PLC单片机 外文翻译 外文文献 英文文献 可编程逻辑控制器PLC.docx

1、PLC单片机 外文翻译 外文文献 英文文献 可编程逻辑控制器PLCProgrammable Logic Controllers (PLC)1. About Programmable Logic Controllers (PLC)Everyone knows theres only one constant in the technology world, and thats change. This is especially evident in the evolution of Programmable Logic Controllers (PLC) and their varied ap

2、plications. From their introduction more than 30 years ago, PLC has become the comer stone of hundreds of thousands of control systems in a wide range of industries.At heart, the PLC is an industrialized computer programmed with highly specialized languages, and it continues to benefit from technolo

3、gical advances in the computer and information technology worlds. The most prominent of which is miniaturization and communications. Programmable logic controllers I/O channel specifications include total number of points, number of inputs and outputs, ability to expand, and maximum number of channe

4、ls. Number of points is the sum of the inputs and the outputs. PLC may be specified by any possible combination of these values. Expandable units may be stacked or linked together to increase total control capacity. Maximum number of channels refers to the maximum total number of input and output ch

5、annels in an expanded system. PLC system specifications to consider include scan time, number of instructions, data memory, and program memory. Scan time is the time required by the PLC to check the states of its inputs and outputs. Instructions are standard operations (such as math functions) avail

6、able to PLC software. Data memory is the capacity for data storage. Program memory is the capacity for control software. Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs. Outputs for PLC include DC, AC

7、, relay, analog, frequency or pulse, transistor, triac. Programming options for PLC include front panel, hand held, and computer. Programmable logic controllers use a variety of software programming languages for control. These include IEC 61131-3, sequential function chart (SFC), function block dia

8、gram (FBD), ladder diagram (LD), structured text (ST), instruction list (IL), relay ladder logic (RLL), flow chart, C, and Basic. The IEC 61131-3 programming environment provides support for five languages specified by the global standard: Sequential Function Chart, Function Block Diagram, Ladder Di

9、agram, Structured Text, and Instruction List. This allows for multi-vendor compatibility and multi-language programming. SFC is a graphical language that provides coordination of program sequences, supporting alternative sequence selections and parallel sequences. FBD uses a broad function library t

10、o build complex procedures in a graphical format. Standard math and logic functions may be coordinated with customizable communication and interface functions. LD is a graphic language for discrete control and interlocking logic. It is completely compatible with FBD for discrete function control. ST

11、 is a text language used for complex mathematical procedures and calculations less well suited to graphical languages. IL is a low-level language similar to assembly .code. It is used in relatively simple logic instructions. Relay Ladder Logic (RLL), or ladder diagrams, is the primary programming la

12、nguage for programmable logic controllers (PLC). Ladder logic programming is a graphical representation of the program designed to look like relay logic. Flow Chart is a graphical language that describes sequential operations in a controller sequence or application. It is used to build modular, reus

13、able function libraries. C is a high level programming language suited to handle the most complex computation, sequential, and data logging tasks. It is typically developed and debugged on a PC. BASIC is a high level language used to handle mathematical, sequential, data capturing and interface func

14、tions. Programmable logic controllers can also be specified with a number of computer interface options, network specifications and features. PLC power options, mounting options and environmental operating conditions are all also important to consider.2. INTRODUCTIONFor simple programming the relay

15、model of the PLC is sufficient. As more complex functions are used the more complex VonNeuman model of the PLC must be used. A VonNeuman computer processes one instruction at a time. Most computers operate this way, although they appear to be doing many things at once. Consider the computer componen

16、ts shown in Figure 1.Figure 1 Simplified Personal Computer ArchitectureInput is obtained from the keyboard and mouse, output is sent to the screen, and the disk and memory are used for both input and output for storage. (Note: the directions of these arrows are very important to engineers, always pa

17、y attention to indicate where information is flowing.) This figure can be redrawn as in Figure 2 to clarify the role of inputs and outputs.Figure 2 An Input-Output Oriented ArchitectureIn this figure the data enters the left side through the inputs. (Note: most engineering diagrams have inputs on th

18、e left and outputs on the right.) It travels through buffering circuits before it enters the CPU. The CPU outputs data through other circuits. Memory and disks are used for storage of data that is not destined for output. If we look at a personal computer as a controller, it is controlling the user

19、by outputting stimuli on the screen, and inputting responses from the mouse and the keyboard.A PLC is also a computer controlling a process. When fully integrated into an application the analogies become;Inputs - the keyboard is analogous to a proximity switch.Input -circuits - the serial input chip

20、 is like a 24Vdc input card.Computer - the 686 CPU is like a PLC CPU unit.Output - circuits - a graphics card is like a triac output card.Outputs - a monitor is like a light.Storage - memory in PLC is similar to memories in personal computers.It is also possible to implement a PLC using a normal Per

21、sonal Computer, although this is not advisable. In the case of a PLC the inputs and outputs are designed to be more reliable and rugged for harsh production environments. 3. OPERATION SEQUENCEAll PLC have four basic stages of operations that are repeated many times per second. Initially when turned

22、on the first time it will check its own hardware and software for faults. If there are no problems it will copy all the input and copy their values into memory, this is called the input scan. Using only the memory copy of the inputs the ladder logic program will be solved once, this is called the lo

23、gic scan. While solving the ladder logic the output values are only changed in temporary memory. When the ladder scan is done the outputs will be updated using the temporary values in memory, this is called the output scan. The PLC now restarts the process by starting a self check for faults. This p

24、rocess typically repeats 10 to 100 times per second as is shown in Figure 3.Figure 3 PLC Scan CycleSELF TEST - Checks to see if all cards error free, reset watch-dog timer, etc. (A watchdog timer will cause an error, and shut down the PLC if not reset within a short period of time - this would indic

25、ate that the ladder logic is not being scanned normally).INPUT SCAN - Reads input values from the chips in the input cards, and copies their values to memory. This makes the PLC operation faster, and avoids cases where an input changes from the start to the end of the program (e.g., an emergency sto

26、p). There are special PLC functions that read the inputs directly, and avoid the input tables.LOGIC SOLVE/SCAN - Based on the input table in memory, the program is executed 1 step at a time, and outputs are updated. This is the focus of the later sections.OUTPUT SCAN - The output table is copied fro

27、m memory to the output chips. These chips then drive the output devices.The input and output scans often confuse the beginner, but they are important. The input scan takes a snapshot of the inputs, and solves the logic. This prevents potential problems that might occur if an input that is used in mu

28、ltiple places in the ladder logic program changed while half way through a ladder scans. This problem could have severe effects on complex programs that are developed later in the book. One side effect of the input scan is that if a change in input is too short in duration, it might fall between inp

29、ut scans and be missed.When the PLC is initially turned on the normal outputs will be turned off. This does not affect the values of the inputs.4The Input and Output ScansWhen the inputs to the PLC are scanned the physical input values are copied into memory. When the outputs to a PLC are scanned th

30、ey are copied from memory to the physical outputs. When the ladder logic is scanned it uses the values in memory, not the actual input or output values. The primary reason for doing this is so that if a program uses an input value in multiple places, a change in the input value will not invalidate t

31、he logic. Also, if output bits were changed as each bit was changed, instead of all at once at the end of the scan the PLC would operate much slower.5 The Logic ScanLadder logic programs are modelled after relay logic. In relay logic each element in the ladder will switch as quickly as possible. But

32、 in a program elements can only be examines one at a time in a fixed sequence. Consider the ladder logic in Figure 4, the ladder logic will be interpreted left-to-right, top-to-bottom. In the figure the ladder logic scan begins at the top rung. At the end of the rung it interprets the top output first, then the output branched below it. On the second rung it solves branches, before moving along the ladder logic rung.Figure 4 Ladder Logic Execution SequenceIt also becomes important when considering output usage. Consider Figure 5, the first line of ladder logic will examine input A