Visible to the public Biblio

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2012
Zoe McCarthy, University of Illinois at Urbana-Champaign, Timothy Bretl, University of Illinois at Urbana-Champaign.  2012.  Mechanics and Manipulation of Planar Elastic Kinematic Chains. IEEE International Conference on Robotics and Automation.

This paper presents a control strategy based on model learning for a self-assembled robotic “swimmer”. The swimmer forms when a liquid suspension of ferro-magnetic micro-particles and a non-magnetic bead are exposed to an alternating magnetic field that is oriented perpendicular to the liquid surface. It can be steered by modulating the frequency of the alternating field. We model the swimmer as a unicycle and learn a mapping from frequency to forward speed and turning rate using locally-weighted projection regression. We apply iterative linear quadratic regulation with a receding horizon to track motion primitives that could be used for path following. Hardware experiments validate our approach.

Zoe McCarthy, University of Illinois at Urbana-Champaign, Timothy Bretl, University of Illinois at Urbana-Champaign.  2012.  Mechanics and Manipulation of Planar Elastic Kinematic Chains. 2012 IEEE International Conference on Robotics and Automation.

In this paper, we study quasi-static manipulation of a planar kinematic chain with a fixed base in which each joint is a linearly-elastic torsional spring. The shape of this chain when in static equilibrium can be represented as the solution to a discrete-time optimal control problem, with boundary conditions that vary with the position and orientation of the last link. We prove that the set of all solutions to this problem is a smooth manifold that can be parameterized by a single chart. For manipulation planning, we show several advantages of working in this chart instead of in the space of boundary conditions, particularly in the context of a sampling-based planning algorithm. Examples are provided in simulation.

Timothy Bretl, University of Illinois at Urbana-Champaign, Zoe McCarthy, University of Illinois at Urbana-Champaign.  2012.  Equilibrium Configurations of a Kirchhoff Elastic Rod under Quasi-static Manipulation. Workshop on the Algorithmic Foundations of Robotics (WAFR).

Consider a thin, flexible wire of fixed length that is held at each end by a robotic gripper. The curve traced by this wire can be described as a local solution to a geometric optimal control problem, with boundary conditions that vary with the position and orientation of each gripper. The set of all local solutions to this problem is the configuration space of the wire under quasi-static manipulation. We will show that this configuration space is a smooth manifold of finite dimension that can be parameterized by a single chart. Working in this chart—rather than in the space of boundary conditions—makes the problem of manipulation planning very easy to solve. Examples in simulation illustrate our approach.

Abdullah Akce, University of Illinois at Urbana-Champaign, James Norton, University of Illinois at Urbana-Champaign, Timothy Bretl, University of Illinois at Urbana-Champaign.  2012.  A Brain-Machine Interface to Navigate Mobile Robots Along Human-Like Paths Amidst Obstacles. IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS).

This paper presents an interface that allows a human user to specify a desired path for a mobile robot in a planar workspace with noisy binary inputs that are obtained at low bit-rates through an electroencephalograph (EEG). We represent desired paths as geodesics with respect to a cost function that is defined so that each path-homotopy class contains exactly one (local) geodesic. We apply max-margin structured learning to recover a cost function that is consistent with observations of human walking paths. We derive an optimal feedback communication protocol to select a local geodesic— equivalently, a path-homotopy class—using a sequence of noisy bits. We validate our approach with experiments that quantify both how well our learned cost function characterizes human walking data and how well human subjects perform with the resulting interface in navigating a simulated robot with EEG.

Dennis Matthews, University of Illinois at Urbana-Champaign, Timothy Bretl, University of Illinois at Urbana-Champaign.  2012.  Experiments in Quasi-Static Manipulation of a Planar Elastic Rod. IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS).

In this paper, we introduce and experimentally validate a sampling-based planning algorithm for quasi-static manipulation of a planar elastic rod. Our algorithm is an immediate consequence of deriving a global coordinate chart of finite dimension that suffices to describe all possible configurations of the rod that can be placed in static equilibrium by fixing the position and orientation of each end. Hardware experiments confirm this derivation in the case where the “rod” is a thin, flexible strip of metal that has a fixed base and that is held at the other end by an industrial robot. We show an example in which a path of the robot that was planned by our algorithm causes the metal strip to move between given start and goal configurations while remaining in quasi-static equilibrium.

Timothy Bretl, University of Illinois at Urbana-Champaign, Zoe McCarthy, University of Illinois at Urbana-Champaign.  2012.  Mechanics and Quasi-Static Manipulation of Planar Elastic Kinematic Chains. IEEE Transactions on Robotics. 29(1)

In this paper, we study quasi-static manipulation of a planar kinematic chain with a fixed base in which each joint is a linearly elastic torsional spring. The shape of this chain when in static equilibrium can be represented as the solution to a discretetime optimal control problem, with boundary conditions that vary with the position and orientation of the last link. We prove that the set of all solutions to this problem is a smooth three-manifold that can be parameterized by a single chart. Empirical results in simulation show that straight-line paths in this chart are uniformly more likely to be feasible (as a function of distance) than straightline paths in the space of boundary conditions. These results, which are consistent with an analysis of visibility properties, suggest that the chart we derive is a better choice of space in which to apply a sampling-based algorithm for manipulation planning. We describe such an algorithm and show that it is easy to implement.

2013
Timothy Bretl, University of Illinois at Urbana-Champaign, Zoe McCarthy, University of Illinois at Urbana-Champaign.  2013.  Quasi-Static Manipulation of a Kirchhoff Elastic Road Based on Geometric Analysis of Equilibrium Configurations. The International Journal of Robotics Research. 33(1)

Consider a thin, flexible wire of fixed length that is held at each end by a robotic gripper. Any curve traced by this wire when in static equilibrium is a local solution to a geometric optimal control problem, with boundary conditions that vary with the position and orientation of each gripper. We prove that the set of all local solutions to this problem over all possible boundary conditions is a smooth manifold of finite dimension that can be parameterized by a single chart. We show that this chart makes it easy to implement a sampling-based algorithm for quasi-static manipulation planning. We characterize the performance of such an algorithm with experiments in simulation.

Published in The International Journal of Robotics Research

Navid Aghasadeghi, University of Illinois at Urbana-Champaign, Huihua Zhao, Texas A&M University, Levi J. Hargrove, Northwestern University, Aaron D. Ames, Texas A&M University, Eric J. Perreault, Northwestern University, Timothy Bretl, University of Illinois at Urbana-Champaign.  2013.  Learning Impedance Controller Parameters for Lower-Limb Prostheses. 2013 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS).

Impedance control is a common framework for control of lower-limb prosthetic devices. This approach requires choosing many impedance controller parameters. In this paper, we show how to learn these parameters for lower-limb prostheses by observation of unimpaired human walkers. We validate our approach in simulation of a transfemoral amputee, and we demonstrate the performance of the learned parameters in a preliminary experiment with a lower-limb prosthetic device.

Miles Johnson, University of Illinois at Urbana-Champaign, Navid Aghasadeghi, University of Illinois at Urbana-Champaign, Timothy Bretl, University of Illinois at Urbana-Champaign.  2013.  Inverse Optimal Control for Deterministic Continuous-time Nonlinear Systems. 52nd Conference on Decision and Control.

Inverse optimal control is the problem of computing a cost function with respect to which observed state and input trajectories are optimal. We present a new method of inverse optimal control based on minimizing the extent to which observed trajectories violate first-order necessary conditions for optimality. We consider continuous-time deterministic optimal control systems with a cost function that is a linear combination of known basis functions. We compare our approach with three prior methods of inverse optimal control. We demonstrate the performance of these methods by performing simulation experiments using a collection of nominal system models. We compare the robustness of these methods by analysing how they perform under perturbations to the system. To this purpose, we consider two scenarios: one in which we exactly know the set of basis functions in the cost function, and another in which the true cost function contains an unknown perturbation. Results from simulation experiments show that our new method is more computationally efficient than prior methods, performs similarly to prior approaches under large perturbations to the system, and better learns the true cost function under small perturbations.

2014
Timothy Bretl, University of Illinois at Urbana-Champaign, Zoe McCarthy, University of Illinois at Urbana-Champaign.  2014.  Quasi-Static Manipulation of a Kirchhoff Elastic Road Based on a Geometric Analysis of Equilibrium Configurations. International Journal of Robotics Research. 33(1)

Consider a thin, flexible wire of fixed length that is held at each end by a robotic gripper. Any curve traced by this wire when in static equilibrium is a local solution to a geometric optimal control problem, with boundary conditions that vary with the position and orientation of each gripper. We prove that the set of all local solutions to this problem over all possible boundary conditions is a smooth manifold of finite dimension that can be parameterized by a single chart. We show that this chart makes it easy to implement a sampling-based algorithm for quasi-static manipulation planning. We characterize the performance of such an algorithm with experiments in simulation.