工科毕业设计英文翻译-PLC.doc

Programmable logic controller
Cynthia Cooper
From Wikipedia, the free encyclopedia
A programmable logic controller or simply programmable controller is a puter used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-puters, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.
Features
Control panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; power supply, controller, relay units for input and output.
The main difference from puters is that PLCs are armored for severe condition (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions plex positioning systems. Some even use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, ic relays or solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to work that plugs into the PLC.
PLCs were invented as replacements for automated systems that would use hundreds or thousands of relays, cam timers, and drum sequencers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels when production models changed.
Many of the