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2002


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Forward models in visuomotor control

Mehta, B., Schaal, S.

J Neurophysiol, 88(2):942-53, August 2002, clmc (article)

Abstract
In recent years, an increasing number of research projects investigated whether the central nervous system employs internal models in motor control. While inverse models in the control loop can be identified more readily in both motor behavior and the firing of single neurons, providing direct evidence for the existence of forward models is more complicated. In this paper, we will discuss such an identification of forward models in the context of the visuomotor control of an unstable dynamic system, the balancing of a pole on a finger. Pole balancing imposes stringent constraints on the biological controller, as it needs to cope with the large delays of visual information processing while keeping the pole at an unstable equilibrium. We hypothesize various model-based and non-model-based control schemes of how visuomotor control can be accomplished in this task, including Smith Predictors, predictors with Kalman filters, tapped-delay line control, and delay-uncompensated control. Behavioral experiments with human participants allow exclusion of most of the hypothesized control schemes. In the end, our data support the existence of a forward model in the sensory preprocessing loop of control. As an important part of our research, we will provide a discussion of when and how forward models can be identified and also the possible pitfalls in the search for forward models in control.

am

link (url) [BibTex]

2002


link (url) [BibTex]


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Kinetics of submonolayer epitaxial growth

Amar, J. G., Family, F., Popescu, M. N.

Computer Physics Communications, 146(1):1-8, 2002 (article)

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[BibTex]

[BibTex]


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Fluids of platelike particles near a hard wall

Harnau, L., Dietrich, S.

Physical Review E, 65(2), 2002 (article)

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[BibTex]

[BibTex]


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Fundamental measure theory for hard-sphere mixtures revisited: the White Bear version

Roth, R., Evans, R., Lang, A., Kahl, G.

Journal of Physics-Condensed Matter, 14(46):12063-12078, 2002 (article)

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[BibTex]

[BibTex]


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Microscopic theory of solvent-mediated long-range forces: Influence of wetting

Archer, A. J., Evans, R., Roth, R.

Europhysics Letters, 59(4):526-532, 2002 (article)

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[BibTex]

[BibTex]


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Colloid aggregation induced by oppositely charged polyions

Harnau, L., Hansen, J. P.

Journal of Chemical Physics, 116(20):9051-9057, 2002 (article)

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[BibTex]

[BibTex]


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Self-consistent rate equation theory of cluster size distribution in aggregation phenomena

Family, F., Popescu, M. N., Amar, J. G.

Physica A, 306(1-4):129-139, 2002 (article)

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[BibTex]

[BibTex]


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Wetting and capillary nematization of binary hard-platelet and hard-rod fluids

Harnau, L., Dietrich, S.

Physical Review E, 66(5), 2002 (article)

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[BibTex]


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Simulating stochastic geometries: morphology of overlapping grains

Brodatzki, U., Mecke, K.

Computer Physics Communications, 147(1-2):218-221, 2002 (article)

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[BibTex]

[BibTex]


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Thermodynamics and phase behavior of the lamellar Zwanzig model

Harnau, L., Rowan, D., Hansen, J. P.

Journal of Chemical Physics, 117(24):11359-11365, 2002 (article)

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[BibTex]

[BibTex]


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Cluster-Computing and Computational Science mit der Wuppertaler Alpha-Linux-Cluster-Engine ALiCE

Arndt, H., Arnold, G., Eicker, N., Fliegner, D., Frommer, A., Hentschke, R., Isalia, F., Kabrede, H., Krech, M., Lippert, T. H., Neff, H., Orth, B., Schilling, K., Schroers, W., Tichy, W.

Praxis der Informationsverarbeitung und Kommunikation, 25(1):21-38, 2002 (article)

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[BibTex]

[BibTex]


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Crossover between strong- and weak-field critical adsorption and the determination of the universal exponent η⊥

Nickel, B., Schlesener, F., Donner, W., Detlefs, C., Dosch, H.

Journal of Chemical Physics, 117(2):902-908, 2002 (article)

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[BibTex]

[BibTex]


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A novel technique for measuring diffusivities of short-lived radioisotopes in solids

Voss, T., Strohm, A., Matics, S., Scharwaechter, P., Frank, W.

Zeitschrift f\"ur Metallkunde, 93(10):1077-1082, 2002 (article)

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[BibTex]

[BibTex]


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The shape of parallel surface: porous media, fluctuating interfaces and complex fluids

Mecke, K. R.

Physica A-Statistical Mechanics and its Applications, 314(1-4):655-662, 2002 (article)

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[BibTex]

[BibTex]


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Strong dependence of percolation thresholds on polydispersity

Mecke, K. R., Seyfried, A.

Europhysics Letters, 58(1):28-34, 2002 (article)

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[BibTex]

[BibTex]


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Self-diffusion of 31Si and 71Ge in relaxed Si0.20Ge0.80 layers

Laitinen, P., Strohm, A., Huikari, J., Nieminen, A., Voss, T., Grodon, C., Riihimäki, I., Kummer, M., Äystö, J., Dendooven, P., Räisänen, J., Frank, W.

Physical Review Letters, 89(8), 2002 (article)

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[BibTex]

[BibTex]


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Self-diffusion of 71Ge and 31Si in Si-Ge alloys

Strohm, A., Voss, T., Frank, W., Laitinen, P., Räisänen, J.

Zeitschrift f\"ur Metallkunde, 93(7):737-744, 2002 (article)

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[BibTex]

[BibTex]


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Reconstruction of complex materials by integral geometric measures

Mecke, K. R.

Journal of Materials Science \& Technology, 18(2):155-158, 2002 (article)

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[BibTex]

[BibTex]


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Conformal map modeling of the pinning transition in Laplacian growth

Hentschel, H. G. E., Popescu, M. N., Family, F.

Physical Review E, 65(3), 2002 (article)

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[BibTex]

[BibTex]


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Geometrically-controlled twist transitions in nematic cells

Patricio, P., Telo da Gama, M. M., Dietrich, S.

Physical Review Letters, 88(24), 2002 (article)

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[BibTex]

[BibTex]


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Entropic torque

Roth, R., van Roij, R., Andrienko, D., Mecke, K. R., Dietrich, S.

Physical Review Letters, 89(8), 2002 (article)

icm

[BibTex]

[BibTex]


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Effective forces in colloidal mixtures: From depletion attraction to accumulation repulsion

Louis, A. A., Allahyarov, E., Löwen, H., Roth, R.

Physical Review E, 65(6), 2002 (article)

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[BibTex]

[BibTex]


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Nucleon mass and pion loops: Renormalization

Oettel, M., Thomas, A. W.

Physical Review C, 66(6), 2002 (article)

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[BibTex]

[BibTex]


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Scalable techniques from nonparameteric statistics for real-time robot learning

Schaal, S., Atkeson, C. G., Vijayakumar, S.

Applied Intelligence, 17(1):49-60, 2002, clmc (article)

Abstract
Locally weighted learning (LWL) is a class of techniques from nonparametric statistics that provides useful representations and training algorithms for learning about complex phenomena during autonomous adaptive control of robotic systems. This paper introduces several LWL algorithms that have been tested successfully in real-time learning of complex robot tasks. We discuss two major classes of LWL, memory-based LWL and purely incremental LWL that does not need to remember any data explicitly. In contrast to the traditional belief that LWL methods cannot work well in high-dimensional spaces, we provide new algorithms that have been tested on up to 90 dimensional learning problems. The applicability of our LWL algorithms is demonstrated in various robot learning examples, including the learning of devil-sticking, pole-balancing by a humanoid robot arm, and inverse-dynamics learning for a seven and a 30 degree-of-freedom robot. In all these examples, the application of our statistical neural networks techniques allowed either faster or more accurate acquisition of motor control than classical control engineering.

am

link (url) [BibTex]

link (url) [BibTex]

1996


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A Kendama learning robot based on bi-directional theory

Miyamoto, H., Schaal, S., Gandolfo, F., Koike, Y., Osu, R., Nakano, E., Wada, Y., Kawato, M.

Neural Networks, 9(8):1281-1302, 1996, clmc (article)

Abstract
A general theory of movement-pattern perception based on bi-directional theory for sensory-motor integration can be used for motion capture and learning by watching in robotics. We demonstrate our methods using the game of Kendama, executed by the SARCOS Dextrous Slave Arm, which has a very similar kinematic structure to the human arm. Three ingredients have to be integrated for the successful execution of this task. The ingredients are (1) to extract via-points from a human movement trajectory using a forward-inverse relaxation model, (2) to treat via-points as a control variable while reconstructing the desired trajectory from all the via-points, and (3) to modify the via-points for successful execution. In order to test the validity of the via-point representation, we utilized a numerical model of the SARCOS arm, and examined the behavior of the system under several conditions.

am

link (url) [BibTex]

1996


link (url) [BibTex]


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One-handed juggling: A dynamical approach to a rhythmic movement task

Schaal, S., Sternad, D., Atkeson, C. G.

Journal of Motor Behavior, 28(2):165-183, 1996, clmc (article)

Abstract
The skill of rhythmic juggling a ball on a racket is investigated from the viewpoint of nonlinear dynamics. The difference equations that model the dynamical system are analyzed by means of local and non-local stability analyses. These analyses yield that the task dynamics offer an economical juggling pattern which is stable even for open-loop actuator motion. For this pattern, two types of pre dictions are extracted: (i) Stable periodic bouncing is sufficiently characterized by a negative acceleration of the racket at the moment of impact with the ball; (ii) A nonlinear scaling relation maps different juggling trajectories onto one topologically equivalent dynamical system. The relevance of these results for the human control of action was evaluated in an experiment where subjects performed a comparable task of juggling a ball on a paddle. Task manipulations involved different juggling heights and gravity conditions of the ball. The predictions were confirmed: (i) For stable rhythmic performance the paddle's acceleration at impact is negative and fluctuations of the impact acceleration follow predictions from global stability analysis; (ii) For each subject, the realizations of juggling for the different experimental conditions are related by the scaling relation. These results allow the conclusion that for the given task, humans reliably exploit the stable solutions inherent to the dynamics of the task and do not overrule these dynamics by other control mechanisms. The dynamical scaling serves as an efficient principle to generate different movement realizations from only a few parameter changes and is discussed as a dynamical formalization of the principle of motor equivalence.

am

link (url) [BibTex]

link (url) [BibTex]