Architecture of an Open-Source Real-Time Distributed Cyber Physical System

Introduction

Cyber physical systems (CPSs) (Guan et al., 2016) are based on a number of nodes connected through a communication network, which can interact with the environment. Application contexts that foresee the use of CPSs are Industry 4.0, building automation and smart grids (Yu & Xue, 2016). In such systems, nodes are connected through a wired or a wireless network, but commonly a typical configuration relies on the combination of both. In many applications, such as those regarding manufacturing systems, determinism on data exchanges between nodes of the system is the most important requirement.

On the other hand, in recent years, open-source software and applications are approaching CPS market. Open-source provides a low cost and effective method to implement inexpensive, performing and bug free applications. The need of open-source components for many application contexts derives from their capability to be highly customizable. This characteristic impacts directly on performance to preform actuations or sensing physical quantities, in terms of determinism and latency. The use of open-source software for real embedded systems has been described in the book (Cibrario Bertolotti, & Hu, 2015). Other examples of use of open-source software will be provided in the next sections. In the PC world, the Linux operating system is an example of widely adopted open-source platform, which offered long-term support and development. These last features are really important for systems such as CPS, which are updated and replaced infrequently. The Linux operating system is often chosen by researchers, users and developers. Moreover, the code of the operating system, and of its software suite, is well documented, allowing to expert users a really high degree of control of any behavioral aspect. From the network point of view, Ethernet is the “de facto” standard for wired networks. Regarding wireless, a number of technologies exist, but the most common, fast and easily interoperable with Ethernet is IEEE 802.11, also known as Wi-Fi. For cost reasons and ease of configuration and installation, these technologies are the best candidates for a CPS based on open-source components. It is worth pointing out that, while the majority of protocols for industrial automation rely on Ethernet, possible candidates in industrial CPS for wireless extensions are IEEE 802.11, Bluetooth or wireless sensor networks based on the IEEE 802.15.4 standard (Lu et al., 2016).

In this chapter, a possible implementation of a CPS based on the Linux operating system is presented. The proposed architecture exploits the RTAI or XENOMAI hard real-time schedulers to guarantee the required degree of determinism of nodes. It makes use of synchronization protocols, in both the wired and the wireless extensions, to provide all the nodes a common view of time. Transmission latency can be reduced in the wired network by using hard real-time protocol stacks (such as, e.g., RTnet) and channel access methods as the time division multiple access (TDMA). Regarding Wi-Fi, redundancy techniques based on the transmission of the very same data packet on two non-overlapping networks allow to reduce both the number of packets lost and latency. Finally, we will show how the proposed architecture simplifies the integration of popular industrial protocols (e.g., EtherCAT or Modbus TCP) within the communication system.