外文翻譯--plc控制系統(tǒng)設計的要點-plc設計(已修改)
2025-06-01 08:23 本頁面
【正文】 Plc control system design elements Originally, the PLC was represented by the acronym PC. There was some confusion with using this acronym as it is monly accepted to represent personal puter. Therefore, PLC is now monly accepted to mean programmable logic contoller. A PLC is a userfriendly, microprocessorbased specialized puter that carries out control functions of many types and leves of plexity. Its purpose is to monitor crucial process parameters and adjust process operations accordingly. It can be programmed, controlled, and operated by a person unskilled in operating puter. Essentially, a PLC’s operator draws the lines and devices of ladder diagrams with a keyboard onto a display screen. The resulting drawing is converted into puter machine language and run as a user program. The puter takes the place of much of the external wiring required for control of a process. The PLC will operate any system that has output devices that go on and off (known as discrete, or digital, outputs). It can also operate any system with variable (analog) outputs. The PLC can be operated on the input side by onoff devices (discrete, or digital) or by variable (analog) input devices. Today, the big unit growth in the PLC industy is at the low endwhere small keeps getting smaller. When a few years ago the micro PLC entered the market, some thougt that these devices had “bottomed out”. Now,nano PLCsgenerally defined as those with 16 or fewer I/Oare spreading. Some can fit into your shirt pocket, being no larger than a deck of cards and at the time of this writing, PLC Direct plants to introduce a PLC the size of a box of TicTac candy that will include many features of current micro models. The first PLC systems evolved from conventional puters in the late 1960s and early 1970s. These first PLCs were installed primarily in automotive plants. Traditionally, The auto plants had to be shut down for up to a month at model changeover time. The early PLCs were used along with other new automation techniques to shorten the changeover time. One of the major timeconsuming changeover procedures had been the wiring of new or revised relay and control panels. The PLC keyboard reprogramming procedure replaced the rewiring of a panel full of wires, relays, timers and other ponents. The new PLCs helped reduce changeover time to a matter of few days. There was a major problem with these early 1970s puter/PLC reprogramming procedures. The programs were plicated and required a highly trained programmer to make the changes. Through the late 1970s, improvements were made in PLC programs to make them somewhat more user friendly。 in 1978, the introduction of the microprocessor chip increased puter power for all kinds of automotion systems and lowered the puting cost. Robotics, automotion devices, and puters of all types, including the PLC, consequently underwent many improvements. PLC programs, written in highlevel language, became more understandable to more people, and PLCs became more affordable. In the 1980s, with more puter power per dollar available, the PLC came into exponentially increasing use. Some large electronics and puter panies and some diverse corporate electronics divisions found that the PLC had bee their greatest volume product. The market for PLCs grew from a volume of $80 million in 1978 to $1 billion pe
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