Analysis and Design of Networked Feedback Loops: A packet-based Approach

The scientific field of Networked Control plays a more and more important role in control engineering. The conventional wired connections between the plant and the controller are replaced by (wireless) communication networks to enable novel control architectures. This is especially important, e.g., if the plant and hence the actuators and sensors are spatially distributed as it is the case in large factories. But also low-level feedback loops where, for example, the sensors are placed on rotating parts are possible applications for Networked Control Systems (NCS).

The main challenge in such feedback loops is to design controllers and perform a stability analysis in the presence of network imperfections such as time-varying delays of the transmitted packets. The present work thus contributes to tackle this field of research.

First, the modeling and simulation of NCS is considered, where special emphasis is placed on the fact, that each transmitted data packet experiences an individual delay in communication channels. This packetized character is often either neglected or not explicitly incorporated in existing approaches, although it might have a significant impact on the closed loop performance.

Based on that, different techniques are proposed for NCS that depend on a trade-off between achievable properties and the assumptions made for the considered networked connections. This means that, for example, a buffering mechanism is used in one approach to mitigate the effect of time-varying delays. Consequently, the introduced conservatism with respect to the network allows to fully exploit the properties of sliding mode techniques to get robust controllers that might also be spatially distributed over the network.

The other extreme case treated in this work is given by NCS with networked channels, where arbitrary but bounded time-varying packet delays can occur. Thus, the actual packet skipping, packet dropping and hold mechanisms at the receiver side play important roles for the stability of the closed loop system. Different criteria for packetized NCS are proposed that allow to prove the stability under the presence of time-varying packet delays and uncertainties in the plant model description.

The third setup constitutes an intermediate case between the previously mentioned cases, where the assumptions are less strict than in the case with arbitrarily varying packet delays. Adaptive control approaches are proposed for such situations in which, e.g., the delays depend on the current task classes in the communication links.

In summary, this work sheds light on effects caused by time-varying packet delays in networked systems and proposes novel methods for the robust stability analysis and controller design of packetized feedback loops. The presented techniques allow to further push the technology towards robust and reliable networked systems.