PLC简介Introductions to PLC英文翻译.doc

Introductions to PLC A PLC(i.e. Programmable Logic Controller)is a device that was invented to replace the necessary sequential relay circuits for machine control. The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The user enters a program, usually via software or programmer, that gives the desired results. PLCs are used in many “real world” applications. If there is industry present, chances are good that there is a PLC present. If you are involved in machining, packaging, material handling, automated assembly or countless other industries, you are probably already using them. If you are not, you are wasting money and time. Almost any application that needs some type of electrical control has a need for a PLC. For example, let’s assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. What if the process also needed to count how many times the switch individually turned on? We need a lot of external counters. As you can see, the bigger the process the more of a need we have for a PLC. We can simply program the PLC to count its inputs and turn the solenoids on for the specified time. We will take a look at what is considered to be the “top 20” PLC instructions. It can be safely estimated that with a firm understanding of these instructions one can solve more than 80%of the applications in existence. That’s right, more than 80%! Of course we’ll learn more than just these instructions to help. You solve almost ALL your potential PLC applications. The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data, as shown in Fig.19.1.We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage lo