Clock Synchronization of Distributed, Real-Time, Industrial Data Acquisition Systems

Abstract

In distributed data acquisition systems for industrial applications, the synchronization of the time references of the nodes is essential to guarantee the right real-time behaviour of the system. All the modern distributed automation plants require a clock synchronization mechanism. The use of distributed architectures in industrial applications based on networks has many benefits, including high system flexibility and scalability. However, this enhancement does not come for free; basically, a high-performance network communication among the nodes is needed and the data exchange across the network requires more complex management software than centralized systems (Felser, 2005). Moreover, a key point in all distributed data acquisition systems is the synchronization of the time reference among the nodes (Eidson & Lee, 2003). A typical automation plant is composed by several distributed nodes, controllers, sensors and actuators that need to sample and control the system in a time coordinated fashion. Considering the real-time applications for data-acquisition, the clock synchronization of the time reference across the network is essential in order to support: • Data fusion of the measures obtained using distributed data acquisition system and sensors (Proft, 2007). • Distribute signal processing which takes the “time” into account. • Coordination of the actions across a distributed set of actuators if a control action has to be taken. • Optimization of the transmission bandwidth in the network, thanks to channel medium access control such as TDMA (Time Division Multiple Access), for both wired and wireless systems (Ueda & Yakoh, 2004). The scientific community has proposed several types of synchronization algorithms: some of them are dedicated to wired networks, with different kind of complexity and accuracy; others are specifically dedicated to satisfy the requirements of wireless networks. The aim of this chapter is the definition of basic concepts in the field of clock synchronization in order to better understand the typical applications of clock synchronization techniques to industrial data acquisition systems. This chapter is organized as follows. A brief introduction to essential notions regarding the time keeping and clock performance metrics is provided in Section 2 and 3. Clock synchronization for distributed data acquisition systems are discussed in Section 4. Real