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1、PLC technique discussion and future developmentAlong with the development of the ages, the technique that is nowadays is also gradually perfect, the competition plays more strong; the operation that list depends the artificial has already cant satisfied with the current manufacturing industry foregr
2、ound, also cant guarantee the request of the higher quantity and high new the image of the technique business enterprise.The people see in produce practice, automate brought the tremendous convenience and the product quantities for people up of assurance, also eased the personnels labor strength, re
3、duce the establishment on the personnel. The target control of the hard realization in many complicated production lines, whole and excellent turn, the best decision etc., well-trained operation work, technical personnel or expert, governor but can judge and operate easily, can acquire the satisfied
4、 result. The research target of the artificial intelligence makes use of the calculator exactly to carry out, imitate these intelligences behavior, moderating the work through persons brain and calculators, with the mode that persons machine combine, for resolve the very complicated problem to look
5、for the best pathThe PLC anti- interference is very and excellent, our root need not concern its service life and the work situation bad, these all problems have already no longer become the topic that we fail, but stay to our is a concern to come to internal resources of make use of the PLC to stre
6、ngthen the control ability of the equipments for us, make our equipments more gentle.PLC language is not we imagine of edit collected materials the language or language of Cs to carry on weaving the distance, but the trapezoid diagram that the adoption is original after the electric appliances to co
7、ntrol, make the electrical engineering teacher while weaving to write the procedure very easy comprehended the PLC language, and a lot of non- electricity professional also very quickly know and go deep into to the PLC.Is PLC one of the advantage above and only, this is also one part that the people
8、 comprehend more and easily, in a lot of equipments, the people have already no longer hoped to see too many control buttons, they damage not only and easily and produce the artificial error easiest, small is not a main error perhaps you can still accept; But lead even is a fatal error greatly is wh
9、at we cant is tolerant of. New technique always for bringing more safe and convenient operation for us, make we a lot of problems for face on sweep but light, do you understand the HMI? Says the HMI here you basically not clear what it is, also have no interest understanding, change one inside text
10、explains it into the touch to hold or man-machine interface you knew, and it combines with the PLC to our larger space.HMI the control not only is reduced the control press button, increase the vivid of the control, more main of it is can sequence of, and at can the change data input to output the f
11、eedback with data, control in the temperature curve of imitate but also can keep the manifestation of view to come out. And can write the function help procedure through a plait to provide the help of various what lies in ones power, the one who make operate reduces the otiose error. Currently the H
12、MI factory is also more and more, the function is also more and more strong, the price is also more and more low, and the noodles of the usage are wide more and more. The HMI foreground can say that think to be good very.The PLC correspondence has already come more body now its value, at the PLC and
13、 correspondence between PLCs, can pass the communication of the information and the share of the dates to guarantee that of the equipments moderates mutually, the result that arrive already to repair with each other. Data conversion the adoption RS232 between PLC connect to come to the transmission
14、data, but the RS232 pick up a people and can guarantee 10 meters only of deliver the distance, if in the distance of 1000 meters we can pass the RS485 to carry on the correspondence, the longer distance can pass the MODEL only to carry on deliver.A line of transmission of the information contains a
15、string of and combine the cent of: The usual PLC is 8 machines, certainly also having 16 machines. We can be an at the time of sending out the data a send out to the other party, also can be 88 send out the data to the other party, an and 8 differentiations are also the as that we say to send out th
16、e data and combine sends out the data. A speed is more and slowly, but as long as 2 or three lines can solve problem, and can use the telephone line to carry on the long range control. But combine the ocular transmission speed is very quick of, it is a string of ocular of 25600%, occupy the advantag
17、e in the short distance, the in view of the fact TTL electricity is even, being limited by the scope of one meter generally, it combine unwell used for the data transmission of the long pull, thus the cost is too expensive.The PLC development has already entered for network ages of correspondence fr
18、om the mode of the one, and together other works control the net plank and I/ O card planks to carry on the share easily. A state software can pass all se hardwires link, more animation picture of keep the view to carries on the control, and cans pass the Internet to carry on the control in the fore
19、ign land, the blast-off that is like the absolute being boat No.5 is to adopt this kind of way to make airship go up the sky.The development of the higher layer needs our continuous effort to obtain. The PLC emergence has already affected a few persons fully, we also obtained more knowledge and prec
20、epts from the top one experience of the generation, coming to the continuous development PLC technique, push it toward higher wave tide.Knowing the available PLC network options and their best applications will ensure an efficient and flexible control system design.The programmable logic controllers
21、 (PLCs) ability to support a range of communication methods makes it an ideal control and data acquisition device for a wide variety of industrial automation and facility control applications. However, there is some confusion because so many possibilities exist. To help eliminate this confusion, let
22、s list what communications are available and when they would be best applied.To understand the PLCs communications versatility, lets first define the terms used in describing the various systems.ASCII: This stands for American Standard Code for Information Interchange. As shown in Fig. 1, when the l
23、etter A is transmitted, for instance, its automatically coded as 65 by the sending equipment. The receiving equipment translates the 65 back to the letter A. Thus, different devices can communicate with each other as long as both use ASCII code.ASCII module: This intelligent PLC module is used for c
24、onnecting PLCs to other devices also capable of communicating using ASCII code as a vehicle.Bus topology: This is a linear local area network (LAN) arrangement, as shown in Fig. 2A, in which individual nodes are tapped into a main communications cable at a single point and broadcast messages. These
25、messages travel in both directions on the bus from the point of connection until they are dissipated by terminators at each end of the bus.CPU: This stands for central processing unit, which actually is that part of a computer, PLC, or other intelligent device where arithmetic and logical operations
26、 are performed and instructions are decoded and executed.Daisy chain: This is a description of the connection of individual devices in a PLC network, where, as shown in Fig. 3, each device is connected to the next and communications signals pass from one unit to the next in a sequential fashion.Dist
27、ributed control: This is an automation concept in which portions of an automated system are controlled by separate controllers, which are located in close proximity to their area of direct control (control is decentralized and spread out over the system).Host computer: This is a computer thats used
28、to transfer data to, or receive data from, a PLC in a PLC/computer network.Intelligent device: This term describes any device equipped with its own CPU.I/O: This stands for inputs and outputs, which are modules that handle data to the PLC (inputs) or signals from the PLC (outputs) to an external dev
29、ice.Kbps: This stands for thousand bits per second, which is a rate of measure for electronic data transfer.Mbps: This stands for million bits per second.Node: This term is applied to any one of the positions or stations in a network. Each node incorporates a device that can communicate with all oth
30、er devices on the network.Protocol: The definition of how data is arranged and coded for transmission on a network.Ring topology. This is a LAN arrangement, as shown in Fig. 2C, in which each node is connected to two other nodes, resulting in a continuous, closed, circular path or loop for messages
31、to circulate, usually in one direction. Some ring topologies have a special loop back feature that allows them to continue functioning even if the main cable is severed.RS232. This is an IEEE standard for serial communications that describes specific wiring connections, voltage levels, and other ope
32、rating parameters for electronic data communications. There also are several other RS standards defined.Serial: This is an electronic data transfer scheme in which information is transmitted one bit at a time.Serial port: This the communications access point on a device that is set up for serial com
33、munications.Star topology. This is a LAN arrangement in which, as shown in Fig. 2B, nodes are connected to one another through a central hub, which can be active or passive. An active hub performs network duties such as message routing and maintenance. A passive central hub simply passes the message
34、 along to all the nodes connected to it.Topology: This relates to a specific arrangement of nodes in a LAN in relation to one another.Transparent: This term describes automatic events or processes built into a system that require no special programming or prompting from an operator.Now that were fam
35、iliar with these terms, lets see how they are used in describing the available PLC network options.PLC network optionsPLC networks provide you with a variety of networking options to meet specific control and communications requirements. Typical options include remote I/O, peer-to-peer, and host com
36、puter communications, as well as LANs. These networks can provide reliable and cost-effective communications between as few as two or as many as several hundred PLCs, computers, and other intelligent devices.Many PLC vendors offer proprietary networking systems that are unique and will not communica
37、te with another make of PLC. This is because of the different communications protocols, command sequences, error-checking schemes, and communications media used by each manufacturer.However, it is possible to make different PLCs talk to one another; whats required is an ASCII interface for the conne
38、ction(s), along with considerable work with software.A remote I/O configuration, as shown in Fig. 4A, has the actual inputs and outputs at some distance from the controller and CPU. This type of system, which can be described as a master-and-slave configuration, allows many distant digital and analo
39、g points to be controlled by a single PLC. Typically, remote I/Os are connected to the CPU via twisted pair or fiber optic cables.Remote I/O configurations can be extremely cost-effective control solutions where only a few I/O points are needed in widely separated areas. In this situation, its not a
40、lways necessary, or practical for that matter, to have a controller at each site. Nor is it practical to individually hard wire each I/O point over long distances back to the CPU. For example, remote I/O systems can be used in acquiring data from remote plant or facility locations. Information such
41、as cycle times, counts, duration or events, etc. then can be sent back to the PLC for maintenance and management reporting.In a remote I/O configuration, the master controller polls the slaved I/O for its current I/O status. The remote I/O system responds, and the master PLC then signals the remote
42、I/O to change the state of outputs as dictated by the control program in the PLCs memory. This entire cycle occurs hundreds of times per second.Peer-to-peer networksPeer-to-peer networks, as shown in Fig. 4B, enhance reliability by decentralizing the control functions without sacrificing coordinated
43、 control. In this type of network, numerous PLCs are connected to one another in a daisy-chain fashion, and a common memory table is duplicated in the memory of each. In this way, when any PLC writes data to this memory area, the information is automatically transferred to all other PLCs in the netw
44、ork. They then can use this information in their own operating programs.With peer-to-peer networks, each PLC in the network is responsible for its own control site and only needs to be programmed for its own area of responsibility. This aspect of the network significantly reduces programming and deb
45、ugging complexity; because all communications occur transparently to the user, communications programming is reduced to simple read-and-write statements.In a peer-to-peer system, theres no master PLC. However, its possible to designate one of the PLCs as a master for use as a type of group controlle
46、r. This PLC then can be used to accept input information from an operator input terminal, for example, sending all the necessary parameters to other PLCs and coordinating the sequencing of various events.Host computer linksPLCs also can be connected with computers or other intelligent devices. In fa
47、ct, most PLCs, from the small to the very large, can be directly connected to a computer or part of a multi drop host computer network via RS232C or RS422 ports. This combination of computer and controller maximizes the capabilities of the PLC, for control and data acquisition, as well as the comput
48、er, for data processing, documentation, and operator interface.In a PLC/computer network, as shown in Fig. 4C, all communications are initiated by the host computer, which is connected to all the PLCs in a daisy-chain fashion. This computer individually addresses each of its networked PLCs and asks
49、for specific information. The addressed PLC then sends this information to the computer for storage and further analysis. This cycle occurs hundreds of times per second.Host computers also can aid in programming PLCs; powerful programming and documentation software is available for program development. Programs then can be written on the computer in relay ladder logic and downloaded into the PLC. In this way, you can create, modify, debug, and monitor
