Author name cluster

Gabriel A. D. Lopes

Possible papers associated with this exact author name in Arrow. This page groups case-insensitive exact name matches and is not a full identity disambiguation profile.

9 papers

1 author row

IROS Conference 2016 Conference Paper

Contact-based language for robotic manipulation planning

Anuj Shah
Gabriel A. D. Lopes
Esmaeil Najafi

This paper proposes a new approach to robotic manipulation planning based on the contact between a set of objects, robots and surfaces. We consider making or breaking contact as the most abstract, yet representative element of a manipulation task. Using this paradigm, a robotic manipulation planner has been developed. Given an environment with robots and objects, a manipulation graph is generated by a set of rules and the available geometrical information. Next, the object manipulation planning is formulated as a graph search problem. Paths on this graph divide a complex manipulation task into sub-tasks, followed by low-level path planning and controller assignment for each sub-task. By sequentially executing these controllers in a hybrid fashion, one achieves the overall manipulation task.

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IROS Conference 2016 Conference Paper

Coordination of monopedal SLIP models towards quadrupedal running

Mohammad Shahbazi
Gabriel A. D. Lopes

This paper presents a coordination controller for the Dual-SLIP model, a novel template for quadrupedal steady and transitional running. The model consists of a pair of “physically-unconnected” Spring-Loaded Inverted Pendulums (SLIPs), each representing a part of the body of a quadruped (see Figure 1). For this model, we propose a spatio-temporal coordination controller that describes the evolution of coordination parameters by simple difference equations. A “time-aware” deadbeat low-level controller is also proposed to realizing the generated control specifications in each SLIP individually. Evaluation of the proposed coordination controller for the Dual-SLIP model in simulation shows that even with remarkably off-phase initial conditions and ground height variation disturbances, quadrupedal bounding, pronking and different transitions between them can be realized.

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ICRA Conference 2015 Conference Paper

Analytical approximation for the double-stance phase of a walking robot

Mohammad Shahbazi
Robert Babuska
Gabriel A. D. Lopes

This paper introduces an approximate analytical solution to the otherwise non-integrable double-stance dynamics of the bipedal spring-loaded inverted pendulum (SLIP). Despite the apparent structural simplicity of the SLIP, the exact analytical solution to its stance dynamics cannot be found. Approximate maps have been proposed for the monoped SLIP runner (encompassing a single-stance phase). Still, even in an approximate form, a solution to the double-stance dynamics of the bipedal SLIP walker remained an open problem. We propose a double-stance map that can be readily utilized especially in the design of control systems for active dynamic walking. The accuracy of the derived map over a feasible range of locomotion properties is analyzed numerically, and a control application based on this solution is presented. Simulations for an arbitrary chosen energy level reveals that the devised controller enlarges the stable walking domain of the standard SLIP considerably.

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IROS Conference 2013 Conference Paper

Design and sensing of a flexible robot leg

Gabriel A. D. Lopes
Fankai Zhang

This paper reports on the work completed towards the design and sensing of a flexible robotic leg with applications to search and rescue and home robotics. Taking inspiration from the tentacles of cephalopods, we have designed and constructed a segmented leg that is driven by a single motor. The overall deformation of the segmented leg is achieved via tendons. Such a design enables the leg to be modular and mountable in any robotic platform that is driven by wheels or tracks. The deformation of the leg is measured in real-time using a fiber optic sensor that can be used for control purposes. We present experimental results for deformation sensing.

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IROS Conference 2011 Conference Paper

Optimal gait switching for legged locomotion

Bart Kersbergen
Gabriel A. D. Lopes
Ton J. J. van den Boom
Bart De Schutter
Robert Babuska

Switching gaits in many-legged robots can present challenges due to the combinatorial nature of the gait space. In this paper we present an intrinsically safe gait switching generator that minimizes the velocity variance of all the legs in stance, allowing for smooth acceleration in legged robots. The gait switching generator is modeled as a max-plus linear discrete event system which is translated to continuous time via a reference trajectory generator.

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IROS Conference 2010 Conference Paper

Motion estimation based on predator/prey vision

David van der Lijn
Gabriel A. D. Lopes
Robert Babuska

We present an unscented Kalman filter based state estimator for a fast moving rigid body (such as a mobile robot) endowed with two video cameras. We focus on forward velocity estimation towards the computation of standard energy cost functions for legged locomotion. Points are chosen as image features and the model of each camera is based on the traditional pinhole projection. The resulting filter's state is composed of the rigid body pose and velocities, together with a measure of depth for each tracked point. By taking inspiration from nature's large predatory and grazing mammals eye configuration, we suggest, via simulation results, a solution for the question of finding the best orientation of two cameras, between side and frontal facing, for velocity estimation in a forward moving robot.

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ICRA Conference 2004 Conference Paper

Automated Gait Adaptation for Legged Robots

Joel Weingarten
Gabriel A. D. Lopes
Martin Buehler
Richard E. Groff
Daniel E. Koditschek

Gait parameter adaptation on a physical robot is an error-prone, tedious and time-consuming process. In this paper we present a system for gait adaptation in our RHex series of hexapedal robots that renders this arduous process nearly autonomous. The robot adapts its gait parameters by recourse to a modified version of Nelder-Mead descent, while managing its self-experiments and measuring the outcome by visual servoing within a partially engineered environment The resulting performance gains extend considerably beyond what we have managed with hand tuning. For example, the best hand tuned alternating tripod gaits never exceeded 0. 8 m/s nor achieved specific resistance below 2. 0. In contrast, Nelder-Mead based tuning has yielded alternating tripod gaits at 2. 7 m/s (well over 5 body lengths per second) and reduced specific resistance to 0. 6 while requiring little human intervention at low and moderate speeds. Comparable gains have been achieved on the much larger ruggedized version of this machine.

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IROS Conference 2004 Conference Paper

Level sets and stable manifold approximations for perceptually driven nonholonomically constrained navigation

Gabriel A. D. Lopes
Daniel E. Koditschek

This paper addresses problems of robot navigation with nonholonomic motion constraints and perceptual cues arising from onboard visual servoing in partially engineered environments. We focus on a unicycle motion model and a variety of artificial beacon constellations motivated by relevance to the autonomous hexapod, RHex. We propose a general hybrid procedure that adapts to the constrained motion setting the standard feedback controller arising from a navigation function in the fully actuated case by switching back and forth between moving "down" and "across" the associated gradient field toward the stable manifold it induces in the constrained dynamics. Guaranteed to avoid obstacles in all cases, we provide some reasonably general sufficient conditions under which the new procedure guarantees convergence to the goal. Simulations are provided for perceptual models previously introduced by other authors.

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ICRA Conference 2003 Conference Paper

Visual registration and navigation using planar features

Gabriel A. D. Lopes
Daniel E. Koditschek

This paper addresses the problem of registering the hexapedal robot, RHex, relative to a known set of beacons, by real-time visual servoing. A suitably constructed navigation function represents the task, in the sense that for a completely actuated machine in the horizontal plane, the gradient dynamics guarantee convergence to the visually cued goal without ever losing sight of the beacons that define it. Since the horizontal plane behavior of RHex can be represented as a unicycle, feeding back the navigation function gradient avoids loss of beacons, but does not yield an asymptotically stable goal. We address new problems arising from the configuration of the beacons and present preliminary experimental results that illustrate the discrepancies between the idealized and physical robot actuation capabilities.

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