1、附录 附录 A 英文文献 Programmable logic controller Cynthia Cooper From Wikipedia, the free encyclopedia A programmable logic controller 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-
2、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 in battery-backed or non-volatile memory. A PLC is an exam
3、ple 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,
4、controller, relay units for input and output. The main difference from other computers 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
5、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, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The input/outp
6、ut 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. PLCs were invented as replacements for automated systems that would use hundreds or thousands of relays, cam timers, and drum sequencers. Often, a single P
7、LC 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 earliest PLCs expressed all decision
8、making logic in simple ladder logic which appeared similar to electrical schematic diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training demands for the existing technicians. Other earl
9、y PLCs used a form of instruction list programming, based on a stack-based logic solver. 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 po
10、wer and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications. Under the IEC 61131-3 standard, PLCs can be program
11、med using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. PLC compared with other control systems PLCs are well-adapted to a range of automation tasks. These are typically industrial processes
12、 in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation.PLCs contain input and output devices compatible with industrial pilot devices and controls.PLC applications are typically highly customized systems so the cost o
13、f 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。 For high vol
14、ume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities. A microcontroller-based design would be appropriate where hundreds or thousands of u
15、nits will be produced and so the development cost (design of power supplies and input/output hardware) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example； millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLCs instead of
