Control methods for safe and efficient cyber-physical systems

The present manuscript represents a report of the main research activities done by the candidate during the three years of his PhD cursus studiorum. The work revolves around the concept of Cyber-Physical Systems (CPS), a class of systems in which the interaction between their digital domain, constituted by connected devices capable of computing, and a physical process plays a fundamental role in their operation. The deep linkage between the cyber and the physical parts of a CPS makes their study appealing for several research fields as Automation and Computer Science, as properties such as stability and robustness are paired with concepts as information integrity and service availability. Due to their very broad definition, CPS are studied and applied in the most heterogeneous domains, spacing from power systems and smart factories to healthcare and autonomous vehicles. The present work explores a total of four case studies that the candidate analysed during his PhD studies, covering energy & power management system, spacecraft control and selfish routing over dynamical networks. The typical goal of a control system designed for a CPS is the one of attaining the desired, optimal, behaviour in the most efficient way possible, while also constraining the system evolution into a region considered to be safe for the system itself and the environment around it. This simple idea is behind the development of the first work presented, which was carried out by the candidate in the scope of the research project H2020 ATENA (regarding Critical Infrastructure Protection). The work develops a control solution for the Risk-Aware and Efficient operation of the Power Distribution Network, exploiting the presence of innovative devices as Electrical Storage Systems. In this application scenario, the candidate designed an Economic Model Predictive Controller (EMPC) for the purpose of increasing the resiliency of the service provision, by automatically reconfiguring the power network in response to, predicted or ongoing, adverse events, as malicious attacks or faults. The specifics of the case study were refined during the project thanks to the continuous interaction with the Israel Electric Company (IEC), principal industrial end user of ATENA. A more user-centric approach is taken for the development of the second controller proposed in this work, as it was designed to manage the heating and power appliances of a smart building, integrating also features as electric vehicles charging and demand side management capabilities. The methodology chosen for this second controller was still EMPC, as it offered the possibility of explicitly consider operational and logical constraints imposed by the specific case study, while also exploiting short-term prediction of the exogenous signals interacting with the system. The third example of CPS studied by the candidate in this work was a multi-body satellite system. Thanks to a collaboration opportunity with the manufacturer Thales Alenia Space Italia, a real case study of interest for the company was analysed, leading to the development of a control scheme for a life-support system that could connect to orbiting satellites to extend their operative life and upgrade their capabilities. The control scheme developed is based on feedback linearization, under which it was proven that the two interconnected spacecraft may operate in parallel without requiring communication or information exchanges. The life-support is in fact able to remove its effects on the original satellite dynamics by applying a compensating control action that reconstructs the original system behaviour. The fourth, and final, study presented was completed in the scope of the H2020 EU-Korea Project 5G-ALLSTAR (regarding the integration of satellite communications and 5G) and deals with the problem of selfish routing and load balancing in heterogeneous networks, to enable 5G Multi-Connectivity. The network studied was modeled as a discrete-time dynamical system, and the proposed control law was proven by Lyapunov arguments to drive the system state into an equilibrium condition that represents an approximation of the Wardrop user equilibrium. The limited availability of network resources was explicitly included in the control design, and the optimality of their usage was obtained in adversarial terms, as the various information flows compete with each other to maximise their connection performances. All the researches included in this work covered different aspects and problems that arise when controlling a CPS. The common aspect that is shared among the four controllers is their focus on the safety and resiliency of the controlled systems, and the optimal usage of the limited available resources, in order to assure efficient and safe system operation and service provision. In the first two researches this aspect is clear, since the optimal exploitation of the available resources (energy) was obtained by a centralised controller capable of predictive optimisation, and it is worth noting that the adversarial load balancing considered in the fourth case study still leads the network in a state in which all of its users cannot utilise better the available resources without cooperating. Even if paying the so-called Price of Anarchy, the load balancing attained can still improve the connection resiliency by enabling Multi-Connectivity (i.e., the routing of a single information flow over multiple paths), a core feature of 5G. Furthermore, the direct improvement of spacecraft resiliency brought by life-support systems as the one discussed in this work (in principle capable even of reactivating a non-operative satellite), is paired with the optimal usage of the limited actuating capabilities typical of spacecrafts, potentially lowering significantly space missions cost. The control methods utilised in this work are taken from different fields of Control Theory, mostly due to the fact that the problems studied were defined starting from applied research projects characterised by very heterogeneous requirements and goals. The rationale behind the choice of the various methodologies utilised is justified for each controller, and the tailoring of their characteristics to the specific applications is discussed in depth in their corresponding chapters.