关于PLC的英文文献.doc

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1. PROGRAMMABLE LOGIC CONTROLLERS
INTRODUCTION
Control engineering has evolved over time. In the past d contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open <non-conducting> when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts
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will be closed <conducting>.
Relays are used to let one power source close a switch for another <often high current> power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure . In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on.
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Programming
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