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2020


Transient coarsening and the motility of optically heated Janus colloids in a binary liquid mixture
Transient coarsening and the motility of optically heated Janus colloids in a binary liquid mixture

Gomez-Solano, J., Roy, S., Araki, T., Dietrich, S., Maciolek, A.

Soft Matter, 16, pages: 8359-8371, Royal Society of Chemistry, August 2020 (article)

Abstract
A gold-capped Janus particle suspended in a near-critical binary liquid mixture can self-propel under illumination. We have immobilized such a particle in a narrow channel and carried out a combined experimental and theoretical study of the non-equilibrium dynamics of a binary solvent around it – lasting from the very moment of switching illumination on until the steady state is reached. In the theoretical study we use both a purely diffusive and a hydrodynamic model, which we solve numerically. Our results demonstrate a remarkable complexity of the time evolution of the concentration field around the colloid. This evolution is governed by the combined effects of the temperature gradient and the wettability, and crucially depends on whether the colloid is free to move or is trapped. For the trapped colloid, all approaches indicate that the early time dynamics is purely diffusive and characterized by composition layers travelling with constant speed from the surface of the colloid into the bulk of the solvent. Subsequently, hydrodynamic effects set in. Anomalously large nonequilibrium fluctuations, which result from the temperature gradient and the vicinity of the critical point of the binary liquid mixture, give rise to strong concentration fluctuations in the solvent and to permanently changing coarsening patterns not observed for a mobile particle. The early time dynamics around initially still Janus colloids produces a force which is able to set the Janus colloid into motion. The propulsion due to this transient dynamics is in the direction opposite to that observed after the steady state is attained.

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link (url) DOI [BibTex]

2020


link (url) DOI [BibTex]


Learning Variable Impedance Control for Contact Sensitive Tasks
Learning Variable Impedance Control for Contact Sensitive Tasks

Bogdanovic, M., Khadiv, M., Righetti, L.

IEEE Robotics and Automation Letters ( Early Access ), IEEE, July 2020 (article)

Abstract
Reinforcement learning algorithms have shown great success in solving different problems ranging from playing video games to robotics. However, they struggle to solve delicate robotic problems, especially those involving contact interactions. Though in principle a policy outputting joint torques should be able to learn these tasks, in practice we see that they have difficulty to robustly solve the problem without any structure in the action space. In this paper, we investigate how the choice of action space can give robust performance in presence of contact uncertainties. We propose to learn a policy that outputs impedance and desired position in joint space as a function of system states without imposing any other structure to the problem. We compare the performance of this approach to torque and position control policies under different contact uncertainties. Extensive simulation results on two different systems, a hopper (floating-base) with intermittent contacts and a manipulator (fixed-base) wiping a table, show that our proposed approach outperforms policies outputting torque or position in terms of both learning rate and robustness to environment uncertainty.

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DOI [BibTex]

DOI [BibTex]


Walking Control Based on Step Timing Adaptation
Walking Control Based on Step Timing Adaptation

Khadiv, M., Herzog, A., Moosavian, S. A. A., Righetti, L.

IEEE Transactions on Robotics, 36, pages: 629 - 643, IEEE, June 2020 (article)

Abstract
Step adjustment can improve the gait robustness of biped robots; however, the adaptation of step timing is often neglected as it gives rise to nonconvex problems when optimized over several footsteps. In this article, we argue that it is not necessary to optimize walking over several steps to ensure gait viability and show that it is sufficient to merely select the next step timing and location. Using this insight, we propose a novel walking pattern generator that optimally selects step location and timing at every control cycle. Our approach is computationally simple compared to standard approaches in the literature, yet guarantees that any viable state will remain viable in the future. We propose a swing foot adaptation strategy and integrate the pattern generator with an inverse dynamics controller that does not explicitly control the center of mass nor the foot center of pressure. This is particularly useful for biped robots with limited control authority over their foot center of pressure, such as robots with point feet or passive ankles. Extensive simulations on a humanoid robot with passive ankles demonstrate the capabilities of the approach in various walking situations, including external pushes and foot slippage, and emphasize the importance of step timing adaptation to stabilize walking.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


Interface-mediated spontaneous symmetry breaking and mutual communication between drops containing chemically active particles
Interface-mediated spontaneous symmetry breaking and mutual communication between drops containing chemically active particles

Singh, D., Domínguez, A., Choudhury, U., Kottapalli, S., Popescu, M., Dietrich, S., Fischer, P.

Nature Communications, 11(2210), May 2020 (article)

Abstract
Symmetry breaking and the emergence of self-organized patterns is the hallmark of com- plexity. Here, we demonstrate that a sessile drop, containing titania powder particles with negligible self-propulsion, exhibits a transition to collective motion leading to self-organized flow patterns. This phenomenology emerges through a novel mechanism involving the interplay between the chemical activity of the photocatalytic particles, which induces Mar- angoni stresses at the liquid–liquid interface, and the geometrical confinement provided by the drop. The response of the interface to the chemical activity of the particles is the source of a significantly amplified hydrodynamic flow within the drop, which moves the particles. Furthermore, in ensembles of such active drops long-ranged ordering of the flow patterns within the drops is observed. We show that the ordering is dictated by a chemical com- munication between drops, i.e., an alignment of the flow patterns is induced by the gradients of the chemicals emanating from the active particles, rather than by hydrodynamic interactions.

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link (url) DOI [BibTex]


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Axisymmetric spheroidal squirmers and self-diffusiophoretic particles

Pöhnl, R., Popescu, M. N., Uspal, W. E.

Journal of Physics: Condensed Matter, 32(16), IOP Publishing, Bristol, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Tracer diffusion on a crowded random Manhattan lattice

Mej\’\ia-Monasterio, C., Nechaev, S., Oshanin, G., Vasilyev, O.

New Journal of Physics, 22(3), IOP Publishing, Bristol, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Wetting transitions on soft substrates

Napiorkowski, M., Schimmele, L., Dietrich, S.

{EPL}, 129(1), EDP Science, Les-Ulis, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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A Real-Robot Dataset for Assessing Transferability of Learned Dynamics Models

Agudelo-España, D., Zadaianchuk, A., Wenk, P., Garg, A., Akpo, J., Grimminger, F., Viereck, J., Naveau, M., Righetti, L., Martius, G., Krause, A., Schölkopf, B., Bauer, S., Wüthrich, M.

IEEE International Conference on Robotics and Automation (ICRA), 2020 (conference) Accepted

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Project Page PDF [BibTex]

Project Page PDF [BibTex]


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Blessing and Curse: How a Supercapacitor Large Capacitance Causes its Slow Charging

Lian, C., Janssen, M., Liu, H., van Roij, R.

Physical Review Letters, 124(7), American Physical Society, Woodbury, N.Y., 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Interplay of quenching temperature and drift in Brownian dynamics

Khalilian, H., Nejad, M. R., Moghaddam, A. G., Rohwer, C. M.

EPL, 128(6), EDP Science, Les-Ulis, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Fractal-seaweeds type functionalization of graphene

Amsharov, K., Sharapa, D. I., Vasilyev, O. A., Martin, O., Hauke, F., Görling, A., Soni, H., Hirsch, A.

Carbon, 158, pages: 435-448, Elsevier, Amsterdam, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Effective pair interaction of patchy particles in critical fluids

Farahmand Bafi, N., Nowakowski, P., Dietrich, S.

The Journal of Chemical Physics, 152(11), American Institute of Physics, Woodbury, N.Y., 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Cassie-Wenzel transition of a binary liquid mixture on a nanosculptured surface

Singh, S. L., Schimmele, L., Dietrich, S.

Physical Review E, 101(5), American Physical Society, Melville, NY, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Adopting the Boundary Homogenization Approximation from Chemical Kinetics to Motile Chemically Active Particles

Popescu, M. N., Uspal, W. E.

In Chemical Kinetics, pages: 517-540, World Scientific, New Jersey, NJ, 2020 (incollection)

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DOI [BibTex]

DOI [BibTex]


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Energy storage in steady states under cyclic local energy input

Zhang, Y., Holyst, R., Maciolek, A.

Physical Review E, 101(1), American Physical Society, Melville, NY, 2020 (article)

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DOI [BibTex]

DOI [BibTex]


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Numerical simulations of self-diffusiophoretic colloids at fluid interfaces

Peter, T., Malgaretti, P., Rivas, N., Scagliarini, A., Harting, J., Dietrich, S.

Soft Matter, 16(14):3536-3547, Royal Society of Chemistry, Cambridge, UK, 2020 (article)

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DOI [BibTex]

DOI [BibTex]

2011


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Collective dynamics of colloids at fluid interfaces

Bleibel, J., Dominguez, A., Oettel, M., Dietrich, S.

European Physical Journal E, 34(11), 2011 (article)

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DOI [BibTex]

2011


DOI [BibTex]


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Anomalous transport of a tracer on percolating clusters

Spanner, M., Höfling, F., Schröder-Turk, G. E., Mecke, K., Franosch, T.

Journal of Physics: Condensed Matter, 23(23), 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Dynamics of colloids in confined geometries

Almenar, L., Rauscher, M.

Journal of Physics: Condensed Matter, 23(18), 2011 (article)

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


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The structure and melting transition of two-dimensional colloidal alloys

Law, A. D., Horozov, T. S., Buzza, D. M. A.

Soft Matter, 7(19):8923-8931, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Second harmonic light scattering from spherical polyelectrolyte brushes

Schürer, B., Hoffmann, M., Wunderlich, S., Harnau, L., Peschel, U., Ballauff, M., Peukert, W.

Journal of Physical Chemistry C, 115, pages: 18302-18309, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Simulation einer fast kritischen binären Flüssigkeit in einem Temperaturgradienten

Single, F.

Universität Stuttgart, Stuttgart, 2011 (mastersthesis)

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[BibTex]

[BibTex]


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Solvation forces in Ising films with long-range boundary fields: density-matrix renormalization-group study

Drzewinski, A., Maciolek, A., Barasinski, A.

Molecular Physics, 109(7-10):1133-1141, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Pulling and pushing a cargo with a catalytically active carrier

Popescu, M. N., Tasinkevych, M., Dietrich, S.

Europhysics Letters, 95(2), 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Critical Casimir forces for Ising films with variable boundary fields

Vasilyev, O., Maciolek, A., Dietrich, S.

Physical Review E, 84(4), 2011 (article)

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DOI [BibTex]


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Study of \pi\pi correlations at LHC and RHIC energies in pp collisions within the quark-gluon string model

Nilsson, M. S., Bravina, L. V., Zabrodin, E. E., Malinina, L. V., Bleibel, J.

Physical Review D, 84(5), 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Micro-rheology on (polymer-grafted) colloids using optical tweezers

Gutsche, C., Elmahdy, M. M., Kegler, K., Semenov, I., Stangner, T., Otto, O., Ueberschär, O., Keyser, U. F., Krüger, M., Rauscher, M., Weeber, R., Harting, J., Kim, Y. W., Lobaskin, V., Netz, R., Kremer, F.

Journal of Physics: Condensed Matter, 23(18), IOP Publishing, Bristol, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Debye-Scherrer rings from block copolymer films with powder-like order

Busch, P., Rauscher, M., Moulin, J.-F., Müller-Buschbaum, P.

Journal of Applied Crystallography, 44(2):370-379, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Effective interactions and equilibrium configurations of colloidal particles on a sessile droplet

Guzowski, J., Tasinkevych, M., Dietrich, S.

Soft Matter, 7(9):4189-4197, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Capillary interactions in Pickering emulsions

Guzowski, J., Tasinkevych, M., Dietrich, S.

Physical Review E, 84(3), 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Critical Casimir forces steered by patterned substrates

Gambassi, A., Dietrich, S.

Soft Matter, 7(4):1247-1253, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Interaction strength between proteins and polyelectrolyte brushes: A small angle X-ray scattering study

Henzler, K., Haupt, B., Rosenfeldt, S., Harnau, L., Narayanan, T., Ballauff, M.

Physical Chemistry Chemical Physics, 13, pages: 17599-17605, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Annealing of single lamella nanoparticles of polyethylene

Rochette, C. N., Rosenfeldt, S., Henzler, K., Polzer, F., Ballauff, M., Tong, Q., Mecking, S., Drechsler, M., Narayanan, T., Harnau, L.

Macromolecules, 44(12):4845-4851, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Struktur dichter ionischer Flüssigkeiten

Dannenmann, O.

Universität Stuttgart, Stuttgart, 2011 (mastersthesis)

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[BibTex]

[BibTex]


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Trapping colloids near chemical stripes via critical Casimir forces

Tröndle, M., Zvyagolskaya, O., Gambassi, A., Vogt, D., Harnau, L., Bechinger, C., Dietrich, S.

Molecular Physics, 109, pages: 1169-1185, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Electrostatic interactions in critical solvents

Bier, M., Gambassi, A., Oettel, M., Dietrich, S.

Europhysics Letters, 95, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Structural investigation of thin diblock copolymer films using time-of-flight grazing-incidence small-angle neutron scattering

Metwalli, E., Moulin, J.-F., Rauscher, M., Kaune, G., Ruderer, M. A., Van Bürck, U., Haese-Seiller, M., Kampmann, R., Müller-Buschbaum, P.

Journal of Applied Crystallography, 44(1):84-92, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Complexation of beta-lactoglobulin fibrils and sulfated polysaccharides

Jones, O. G., Handschin, S., Adamcik, J., Harnau, L., Bolisetty, S., Mezzenga, R.

Biomacromolecules, 12, pages: 3056-3065, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Parallelisierung Stokesscher Dynamik für Graphikprozessoren zur Simulation kolloidaler Suspensionen

Kopp, M.

Universität Stuttgart, Stuttgart, 2011 (mastersthesis)

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[BibTex]

[BibTex]


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Diffusion in Wandnähe

Müller, J.

Universität Stuttgart, Stuttgart, 2011 (mastersthesis)

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[BibTex]

[BibTex]


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Magnetization of multicomponent ferrofluids

Szalai, I., Dietrich, S.

Journal of Physics: Condensed Matter, 23(32), 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Space-resolved dynamics of a tracer in a disordered solid

Franosch, T., Spanner, M., Bauer, T., Schröder-Turk, G. E., Höfling, F.

Journal of Non-Crystalline Solids, 357(2):472-478, 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Long-wavelength anomalies in the asymptotic behavior of mode-coupling theory

Schnyder, S. K., Höfling, F., Franosch, T., Voigtmann, T.

Journal of Physics: Condensed Matter, 23(23), 2011 (article)

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DOI [BibTex]

DOI [BibTex]


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Toward simple control for complex, autonomous robotic applications: combining discrete and rhythmic motor primitives

Degallier, S., Righetti, L., Gay, S., Ijspeert, A.

Autonomous Robots, 31(2-3):155-181, October 2011 (article)

Abstract
Vertebrates are able to quickly adapt to new environments in a very robust, seemingly effortless way. To explain both this adaptivity and robustness, a very promising perspective in neurosciences is the modular approach to movement generation: Movements results from combinations of a finite set of stable motor primitives organized at the spinal level. In this article we apply this concept of modular generation of movements to the control of robots with a high number of degrees of freedom, an issue that is challenging notably because planning complex, multidimensional trajectories in time-varying environments is a laborious and costly process. We thus propose to decrease the complexity of the planning phase through the use of a combination of discrete and rhythmic motor primitives, leading to the decoupling of the planning phase (i.e. the choice of behavior) and the actual trajectory generation. Such implementation eases the control of, and the switch between, different behaviors by reducing the dimensionality of the high-level commands. Moreover, since the motor primitives are generated by dynamical systems, the trajectories can be smoothly modulated, either by high-level commands to change the current behavior or by sensory feedback information to adapt to environmental constraints. In order to show the generality of our approach, we apply the framework to interactive drumming and infant crawling in a humanoid robot. These experiments illustrate the simplicity of the control architecture in terms of planning, the integration of different types of feedback (vision and contact) and the capacity of autonomously switching between different behaviors (crawling and simple reaching).

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


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Learning Force Control Policies for Compliant Manipulation

Kalakrishnan, M., Righetti, L., Pastor, P., Schaal, S.

In 2011 IEEE/RSJ International Conference on Intelligent Robots and Systems, pages: 4639-4644, IEEE, San Francisco, USA, sep 2011 (inproceedings)

Abstract
Developing robots capable of fine manipulation skills is of major importance in order to build truly assistive robots. These robots need to be compliant in their actuation and control in order to operate safely in human environments. Manipulation tasks imply complex contact interactions with the external world, and involve reasoning about the forces and torques to be applied. Planning under contact conditions is usually impractical due to computational complexity, and a lack of precise dynamics models of the environment. We present an approach to acquiring manipulation skills on compliant robots through reinforcement learning. The initial position control policy for manipulation is initialized through kinesthetic demonstration. We augment this policy with a force/torque profile to be controlled in combination with the position trajectories. We use the Policy Improvement with Path Integrals (PI2) algorithm to learn these force/torque profiles by optimizing a cost function that measures task success. We demonstrate our approach on the Barrett WAM robot arm equipped with a 6-DOF force/torque sensor on two different manipulation tasks: opening a door with a lever door handle, and picking up a pen off the table. We show that the learnt force control policies allow successful, robust execution of the tasks.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


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Control of legged robots with optimal distribution of contact forces

Righetti, L., Buchli, J., Mistry, M., Schaal, S.

In 2011 11th IEEE-RAS International Conference on Humanoid Robots, pages: 318-324, IEEE, Bled, Slovenia, 2011 (inproceedings)

Abstract
The development of agile and safe humanoid robots require controllers that guarantee both high tracking performance and compliance with the environment. More specifically, the control of contact interaction is of crucial importance for robots that will actively interact with their environment. Model-based controllers such as inverse dynamics or operational space control are very appealing as they offer both high tracking performance and compliance. However, while widely used for fully actuated systems such as manipulators, they are not yet standard controllers for legged robots such as humanoids. Indeed such robots are fundamentally different from manipulators as they are underactuated due to their floating-base and subject to switching contact constraints. In this paper we present an inverse dynamics controller for legged robots that use torque redundancy to create an optimal distribution of contact constraints. The resulting controller is able to minimize, given a desired motion, any quadratic cost of the contact constraints at each instant of time. In particular we show how this can be used to minimize tangential forces during locomotion, therefore significantly improving the locomotion of legged robots on difficult terrains. In addition to the theoretical result, we present simulations of a humanoid and a quadruped robot, as well as experiments on a real quadruped robot that demonstrate the advantages of the controller.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


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Learning Motion Primitive Goals for Robust Manipulation

Stulp, F., Theodorou, E., Kalakrishnan, M., Pastor, P., Righetti, L., Schaal, S.

In IEEE/RSJ International Conference on Intelligent Robots and Systems, pages: 325-331, IEEE, San Francisco, USA, sep 2011 (inproceedings)

Abstract
Applying model-free reinforcement learning to manipulation remains challenging for several reasons. First, manipulation involves physical contact, which causes discontinuous cost functions. Second, in manipulation, the end-point of the movement must be chosen carefully, as it represents a grasp which must be adapted to the pose and shape of the object. Finally, there is uncertainty in the object pose, and even the most carefully planned movement may fail if the object is not at the expected position. To address these challenges we 1) present a simplified, computationally more efficient version of our model-free reinforcement learning algorithm PI2; 2) extend PI2 so that it simultaneously learns shape parameters and goal parameters of motion primitives; 3) use shape and goal learning to acquire motion primitives that are robust to object pose uncertainty. We evaluate these contributions on a manipulation platform consisting of a 7-DOF arm with a 4-DOF hand.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


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Inverse Dynamics Control of Floating-Base Robots with External Constraints: a Unified View

Righetti, L., Buchli, J., Mistry, M., Schaal, S.

In 2011 IEEE International Conference on Robotics and Automation, pages: 1085-1090, IEEE, Shanghai, China, 2011 (inproceedings)

Abstract
Inverse dynamics controllers and operational space controllers have proved to be very efficient for compliant control of fully actuated robots such as fixed base manipulators. However legged robots such as humanoids are inherently different as they are underactuated and subject to switching external contact constraints. Recently several methods have been proposed to create inverse dynamics controllers and operational space controllers for these robots. In an attempt to compare these different approaches, we develop a general framework for inverse dynamics control and show that these methods lead to very similar controllers. We are then able to greatly simplify recent whole-body controllers based on operational space approaches using kinematic projections, bringing them closer to efficient practical implementations. We also generalize these controllers such that they can be optimal under an arbitrary quadratic cost in the commands.

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link (url) DOI [BibTex]

link (url) DOI [BibTex]