3,198 research outputs found
Guaranteed manipulator precision via interval analysis of inverse kinematics.
International audienceThe paper presents a new methodology for solving the inverse problem of manipulator precision design. Such design problems are often encountered when the end-effector uncertainty bounds are given, but it is not clear how to allocate precision bounds on individual robot axes. The approach presented in this paper uses interval analysis as a tool for uncertainty modeling and computational analysis. In prior work, the exponential formulation of the forward kinematics map was extended to intervals. Here, we use this result as an inclusion function in the computation of solutions to set-valued inverse kinematic problems. Simulation results are presented in two case studies to illustrate how we can go from an uncertainty interval at the end-effector to a design domain of allowable uncertainties at individual joints and links. The proposed method can be used to determine the level of precision needed in the design of a manipulator such that a predefined end-effector precision can be guaranteed. Also, the approach is general as such it can be easily extended to any degree-of-freedom and kinematic configuration
Heterocyst placement strategies to maximize growth of cyanobacterial filaments
Under conditions of limited fixed-nitrogen, some filamentous cyanobacteria
develop a regular pattern of heterocyst cells that fix nitrogen for the
remaining vegetative cells. We examine three different heterocyst placement
strategies by quantitatively modelling filament growth while varying both
external fixed-nitrogen and leakage from the filament. We find that there is an
optimum heterocyst frequency which maximizes the growth rate of the filament;
the optimum frequency decreases as the external fixed-nitrogen concentration
increases but increases as the leakage increases. In the presence of leakage,
filaments implementing a local heterocyst placement strategy grow significantly
faster than filaments implementing random heterocyst placement strategies. With
no extracellular fixed-nitrogen, consistent with recent experimental studies of
Anabaena sp. PCC 7120, the modelled heterocyst spacing distribution using our
local heterocyst placement strategy is qualitatively similar to experimentally
observed patterns. As external fixed-nitrogen is increased, the spacing
distribution for our local placement strategy retains the same shape while the
average spacing between heterocysts continuously increases.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in Physical Biology. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from it. The definitive publisher-authenticated version
will be available onlin
Explicit force control V.S. impedance control for micromanipulation.
International audienceThis paper presents a study of different force control schemes for controlling contact during manipulation tasks at the microscale. Explicit force control and impedance control are compared in a contact transition scenario consisting of a compliant microforce sensor mounted on a microrobotic positioner, and a compliant microstructure fabricated using Silicon MEMS. A traditional double mass-spring-damper model of the overall robot is employed to develop the closed-loop force controllers. Specific differences between the two control schemes due to the microscale nature of contact are highlighted in this paper from the experimental results obtained. The limitations and tradeoffs of the two control laws at the microscale due to the presence of backlash are discussed. A simple method to deal with the pull-off force effects specific to the microscale is proposed. Future improvements of the impedance control schemes to include adaptation are discussed in order to handle objects with unknown stiffness
A Processual Approach To Friction in Quadruple Helix Collaborations
R&D collaborations between industry, government, civil society, and research (also known as âquadruple helix collaborationsâ (QHCs)) have recently gained attention from R&D theorists and practitioners. In aiming to come to grips with their complexity, past models have generally taken a stakeholder-analytical approach based on stakeholder types. Yet stakeholder types are difficult to operationalise. We therefore argue that a processual model is more suited for studying the interaction in QHCs because it eschews matters of titles and identities. We develop such a model in which the QHC is represented as a process of generating four types of value: research value, market value, political value, and societal value. We then apply this processual model in analysing real-life cases of friction in QHCs. Friction is seen, not as an interpersonal clash, but as a discrepancy between two or more value-creation processes that compete for limited resources (some over-performing while others under-performing)
Composition with Target Constraints
It is known that the composition of schema mappings, each specified by
source-to-target tgds (st-tgds), can be specified by a second-order tgd (SO
tgd). We consider the question of what happens when target constraints are
allowed. Specifically, we consider the question of specifying the composition
of standard schema mappings (those specified by st-tgds, target egds, and a
weakly acyclic set of target tgds). We show that SO tgds, even with the
assistance of arbitrary source constraints and target constraints, cannot
specify in general the composition of two standard schema mappings. Therefore,
we introduce source-to-target second-order dependencies (st-SO dependencies),
which are similar to SO tgds, but allow equations in the conclusion. We show
that st-SO dependencies (along with target egds and target tgds) are sufficient
to express the composition of every finite sequence of standard schema
mappings, and further, every st-SO dependency specifies such a composition. In
addition to this expressive power, we show that st-SO dependencies enjoy other
desirable properties. In particular, they have a polynomial-time chase that
generates a universal solution. This universal solution can be used to find the
certain answers to unions of conjunctive queries in polynomial time. It is easy
to show that the composition of an arbitrary number of standard schema mappings
is equivalent to the composition of only two standard schema mappings. We show
that surprisingly, the analogous result holds also for schema mappings
specified by just st-tgds (no target constraints). This is proven by showing
that every SO tgd is equivalent to an unnested SO tgd (one where there is no
nesting of function symbols). Similarly, we prove unnesting results for st-SO
dependencies, with the same types of consequences.Comment: This paper is an extended version of: M. Arenas, R. Fagin, and A.
Nash. Composition with Target Constraints. In 13th International Conference
on Database Theory (ICDT), pages 129-142, 201
Search for massive rare particles with the SLIM experiment
The search for magnetic monopoles in the cosmic radiation remains one of the
main aims of non-accelerator particle astrophysics. Experiments at high
altitude allow lower mass thresholds with respect to detectors at sea level or
underground. The SLIM experiment is a large array of nuclear track detectors at
the Chacaltaya High Altitude Laboratory (5290 m a.s.l.). The results from the
analysis of 171 m exposed for more than 3.5 y are here reported. The
completion of the analysis of the whole detector will allow to set the lowest
flux upper limit for Magnetic Monopoles in the mass range 10 - 10
GeV. The experiment is also sensitive to SQM nuggets and Q-balls, which are
possible Dark Matter candidates.Comment: Presented at the 29-th ICRC, Pune, India (2005
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