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1、英文资料翻译题 目 Programmable Logic Controllers (PLC) 院 别 中德机电学院 专 业 机电一体化技术 班 级 学生姓名 学 号 指导教师 2012年 4 月Programmable Logic Controllers (PLC)1、Motivation Programmable Logic Controllers (PLC), a computing device invented by Richard E. Morley in 1968, have been widely used in industry including manufacturing
2、systems, transportation systems, chemical process facilities, and many others. At that time, the PLC replaced the hardwired logic with soft-wired logic or so-called relay ladder logic (RLL), a programming language visually resembling the hardwired logic, and reduced thereby the configuration time fr
3、om 6 months down to 6 days Moody and Morley, 1999. Although PC based control has started to come into place, PLC based control will remain the technique to which the majority of industrial applications will adhere due to its higher performance, lower price, and superior reliability in harsh environm
4、ents. Moreover, according to a study on the PLC market of Frost and Sullivan 1995, an increase of the annual sales volume to 15 million PLCs per year with the hardware value of more than 8 billion US dollars has been predicted, though the prices of computing hardware is steadily dropping. The invent
5、or of the PLC, Richard E Morley, fairly considers the PLC market as a 5-billion industry at the present time. Though PLCs are widely used in industrial practice, the programming of PLC based control systems is still very much relying on trial-and-error. Alike software engineering, PLC software desig
6、n is facing the software dilemma or crisis in a similar way. Morley himself emphasized this aspect most forcefully by indicating Moody and Morley, 1999, p. 110:If houses were built like software projects, a single woodpecker could destroy civilization.” Particularly, practical problems in PLC progra
7、mming are to eliminate software bugs and to reduce the maintenance costs of old ladder logic programs. Though the hardware costs of PLCs are dropping continuously, reducing the scan time of the ladder logic is still an issue in industry so that low-cost PLCs can be used. In general, the productivity
8、 in generating PLC is far behind compared to other domains, for instance, VLSI design, where efficient computer aided design tools are in practice. Existent software engineering methodologies are not necessarily applicable to the PLC based software design because PLC-programming requires a simultane
9、ous consideration of hardware and software. The software design becomes, thereby, more and more the major cost driver. In many industrial design projects, more than 50% of the manpower allocated for the control system design and installation is scheduled for testing and debugging PLC programs Rockwe
10、ll, 1999. In addition, current PLC based control systems are not properly designed to support the growing demand for flexibility and reconfigurability of manufacturing systems. A further problem, impelling the need for a systematic design methodology, is the increasing software complexity in large-s
11、cale projects. PLCs (programmable logic controllers) are the control hubs for a wide variety of automated systems and processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches and relays to control equipment. They are programmable via software
12、interfaced via standard computer interfaces and proprietary languages and network options. Programmable logic controllers I/O channel specifications include total number of points, number of inputs and outputs, ability to expand, and maximum number of channels. Number of points is the sum of the inp
13、uts and the outputs. PLCs 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 channels in an expanded system. PLC system
14、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) available to PLC software. Data memory is the
15、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 PLCs include DC, AC, relay, analog, frequency or pulse, tra
16、nsistor, and triac. Programming options for PLCs 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 diagram (FBD), ladder diagram (LD), st
17、ructured 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 Diagram, Structured Text, and Instruc
18、tion 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 to build complex procedures in a gra
19、phical 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 is a text language used for comple
20、x 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 language for programmable logic contro
21、llers (PLCs). 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, reusable function libraries. C is a hig
22、h 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 functions. Programmable logic controlle
23、rs 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、Resume A PLC (programmable Logic Controller) is a device that was invented to rep
24、lace the necessary sequential relay circuits for control.The PLC works by looking at its input and depending upon their state, turning on/off its outputs. The user enters a program, usually via software or programmer, which gives the desired results. PLC is used in many real world applications. If t
25、here is industry present, chance are good that there is a PLC present. If you are involved in machining, packing, material handling, automated assembly or countless other industries, you are probably already using them. If you are not, you are wasting money and time. Almost any application that need
26、s some type of electrical control has a need for a PLC. For example, lets assume that when a switch turns on we want to turn a solenoid on for 5 second and then turn it off regardless of how long the switch is on for. We can do this with a simple external timer. But what if the process included 10 s
27、witches and solenoids? We should need 10 external times. What if the process also needed to count how many times the switch individually turned on? We need a lot of external counters. As you can see the bigger the process the more of a need we have for a PLC. We can simply program the PLC to count i
28、ts input and turn the solenoids on for the specified time. We will take a look at what is considered to be the top 20 PLC instructions. It can be safely estimated that with a firm understanding of these instructions one can solve more than 80% of the applications in existence. Of course we will lear
29、n more than just these instruction to help you solve almost ALL potential PLC applications. The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data. We can actually consider the PLC to be a box full of hundreds or thousands of separate relay, counters, t
30、imes and data storage locations,Do these counters,timers, etc. really exist? No,they dont physically exist but rather they simulated and be considered software counters, timers, etc. .These internal relays are simulated through bit locations in registers. What does each part do? Let me tell you. Inp
31、ut Relays These are connected to the outside world.They physically exsit and receive signals from switches,sensors,ect.Typically they are not relays but rather they are transistors. Internal Utility Relays These do not receive signals from the outside world nor do they physically exist.they are simu
32、lated relays and are what enables a PLC to eliminate external relays.There are also some special relays that are dedicated to performing only one task.Some are always on while some are always off.Some are on only once during power-on and are typically used for initializing data that was stored. Coun
33、ters These again do not physically exist. They are simulated counters and they can be programmed to count pulses.Typically these counters can count up,down or both up and down.Since they are simulated,they are limited in their counting speed.Some manufacturers also include high-speed counters that a
34、re hardware based.We think of these as physically existing.Most times these counters can count up,down or up and down. TimersThese also do not physically exist.They come in many varieties and increments.The most common type is an on-delay type.Others include off-delays and both retentive and non-ret
35、entive types.Increments vary from 1ms through 1s. Output Relays There are connected to the outside world.They physically exist and send on/off signals to solenoids,lights,etc.They can be transistors,relays,or triacs depending upon the model chosen. Data Storage Typically there are registers assigned
36、 to simply store data.They are usually used as temporary storage for math or data manipulation.They can also typically be used to store data when power is removed form the PLC.Upon power-up they will still have the same contents as before power was moved.Very convenient and necessary! A PLC works by
37、 continually scanning a program.We can think of this scan cycle as consisting of 3 important steps.There are typically more than 3 but we can focus on the important parts and not worry about the others,Typically the others are checking the system and updating the current internal counter and timer v
38、alues. Step 1 is to check input status,First the PLC takes a look at each input to determine if it is on off.In other words,is the sensor connected to the first input on?How about the third.It records this data into its memory to be used during the next step. Step 2 is to execute program.Next the PL
39、C executes your program one instruction at a time.Maybe your program said that if the first input was on then it should turn on the first output.Since it already knows which inputs are on/off from the previous step,it will be able to decide whether the first output should be turned on based on the s
40、tate of the first input.It will store the execution results for use later during the next step. Step 3 is to update output status.Finally the PLC updates the status the outputs.It updates the outputs based on which inputs were on during the first step and the results executing your program during th
41、e second step.Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true. After the third step the PLC goes back to step one repeats the steps continuously.One scan time is defined
42、as the time it takes to execute the 3 steps continuously.One scan time is defined as the time it takes to execute the 3 steps listed above.Thus a practical system is controlled to perform specified operations as desired. 3、 PLC Status The lack of keyboard, and other input-output devices is very noti
43、ceable on a PLC. On the front of the PLC there are normally limited status lights. Common lights indicate. power on - this will be on whenever the PLC has power. program running - this will often indicate if a program is running, or if no program is running. fault - this will indicate when the PLC h
44、as experienced a major hardware or software problem. These lights are normally used for debugging. Limited buttons will also be provided for PLC hardware. The most common will be a run/program switch that will be switched to program when maintenance is being conducted, and back to run when in produc
45、tion. This switch normally requires a key to keep unauthorized personnel from altering the PLC program or stopping execution. A PLC will almost never have an on-off switch or reset button on the front. This needs to be designed into the remainder of the system. The status of the PLC can be detected
46、by ladder logic also. It is common for programs to check to see if they are being executed for the first time, as shown in Figure 1. The first scan input will be true on the very first time the ladder logic is scanned, but false on every other scan. In this case the address for first scan in a PLC-5
47、 is S2:1/14. With the logic in the example the first scan will seal on light, until clear is turned on. So the light will turn on after the PLC has been turned on, but it will turn off and stay off after clear is turned on. The first scan bit is also referred to at the first pass bit.Figure 1 An pro
48、gram that checks for the first scan of the PLC4、Memory Types There are a few basic types of computer memory that are in use today. RAM (Random Access Memory) - this memory is fast, but it will lose its contents when power is lost, this is known as volatile memory. Every PLC uses this memory for the central CPU when running the PLC. ROM (Read Only Memory) - this memory is permanent and cannot be erased. It is often used for storing the operating system for the PLC. EPROM (Erasable Programmable Read Only Memory) - this is memory
