Relay pursuit of a maneuvering target using dynamic Voronoi diagrams

This paper addresses the problem of the pursuit of a maneuvering target by a group of pursuers distributed in the plane. This pursuit problem is solved by associating it with a Voronoi-like partitioning problem that characterizes the set of initial positions from which the target can be intercepted by a given pursuer faster than any other pursuer from the same group. In the formulation of this partitioning problem, the target does not necessarily travel along prescribed trajectories, as it is typically assumed in the literature, but, instead, it can apply an “evading” strategy in an effort to delay or, if possible, escape capture. We characterize an approximate solution to this problem by associating it with a standard Voronoi partitioning problem. Subsequently, we propose a relay pursuit strategy, that is, a special group pursuit scheme such that, at each instant of time, only one pursuer is assigned the task of capturing the maneuvering target. During the course of the relay pursuit, the pursuer–target assignment changes dynamically with time based on the (time varying) proximity relations between the pursuers and the target. This proximity information is encoded in the solution of the Voronoi-like partitioning problem. Simulation results are presented to highlight the theoretical developments.     

Water-Lubricated Ni-Based Composite(Ni-Al_2O_3,Ni-SiC and Ni-ZrO_2)Thin Film Coatings for Industrial Applications

In this work,pure nickel and Ni-based nanocomposite coatings(N1-Al_2O_3,Ni-SiC and Ni-ZrO_2) were produced on steel substrate by using pulse electrodeposition technique.The industrial performance tests were conducted to evaluate the wear resistance,corrosion resistance,adhesion strength and wettability behaviour of newly developed coatings.Rolling contact ball-on-disc tribometer was used to assess anti-wear behaviour of these coatings under waterlubricated contacts.The results showed that the wear- and corrosion resistance properties of nickel alumina and Ni-SiC composite coatings significantly improved than that of pure Ni and Ni-ZrO_2 coatings.The adhesion and wettability results of Ni-Al_2O_3 composite showed better performance when compared to the rest of the coatings.The effects of incorporating nanoparticles on the surface microstructure,interface adhesion and distribution of the particles were also investigated.The coatings were characterized by using scanning electron microscopy,X-ray diffraction analysis and 3D white light interferometry.The wear failure behaviour of these coatings was further examined by post-test surface observation under optical microscope.     

Decentralized spatial partitioning for multi-vehicle systems in spatiotemporal flow-field

We consider the problem of characterizing a generalized Voronoi diagram/partition of a convex polygon in a two-dimensional Euclidean space that encodes information about the proximity relations between a team of aerial/marine vehicles and arbitrary points in the partition space. These proximity relations are determined by the time required for each vehicle to reach an arbitrary point (time-to-go) in the partition space when driven by a locally optimal feedback control law in the presence of a spatiotemporal drift field. The main contribution of this work is the presentation of a partitioning algorithm, which is decentralized, in the sense that each vehicle can independently compute its corresponding cell from the generalized Voronoi partition without computing or receiving information about the cells of the other vehicles. Finally, we present numerical simulations using data from real drift fields to illustrate the key features of the decentralized solution to the proposed class of spatial partitioning problems.     

The Zermelo–Voronoi diagram: A dynamic partition problem

We consider a Voronoi-like partition problem in the plane for a given finite set of generators. Each element in this partition is uniquely associated with a particular generator in the following sense: an agent that resides within a set of the partition at a given time will arrive at the generator associated with this set faster than any other agent that resides anywhere outside this set at the same instant of time. The agent’s motion is affected by the presence of a temporally varying drift, which is induced by local winds/currents. As a result, the minimum time to a destination is not equivalent to the minimum distance traveled. This simple fact has important ramifications over the partitioning problem. It is shown that this problem can be interpreted as a Dynamic Voronoi Diagram problem, where the generators are not fixed, but rather they are moving targets to be reached in minimum time. The problem is solved by first reducing it to a standard Voronoi Diagram by means of a time-varying coordinate transformation. We then extend the approach to solve the dual problem where the generators are the initial locations of a given set of agents distributed over the plane, such that each element in the partition consists of the terminal positions that can be reached by the corresponding agent faster than any other agent.     

Consolidation effectiveness of modified Si-based nanocomposites applied to limestones

One of the main issues in the field of Monument Protection is the degradation of limestones as a result of the action of various weathering mechanisms. The modification of widely used silicon-based materials for stone consolidation is intended to overcome the well-known drawbacks of these materials, such as shrinkage and cracking tendency during drying. The addition of nano-dispersions into a silica matrix aims to enhance their effectiveness in several ways, by improving their properties and their viscoelastic behavior. The objective of the current research was the application and evaluation of Si-based modified nanocomposites of optimized composition. The materials were applied to two types of porous stone and the assessment of their compatibility and performance was carried out by using both laboratory techniques and methods (SEM, MIP, TMA, Water Absorption by Capillarity, determination of Water Vapor Permeability) and non-destructive techniques (Ultrasound Velocity determination, Colorimetry). To study the resistance of the treated samples to soluble salt crystallization, accelerated aging tests were performed in sodium sulfate cycles. The modified consolidants consist of an ethyl silicate matrix reinforced with colloidal silica (SiO₂) nano-particles and titania (TiO₂) particles. Based on the results, the consolidating material does not significantly alter the characteristics of the microstructure and the appearance of stones, allowing the passage of water vapor, while increasing their mechanical properties. Furthermore, the accelerated ageing tests revealed that the treated samples have a higher resistance to the action and crystallization of soluble salts in comparison to untreated.     

Time-optimal control of a self-propelled particle in a spatiotemporal flow field

We address a minimum-time problem that constitutes an extension of the classical Zermelo navigation problem in higher dimensions. In particular, we address the problem of steering a self-propelled particle to a prescribed terminal position with free terminal velocity in the presence of a spatiotemporal flow field. Furthermore, we assume that the norm of the rate of change of the particle's velocity relative to the flow is upper bounded by an explicit upper bound. To address the problem, we first employ Pontryagin's minimum principle to parameterise the set of candidate time-optimal control laws in terms of a parameter vector that belongs to a compact set. Subsequently, we develop a simple numerical algorithm for the computation of the minimum time-to-come function that is tailored to the particular parametrisation of the set of the candidate time-optimal control laws of our problem. The proposed approach bypasses the task of converting the optimal control problem to a parameter optimisation problem, which can be computationally intense, especially when one is interested in characterising the optimal synthesis of the minimum-time problem. Numerical simulations that illustrate the theoretical developments are presented.     

Partial‐state stabilization and optimal feedback control

In this paper, we develop a unified framework to address the problem of optimal nonlinear analysis and feedback control for partial stability and partial‐state stabilization. Partial asymptotic stability of the closed‐loop nonlinear system is guaranteed by means of a Lyapunov function that is positive definite and decrescent with respect to part of the system state, which can clearly be seen to be the solution to the steady‐state form of the Hamilton–Jacobi–Bellman equation and hence guaranteeing both partial stability and optimality. The overall framework provides the foundation for extending optimal linear‐quadratic controller synthesis to nonlinear nonquadratic optimal partial‐state stabilization. Connections to optimal linear and nonlinear regulation for linear and nonlinear time‐varying systems with quadratic and nonlinear nonquadratic cost functionals are also provided. Finally, we also develop optimal feedback controllers for affine nonlinear systems using an inverse optimality framework tailored to the partial‐state stabilization problem and use this result to address polynomial and multilinear forms in the performance criterion. Copyright © 2015 John Wiley & Sons, Ltd.     

Physical and mechanical characteristics of TEOS-modified nanocomposites

Journal of Polymer Research - Sodium 2,2′-methylene-bis(4,6-di-tertbutylphenyl)-phosphate (NA11) is an α-type nucleating agent for improving the mechanical and crystallization...     

Correlation of physicochemical and mechanical properties of historical mortars and classification by multivariate statistics

This work uses multivariate statistics in an attempt to classify historical mortars in more or less distinct groups, depending on their physicochemical characteristics. Four types of mortars are studied: “typical lime,” “cementitious,” “crushed brick” and Portland cement. Fifty samples in total were analysed by thermal analyses (differential thermal analysis [DTA] and thermogravimetric analysis [TGA]), mercury intrusion porosimetry and mechanical strength tests. The results give us useful information on the understanding of the technology of historical mortars and planning syntheses for restoration ones. The inverse hydraulicity ratio (CO₂/structurally bound water, SBW) is correlated to CO₂ content (%) as measured by thermal analysis. The tensile strength increases with the amount of hydrated phases and the mechanical properties of the aggregate and the binder. Medians, ranges and extremely rare values were determined for each property showing compact groups. These groups were discriminated by principal component analysis (PCA) giving a tool for characterisation of historical mortars.