Networked and event-triggered control systems
Networked and Event-Triggered Control Systems In this thesis, control algorithms are studied that are tailored for platforms with limited computation and communication resources. The interest in such control algorithms is motivated by the fact that nowadays control algorithms are implemented on small and inexpensive embedded microprocessors and that the sensors, actuators and controllers are connected through multipurpose communication networks. To handle the fact that computation power is no
... nger abundant and that communication networks do not have infinite bandwidth, the control algorithms need to be either robust for the deficiencies induced by these constraints, or they need to optimally utilise the available computation and communication resources. In this thesis, methodologies for the design and analysis of control algorithms with such properties are developed. Networked Control Systems: In the first part of the thesis, so-called networked control systems (NCSs) are studied. The control algorithms studied in this part of the thesis can be seen as conventional sampled-data controllers that need to be robust against the artefacts introduced by using a finite bandwidth communication channel. The network-induced phenomena that are considered in this thesis are time-varying transmission intervals, time-varying delays, packet dropouts and communication constraints. The latter phenomenon causes that not all sensor and actuator data can be transmitted simultaneously and, therefore, a scheduling protocol is needed to orchestrate when to transmit what data over the network. To analyse the stability of the NCSs, a discrete-time modelling framework is presented and, in particular, two cases are considered: in the first case, the transmission intervals and delays are assumed to be upper and lower bounded, and in the second case, they are described by a sequence of continuous random variables. Both cases are relevant. The former case requires a less detailed description of the network behaviour than the latter case, while the latter results in a less conservative stability analysis than the former. This allows to make a tradeoff between modelling accuracy (of network-induced effects) and conservatism in the stability analysis. In both cases, linear plants and controllers are considered and the NCS is modelled as a discrete-time switched linear parameter-varying system. To assess the stability of this system, novel polytopic overapproximations are developed, which allows the stability of the NCS to be studied using a finite number of linear matrix inequalities. It will be shown that this approach reduces conservatism significantly with respect to existing results in the literature and allows for studying larger classes of controllers, including discrete-time dynamical output-based controllers. Hence, the main contribution of this part of the thesis is the de-viii Summary velopment of a new and general framework to analyse the stability of NCSs subject to four network-induced phenomena in a hardly conservative manner. x Summary A current trend in control engineering is to no longer implement control algorithms on dedicated computation platforms having dedicated communication channels. Instead, control algorithms are nowadays implemented on embedded microprocessors  , which communicate with the sensors and actuators using (shared) communication networks. This results in larger flexibility and maintainability of the control system, as modifying control algorithms and adding control loops becomes easier. These advantages form some of the reasons why this control architecture is applied in conventional passenger cars, in which more and more data is transmitted over a controller area network (CAN)  . Furthermore, besides the enhanced flexibility and maintainability, the embedded and networked control architecture allows the control system to have less wiring, with the extremum of being completely wireless. This is especially beneficial for large-scale systems, e.g., mines  , manufacturing/production lines , chemical plants  , water distribution networks  and distributed power generation systems  . In some cases even, wiring the control systems is impossible, e.g., in cooperative control of unmanned aerial vehicles (UAVs)  , vehicle platoons on motorways [48, 106] , or in tele-operated haptic systems [70, 93] . Hence, control systems that use embedded microprocessors and communication networks can already be found in a large variety of practical applications and the deployment of these control systems is believed to even grow in the near future. In fact, the development of control strategies that are tailored for embedded microprocessors and communication networks is considered as one of the important challenges in control theory  , as this will further reinforce this trend. This introduces nonzero and time-varying transmission delays. Communication constraints: When several sensors and actuators have to communicate over a shared network, it is generally impossible to transmit all sensor and actuator signals simultaneously. This introduces the need for a scheduling protocol that orchestrates when a node is given access to the network and is allowed to transmit its data. It is generally known that any of these phenomena can degrade closed-loop performance or, even worse, can harm closed-loop stability of the control system, see, e.g.,  . It is therefore important to know how these effects influence sions of existing ETMs in order to guarantee a nonzero minimum time between two subsequent events. Furthermore, since sensors and actuators, which can be grouped into nodes, can be physically distributed, centralised ETMs are often prohibitive and, therefore, there is a need for decentralised ETMs. We will propose a novel output-based decentralised ETC strategy that has nonzero minimum inter-event times and we will study the closed-loop stability and the L ∞ -performance of the resulting ETCS. We provide a computational procedure to compute a lower bound on the minimum inter-event time of each node. Furthermore, we will model the event-triggered control system as an impulsive system, thereby explicitly describing the behaviour of the event-triggered control system, which leads to improved stability guarantees, compared to the existing results in the ETC literature. The second contribution in the area of ETC is the proposition of an ETC strategy that alleviates the need for dedicated hardware for its implementation.