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1、RelaysThe Programmable Logic ControllerEarly machines were controlled by mechanical means using cams, gears, levers and other basic mechanical devices. As the complexity grew, so did the need for a more sophisticated control system. This system contained wired relay and switch control elements. Thes
2、e elements were wired as required to provide the control logic necessary for the particular type of machine operation. This was acceptable for a machine that never needed to be changed or modified, but as manufacturing techniques improved and plant changeover to new products became more desirable an
3、d necessary, a more versatile means of controlling this equipment had to be developed. Hardwired relay and switch logic was cumbersome and time consuming to modify. Wiring had to be removed and replaced to provide for the new control scheme required. This modification was difficult and time consumin
4、g to design and install and any small bug in the design could be a major problem to correct since that also required rewiring of the system. A new means to modify control circuitry was needed. The development and testing ground for this new means was the U.S. auto industry. The time period was the l
5、ate 1960s and early 1970s and the result was the programmable logic controller, or PLC. Automotive plants were confronted with a change in manufacturing techniques every time a model changed and, in some cases, for changes on the same model if improvements had to be made during the model year. The P
6、LC provided an easy way to reprogram the wiring rather than actually rewiring the control system.The PLC that was developed during this time was not very easy to program. The language was cumbersome to write and required highly trained programmers. These early devices were merely relay replacements
7、and could do very little else. The PLC has at first gradually, and in recent years rapidly developed into a sophisticated and highly versatile control system component. Units today are capable of performing complex math functions including numerical integration and differentiation and operate at the
8、 fast microprocessor speeds now available. Older PLCs were capable of only handling discrete inputs and outputs , while todays systems can accept and generate analog voltages and currents as well as a wide range of voltage levels and pulsed signals. PLCs are also designed to be rugged. Unlike their
9、personal computer cousin, they can typically withstand vibration, shock, elevated temperatures, and electrical noise to which manufacturing equipment is exposed.As more manufacturers become involved in PLC production and development, and PLC capabilities expand, the programming language is also expa
10、nding. This is necessary to allow the programming of these advanced capabilities. Also, manufacturers tend to develop their own versions of ladder logic language . This complicates learning to program PLCs in general since one language cannot be learned that is applicable to all types. However, as w
11、ith other computer languages, once the basics of PLC operation and programming in ladder logic are learned, adapting to the various manufacturers devices is not a complicated process. Most system designers eventually settle on one particular manufacturer that produces a PLC that is personally comfor
12、table to program and has the capabilities suited to his or her area of applications.It should be noted that in usage, a programmable logic controller is generally referred to as a PLC or programmable controller. Although the term programmable controller is generally accepted, it is not abbreviated P
13、C because the abbreviation PC is usually used in reference to a personal computer. As we will see in this chapter, a PLC is by no means a personal computer.Programmable controllers are much like personal computers in that the user can be overwhelmed by thevast array of options and configurations ava
14、ilable. Also, like personal computers, the bestteacher of which one to select is experience. As one gains experience with the variousoptions and configurations available, it becomes less confusing to be able to select the unitthat will best perform in a particular application.The typical system comp
15、onents for a modularized PLC are:1. Processor.The processor , as in the self contained units,is generally specified according to memory required for the program to beimplemented. In the modularized versions, capability can also be a factor.This includes features such as higher math functions, PID co
16、ntrol loops andoptional programming commands. The processor consists of themicroprocessor, system memory, serial communication ports for printer, PLCLAN link and external programming device and, in some cases, the systempower supply to power the processor and I/O modules.2. Mounting rack.This is usually a met
