PLC相关的外文英语文献与翻译.doc

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1、Four short words sum up what has lifted most successful individuals above the crowd: a little bit more.-author-datePLC相关的外文英语文献与翻译The Programmable Logic ControllerRelaysThe Programmable Logic ControllerEarly machines were controlled by mechanical means using cams, gears, levers and other basic mecha

2、nical devices. As the complexity grew, so did the need for a more sophisticated control system. This system contained wired relay and switch control elements. These elements were wired as required to provide the control logic necessary for the particular type of machine operation. This was acceptabl

3、e 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 and necessary, a more versatile means of controlling this equipment had to be developed. Hardwired relay and switch logic was cumbersome an

4、d 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 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 syst

5、em. 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 late 1960s and early 1970s and the result was the programmable logic controller, or PLC. Automotive plants were confronted with a change i

6、n 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 PLC provided an easy way to reprogram the wiring rather than actually rewiring the control system.The PLC that was developed during this t

7、ime was not very easy to program. The language was cumbersome to write and required highly trained programmers. These early devices were merely relay replacements and could do very little else. The PLC has at first gradually, and in recent years rapidly developed into a sophisticated and highly vers

8、atile control system component. Units today are capable of performing complex math functions including numerical integration and differentiation and operate at the fast microprocessor speeds now available. Older PLCs were capable of only handling discrete inputs and outputs (that is, on-off type sig

9、nals), 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 personal computer cousin, they can typically withstand vibration, shock, elevated temperatures, and electri

10、cal 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 expanding. This is necessary to allow the programming of these advanced capabilities. Also, manufacturers tend

11、to develop their own versions of ladder logic language (the language used to program PLCs). This complicates learning to program PLCs in general since one language cannot be learned that is applicable to all types. However, as with other computer languages, once the basics of PLC operation and progr

12、amming 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 comfortable to program and has the capabilities suited to his or her area of a

13、pplications.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 “PC” because the abbreviation “PC” is usually used in reference to

14、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 programmable logic controllers) are much like personal computers in that the user can be overwhelmed by the vast array of options and configurations ava

15、ilable. Also, like personal computers, the best teacher of which one to select is experience. As one gains experience with the various options and configurations available, it becomes less confusing to be able to select the unit that will best perform in a particular application. The typical system

16、components for a modularized PLC are:1. Processor.The processor (sometimes call a CPU), 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 h

17、igher math functions, PID control loops and optional programming commands. The processor consists of the microprocessor, system memory, serial communication 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 modu

18、les.2. Mounting rack.This is usually a metal framework with a printed circuit board backplane which provides means for mounting the PLC input/output (I/O) modules and processor. Mounting racks are specified according to the number of modules required to implement the system. The mounting rack provid

19、es data and power connections to the processor and modules via the backplane. For CPUs that do not contain a power supply, the rack also holds the modular power supply. There are systems in which the processor is mounted separately and connected by cable to the rack. The mounting rack can be availab

20、le to mount directly to a panel or can be installed in a standard 19 wide equipment cabinet. Mounting racks are cascadable so several may be interconnected to allow a system to accommodate a large number of I/O modules.3. Input and output modules.Input and output (I/O) modules are specified accordin

21、g to the input and output signals associated with 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 modul

22、e. Modules are specified as input or output but 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.

23、Analog input and output modules are available and 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 avai

24、lable which can directly accept thermocouple inputs 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

25、 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 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

26、 words are generally numbers (BCD or Binary) which 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 specializ

27、ed communication modules to allow for the transfer 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 th

28、e manufacturers PLC being utilized in the application. 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

29、 the sum of all the currents needed by the other 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 throug

30、h the addition of a second power source. This is 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

31、 edit the program to be executed. In its simplest 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

32、, edit and download the program to the PLC. This 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,

33、 timers and other items not visible externally 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

34、is via a cable connected to a special programming 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 pa

35、rts 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 the program commands retrieved from memory and acts on those commands. In present day PLCs this unit is a mi

36、croprocessor based system. The CPU is housed in 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

37、memory is generally kept alive with an on-board battery so that ladder programming is not lost when the system power is removed. This battery can be a standard dry cell or rechargeable nickel-cadmium type. Newer PLC units are now available with Electrically Erasable Programmable Read Only Memory (EE

38、PROM) which does not require a battery. Memory is also housed in the processor module in modular systems.Input units can be any of several different types depending on input signals expected as described above. The input section can accept discrete or analog signals of various voltage and current le

39、vels. Present day controllers offer discrete signal inputs of both AC and DC voltages from TTL to 250 VDC and from 5 to 250 VAC. Analog input units can accept input levels such as 10 VDC, 5 VDC and 4-20 ma. current loop values. Discrete input units present each input to the CPU as a single 1 or 0 wh

40、ile analog input units contain analog to digital conversion circuitry and present the input voltage to the CPU as binary number normalized to the maximum count available from the unit. The number of bits representing the input voltage or current depends upon the resolution of the unit. This number g

41、enerally contains a defined number of magnitude bits and a sign bit. Register input units present the word input to the CPU as it is received (Binary or BCD).Output units operate much the same as the input units with the exception that the unit is either sinking (supplying a ground) or sourcing (pro

42、viding a voltage) discrete voltages or sourcing analog voltage or current. These output signals are presented as directed by the CPU. The output circuit of discrete units can be transistors for TTL and higher DC voltage or Triacs for AC voltage outputs. For higher current applications and situations

43、 where a physical contact closure is required, mechanical relay contacts are available. These higher currents, however, are generally limited to about 2-3 amperes. The analog output units have internal circuitry which performs the digital to analog conversion and generates the variable voltage or cu

44、rrent output.The first thing the PLC does when it begins to function is update I/O. This means that all discrete input states are recorded from the input unit and all discrete states to be output are transferred to the output unit. Register data generally has specific addresses associated with it fo

45、r both input and output data referred to as input and output registers. These registers are available to the input and output modules requiring them and are updated with the discrete data. Since this is input/output updating, it is referred to as I/O Update. The updating of discrete input and output

46、 information is accomplished with the use of input and output image registers set aside in the PLC memory. Each discrete input point has associated with it one bit of an input image register. Likewise, each discrete output point has one bit of an output image register associated with it. When I/O up

47、dating occurs, each input point that is ON at that time will cause a 1 to be set at the bit address associated with that particular input. If the input is off, a 0 will be set into the bit address. Memory in todays PLCs is generally configured in 16 bit words. This means that one word of memory can

48、store the states of 16 discrete input points. Therefore, there may be a number of 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 all discrete inputs and the status of the output image register

49、is transferred to the output unit. This transfer of information typically only occurs at I/O update. It may be forced to occur at other times in PLCs which have an Immediate I/O Update command. This command will force the PLC to update the I/O at other times although this would be a special case.Before a study of PLC programming can begin, it is important to gain a fundamental understanding of the various t