Consecutive Dropouts Research Articles

Encoding-Decoding Mechanism-Based Finite-Level Quantized Iterative Learning Control With Random Data Dropouts

Learning control is investigated to solve the tracking problem for linear systems via unreliable networks with random data dropouts. By using an encoding-decoding mechanism-based finite-level uniform quantizer, the communication burden is remarkably reduced while retaining a precise tracking performance. An intermittent update principle is adopted in both the encoding-decoding mechanism and the learning control algorithm to handle the effects of data dropouts. A special case, in which no consecutive dropouts exist along the iteration axis, is provided first and then extended to a general case, in which bounded consecutive data dropouts are allowed. A precise analysis of the maximum range of the finite-level quantizer and the associated asymptotical convergence is conducted. Illustrative simulations demonstrate the validity of the proposed framework.

New Constrained Predictive PID controller for packet dropouts in Wireless Networked Control Systems

A new constrained predictive PID controller is presented to achieve stability and performance robustness in Wireless Networked Control Systems (WNCS), where the communication is subject to dropouts in both communication directions: sensor to control and control to actuator transmission. The control strategy is based on a new PID controller with similar properties to Model-Based Predictive Control (MBPC). A Kalman filter used for output prediction and a consecutive dropouts compensator have also been added to the control scheme. The purpose of this approach is to develop an estimation algorithm and a control system that maintain information of the sensor packets and the control actions. Several experiments using the TrueTime network simulator showed that the predictive PID controller performs as good as the MBPC scheme with the advantage of having a simple structure.

Controllability of linear systems subject to packet losses

In this paper we study the controllability properties of discrete-time linear systems subject to packet losses. We tackle the problem from a switching systems perspective in which available information known on the packet loss signal, e.g., there cannot be more than a given maximum number of consecutive losses, is modelled through an automaton. For the resulting constrained switching system, we reformulate the controllability problem into an easier-to-study formulation through an algebraic characterization.We show that the particular case where the packet loss signal does not contain more than N consecutive dropouts (N G N) boils down to a similar controllability problem with switching delays previously studied in the literature. For the general case, i.e., for an arbitrary automaton describing the lossy behaviour, we exploit the algebraic characterization and establish that our controllability problem of constrained switching systems is algorithmically solvable. This latter result is obtained by connecting it with the celebrated Skolem Theorem from linear algebra.

Experimental and numerical study of the symbolic dynamics of a modulated external-cavity semiconductor laser

We study the symbolic dynamics of a stochastic excitable optical system with periodic forcing. Specifically, we consider a directly modulated semiconductor laser with optical feedback in the low frequency fluctuations (LFF) regime. We use a method of symbolic time-series analysis that allows us to uncover serial correlations in the sequence of intensity dropouts. By transforming the sequence of inter-dropout intervals into a sequence of symbolic patterns and analyzing the statistics of the patterns, we unveil correlations among several consecutive dropouts and we identify clear changes in the dynamics as the modulation amplitude increases. To confirm the robustness of the observations, the experiments were performed using two lasers under different feedback conditions. Simulations of the Lang-Kobayashi (LK) model, including spontaneous emission noise, are found to be in good agreement with the observations, providing an interpretation of the correlations present in the dropout sequence as due to the interplay of the underlying attractor topology, the external forcing, and the noise that sustains the dropout events.

Stability of Sequence-Based Control With Random Delays and Dropouts

We study networked control of non-linear systems where system states and tentative plant input sequences are transmitted over unreliable communication channels. The sequences are calculated recursively by using a pre-designed nominally stabilizing state-feedback control mapping to plant state predictions. The controller does not require receipt acknowledgments or knowledge of delay or dropout distributions. For the i.i.d. case, in which case the numbers of consecutive dropouts are geometrically distributed, we show how the resulting closed loop system can be modeled as a Markov non-linear jump system and establish sufficient conditions for stochastic stability.

Design and Application of Suboptimal Mixed $H_{2}/H_{\infty}$ Controllers for Networked Control Systems

This brief tackles the problem of designing suboptimal H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> controllers for linear networked control systems (NCS) subject to time-varying delays and packet dropouts. The formulation provides state feedback NCS controllers allowing to tradeoff performance and disturbance rejection. The control objective consists in designing an H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> suboptimal control minimizing a quadratic performance index, with a disturbance rejection constraint (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> constraint). To characterize the network, only the lower and upper bounds for the delay, as well as the maximum number of consecutive dropouts are required. The approach relies on the formulation of the problem in terms of the minimization of a single scalar parameter, that can be cast as a standard linear matrix inequality (LMI) problem, yielding a suboptimal cost-guaranteed solution. As a difference from previous works, the solution provided is independent of initial conditions. Stability and robustness properties of the proposed controller are theoretically demonstrated and tested on an experimental testbed consisting in the stabilization of a robot arm in the proximities of the unstable upright position. The application shows good performance and disturbance rejection capabilities even for stringent network conditions.

Power dropout control by optical phase modulation in a chaotic semiconductor laser

The effect of periodic phase modulation of light on a chaotic external-cavity semiconductor laser working in a regime of low-frequency fluctuations (LFFs) is studied numerically. It is observed that the phase modulation changes the time period between consecutive dropouts in the emitted laser intensity. A new variable ΦL is defined as the phase of the laser's LFFs, which increases in time with 2π after each power dropout event. The phase ΦPM of the periodic phase modulator is unfolded on the real axis and increases linearly at a rate given by the modulating frequency. It is shown that the phase difference between the laser and the modulator ΔΦ(t)=mΦL(t)−nΦPM(t), where m and n are integers, remains constant in time, leading to phase-synchronized states, for specific values of the modulating frequency and amplitude.

Final comments by the authors

are a little different when it comes for feedback control and packet dropouts between controller and actuator. As demonstrated by [7], depending on the packet dropout rates and the weights of the optimization variable, better results can be obtained with zero-input or with hold-input schemes. Hence, the design of the mode-dependent filter could be simplified by considering a null input whenever a dropout occurs, but some simulation should be done before deciding what the best strategy for the control input is. The output feedback control is built based on an internal model state observer interconnected with a state gain. For themode-dependentH2 control problem, a Separation Principle has been proved by [1], but there is no similar result for the H∞ control problem, to the best of my knowledge. Therefore, it would make more sense to work with a full-order dynamic output feedback controller, as indicated in [4]. Finally, it seems to me that using a limit for the number of consecutive dropouts in the communication channels, indicated in the article as lk and hk , has increased too much the complexity of the problem. By considering an upper bound for that variable, the stochastic nature of the dropout is not Bernoulli anymore and the formulas for the expected values should be different. Moreover, most of the complexity on the Lyapunov function used by the authors comes from the fact that their closed loop system is represented as a discrete-timeMarkov jump systemwith time-delays. In summary, the association of a mode-dependent dynamic output feedback controller with the quantization effects and the discussion of when to apply hold-input or zero-input strategies would be a good direction for future work by the authors, given the developments they have achieved in the article under discussion.