Cross-Layer Adaptive Feedback Scheduling of Wireless Control Systems

There is a trend towards using wireless technologies in networked control systems. However, the adverse properties of the radio channels make it difficult to design and implement control systems in wireless environments. To attack the uncertainty in available communication resources in wireless control systems closed over WLAN, a cross-layer adaptive feedback scheduling (CLAFS) scheme is developed, which takes advantage of the co-design of control and wireless communications. By exploiting cross-layer design, CLAFS adjusts the sampling periods of control systems at the application layer based on information about deadline miss ratio and transmission rate from the physical layer. Within the framework of feedback scheduling, the control performance is maximized through controlling the deadline miss ratio. Key design parameters of the feedback scheduler are adapted to dynamic changes in the channel condition. An event-driven invocation mechanism for the feedback scheduler is also developed. Simulation results show that the proposed approach is efficient in dealing with channel capacity variations and noise interference, thus providing an enabling technology for control over WLAN.Comment: 17 pages, 12 figures; Open Access at http://www.mdpi.org/sensors/papers/s8074265.pd

Optimistic minimax search for noncooperative switched control with or without dwell time

International audienceWe consider adversarial problems in which two agents control two switching signals, the first agent aiming to maximize a discounted sum of rewards, and the second aiming to minimize it. Both signals may be subject to constraints on the dwell time after a switch. We search the tree of possible mode sequences with an algorithm called optimistic minimax search with dwell time (OMSd), showing that it obtains a solution close to the minimax-optimal one, and we characterize the rate at which the suboptimality goes to zero. The analysis is driven by a novel measure of problem complexity, and it is first given in the general dwell-time case, after which it is specialized to the unconstrained case. We exemplify the framework for networked control systems where the minimizer signal is a discrete time delay on the control channel, and we provide extensive simulations and a real-time experiment for nonlinear systems of this type

Discussion on: “Development and Experimental Verification of a Mobile Client-Centric Networked Controlled System”

The paper under discussion [12] presents the analytical results underlying the authors ’ design of a networked control system (NCS) operating over a cellular network. Their formulation fits the common framework [7, 14], in which there are transmission delays present in the sensor/controller and controller/actuator paths. One key difference between this work and existing literature relates to the use of a client-centric system, which is mandated by security issues. However, this seems to make little to no functional difference in the implementation or resulting analysis. The chief theoretical results of the paper are based upon linear matrix inequality (LMI) theory. In particular, the authors have provided an interesting, LMI-derived analytical result for ascertainin

European Journal of Control (2005)11:242–254 # 2005 EUCA Discussion on: ‘‘Development and Experimental Verification of a Mobile Client-Centric Networked Controlled System’’

Tzes et al. present the experimental verification of a mobile client-centric networked control system (NCS) implemented over a General Purpose Radio Service (GPRS) communication channel. The system is set up by defining a client, the controller, which sends information over the network to the server, the plant and actuator. The characteristics of GPRS make the system different from other NCSs studied, which are often implemented over Ethernet or DeviceNet. For security reasons, mobile-phone service providers preclude a server from independently sending information back to the client. In this network, all actions must be initiated by the controller creating a client-centric NCS. The mobile NCS time delays are composed of both encoding/decoding delays and as well the transmission times of data through the mobile network. The transmission delays are highly uncertain and depend on a number of factors that include the number of users, loss of packets and the existence of higher priority transmissions. As explained in the paper, GSPR transmissions have a lower priority than GSM-based voice calls. A useful contribution of the paper is the characterization of transmission delays in both UDP and FTP connections through GSPR. The measurements show that the average bit rates are significantly lower than the theoretical limits of GSPR. The stability of the closed-loop is guaranteed by implementing results from time-delayed systems and by solving a set of linear matrix inequalities. The authors state that the maximum time delay, max, calculated is conservative. Walsh, et al. [14] present a