PLC相关的外文英语文献与翻译

1、邢唷? ?su?r欹? 餜?躾bjbj鲟鲟?攰攰TP$? 8S)?$?-?4?$? l顤)? !N?)0)? .o? .?2 .?)? .?: RelaysThe Programmable Logic ControllerEarly machines were controlled by mechanical means using cams, gears, levers and otherbasic mechanical devices. As the complexity grew, so did the need for a moresophisticated control syste

2、m. This system contained wired relay and switch controlelements. These elements were wired as required to provide the control logic necessaryfor the particular type of machine operation. This was acceptable for a machine thatnever needed to be changed or modified, but as manufacturing techniques imp

3、roved andplant changeover to new products became more desirable and necessary, a more versatilemeans of controlling this equipment had to be developed. Hardwired relay and switchlogic was cumbersome and time consuming to modify. Wiring had to be removed and replacedto provide for the new control sch

4、eme required. This modification was difficult andtime consuming to design and install and any small bug in the design could be a majorproblem to correct since that also required rewiring of the system. A new means to modifycontrol circuitry was needed. The development and testing ground for this new

5、 meanswas the U.S. auto industry. The time period was the late 1960s and early 1970s andthe result was the programmable logic controller, or PLC. Automotive plants wereconfronted with a change in manufacturing techniques every time a model changed and,in some cases,for changes on the same model if i

6、mprovements had to be made duringthe model year.The PLC provided an easy way to reprogram the wiring rather thanactually rewiring the control system.The PLC that was developed during this time was not very easy to program. The languagewas cumbersome to write and required highly trained programmers.

7、These early deviceswere merely relay replacements and could do very little else. The PLC has at firstgradually, and in recent years rapidly developed into a sophisticated and highlyversatile control system component. Units today are capable of performing complex mathfunctions including numerical int

8、egration and differentiation and operate at the fastmicroprocessor speeds now available. Older PLCs were capable of only handling discreteinputs and outputs (that is, on-off type signals), while todays systems can acceptand generate analog voltages and currents as well as a wide range of voltage lev

9、elsand pulsed signals. PLCs are also designed to be rugged. Unlike their personal computercousin, they can typically withstand vibration, shock, elevated temperatures, andelectrical noise to which manufacturing equipment is exposed.As more manufacturers become involved in PLC production and developm

10、ent, and PLCcapabilities expand, the programming language is also expanding. This is necessary toallow the programming of these advanced capabilities. Also, manufacturers tend todevelop their own versions of ladder logic langua wide equipment cabinet. Mountingracks are cascadable so several may be i

11、nterconnected to allow a system to accommodatea large number of I/O modules.3. Input and output modules.Input and output (I/O) modules are specified according to the input and output signalsassociated with the particular application. These modules fall into the categories ofdiscrete, analog, high sp

12、eed counter or register types.Discrete I/O modules are generally capable of handling 8 or 16 and, in some cases 32,on-off type inputs or outputs per module. Modules are specified as input or output butgenerally not both although some manufacturers now offer modules that can be configuredwith both in

13、put and output points in the same unit. The module can be specified as AConly, DC only or AC/DC along with the voltage values for which it is designed.Analog input and output modules are available and are specified according to the desiredresolution and voltage or current range. As with discrete mod

14、ules, these are generallyinput or output; however some manufacturers provide analog input and output in the samemodule. Analog modules are also available which can directly accept thermocouple inputsfor temperature measurement and monitoring by the PLC.Pulsed inputs to the PLC can be accepted using

15、a high speed countermodule. This modulecan be capable of measuring the frequency of an inputsignal from a tachometer or otherfrequency generating device. These modules can also count the incoming pulses if desired.Generally, both frequency and count are available from the same module at the same tim

16、eif both are required in the application.Register input and output modules transfer 8 or 16 bit words of information to and fromthe PLC. These words are generally numbers (BCD or Binary) which are generated fromthumbwheel switches or encoder systems for input or data to be output to a display device

17、by the PLC.Other types of modules may be available depending upon the manufacturer of the PLC andits capabilities. These include specialized communication modules to allow for thetransfer of information from one controller to another. One new development is an I/OModule which allows the serial trans

18、fer of information to remote I/O units that canbe as far as 12,000 feet away.4. Power supply.The power supply specified depends upon the manufacturers PLC being utilized in theapplication. As stated above, in some cases a power supply capable of delivering allrequired power for the system is furnish

19、ed as part of the processor module. If the powersupply is a separate module, it must be capable of delivering a current greater thanthe sum of all the currents needed by the other modules. For systems with the powersupply inside the CPU module, there may be some modules in the system which requireex

20、cessive power not available from the processor either because of voltage or currentrequirements that can only be achieved through the addition of a second power source.This is generally true if analog or external communication modules are present sincethese require ?DC supplies which, in the case of

21、 analog modules, must be well regulated.5. Programming unit.The programming unit allows the engineer or technician to enter and edit the programto be executed. In its simplest form it can be a hand held device with a keypad forprogram entry and a display device (LED or LCD)for viewing program steps

22、or functions,as shown. More advanced systems employ a separate personal computer which allows theprogrammer to write, view, edit and download the program to the PLC. This is accomplishedwith proprietary software available from the PLC manufacturer. This software alsoallows the programmer or engineer

23、 to monitor the PLC as it is running the program. Withthis monitoring system, such things as internal coils, registers, timers and other itemsnot visible externally can be monitored to determine proper operation. Also, internalregister data can be altered if required to fine tune program operation.

24、This can beadvantageouswhendebuggingtheprogram.Communicationwiththeprogrammablecontroller with this system is via a cable connected to a special programming port onthe controller. Connection to the personal computer can be through a serial port orfrom a dedicated card installed in the computer.A Pro

25、grammable Controller is a specialized computer. Since it is a computer, it hasall the basic component parts that any other computer has; a Central Processing Unit,Memory, Input Interfacing and Output Interfacing.The Central Processing Unit (CPU) is the control portion of the PLC. It interprets thepr

26、ogram commands retrieved from memory and acts on those commands. In present day PLCsthis unit is a microprocessor based system. The CPU is housed in the processor moduleof modularized systems.Memory in the system is generally of two types; ROM and RAM. The ROM memory containsthe program information

27、that allows the CPU to interpret and act on the Ladder Logicprogram stored in the RAM memory. RAM memory is generally kept alive with an on-boardbattery so that ladder programming is not lost when the system power is removed. Thisbattery can be a standard dry cell or rechargeable nickel-cadmium type

28、. Newer PLC unitsare now available with Electrically Erasable Programmable Read Only Memory (EEPROM)which does not require a battery. Memory is also housed in the processor module inmodular systems.input. If the input is off, a 0 will be set into the bit address. Memoryin todays PLCs is generally co

29、nfigured in 16 bit words. This means that one word ofmemory can store the states of 16 discrete input points. Therefore, there may be a numberof words of memory set aside as the input and output image registers. At I/O update,the status of the input image register is set according to the state of al

30、l discreteinputs and the status of the output image register is transferred to the output unit.This transfer of information typically only occurs at I/O update. It may be forced tooccur at other times in PLCs which have an Immediate I/O Update command. This commandwill force the PLC to update the I/

31、O at other times although this would be a specialcase.Before a study of PLC programming can begin, it is important to gain a fundamentalunderstanding of the various types of PLCs available, the advantages and disadvantagesof each, and the way in which a PLC executes a program. The open frame, shoebo

32、x, andmodular PLCs are each best suited to specific types of applications based on theenvironmental conditions, number of inputs and outputs, ease of expansion, and methodof entering and monitoring the program. Additionally, programming requires a priorknowledge of the manner in which a PLC receives

33、 input information, executes a program,and sends output information. With this information, we are now prepared to begin astudy of PLC programming techniques.When writing programs for PLCs, it is beneficial to have a background in ladderdiagramming for machine controls. This is basically the materia

34、l that was covered inChapter 1 of this text. The reason for this is that at a fundamental level, ladder logicprograms for PLCs are very similar to electrical ladder diagrams. This is nocoincidence.The engineers that developed the PLC programming language were sensitiveto the fact that most engineers

35、, technicians and electricians who work with electricalmachines on a day-to-day basis will be familiar with this method of representing controllogic. This would allow someone new to PLCs, but familiar with control diagrams, tobe able to adapt very quickly to the programming language. It is likely th

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