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Über dieses Buch

This book introduces the fundamentals of DCS, and shows how to include wireless technology in their design while guaranteeing the desired operation characteristics. The text also presents insights and results gained from extensive practical experience in implementing and testing systems within a specific industrial setting. Features: examines the operations that the DCS implements, covering human-machine interfaces, diagnostics and maintenance interfaces, and controllers; discusses industrial control system and wireless network protocols; reviews scheduling in wireless sensor networks; describes a latency model for heterogeneous DCS with wired and wireless parts, that predicts monitoring, command, and closed loop latencies; explains how to plan operation timings systematically; introduces measures and metrics for performance monitoring and debugging, and describes how to add these to a system; presents experimental results to validate the planning approach, based on an application test-bed.



Chapter 1. Introduction

In industrial sites, sensors and actuators of the Distributed Control Systems (DCS) are connected to some intermediate equipment through a huge web of cables spanning many kilometers and costing a fortune. About a decade ago, industry suppliers have started deploying wireless sensor and actuation solutions, which are easier to deploy and less costly than totally cabled ones. These solutions are based on small-embedded devices that are able to sense and actuate, as well as communicate and compute. The promise is that, if wireless solutions are proved reliable enough, they can revolutionize critical applications by allowing sensing and actuation at significantly lower costs. This first chapter starts by discussing issues and challenges arising from the inclusion of wireless sensors in industrial networks. Then the chapter presents a small description of all chapters included in this book. Each chapter describes one specific issue that leads the reader to better understand the concepts behind heterogeneous distributed control systems, as well as scheduling and planning in those systems. After reading this chapter, the reader will know the main concepts and issues behind heterogeneous DCS with wireless components.
José Cecílio, Pedro Furtado

Chapter 2. Industrial Control Systems: Concepts, Components, and Architectures

Recent advances in industrial networking systems have started to approximate industrial and standard/commercial networks. However, they have different requirements, which results in the need for different architectures. The most essential difference is that industrial networks are connected to physical equipment in some form and are used to control and monitor real-world actions and conditions. In this chapter we describe the main concepts, components, and architectures of industrial networking systems, mainly Distributed Control Systems (DCS). A set of generic concepts and terms related to heterogeneous sensors and actuator networks are discussed, as well as an overview of the architectures of industrial networks and DCS. Since those networks can be used in numerous scenarios, we also review application scenarios for DCS and discuss network topologies, low-level transmission mediums, network communication components, and control components. The chapter ends with a description of the overall organization of DCS when those include wireless sub-networks.
José Cecílio, Pedro Furtado

Chapter 3. Distributed Control System Operations

Typically, industrial Distributed Control Systems are programmable electronic cyber-physical systems that perform a set of operations continuously. Those systems are designed to perform real-time monitoring, continuous recording, and logging of plant status and process parameters. They implement automatic process control and batch/sequence control during start-up, normal operation, shutdown, and disturbance, i.e., control within normal operating limits. When they are deployed in hazardous environments, they should be able to detect hazardous conditions and to terminate or mitigate hazards (i.e., control within safe operating limits). In this chapter we take a closer look at the operations that the DCS implements. It focuses on the software components and functionalities that are implemented in those systems. These include Human–Machine Interfaces, Diagnostics and Maintenance Interfaces, and Controllers. Controllers are fundamental software components through which users and automated logic interact with the whole cyber-physical system. The main operations implemented in a DCS are overviewed.
José Cecílio, Pedro Furtado

Chapter 4. Industrial Protocols and Planning Considerations

Industrial control system protocols are needed to deploy industrial networks. In this chapter, we discuss those protocols and current solutions for planning within those systems. We review the concepts behind the fieldbus and fieldbus follower standards, and we describe the currently available guidelines for planning over fieldbus. Planning is mainly based on guidelines and ad-hoc engineering decisions. We also review wireless industrial network protocols and the guidelines that can be followed regarding planning for those networks. After this chapter, the reader will understand the industrial protocols for both wired and wireless networking and will also understand current solutions to plan operation timings.
José Cecílio, Pedro Furtado

Chapter 5. Scheduling of Wireless Sensor Networks

One key issue in networked systems that influences whether the deployed system will be able to provide timing guarantees is the Medium Access Control (MAC) protocol and its configurations. MAC protocols can be classified broadly into two categories: contention- and schedule-based. The contention-based protocols can easily adjust to the topology changes as new nodes may join and others may die after deployment. These protocols are based on Carrier Sense Multiple Access (CSMA) mechanisms and have higher costs due to potential interference, message collisions, overhearing, and idle listening than the schedule-based counterparts. Schedule-based protocols can avoid those problems by defining precise schedules, but they have strict time synchronization requirements. In this chapter we discuss scheduling approaches and mechanisms in wireless sensor networks. It sheds light on the medium access layer protocols, explains the mechanism of time-division medium access protocols, and finally how schedules are achieved within those protocols. The capacity to define schedules in wireless sensor networks is very important for planning operation-timings in the whole DCS.
José Cecílio, Pedro Furtado

Chapter 6. Latency Modeling for Distributed Control Systems with Wired and Wireless Sensors

In this chapter we describe a latency model for end-to-end operation over hybrid wired and wireless industrial Distributed Control Systems (DCS). Since nodes are configurable and the DCS may include actuators, the latency model can be decomposed in two parts: monitoring latency model (upstream), which corresponds to the latency that is measured from the sensing node to the control station, and commanding latency model (downstream), used to access the latency of sending a configuration command or the latency associated with an actuation command resulting from a closed-loop operation. This chapter describes a latency model for heterogeneous DCS with wired and wireless parts. The model predicts monitoring latencies, command latencies, and closed loop latencies. Based on the model, we describe the approach for prediction of maximum latencies.
José Cecílio, Pedro Furtado

Chapter 7. Planning for Distributed Control Systems with Wired and Wireless Sensors

Since real-time operations in DCS must be pre-planned, this chapter describes how to plan operation timings systematically. The planning approach is based on the construction of schedules and the latency model. Based on user inputs and a first network layout, the approach determines maximum WSN latencies first, computes the required number and placement of downstream slots to meet command and closed loop latencies, and then verifies whether the latency requirements are met. The network is successively partitioned and latencies verified until the required latencies are achieved.
José Cecílio, Pedro Furtado

Chapter 8. Performance and Debugging

Some measures and metrics are needed to evaluate heterogeneous Distributed Control Systems. This chapter refers to measures and metrics for performance monitoring and debugging. How to add performance monitoring and debugging to a system is also described. Starting from specified operation timing requirements, a performance-monitoring tool should verify latencies for the various paths and operations, operation-timing delays, and compliance with pre-specified bounds, messages, and packet losses. It should also compute statistical information on timings compliance. The chapter ends with the definition of the modules and user interfaces that should be added for performance monitoring and debugging.
José Cecílio, Pedro Furtado

Chapter 9. Application Scenario: An Industrial Case Study

In order to demonstrate the usability of some models and planning approaches discussed in this book, in this chapter we present experimental results to validate those approaches, based on an application test-bed. A heterogeneous DCS was built by adding wireless sensor networks to a cabled setup. We experimented with various operations and operation-timing requirements, planning for network layouts that meet operation-timing requirements. Maximum operation-timing bounds are specified for sensing, actuations, and closed loops, including examples of multiple simultaneous closed loops. We show that the models predict latencies adequately and generate useful network layouts. Finally, we use a performance monitoring approach, as described in Chap.​ 8, to control timing bounds and detect link malfunctions.
José Cecílio, Pedro Furtado


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