80 research outputs found
Planar image based visual servoing as a navigation problem
We describe a hybrid planar image-based servo algorithm which, for a simplified planar convex rigid body, converges to a static goal for all initial conditions within the workspace of the camera. This is achieved by using the sequential composition of a palette of continuous image based controllers. Each sub-controller, based on a specified set of collinear feature points, is shown to converge for all initial configurations in which the feature points are visible. Furthermore, the controller guarantees that the body will maintain a visible orientation, i.e. the feature points will always be in view of the camera. This is achieved by introducing a change of coordinates from SE(2) to an image plane measurement of three points, and imposing a navigation function in that coordinate system. Our intuition suggests that appropriately generalized versions of these ideas may be extended to SE(3
Toward global visual servos and estimators for rigid bodies
We describe work-in-progress toward a nonlinear image-based rigid body dynamic triangulator which we believe tracks a moving target from essentially all initial conditions (all initial conditions except a set of measure zero). The dynamic triangulator depends on the goal state only through its image plane position and velocity and requires a navigation function, imposed directly upon image features, to serve as a regressor for a gradient-like state update law
Walking dynamics are symmetric (enough)
Many biological phenomena such as locomotion, circadian cycles, and breathing
are rhythmic in nature and can be modeled as rhythmic dynamical systems.
Dynamical systems modeling often involves neglecting certain characteristics of
a physical system as a modeling convenience. For example, human locomotion is
frequently treated as symmetric about the sagittal plane. In this work, we test
this assumption by examining human walking dynamics around the steady-state
(limit-cycle). Here we adapt statistical cross validation in order to examine
whether there are statistically significant asymmetries, and even if so, test
the consequences of assuming bilateral symmetry anyway. Indeed, we identify
significant asymmetries in the dynamics of human walking, but nevertheless show
that ignoring these asymmetries results in a more consistent and predictive
model. In general, neglecting evident characteristics of a system can be more
than a modeling convenience---it can produce a better model.Comment: Draft submitted to Journal of the Royal Society Interfac
An observability result related to active sensing
For a general class of translationally invariant systems with a specific
category of nonlinearity in the output, this paper presents necessary and
sufficient conditions for global observability. Critically, this class of
systems cannot be stabilized to an isolated equilibrium point by dynamic output
feedback. These analyses may help explain the active sensing movements made by
animals when they perform certain motor behaviors, despite the fact that these
active sensing movements appear to run counter to the primary motor goals. The
findings presented here establish that active sensing underlies the maintenance
of observability for such biological systems, which are inherently nonlinear
due to the presence of the high-pass sensor dynamics
Rigid body visual servoing using navigation functions
Visual servo controllers in the literature rarely achieve provably large domains of attraction, and seldom address two important sensor limitations: (i) susceptibility to self-occlusions and (ii) finite field of view (FOV). We tackle the problem of global, occlusion-free visual servoing of a fully actuated rigid body by recourse to navigation functions on a compact manifold which encode these restrictions as control obstacles. For occlusion free rigid body servoing, the manifold of interest is the visible set of rigid body configurations, that is, those for which the feature points are within the field of view and unoccluded by the body. For a set of coplanar feature points on one face of a convex polyhedron, we show that a slightly conservative subset of the visible set has a simple topology amenable to analytical construction of a navigation function. We construct the controller via a closed form coordinate transformation from our problem domain into the topological model space and conclude with simulation results
Feedback Control as a Framework for Understanding Tradeoffs in Biology
Control theory arose from a need to control synthetic systems. From
regulating steam engines to tuning radios to devices capable of autonomous
movement, it provided a formal mathematical basis for understanding the role of
feedback in the stability (or change) of dynamical systems. It provides a
framework for understanding any system with feedback regulation, including
biological ones such as regulatory gene networks, cellular metabolic systems,
sensorimotor dynamics of moving animals, and even ecological or evolutionary
dynamics of organisms and populations. Here we focus on four case studies of
the sensorimotor dynamics of animals, each of which involves the application of
principles from control theory to probe stability and feedback in an organism's
response to perturbations. We use examples from aquatic (electric fish station
keeping and jamming avoidance), terrestrial (cockroach wall following) and
aerial environments (flight control in moths) to highlight how one can use
control theory to understand how feedback mechanisms interact with the physical
dynamics of animals to determine their stability and response to sensory inputs
and perturbations. Each case study is cast as a control problem with sensory
input, neural processing, and motor dynamics, the output of which feeds back to
the sensory inputs. Collectively, the interaction of these systems in a closed
loop determines the behavior of the entire system.Comment: Submitted to Integr Comp Bio
Isotopic composition (238U/235U) of some commonly used uranium reference materials
We have determined 238U/235U ratios for a suite of commonly used natural (CRM 112a, SRM 950a, and HU-1) and synthetic (IRMM 184 and CRM U500) uranium reference materials by thermal ionisation mass-spectrometry (TIMS) using the IRMM 3636 233U-236U double spike to accurately correct for mass fractionation. Total uncertainty on the 238U/235U determinations is estimated to be < 0.02% (2σ). These natural 238U/235U values are different from the widely used ‘consensus’ value (137.88), with each standard having lower 238U/235U values by up to 0.08%. The 238U/235U ratio determined for CRM U500 and IRMM 184 are within error of their certified values; however, the total uncertainty for CRM U500 is substantially reduced (from 0.1% to 0.02%). These reference materials are commonly used to assess mass spectrometer performance and accuracy, calibrate isotope tracers employed in U, U-Th and U-Pb isotopic studies, and as a reference for terrestrial and meteoritic 238U/235U variations. These new 238U/235U values will thus provide greater accuracy and reduced uncertainty for a wide variety of isotopic determinations
Nodal dynamics, not degree distributions, determine the structural controllability of complex networks
Structural controllability has been proposed as an analytical framework for
making predictions regarding the control of complex networks across myriad
disciplines in the physical and life sciences (Liu et al.,
Nature:473(7346):167-173, 2011). Although the integration of control theory and
network analysis is important, we argue that the application of the structural
controllability framework to most if not all real-world networks leads to the
conclusion that a single control input, applied to the power dominating set
(PDS), is all that is needed for structural controllability. This result is
consistent with the well-known fact that controllability and its dual
observability are generic properties of systems. We argue that more important
than issues of structural controllability are the questions of whether a system
is almost uncontrollable, whether it is almost unobservable, and whether it
possesses almost pole-zero cancellations.Comment: 1 Figures, 6 page
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