机电专业英语 教学课件 作者 杨寿智 刘水平 机电专业英语.Lesson 28.pptVIP

Lesson 28 Programmable Logic Controllers Hardware Text Introduction A programmable logic controller（PLC）is a small, self-contained, rugged computer designed to control processes and events in an industrial environment—that is, to take over the job previously done with relay logic controllers①. Figure28.1 shows a number of different types of PLCs. Physically, they range in size from that of a camera to that of a shoe box（and sometimes larger）. Wires from switches, sensors, and other input devices are attached directly to the PLC; wires driving lights, small motors, motor starters, and other output devices are also connected directly to the PLC（Figure 28.2）. Each PLC contains a microprocessor that has been programmed to drive the output terminals in a specified manner, based on the signals from the input terminals②. The PLC program is usually developed on a separate computer such as a personal computer（PC）,using special software provided by the PLC manufacturer. Once the program has been written, it is transferred, or downloaded, into the PLC. From this point on, the PLC may operate on its own, as a completely independent controller. PLC Hardware Figure 28.3 shows the block diagram for a PLC and includes the fundamental parts found in any microprocessor system. These major functional blocks are explained next. Power Supply PLCs are usually powered directly from 120 or 240 Vac. The power supply converts the AC into DC voltages for the internal microprocessor components. It may also provide the user with a source of reduced voltage to drive switches, small relays, indicator lamps, and the like. Processor The processor is a microprocessor-based CPU and is the part of the PLC that is capable of reading and executing the program instructions, one-by-one（such as the rungs of a ladder logic program）. Program Memory The program memory receives and holds the downloaded program instructions from the programming device. If this memory is standard RAM, the program