SIMULATION OF NETWORKED CONTROL SYSTEMS USING TRUETIME

The installation of industrial fieldbuses in new industrial applications and plants calls for new approaches to the designing of the control system. The modern networked control systems are usual decentralized systems interconnected by industrial network cables or wirelessly. In this paper we describe networked control in industrial applications and the possibility to simulate the control systems by TrueTime. 1 Networked Control Systems Nowadays we can see the trend of passing from the traditional centralized control to distributed control systems. This calls for a change of the usual design approaches to the control systems. The traditional automation is fusing with the technologies known from informatics and computer networks [1]. According to [2], a Networked Control System (NCS) is a distributed control structure where the communication between the nodes of the control system is provided by a communication network. The basic elements of a NCS are sensors, controllers, actuators and the communication network. A NCS can be a one-level or a multi-level NCS. The one-level NCS is a system where the communication nodes are interconnected in one level of the whole manufacturing process of the company. An example of a one-level NCS is in Fig. 1. Multiple interconnected levels of the company’s manufacturing process form a multi-level NCS. There are the fieldbuses on the bottom levels of the process and the company’s LAN on the higher levels, where the data from the industrial layer are handled for economical or technological reasons. The reasons for using a network infrastructure similar to computer networks are exactly the same as for using networks in buildings. To these reasons belongs the relatively cheap installation cost, use of standardized hardware, transparent infrastructure, ability to expand the system in the future, or a good resistance to interferences. The use of a network is very cost effective. Figure 1: The bottom level of the company’s manufacturing process (Hybrid NCS) According to the communication point of view, the NCSs can be divided into straight and hierarchical [1]. In a straight NCS, the control loop consists of a controller, sensors and actuators connected through a network. In the hierarchical NCS the system consists of local straight control loops connected to a superior controller. This type of NCS is used in more complicated systems, like in mobile robotics. The nodes in the NCSs are either connected by cables (fieldbuses, Ethernet) or wirelessly (ZigBee, iWLAN). 1.1 Wired NCS technologies The most common, most reliable and most secure way of connecting nodes communicating over an industrial network is by cables. There are several technologies for use in industrial applications. Each of them is supported by a number of device manufacturers. As we have presented, the benefits of the networks are mainly the cost effectiveness and reliability. Each technology respects these needs. The main differences are in the transfer rates and the communication protocols. The older technologies like the CAN bus or Profibus were designed from scratch for the industrial needs (reliability, resistance to interferences) and their limits are unveiling in the last years, e.g. the maximum possible number of connected nodes, the maximum transfer rate. Because these limitations, there has been an initiative to use the existing Ethernet technology used in LANs. The Ethernet is perspective because of its high transfer rate, ease of installation, cheap hardware (compared to the hardware used in existing fieldbuses) and resistance to interferences. Some of the hardware manufacturers have been developing their own industrial implementation of the Ethernet. Examples of the implementations are Profinet by Siemens, EtherCAT by Beckhoff or Ethernet Powerlink by B&R. The differences of the technologies are especially in their transfer protocols and the medium access. Their common characteristic is the physical layer (cables, switches). An example of an EtherCAT network topology is in Fig. 2. Figure 2: Example of an EtherCAT topology [3] 1.2 Wireless NCS technologies Following the successful implementation of wired Ethernet technologies in industry the manufacturers have been developing also the industrial implementation of the wireless data transfer technologies. The reason for the use of wireless data transfers is similar to the use of fieldbuses and it is the cost effectiveness in some applications. The benefits are reduced installation cost, lower operating cost, installation flexibility, and scalability [4]. A proper industrial application and a proper wireless technology must be chosen for the right control behavior of the system. Factors influencing the functionality of the wireless system are for example the environment interference or the safety of the data transfer. The typical industrial wireless technologies are ZigBee and industrial WLAN (an industrial modification of the 802.11 b/g standard). According to [5] the wireless technologies bring to users low cost access to additional measurements that would otherwise be too expensive to be installed. These additional measurements sometimes contribute to increasing of the safety of the plant. According to [6] the wireless applications in industry can be used in the fields of Plant safety Increasing the reliability of the plant Optimization of the production process 1.3 The need for co-simulation during NCS development Computer-based control systems and networked control systems are hybrid systems where continuous time-driven dynamics and discrete event-driven dynamics interact. The temporal nondeterminism introduced by computing and communication in the form of delays and jitter can lead to significant performance degradation. Software tools are needed to analyze and simulate how the timing affects the control performance [7]. The wireless components of the industrial system are often moving around in the environment or a sensor is monitoring the presence of moving objects in the environment. These systems interact with their environment. These applications are usually very complex and a simulation tool during their development is very helpful. It should be possible to simultaneously simulate the computations that take place within the nodes, the wired and wireless communication between the nodes, the sensor and actuator dynamics, the dynamics of the mobile robots, and the dynamics of the environment, including the physical systems under control [8].