PLC相关外文英语文献与翻译.docx

1、PLC相关外文英语文献与翻译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

2、elements. These 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 mor

3、e desirable and 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

4、 time consuming 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 pe

5、riod was the late 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 mod

6、el year. The PLC 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

7、 replacements 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

8、operate at the fast microprocessor speeds now available. Older PLCs were capable of only handling discrete inputs and outputs (that is, on-off type signals), while todays systems can accept and generate analog voltages and currents as well as a wide range of voltage levels and pulsed signals. PLCs a

9、re also designed to be rugged. Unlike their 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 e

10、xpand, the programming language is also expanding. This is necessary to allow the programming of these advanced capabilities. Also, manufacturers tend to develop their own versions of ladder logic language (the language used to program PLCs). This complicates learning to program PLCs in general sinc

11、e one language cannot be learned that is applicable to all types. However, as with 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 sett

12、le on one particular manufacturer that produces a PLC that is personally comfortable 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”. Altho

13、ugh the term “programmable controller” is generally accepted, it is not abbreviated “PC” 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 (the shortened name used for p

14、rogrammable logic controllers) are much like personal computers in that the user can be overwhelmed by the vast array of options and configurations available. Also, like personal computers, the best teacher of which one to select is experience. As one gains experience with the various options and co

15、nfigurations available, it becomes less confusing to be able to select the unit that will best perform in a particular application. The typical system components for a modularized PLC are:1. Processor.The processor (sometimes call a CPU), as in the self contained units, is generally specified accord

16、ing 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 control loops and optional programming commands. The processor consists of the microprocessor, system memory, serial commun

17、ication ports for printer, PLC LAN link and external programming device and, in some cases, the system power supply to power the processor and I/O modules.2. Mounting rack.3. Input and output modules.Input and output (I/O) modules are specified according to the input and output signals associated wi

18、th the particular application. These modules fall into the categories of discrete, analog, high speed 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 but

19、 generally not both although some manufacturers now offer modules that can be configured with both input and output points in the same unit. The module can be specified as AC only, DC only or AC/DC along with the voltage values for which it is designed.Analog input and output modules are available a

20、nd are specified according to the desired resolution and voltage or current range. As with discrete modules, these are generally input or output; however some manufacturers provide analog input and output in the same module. Analog modules are also available which can directly accept thermocouple in

21、puts for temperature measurement and monitoring by the PLC.Pulsed inputs to the PLC can be accepted using a high speed countermodule. This module can be capable of measuring the frequency of an inputsignal from a tachometer or other frequency generating device. These modules can also count the incom

22、ing pulses if desired. Generally, both frequency and count are available from the same module at the same time if both are required in the application.Register input and output modules transfer 8 or 16 bit words of information to and from the PLC. These words are generally numbers (BCD or Binary) wh

23、ich are generated from thumbwheel switches or encoder systems for input or data to be output to a display device by the PLC. Other types of modules may be available depending upon the manufacturer of the PLC and its capabilities. These include specialized communication modules to allow for the trans

24、fer of information from one controller to another. One new development is an I/O Module which allows the serial transfer of information to remote I/O units that can be as far as 12,000 feet away.4. Power supply.The power supply specified depends upon the manufacturers PLC being utilized in the appli

25、cation. As stated above, in some cases a power supply capable of delivering all required power for the system is furnished as part of the processor module. If the power supply is a separate module, it must be capable of delivering a current greater than the sum of all the currents needed by the othe

26、r modules. For systems with the power supply inside the CPU module, there may be some modules in the system which require excessive power not available from the processor either because of voltage or current requirements that can only be achieved through the addition of a second power source. This i

27、s generally true if analog or external communication modules are present since these require DC supplies which, in the case of analog modules, must be well regulated.5. Programming unit.The programming unit allows the engineer or technician to enter and edit the program to be executed. In its simple

28、st form it can be a hand held device with a keypad for program entry and a display device (LED or LCD) for viewing program steps or functions, as shown. More advanced systems employ a separate personal computer which allows the programmer to write, view, edit and download the program to the PLC. Thi

29、s is accomplished with proprietary software available from the PLC manufacturer. This software also allows the programmer or engineer to monitor the PLC as it is running the program. With this monitoring system, such things as internal coils, registers, timers and other items not visible externally

30、can be monitored to determine proper operation. Also, internal register data can be altered if required to fine tune program operation. This can be advantageous when debugging the program. Communication with the programmable controller with this system is via a cable connected to a special programmi

31、ng port on the controller. Connection to the personal computer can be through a serial port or from a dedicated card installed in the computer.A Programmable Controller is a specialized computer. Since it is a computer, it has all the basic component parts that any other computer has; a Central Proc

32、essing Unit, Memory, Input Interfacing and Output Interfacing.The Central Processing Unit (CPU) is the control portion of the PLC. It interprets the program commands retrieved from memory and acts on those commands. In present day PLCs this unit is a microprocessor based system. The CPU is housed in

33、 the processor module of modularized systems.Memory in the system is generally of two types; ROM and RAM. The ROM memory contains the program information that allows the CPU to interpret and act on the Ladder Logic program stored in the RAM memory. RAM memory is generally kept alive with an on-board battery so that ladder programming i