Network-based control systems: A tutorial

For many years now, data networking technologies have been widely applied in the control of industrial and military applications. These applications include manufacturing plants, automobiles, and aircrafts. Connecting the control system components in these applications, such as sensors, controllers, and actuators, via a network can effectively reduce the complexity of the systems with nominal economical investments. Furthermore, the applications connected through a network can be remotely controlled from a long-distance source. Traditionally, the networks used in the aforementioned applications are specific industrial networks, such as CAN (Controller Area Networks), and PROFIBUS. However, general data networks such as Ethernet and Internet are rapidly advancing to be the networks of choices for many applications due to their flexibility and lower costs. There are two general structures to design a control system through a network. The first structure is to have several subsystems, in which each of the subsystem contains a set of sensors, a set of actuators, and a controller by itself. These system components are attached to the same control plant. In this case, a subsystem controller receives a set point from the central controller. Another structure is to connect a set of sensors and a set of actuators to a network directly. Sensors and actuators in this case are attached to a plant, while a controller is separated from the plant via a network connection to perform a closed-loop control over the network. Both structures have different advantages. The first structure is more modular. A control loop is simpler to be reconfigured. The second structure has better interaction because data are transmitted to components directly. A controller in the second structure can observe and process every measurement, whereas a (central) controller in the first structure may have to wait until the set point is satisfied to transfer the complete measurements, status signals, or alarm signals. A control system in the second structure is so-called networked control system or network-based control system depending on different authors' preference. A challenging problem in control of networked-based system is network delay effects. The time to read a sensor measurement and to send a control signal to an actuator through the network depends on network characteristics such as their topologies, routing schemes, etc. Therefore, the overall performance of a network-based control system can be significantly affected by network delays. The severity of the delay problem is aggravated when data loss occurs during a transmission. Moreover, the delays do not only degrade the performance of a network-based control system, but also can destabilize the system. This tutorial presents fundamental details of network-based control and recent network-based control techniques for handling the network delays. The techniques are based on various concepts such as state augmentation, queuing and probability theory, nonlinear control and perturbation theory, and scheduling. A general structure of a network-based control system, delay types, and delay behaviors are also described in this tutorial. In addition, advantages and disadvantages of these techniques are discussed.