可编程逻辑控制器外文

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可编程逻辑控制器
    主题
PLC的历史
梯形图和继电器
PLC的编程 
PLC的运行
实例
   任务
熟悉PLC的一般事件
能够编写简单的梯形逻辑程序
理解PLC的运行
2.1说明
控制系统的发展已经有很长的历史。过去，手工控制是控制系统的主要方法，再近一些，电气就被用于控制，而早期的电气控制是基于继电器的。这些继电器不需要机械开关来接通和关断电源。使用继电器来制作一些简单的逻辑控制是很平常的。随着低成本计算机的发展，已经引发了新一轮的革命，可编程控制器（PLC）的产生。PLC出现于20世纪70年代，现在已经成为制造加工控制中最普遍的选择。
PLC已经在工厂车间受到欢迎，还可能在将要到来的未来占据统治地位。所有的这些都要归功于它所具有的优势。
低成本有效地控制复杂的系统
使用灵活且能简单快捷地运用于控制其他系统
它的计算能力能够被用于更加复杂的控制
可靠的零部件保证了其使用寿命
2.1.1逻辑梯形图
   逻辑梯形图是用于PLC控制的主要编程方法。正如前面所述，逻辑梯形图已经发展成为模拟的逻辑继电器。使用逻辑延迟图是一个有策略的决定。通过使用逻辑延迟图作为主要的编程方法，工程师和销售人员所需要的大量束缚将会被大幅地减少。

AMMABLE LOGIC CONTROLLERS
Topics:
  PLC History
  Ladder Logic and Relays
  PLC Programming
  PLC Operation
  An Example
Objectives:
  Know general PLC issues
  To be able to write simple ladder logic programs
  Understand the operation of a PLC
2.1 INTRODUCTION
Control engineering has evolved over time. In the past humans were the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls.
PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer.
  Cost effective for controlling complex systems.
  Flexible and can be reapplied to control other systems quickly and easily.
  Computational abilities allow more sophisticated control.
  Trouble shooting aids make programming easier and reduce downtime.
  Reliable components make these likely to operate for years before failure.
2.1.1 Ladder Logic
Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced.

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