Measurement and Control Basics 3rd Edition (complete book)

The standard programming terminal is a personal computer, such as that shown in Figure 11-20, in which the programming software is loaded on the hard drive. These units can perform program editing and storage.

They also have added features such as the capacity to print out programs and to be connected to local area networks (LAN). LANs give the programmer or engineer access to any programmable controller in the communications network, so he or she can monitor or control any device in the network. Normally, laptop PCs are used as the programming terminal because they are light and portable and can be easily used in the field when the control program is tested, started up, and modified.

Power Supply

The power supply converts ac line voltages to dc voltages in order to power the electronic circuits in a programmable controller system. These power supplies rectify, filter, and regulate voltages and currents so as to supply the correct amounts of voltage and current to the system. The power supply normally converts 120 Vac or 240 Vac line voltage into direct current (dc) voltages such as +5 Vdc, –15 Vdc, or +15 Vdc.

You may integrate the power supply for a programmable controller system with the processor, memory, and I/O modules into a single housing. It may also be a separate unit connected to the system through a cable. As a system expands to include more I/O modules or special-function modules, most programmable controllers require an additional or auxiliary power supply to meet the increased power demand. Programmable controller power supplies are usually designed to eliminate the electrical noise present on the ac power and signal lines of industrial plants so it does not introduce errors into the control system. The power supplies are also designed to operate properly in the highertemperature and humidity environments present in most industrial applications.

Graphical User Interface

Two methods are commonly used to provide operators with color process graphics displays in programmable controller-based control systems. The first is to hardwire the programmable controller I/O modules to a graphics display panel using hardwired lights and digital indicators. This method is cost effective in a small system that will not be changed. It is generally not recommended for larger control systems that will be expanded in the future. The second method is to use an industrial-grade personal computer in combination with process color graphics software. The advantages of this personal computer-based method is that you can easily modify the process display screens for process changes, and the

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computer can perform other functions such as listing alarms, generating reports, and running programmable controller software.

The best way to explain these features is to use a personal computer-based graphical user interface (GUI) system on a typical process control system, as shown in Figure 11-21. The software for vendor-supplied GUI color displays is normally menu driven and relatively easy to use. The process display screens are usually based on the process and instrument drawings for the process being controlled.

For example, if we built a process display based on a dehydration process, the computer-generated display would be similar to the one shown in Figure 11-21. An advantage of a PC-based GUI is that process data and alarm messages can be displayed on the screen.

In the GUI graphics shown in Figure 11-21, Tower 1 is shown in service so that the Process Gas Inlet valve FV-1 and the Process Gas Outlet valve FV-7 are open and Tower 1’s Heating Gas Inlet valve FV-3 and Heating Gas Outlet Valve FV-5 are closed. Tower 2 is shown in regeneration, with valves FV-4 and FV-6 opened and valves FV-2 and FV-8 closed. Figure 11-21 shows the open valves with no fill color and the closed valves filled in with black. We can also display process conditions, such as tower pressure high or low, on the graphics screen, as shown in Figure 11-21.

The process values, such as valves open and closed, are transmitted to the GUI application with special communications interface software called drivers. You normally configure the GUI software using a crossreferenced table that lists the PLC input and output addresses that will drive or animate each point or information window on the graphic displays. Typically, many communications drivers are provided with the GUI software so it can interface with the various PLC manufacturer and PLC types. The GUI programmer simply selects the correct PLC manufacturer and model number for the application.

Plantwide Computer-based System

The modern plantwide data collection and control system combines features of DCS and programmable controllers. It also uses high-speed local area networks (LANs) to integrate all hardware and software capabilities into a complete production and business operations system.

A local area network (LAN) is a user-owned and -operated data transmission system that operates within a building or set of buildings. LANs allow many different machines and processes to exchange large amounts of information at high speed over a limited distance. LANs link together

Figure 11-21. Dehydration GUI display

communicating devices--such as computers, programmable controllers, process controllers, terminals, printers, and mass storage units--within a single process or manufacturing building or plant.

Communication networks enable computers to share data resources, hardware resources, and software. For example, a typical manufacturing facility might tie together the process control system computers and a central computer used by purchasing to accelerate the order process for the plant’s raw materials.

Figure 11-22 shows a typical plantwide computer-based control and data collection system for an industrial facility. It consists of three levels of control and data collection with three associated communication networks. The highest level (Level I) is the information network used by groups at the plant like accounting and purchasing. This network has the highest-

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Figure 11-22. Block diagram of plantwide control and data collection system

speed communication (100 Mbps and higher) network because it must handle large amounts of data and information. The middle level (Level II) is used to perform the automation and control of the industrial plant and normally has a communications network with speeds from 1 Mbps to 100 Mbps. This network is slightly slower than the level I network because process and machine control require somewhat lower data rates. The lowest level in the plant (Level III) is used to connect programmable control and other controllers directly to the field devices, such as weight, flow, pressure, level transmitters. The Level III switches and communication speed are generally less than 1 Mbps.

EXERCISES

11.1List some of the disadvantages of the early centralized control computers.

11.2Discuss the purpose and function of the four levels in a typical distributed control system.

11.3What are the functions and features of a typical DCS?

11.4Explain the operation and purpose of the processor in a typical programmable controller system.

11.5What is the main purpose of the input and output system in a PLC system?

11.6List some discrete input devices typically found in process industries.

11.7List some typical discrete output devices encountered in industrial applications.

11.8List some typical analog signal values found in process applications.

11.9Explain the difference between volatile and nonvolatile memory.

11.10List some common applications for personal computers in programmable controller systems.

11.11Which device is most commonly used to program PLCs?

11.12Discuss the three basic instructions used in ladder logic programs.

11.13What is power flow in a relay ladder diagram?

11.14Explain the concept of logical power flow in a ladder diagram program.

11.15What are the two basic structures used in STL instruction statements?

BIBLIOGRAPHY

1.Allen-Bradley Company. Micro Mentor – Understanding and Applying Micro Programmable Controller, Allen-Bradley Company, 1995.

2.Burton, D. P., and A. L. Dexter. Microprocessor Systems Handbook, Norwood, MA: Analog Devices, 1979.

3.Gilbert, R. A., and J. A. Llewellyn. Programmable Controllers — Practices and Concepts, Rockville, MD: Industrial Training Corporation, 1985.

4.Harrison, T. J. (ed.). Minicomputers in Industrial Control: An Introduction, Research Triangle Park, NC: ISA, 1978.

5.Jones, C. T., and L. A. Bryan. Programmable Controllers — Concepts and Applications, Atlanta, GA: International Programmable Controllers, 1983.

6.Peatman, J. B. Microprocessor-Based Design, New York: McGraw-Hill, 1979.

7.Webb, J. W., and R. A. Reis. Programmable Controller – Principles and Applications, 3rd ed., Prentice-Hall, 1995.

8.Williams, T. J. The Use of Digital Computers in Process Control, Research Triangle Park, NC: ISA, 1984.

342 Measurement and Control Basics

INSTRUMENT LINE SYMBOLS

(1) INSTRUMENT SUPPLY

OR CONNECTION TO PROCESS

(2) UNDEFINED SIGNAL

(3) PNUEMATIC SIGNAL

(5) HYDRAULIC SIGNAL

(6) CAPILLARY TUBE

(7) ELECTROMAGNETIC OR SONIC SIGNAL (GUIDED)

(8) ELECTROMAGNETIC OR SONIC SIGNAL (NOT GUIDED)

(9) INTERNAL SYSTEM LINK (SOFTWARE OR DATA LINK)

(10) MECHANICAL LINK

*The following abbreviations can be used to denote the types of supply: AS—Air Supply, IA—Instrument Air, PA—Plant Air, ES—Electric Supply, GS—Gas Supply, HS—Hydraulic Supply, NS—Nitrogen Supply, SS—Steam Supply, WS—Water Supply.

Table A-1. Instrument Line Symbols (1)

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FURTHER INFORMATION MAY BE ADDED ADJACENT TO THE BODY SYMBOL EITHER BY NOTE OR CODE NUMBER.

Figure A-2. Control valve body symbols, damper symbols. (Courtesy of ISA)