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2015


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Experimental investigation of optimal adhesion of mushroomlike elastomer microfibrillar adhesives

Marvi, H., Song, S., Sitti, M.

Langmuir, 31(37):10119-10124, American Chemical Society, August 2015 (article)

Abstract
Optimal fiber designs for the maximal pull-off force have been indispensable for increasing the attachment performance of recently introduced gecko-inspired reversible micro/nanofibrillar adhesives. There are several theoretical studies on such optimal designs; however, due to the lack of three-dimensional (3D) fabrication techniques that can fabricate such optimal designs in 3D, there have not been many experimental investigations on this challenge. In this study, we benefitted from recent advances in two-photon lithography techniques to fabricate mushroomlike polyurethane elastomer fibers with different aspect ratios of tip to stalk diameter (β) and tip wedge angles (θ) to investigate the effect of these two parameters on the pull-off force. We found similar trends to those predicted theoretically. We found that β has an impact on the slope of the force-displacement curve while both β and θ play a role in the stress distribution and crack propagation. We found that these effects are coupled and the optimal set of parameters also depends on the fiber material. This is the first experimental verification of such optimal designs proposed for mushroomlike microfibers. This experimental approach could be used to evaluate a wide range of complex microstructured adhesive designs suggested in the literature and optimize them.

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DOI [BibTex]

2015


DOI [BibTex]


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pH-taxis of biohybrid microsystems

Zhuang, J., Carlsen, R. W., Sitti, M.

Scientific reports, 5, Nature Publishing Group, June 2015 (article)

Abstract
The last decade has seen an increasing number of studies developing bacteria and other cell-integrated biohybrid microsystems. However, the highly stochastic motion of these microsystems severely limits their potential use. Here, we present a method that exploits the pH sensing of flagellated bacteria to realize robust drift control of multi-bacteria propelled microrobots. Under three specifically configured pH gradients, we demonstrate that the microrobots exhibit both unidirectional and bidirectional pH-tactic behaviors, which are also observed in free-swimming bacteria. From trajectory analysis, we find that the swimming direction and speed biases are two major factors that contribute to their tactic drift motion. The motion analysis of microrobots also sheds light on the propulsion dynamics of the flagellated bacteria as bioactuators. It is expected that similar driving mechanisms are shared among pH-taxis, chemotaxis, and thermotaxis. By identifying the mechanism that drives the tactic behavior of bacteria-propelled microsystems, this study opens up an avenue towards improving the control of biohybrid microsystems. Furthermore, assuming that it is possible to tune the preferred pH of bioactuators by genetic engineering, these biohybrid microsystems could potentially be applied to sense the pH gradient induced by cancerous cells in stagnant fluids inside human body and realize targeted drug delivery.

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DOI Project Page [BibTex]

DOI Project Page [BibTex]


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Structural optimization for flexure-based parallel mechanisms–Towards achieving optimal dynamic and stiffness properties

Lum, G. Z., Teo, T. J., Yeo, S. H., Yang, G., Sitti, M.

Precision Engineering, 42, pages: 195-207, Elsevier, May 2015 (article)

Abstract
Flexure-based parallel mechanisms (FPMs) are a type of compliant mechanisms that consist of a rigid end-effector that is articulated by several parallel, flexible limbs (a.k.a. sub-chains). Existing design methods can enhance the FPMs’ dynamic and stiffness properties by conducting a size optimization on their sub-chains. A similar optimization process, however, was not performed for their sub-chains’ topology, and this may severely limit the benefits of a size optimization. Thus, this paper proposes to use a structural optimization approach to synthesize and optimize the topology, shape and size of the FPMs’ sub-chains. The benefits of this approach are demonstrated via the design and development of a planar X − Y − θz FPM. A prototype of this FPM was evaluated experimentally to have a large workspace of 1.2 mm × 1.2 mm × 6°, a fundamental natural frequency of 102 Hz, and stiffness ratios that are greater than 120. The achieved properties show significant improvement over existing 3-degrees-of-freedom compliant mechanisms that can deflect more than 0.5 mm and 0.5°. These compliant mechanisms typically have stiffness ratios that are less than 60 and a fundamental natural frequency that is less than 45 Hz.

pi

DOI [BibTex]

DOI [BibTex]


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Controlled surface topography regulates collective 3D migration by epithelial–mesenchymal composite embryonic tissues

Song, J., Shawky, J. H., Kim, Y., Hazar, M., LeDuc, P. R., Sitti, M., Davidson, L. A.

Biomaterials, 58, pages: 1-9, Elsevier, April 2015 (article)

Abstract
Cells in tissues encounter a range of physical cues as they migrate. Probing single cell and collective migratory responses to physically defined three-dimensional (3D) microenvironments and the factors that modulate those responses are critical to understanding how tissue migration is regulated during development, regeneration, and cancer. One key physical factor that regulates cell migration is topography. Most studies on surface topography and cell mechanics have been carried out with single migratory cells, yet little is known about the spreading and motility response of 3D complex multi-cellular tissues to topographical cues. Here, we examine the response to complex topographical cues of microsurgically isolated tissue explants composed of epithelial and mesenchymal cell layers from naturally 3D organized embryos of the aquatic frog Xenopus laevis. We control topography using fabricated micropost arrays (MPAs) and investigate the collective 3D migration of these multi-cellular systems in these MPAs. We find that the topography regulates both collective and individual cell migration and that dense MPAs reduce but do not eliminate tissue spreading. By modulating cell size through the cell cycle inhibitor Mitomycin C or the spacing of the MPAs we uncover how 3D topographical cues disrupt collective cell migration. We find surface topography can direct both single cell motility and tissue spreading, altering tissue-scale processes that enable efficient conversion of single cell motility into collective movement.

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DOI [BibTex]

DOI [BibTex]


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Transfer Printing of Metallic Microstructures on Adhesion-Promoting Hydrogel Substrates

Wu, H., Sariola, V., Zhu, C., Zhao, J., Sitti, M., Bettinger, C. J.

Advanced Materials, 27(22):3398-3404, April 2015 (article)

Abstract
Fabrication schemes that integrate inorganic microstructures with hydrogel substrates are essential for advancing flexible electronics. A transfer printing process that is made possible through the design and synthesis of adhesion-promoting hydrogels as target substrates is reported. This fabrication technique may advance ultracompliant electronics by melding microfabricated structures with swollen hydrogel substrates.

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DOI [BibTex]

DOI [BibTex]


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Biomedical applications of untethered mobile milli/microrobots

Sitti, M., Ceylan, H., Hu, W., Giltinan, J., Turan, M., Yim, S., Diller, E.

Proceedings of the IEEE, 103(2):205-224, IEEE, March 2015 (article)

Abstract
Untethered robots miniaturized to the length scale of millimeter and below attract growing attention for the prospect of transforming many aspects of health care and bioengineering. As the robot size goes down to the order of a single cell, previously inaccessible body sites would become available for high-resolution in situ and in vivo manipulations. This unprecedented direct access would enable an extensive range of minimally invasive medical operations. Here, we provide a comprehensive review of the current advances in biomedical untethered mobile milli/microrobots. We put a special emphasis on the potential impacts of biomedical microrobots in the near future. Finally, we discuss the existing challenges and emerging concepts associated with designing such a miniaturized robot for operation inside a biological environment for biomedical applications.

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DOI [BibTex]

DOI [BibTex]


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Collective 3D Migration of Embryonic Epithelial Mesenchymal Composite Tissues are Regulated by Surface Topology

Song, J., Shawky, J., Kim, Y. T., Hazar, M., Sitti, M., LeDuc, P. R., Davidson, L. A.

Biophysical Journal, 108(2):455a, Elsevier, January 2015 (article)

Abstract
Cells in tissues encounter a range of physical cues as they migrate. Probing single cell and collective migratory responses to physically defined three-dimensional (3D) microenvironments and the factors that modulate those responses are critical to understanding how tissue migration is regulated during development, regeneration, and cancer. One key physical factor that regulates cell migration is topology. Most studies on surface topology and cell mechanics have been carried out with single migratory cells, yet little is known about the spreading and motility response of 3D complex multicellular tissues to topological cues. Here, we examine the behaviors of microsurgically isolated tissue explants composed of epithelial and mesenchymal cell layers from naturally 3D organized embryos of the aquatic frog Xenopus laevis to complex topological cues. We control topology using fabricated micropost arrays (MPAs) with different diameters (e.g., different spacing gaps) and investigate the collective 3D migration of these multicellular systems in these MPAs. Our topographical controlled approach for cellular application enables us to achieve a high degree of control over micropost positioning and geometry via simple, accurate, and repeatable microfabrication processes. We find that the topology regulates both collective and individual cell migration and that dense MPAs reduce but do not eliminate tissue spreading. By modulating cell size through the cell cycle inhibitor Mitomycin C or the spacing within MPAs we discover a role for topology in disrupting collective enhancement of cell migration. We find 3D topological cues can direct both single cell motility and tissue spreading, altering tissue-scale processes that enable efficient conversion of single cell motility into collective movement.

pi

DOI [BibTex]

DOI [BibTex]


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Three-dimensional heterogeneous assembly of coded microgels using an untethered mobile microgripper

Chung, S. E., Dong, X., Sitti, M.

Lab on a Chip, 15(7):1667-1676, Royal Society of Chemistry, January 2015 (article)

Abstract
Three-dimensional (3D) heterogeneous assembly of coded microgels in enclosed aquatic environments is demonstrated using a remotely actuated and controlled magnetic microgripper by a customized electromagnetic coil system. The microgripper uses different ‘stick–slip’ and ‘rolling’ locomotion in 2D and also levitation in 3D by magnetic gradient-based pulling force. This enables the microrobot to precisely manipulate each microgel by controlling its position and orientation in all x–y–z directions. Our microrobotic assembly method broke the barrier of limitation on the number of assembled microgel layers, because it enabled precise 3D levitation of the microgripper. We used the gripper to assemble microgels that had been coded with different colours and shapes onto prefabricated polymeric microposts. This eliminates the need for extra secondary cross-linking to fix the final construct. We demonstrated assembly of microgels on a single micropost up to ten layers. By increasing the number and changing the distribution of the posts, complex heterogeneous microsystems were possible to construct in 3D.

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DOI Project Page [BibTex]

DOI Project Page [BibTex]


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Integrating mechanism synthesis and topological optimization technique for stiffness-oriented design of a three degrees-of-freedom flexure-based parallel mechanism

Lum, G. Z., Teo, T. J., Yang, G., Yeo, S. H., Sitti, M.

Precision Engineering, 39, pages: 125-133, Elsevier, January 2015 (article)

Abstract
This paper introduces a new design approach to synthesize multiple degrees-of-freedom (DOF) flexure-based parallel mechanism (FPM). Termed as an integrated design approach, it is a systematic design methodology, which integrates both classical mechanism synthesis and modern topology optimization technique, to deliver an optimized multi-DOF FPM. This design approach is separated into two levels. At sub-chain level, a novel topology optimization technique, which uses the classical linkage mechanisms as DNA seeds, is used to synthesize the compliant joints or limbs. At configuration level, the optimal compliant joints are used to form the parallel limbs of the multi-DOF FPM and another stage of optimization was conducted to determine the optimal space distribution between these compliant joints so as to generate a multi-DOF FPM with optimized stiffness characteristic. In this paper, the design of a 3-DOF planar motion FPM was used to demonstrate the effectiveness and accuracy of this proposed design approach.

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DOI [BibTex]


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Actively controlled fibrillar friction surfaces

Marvi, H, Han, Y, Sitti, M

Applied Physics Letters, 106(5):051602, AIP Publishing, January 2015 (article)

Abstract
In this letter, we propose a technique by which we can actively adjust frictional properties of elastic fibrillar structures in different directions. Using a mesh attached to a two degree-of-freedom linear stage, we controlled the active length and the tilt angle of fibers, independently. Thus, we were able to achieve desired levels of friction forces in different directions and significantly improve passive friction anisotropies observed in the same fiber arrays. The proposed technique would allow us to readily control the friction anisotropy and the friction magnitude of fibrillar structures in any planar direction.

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DOI [BibTex]

DOI [BibTex]


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Active Reward Learning with a Novel Acquisition Function

Daniel, C., Kroemer, O., Viering, M., Metz, J., Peters, J.

Autonomous Robots, 39(3):389-405, 2015 (article)

am ei

link (url) DOI [BibTex]

link (url) DOI [BibTex]


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Learning Movement Primitive Attractor Goals and Sequential Skills from Kinesthetic Demonstrations

Manschitz, S., Kober, J., Gienger, M., Peters, J.

Robotics and Autonomous Systems, 74, Part A, pages: 97-107, 2015 (article)

am ei

link (url) DOI [BibTex]

link (url) DOI [BibTex]


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Bayesian Optimization for Learning Gaits under Uncertainty

Calandra, R., Seyfarth, A., Peters, J., Deisenroth, M.

Annals of Mathematics and Artificial Intelligence, pages: 1-19, 2015 (article)

am ei

DOI [BibTex]

DOI [BibTex]

2009


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Magnetic mobile micro-robots

Pawashe, C., Floyd, S., Sitti, M.

7eme Journees Nationales de la Recherche en Robotique, 2009 (article)

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[BibTex]

2009


[BibTex]


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Gecko-Inspired Directional and Controllable Adhesion

Murphy, M. P., Aksak, B., Sitti, M.

Small, 5(2):170-175, WILEY-VCH Verlag, 2009 (article)

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Project Page [BibTex]

Project Page [BibTex]


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Multiple magnetic microrobot control using electrostatic anchoring

Pawashe, C., Floyd, S., Sitti, M.

Applied Physics Letters, 94(16):164108, AIP, 2009 (article)

pi

[BibTex]

[BibTex]


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Wet self-cleaning of biologically inspired elastomer mushroom shaped microfibrillar adhesives

Kim, S., Cheung, E., Sitti, M.

Langmuir, 25(13):7196-7199, ACS Publications, 2009 (article)

pi

[BibTex]

[BibTex]


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Robot ceiling climbers harness new tricks

Marks, Paul

New Scientist, 202(2705):18-19, Reed Business Information, 2009 (article)

pi

[BibTex]

[BibTex]


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Biologically-Inspired Patterned and Coated Adhesives for Medical Devices

Glass, P, Chung, H, Lee, C, Tworkoski, E, Washburn, NR, Sitti, M

Journal of Medical Devices, 3(2):027537, American Society of Mechanical Engineers, 2009 (article)

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[BibTex]

[BibTex]


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Modeling and experimental characterization of an untethered magnetic micro-robot

Pawashe, C., Floyd, S., Sitti, M.

The International Journal of Robotics Research, 28(8):1077-1094, Sage Publications, 2009 (article)

pi

[BibTex]

[BibTex]


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Towards automated nanoassembly with the atomic force microscope: A versatile drift compensation procedure

Krohs, F., Onal, C., Sitti, M., Fatikow, S.

Journal of Dynamic Systems, Measurement, and Control, 131(6):061106, American Society of Mechanical Engineers, 2009 (article)

pi

[BibTex]

[BibTex]


Valero-Cuevas, F., Hoffmann, H., Kurse, M. U., Kutch, J. J., Theodorou, E. A.

IEEE Reviews in Biomedical Engineering – (All authors have equally contributed), (2):110?135, 2009, clmc (article)

Abstract
Computational models of the neuromuscular system hold the potential to allow us to reach a deeper understanding of neuromuscular function and clinical rehabilitation by complementing experimentation. By serving as a means to distill and explore specific hypotheses, computational models emerge from prior experimental data and motivate future experimental work. Here we review computational tools used to understand neuromuscular function including musculoskeletal modeling, machine learning, control theory, and statistical model analysis. We conclude that these tools, when used in combination, have the potential to further our understanding of neuromuscular function by serving as a rigorous means to test scientific hypotheses in ways that complement and leverage experimental data.

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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.

am

link (url) [BibTex]

link (url) [BibTex]


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Local dimensionality reduction for non-parametric regression

Hoffman, H., Schaal, S., Vijayakumar, S.

Neural Processing Letters, 2009, clmc (article)

Abstract
Locally-weighted regression is a computationally-efficient technique for non-linear regression. However, for high-dimensional data, this technique becomes numerically brittle and computationally too expensive if many local models need to be maintained simultaneously. Thus, local linear dimensionality reduction combined with locally-weighted regression seems to be a promising solution. In this context, we review linear dimensionality-reduction methods, compare their performance on nonparametric locally-linear regression, and discuss their ability to extend to incremental learning. The considered methods belong to the following three groups: (1) reducing dimensionality only on the input data, (2) modeling the joint input-output data distribution, and (3) optimizing the correlation between projection directions and output data. Group 1 contains principal component regression (PCR); group 2 contains principal component analysis (PCA) in joint input and output space, factor analysis, and probabilistic PCA; and group 3 contains reduced rank regression (RRR) and partial least squares (PLS) regression. Among the tested methods, only group 3 managed to achieve robust performance even for a non-optimal number of components (factors or projection directions). In contrast, group 1 and 2 failed for fewer components since these methods rely on the correct estimate of the true intrinsic dimensionality. In group 3, PLS is the only method for which a computationally-efficient incremental implementation exists. Thus, PLS appears to be ideally suited as a building block for a locally-weighted regressor in which projection directions are incrementally added on the fly.

am

link (url) [BibTex]

link (url) [BibTex]


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Enhanced adhesion by gecko-inspired hierarchical fibrillar adhesives

Murphy, M. P., Kim, S., Sitti, M.

ACS applied materials \& interfaces, 1(4):849-855, American Chemical Society, 2009 (article)

pi

Project Page [BibTex]

Project Page [BibTex]


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Miniature devices: Voyage of the microrobots

Sitti, M.

Nature, 458(7242):1121-1122, Nature Publishing Group, 2009 (article)

pi

[BibTex]

[BibTex]


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Dry spinning based spinneret based tunable engineered parameters (STEP) technique for controlled and aligned deposition of polymeric nanofibers

Nain, A. S., Sitti, M., Jacobson, A., Kowalewski, T., Amon, C.

Macromolecular rapid communications, 30(16):1406-1412, WILEY-VCH Verlag, 2009 (article)

pi

[BibTex]

[BibTex]


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Two-dimensional contact and noncontact micromanipulation in liquid using an untethered mobile magnetic microrobot

Floyd, S., Pawashe, C., Sitti, M.

IEEE Transactions on Robotics, 25(6):1332-1342, IEEE, 2009 (article)

pi

[BibTex]

[BibTex]


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A scaled bilateral control system for experimental one-dimensional teleoperated nanomanipulation

Onal, C. D., Sitti, M.

The International Journal of Robotics Research, 28(4):484-497, Sage Publications, 2009 (article)

pi

[BibTex]

[BibTex]


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A Swallowable Tethered Capsule Endoscope for Diagnosing Barrett’s Esophagus

Glass, P., Sitti, M., Pennathur, A., Appasamy, R.

Gastrointestinal Endoscopy, 69(5):AB106, Mosby, 2009 (article)

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Project Page [BibTex]

Project Page [BibTex]


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Adhesion of biologically inspired polymer microfibers on soft surfaces

Cheung, E., Sitti, M.

Langmuir, 25(12):6613-6616, ACS Publications, 2009 (article)

pi

[BibTex]

[BibTex]


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Dangling chain elastomers as repeatable fibrillar adhesives

Sitti, M., Cusick, B., Aksak, B., Nese, A., Lee, H., Dong, H., Kowalewski, T., Matyjaszewski, K.

ACS applied materials \& interfaces, 1(10):2277-2287, American Chemical Society, 2009 (article)

pi

[BibTex]

[BibTex]


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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

am

PDF [BibTex]

PDF [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.

am

link (url) [BibTex]

link (url) [BibTex]


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Reversible dry micro-fibrillar adhesives with thermally controllable adhesion

Kim, S., Sitti, M., Xie, T., Xiao, X.

Soft Matter, 5(19):3689-3693, Royal Society of Chemistry, 2009 (article)

pi

Project Page [BibTex]

Project Page [BibTex]


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Enhanced reversible adhesion of dopamine methacrylamide-coated elastomer microfibrillar structures under wet conditions

Glass, P., Chung, H., Washburn, N. R., Sitti, M.

Langmuir, 25(12):6607-6612, ACS Publications, 2009 (article)

pi

Project Page [BibTex]

Project Page [BibTex]

2005


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Composite adaptive control with locally weighted statistical learning

Nakanishi, J., Farrell, J. A., Schaal, S.

Neural Networks, 18(1):71-90, January 2005, clmc (article)

Abstract
This paper introduces a provably stable learning adaptive control framework with statistical learning. The proposed algorithm employs nonlinear function approximation with automatic growth of the learning network according to the nonlinearities and the working domain of the control system. The unknown function in the dynamical system is approximated by piecewise linear models using a nonparametric regression technique. Local models are allocated as necessary and their parameters are optimized on-line. Inspired by composite adaptive control methods, the proposed learning adaptive control algorithm uses both the tracking error and the estimation error to update the parameters. We first discuss statistical learning of nonlinear functions, and motivate our choice of the locally weighted learning framework. Second, we begin with a class of first order SISO systems for theoretical development of our learning adaptive control framework, and present a stability proof including a parameter projection method that is needed to avoid potential singularities during adaptation. Then, we generalize our adaptive controller to higher order SISO systems, and discuss further extension to MIMO problems. Finally, we evaluate our theoretical control framework in numerical simulations to illustrate the effectiveness of the proposed learning adaptive controller for rapid convergence and high accuracy of control.

am

link (url) [BibTex]

2005


link (url) [BibTex]


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A model of smooth pursuit based on learning of the target dynamics using only retinal signals

Shibata, T., Tabata, H., Schaal, S., Kawato, M.

Neural Networks, 18, pages: 213-225, 2005, clmc (article)

Abstract
While the predictive nature of the primate smooth pursuit system has been evident through several behavioural and neurophysiological experiments, few models have attempted to explain these results comprehensively. The model we propose in this paper in line with previous models employing optimal control theory; however, we hypothesize two new issues: (1) the medical superior temporal (MST) area in the cerebral cortex implements a recurrent neural network (RNN) in order to predict the current or future target velocity, and (2) a forward model of the target motion is acquired by on-line learning. We use stimulation studies to demonstrate how our new model supports these hypotheses.

am

link (url) [BibTex]

link (url) [BibTex]


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Parametric and Non-Parametric approaches for nonlinear tracking of moving objects

Hidaka, Y, Theodorou, E.

Technical Report-2005-1, 2005, clmc (article)

am

PDF [BibTex]

PDF [BibTex]

1999


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< 研究速報>(< 小特集> マイクロマシン)

Sitti, M., 橋本秀紀,

生産研究, 51(8):651-653, 東京大学, 1999 (article)

pi

[BibTex]

1999


[BibTex]


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Micro/Nano Manipulation Using Atomic Force Microscope.

Sitti, M., Hashimoto, H.

生産研究, 51(8):651-653, 東京大学生産技術研究所, 1999 (article)

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[BibTex]

[BibTex]


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Is imitation learning the route to humanoid robots?

Schaal, S.

Trends in Cognitive Sciences, 3(6):233-242, 1999, clmc (article)

Abstract
This review will focus on two recent developments in artificial intelligence and neural computation: learning from imitation and the development of humanoid robots. It will be postulated that the study of imitation learning offers a promising route to gain new insights into mechanisms of perceptual motor control that could ultimately lead to the creation of autonomous humanoid robots. This hope is justified because imitation learning channels research efforts towards three important issues: efficient motor learning, the connection between action and perception, and modular motor control in form of movement primitives. In order to make these points, first, a brief review of imitation learning will be given from the view of psychology and neuroscience. In these fields, representations and functional connections between action and perception have been explored that contribute to the understanding of motor acts of other beings. The recent discovery that some areas in the primate brain are active during both movement perception and execution provided a first idea of the possible neural basis of imitation. Secondly, computational approaches to imitation learning will be described, initially from the perspective of traditional AI and robotics, and then with a focus on neural network models and statistical learning research. Parallels and differences between biological and computational approaches to imitation will be highlighted. The review will end with an overview of current projects that actually employ imitation learning for humanoid robots.

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link (url) [BibTex]

link (url) [BibTex]


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Virtual Reality-Based Teleoperation in the Micro/Nano World.

Sitti, M., Hashimoto, H.

生産研究, 51(8):654-656, 東京大学生産技術研究所, 1999 (article)

pi

[BibTex]

[BibTex]


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Segmentation of endpoint trajectories does not imply segmented control

Sternad, D., Schaal, D.

Experimental Brain Research, 124(1):118-136, 1999, clmc (article)

Abstract
While it is generally assumed that complex movements consist of a sequence of simpler units, the quest to define these units of action, or movement primitives, still remains an open question. In this context, two hypotheses of movement segmentation of endpoint trajectories in 3D human drawing movements are re-examined: (1) the stroke-based segmentation hypothesis based on the results that the proportionality coefficient of the 2/3 power law changes discontinuously with each new â??strokeâ?, and (2) the segmentation hypothesis inferred from the observation of piecewise planar endpoint trajectories of 3D drawing movements. In two experiments human subjects performed a set of elliptical and figure-8 patterns of different sizes and orientations using their whole arm in 3D. The kinematic characteristics of the endpoint trajectories and the seven joint angles of the arm were analyzed. While the endpoint trajectories produced similar segmentation features as reported in the literature, analyses of the joint angles show no obvious segmentation but rather continuous oscillatory patterns. By approximating the joint angle data of human subjects with sinusoidal trajectories, and by implementing this model on a 7-degree-of-freedom anthropomorphic robot arm, it is shown that such a continuous movement strategy can produce exactly the same features as observed by the above segmentation hypotheses. The origin of this apparent segmentation of endpoint trajectories is traced back to the nonlinear transformations of the forward kinematics of human arms. The presented results demonstrate that principles of discrete movement generation may not be reconciled with those of rhythmic movement as easily as has been previously suggested, while the generalization of nonlinear pattern generators to arm movements can offer an interesting alternative to approach the question of units of action.

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link (url) [BibTex]

link (url) [BibTex]


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Teleoperated nano scale object manipulation

Sitti, M., Hashimoto, H.

Recent Advances on Mechatronics, pages: 322-335, Singapore: Springer-Verlag, 1999 (article)

pi

[BibTex]

[BibTex]