Fundamentals of Distributed Control Systems / Digital Automation Systems Helen Beecroft Jim Cahill Extracted from Fundamentals of Industrial Control, 2nd Edition Copyright 2005 ISA – The Instrumentation, Systems, and Automation Society This file is copyright protected and no authorization is granted for resale or distribution. Original purchaser is authorized to print one copy for personal use.
8 Distributed Control Systems / Digital Automation Systems I n broad terms, instrumentation can be divided into two groups—primary and secondary. Primary instrumentation consists of the sensors and final control elements that are located near the process being controlled. This is an area in which quiet improvements occur continuously. Secondary instrumentation is essentially what one sees in a control room: the equipment used to indicate, alarm, record, and control. Indicating, alarming, and recording support the principal function, which is controlling. In a continuous process environment, control in its simplest terms means keeping the process variable equal to the set point. In a batch process environment, control means keeping the process variable equal to the set point as well as ensuring that all physical events are synchronize with the sequence of events of the process recipe. Advanced control means determining correct set points or an ideal process recipe. If the control system design ensures that the right set points are being effectively maintained, then consistent and efficient process performance is the result. Tremendous technological advances in hardware and software have enabled process automation system suppliers to move far beyond systems based on 4–20 mA signals and proprietary communications. Microprocessors now can be embedded virtually anywhere, delivering information not previously available in the eras of the PLC or the distributed control system (DCS). Today’s digital automation systems (DAS) are unprecedented in speed, scope and scale. Introduction The technological advances in instrumentation that have improved the performance of conventional control applications include the following: (1) Pneumatic telemetry permitted the birth of centralized control rooms. (2) Electronic analog controls improved accuracy and allowed control panels to be arranged more compactly. 1
Distributed Control Systems / Digital Automation Systems (3) Digital technology (minicomputers) introduced sophisticated and advanced controls, which made it possible to do logical alarming and indication through CRTs. (4) Distributed control systems (DCS) initially lowered installation costs because DCS enabled control modules to be interconnected and grouped, lowered maintenance costs, improved system reliability, and made it easier to configure the process concept and expand the system. (5) Today’s digital automation systems are built using: • accepted industry standards • totally interoperable architecture • the communications architecture and bandwidth to support the vast intelligence found within digital devices • comprehensive batch solutions • embedded advanced control • OPC, XML, and web services to integrate the process with the global enterprise • software to facilitate the migration from existing DCS/PLC systems to DASs. Figure 8-1 shows other key developments and illustrates the general progression of industrial control from its mechanical regulator beginnings to state-of-the-art distributed control systems and digital automation systems. Figure 8-1. Industrial Control from Mechanical Regulator to DAS [Ref. 1] 2
Introduction Overview Because of its complete dependence on computer technology, the DCS or DAS is clearly software intensive. Consequently, practitioners cannot neglect the software aspect. Nevertheless, we will not discuss advanced computer-related topics such as artificial intelligence, management information systems, optimization, simulation, and modeling in any great detail in this chapter. The implementation of advanced computer capabilities varies widely not only in terms of the way such capabilities are used but where and why they are used. Which specific choices are selected depend on the philosophy and operating needs of the individual plant. Most DCSs or DASs provide similar capabilities in a comparatively cost-effective way for a given project. However, they may vary greatly in terms of cost of ownership. Simple things such as the ease with which the system can be started up (and restarted after a shutdown) may have profound effects on the overall cost of a DCS or DAS project. Adding a seemingly small number of extra modules or even a software upgrade can double project costs and delay startup. A DCS or DAS purchase requires careful evaluation and planning for the future. Other factors, such as site preparation costs, ease of expansion, product obsolescence, the upgradability of hardware and software, backward and forward systems’ compatibility, maintenance, training, and integratibility with other computers, are important issues when evaluating any DCS or DAS. DCSs and DASs can also vary widely in terms of reliability and availability. Some suppliers offer proven, off-the-shelf products; some products are still in the developmental stage. Software that is continually promised but never delivered has come to be known as “vaporware.” “Vendor support requirements” is a term for how much an owner can do alone before returning it to the vendor to do. Any of these factors could make the difference between an easily implemented, cost-effective computer system and a financial “black hole.” Often, apparently low-cost systems turn into overbudget problems because the right questions were not asked in the beginning. Thus, this chapter presents DCS and DAS basics in a broader scope, from hardware and software to startup, expansion, maintenance, upgrades, and purchasing strategies. We do this in acknowledgement of the fact that a DCS or DAS is a long-term, living investment rather than simply a one-time computer purchase. We conclude this chapter by focusing on solutions for migrating from legacy systems to a digital automation system. DCS Defined As implied by its name, a distributed control system is one whose functions are distributed rather than centralized. A DCS consists of a number of microprocessor-based modules that work together to control and monitor a plant's operations. The modules are distributed geographically. This reduces field-wiring and installation costs. It also reduces risk by distributing the control function throughout a number of small modules rather than concentrating it in one large module. A DCS is a computer network. It differs from an office or personal computer network in that a DCS does real-time computer processing rather than the transactional processing performed by business computers. The difference between realtime and business computers is the way they execute their programs. Business computers typically do a single program operation at a time. The program will start with some fixed data, perform a complex set of calculations, and provide a set of results. Once the program has done its job, it stops until it is instructed to run again with new data. An example would be the monthly processing of invoices by a utility company. The real-time computer also executes its program by using fixed data, performing calculations, and providing a set of results. The difference, however, is 3