All Years Seminars
[INMA] 2011-12-13 (14.00) : Adaptive control of grip force to compensate for static and dynamic torques during object manipulation
At Euler building
Speaker :
Giard, Thibault
Abstract :
Manipulating a cup by the handle requires compensating for the torque
induced by the moment of the mass of the cup relative to the location
of the handle. In the present study, we investigated the control
strategy of subjects asked to perform grip-lift movements with an
object with center of mass located away from the grip
axis. Participants were asked to lift the manipulandum with a
two-fingers precision grip and stabilize it in front of a visual
target. Subjects showed a gradual and slow adaptation of the grip
force scaling across trials: the grip force tended to decrease slowly
and the temporal coordination between grip force and load torque rates
displayed gradually better-coordinated patterns. Importantly, this
adaptation was much slower than the stabilization of the same
parameters measured either when no torque came into play or after
previous adaptation to the presence of a torque. In contrast, the
maximum rotation induced by the torque was efficiently controlled
after only few trials, and an unexpected decrease in the tangential
torque produced significant overcompensation. An unexpected increase
in torque produced a consistent opposite effect. This shows that the
compensation for the dynamic torque was based on an anticipatory
dynamic counter-torque produced by the arm and wrist motor
commands. The comparatively slow stabilization of grip force control
suggests a specific adaptation process engaged by the presence of the
torque. This paradigm including tangential torques clearly constitutes
a powerful tool to extract the adaptive component of grip control
during object manipulation.
[INMA] 2011-12-06 (14.00) : Modeling, analysis and design of multi-hop control networks
At Euler building
Speaker :
Alessandro D'Innocenzo (University of L'Aquila)
Abstract :
A multi-hop Control Network (MCN) consists of a plant where the
communication between sensors, actuators and computational units is
supported by a (wireless) multi-hop communication network, and data
flow is performed using scheduling and routing of sensing and
actuation data. We propose a mathematical framework for modeling and
analyzing systems consisting of multiple control loops closed over a
multi-hop (wireless) communication network. We separate control,
topology, routing, and scheduling and propose formal syntax and
semantics for the dynamics of the composed system, providing an
explicit translation of multi-hop control networks to switched
systems. We propose formal models for analyzing robustness of
multi-hop control networks, where data is exchanged through a
multi-hop communication network subject to disruptions. When
communication disruptions are long, compared to the speed of the
control system, we propose to model them as permanent link
failures. We show that the complexity of analyzing such failures is
NP-hard, and discuss a way to overcome this limitation for practical
cases using compositional analysis. For typical packet transmission
errors, we propose a transient error model where links fail for one
time slot independently of the past and of other links. We provide
sufficient conditions for almost sure stability in presence of
transient link failures, and give efficient decision procedures. We
deal with errors that have random time span and show that, under some
conditions, the permanent failure model can be used as a reliable
abstraction. Our approach is compositional, namely it addresses the
problem of designing scalable scheduling and routing policies for
multiple control loops closed on the same multi-hop control
network. We describe how the translation of multi-hop control networks
to switched systems can be automated, and use it to solve control and
networking co-design challenges in some representative examples, and
to propose a scheduling solution in a mineral floatation control
problem that can be implemented on a time triggered communication
protocols for wireless networks. We address the control design problem
on a MCN that is subject to link failures, where the plant is a SISO
LTI system. We first characterize controllability and observability of
a MCN, by means of necessary and sufficient conditions on the plant
dynamics and on the communication scheduling and routing. We provide a
methodology to design scheduling and routing, in order to satisfy
controllability and observability of a MCN for any link failure that
preserves connectivity of the radio connectivity graph. Then, we
characterize the problem of detecting the failure of links of the
radio connectivity graph: we provide necessary and sufficient
conditions on the plant dynamics and on the communication protocol,
and we provide a methodology to design the network topology,
scheduling and routing to satisfy the above conditions.
[INMA] 2011-11-22 (14.00) : Morse Theory and Formation Control
At Euler building
Speaker :
Brian D O ANDERSON (Australian
National University)
Abstract : Formation shape control for a collection of point agents is concerned
with devising decentralized control laws which will ensure that the
formation will move so that certain inter-agent distances assume
prescribed values. A number of algorithms based on steepest descent of
an error function have been suggested for various problems, and all
display the existence of incorrect equilibria, though often the
equilibria are saddle points or unstable. This talk introduces Morse
theory as a tool for analyzing the number of such equilibria.
Initially a simpler problem is reviewed: that of navigating a point
agent using a steepest descent law to a point in a closed
two-dimensional region containing obstacles. A key conclusion is that
for both the navigation problem and for the formation shape control
problem, there will always be incorrect equilibria associated with any
steepest descent law.
[INMA] 2011-11-15 (14.00) : Should Belgium be split? A perspective from complex networks, mobile phone data and privacy
At Euler building
Speaker :
Blondel, Vincent
Abstract : Many complex networks have their nodes distributed in space. In this talk, we describe some recent results on spatially distributed networks. In particular, we describe results obtained from a community detection method on a large network constructed from communications between millions of mobile phone users. We quantify in this network the decrease with distance of connection probability between mobile phone users and we describe a conjecture about a possible explanation for the observed decrease. Related to this observation, we will also analyse a surprising connection between how people relocate in the US and the eigenvectors of a large matrix. Finally, we will conclude our talk with comments on privacy issues in data collection in large social networks.
[INMA] 2011-11-08 (14.00) : Dichotomic collective behaviors in networks of pulse-coupled oscillators
At Euler building
Speaker :
Mauroy, Alexandre
Abstract : The study of networks of pulse-coupled firing oscillators is a general and simple paradigm to investigate a wealth of natural phenomena (brain neurons, earthquakes, animal behaviors, etc.). In this framework, the oscillators of the network interact through an instantaneous impulsive coupling: whenever an oscillator fires, it sends out a pulse which instantaneously increments the state of the other oscillators by a constant value.
It is a remarkable fact that networks of pulse-coupled oscillators usually exhibit a dichotomic behavior: either the oscillators achieve perfect synchrony or they converge toward an anti-synchronized configuration. Interestingly, theoretical results confirm the dichotomic behavior for a large class of oscillators, but also suggest that the dichotomic behavior is not a general feature of every network of pulse-coupled oscillators.
In this talk, I will present stability results, both for finite and infinite populations of pulse-coupled oscillators, as well as several related open problems.
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