Because of the reliability problems and high cost of the control process computer systems of the 1960s, there were few new process computer projects in the early 1970s. The rare projects that were started in this period were based on medium-priced minicomputers that were designed to be small in size. At the same time, two developments occurred in electronics that profoundly changed the application of digital computers to process control.
The first was the development of integrated circuits and microprocessors. The second was the release of the distributed control system (DCS) by Honeywell in 1969. This new design concept was based on the idea of
widely distributing the control to computer modules. Each of these modules controlled several instrument loops, generally one to four. They were connected by a single high-speed data communications link, called a data highway, that made possible communications between each of the computer modules and the central operator console. This design allowed the operator to monitor the operation of each local process.
In the mid-1970s, microprocessor-based modules replaced hardwired computer modules. The typical DCS had the configuration shown in Figure1. Today’s distributed control systems are much more powerful and
faster than the first systems because of improvements in microprocessors and other electronic circuits.
Fig. 1: Typical DCS configuration.
Distributed control systems today consist of one or more levels of control and information collection, as shown in Figure 2. The lowest level is process control and measurement on the plant floor. At this level, microprocessor- based controllers such as programmable controllers execute loop control, perform logic functions, collect and analyze process data, and communicate with other devices and to other levels in the system.
In Figure 2 , the process data collected at level 1 is transferred to level 2. At this level, process operators and engineers use operator consoles that have a keyboard, mouse, and video display to view and adjust the various processes being controlled and monitored by the system. Also, at level 3, process and control engineers implement advanced control functions and strategies, and members of the operations management team perform advanced data collection and analysis on process information. The various plant management systems—such as inventory management and control, billing and invoicing, and statistical quality control—exist at level 3. The highest level (level 4) is used in large industrial plants to provide corporate management with extensive process and operations information.
Fig. 2: Distributed Control System Level