Distributed Control Systems

The DCS is the dominant form of instrumentation used for process control (Liptak 1994). The equipment in a DCS is separated by function and is installed in two different working areas of a process installation. The equipment that an operator uses to monitor process conditions and manipulate the set points of the process operation is located in a central control room. From this location, the operator can (1) view the information transmitted from the processing areas on a CRT and (2) change control conditions from a keyboard. The controlling portions of the system, which are distributed at various locations throughout the processing area, perform the following two functions at each location:

• Measure analog variable and discrete input

• Generate output signals to actuators that change process conditions

Input and output signals can be analog or digital. By means of electric transmission, the system communicates information between the central location and the remote controller locations.

Figure 3.13.1 shows a generic arrangement for the components in a DCS. The operator's console in the control room is called the high-level operator's interface (HLOI). It can be connected through a shared communication facility (data highway) to several distributed system components. These components can be located either in rooms adjacent to the control room or in the field. Such distributed local control units (LCUs) can also have a limited amount of display capability and are called the low-level operator's interface (LLOI).

FIG. 3.13.1 A typical DCS. The panel boards and consoles are eliminated, and the communications are over a shared data highway, which minimizes the quantity of wiring while allowing unlimited reconfiguration flexibility. (Reprinted, with permission, from M.P. Lukas, 1986, Distributed control systems, Van Nostrand Reinhold Co.)

FIG. 3.13.1 A typical DCS. The panel boards and consoles are eliminated, and the communications are over a shared data highway, which minimizes the quantity of wiring while allowing unlimited reconfiguration flexibility. (Reprinted, with permission, from M.P. Lukas, 1986, Distributed control systems, Van Nostrand Reinhold Co.)

The console or HLOI is the work center for an operator. In this area, the operator follows a process and uses the fast and accurate translation of raw data into useful trends and patterns to decide the required actions. One CRT is usually dedicated to each section of a plant; each of these CRTs requires an operator's continuous attention. The HLOI also includes keyboards, usually one for each CRT, which allow the operator to enter set points or other parameters or to closely examine particular portions of the process for further information. The HLOI peripherals include disks, tapes or other recorders, and printer units.

The DCS operator depends on the CRT displays for plant information. Three principal types of displays are the group display, the overview display, and the detailed display. A graphic display capacity shows a picture on the screen so that the operator can look at a portion of the process more realistically than by watching a row of bar graphs. Figure 3.13.2 is a graphic display representation of a fractionation column. The display includes process and operation information, and it can be interactive, dynamically changing as real-time information changes.

Trend displays are the DCS equivalents of chart records. They are a profile of the values of process variables showing the changes that occur over a period of time. Some detail displays (see Figure 3.13.3) include a real-time trend graph of the process variable values during a selected period. In some displays, several trend graphs can be displayed at the same time, allowing a comparison of the history of several variables. Trends over longer periods (up to a week) can be saved on floppy-disk memory and displayed on command.

A single best distributed control solution does not exist. The right control system for an application is a function of the process to be controlled (Funk and McAllister 1989).

In the broadest sense, manufacturing processes are either continuous (e.g., a petroleum refinery or an ethylene plant), or batch (e.g., specialty chemicals or pharmaceuti-

FIG. 3.13.2 Graphic display.
FIG. 3.13.3 Detail display.

cals). In reality, few plants are purely batch or continuous. Instead, they often fall in between and blend some aspects of both.

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