外文翻译-可编程逻辑控制器.doc

1、翻译部分英文原文Programmable Logic Controller As mentioned previously, the programmable logic controller (PLC or PC) is by far the most important commercially available logic control device. Its development began in 1968 when General Motors Corp. solicited the electronics industry for an alternative to the

2、eletrome-chanical relay logic systems used to control machining, materials handling, and assembly in automobile manufacture. Their relay systems had become very large and complex and required a great amount of rewiring and even outright scrapping when changes in manufacturing processes were needed.

3、This became particularly critical during the years model changeovers typical of the auto industry. A number of manufacturers responded and the software-based programmable controller was born. It could duplicate all the logic control functions of relay networks, but implemented them in software (a pr

4、ogram), just like a computer. This meant that process revisions or complete model changeovers did not now require scrapping or tedious rewiring of relay systems, but instead needed only relatively quick and easy changes in the PLC s program.In 1969, PLCs were first actually used in the auto industry

5、 and by 1971 their utility in other industries was recognized and applied. By 1973 “smart” PLCs using arithmetic operations, printer control, matrix operations, CRT interface and the like began to appear. The capability to control “continuous” processes involving such things as pressure, temperature

6、, and flow feedback control loops using proportional, integral, and derivative control was added in about 1975, greatly increasing potential applications, since one machine could now be used to handle the logical and the continuous feedback aspects of system control in an integrated fashion. About 1

7、976, the coordination of several individual PLCs in a factory into an integrated computer managed overall manufacturing system began. To complement these advances int0 operations of larger and larger scale, very small PLCs (based on microprocessors) were now developed to economically replace systems

8、 of only 8 or 10 relays. Systems now range from those costing several hundred dollars and replacing 8 or 10 relays to $ 50, 000 items replacing hundreds of relays and a minicomputer.Although understanding the internal details of PLCs is neither necessary to use them nor appropriate for this text, Fi

9、g. I shows a general overview of the basic elements. All programmable controllers contain a central processing unit (CPU), memory (the established program), power supply, input /output(I/O) modules, and programming device for programming, editing, and trouble-shooting. The CPU, upon receiving instru

10、ctions from the memory (together with feedback on the status of the input/output devices), generates commands, to the outputs by means of the output module. These commands control the output elements on a machine or process. Devices such as relay coils, solenoid valves, indicator lamps, and motor st

11、arters are typical loads to be controlled. The machine or process input devices transmit status signal to the input modules, which in turn generate logic signals for use by the CPU. In this way the CPU monitors elements such as push buttons, selector switches, and relay contacts on a machine or proc

12、ess.The programming device is often detachable and shared among several PLCs since it is needed only when a program is being developed or altered. Once an operating program is in memory, the programmer can be removed. Although the structure of Fig. I is strongly reminiscent of general purpose comput

13、ers, recall that although the PLC is a computer-like device, it is industrially hardened for the factory-floor environment and the programming device speaks a high-level language (usually relay-ladder-network diagrams) making it easy to use by noncomputer specialists. These features have really been

14、 the key to its success. The larger PLCs are usually supplied with a CRT display for the programmer that presents a graphic picture whose appearance cosely resembles a traditional relay-ladder-network diagram. One can thus “draw” the control diagram one element at a time and actually see the element

15、 connections being established on the CRT display. A keyboard that has a key for each needed logic operation is used to enter the ladder diagram into the CRT display, one element at a time. When the CRT display show the desired final diagram, the PLC has been programmed and can be used to control th

16、e system. The PLC provides a variety of input/output modules to which the external input and output devices can be connected by ordinary hand wiring.Although I do not intend to present comprehensive performance specifications, I will mention the concept of scan time. A PLC simulates the action of relays in a serial rather than parallel fashion. In an actual relay network, many relays can be reacting simultaneously, whereas a PLC must “scan” through the entire network to