2,922 research outputs found
Geometric origin of mechanical properties of granular materials
Some remarkable generic properties, related to isostaticity and potential
energy minimization, of equilibrium configurations of assemblies of rigid,
frictionless grains are studied. Isostaticity -the uniqueness of the forces,
once the list of contacts is known- is established in a quite general context,
and the important distinction between isostatic problems under given external
loads and isostatic (rigid) structures is presented. Complete rigidity is only
guaranteed, on stability grounds, in the case of spherical cohesionless grains.
Otherwise, the network of contacts might deform elastically in response to load
increments, even though grains are rigid. This sets an uuper bound on the
contact coordination number. The approximation of small displacements (ASD)
allows to draw analogies with other model systems studied in statistical
mechanics, such as minimum paths on a lattice. It also entails the uniqueness
of the equilibrium state (the list of contacts itself is geometrically
determined) for cohesionless grains, and thus the absence of plastic
dissipation. Plasticity and hysteresis are due to the lack of such uniqueness
and may stem, apart from intergranular friction, from small, but finite,
rearrangements, in which the system jumps between two distinct potential energy
minima, or from bounded tensile contact forces. The response to load increments
is discussed. On the basis of past numerical studies, we argue that, if the ASD
is valid, the macroscopic displacement field is the solution to an elliptic
boundary value problem (akin to the Stokes problem).Comment: RevTex, 40 pages, 26 figures. Close to published paper. Misprints and
minor errors correcte
Haptic Perception of Object Curvature in Parkinson's Disease
The haptic perception of the curvature of an object is essential for adequate object manipulation and critical for our guidance of actions. This study investigated how the ability to perceive the curvature of an object is altered by Parkinson's disease (PD).Eight healthy subjects and 11 patients with mild to moderate PD had to judge, without vision, the curvature of a virtual "box" created by a robotic manipulandum. Their hands were either moved passively along a defined curved path or they actively explored the curved curvature of a virtual wall. The curvature was either concave or convex (bulging to the left or right) and was judged in two locations of the hand workspace--a left workspace location, where the curved hand path was associated with curved shoulder and elbow joint paths, and a right workspace location in which these joint paths were nearly linear. After exploring the curvature of the virtual object, subjects had to judge whether the curvature was concave or convex. Based on these data, thresholds for curvature sensitivity were established. The main findings of the study are: First, 9 out 11 PD patients (82%) showed elevated thresholds for detecting convex curvatures in at least one test condition. The respective median threshold for the PD group was increased by 343% when compared to the control group. Second, when distal hand paths became less associated with proximal joint paths (right workspace), haptic acuity was reduced substantially in both groups. Third, sensitivity to hand trajectory curvature was not improved during active exploration in either group.Our data demonstrate that PD is associated with a decreased acuity of the haptic sense, which may occur already at an early stage of the disease
Recommended from our members
Image Understanding and Robotics Research at Columbia University
Over the past year, the research investigations of the Vision/Robotics Laboratory at Columbia University have reflected the interests of its four faculty members, two staff programmers, and 16 Ph.D. students. Several of the projects involve other faculty members in the department or the university, or researchers at AT&T, IBM, or Philips. We list below a summary of our interests and results, together with the principal researchers associated with them. Since it is difficult to separate those aspects of robotic research that are purely visual from those that are vision-like (for example, tactile sensing) or vision-related (for example, integrated vision-robotic systems), we have listed all robotic research that is not purely manipulative. The majority of our current investigations are deepenings of work reported last year; this was the second year of both our basic Image Understanding contract and our Strategic Computing contract. Therefore, the form of this year's report closely resembles last year's. Although there are a few new initiatives, mainly we report the new results we have obtained in the same five basic research areas. Much of this work is summarized on a video tape that is available on request. We also note two service contributions this past year. The Special Issue on Computer Vision of the Proceedings of the IEEE, August, 1988, was co-edited by one of us (John Kender [27]). And, the upcoming IEEE Computer Society Conference on Computer Vision and Pattem Recognition, June, 1989, is co-program chaired by one of us (John Kender [23])
Grasping Points Determination Using Visual Features
This paper discusses some issues for generating point of contact using visual features. To address these issues, the paper is divided into two sections: visual features extraction and grasp planning. In order to provide a suitable description of object contour, a method for grouping visual features is proposed. A very important aspect of this method is the wa
Exact solutions for hydrodynamic interactions of two squirming spheres
We provide exact solutions of the Stokes equations for a squirming sphere
close to a no-slip surface, both planar and spherical, and for the interactions
between two squirmers, in three dimensions. These allow the hydrodynamic
interactions of swimming microscopic organisms with confining boundaries, or
each other, to be determined for arbitrary separation and, in particular, in
the close proximity regime where approximate methods based on point singularity
descriptions cease to be valid. We give a detailed description of the circular
motion of an arbitrary squirmer moving parallel to a no-slip spherical boundary
or flat free surface at close separation, finding that the circling generically
has opposite sense at free surfaces and at solid boundaries. While the
asymptotic interaction is symmetric under head-tail reversal of the swimmer, in
the near field microscopic structure can result in significant asymmetry. We
also find the translational velocity towards the surface for a simple model
with only the lowest two squirming modes. By comparing these to asymptotic
approximations of the interaction we find that the transition from near- to
far-field behaviour occurs at a separation of about two swimmer diameters.
These solutions are for the rotational velocity about the wall normal, or
common diameter of two spheres, and the translational speed along that same
direction, and are obtained using the Lorentz reciprocal theorem for Stokes
flows in conjunction with known solutions for the conjugate Stokes drag
problems, the derivations of which are demonstrated here for completeness. The
analogous motions in the perpendicular directions, i.e. parallel to the wall,
currently cannot be calculated exactly since the relevant Stokes drag solutions
needed for the reciprocal theorem are not available.Comment: 27 pages, 7 figure
Sensor system and related models to determine irregular shaped 3-D objects
This work comprises several parts, the initial part o f which is a review o f the techniques
in use at present for measuring shape and characterising products. The major work
details a ring sensor system, which consists o f a large number o f transmitters and
receivers alternately arranged on the circumference o f a metal annulus. Using a modified
polar co-ordinate system and trigonometric functions, two enveloping spirals o f an
object can be determined. One or both spirals can then be used for further data analysis.
Each spiral consists o f intersections between enveloping chords and parts o f the chords.
The area surrounding the object is segmented and properties such as volume and axis
measurements can be determined. A mode! was developed to simulate artificial objects
o f various shapes. Simulation tests were carried out to determine the limits o f the system
concerning position within the ring, shape and speed o f the object and resolution o f the
ring. A ring was manufactured for actual tests, which were carried out mainly on
potatoes to confirm the possible use in practice and to show the relative merits compared
with existing systems. Interesting side issues are introduced, such as the low number of
primary data, possibilities of further reduction using differential coding, and the
consumption time of the algorithms. Finally, a model for the simulation o f more than
one object in the ring at the same time is introduced and a possible way o f separation is
investigated
Recommended from our members
Image Understanding and Robotics Research at Columbia University
The research investigations of the Vision/Robotics Laboratory at Columbia University reflect the diversity of interests of its four faculty members, two staff programmers and 15 Ph.D. students. Several of the projects involve either a visiting computer science post-doc, other faculty members in the department or the university, or researchers at AT&T Bell Laboratories or Philips laboratories. We list below a summary of our interest and results, together with the principal researchers associated with them. Since it is difficult to separate those aspects of robotic research that are purely visual from those that are vision-like (for example, tactile sensing) or vision-related (for example, integrated vision-robotic systems), we have listed all robotic research that is not purely manipulative
- …