A UNIFIED APPROACH TO TWO-TIME-SCALE CONTROL SYSTEMS DESIGN: A TUTORIAL

ABSTRACT The goal of the tutorial is to give an overview of the newest unified design methodology of continuous-time or discretetime nonlinear control systems which guarantees desired transient performances in the presence of plant parameter variations and unknown external disturbances. The tutorial presents the up-to-date coverage of fundamental issues and recent research developments in design of nonlinear control systems with the highest derivative in feedback. The discussed design methodology allows us to provide effective control of nonlinear systems on the assumption of uncertainty. The approach is based on an application of a dynamical control law with the highest derivative of the output signal in the feedback loop. A distinctive feature of the control systems thus designed is that two-time-scale motions are forced in the closed-loop system. Stability conditions imposed on the fast and slow modes, and a sufficiently large mode separation rate, can ensure that the full-order closed-loop system achieves desired properties: the output transient performances are as desired, and they are insensitive to parameter variations and external disturbances. A general design methodology for control systems with the highest derivative in feedback for continuous-time systems, as well as corresponding discrete-time counterpart, will be presented during this tutorial. The method of singular perturbation is used to analyze the closed-loop system properties throughout. KEY WORDS Control under uncertainty, singular perturbation method

Modelling of low-energy/low-velocity impact on Nomex honeycomb sandwich structures with metallic skins

In the aircraft industry, manufacturers have to decide quickly whether an impacted sandwich needs repairing or not. Certain computation tools exist at present but they are very time-consuming and they also fail to perfectly model the physical phenomena involved in an impact. In a previous publication, the authors demonstrated the possibility of representing the NomexTM honeycomb core by a grid of nonlinear springs and have pointed out both the structural behaviour of the honeycomb and the influence of core-skin boundary conditions. This discrete approach accurately predicts the static indentation on honeycomb core alone and the indentation on sandwich structure with metal skins supported on rigid flat support. In this study, the domain of validity of this approach is investigated. It is found that the approach is not valid for sharp projectiles on thin skins. In any case, the spring elements used to model the honeycomb cannot take into account the transverse shear that occurs in the core during the bending of a sandwich. To overcome this strong limitation, a multi-level approach is proposed in the present article. In this approach, the sandwich structure is modelled by Mindlin plate elements and the computed static contact law is implemented in a nonlinear spring located between the impactor and the structure. Thus, it is possible to predict the dynamic structural response in the case of low-velocity/low-energy impact on metal-skinned sandwich structures. A good correlation with dynamic experimental tests is achieved

Aeronautical engineering: A continuing bibliography with indexes (supplement 227)

This bibliography lists 418 reports, articles, and other documents introduced into the NASA scientific and technical information system in May, 1988

Autonomous Vehicles

This edited volume, Autonomous Vehicles, is a collection of reviewed and relevant research chapters, offering a comprehensive overview of recent developments in the field of vehicle autonomy. The book comprises nine chapters authored by various researchers and edited by an expert active in the field of study. All chapters are complete in itself but united under a common research study topic. This publication aims to provide a thorough overview of the latest research efforts by international authors, open new possible research paths for further novel developments, and to inspire the younger generations into pursuing relevant academic studies and professional careers within the autonomous vehicle field

NASA Tech Briefs, December 1990

Topics: New Product Ideas; NASA TU Services; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences; Life Sciences

Fourth NASA Workshop on Computational Control of Flexible Aerospace Systems, part 1

The proceedings of the workshop are presented. Some areas of discussion are as follows: modeling, systems identification, and control of flexible aircraft, spacecraft, and robotic systems

Experimental investigation of liquid fragmentation in hypersonic cross flow

This thesis presents an experimental investigation carried out to study penetration and fragmentation of liquid injected into Mach 6 hypersonic cross flow. Flow topology, shock and vortex systems, fragmentation and atomization mechanisms are investigated using high-speed photography, Schlieren photography, flow visualization and Phase Doppler Interferometry techniques. All experiments are conducted at the H-3 Mach 6 wind tunnel facility of the von Karman Institute. Water is used for all tests. Freestream conditions of air flow are kept constant. The variation of the injector geometry and the effect of momentum flux ratio are studied throughout the experimental campaign. Droplet size measurements are analyzed and treated to characterize the atomization process of the liquid jet. The Sauter Mean Diameter and the standard deviation of the droplet size distribution are calculated and presented as a function of location and momentum flux ratio. The obtained Sauter Mean Diameter distribution is compared with the theory available in the literature for lower cross flow speed cases. The whipping phenomenon observed for the low momentum flux ratio liquid injections is explained by frequency maps, which allow one to see the flow domains with similar frequency content. This analysis proposes that the penetration of liquid jet determines the shape of the bow shock, which determines the location and angle of the separation shock. The separation shock is observed to penetrate into liquid phase, playing an important role in fragmentation of liquid, thus changing the penetration height and the shape of the bow shock. A continuous interaction between the liquid penetration, bow shock, separation shock and liquid fragmentation is believed to be the mechanism responsible of the whipping phenomenon. The fragmentation of liquid exposed to Mach 6 air flow is also investigated. Experiments are conducted using water-filled balloons mounted on sharp and blunt leading edge supports. The water-filled balloons are exposed to Mach 6 air flow and high speed camera measurements are taken during the bursting of the balloon, to study the fragmentation of water. Shock patterns and flow topology are visualized by Schlieren photography

Aeronautical Engineering: A continuing bibliography, 1982 cumulative index

This bibliography is a cumulative index to the abstracts contained in NASA SP-7037 (145) through NASA SP-7037 (156) of Aeronautical Engineering: A Continuing Bibliography. NASA SP-7037 and its supplements have been compiled through the cooperative efforts of the American Institute of Aeronautics and Astronautics (AIAA) and the National Aeronautics and Space Administration (NASA). This cumulative index includes subject, personal author, corporate source, contract, and report number indexes

Nondestructive Testing (NDT)

The aim of this book is to collect the newest contributions by eminent authors in the field of NDT-SHM, both at the material and structure scale. It therefore provides novel insight at experimental and numerical levels on the application of NDT to a wide variety of materials (concrete, steel, masonry, composites, etc.) in the field of Civil Engineering and Architecture