Robust Control Mechanism Research Articles

Narrative review: evolving concepts in potassium homeostasis and hypokalemia.

Humans are intermittently exposed to large variations in potassium intake, which range from periods of fasting to ingestion of potassium-rich meals. These fluctuations would abruptly alter plasma potassium concentration if not for rapid mechanisms, primarily in skeletal muscle and the liver, that buffer the changes in plasma potassium concentration by means of transcellular potassium redistribution and feedback control of renal potassium excretion. However, buffers have capacity limits, and even robust feedback control mechanisms require that the perturbation occur before feedback can initiate corrective action. In contrast, feedforward control mechanisms sense the effect of disturbances on the system's homeostasis. This review highlights recent experimental insights into the participation of feedback and feedforward control mechanisms in potassium homeostasis. New data make clear that feedforward homeostatic responses activate when decreased potassium intake is sensed, even when plasma potassium concentration is still within the normal range and before frank hypokalemia ensues, in addition to the classic feedback activation of renal potassium conservation when plasma potassium concentration decreases. Given the clinical importance of dyskalemias in patients, these novel experimental paradigms invite renewed clinical inquiry into this important area.

An adaptive fuzzy-based CAC scheme for uplink and downlink congestion control in converged IP and DVB-S2 networks

This paper introduces a robust buffer occupancy-based connection admission control (CAC) mechanism to alleviate both uplink and downlink congestions in converged IP and broadcasting networks. The scheme also ensures a fair share of downlink bandwidth among competing satellite terminals (subnetworks) in the event of congestion. The proposed scheme is dubbed Weighted Fair CAC (W-FCAC). It accepts or rejects connections based on an adaptive fuzzy-based approach. The use of the fuzzy-based mechanism is for the purpose of overcoming issues related to instantaneous link capacity assessment, flow characterization and the associated high computational complexity, and use of traffic descriptors for new flows. Additionally, the adaptive fuzzy logic makes the proposed CAC approach robust to traffic dynamics. These features make the scheme highly suitable for DVB-S2 environments where the link capacity frequently fluctuates due to the adaptive coding/modulation of the physical layer during noisy periods. Simulation results elucidate that the proposed W-FCAC scheme prevents downlink congestion and fairly allocates network resources among satellite terminals. It also minimizes the frequency of congestion events while maintaining efficient utilization of network resources.

A robust decision feedback equalizer for ATSC DTV receivers

Least mean square (LMS) decision feedback equalizer (DFE) is preferred as an effective solution to coping with inter-symbol interference (ISI) for ATSC digital television (DTV) receivers. In DTV transmission environment, echo delay often covers several hundreds symbols, which leads to very large-scale equalizer. One consequence of the large-scale equalizer is the very slow convergence, which combined with error propagation, inherent drawback of DFE, seriously deteriorates the performance of the receivers, especially in severe channels. More working modes and corresponding robust control mechanism were given to help the equalizer converge to the stable state smoothly. Simulation results show that the improved equalizer can perform better, especially in the severe channels.

An advanced LMI-based-LQR design for voltage control of grid-connected wind farm

This paper presents an innovative voltage control scheme for a grid-connected wind farm. In wind power generation systems, operating conditions are changing continually by wind speed fluctuations and load changes. Therefore, a robust control mechanism is necessary. To enable a linear and robust control framework, the overall system is represented by a set of reduced-order linear systems that cover an operating range of interest determined by variations of the load. A control-design technique known as the linear quadratic regulator (LQR) can be conveniently utilized for multi-input multi-output systems. However, to make this approach applicable simultaneously to several linear systems, the LQR problem needs to be reformulated for finding a common Lyapunov function for the set of considered linear systems. This is accomplished by representing the underlying control optimization problem in terms of a system of linear-matrix-inequality (LMI) constraints and matrix equations that are simultaneously solved. The solution of LMI equations involves a form of quadratic Lyapunov function that not only gives the stability property of the controlled system but can also be used for achieving certain performance specifications. In addition, to make the control design applicable to realistic systems, with noise and disturbances in the measured signals, we consider a state observer. A candidate wind farm site on Vancouver Island, Canada, is conducted for simulation study. The proposed methodology is also flexible and readily applicable to larger wind farms of different configurations.

From Corporate Governance to Metagovernance - A Holistic Framework

Corporate and financial market history has demonstrated that the approaches to corporate governance set out in the standard economics and finance literature may require further development. This paper presents a generic and holistic framework that views corporate governance as part of the governance infrastructure in an economic system. The framework is based on a multi-disciplinary approach that attempts to acknowledge the deviations from the efficient markets hypothesis and principal-agent theory that have been observed. In doing so, it is hoped that using this framework will provide greater insights and understanding about the dynamics of corporate governance as well as corporate misgovernance. Corporate scandals affect trust and confidence. This paper proposes that trust and confidence rest on a seat of governance, which is held up by three interdependent and dynamic supports - corporate governance (within firms), reputational intermediaries (banks, institutional investors, auditors, media, ratings agencies etc) and proactive monitors (generic term referring to all bodies that devise, monitor and enforce policies and laws). Events from corporate and financial market history demonstrate that a key proactive monitor may be required to play a more dynamic and pivotal role as an agent of metagovernance. Through, what the author terms as market command with robust control mechanisms, this authority can co-ordinate and influence the supports of governance. Metagovernance would enable a dynamic assessment of the integrity of the governance infrastructure in order to protect and stimulate wealth generation; while ensuring that any risk posed to trust and confidence nationally and globally is at an acceptable level.

Experience-Based Admission Control in the Presence of Traffic Changes

This article investigates the transient behavior of experience-based admission control (EBAC) in case of traffic changes. EBAC is a robust and resource-efficient admission control (AC) mechanism used for reservation overbooking of link capacities in packet-based networks. Recent analyses gave a proof of concept for EBAC and showed its efficiency and robustness through steady state simulation on a single link carrying traffic with constant properties. The contribution of this paper is an examination of the memory from which EBAC gains its experience and which strongly influences the behavior of EBAC in stationary and nonstationary state. For the latter, we investigate the transient behavior of the EBAC mechanism through simulation of strong traffic changes which are characterized by either a sudden decrease or increase of the traffic intensity. Our results show that the transient behavior of EBAC partly depends on its tunable memory and that it copes well with even strongly changing traffic characteristics.

Simultaneous Measurement of Ca 2+ and Cellular Dynamics: Combined Scanning Ion Conductance and Optical Microscopy to Study Contracting Cardiac Myocytes

We have developed a distance modulated protocol for scanning ion conductance microscopy to provide a robust and reliable distance control mechanism for imaging contracting cells. The technique can measure rapid changes in cell height from 10 nm to several micrometers, with millisecond time resolution. This has been demonstrated on the extreme case of a contracting cardiac myocyte. By combining this method with laser confocal microscopy, it was possible to simultaneously measure the nanometric motion of the cardiac myocyte, and the local calcium concentration just under the cell membrane. Despite large cellular movement, simultaneous tracking of the changes in cell height and measurement of the intracellular Ca 2+ near the cell surface is possible while retaining the cell functionality.

Performance of TCP applications over ATM networks with ABR and UBR services—a simulation analysis

This paper aims to present a comparative simulation study of the performance of TCP traffic over ATM networks with UBR and ABR services; to identify weaknesses of currently most promising ATM congestion control schemes; and to indicate the requirements for a fairer, simpler and more robust congestion control mechanism. Variety of congestion control schemes under various network configurations and traffic patterns are simulated for a comprehensive analysis. Simulation results show that Early Packet Discard (EPD) wastes buffer capacity and has no provision for fairness, Intelligent Marking [K.Y. Siu, H.Y. Tzeng, Computer Communication Review 24 (5) (1995) 81–106] does not provide adequate fairness among competitive connections, and ERICA+ [R. Jain, S. Kalyanaraman, Y. Goyal, S. Fahmy, R. Viswanathan, ERICA switch algorithm: a complete description, ATM Forum Contribution 96-1172, 1996] is too restrictive and unnecessarily complicated. The paper also indicates that TCP flow control and ATM congestion control do not cooperate well when buffer overflows and retransmission occurs, necessitating further control mechanisms for improving performance.

Intelligent Control Paradigm for Dynamic Discrete EventSystem Simulation

Simulation based control of discrete event systems has been a potential approach to support decision-making in the manufacturing scenario. In this paper, a knowledge intensive simulation modelling approach for a discrete even system is investigated. Based on the proposed simulation model, a robust control mechanism is presented that is believed to add significant value to discrete event dynamic system. The algorithm utilises neural network feedforward control plus robust proportional derivative feedback control to achieve control performance and output stability. The novel simulation approach, as well as the proposed controller, is implemented in an Extend TM environment and the effectiveness and usefulness of the proposed controller are verified, industrially, in the hard disk drive assembly process, a significant component of the Singapore manufacturing economy.

INTELLIGENT CONTROL OF AN INTERNAL SUPPLY CHAIN IN THE HARD DISK DRIVE INDUSTRY

Simulation-based control of discrete event systems has been a potential approach to support decision-making in the manufacturing scenario. In this paper, a new simulation-based robust control mechanism is presented. The proposed algorithm utilizes neural network feedforward control plus robust proportional derivative feedback control to achieve significant control performance and output stability. A new learning mechanism and topological structure for neural networks and a new robust controller is investigated. The stability of the proposed controller is proved in detail. The proposed controller is implemented in an ExtendTM environment and the effectiveness and usefulness of the proposed controller are verified in the hard disk drive assembly process, a significant component of Singapore's manufacturing economy.

Virtual Cognitive Systems (VCS): Neural-Fuzzy Logic Approach

Over the last decade or so, several parallel advances have been made in the two distinct disciplines: the fuzzy logic and the neural networks. As the names imply, the fuzzy logic provides a mathematical strength to the emulation of certain perceptual and linguistic attributes associated with the human cognition, whereas the science of neural networks provides new computing morphologies with learning and adaptive capabilities.An integration of these two fields: neural networks and fuzzy logic, has the potential of generating a new discipline - the virtual intelligence (VI) with a new generation of computIng systems - the virtual cognitive systems (VCS). Such a system will, hopefully, recapture certain aspects of human cognition which will result in robust sensors and control mechanisms. In this paper, we briefly explore the possibilities of such a new system - the virtual cognitive system, in the service of humanity.

Evaluation of timestamp-based concurrency control mechanisms incorporating livelock avoidance

A simulation model of a distributed database system is defined; it is then used to analyze the behavior of various concurrency control mechanisms based on timestamp ordering. The model can reflect different network configurations, different data distribution/replication schemes, and different users' activity characteristics. Various concurrency control algorithms are accepted, and their relative performance can be compared. This model is used to show that the standard transaction restart policy used by time-stamp-based algorithms leads to infinite cyclic restarts of transactions. To remedy this problem, three alternative restart techniques are defined and are formally shown to guarantee the absence of cyclic restarts. Simulation is used to determine the relative advantages of the resulting robust concurrency control mechanisms.

Performance analysis of a LAN/WAN bridging architecture

The performance of local area networks (LANs) bridged through a virtual-circuit-based wide area network (WAN) is analyzed. The focus is on understanding end-to-end performance issues from an applications point of view. Results that can be used to traffic engineer the network to meet end-user service objectives on interactive response time and bulk data transfer throughput are derived. It is demonstrated that the use of a windowing protocol in the wide area network provides a robust control mechanism for regulating traffic and delivering high performance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Access control for network directory systems

Directory Systems to support communications networks are currently the subject of serious international standardization efforts. The most recent CCITT and ISO proposals for Directory Systems are summarized, and within this context the requirements for flexible and robust access control mechanisms are examined. The existence of varied access control policies at the international, national and local levels and the need to maintain the integrity of Directory System information are identified as major factors determining access control procedures. Techniques are suggested for access control mechanisms capable of capturing and enforcing the relevant access control semantics at each network site.

The decay of timber and its prevention

In this paper, the issue of output voltage regulation in buck type dc-dc converters is addressed using a current sensorless control technique. The proposed strategy integrates a finite time current observer with an adaptive backstepping control scheme to yield a cost-effective and robust control mechanism. The overall controller stability in the sense of Lyapunov is proved. Applicability of the proposed control is verified experimentally on a buck converter in the laboratory. The control scheme is implemented on dSPACE DS1103 platform based on DSP TM320F240 processor. To examine the efficacy of the proposed method, the buck converter is subjected to a wide change in input voltage, load resistance and reference voltage. For comparison purpose, a conventional adaptive backstepping control scheme is evaluated under identical conditions of experimental study to examine the merit of the proposed control. The results obtained reveal that the proposed control is prompt in rejecting perturbations and achieves a smooth, reliable and satisfactory output voltage regulation with faithful and time bound estimation of inductor current. Thereby, this investigation demonstrates the validity of the proposed control in maintaining a stringent output voltage regulation in buck converters.