关于PLC的英文文献.doc

1. PROGRAMMABLE LOGIC CONTROLLERS
INTRODUCTION
Control engineering has evolved over time. In the past humans was 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 mon to use relays to make simple logical control decisions. The development of low puter has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has e the mon choice for manufacturing controls. PLC have been gaining popularity on the factory floor and will probably remain predominant for some time e. Most of this is because of the advantages they offer.
. Cost effective for plex 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.
. ponents make these likely to operate for years before failure.
Ladder Logic
Ladder logic is the main programming method used for PLC. 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