Radio link modelling for co-design of wireless control systems in industrial automation

Wireless networked control systems (WNCSs) are receiving ever growing attention from both researchers and practitioners thanks to the introduction and wide spreading of the concepts of the IoT (Internet of things) and 5G (fifth generation of cellular mobile communications). The main advantage of WNCSs is given by the full exploitation of a wireless communication for sensors, actuators, and control data, thus eliminating the cost and time-consuming problems arising by cabled connections implementation. This aspect clearly represents an enabling technology for industrial automation in the context of Industry 4.0, that can exploit the benefits offered by a WNCS such as ease of maintenance and installation, low costs, large flexibility, and possible enhancement of safety. On the other hand, WNCSs for industrial automation face unique challenges. Indeed, data exchange exploits a medium characterized by dynamically changing conditions due to interference, and time varying fading. This deficiency makes it challenging for WNCSs to meet the stringent reliable and low latency constraints of industrial control applications especially in the case of multi-hop communication over mesh networks. The design of WNCSs is a challenging task since it must consider these imperfections by integrating wireless networks models and control algorithms thus implementing the cyberphysical co-design approach. Moreover, in order to cope with the automation-specific needs for quantifiable reliable, timely and efficient communication, the interdependences between control and communication protocols and systems, must be taken into account to guarantee the joint tuning of the critical interactive variables. Furthermore, the lack of analytic methods for achieving real-time performance in WNCSs hinders their adoption in control systems. Therefore, this thesis aims at filling this gap by tackling the problem of deriving an accurate analytic communication link model that accounts for the aforementioned imperfections. The thesis addresses the modelling and design challenge by focusing on different wireless standards like IEEE 802.15.4, WirelessHART, ISA-100.11a and IEEE 804.15.4e, which are networking protocol stacks of wide interest for wireless industrial automation. Specifically, since the aforementioned standards share the same physical layer, we first develop and validate a Markov link model that abstracts the wireless standard radio link subject to channel impairments and interference. The link quality metrics introduced in the theoretical framework are validated in order to enable the accurate representation of the average and extreme behaviour of the radio link. By adopting these metrics, it is straightforward to handle a consistent finite-state abstraction. Based on such a model, we then derive a stationary Markov jump linear system model that captures the dynamics of a control loop closed over the radio link. Subsequently, we show that our modelling framework allows to discover and manage the challenging subtleties arising from bursty behaviour. A relevant theoretical outcome consists in designing a controller that guarantees stability and improves control performance of the closed-loop system, where other approaches based on a simplified channel model fail. An extensive parametric analysis has been done on the model with the variation of different parameters.