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2015


Scalable Robust Principal Component Analysis using {Grassmann} Averages
Scalable Robust Principal Component Analysis using Grassmann Averages

Hauberg, S., Feragen, A., Enficiaud, R., Black, M.

IEEE Trans. Pattern Analysis and Machine Intelligence (PAMI), December 2015 (article)

Abstract
In large datasets, manual data verification is impossible, and we must expect the number of outliers to increase with data size. While principal component analysis (PCA) can reduce data size, and scalable solutions exist, it is well-known that outliers can arbitrarily corrupt the results. Unfortunately, state-of-the-art approaches for robust PCA are not scalable. We note that in a zero-mean dataset, each observation spans a one-dimensional subspace, giving a point on the Grassmann manifold. We show that the average subspace corresponds to the leading principal component for Gaussian data. We provide a simple algorithm for computing this Grassmann Average (GA), and show that the subspace estimate is less sensitive to outliers than PCA for general distributions. Because averages can be efficiently computed, we immediately gain scalability. We exploit robust averaging to formulate the Robust Grassmann Average (RGA) as a form of robust PCA. The resulting Trimmed Grassmann Average (TGA) is appropriate for computer vision because it is robust to pixel outliers. The algorithm has linear computational complexity and minimal memory requirements. We demonstrate TGA for background modeling, video restoration, and shadow removal. We show scalability by performing robust PCA on the entire Star Wars IV movie; a task beyond any current method. Source code is available online.

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preprint pdf from publisher supplemental Project Page [BibTex]

2015


preprint pdf from publisher supplemental Project Page [BibTex]


{SMPL}: A Skinned Multi-Person Linear Model
SMPL: A Skinned Multi-Person Linear Model

Loper, M., Mahmood, N., Romero, J., Pons-Moll, G., Black, M. J.

ACM Trans. Graphics (Proc. SIGGRAPH Asia), 34(6):248:1-248:16, ACM, New York, NY, October 2015 (article)

Abstract
We present a learned model of human body shape and pose-dependent shape variation that is more accurate than previous models and is compatible with existing graphics pipelines. Our Skinned Multi-Person Linear model (SMPL) is a skinned vertex-based model that accurately represents a wide variety of body shapes in natural human poses. The parameters of the model are learned from data including the rest pose template, blend weights, pose-dependent blend shapes, identity-dependent blend shapes, and a regressor from vertices to joint locations. Unlike previous models, the pose-dependent blend shapes are a linear function of the elements of the pose rotation matrices. This simple formulation enables training the entire model from a relatively large number of aligned 3D meshes of different people in different poses. We quantitatively evaluate variants of SMPL using linear or dual-quaternion blend skinning and show that both are more accurate than a Blend-SCAPE model trained on the same data. We also extend SMPL to realistically model dynamic soft-tissue deformations. Because it is based on blend skinning, SMPL is compatible with existing rendering engines and we make it available for research purposes.

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pdf video code/model errata DOI Project Page Project Page [BibTex]

pdf video code/model errata DOI Project Page Project Page [BibTex]


Proceedings of the 37th German Conference on Pattern Recognition
Proceedings of the 37th German Conference on Pattern Recognition

Gall, J., Gehler, P., Leibe, B.

Springer, German Conference on Pattern Recognition, October 2015 (proceedings)

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GCPR conference website [BibTex]

GCPR conference website [BibTex]


Experimental investigation of optimal adhesion of mushroomlike elastomer microfibrillar adhesives
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]

DOI [BibTex]


Dyna: A Model of Dynamic Human Shape in Motion
Dyna: A Model of Dynamic Human Shape in Motion

Pons-Moll, G., Romero, J., Mahmood, N., Black, M. J.

ACM Transactions on Graphics, (Proc. SIGGRAPH), 34(4):120:1-120:14, ACM, August 2015 (article)

Abstract
To look human, digital full-body avatars need to have soft tissue deformations like those of real people. We learn a model of soft-tissue deformations from examples using a high-resolution 4D capture system and a method that accurately registers a template mesh to sequences of 3D scans. Using over 40,000 scans of ten subjects, we learn how soft tissue motion causes mesh triangles to deform relative to a base 3D body model. Our Dyna model uses a low-dimensional linear subspace to approximate soft-tissue deformation and relates the subspace coefficients to the changing pose of the body. Dyna uses a second-order auto-regressive model that predicts soft-tissue deformations based on previous deformations, the velocity and acceleration of the body, and the angular velocities and accelerations of the limbs. Dyna also models how deformations vary with a person’s body mass index (BMI), producing different deformations for people with different shapes. Dyna realistically represents the dynamics of soft tissue for previously unseen subjects and motions. We provide tools for animators to modify the deformations and apply them to new stylized characters.

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pdf preprint video data DOI Project Page Project Page [BibTex]

pdf preprint video data DOI Project Page Project Page [BibTex]


Linking Objects to Actions: Encoding of Target Object and Grasping Strategy in Primate Ventral Premotor Cortex
Linking Objects to Actions: Encoding of Target Object and Grasping Strategy in Primate Ventral Premotor Cortex

Vargas-Irwin, C. E., Franquemont, L., Black, M. J., Donoghue, J. P.

Journal of Neuroscience, 35(30):10888-10897, July 2015 (article)

Abstract
Neural activity in ventral premotor cortex (PMv) has been associated with the process of matching perceived objects with the motor commands needed to grasp them. It remains unclear how PMv networks can flexibly link percepts of objects affording multiple grasp options into a final desired hand action. Here, we use a relational encoding approach to track the functional state of PMv neuronal ensembles in macaque monkeys through the process of passive viewing, grip planning, and grasping movement execution. We used objects affording multiple possible grip strategies. The task included separate instructed delay periods for object presentation and grip instruction. This approach allowed us to distinguish responses elicited by the visual presentation of the objects from those associated with selecting a given motor plan for grasping. We show that PMv continuously incorporates information related to object shape and grip strategy as it becomes available, revealing a transition from a set of ensemble states initially most closely related to objects, to a new set of ensemble patterns reflecting unique object-grip combinations. These results suggest that PMv dynamically combines percepts, gradually navigating toward activity patterns associated with specific volitional actions, rather than directly mapping perceptual object properties onto categorical grip representations. Our results support the idea that PMv is part of a network that dynamically computes motor plans from perceptual information. Significance Statement: The present work demonstrates that the activity of groups of neurons in primate ventral premotor cortex reflects information related to visually presented objects, as well as the motor strategy used to grasp them, linking individual objects to multiple possible grips. PMv could provide useful control signals for neuroprosthetic assistive devices designed to interact with objects in a flexible way.

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publisher link DOI Project Page [BibTex]

publisher link DOI Project Page [BibTex]


pH-taxis of biohybrid microsystems
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]


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

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DOI [BibTex]

DOI [BibTex]


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


Transfer Printing of Metallic Microstructures on Adhesion-Promoting Hydrogel Substrates
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]


Biomedical applications of untethered mobile milli/microrobots
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]


Multi-view and 3D Deformable Part Models
Multi-view and 3D Deformable Part Models

Pepik, B., Stark, M., Gehler, P., Schiele, B.

Pattern Analysis and Machine Intelligence, 37(11):14, IEEE, March 2015 (article)

Abstract
As objects are inherently 3-dimensional, they have been modeled in 3D in the early days of computer vision. Due to the ambiguities arising from mapping 2D features to 3D models, 3D object representations have been neglected and 2D feature-based models are the predominant paradigm in object detection nowadays. While such models have achieved outstanding bounding box detection performance, they come with limited expressiveness, as they are clearly limited in their capability of reasoning about 3D shape or viewpoints. In this work, we bring the worlds of 3D and 2D object representations closer, by building an object detector which leverages the expressive power of 3D object representations while at the same time can be robustly matched to image evidence. To that end, we gradually extend the successful deformable part model [1] to include viewpoint information and part-level 3D geometry information, resulting in several different models with different level of expressiveness. We end up with a 3D object model, consisting of multiple object parts represented in 3D and a continuous appearance model. We experimentally verify that our models, while providing richer object hypotheses than the 2D object models, provide consistently better joint object localization and viewpoint estimation than the state-of-the-art multi-view and 3D object detectors on various benchmarks (KITTI [2], 3D object classes [3], Pascal3D+ [4], Pascal VOC 2007 [5], EPFL multi-view cars [6]).

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DOI Project Page [BibTex]

DOI Project Page [BibTex]


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

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DOI [BibTex]

DOI [BibTex]


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


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


Actively controlled fibrillar friction surfaces
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]


{Spike train SIMilarity Space} ({SSIMS}): A framework for single neuron and ensemble data analysis
Spike train SIMilarity Space (SSIMS): A framework for single neuron and ensemble data analysis

Vargas-Irwin, C. E., Brandman, D. M., Zimmermann, J. B., Donoghue, J. P., Black, M. J.

Neural Computation, 27(1):1-31, MIT Press, January 2015 (article)

Abstract
We present a method to evaluate the relative similarity of neural spiking patterns by combining spike train distance metrics with dimensionality reduction. Spike train distance metrics provide an estimate of similarity between activity patterns at multiple temporal resolutions. Vectors of pair-wise distances are used to represent the intrinsic relationships between multiple activity patterns at the level of single units or neuronal ensembles. Dimensionality reduction is then used to project the data into concise representations suitable for clustering analysis as well as exploratory visualization. Algorithm performance and robustness are evaluated using multielectrode ensemble activity data recorded in behaving primates. We demonstrate how Spike train SIMilarity Space (SSIMS) analysis captures the relationship between goal directions for an 8-directional reaching task and successfully segregates grasp types in a 3D grasping task in the absence of kinematic information. The algorithm enables exploration of virtually any type of neural spiking (time series) data, providing similarity-based clustering of neural activity states with minimal assumptions about potential information encoding models.

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pdf: publisher site pdf: author's proof DOI Project Page [BibTex]

pdf: publisher site pdf: author's proof DOI Project Page [BibTex]


Sensory synergy as environmental input integration
Sensory synergy as environmental input integration

Alnajjar, F., Itkonen, M., Berenz, V., Tournier, M., Nagai, C., Shimoda, S.

Frontiers in Neuroscience, 8, pages: 436, 2015 (article)

Abstract
The development of a method to feed proper environmental inputs back to the central nervous system (CNS) remains one of the challenges in achieving natural movement when part of the body is replaced with an artificial device. Muscle synergies are widely accepted as a biologically plausible interpretation of the neural dynamics between the CNS and the muscular system. Yet the sensorineural dynamics of environmental feedback to the CNS has not been investigated in detail. In this study, we address this issue by exploring the concept of sensory synergy. In contrast to muscle synergy, we hypothesize that sensory synergy plays an essential role in integrating the overall environmental inputs to provide low-dimensional information to the CNS. We assume that sensor synergy and muscle synergy communicate using these low-dimensional signals. To examine our hypothesis, we conducted posture control experiments involving lateral disturbance with 9 healthy participants. Proprioceptive information represented by the changes on muscle lengths were estimated by using the musculoskeletal model analysis software SIMM. Changes on muscles lengths were then used to compute sensory synergies. The experimental results indicate that the environmental inputs were translated into the two dimensional signals and used to move the upper limb to the desired position immediately after the lateral disturbance. Participants who showed high skill in posture control were found to be likely to have a strong correlation between sensory and muscle signaling as well as high coordination between the utilized sensory synergies. These results suggest the importance of integrating environmental inputs into suitable low-dimensional signals before providing them to the CNS. This mechanism should be essential when designing the prosthesis’ sensory system to make the controller simpler

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link (url) DOI [BibTex]

link (url) 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)

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


Metric Regression Forests for Correspondence Estimation
Metric Regression Forests for Correspondence Estimation

Pons-Moll, G., Taylor, J., Shotton, J., Hertzmann, A., Fitzgibbon, A.

International Journal of Computer Vision, pages: 1-13, 2015 (article)

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springer PDF Project Page [BibTex]

springer PDF Project Page [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)

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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)

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DOI [BibTex]

DOI [BibTex]


Formation control driven by cooperative object tracking
Formation control driven by cooperative object tracking

Lima, P., Ahmad, A., Dias, A., Conceição, A., Moreira, A., Silva, E., Almeida, L., Oliveira, L., Nascimento, T.

Robotics and Autonomous Systems, 63(1):68-79, 2015 (article)

Abstract
In this paper we introduce a formation control loop that maximizes the performance of the cooperative perception of a tracked target by a team of mobile robots, while maintaining the team in formation, with a dynamically adjustable geometry which is a function of the quality of the target perception by the team. In the formation control loop, the controller module is a distributed non-linear model predictive controller and the estimator module fuses local estimates of the target state, obtained by a particle filter at each robot. The two modules and their integration are described in detail, including a real-time database associated to a wireless communication protocol that facilitates the exchange of state data while reducing collisions among team members. Simulation and real robot results for indoor and outdoor teams of different robots are presented. The results highlight how our method successfully enables a team of homogeneous robots to minimize the total uncertainty of the tracked target cooperative estimate while complying with performance criteria such as keeping a pre-set distance between the teammates and the target, avoiding collisions with teammates and/or surrounding obstacles.

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DOI [BibTex]

DOI [BibTex]


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Efficient single-cell poration by microsecond laser pulses

Fan, Q., Hu, W., Ohta, A. T.

Lab on a Chip, 15(2):581-588, Royal Society of Chemistry, 2015 (article)

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[BibTex]

[BibTex]

2008


A non-parametric {Bayesian} alternative to spike sorting
A non-parametric Bayesian alternative to spike sorting

Wood, F., Black, M. J.

J. Neuroscience Methods, 173(1):1–12, August 2008 (article)

Abstract
The analysis of extra-cellular neural recordings typically begins with careful spike sorting and all analysis of the data then rests on the correctness of the resulting spike trains. In many situations this is unproblematic as experimental and spike sorting procedures often focus on well isolated units. There is evidence in the literature, however, that errors in spike sorting can occur even with carefully collected and selected data. Additionally, chronically implanted electrodes and arrays with fixed electrodes cannot be easily adjusted to provide well isolated units. In these situations, multiple units may be recorded and the assignment of waveforms to units may be ambiguous. At the same time, analysis of such data may be both scientifically important and clinically relevant. In this paper we address this issue using a novel probabilistic model that accounts for several important sources of uncertainty and error in spike sorting. In lieu of sorting neural data to produce a single best spike train, we estimate a probabilistic model of spike trains given the observed data. We show how such a distribution over spike sortings can support standard neuroscientific questions while providing a representation of uncertainty in the analysis. As a representative illustration of the approach, we analyzed primary motor cortical tuning with respect to hand movement in data recorded with a chronic multi-electrode array in non-human primates.We found that the probabilistic analysis generally agrees with human sorters but suggests the presence of tuned units not detected by humans.

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pdf preprint pdf from publisher PubMed [BibTex]

2008


pdf preprint pdf from publisher PubMed [BibTex]


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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.

am ei

link (url) DOI [BibTex]

link (url) DOI [BibTex]


Neural control of computer cursor velocity by decoding motor cortical spiking activity in humans with tetraplegia
Neural control of computer cursor velocity by decoding motor cortical spiking activity in humans with tetraplegia

(J. Neural Engineering Highlights of 2008 Collection)

Kim, S., Simeral, J., Hochberg, L., Donoghue, J. P., Black, M. J.

J. Neural Engineering, 5, pages: 455–476, 2008 (article)

Abstract
Computer-mediated connections between human motor cortical neurons and assistive devices promise to improve or restore lost function in people with paralysis. Recently, a pilot clinical study of an intracortical neural interface system demonstrated that a tetraplegic human was able to obtain continuous two-dimensional control of a computer cursor using neural activity recorded from his motor cortex. This control, however, was not sufficiently accurate for reliable use in many common computer control tasks. Here, we studied several central design choices for such a system including the kinematic representation for cursor movement, the decoding method that translates neuronal ensemble spiking activity into a control signal and the cursor control task used during training for optimizing the parameters of the decoding method. In two tetraplegic participants, we found that controlling a cursor’s velocity resulted in more accurate closed-loop control than controlling its position directly and that cursor velocity control was achieved more rapidly than position control. Control quality was further improved over conventional linear filters by using a probabilistic method, the Kalman filter, to decode human motor cortical activity. Performance assessment based on standard metrics used for the evaluation of a wide range of pointing devices demonstrated significantly improved cursor control with velocity rather than position decoding.

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pdf preprint pdf from publisher [BibTex]

pdf preprint pdf from publisher [BibTex]


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ENHANCED ADHESION OF PDMS SURFACES FUNCTIONALIZED BY POLY (n-BUTYL ACRYLATE) BRUSHES INSPIRED BY GECKO FOOT HAIRS

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

Polymer Preprints, 49(2):107, 2008 (article)

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[BibTex]

[BibTex]


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Design and development of the lifting and propulsion mechanism for a biologically inspired water runner robot

Floyd, S., Sitti, M.

IEEE transactions on robotics, 24(3):698-709, IEEE, 2008 (article)

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[BibTex]

[BibTex]


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Control of Cell Behavior by Aligned Micro/Nanofibrous Biomaterial Scaffolds Fabricated by Spinneret-Based Tunable Engineered Parameters (STEP) Technique

Nain, A. S., Phillippi, J. A., Sitti, M., MacKrell, J., Campbell, P. G., Amon, C.

Small, 4(8):1153-1159, Wiley Online Library, 2008 (article)

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[BibTex]

[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]


Brownian Warps for Non-Rigid Registration
Brownian Warps for Non-Rigid Registration

Mads Nielsen, Peter Johansen, Andrew Jackson, Benny Lautrup, Soren Hauberg

Journal of Mathematical Imaging and Vision, 31, pages: 221-231, Springer Netherlands, 2008 (article)

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Publishers site PDF [BibTex]

Publishers site PDF [BibTex]


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Rolling and spinning friction characterization of fine particles using lateral force microscopy based contact pushing

Sümer, B., Sitti, M.

Journal of Adhesion Science and Technology, 22(5-6):481-506, Taylor & Francis Group, 2008 (article)

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[BibTex]

[BibTex]


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Modeling the soft backing layer thickness effect on adhesion of elastic microfiber arrays

Long, R., Hui, C., Kim, S., Sitti, M.

Journal of Applied Physics, 104(4):044301, AIP, 2008 (article)

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Project Page [BibTex]

Project Page [BibTex]


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Cross-talk compensation in atomic force microscopy

Onal, C. D., Sümer, B., Sitti, M.

Review of scientific instruments, 79(10):103706, AIP, 2008 (article)

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[BibTex]

[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]


 An Efficient Algorithm for Modelling Duration in Hidden Markov Models, with a Dramatic Application
An Efficient Algorithm for Modelling Duration in Hidden Markov Models, with a Dramatic Application

Soren Hauberg, Jakob Sloth

Journal of Mathematical Imaging and Vision, 31, pages: 165-170, Springer Netherlands, 2008 (article)

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Publishers site Paper site PDF [BibTex]

Publishers site Paper site PDF [BibTex]


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Adhesion of biologically inspired oil-coated polymer micropillars

Cheung, E., Sitti, M.

Journal of Adhesion Science and Technology, 22(5-6):569-589, Taylor & Francis Group, 2008 (article)

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[BibTex]

[BibTex]


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Vision-based feedback strategy for controlled pushing of microparticles

Lynch, N. A., Onal, C. D., Schuster, E., Sitti, M.

Journal of Micro-Nano Mechatronics, 4(1-2):73-83, Springer-Verlag, 2008 (article)

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[BibTex]

[BibTex]


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Effect of quantity and configuration of attached bacteria on bacterial propulsion of microbeads

Behkam, B., Sitti, M.

Applied Physics Letters, 93(22):223901, AIP, 2008 (article)

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[BibTex]

[BibTex]


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A library for locally weighted projection regression

Klanke, S., Vijayakumar, S., Schaal, S.

Journal of Machine Learning Research, 9, pages: 623-626, 2008, clmc (article)

Abstract
In this paper we introduce an improved implementation of locally weighted projection regression (LWPR), a supervised learning algorithm that is capable of handling high-dimensional input data. As the key features, our code supports multi-threading, is available for multiple platforms, and provides wrappers for several programming languages.

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link (url) [BibTex]

link (url) [BibTex]


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Preface to the Journal of Micro-Nano Mechatronics

Dario, P., Fukuda, T., Sitti, M.

Journal of Micro-Nano Mechatronics, 4(1-2):1-1, Springer-Verlag, 2008 (article)

pi

[BibTex]

[BibTex]


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A legged anchoring mechanism for capsule endoscopes using micropatterned adhesives

Glass, P., Cheung, E., Sitti, M.

IEEE Transactions on Biomedical Engineering, 55(12):2759-2767, IEEE, 2008 (article)

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Project Page [BibTex]

Project Page [BibTex]


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Optimization strategies in human reinforcement learning

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

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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Dynamic modeling of stick slip motion in an untethered magnetic microrobot

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

Proceedings of Robotics: Science and Systems IV, Zurich, Switzerland, 2008 (article)

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[BibTex]

[BibTex]

2007


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Learning static Gestalt laws through dynamic experience

Ostrovsky, Y., Wulff, J., Sinha, P.

Journal of Vision, 7(9):315-315, ARVO, June 2007 (article)

Abstract
The Gestalt laws (Wertheimer 1923) are widely regarded as the rules that help us parse the world into objects. However, it is unclear as to how these laws are acquired by an infant's visual system. Classically, these “laws” have been presumed to be innate (Kellman and Spelke 1983). But, more recent work in infant development, showing the protracted time-course over which these grouping principles emerge (e.g., Johnson and Aslin 1995; Craton 1996), suggests that visual experience might play a role in their genesis. Specifically, our studies of patients with late-onset vision (Project Prakash; VSS 2006) and evidence from infant development both point to an early role of common motion cues for object grouping. Here we explore the possibility that the privileged status of motion in the developmental timeline is not happenstance, but rather serves to bootstrap the learning of static Gestalt cues. Our approach involves computational analyses of real-world motion sequences to investigate whether primitive optic flow information is correlated with static figural cues that could eventually come to serve as proxies for grouping in the form of Gestalt principles. We calculated local optic flow maps and then examined how similarity of motion across image patches co-varied with similarity of certain figural properties in static frames. Results indicate that patches with similar motion are much more likely to have similar luminance, color, and orientation as compared to patches with dissimilar motion vectors. This regularity suggests that, in principle, common motion extracted from dynamic visual experience can provide enough information to bootstrap region grouping based on luminance and color and contour continuation mechanisms in static scenes. These observations, coupled with the cited experimental studies, lend credence to the hypothesis that static Gestalt laws might be learned through a bootstrapping process based on early dynamic experience.

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link (url) DOI [BibTex]

2007


link (url) DOI [BibTex]


Neuromotor prosthesis development
Neuromotor prosthesis development

Donoghue, J., Hochberg, L., Nurmikko, A., Black, M., Simeral, J., Friehs, G.

Medicine & Health Rhode Island, 90(1):12-15, January 2007 (article)

Abstract
Article describes a neuromotor prosthesis (NMP), in development at Brown University, that records human brain signals, decodes them, and transforms them into movement commands. An NMP is described as a system consisting of a neural interface, a decoding system, and a user interface, also called an effector; a closed-loop system would be completed by a feedback signal from the effector to the brain. The interface is based on neural spiking, a source of information-rich, rapid, complex control signals from the nervous system. The NMP described, named BrainGate, consists of a match-head sized platform with 100 thread-thin electrodes implanted just into the surface of the motor cortex where commands to move the hand emanate. Neural signals are decoded by a rack of computers that displays the resultant output as the motion of a cursor on a computer monitor. While computer cursor motion represents a form of virtual device control, this same command signal could be routed to a device to command motion of paralyzed muscles or the actions of prosthetic limbs. The researchers’ overall goal is the development of a fully implantable, wireless multi-neuron sensor for broad research, neural prosthetic, and human neurodiagnostic applications.

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pdf [BibTex]

pdf [BibTex]


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Bacterial flagella-based propulsion and on/off motion control of microscale objects

Behkam, B., Sitti, M.

Applied Physics Letters, 90(2):023902, AIP, 2007 (article)

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[BibTex]

[BibTex]


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Friction of partially embedded vertically aligned carbon nanofibers inside elastomers

Aksak, B., Sitti, M., Cassell, A., Li, J., Meyyappan, M., Callen, P.

Applied Physics Letters, 91(6):061906, AIP, 2007 (article)

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[BibTex]

[BibTex]


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Enhanced friction of elastomer microfiber adhesives with spatulate tips

Kim, S., Aksak, B., Sitti, M.

Applied Physics Letters, 91(22):221913, AIP, 2007 (article)

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Project Page [BibTex]

Project Page [BibTex]