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IEEE-RAS International Conference on Humanoid Robots

作者： Markus Giftthaler Jonas Buchli Agile & Dexterous Robotics Lab Institute of Robotics and Intelligent Systems ETH Zürich Switzerland

ISBN: (纸本)9781538646793

This paper presents a state and state-input constrained variant of the discrete-time iterative Linear Quadratic Regulator (iLQR) algorithm, with linear time-complexity in the number of time steps. The approach is based on a projection of the control input onto the nullspace of the linearized constraints. We derive a fully constraint-compliant feedforward-feedback control update rule, for which we can solve efficiently with Riccati-style difference equations. We assume that the relative degree of all constraints in the discrete-time system model is equal to one, which often holds for robotics problems employing rigid-body dynamic models. Simulation examples, including a 6 DoF robotic arm, are given to validate and illustrate the performance of the method.

关键词： Optimal control Manganese Robots System dynamics Mathematical model Heuristic algorithms Trajectory

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Automatic Differentiation of Rigid Body Dynamics for Optimal Control and Estimation

arXiv

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arXiv 2017年

作者： Giftthaler, Markus Neunert, Michael Stäuble, Markus Frigerio, Marco Semini, Claudio Buchli, Jonas Agile & Dexterous Robotics Lab Institute of Robotics and Intelligent Systems ZürichETH Switzerland Dept. of Advanced Robotics Istituto Italiano di Tecnologia Genova Italy

Many algorithms for control, optimization and estimation in robotics depend on derivatives of the underlying system dynamics, e.g. to compute linearizations, sensitivities or gradient directions. However, we show that when dealing with Rigid Body Dynamics, these derivatives are difficult to derive analytically and to implement efficiently. To overcome this issue, we extend the modelling tool ‘RobCoGen’ to be compatible with Automatic Differentiation. Additionally, we propose how to automatically obtain the derivatives and generate highly efficient source code. We highlight the flexibility and performance of the approach in two application examples. First, we show a Trajectory Optimization example for the quadrupedal robot HyQ, which employs auto-differentiation on the dynamics including a contact model. Second, we present a hardware experiment in which a 6 DoF robotic arm avoids a randomly moving obstacle in a go-to task by fast, dynamic replanning. This paper is an extended version of [1]. Copyright © 2017, The Authors. All rights reserved.

关键词： Aerodynamics

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Design, development and experimental assessment of a robotic end-effector for non-standard concrete applications

Design, development and experimental assessment of a robotic...

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IEEE International Conference on robotics and Automation (ICRA)

作者： Nitish Kumar Norman Hack Kathrin Doerfler Alexander Nikolas Walzer Gonzalo Javier Rey Fabio Gramazio Matthias Daniel Kohler Jonas Buchli The Agile & Dexterous Robotics Lab The Institute of Robotics and Intelligent Systems ETH Zurich Switzerland The Chair of Architecture and Digital Fabrication The Institute of Technology in Architecture ETH Zurich Switzerland Gonzalo Javier Rey is with Moog Inc

Despite the recent advances in, and the adoption of robotic technologies in the construction industry, the architectural processes which demand a high degree of geometric freedom still remain largely labour intensive and manual. This is due to the inherent difficulties in robotizing the current implementation of such processes coupled with the lack of alternate robotic technologies. A specific example, which is also the focus of this paper, is that of building a steel reinforced concrete structure, with varying curvature or cross-section. This process still remains rather manual and requires extensive support of customized form-work. In this paper, first we describe an alternate novel robotic fabrication process for building steel wire meshes which act as both reinforcement and formwork. The robotization of such a process is discussed with the use of a previously developed mobile robotic system. Based on the specifications derived from the process, design of a novel custom designed robotic end-effector, enabling this process, is detailed. Automation of the full robotic system comprising the mobile robotic system and the robotic end-effector is discussed from simulation to control. Through experimental evaluation of the robotic system, we demonstrate the ability to fully automate the construction of non-standard steel reinforced steel meshes of varying curvature and cell sizes.

关键词： Wires Steel Concrete Service robots Fabrication Three-dimensional displays

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Mobile robotic fabrication at 1:1 scale: the In situ Fabricator

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Construction robotics 2017年第1期1卷 3-14页

作者： Markus Giftthaler Timothy Sandy Kathrin Dörfler Ian Brooks Mark Buckingham Gonzalo Rey Matthias Kohler Fabio Gramazio Jonas Buchli Agile & Dexterous Robotics Lab ETH Zürich Institute of Robotics and Intelligent Systems Zürich Switzerland ETH Zürich Chair of Architecture and Digital Fabrication Zürich Switzerland Moog Inc. Buffalo USA Renishaw PLC. Gloucester UK

This paper presents the concept of an In situ Fabricator, a mobile robot intended for on-site manufacturing, assembly and digital fabrication. We present an overview of a prototype system, its capabilities, and highlight the importance of high-performance control, estimation and planning algorithms for achieving desired construction goals. Next, we detail on two architectural application scenarios: first, building a full-size undulating brick wall, which required a number of repositioning and autonomous localisation manoeuvres. Second, the mesh mould concrete process, which shows that an In situ Fabricator in combination with an innovative digital fabrication tool can be used to enable completely novel building technologies. Subsequently, important limitations of our approach are discussed. Based on that, we identify the need for a new type of robotic actuator, which facilitates the design of novel full-scale construction robots. We provide brief insight into the development of this actuator and conclude the paper with an outlook on the next-generation In situ Fabricator, which is currently under development.

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Numerical search for local (partial) differential flatness

Numerical search for local (partial) differential flatness

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IEEE/RSJ International Conference on intelligent Robots and systems

作者： Carmelo Sferrazza Diego Pardo Jonas Buchli Agile Dexterous Robotics Lab at the Institute of Robotics and Intelligent Systems ETH Zurich Switzerland

ISBN: (纸本)9781509037636

Differential flatness is a property of certain systems that greatly simplifies the generation of optimal and dynamically feasible trajectories. Using a differentially flat model, there is no need to integrate the system dynamics to retrieve the states and the constraints of the optimization problem are simpler. Recently, the concept of partial differential flatness has been introduced covering a broader class of systems. In particular, it allows to reduce the need for integration by limiting it to a subset of the states. However, finding an analytical expression for the (partial) differential flatness requires the manipulation of the equations of motion in a very specific manner such that a series of properties are fulfilled. In general, finding such analytical model is not straightforward nor compatible with algorithmic models. In order to tackle this problem, in this paper we present a numerical method to find a (partially) differentially flat model of a system around a collection of states and inputs trajectories. We present results on three underactuated nonlinear systems (cart-pole, planar ballbot and a 3D quadrotor). As use case examples, we show online trajectory re-planning tasks. The validity of the trajectories obtained with the locally flat models is verified by forward integrating the original equations of motion together with an optimal stabilizer.

关键词： Trajectory Mathematical model Numerical models Robots Optimization Analytical models System dynamics

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On the use of positive feedback for improved torque control

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Control Theory and Technology 2015年第3期13卷 266-285页

作者： Houman DALLALI Gustavo A.MEDRANO-CERDA Michele FOCCHI Thiago BOAVENTURA Marco FRIGERIO Claudio SEMINI Jonas BUCHLI Darwin G.CALDWELL Department of Advanced Robotics Istituto Italiano di Tecnologia(IIT) Agile and Dexterous Robotics Lab Institute of Robotics and Intelligent SystemsETH

This paper considers the torque control problem for robots with flexible joints driven by electrical actuators. It is shown that the achievable closed-loop tracking bandwidth using PI torque controllers may be limited due to transmission zeros introduced by the load dynamics. This limitation is overcome by using positive feedback from the load motion in unison with PI torque controllers. The positive feedback is given in terms of load velocity, acceleration and jerk. Stability conditions for designing decentralized PI torque controllers are derived in terms of Routh-Hurwitz criteria. Disturbance rejection properties of the closed system are characterized and an analysis is carried out investigating the use of approximate positive feedback by omitting acceleration and/or jerk signals. The results of this paper are illustrated for a two DoF （degrees of freedom） system. Experimental results for a one DoF system are also included.

关键词： Force-torque control load motion compensation decentralized control

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Unified motion control for dynamic quadrotor maneuvers demonstrated on slung load and rotor failure tasks

Unified motion control for dynamic quadrotor maneuvers demon...

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IEEE International Conference on robotics and Automation (ICRA)

作者： Cédric de Crousaz Farbod Farshidian Michael Neunert Jonas Buchli Agile & Dexterous Robotics Lab Institute of Robotics and Intelligent Systems ETH Zürich Switzerland

ISBN: (纸本)9781479969241

In recent years impressive results have been presented illustrating the potential of quadrotors to solve challenging tasks. Generally, the derivation of the controllers involve complex analytical manipulation of the dynamics and are very specific to the task at hand. In addition, most approaches construct a trajectory and then design a stabilizing controller in a separate step, whereas a fully optimal solution requires finding both simultaneously. In this paper, a generalized approach is presented using an iterative optimal control algorithm. A series of complex tasks are thus solved using the same algorithm without the need for manual manipulation of the system dynamics, heuristic simplifications, or manual trajectory generation. First, aggressive maneuvers are performed by requiring the quadrotor to pass with a slung load through a window not high enough for the load to pass while hanging straight down. Second, go-to-goal tasks with single and double rotor failure are demonstrated. The adaptability and applicability of this unified approach to such diverse tasks with a nonlinear, underactuated, constrained, and in the case of the slung load, hybrid quadrotor systems is thus shown.

关键词： Rotors Trajectory Heuristic algorithms Load modeling Algorithm design and analysis Cost function Switches

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Planning and execution of dynamic whole-body locomotion for a hydraulic quadruped on challenging terrain

Planning and execution of dynamic whole-body locomotion for ...

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IEEE International Conference on robotics and Automation (ICRA)

作者： Alexander W. Winkler Carlos Mastalli Ioannis Havoutis Michele Focchi Darwin G. Caldwell Claudio Semini Department of Advanced Robotics Istituto Italiano di Tecnologia Genova Italy Agile & Dexterous Robotics Lab Institute of Robotics and Intelligent Systems ETH Zurich Switzerland

We present a framework for dynamic quadrupedal locomotion over challenging terrain, where the choice of appropriate footholds is crucial for the success of the behaviour. We build a model of the environment on-line and on-board using an efficient occupancy grid representation. We use Any-time-Repairing A* (ARA*) to search over a tree of possible actions, choose a rough body path and select the locally-best footholds accordingly. We run a n-step lookahead optimization of the body trajectory using a dynamic stability metric, the Zero Moment Point (ZMP), that generates natural dynamic whole-body motions. A combination of floating-base inverse dynamics and virtual model control accurately executes the desired motions on an actively compliant system. Experimental trials show that this framework allows us to traverse terrains at nearly 6 times the speed of our previous work, evaluated over the same set of trials.

关键词： Dynamics Trajectory Acceleration Planning Vehicle dynamics Legged locomotion

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Learning of Closed-Loop Motion Control

Learning of Closed-Loop Motion Control

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IEEE/RSJ International Conference on intelligent Robots and systems

作者： Farbod Farshidian Michael Neunert Jonas Buchli Agile & Dexterous Robotics Lab at the Institute of Robotics and Intelligent Systems ETH Zurich Switzerland

ISBN: (纸本)9781479969357

Learning motion control as a unified process of designing the reference trajectory and the controller is one of the most challenging problems in robotics. The complexity of the problem prevents most of the existing optimization algorithms from giving satisfactory results. While model-based algorithms like iterative linear-quadratic-Gaussian (iLQG) can be used to design a suitable controller for the motion control, their performance is strongly limited by the model accuracy. An inaccurate model may lead to degraded performance of the controller on the physical system. Although using machine learning approaches to learn the motion control on real systems have been proven to be effective, their performance depends on good initialization. To address these issues, this paper introduces a two-step algorithm which combines the proven performance of a model-based controller with a model-free method for compensating for model inaccuracy. The first step optimizes the problem using iLQG. Then, in the second step this controller is used to initialize the policy for our PI~2-01 reinforcement learning algorithm. This algorithm is a derivation of the PI~2 algorithm enabling more stable and faster convergence. The performance of this method is demonstrated both in simulation and experimental results.

关键词： Motor Control Controllers Optimization algorithms degraded performance Learning initialization

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