本科毕业设计论文--可编程序控制器--外文文献翻译.doc

英语原文：

Programmable logic controller

A programmable logic controller (PLC) or simply programmable controller is a digital

computer used for automation of industrial processes, such as control of machinery on factory

assembly lines. Unlike general-purpose computers, 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

arrangements. These connect the PLC to sensors and actuators. PLC read limit switches,

analog process variables (such as temperature and pressure), and the positions of complex

positioning systems. Some even use machine vision. On the actuator side, PLC operate

electric motors, pneumatic or hydraulic cylinders, magnetic 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 a computer network that plugs into the PLC.

PLC 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

The functionality of the PLC has evolved over the years to include sequential relay

control, motion control, process control, distributed control systems and networking. The data

handling, storage, processing power and communication capabilities of some modern PLC are

approximately equivalent to desktop computers. PLC-like programming combined with

remote I/O hardware, allow a general-purpose desktop computer to overlap some PLC in

certain applications.

PLC compared with other control systems

system is high relative to the total cost of the automation, and where changes to the system

would be expected during its operational life. PLC contain input and output devices

compatible with industrial pilot devices and controls; little electrical design is required, and

the design problem centers on expressing the desired sequence of operations in ladder logic

notation. PLC applications are typically highly customized systems so the cost of a packaged

PLC is low compared to the cost of a specific custom-built controller design. On the other

hand, in the case of mass-produced goods, customized control systems are economic due to

the lower cost of the components, which can be optimally chosen instead of a "generic"

solution, and where the non-recurring engineering charges are spread over thousands of

Very complex process control, such as used in the chemical industry, may require

algorithms and performance beyond the capability of even high-performance PLC. Very

high-speed or precision controls may also require customized solutions; for example, aircraft

flight controls.

PLC may include logic for single-variable feedback analog control loop, a "proportional,

integral, derivative" or "PID controller." A PID loop could be used to control the temperature

of a manufacturing process, for example. Historically PLC were usually configured with only

a few analog control loops; where processes required hundreds or thousands of loops, a

distributed control system (DCS) would instead be used. However, as PLC have become more

powerful, the boundary between DCS and PLC applications has become less clear-cut.

Digital and analog signals

Digital or discrete signals behave as binary switches, yielding simply an On or Off signal

(1 or 0, True or False, respectively). Push buttons, limit switches, and photoelectric sensors

voltage or current, where a specific range is designated as On and another as Off. For example,

a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below

2VDC representing Off, and intermediate values undefined. Initially, PLC had only discrete