自动化英文文献翻译(2)

Backup and maintenance methods can make a real system very complicated. Programming

PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Often, a single PLC can be programmed to replace thousands of relays.

Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.

Recently, the International standard

IEC 61131-3

has become popular. IEC 61131-3

currently defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list, similar to assembly language) and SFC (Sequential function chart). These techniques emphasize logical organization of operations.

While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible. History Origin

The PLC was invented in response to the needs of the American automotive manufacturing industry. Programmable controllers were initially adopted by the automotive industry where software revision replaced the re-wiring of hard-wired control panels when production models changed.

Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was accomplished using hundreds or thousands of relays, cam timers, and drum

sequencers and dedicated closed-loop controllers. The process for updating such facilities for the

yearly model change-over was very time consuming and expensive, as the relay systems needed to be rewired by skilled electricians.

In 1968 GM Hydramatic (the automatic transmission division of General Motors) issued a

request for proposal for an electronic replacement for hard-wired relay systems.

The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates' eighty-fourth project, was the result. Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked on that project was Dick Morley, who is considered to be the \of the PLC. The Modicon brand was sold in 1977 to Gould Electronics, and later acquired by German Company AEG and then by French Schneider Electric, the current owner.

One of the very first 084 models built is now on display at Modicon's headquarters in

North Andover, Massachusetts. It was presented to Modicon by GM, when the unit was retired after

nearly twenty years of uninterrupted service.

The automotive industry is still one of the largest users of PLCs, and Modicon still numbers some of its controller models such that they end with eighty-four. Development

Early PLCs were designed to replace relay logic systems. These PLCs were programmed in \ladder logic\which strongly resembles a schematic diagram of relay logic. Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional programming languages such as BASIC and C. Another method is

State Logic, a Very High Level Programming

Language designed to program PLCs based on State Transition Diagrams.

Many of the earliest PLCs expressed all decision 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 early PLCs used a form of instruction list programming, based on a stack-based logic solver. Programming

Early PLCs, up to the mid-1980s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were very minimal due to lack of memory capacity. The very oldest PLCs used non-volatile magnetic core memory.

Functionality

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 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. Suppliers

Well known PLC brands include Siemens, Allen-Bradley, IDEC, ABB, Mitsubishi, Omron, Honeywell,

Schneider Electric, Saia-Burgess Controls, and General Electric.

可编程控制器

摘自Wikipedia

PLC和输入输出接口的排布

一台PLC或者称为程序控制器是用于机电一体化进程的全自动工控机，如对机械装配流水线、汽车驾乘、照明系统的控制。PLC可用于许多不同的仪器设备上，比如：包装机械和半导体仪器。PLC不同于平常使用的计算机，它的开发源自于大量的输入与输出信号的排布，能够在较大的温度范围下保持良好的工作状态，不受电磁噪音的干扰，不受震动和撞击的影响。用于控制机械运作的程序被存储在使用电池的记忆体或者具有闪存作用的存储器中。PLC是典型的在线系统，在有输入信 …… 此处隐藏：5538字，全部文档内容请下载后查看。喜欢就下载吧 ……