172 research outputs found
Tribological Behavior of Very Thin Confined Films
The tribological properties of two smooth surfaces in the presence of a thin
confined film are investigated with a generic model for the interaction between
two surfaces and with computer simulations. It is shown that at large normal
contact pressures, an ultra thin film automatically leads to static friction
between two flat surfaces - even if the surfaces are incommensurate.
Commensurability is nevertheless the key quantity to understand the
tribological behavior of the contact. Qualitative differences between
commensurate and incommensurate contacts remain even in the presence of a thin
film. The differences mainly concern the thermal diffusion of the contact and
the transition between smooth sliding and stick-slip.Comment: 7 pages, 6 figure
Optimal Formation Design for Imaging and Fuel Usage
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77028/1/AIAA-20080-363.pd
Revisiting the orbital tracking problem
Consider a space object in an orbit about the earth. An uncertain initial
state can be represented as a point cloud which can be propagated to later
times by the laws of Newtonian motion. If the state of the object is
represented in Cartesian earth centered inertial (Cartesian-ECI) coordinates,
then even if initial uncertainty is Gaussian in this coordinate system, the
distribution quickly becomes non-Gaussian as the propagation time increases.
Similar problems arise in other standard fixed coordinate systems in
astrodynamics, e.g. Keplerian and to some extent equinoctial. To address these
problems, a local "Adapted STructural (AST)'' coordinate system has been
developed in which uncertainty is represented in terms of deviations from a
"central state".
Given a sequence of angles-only measurements, the iterated nonlinear extended
(IEKF) and unscented (IUKF) Kalman filters are often the most appropriate
variants to use. In particular, they can be much more accurate than the more
commonly used non-iterated versions, the extended (EKF) and unscented (UKF)
Kalman filters, especially under high eccentricity. In addition, iterated
Kalman filters can often be well-approximated by two new closed form filters,
the observation-centered extended (OCEKF) and unscented (OCUKF) Kalman filters.Comment: 37 pages, 8 figures, 2 table
Trusted Distributed Autonomy Demonstration Experiment: Simulation Based Results
The goal of the Trusted Distributed Autonomy Demonstration Experiment (TDADE) is to develop and demonstrate a fully autonomous, distributed, trusted, and adaptive onboard autonomy software solution that enables resilient management of resources in proliferated space constellations, to meet complex Missions at Scale
On-Line Symbolic Constraint Embedding for Simulation of Hybrid Dynamical Systems
In this paper we present a simulator designed to handle multibody systems with changing constraints, wherein the equations of motion for each of its constraint configurations are formulated in minimal ODE form with constraints embedded before they are passed to an ODE solver. The constraint-embedded equations are formulated symbolically according to a re-combination of terms of the unconstrained equations, and this symbolic process is undertaken on-line by the simulator. Constraint-embedding undertaken on-the-fly enables the simulation of systems with an ODE solver for which constraints are not known prior to simulation start or for which the enumeration of all constraint conditions would be unwieldy because of their complexity or number. Issues of drift associated with DAE solvers that usually require stabilization are sidestepped with the constraint-embedding approach. We apply nomenclature developed for hybrid dynamical systems to describe the system with changing constraints and to distinguish the roles of the forward dynamics solver, a collision detector, and an impact resolver. We have prototyped the simulator in MATLAB and demonstrate the design using three representative examples.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43263/1/11044_2005_Article_269.pd
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