1,721,055 research outputs found
A New Cell-Subdivision Approach to Plan Free Translations in Cluttered Environments
No full textThis paper presents a thorough discussion of the potential of a new cell-subdivision approach to plan translations of a convex polygon in a cluttered environment, where the focus is on planning simple motions on the basis of a fine-grained description of the workspace. A free path is planned in two main stages. The first stage exploits a plane-sweep paradigm in order to build a cell subdivision holding much relevant topological information on the free space and organizing a set of polygonal chains that approximate the boundaries of the configuration space obstacles. Then, the computations in the second stage are driven by an A* scheme designed to search the cell subdivision. During the search the bounding chains are subject to further refinements, but the cell graph is no longer modified. Among the remarkable features of the proposed technique we can mention: simple interface with the geometric modeler, based on two collision-detection primitives; small number of cells and adjacencies; incremental characterization of the free space. A few numerical results suggest that the new technique should be worth considering for applications, where appropriate; in particular, it seems to perform better than other approaches based on quadtrees. Moreover, it is quite interesting to observe that the cost of finding collision-free paths grows with the number of convex obstacles, whereas it is almost independent of the overall number of sides: we can interpret this result as supporting the choice of representing the obstacles decomposed into convex components. A succinct comparison between algorithmic and human intuitive path planning is also discussed in order to appraise the rate of redundant information processed by the algorithm, but we can also see that human planners behave significantly better only when the solutions are easy to find
Fast convex minimization to detect collisions between polyhedra
No full textThe subject of this paper is a fast algorithm for detecting collisions of two convex polyhedra translating in the space. A major feature is the novelty of the approach: collision detection for two convex bodies is reduced to collision detection for pairs of planar sections and minimization of a bivariate convex function; furthermore, most of the subproblems are solved using two-dimensional geometry. As proved by previous theoretical work, on this basis it is possible to design an algorithm, which runs in O(log^2n) time in the average and O(log^3n) in the worst case, where n is the total number of vertices. Here the focus is on a more practical version of the algorithm, which is particularly suited to plan collision-free paths on the basis of fine-grain descriptions of the objects in the workspace, as it is the case for the systems supported by sophisticated geometric modelers. After explaining the main ideas underlying the approach, a set of experimental results are presented and discussed in some depth
A Practical Motion Planning Strategy Based on a Plane-Sweep Approach
No full textWe discuss a practical motion planning strategy based on a two-step approach. First, an approximation of the C-space is built by a plane-sweep algorithm. Then, the search for a solution path drives the necessary refinement steps. Our claim is that an approach based on the incremental characterization of the C-space can be competitive with the best proposed motion planning techniques. We substantiate this claim in the simple case of planning translations of a convex body in the plane. Since the shape of the free space is incrementally recognized by probing the space via collision detection, every item of geometric information is obtained from the analysis of contact configurations involving convex bodies. At any stage the probes provide a partial characterization, represented by a simple cell subdivision and a suitable set of chains approximating the boundaries of the grown obstacles. The cells and their adjacencies do not change during the refinement step, so that the search strategy is straightforward. Although the performances are not optimal in theory, the planning algorithm shows a good behaviour, as demonstrated by a few experiments where it is compared with a quadtree-based strategy
Flexible exploitation of space coherence to detect collisions of convex polyhedra
No full textThe subject of this paper is a fast algorithm to compute collision translations for pairs of convex polyhedra with some interesting features. From a theoretical viewpoint, besides the novelty of the approach, the polylog asymptotic trend in the average case is as good as that of the best algorithms proposed to solve similar problems. On the other hand, the measured performances to detect possible collisions from scratch are satisfactory, and this is especially true in the cases where the bodies do not collide. But the most peculiar feature is a simple and flexible mechanism to exploit spatial coherence in a continuous range, which distinguishes this algorithm from all the other proposals we know. Furthermore, the nature of the approach is such that the self-tuning capability is attained at negligible additional costs even for unrelated collision tests. After a brief outline of the main ideas characterizing the approach, a set of numerical results are summarized. The proposed algorithm may be appropriate to plan collision-free paths, both on-line and off-line, on the basis of fine-grain descriptions of the objects in the workspace
How different human muscle models affect the estimation of lower limb joint stiffness during running
No full textThe dynamics of muscle force generation is directly related
to the movement dynamics of the skeletal system. Thus, modelling muscle dynamics is important to fully understand the control of movement
in humans. Abnormal movements caused by neuromuscular diseases such
as stroke, Parkinson's disease, or multiple sclerosis to name a few have all
in common the presence of some abnormal muscle tone. Muscle tone can
be effectively represented via short-range stiffness. Since stiffness is difficult to measure in real time, it is convenient to use numerical models to
assess muscle stiffness as function of muscle dynamics. In this work, two
different implementations of the Hill-type muscle model are considered
to estimate the lower limb joint stiffness during running. The obtained
results are discussed to evaluate how the choices of muscle models affect the estimation of lower limb joint stiffness. We found that stiffness
estimates are strongly dependent on the adopted muscle model. We observed different magnitude and timing of the estimated stiffness time
profile with respect to each gait phase, as function of the model used.
Furthermore, the two models produced substantially different joint stiffness time profiles for the ankle joint
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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