【正文】 子公司. S7200可編程控制器產(chǎn)品目錄, 2005 [13] 北京亞控自動(dòng)化軟件科技有限公司. “組態(tài)王”使用手冊(cè), 2002[14] . 西安: 西安電子科技大學(xué)出版社, 2004[15] 汪曉平. PLC可編程控制器系統(tǒng)開(kāi)發(fā)實(shí)例導(dǎo)航. 北京: 人民郵電出版社, 2004[16] 張萬(wàn)忠. 可編程控制器應(yīng)用技術(shù). 北京: 化學(xué)工業(yè)出版社, [17] 周美蘭等. PLC電氣控制與組態(tài)設(shè)計(jì). 北京: 科學(xué)出版社, 2000[18] . 北京: 電子出版社, 2006[19] 李長(zhǎng)久. PLC 原理及應(yīng)用. 北京: 機(jī)械工業(yè)出版社, [20] 周美蘭等. PLC電氣控制與組態(tài)設(shè)計(jì). 北京: 科學(xué)出版社, 2000外文資料翻譯 PROGRAMMABLE LOGIC CONTROLLERS INTRODUCTION Control engineering has evolved over time. In the past humans was the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is mon to use relays to make simple logical control decisions. The development of low cost puter has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has bee the most mon choice for manufacturing controls. PLC have been gaining popularity on the factory floor and will probably remain predominant for some time to e. Most of this is because of the advantages they offer. Cost effective for controlling plex systems. Flexible and can be reapplied to control other systems quickly and easily. Computational abilities allow more sophisticated control. Trouble shooting aids make programming easier and reduce downtime. Reliable ponents make these likely to operate for years before failure. LADDER LOGIC Ladder logic is the main programming method used for PLC. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and trades people was greatly reduced. Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure . When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (nonconducting) when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed (conducting). Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure . In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This can be read logically as C will be on if A is off and B is on. PROGRAMThe first PLC were programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a puter but, this method has stuck and it is the most mon technique for programming PLC today. An example of ladder logic can be seen in Figure . To interpret this diagram imagines that the power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the neutral rail. In the figure there are two rungs, and on each rung there are binations of inputs (two vertical lines) and outputs (circles). If the inputs are opened or closed in the right bination the power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail. An input can e from a sensor, switch, or any other type of sensor. An output will be some device outside the PLC that is switched on or off, such as lights or motors.In the top rung the contacts are normally open and normally closed, which means if input A is on and input B is off, then power will flow through the output and activate it.The second rung of Figure is more plex, there are actually multiple binations of inputs that will result in the output Y turning on. On the left most part of the rung, power could flow through the top if C is off and D is on. Power could also (and simultaneously) flow through the bottom if both E and F are true. This would get power half way across the rung, and then if G or H is true the power will be delivered to output Y. In later chapters we will examine how to interpret and construct these diagrams. There are other methods for programming PLC. One of the earliest techniques involved mnemonic instructions. An example of mnemonics is shown in Figure . In this example the instructions are read one line at a time from top to bottom. The first line 00000 has the instruction LDN (input load and not) for input 00001. If the input is off it remembers a 0, if the input is on it remembers a 1 (note: this is the reverse of the LD). The AND statement recalls the last two numbers remembered and if they are both true the result is a 1。 2. The provisions of cleaning equipment on a regular basis(1) Every six months or a quarter of the PLC to clean, cut off power to the PLC power supply to the power rack, CPU board and input / output board in turn be removed to purge, clean and then in turn installed in situ, will all link to