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

Abstract

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 ascertaining the stability of a networked control system with delays. Some existing work [4] has pursued similar derivations for systems with time-varying delays without applying the results directly to networked control systems; a complementary approach [1] provided LMIs that could be used to guarantee the stability of an undelayed networked control system under packet losses. Another key point in the work here is that the authors assume that the application of control signals and delivery of state information happens on the boundaries of sampling intervals. This approach is advantageous to their analytical approach, which augments the system with delayed state measurements up to a certain bound. Moreover, it provides results for situations in which the delay is greater than the sampling period, a scenario that is often neglected in NCS research, but fairly common in wide-area networks. In summary, their different viewpoints plus the development of an endto-end system under quite restrictive bandwidth and delay constraints is of obvious value to the progress toward a larger vision of networked control. The Bigger Picture