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Hans-Hellmut Nagel

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14 papers
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14

ICRA Conference 2001 Conference Paper

Machine-Vision-Based Estimation of Pose and Size Parameters from a Generic Workpiece Description

  • Ferit Keçeci
  • Hans-Hellmut Nagel

Automatic disassembly of used products can not assume that CAD databases will provide precise knowledge about the pose, size, and shape of components to be manipulated because some of the components may have been repaired, repositioned, or replaced, thereby possibly invalidating the original construction data. In principle, the missing information can be provided by machine vision which often becomes more robust if it can rely on some knowledge about the workpiece to be manipulated, for example, in the form of generic descriptions. Use of the latter necessitates, however, that not only an unknown pose, but in addition unknown size and shape parameters of a component have to be determined. A solution to the automatic determination of location, orientation, size, and shape of truncated cones is demonstrated by the example of shock absorber lids in the engine compartment of a used car.

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ICRA Conference 2001 Conference Paper

Robust Visual Servoing: Examination of Cameras Under Different Illumination Conditions

  • Alexander Bachem
  • Thomas Müller 0004
  • Hans-Hellmut Nagel

Experience has shown that changes in illumination conditions may influence the robustness of robot control based on machine vision even if such illumination changes appear practically negligible to a human. CMOS cameras have become available with a larger dynamic range than conventional (high quality) CCD cameras. The use of such higher dynamic range cameras for robot control by machine vision is compared with the use of conventional cameras under various illumination conditions. The task to be performed by the robot consists in the removal of a battery from the engine compartment of a used car. In all cases, the servo task could be accomplished successfully. The stop of CCD cameras had to be adjusted manually, however, whereas an automatic adaptation to the current lighting conditions could be implemented for the other camera type, thus allowing us to deal automatically with significant changes of illumination.

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ICRA Conference 1998 Conference Paper

Improving Visually Servoed Disassembly Operations by Automatic Camera Placement

  • Ferit Keçeci
  • Martin Tonko
  • Hans-Hellmut Nagel
  • Volker Gengenbach

It is advantageous to use an independently mobile camera in visually servoed disassembly operations since its degrees of freedom can be used to bring the camera into an optimal view-pose. Apart from the vision-based closed-loop control that guides the disassembly robot, a second vision-based closed-loop control can be set up which upgrades the camera from a passive to an active sensor and optimizes the view-pose according to changes in the scene domain. The objective of this contribution is to present an approach to model-based pose initialization and pose refinement steps of an active camera which is able to simultaneously observe and track several objects moving to some extent independently from each other. In both steps, constraints regarding object occlusions, the limited field-of-view, and the uncertainty of pose estimation are exploited to determine an optimal camera pose. The feasibility of this approach to model-based pose initialization is experimentally validated and documented for the case of the manipulation of a shock absorber lid.

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ICRA Conference 1997 Conference Paper

Towards visually servoed manipulation of car engine parts

  • Martin Tonko
  • Kart Schäfer
  • Frank Heimes
  • Hans-Hellmut Nagel

The disassembly of engines of used cars based on visually servoed manipulators requires a state-of-the-art vision system which is able to evaluate images of dirty, partly occluded or possibly rearranged quasi- or non-polyhedral parts in real-time. As the pose of a stationary rigid part has, in general, six degrees of freedom, at least six degrees of freedom of a manipulator have to be visually controlled. Two pose estimators relying on different types of image features are discussed within a common framework. A position-based solution to the visually servoed manipulation of non-polyhedrals is presented that extends to real time and meets all given requirements. Experiments with the manipulation of a battery, which is partly occluded by wires, show the feasibility of the proposed approach.

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IROS Conference 1997 Conference Paper

Visual tracking of an end-effector by adaptive kinematic prediction

  • Andreas Ruf
  • Martin Tonko
  • Radu Horaud
  • Hans-Hellmut Nagel

Presents results of a model-based approach to visual tracking and pose estimation for a moving polyhedral tool in position-based visual servoing. This enables the control of a robot in look-and-move mode to achieve six degree of freedom goal configurations. Robust solutions of the correspondence problem-known as "matching" in the static case and "tracking" in the dynamic one-are crucial to the feasibility of such an approach in real-world environments. The object's motion along an arbitrary trajectory in space is tracked using visual pose estimates through consecutive images. Subsequent positions are predicted from robot joint angle measurements. To deal with inaccurate models and to relax calibration requirements, adaptive online calibration of the kinematic chain is proposed. The kinematic predictions enable unambiguous feature matching by a pessimistic algorithm. The performance of the suggested algorithms and the robustness of the proposed system are evaluated on real image sequences of a moving gripper. The results fulfill the requirements of visual servoing, and the computational demands are sufficiently low to allow for real-time implementation.

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IROS Conference 1997 Conference Paper

Visually servoed gripping of a used car battery

  • Martin Tonko
  • J. Schurmann
  • Kart Schäfer
  • Hans-Hellmut Nagel

This contribution presents a position-based approach to visual servoing that allows the manipulation of quasi- or nonpolyhedral objects in a complex scene, namely the engine compartment of a used car. During a manipulation, the moving disassembly robot with its end-effector-mounted tool as well as the-possibly moving-object are observed by an independently mobile, binocular camera configuration. A simple proportional control law is used to guide the robot to its manipulation position. Vision-based tracking as implemented here tolerates mutual occlusions of tool and object. Moreover, it does not require additional feature configurations such as white circular patterns on homogeneous black background to reliably pursue tool and object. We demonstrate simultaneous movements of tool, object and independently mobile camera configuration during 6-DOF closed-loop control.

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ICRA Conference 1996 Conference Paper

Automatic dismantling integrating optical flow into a machine vision-controlled robot system

  • Volker Gengenbach
  • Hans-Hellmut Nagel
  • Martin Tonko
  • Kart Schäfer

In order to automatically run a dismantling task, first a workpiece has to be recognized and its pose has to be estimated. This step is solved by a model-based recognition process using the information from three stationary cameras. Our workpiece model could be extracted from CAD-data and consists of planar patches and circular features, where the circular features are suitable to model parts of connection elements like screws. Using the initial estimation of the workpiece pose we control the robot by evaluating the image sequence given by a camera mounted on the robot hand. We use a multi-level 3D tracking approach to continuously estimate the workpiece pose using an iterative extended Kalman-filter process and integrate velocity estimation based on optical flow. To provide enough computing power for real-time image processing, we combine our dedicated image processing hardware with a heterogeneous parallel computer. Our approach has been tested in numerous experiments on loosening screws with a powered screwdriver and dismantling a workpiece consisting of two blocks of different sizes connected by two screws.

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AIJ Journal 1995 Journal Article

Estimation of optical flow based on higher-order spatiotemporal derivatives in interlaced and non-interlaced image sequences

  • Michael Otte
  • Hans-Hellmut Nagel

This contribution 1 1 Partial results have been presented in a preliminary version at the ECCV '94 [42]. investigates local differential techniques for estimating optical flow and its derivatives based on the brightness change constraint. By using the tensor calculus representation we build the Taylor expansion of the gray-value derivatives as well as of the optical flow in a spatiotemporal neighborhood. Such a formulation provides a unifying framework for all existing local differential approaches and allows to derive new systems of equations for the estimation of the optical flow and of its derivatives. We also tested various optical flow estimation approaches on real image sequences recorded by a calibrated camera which was fixed on the arm of a robot. By moving the arm of the robot along a precisely defined trajectory, we can determine the true displacement rate of scene surface elements projected into the image plane and compare it quantitatively with the results of different optical flow estimators. Since the optical flow estimators are based on gray-value derivatives of up to fourth-order, we were forced to develop modified Gaussian derivative filters to obtain acceptable estimates for the derivatives. Further, we show quantitatively that these filters contribute to a much more robust optical flow estimation. In addition, successive lines of TV-cameras have an offset in time due to the interlace technique. We demonstrate the adaptation of filter kernels for estimating higher-order spatiotemporal derivatives in interlaced image sequences.

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AIJ Journal 1987 Journal Article

On the estimation of optical flow: Relations between different approaches and some new results

  • Hans-Hellmut Nagel

A common basis is suggested for the optical flow estimation approaches of Nagel (1983), Haralick and Lee (1983) and Tretiak and Pastor (1984). Based on a discussion of these approaches, an exact solution for the system of partial differential equations proposed by Horn and Schunck (1981) is given at gray value corners and extrema. The insight gained by this solution results in a modification of the “oriented smoothness” approach of Nagel (1983) which thereby becomes considerably simpler. In addition, the optical flow estimation approach of Hildreth (1983, 1984) can be shown to represent a kind of special case of this modified “oriented smoothness” approach in a more direct manner than discussed in Nagel (1984).

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v2026.09.13