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So far we have been concerned in this chapter with the operation of a single controller. Such a controller is referred to as a stand-alone controller because it is not communicating with other controllers, but only with the one control loop of which it is a part. Present-day microcomputer-based control systems have the capability of communicating with other controllers through a network, which is called distributed control. Figure 35.6 shows one version of the communication linkages that are usually present in a distributed control system. Each manufacturer of distributed control systems has a different way of organizing them. A distributed control system is intended to be used for a large processing facility that involves as many as fifty to one hundred loops. Examples include a refinery, a brewery, a power plant, and the like. In Fig. 35.6, the modules of control equipment that communicate with each other are as follows. Control processor (CP) Applications processor (AP) Workstation (W!$) Fieldbus module (FBM) The first three of these modules communicate with each other through a nodebus or data highway, as it has been called. The fieldbus modules serve as devices that interface with transducers and valves in the process. The control processor contains the blocks described earlier (analog input, analog output, control, linearization, etc.) that are connected together by softwiring to provide the control algorithm required for each loop. Communication between the control processor and the process (a distance away) in the field takes place in the fieldbus module. l%vo types of fieldbus modules are available. One type provides a set of analog inputs and a set of analog outputs that send to and receive from the field continuous signals (4-20 ma). The other type of module
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Process
FIGURE 35-6 Apical connections in a distributed control system: CP: control processor, AP: applications processor, WS: workstation, FBM: fieldbus module.
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sends to and receives from the field digital signals that often take the form of switch-contact closures~ The application processor is a microprocessor (or computer) in which the programs (or software) are stored for performing the many tasks described earlier and for managing the communication among modules. The workstation module is connected to a keyboard, a mouse, a monitor, and a printer for use by process operators to interact with the system. At the workstation, the process operator can call up on the screen various displays, change set points and controller parameters, switch from automatic to manual, acknowledge alarms, and perform other tasks needed to operate a control system consisting of many loops. A control system can also be configured as an offline task at the workstation. After configuration, the configured control system is downloaded to the control processor. If necessary, more than one workstation can be attached to the nodebus in order to provide communication at several locations in a plant. If more than one workstation is used, only one of them should have the authority at a given time to be in charge of the control system.
SUMMARY
During the past 15 years, the computer has greatly changed the nature of industrial process control equipment. The microprocessor has become the heart of control instruments and the computer programs stored in the memory of the hardware have provided many functions besides the basic control algorithm. When the pneumatic controller was the predominant type, one purchased a controller with very specific attributes (e.g., mode of control, type of measured variable, chart speed, etc.}. The microprocessor-based control instruments available today contain not only the conventional control algorithms, but many other functions such as simulation of basic transfer functions (e.g., lead-lag and transport lag), display-building, mathematical functions, process and diagnostic alarms and data acquisition. The modem instruments also provide logic functions (comparators, timers, counters, etc.) for use in batch control and plant start-up and shutdown. Recently, self-tuning algorithms have been added to the microprocessor-based instruments. In this chapter, some of the special features of modem controllers were discussed (e.g., limiting, tracking, and anti-reset windup). Any controller having integral action can cause reset windup under certain conditions when the error persists for a long time. The result of such a phenomenon is a transient that has large overshoot. Manufacturers of control instruments now offer several methods for reducing reset windup; the one presented in this chapter was use of external feedback. Before computer control appeared, most process loops were served by individual controllers with signals to and from these controllers being collected on a large panel board in a special control room. To obtain communication between the control room and the controllers required much wiring and piping (for pneumatic systems). Today, microprocessor-based control systems have the capability of communicating with other control instruments through networks, called dis-
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