All Years Seminars
[INMA] 2012-05-15 (14.00) : Development of safe autonomous systems: a perspective
At Euler building
Speaker :
Holger VOOS (University of Luxembourg)
Abstract :
In the near future, systems with a high degree of autonomy like mobile service robots are expected to provide services in many area of life. They have to execute demanding and complex tasks in a dynamic unstructured environment, collaborate with human users in a natural and intuitive way and adapt themselves to varying conditions. While much effort in research is spent on the investigation and realization of artificial intelligence and autonomy, it is often neglected that a higher degree of autonomy also results in higher safety requirements, especially if these autonomous systems have to interact closely with human users.
This presentation focuses on a perspective for the development of safe autonomous systems. While processes and measures for the design of safety-critical technical systems in general exist, both the structural and behavioral complexity of an autonomous system requires measures to ensure safety that are far beyond the currently existing approaches. However, the application of methods like simulation, testing or formal verification during design time turned out to be rather difficult for the development of safe autonomous systems. Therefore this presentation also considers a supervisory approach where safe autonomous behavior of a system should be achieved via online risk assessment and risk-sensitive action planning and generation.
[INMA] 2012-05-08 (14.00) : Tactile detection of slip: fine characterization of skin deformation during the onset of slip
At Euler building
Speaker :
Delhaye, Benoit
Abstract :
In daily life, we manipulate objects that have different physical properties (mass, surface, shape). Most of the time, we succeed in this task and prevent the objects dropping from the hand. This is explained by the fine coordination between the load force (LF), the tangential force due to the object weight and inertia, and the grip force (GF), the normal force applied to the objects to counteract the LF. This fine coordination is possible with the help of the cutaneous feedback from the fingertips mechanoreceptors. These respond to the stresses and strains created by the deformation of the skin during object grasping and manipulation. Measuring the fingertip skin deformation before and during slipping may thus contribute to better understand how we can prevent the objects from slipping during their manipulation.
In this study, we used an imaging system to record the fingertip deformation during tangential sliding movements of the fingertips on a glass. The constraints were applied passively to the fingertip of subjects in six different directions : front-rear translations, left-right translations and clockwise-counterclockwise rotations. A robotic system controlled the normal force applied to the fingertip and the tangential speed. The contact forces and torques were recorded. Using computer vision techniques, the contact area was extracted from the images and the relative velocity field between the glass and the fingertip contact area was computed during the whole sliding phase. An ellipse was fitted to the contact area during the whole slip in order to measure the displacement (for translations) and the tilt angle (for rotations) of the contact area.
Results showed that the skin deforms in a complex way from a full
stuck state to a full slipping contact. During this transition, while
the tangential force was monotonically increasing, the skin
progressively slipped on the plate, beginning from the border of the
contact area and ending at the central zone, and forming a velocity
gradient from the border to the center. This behavior was observed for
both translational and rotational movements, and was further
characterized...
This is joint work with P. Lefèvre and J-L.Thonnard.
[INMA] 2012-04-24 (14.00) : Discontinuous Observers with strong convergence properties and some applications
At Euler building
Speaker :
Jaime Moreno (UNAM, Mexico)
Abstract :
Despite of the fact that the (global and instantaneous) observability
property implies that the state of a system can be recovered in an
arbitrarily small time (for every state), smooth observers are able to
provide an estimation of the states that converges only asymptotically and
that it is only practically convergent in the presence of uncertainties or
persistent perturbations.
In order to achieve the desired properties of convergence in finite time,
robustness against persistent perturbations and convergence time
independent of the initial conditions, it is necessary to provide the
observer with strong nonlinearities, i.e. discontinuities and not globally
Lipschitz nonlinearities. In the talk some recent developments in this
direction will be presented and the results will be illustrated by means
of some applications.
[INMA] 2012-04-17 (14.00) : LQ-optimal control of infinite dimensional linear systems with boundary control and observation
At Euler building
Speaker :
Jérémy DEHAYE (FUNDP Namur)
Abstract :
Distributed parameter systems based on partial differential equations can be used to describe the behaviour of many physical setups, e.g. convection-diffusion-reaction systems with a linear or linearized reaction rate and inputs and sensors located on the spatial boundary. These systems can generally be written as abstract infinite-dimensional linear differential systems with boundary control and observation, and the associated operators are thus unbounded.
Under these conditions, difficulties can arise when trying to solve some specific control problems, including the linear-quadratic optimal control problem and the design of the associated stabilizing state feedback. In particular, the method of spectral factorization by symmetric extraction provides an alternative to the resolution of the operator Riccati equation for the LQ-optimal control problem but its convergence relies on the fact that the control and observation operators are bounded.
In order to solve this problem, we consider, under appropriate conditions on the initial equations, a change of variables and an extended abstract linear differential system with bounded control and observation operators. An adapted cost functional can be associated with this extended system and can be interpreted in the framework of the initial one. Solving the LQ-optimal control problem for this cost functional provides a stabilizing feedback at the cost of an approximation in the measurement of the output and the feedback not being the optimal one in general.
As an application, we try to solve the LQ-optimal control problem for a convection-diffusion-reaction system with both distributed and boundary control, and boundary observation. The involved differential operator belongs to the class of Sturm-Liouville operators that are densely defined in the Lebesgue space of square-integrable functions. These systems are used in the modelling of e.g. biochemical processes, such as nonisothermal tubular reactors with axial dispersion, biochemical reactors, UV disinfection and many others.
The main tools of functional analysis used in this framework are the
theories of C_0-semigroups and Riesz-spectral systems, playing a
central role when Sturm-Liouville systems are involved.
[INMA] 2012-03-20 (14.00) : Limits of Community Detection
At Euler building
Speaker :
Vincent TRAAG (UCL)
Abstract :
Many networks can be decomposed into several relatively dense subgraphs, that
are only sparsely connected in between. One of the more popular methods is that
of modularity, which compares the given network to a random null-model. Several
null-models have been suggested, but they all suffer from a similar drawback:
what communities will be found depends on the size of the network. This problem
is known as the resolution-limit. However, it is not well understood what it
entails exactly. We will provide a definition of so called
"resolution-limit-free" methods that captures this notion more rigorously. Using
this definition, we can show that only few sensible methods exist that exhibit
this property. For one of these resolution-limit-free methods, we will provide
an efficient algorithm, and show it performs quite well in various benchmarks.
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