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1、外文原文：Introduction to PLCIn order to use the advantages of all those controllers and eliminate the difficulties, the programmable logic controllers were invented. programmable logic controllers (PLCs), also referred to as programmable controllers, are in the computer family.A PLC was replacement for
2、relay devices. They are programmed using a ladder diagram,which is standard electric wiring diagram. As PLC become more flexible, high-level as well as low-level languages are available to PLC programmers. PLC have the flexibility of computers as well as a standard and ersy interface with processes
3、and other devices. They are used in commercial and applications. They are widely acceped in industry for controlling from a single device to a complex manufacturing facility.programmable logic controllers were first introduced in 1968 as a substitute for hardwired relay panels. The original intent w
4、as to replace a mechanical switching device (relay modules).However,since 1968, the capabilities of the PLC were to replace relay panels,modern PLC have many more functions. Their use extends from simple process control to manufacturing system controls and monitoring.They are used for high-speed dig
5、ital processing, high-speed digital communication, high-level computer-language support, and, of course, for basic control.PLC vary in size and power. A large PLC can have up to 10000 I/O points and support all the functions discussed earlier. There are expansion slots to accommodate PC and oter com
6、munitions decices. For many applications, a small PLC is sufficient. The speed of PLC is constantly improving, even the low-end PLC perform at high speed. One to two microsec / kbyte of memory speed is very common.Programmable logic controllers ArchitectureThe basic components in a relay panel circu
7、it and the design of a relay ladder diagram have been discussed. In the implementation of such a circuit, not only doesthe logic problem have to be thought out, but alse the electrical compatibility and wiring layout have to be considered. Breadboarding a relay panel circuit is a tedious task; it re
8、quires a lot of careful planning and work. The debugging and changing of a circuit is even more difficult. Programmable logic controllers replace most of this wiring by software programming. Therefore, the task is made much easier, because the wires and the moving mechanical components (relay contac
9、ts) are mostly replaced by software, the system is much more reliable.Like a general-purpose computer, a programmable controller consists of five major parts; CPU (processor), memory, input/out (I/O), power supply, and peripherals.Programmable logic controllers Functions and Configurations.1 Basic P
10、LC OperationPLC consist of input modules or poits, a Central Processing Unit (CPU), and output modules or points. An input accepts a variety of digital oranalog signals from various field devices (sensors) and converts them into a logic signal that can be used by the CPU. The CPU makes decisions and
11、 executes control instructions based on program instructions in memory. Output modules convert convert control instructions from the CPU into a digital or analog signal that can be used to control various field devices (actuators). A programming device is used to input the desired instructions. Thes
12、e instructions determine what the PLC will do for a specific input. An operator interface device allows process information to be displayed and new control paramenters to be entered. Pushbuttons (sensors), connected to PLC inputs, can be used to start and stop a motor connected to a PLC through a mo
13、tor starter (actuator).2 Hard-Wired Control Prior to PLC many of these control tasks were solved by contactors or relay controls. This is often referred to as hardwired control. Circuit diagrams had to be designed. Electrical components specified and installed, and wiring lists created. Electricians
14、 would then wire the components necessary to perform a specific task. If an error was made the wires had to be reconnected correctly. A change in function or expansion required wxtensive component changes and rewiring.3 Advantages of PLC The same, as well as more complex tasks, can be done with a PL
15、C. Wiring between devices and relay cintacts is done in the PLC program. Hard-wiring, though still required to connect field device, is less intensive. Modifying the application and correcting errors are easier to handle. It is easier to create and chang a program in a PLC than it is to wire and rew
16、ire a circuit.Following are just a few of the advantages of PLC:Smaller physical size than hard-wire solution.Easier and faster to make changes.PLC have integrated diagnostics and override dunction.Applications can be immediately documented.Applications can be duplicated faster and less expensively.4 Siements PLCSiements makes several PLC product lines in the SIMATIC S7 family. They are: S7-200, S7-300, S7-400.(1) S7-200The S7-200 is
