Stability of systems with nonlinear feedback through randomly time-varying delays

In a large class of multiloop control systems, many feedback loops are "closed" through a time-shared digital computer, using algorithms which require information from sources which are sampled at a rate which is not synchronized with the sampling of the individual "plants." This missynchronization, in conjunction with variations in the computer's task load caused by "interrupts," results in a randomly time-varying delay in the closing of the various feedback loops. Consequently, the dynamics of each controlled "plant" in such a system may be modeled by means of a stochastic delay-differential equation. This paper presents some new research results concerning the sample stability (as opposed to statistical, or ensemble stability) of nonlinear stochastic delay-differential equations.     

Output feedback control of switched nonlinear systems using multiple Lyapunov functions

This work presents a hybrid nonlinear control methodology for a broad class of switched nonlinear systems with input constraints. The key feature of the proposed methodology is the integrated synthesis, via multiple Lyapunov functions, of “lower-level” bounded nonlinear feedback controllers together with “upper-level” switching laws that orchestrate the transitions between the constituent modes and their respective controllers. Both the state and output feedback control problems are addressed. Under the assumption of availability of full state measurements, a family of bounded nonlinear state feedback controllers are initially designed to enforce asymptotic stability for the individual closed-loop modes and provide an explicit characterization of the corresponding stability region for each mode. A set of switching laws are then designed to track the evolution of the state and orchestrate switching between the stability regions of the constituent modes in a way that guarantees asymptotic stability of the overall switched closed-loop system. When complete state measurements are unavailable, a family of output feedback controllers are synthesized, using a combination of bounded state feedback controllers, high-gain observers and appropriate saturation filters to enforce asymptotic stability for the individual closed-loop modes and provide an explicit characterization of the corresponding output feedback stability regions in terms of the input constraints and the observer gain. A different set of switching rules, based on the evolution of the state estimates generated by the observers, is designed to orchestrate stabilizing transitions between the output feedback stability regions of the constituent modes. The differences between the state and output feedback switching strategies, and their implications for the switching logic, are discussed and a chemical process example is used to demonstrate the proposed approach.     

Some issues of structure and control for linear time-invariant systems

The objective of this paper is to emphasise the role played by particular structures in the solution of some control problems. The so-called “structural approach” relies on various indicators of dynamical systems such as, for instance, finite and infinite zeros, kernel indices, …. The fundamental invariance properties of these structures under the action of some transformations groups (e.g. feedback) are at the origin of their key role. Structural solutions to “classical” control problems, such as disturbance rejection, model matching and non-interaction are now rather well known: zeros at infinity play a role in the existence of “proper” solutions, while finite (invariant) zeros allow for the characterisation of “fixed poles”, whose location in the complex plane gives answer to pole placement limitations (including stability). Among the recent contributions to this structural approach, a particular attention is here devoted to:

- “Partial” versions of some of these control problems: The control objective only concerns a finite number of (and not necessarily all) the first Markov parameters of the transfer function matrix of the controlled system (e.g. to be zero for disturbance rejection or model matching, to be diagonal for non-interaction). Some interesting new issues in the dual context of failure detection are also sketched.

- Generalised solutions: Based on proportional and derivative feedback laws, with new issues in the context of systems with variable internal structures, and also for systems with delays.

Geometric concepts, such as invariant and almost invariant subspaces, and algebraic counterparts, such as factorisations on some special rings, are intermediary tools which support the characterisations of those particular structures and which allow for a structural treatment of the considered control and/or observation problems.

The results are here presented without proof: references are given to previous published results (in most cases in books and journals which are easily available), and some simple examples are used to illustrate non-standard notions (among which systems with variable internal structure, and time domain left invertibility).

Most of the results here presented rely on long and intensive collaborations between the author and various colleagues.     

Retrieval of movie scenes by semantic matrix and automatic feature weight update

A new semantic-based video scene retrieval method is proposed in this paper. Twelve low-level features extracted from a video clip are represented in a genetic chromosome and target videos that user has in mind are retrieved by the interactive genetic algorithm through the feedback iteration. In this procedure, high-level semantic relevance between retrieved videos is accumulated with so-called semantic relevance matrix and semantic frequency matrix for each iteration, and they are combined with an automatic feature weight update scheme to retrieve more target videos at the next iteration. Experiments over 300 movie scene clips extracted from latest well-known movies, showed an user satisfaction of 0.71 at the fourth iteration for eight queries such as “gloominess”, “happiness”, “quietness”, “action”, “conversation”, “explosion”, “war”, and “car chase”.     

General structure and characteristics of quick response production system

The diversification and shorter production cycles urge manufacturers to shift their production systems from “make to stock” to “make to order” or an intermediate production system between them. The present paper deals with a production system developed by some manufacturers to meet requirements in the last decade. The system, which we call “Quick response to orders production system” or in brief “quick response production system (QRPS)”, is characterized by two key factors, i.e., “acceptable response time” and “semifinished product.” An acceptable response time is decided through the explicit or implicit approval of customers beforehand and manufacturers guarantee to ship products within an acceptable response time in a stochastic sense by processing the semifinished product according to customer order. As a result, manufacturers are able to reduce the inventory of finished products, while customers are able to enjoy the diversification of products and a stable supply from manufacturers. The general structure and characteristics of QRPS are discussed primarily from a quantitative point of view and a numerical example is shown to assist in understanding the design and operation of QRPS.     

Optimal control of demand-paging systems

Demand-paging systems are characterized as stochastic control processes, and optimal page replacement decisions are determined by means of dynamic programming. This approach is distinguished from others by its utilization of page structure information, which may be either supplied a priori or else dynamically learned. The main result is an optimal realizable solution for a general class of replacement problems. The resulting algorithm subsumes others (including “A₀”) as special cases.     

A “mixed” small gain and passivity theorem in the frequency domain

We show that the negative feedback interconnection of two causal, stable, linear time-invariant systems, with a “mixed” small gain and passivity property, is guaranteed to be finite-gain stable. This “mixed” small gain and passivity property refers to the characteristic that, at a particular frequency, systems in the feedback interconnection are either both “input and output strictly passive”; or both have “gain less than one”; or are both “input and output strictly passive” and simultaneously both have “gain less than one”. The “mixed” small gain and passivity property is described mathematically using the notion of dissipativity of systems, and finite-gain stability of the interconnection is proven via a stability result for dissipative interconnected systems.     

Global stabilization of linear discrete-time systems with bounded feedback

This paper deals with the problem of global stabilization of linear discrete time systems by means of bounded feedback laws. The main result proved is an analog of one proved for the continuous time case by the authors, and shows that such stabilization is possible if and only if the system is stabilizable with arbitrary controls and the transition matrix has spectral radius less than or equal to one. The proof provides in principle an algorithm for the construction of such feedback laws, which can be implemented either as cascades or as parallel connections (“single hidden layer neural networks”) of simple saturation functions.     

Overload effects and their prevention

The so-called “thrashing effect”, well known from virtual storage, but also reported from data-base systems and packet switching networks, has turned out to be a common phenomenon of large systems with concurrent processing. It simply means that beyond a saturation point an increase of the load (e.g. number of jobs) leads to a (sometimes sudden) decrease in performance (e.g. throughput). With growing size and complexity of computer systems and the general trend towards distribution, overload phenomena of different origin can interfere and superimpose mutually, resulting in a composite overload effect that can hardly be broken down into its constituents. Because the complexity of such systems defies detailed modeling, it is more appropriate to look at those systems in a more macroscopic, behavioral way, considering only the two externally measurable variables “load” and “throughput”. The resulting abstraction from internal details can smooth the way to a more general treatment and application. The article deals with such overload phenomena and their prevention in a general way using a control-theoretic approach. Special emphasis is placed on dynamic behavior, where load characteristics are changing with time, making feedback mechanisms necessary. The problem is approached as a dynamic optimum search problem for which different algorithms are presented and compared by simulation.     

A heuristic index for selecting similar categories in multiple correspondence analysis applied to living donor kidney transplantation

This work introduces a heuristic index (the “tolerance distance”) to define the “closeness” of two variable categories in multiple correspondence analysis (MCA). This index is a weighted Euclidean distance where weightings are based on the “importance” of each MCA axis, and variable categories were considered to be associated when their distances were below the tolerance distance. This approach was applied to a renal transplantation data. The analysed variables were allograft survival and 13 of its putative predictors. A bootstrap-based stability analysis was employed for assessing result reliability. The method identified previously detected associations within the database, such as that between race of donors and recipients, and that between HLA match and Cyclosporine use. A hierarchical clustering algorithm was also applied to the same data, allowing for interpretations similar to those based on MCA. The defined tolerance distance could thus be used as an index of “closeness” in MCA, hence decreasing the subjectivity of interpreting MCA results.     

Drill wear sensing and failure prediction for untended machining

Real-time control of drilling was carried out by measuring the thrust force and determining its gradient. Using a microcomputer-based feedback control system, experiments were carried out under different cutting conditions to test the effectiveness of the thrust force gradient in predicting failure. The system was able to predict failure due to excessive wear commonly encountered with 5 and 8 mm drills. With such drills, excessive weat at the outer corner led to an increase in the local temperature which in turn increased the wear. This led to very high temperatures (>600°C), causing local welding of the drill material to the peripheral surface of the hole being drilled. Furthermore, the high temperatures reduced the compressive yield strength of the drill material, causing sub-surface fracture to occur under the influence of the cutting loads. This cyclic phenomenon of “seizure” due to local welding and “release” due to shear fracture (i.e. “stick-slip”) caused sharp fluctuations in the thrust force under constant feed.

This paper discusses the effectiveness of the control system described above in predicting failure due to the excessive wear common to large drills. This system is also contrasted with another based on vibration measurements which has been successfully used to predict failure due to fracture common with small drills. This paper also presents other experimental sensor schemes in the literature. Finally, this paper proposes a framework for an “intelligent” machining process control system driven by multiple sensors, which would facilitate untended machining.     

Industrial integration of graphical and formal specifications

This paper discusses an approach to formal methods technology exploitation which introduces formal notations into critical systems development processes. The provision of “Ω functions” that map graphical specifications into axiomatic formal specifications, which can then be subjected to proof, is explored as a means of achieving this goal. Such functions must treat a useful subset of the graphical language and be compatible with industrialized tools. Experiences in developing this approach for industrial application are discussed by means of an example mapping function known as Ω1. This function maps a subset of statecharts into Real Time Logic.     

Combined effects of in-plane boundary conditions and stiffening on buckling of eccentrically stringer-stiffened cylindrical panels

Donnel type stability equations for buckling of stringer stiffened cylindrical panels under combined axial compression and hydrostatic pressure are solved by the displacement approach of [6], The solution is employed for a parametric study over a wide range of panel and stringer geometries to evaluate the combined influence of panel configurations and boundary conditions along the straight edges on the buckling behavior of the panel relative to a complete “counter” cylinder (i.e. a cylinder with identical skin and stiffener parameters).

The parametric studies reveal a “sensitivity” to the “weak in shear”, N_x = N_xφ = 0, along the straight edges, SS1 boundary conditions type where the panel buckling loads are always smaller than those predicted for a complete “counter” cylinder. In the case of “classical”, SS3 B.Cs., there always exist values of panel width, 2φ₀, for which ρ = 1, i.e. the panel buckling load equals that of the complete “counter” cylinder. For SS2 and SS4 B.Cs. types, the nature by which the panel critical load approaches that of the complete cylinder appears to be panel configuration dependent.

Utilization of panels for the experimental determination of a complete cylinder buckling load is found to be satisfactory for relatively very lightly and heavily stiffened panels, as well as for short panels, (L/R) = 0.2 and 0.5. Panels of moderate length and stiffening have to be debarred, since they lead to nonconservative buckling load predictions.     

Constrained multibody system dynamics an automated approach

The governing equations for constrained multibody systems are formulated in a manner suitable for their automated, numerical development and solution. Specifically, the “closed loop” problem of multibody chain systems is addressed.

The governing equations are developed by modifying dynamical equations obtained from Lagrange's form of d'Alembert's principle. This modification, which is based upon a solution of the constraint equations obtained through a “zero eigenvalues theorem,” is, in effect, a contraction of the dynamical equations.

It is observed that, for a system with n generalized coordinates and m constraint equations, the coefficients in the constraint equations may be viewed as “constraint vectors” in n-dimensional space. Then, in this setting the system itself is free to move in the n−m directions which are “orthogonal” to the constraint vectors.     

A Game Theoretic Approach to the Analysis of Dynamic Networks

A model of dynamic networks is introduced which incorporates three kinds of network changes: deletion of nodes (by faults or sabotage), restoration of nodes (by actions of “repair”), and creation of nodes (by actions that extend the network). The antagonism between the operations of deletion and restoration resp. creation is modelled by a game between the two agents “Destructor” and “Constructor”. In this framework of dynamic model-checking, we consider as specifications (“winning conditions” for Constructor) elementary requirements on connectivity of those networks which are reachable from some initial given network. We show some basic results on the (un-)decidability and hardness of dynamic model-checking problems.     

User feedback based enhancement in web search quality

Of late, there has been a paradigm shift in web searching from the content based searching to the connectivity based or more commonly known as hyperlink based (or simply link based) searching. But, both the content based approach as well as the link based approach are objective ones, which are totally dependent on the effectiveness of their “feature extraction” mechanisms, with no apparent consideration to the preference of the searcher. In this work, a “user satisfaction” guided web search procedure is proposed. We calculate the importance weight of each document viewed by the user based on the feedback vector obtained from his actions. This document weight is then used to update the index database in such a way that the documents being consistently preferred go up the ranking, while the ones being neglected go down. Our simulation results show a steady rise in the satisfaction levels of the modeled users as more and more learning goes into our system. We also propose a couple of novel additions to the web search querying techniques.     

Robust adaptive fuzzy tracking control for a class of perturbed strict-feedback nonlinear systems via small-gain approach

This paper presents a novel robust adaptive fuzzy tracking controller (RAFTC) for a wide class of perturbed strict-feedback nonlinear systems with both unknown system and virtual control gain nonlinearities. For unknown system nonlinearities, two types for them are included: one naturally satisfies the “triangularity condition” and may possess a class of unstructured uncertain functions which are not linearly parameterized, while the other is partially known and consists of parametric uncertainties and known “bounding functions”. The Takagi–Sugeno type fuzzy logic systems are used to approximate unknown system nonlinearities and a systematic design procedure is developed for synthesis of RAFTC by combining the backstepping technique and generalized small-gain approach. The algorithm proposed is highlighted by three advantages: (i) the semi-global uniform ultimate bound of RAFTC in the presence of perturbed uncertainties and unknown virtual control gain nonlinearities can be guaranteed, (ii) the adaptive mechanism with minimal learning parameterizations is obtained and (iii) the possible controller singularity problem in some of the existing adaptive control schemes with feedback linearization techniques can be removed. Performance and limitations of proposed method are discussed and illustrated with simulation results.     

Groupware in Practice: Expected and Realized Benefits

This article examines the usage of groupware at 22 consulting firms and attempts to answer the questions: “What groupware functions are used by consultancies?” “What benefits were expected and to what degree were these benefits realized?” and “What factors accounted for any realization gaps?”     

Color enhancement of highly correlated images. I. Decorrelation and HSI contrast stretches

Conventional enhancements for the color display of multispectral images are based on independent contrast modifications or “stretches” of three input images. This approach is not effective if the image channels are highly correlated or if the image histograms are strongly bimodal or more complex. Any of several procedures that tend to “stretch” color saturation while leaving hue unchanged may better utilize the full range of colors for the display of image information. Two conceptually different enhancements are discussed: the “decorrelation stretch”, based on principal-component (PC) analysis, and the “stretch” of “hue”-“saturation”-intensity (HSI) transformed data. The PC transformation is scene-dependent, but the HSI transformation is invariant. Examples of images enhanced by conventional linear stretches, decorrelation stretch, and by stretches of HSI transformed data are compared. Schematic variation diagrams or two- and three-dimensional histograms are used to illustrate the “decorrelation stretch” method and the effect of the different enhancements.     

Prioritization of human capital measurement indicators using fuzzy AHP

People in an organization constitute an important and essential asset which tremendously contributes to development and growth of that company by the help of their collective attitudes, skills and abilities. This is why the human capital (HC) can be considered the most important sub-dimension of the intellectual capital. Since you cannot manage what you cannot control, and you cannot control what you do not measure, the measurement of HC is a very important issue. This study aims at defining a methodology to improve the quality of prioritization of HC measurement indicators under fuzziness. To do so, a methodology based on the extent fuzzy analytic hierarchy process (AHP) is proposed. Within the model, five main attributes; talent, strategical integration, cultural relevance, knowledge management, and leadership; their sub-attributes, and 20 indicators are defined. The proposed model can be used for any country. However, the results obtained in the numerical example reflect the situation of HC in Turkey, since the experts are asked to make their evaluations considering the cultural characteristics of Turkey. The results of the study indicate that “creating results by using knowledge”, “employees’ skills index”, “sharing and reporting knowledge”, and “succession rate of training programs” are the four most important measurement indicators for the HC in Turkey.