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2009


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Incorporating Muscle Activation-Contraction dynamics to an optimal control framework for finger movements

Theodorou, Evangelos A., Valero-Cuevas, Francisco J.

Abstracts of Neural Control of Movement Conference (NCM 2009), 2009, clmc (article)

Abstract
Recent experimental and theoretical work [1] investigated the neural control of contact transition between motion and force during tapping with the index finger as a nonlinear optimization problem. Such transitions from motion to well-directed contact force are a fundamental part of dexterous manipulation. There are 3 alternative hypotheses of how this transition could be accomplished by the nervous system as a function of changes in direction and magnitude of the torque vector controlling the finger. These hypotheses are 1) an initial change in direction with a subsequent change in magnitude of the torque vector; 2) an initial change in magnitude with a subsequent directional change of the torque vector; and 3) a simultaneous and proportionally equal change of both direction and magnitude of the torque vector. Experimental work in [2] shows that the nervous system selects the first strategy, and in [1] we suggest that this may in fact be the optimal strategy. In [4] the framework of Iterative Linear Quadratic Optimal Regulator (ILQR) was extended to incorporate motion and force control. However, our prior simulation work assumed direct and instantaneous control of joint torques, which ignores the known delays and filtering properties of skeletal muscle. In this study, we implement an ILQR controller for a more biologically plausible biomechanical model of the index finger than [4], and add activation-contraction dynamics to the system to simulate muscle function. The planar biomechanical model includes the kinematics of the 3 joints while the applied torques are driven by activation?contraction dynamics with biologically plausible time constants [3]. In agreement with our experimental work [2], the task is to, within 500 ms, move the finger from a given resting configuration to target configuration with a desired terminal velocity. ILQR does not only stabilize the finger dynamics according to the objective function, but it also generates smooth joint space trajectories with minimal tuning and without an a-priori initial control policy (which is difficult to find for highly dimensional biomechanical systems). Furthemore, the use of this optimal control framework and the addition of activation-contraction dynamics considers the full nonlinear dynamics of the index finger and produces a sequence of postures which are compatible with experimental motion data [2]. These simulations combined with prior experimental results suggest that optimal control is a strong candidate for the generation of finger movements prior to abrupt motion-to-force transitions. This work is funded in part by grants NIH R01 0505520 and NSF EFRI-0836042 to Dr. Francisco J. Valero- Cuevas 1 Venkadesan M, Valero-Cuevas FJ. 
Effects of neuromuscular lags on controlling contact transitions. 
Philosophical Transactions of the Royal Society A: 2008. 2 Venkadesan M, Valero-Cuevas FJ. 
Neural Control of Motion-to-Force Transitions with the Fingertip. 
J. Neurosci., Feb 2008; 28: 1366 - 1373; 3 Zajac. Muscle and tendon: properties, models, scaling, and application to biomechanics and motor control. Crit Rev Biomed Eng, 17 4. Weiwei Li., Francisco Valero Cuevas: ?Linear Quadratic Optimal Control of Contact Transition with Fingertip ? ACC 2009

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PDF [BibTex]

2009


PDF [BibTex]


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Inertial parameter estimation of floating-base humanoid systems using partial force sensing

Mistry, M., Schaal, S., Yamane, K.

In IEEE-RAS International Conference on Humanoid Robots (Humanoids 2009), Paris, Dec.7-10, 2009, clmc (inproceedings)

Abstract
Recently, several controllers have been proposed for humanoid robots which rely on full-body dynamic models. The estimation of inertial parameters from data is a critical component for obtaining accurate models for control. However, floating base systems, such as humanoid robots, incur added challenges to this task (e.g. contact forces must be measured, contact states can change, etc.) In this work, we outline a theoretical framework for whole body inertial parameter estimation, including the unactuated floating base. Using a least squares minimization approach, conducted within the nullspace of unmeasured degrees of freedom, we are able to use a partial force sensor set for full-body estimation, e.g. using only joint torque sensors, allowing for estimation when contact force measurement is unavailable or unreliable (e.g. due to slipping, rolling contacts, etc.). We also propose how to determine the theoretical minimum force sensor set for full body estimation, and discuss the practical limitations of doing so.

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link (url) [BibTex]

link (url) [BibTex]


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On-line learning and modulation of periodic movements with nonlinear dynamical systems

Gams, A., Ijspeert, A., Schaal, S., Lenarčič, J.

Autonomous Robots, 27(1):3-23, 2009, clmc (article)

Abstract
Abstract  The paper presents a two-layered system for (1) learning and encoding a periodic signal without any knowledge on its frequency and waveform, and (2) modulating the learned periodic trajectory in response to external events. The system is used to learn periodic tasks on a humanoid HOAP-2 robot. The first layer of the system is a dynamical system responsible for extracting the fundamental frequency of the input signal, based on adaptive frequency oscillators. The second layer is a dynamical system responsible for learning of the waveform based on a built-in learning algorithm. By combining the two dynamical systems into one system we can rapidly teach new trajectories to robots without any knowledge of the frequency of the demonstration signal. The system extracts and learns only one period of the demonstration signal. Furthermore, the trajectories are robust to perturbations and can be modulated to cope with a dynamic environment. The system is computationally inexpensive, works on-line for any periodic signal, requires no additional signal processing to determine the frequency of the input signal and can be applied in parallel to multiple dimensions. Additionally, it can adapt to changes in frequency and shape, e.g. to non-stationary signals, such as hand-generated signals and human demonstrations.

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link (url) [BibTex]

link (url) [BibTex]


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Carbon Materials

Hirscher, M.

In Encyclopedia of Electrochemical Power Sources, pages: 484-487, Elsevier, Amsterdam [et al.], 2009 (incollection)

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[BibTex]

[BibTex]


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Theorie der elektronischen Zustände in oxidischen magnetischen Materialien

Kostoglou, C.

Universität Stuttgart, Stuttgart, 2009 (phdthesis)

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[BibTex]

[BibTex]


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Magnetooptische Untersuchungen an Ferromagnet- und Supraleiter-Nanosystemen und deren Hybriden

Treiber, S.

Universität Stuttgart, Stuttgart, 2009 (mastersthesis)

mms

[BibTex]

[BibTex]


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Superconducting phase formation in random neck syntheses: a study of the Y-Ba-Cu-O system by magneto-optics and magnetometry

Willems, J. B., Albrecht, J., Landau, I. L., Hulliger, J.

{Superconductor Science and Technology}, 22, 2009 (article)

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DOI [BibTex]

DOI [BibTex]


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Determination of spin moments from magnetic EXAFS

Popescu, V., Gü\ssmann, M., Fähnle, M., Schütz, G.

{Physical Review B}, 79, 2009 (article)

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DOI [BibTex]

DOI [BibTex]


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Linewidth of ferromagnetic resonance for systems with anisotropic damping

Seib, J., Steiauf, D., Fähnle, M.

{Physical Review B}, 79, 2009 (article)

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DOI [BibTex]

DOI [BibTex]


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Structural and magnetic deconvolution of FePt/FeOx-nanoparticles using x-ray magnetic circular dichroism

Nolle, D., Goering, E., Tietze, T., Schütz, G., Figuerola, A., Manna, L.

{New Journal of Physics}, 11, 2009 (article)

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DOI [BibTex]

DOI [BibTex]


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Magnetic imaging with femtosecond temporal resolution

Li, J., Lee, M.-S., He, W., Redeker, B., Remhof, A., Amaladass, E., Hassel, C., Eimüller, T.

{Review of Scientific Instruments}, 80(7), 2009 (article)

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DOI [BibTex]

DOI [BibTex]


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Elliott-Yafet mechanism and the discussion of femtosecond magnetization dynamics

Steiauf, D., Fähnle, M.

{Physical Review B}, 79, 2009 (article)

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DOI [BibTex]

DOI [BibTex]


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Investigation of the stability of Mn12 single molecule magnets

Voss, S., Fonin, M., Burova, L., Burgert, M., Dedkov, Y. S., Preobrajenski, A. B., Goering, E., Groth, U., Kaul, A. R., Ruediger, U.

{Applied Physics A}, 94(3):491-495, 2009 (article)

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DOI [BibTex]

DOI [BibTex]

2008


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Learning to control in operational space

Peters, J., Schaal, S.

International Journal of Robotics Research, 27, pages: 197-212, 2008, clmc (article)

Abstract
One of the most general frameworks for phrasing control problems for complex, redundant robots is operational space control. However, while this framework is of essential importance for robotics and well-understood from an analytical point of view, it can be prohibitively hard to achieve accurate control in face of modeling errors, which are inevitable in com- plex robots, e.g., humanoid robots. In this paper, we suggest a learning approach for opertional space control as a direct inverse model learning problem. A first important insight for this paper is that a physically cor- rect solution to the inverse problem with redundant degrees-of-freedom does exist when learning of the inverse map is performed in a suitable piecewise linear way. The second crucial component for our work is based on the insight that many operational space controllers can be understood in terms of a constrained optimal control problem. The cost function as- sociated with this optimal control problem allows us to formulate a learn- ing algorithm that automatically synthesizes a globally consistent desired resolution of redundancy while learning the operational space controller. From the machine learning point of view, this learning problem corre- sponds to a reinforcement learning problem that maximizes an immediate reward. We employ an expectation-maximization policy search algorithm in order to solve this problem. Evaluations on a three degrees of freedom robot arm are used to illustrate the suggested approach. The applica- tion to a physically realistic simulator of the anthropomorphic SARCOS Master arm demonstrates feasibility for complex high degree-of-freedom robots. We also show that the proposed method works in the setting of learning resolved motion rate control on real, physical Mitsubishi PA-10 medical robotics arm.

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link (url) DOI [BibTex]

2008


link (url) DOI [BibTex]


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Adaptation to a sub-optimal desired trajectory

M. Mistry, E. A. G. L. T. Y. S. S. M. K.

Advances in Computational Motor Control VII, Symposium at the Society for Neuroscience Meeting, Washington DC, 2008, 2008, clmc (article)

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PDF [BibTex]

PDF [BibTex]


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Human movement generation based on convergent flow fields: A computational model and a behavioral experiment

Hoffmann, H., Schaal, S.

In Advances in Computational Motor Control VII, Symposium at the Society for Neuroscience Meeting, Washington DC, 2008, 2008, clmc (inproceedings)

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link (url) [BibTex]

link (url) [BibTex]


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Ab-initio Elektronentheorie der magnetischen Anisotropie im System FePt mit der Clusterentwicklungsmethode

Subkow, S.

Universität Stuttgart, Stuttgart, 2008 (mastersthesis)

mms

[BibTex]


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Texture, microstructure and mechanical properties of equiaxed ultrafine-grained Zr fabricated by accumulative roll bonding

Jiang, L., Perez-Prado, M. T., Gruber, P. A., Arzt, E., Ruano, O. A., Kassner, M. E.

{Acta Materialia}, 56(6):1228-1242, 2008 (article)

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[BibTex]

[BibTex]


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Theory of size mismatched alloy systems: many-body Kanzaki forces

Shchyglo, O., Diaz-Ortiz, A., Udyansky, A., Bugaev, V. N., Reichert, H., Dosch, H., Drautz, R.

{Journal of Physics: Condensed Matter}, 20, pages: 1-9, 2008 (article)

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[BibTex]

[BibTex]


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Pinning of domain walls in composite particles

Kronmüller, H., Goll, D.

{Physica B}, 403, pages: 237-241, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Chaotic vortex dynamics and low current phases in the remanent state of MgB2 thin films

Albrecht, J., Audehm, P., Djupmyr, M.

{Superconductor Science and Technology}, 21, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Continuous and discontinuous grain-boundary wetting in ZnxAl1-x

Straumal, B. B., Gornakova, A. S., Kogtenkova, O. A., Protasova, S. G., Sursaeva, V. G., Baretzky, B.

{Physical Review B}, 78, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Magnetic circular dichroism in two-photon absorption and depth-resolved magnetic microscopy

Seib, J., Fähnle, M.

{Physical Review B}, 77, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Heat of adsorption for hydrogen in microporous high-surface-area materials

Schmitz, B., Müller, U., Trukhan, N., Schubert, M., Férey, G., Hirscher, M.

{ChemPhysChem}, 9, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Untersuchungen der Desorption von Wasserstoff in metall-organischen Gerüsten

Panella, B., Hönes, K., Müller, U., Trukhan, N., Schubert, M., Pütter, H., Hirscher, M.

{Angewandte Chemie}, 120, pages: 2169-2173, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Influence of domain wall pinning on the dynamic behavior of magnetic vortex structures: Time-resolved scanning x-ray transmission microscopy in NiFe thin film structures

Vansteenkiste, A., De Baerdemaeker, J., Chou, K. W., Stoll, H., Curcic, M., Tyliszczak, T., Woltersdorf, G., Back, C. H., Schütz, G., Van Waeyenberge, B.

{Physical Review B}, 77, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Temperature dependence of mechanical properties in ultrathin Au films with and without passivation

Gruber, P. A., Olliges, S., Arzt, E., Spolenak, R.

{Journal of Materials Research}, 23(9):2406-2419, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Structure, phase composition, and microhardness of carbon steels after high-pressure torsion

Mazilkin, A. A., Straumal, B., Protasova, S. G., Dobatkin, S. V., Baretzky, B.

{Journal of Materials Science}, 43, pages: 3800-3805, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Operational space control: A theoretical and emprical comparison

Nakanishi, J., Cory, R., Mistry, M., Peters, J., Schaal, S.

International Journal of Robotics Research, 27(6):737-757, 2008, clmc (article)

Abstract
Dexterous manipulation with a highly redundant movement system is one of the hallmarks of hu- man motor skills. From numerous behavioral studies, there is strong evidence that humans employ compliant task space control, i.e., they focus control only on task variables while keeping redundant degrees-of-freedom as compliant as possible. This strategy is robust towards unknown disturbances and simultaneously safe for the operator and the environment. The theory of operational space con- trol in robotics aims to achieve similar performance properties. However, despite various compelling theoretical lines of research, advanced operational space control is hardly found in actual robotics imple- mentations, in particular new kinds of robots like humanoids and service robots, which would strongly profit from compliant dexterous manipulation. To analyze the pros and cons of different approaches to operational space control, this paper focuses on a theoretical and empirical evaluation of different methods that have been suggested in the literature, but also some new variants of operational space controllers. We address formulations at the velocity, acceleration and force levels. First, we formulate all controllers in a common notational framework, including quaternion-based orientation control, and discuss some of their theoretical properties. Second, we present experimental comparisons of these approaches on a seven-degree-of-freedom anthropomorphic robot arm with several benchmark tasks. As an aside, we also introduce a novel parameter estimation algorithm for rigid body dynamics, which ensures physical consistency, as this issue was crucial for our successful robot implementations. Our extensive empirical results demonstrate that one of the simplified acceleration-based approaches can be advantageous in terms of task performance, ease of parameter tuning, and general robustness and compliance in face of inevitable modeling errors.

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link (url) [BibTex]

link (url) [BibTex]


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Movement reproduction and obstacle avoidance with dynamic movement primitives and potential fields

Park, D., Hoffmann, H., Pastor, P., Schaal, S.

In IEEE International Conference on Humanoid Robots, 2008., 2008, clmc (inproceedings)

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PDF [BibTex]

PDF [BibTex]


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Magnetism of amorphous and highly anisotropic multilayer systems on flat substrates and nanospheres

Amaladass, E. P.

Universität Stuttgart, Stuttgart, 2008 (phdthesis)

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link (url) [BibTex]


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Wetting and premelting of triple junctions and grain boundaries in the Al-Zn alloys

Straumal, B., Kogtenkova, O., Protasova, S., Mazilkin, A., Zieba, P., Czeppe, T., Wojewoda-Budka, J., Faryna, M.

In 495, pages: 126-131, Alicante, Spain, 2008 (inproceedings)

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DOI [BibTex]

DOI [BibTex]


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Interfaces in driven Ising models: shear enhances confinement

Smith, T. H. R., Vasilyev, O., Abraham, D. B., Maciolek, A., Schmidt, M.

{Physical Review Letters}, 101(6), 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Bose-Theorie der Dämpfung der Bewegung einer magnetischen Domänenwand

Hähnel, D.

Universität Stuttgart, Stuttgart, 2008 (mastersthesis)

mms

[BibTex]

[BibTex]


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Planar high-Tc superconducting quantum interference device gradiometer for simultaneous measurements of two magnetic field gradients

Steppke, A., Becker, C., Grosse, V., Dörrer, L., Schmidl, F., Seidel, P., Djupmyr, M., Albrecht, J.

{Applied Physics Letters}, 92, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Terminal contact elements of insect attachment devices studied by transmission X-ray microscopy

Eimüller, T., Guttmann, P., Gorb, S. N.

{Journal of Experimental Biology}, 211, pages: 1958-1963, 2008 (article)

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[BibTex]

[BibTex]


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Increase of Co solubility with decreasing grain size in ZnO

Straumal, B. B., Mazilkin, A. A., Protasova, S. G., Myatiev, A. A., Straumal, P. B., Baretzky, B.

{Acta Materialia}, 56, pages: 6246-6256, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Hydrogen adsorption (Carbon, Zeolites, Nanocubes)

Hirscher, M., Panella, B.

In Hydrogen as a Future Energy Carrier, pages: 173-188, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008 (incollection)

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[BibTex]

[BibTex]


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Uncompensated moments in the MnPd/Fe exchange bias system

Brück, S., Schütz, G., Goering, E., Ji, X., Krishnan, K. M.

{Physical Review Letters}, 101, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Aqueous deposition of ultraviolet luminescent columnar tin-doped indium hydroxide films

Qiu, Y., Bellina, P., Jeurgens, L. P. H., Leineweber, A., Welzel, U., Gerstel, P., Jiang, L., van Aken, P. A., Bill, J., Aldinger, F.

{Advanced Functional Materials}, 18, pages: 2572-2583, 2008 (article)

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[BibTex]

[BibTex]


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Unified theory of near-adiabatic magnetization dynamics for collinear and noncollinear magnetization

Steiauf, D., Seib, J., Fähnle, M.

{Physical Review B}, 78, 2008 (article)

mms

DOI [BibTex]

DOI [BibTex]


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Hardmetals with nanograin reinforced binder: binder fine structure and hardness

Konyashin, I., Ries, B., Lachmann, F., Cooper, R., Mazilkin, A., Straumal, B., Aretz, A., Babaev, V.

{International Journal of Refractory Metals \& Hard Materials}, 26, pages: 583-588, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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The dual role of uncertainty in force field learning

Mistry, M., Theodorou, E., Hoffmann, H., Schaal, S.

In Abstracts of the Eighteenth Annual Meeting of Neural Control of Movement (NCM), Naples, Florida, April 29-May 4, 2008, clmc (inproceedings)

Abstract
Force field experiments have been a successful paradigm for studying the principles of planning, execution, and learning in human arm movements. Subjects have been shown to cope with the disturbances generated by force fields by learning internal models of the underlying dynamics to predict disturbance effects or by increasing arm impedance (via co-contraction) if a predictive approach becomes infeasible. Several studies have addressed the issue uncertainty in force field learning. Scheidt et al. demonstrated that subjects exposed to a viscous force field of fixed structure but varying strength (randomly changing from trial to trial), learn to adapt to the mean disturbance, regardless of the statistical distribution. Takahashi et al. additionally show a decrease in strength of after-effects after learning in the randomly varying environment. Thus they suggest that the nervous system adopts a dual strategy: learning an internal model of the mean of the random environment, while simultaneously increasing arm impedance to minimize the consequence of errors. In this study, we examine what role variance plays in the learning of uncertain force fields. We use a 7 degree-of-freedom exoskeleton robot as a manipulandum (Sarcos Master Arm, Sarcos, Inc.), and apply a 3D viscous force field of fixed structure and strength randomly selected from trial to trial. Additionally, in separate blocks of trials, we alter the variance of the randomly selected strength multiplier (while keeping a constant mean). In each block, after sufficient learning has occurred, we apply catch trials with no force field and measure the strength of after-effects. As expected in higher variance cases, results show increasingly smaller levels of after-effects as the variance is increased, thus implying subjects choose the robust strategy of increasing arm impedance to cope with higher levels of uncertainty. Interestingly, however, subjects show an increase in after-effect strength with a small amount of variance as compared to the deterministic (zero variance) case. This result implies that a small amount of variability aides in internal model formation, presumably a consequence of the additional amount of exploration conducted in the workspace of the task.

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[BibTex]

[BibTex]


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Dynamic movement primitives for movement generation motivated by convergent force fields in frog

Hoffmann, H., Pastor, P., Schaal, S.

In Adaptive Motion of Animals and Machines (AMAM), 2008, clmc (inproceedings)

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PDF [BibTex]

PDF [BibTex]


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Interfaces in confined Ising models: Kawasaki, Glauber and sheared dynamics

Smith, T. H. R., Vasilyev, O., Abraham, D. B., Maciolek, A., Schmidt, M.

{Journal of Physics: Condensed Matter}, 20(49), 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Critical fields of an exchange coupled two-layer composite particle

Goll, D., Kronmüller, H.

{Physica B}, 403, pages: 1854-1859, 2008 (article)

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DOI [BibTex]

DOI [BibTex]


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Coercivity and domain structure of nanograined Fe-C alloys after high-pressure torsion

Protasova, S. G., Straumal, B., Dobatkin, S. V., Goll, D., Schütz, G., Baretzky, B., Mazilkin, A. A., Nebrasov, A. N.

{Journal of Materials Science}, 43, pages: 3775-3781, 2008 (article)

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DOI [BibTex]

DOI [BibTex]