Nonlinear output‐feedback control of torsional vibrations in drilling systems

This paper considers the design of a nonlinear observer‐based output‐feedback controller for oil‐field drill‐string systems aiming to eliminate (torsional) stick–slip oscillations. Such vibrations decrease the performance and reliability of drilling systems and can ultimately lead to system failure. Current industrial controllers regularly fail to eliminate stick–slip vibrations under increasingly challenging operating conditions caused by the tendency towards drilling deeper and inclined wells, where multiple vibrational modes play a role in the occurrence of stick–slip vibrations. As a basis for controller synthesis, a multi‐modal model of the torsional drill‐string dynamics for a real rig is employed, and a bit–rock interaction model with severe velocity‐weakening effect is used. The proposed model‐based controller design methodology consists of a state‐feedback controller and a (nonlinear) observer. Conditions, guaranteeing asymptotic stability of the desired equilibrium, corresponding to nominal drilling operation, are presented. The proposed control strategy has a significant advantage over existing vibration control systems as it can effectively cope with multiple modes of torsional vibration. Case study results using the proposed control strategy show that stick–slip oscillations can indeed be eliminated in realistic drilling scenarios in which industrial controllers fail to do so. Moreover, key robustness aspects of the control system involving the robustness against uncertainties in the bit–rock interaction and changing operational conditions are evidenced. Copyright © 2017 John Wiley & Sons, Ltd.     

A virtual structure approach to formation control of unicycle mobile robots using mutual coupling

In this article, the formation control problem for unicycle mobile robots is studied. A distributed virtual structure control strategy with mutual coupling between the robots is proposed. The rationale behind the introduction of the coupling terms is the fact that these introduce additional robustness of the formation with respect to perturbations as compared to typical leader–follower approaches. The applicability of the proposed approach is shown in simulations and experiments with a group of wirelessly controlled mobile robots.     

Optimization of sintering conditions for improved microstructural and mechanical properties of dense Ce0.8Gd0.2O2-δ-FeCo2O4 oxygen transport membranes

Ce_0.8Gd_0.2O_2-δ-FeCo₂O₄ composite is an excellent oxygen transport membrane material with good chemical stability for applications in oxygen separation and membrane reactors. To improve microstructural and mechanical properties, sintering profiles for Ce_0.8Gd_0.2O_2-δ-FeCo₂O₄ composites were optimized. Different sintering temperatures are selected based on our study of phase interactions among the initial powder mixtures using high-temperature X-ray diffraction. The results reveal that the phase interaction at ～1050 ℃ accelerates densification process, and a further increase of sintering temperature to 1200 ℃ contributes to the homogenization of the pore distribution. A higher density and an improved homogeneity of pore distribution result in enhanced mechanical strength. However, the density decreases once the sintering temperature reaches 1350 ℃. Hence, the optimal sintering temperature considering both microstructural and mechanical properties appears to be 1200 ℃. Sintering at this temperature results in a microstructure with a density exceeding 99 % with only small surface defects and a high average flexural strength of approximately 266 MPa.     

High temperature compressive creep behavior of BaCe0.65Zr0.2Y0.15O3−δ in air and 4% H2/Ar

The proton conductive material BaCe_0.65Zr_0.2Y_0.15O_3−δ has great potential for the separation and purification of hydrogen. However, due to the demanding application conditions regarding both temperature and atmosphere, the elevated temperature structural stability needs to be characterized and warranted. Hence, in this research work, the elevated temperature compressive creep behavior of BaCe_0.65Zr_0.2Y_0.15O_3−δ in the temperature regime of 850°C to 1200°C was studied in both air and 4% H₂/Ar as a function of the applied stress. The results indicate different creep mechanisms depending on atmosphere and temperature range. While dislocation creep was observed in 4% H₂/Ar over the full range, a dislocation creep mechanism was observed in air at temperatures ≤1050°C and a diffusional creep mechanism at temperature ≥1100°C. A detailed microstructural analysis of the post-creep test specimens revealed that the exposure to oxygen leads to localized stoichiometric changes and a decomposition at the surface.     

Flutter and bifurcation instability analysis of fluid-conveying micro-pipes sandwiched by magnetostrictive smart layers under thermal and magnetic field

International Journal of Mechanics and Materials in Design - Fluid-conveying micro/Nano structures are key tools in MEMS and NEMS applications especially for drug delivery systems to attack a...     

Definition of requirements for accessing multilingual information opinions

Multimedia Tools and Applications - In this paper, we will present a study concerning the understanding of the needs of people using Internet in order to access to multilingual information. In...     

A high-vacuum deposition system for in situ and real-time electrical characterization of organic thin-film transistors

We present a home-built high-vacuum system for performing organic semiconductor thin-film growth and its electrical characterization during deposition (real-time) or after deposition (in situ). Since the environment conditions remain unchanged during the deposition and electrical characterization process, a direct correlation between growth mode and electrical properties of thin film can be obtained. Deposition rate and substrate temperature can be systematically set in the range 0.1-10 ML∕min and RT-150 °C, respectively. The sample-holder configuration allows the simultaneous electrical monitoring of up to five organic thin-film transistors (OTFTs). The OTFTs parameters such as charge carrier mobility μ, threshold voltage V(TH), and the on-off ratio I(on)∕I(off) are studied as a function of the semiconductor thickness, with a submonolayer accuracy. Design, operation, and performance of the setup are detailed. As an example, the in situ and real-time electrical characterization of pentacene TFTs is reported.