A Hybrid Scheduling Protocol to Improve Quality of Service in Networked Control Systems Ahmed Elmahdi ∗ , Ahmad F Taha ∗ , Stefen Hui † , and Stanislaw H. ˙ Zak ∗ Abstract— In many networked control systems (NCSs) only one node is allowed to use the shared medium at any given time. This network constraint can adversely affect the performance of the system and its stability. There are two types of network schedulers, static and dynamic. Static schedulers, such as the token ring protocol, have problems handling large-scale systems. On the other hand, dynamic schedulers, such as try-once- discard (TOD) cannot guarantee good quality of service for each node, especially the low-priority ones. In this paper, a hybrid scheduler, which is a combination of dynamic and static protocols, is proposed. This scheduler improves the medium access strategy in large-scale control systems. We refer to this scheduler as the traffic-division arbitration (TDA) protocol. The network-induced delay error bound and the system stability of the NCS using the proposed scheduler are investigated. Simulations illustrate the performance of the proposed scheduler and difference from TOD are shown. We use two different decision functions to prioritize the scheduling criteria of the protocol. I. INTRODUCTION A networked control system (NCS) is a control system that closes its control loops through a network, which can be the Internet or any network. An essential feature of the NCS is that signals such as reference input or plant output are exchanged between control system components such as actuators, sensors, or a controller through a network. This requires a scheduling protocol for the NCS to control traffic flow. In this paper, we propose a scheduling protocol for NCSs. In the next subsection, we discuss some NCS applications and the importance of scheduling protocols in the NCS operation. A. Remarks on Applications of Networked Control Systems Control systems are characterized by the three main com- ponents: sensors to measure input/output signals, controllers to provide commands to the actuators, and actuators to execute the controllers’ commands. A control system can be modeled by a transfer function or a state-space model. In many applications, controllers and plants are geographically separated which requires adding a real time network to the control system. A defining attribute of the NCS is that feed- back and control signals are exchanged among the system’s components in the form of information packets through a network [1]. In Figure 1 we illustrate a typical NCS setup. Many modern control systems are inherently NCSs. The advantages of connecting the system components via network ∗ School of Electrical and Computer Engineer- ing, Purdue University, West Lafayette, Indiana 47907 (aelmahdi,tahaa,zak)@purdue.edu † Department of Mathematical Sciences, San Diego State University, San Diego, California 92182 hui@math.sdsu.edu compared with traditional point-to-point control systems are modularity and flexibility of the system design, the simplicity of implementation, such as reduced system wiring and con- figuration tools and ease of diagnosing and maintaining the system [4]. NCSs applications are found in manufacturing plants, auto- mobiles, air conditioning/cooling systems, elevators, building automation, medical equipment and devices, remote surgery, mobile sensor networks, robotics, and many other industrial applications. New cars have built-in NCSs. According to Walsh et al [2], [5], a modern car can have in fact several controller area networks (CANs): high speed CAN in front of the firewall for the engine, transmission, and traction control and low speed CAN for locks, windows, and other devices. Integrating computer and communication networks with con- trol systems having different operations and functionality is a new trend in the current industrial applications [3]. The addition of a network to the system increases its complexity. Important issues that an NCS designer must address are: network’s fairness, stability, delays, and error analysis. In this paper, we propose a scheduling protocol that addresses some of the challenges mentioned above, such as Quality of Service (QoS) and network-induced delays. B. NCS Limitations Although an NCS can improve system reliability, reduce weight, space, power and wiring requirements, there are con- straints that somewhat limits its applications. Multiple-packet transmission, data packet drop-outs and finite bandwidth, are all problems that have to be addressed when an NCS is used. These problems can cause signal delay and distortion, affect stability and fairness of the system [6]. The distribution and characteristics of the network-induced delay and the signal distortion in an NCS are determined by its medium access control (MAC) protocol. The computation- time delay of a controller computer, the time taken to execute programs that implement control algorithms at the controller’s nodes or process data at sensors or actuator nodes, is another effective source of time delay [4], [11]. The insertion of a communication network in the feedback control loop makes the analysis and design of an NCS more complex. Conventional control theories with many ideal assumptions, such as synchronized control and non-delayed sensing and actuation, must be re-evaluated before they can be applied. Basically, the primary objective of NCS analysis and design is to develop a system that efficiently uses the finite bus capacity while maintaining reasonable closed-loop control system performance. Furthermore, because the time 98 Fiftieth Annual Allerton Conference Allerton House, UIUC, Illinois, USA October 1 - 5, 2012 978-1-4673-4539-2/12/$31.00 ©2012 IEEE