Optimal Constant Stiffness: Experiments on a Soft Exoskeleton
Abstract: Variable Stiffness Actuators (VSAs) have been around for quite some time; however, this crucial property has not been exploited in legged robot control task. In this talk, we present a real-time balance controller which utilizes the variable physical ankle stiffness by means of ZMP feedback. Inspired by the findings in biomechanics literature, a soft exoskeleton with pneumatic muscles were built and used as the testbed. The proposed variable ankle stiffness strategy was compared to optimal constant stiffness strategy. As the result, the proposed method outperformed the optimal constant stiffness method in terms of disturbance rejection, ZMP/CoM position regulation, and mechanical power; while air mass consumption rate increased about 70%. To the authors' knowledge, this is the first implementation of variable physical stiffness in the legged robot control literature. Details regarding this study will be discussed during the talk.

ClaudioSemini Claudio Semini, Dynamic Legged Systems (Head), Dept. of Advanced Robotics, Istituto Italiano di Tecnologia (IIT), Italy

Talk: Active impedance for increased Performance and Versatility of Legged Robots
Abstract: This talk will focus on the benefits of active impedance for versatile legged robots. To improve the performance and versatility of legged robots in real-world settings, it is crucial to properly control the interaction forces between the robot and the environment. We will focus on active impedance/compliance. Recent experiments on IIT’s fully torque-controlled hydraulic robot HyQ are used to illustrate the importance of active impedance for legged robots. HyQ has recently demonstrated a large set of behaviors ranging from highly dynamic motions to planned navigation on rough terrain, e.g. flying trot, punch bag disturbance rejection, and walking over stepping stones.


This workshop is organized and supported by the H2020 Topic Group on Field/Service Robots in unstructured Environments.
Futhermore this workshop is co-organized by the IEEE-RAS Technical Committee on Mechanisms and Design.


Last Updated on Saturday, 25 July 2015 20:46


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