附录二 外文文献及其翻译
Programmable logic controller
Cynthia cooper
From Wikipedia, the free encyclopedia
A programmable logic controller or simply programmable controller is a digital computer used for  of industrial processes, such as control of machinery on factory . 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 in battery-backed or  memory. A PLC is an example of a  system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.
Featurescontroller翻译中文
Control panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; , controller,  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  (I/O) arrangements. These connect the PLC to  and . PLCs read limit , analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some even use . On the actuator side, PLCs operate ,  or  cylinders, magnetic  or , 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.
PLCs were invented as replacements for automated systems that would use hundreds or thousands of , , and . Often, a single PLC can be programmed to replace thousands of . 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 making logic in simple  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 early PLCs used a form of  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, ,  and . The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to . PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.
Under the  standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called  is available on certain programmable controllers.
PLC compared with other control systems
PLCs are well-adapted to a range of  tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the  contain input and output devices compatible with industrial pilot devices and  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。
For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer  would be controlled by an electromechanical  costing only a few dollars in production quantities.
A -based design would be appropriate where hundreds or thousands of units 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 custom-designed controls, because the volumes are low and the development cost would be uneconomic.
Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls.

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。