【正文】 er 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。黃北剛. 工廠電氣控制電路實(shí)例詳解. 北京: 化學(xué)工業(yè)出版社, 我再次向他們表達(dá)謝意，祝他們身體健康，工作順利！參考文獻(xiàn)[1] 李道霖. 電氣控制與PLC原理及應(yīng)用. 北京: 電子工業(yè)出版社, [2] 孫海維. SIMATIC可編程控制器及應(yīng)用. 北京: 機(jī)械工業(yè)出版社, [3] 廖常初. 可編程控制應(yīng)用技術(shù). 重慶: 重慶大學(xué)出版社, [4] 馮立明. 電鍍工藝與設(shè)備化學(xué). 北京: 北京工業(yè)出版社, [5] 孫平. 可編程控制器原理及應(yīng)用. 北京: 高等教育出版社, [6] 常曉玲. 電氣控制系統(tǒng)與可編程控制器. 北京: 機(jī)械工業(yè)出版社, 2007總之，這此的設(shè)計(jì)教會了我很多，不僅僅是專業(yè)知識得到擴(kuò)展，更多的是發(fā)現(xiàn)問題，解決問題的能力和獲取知識的能力得到鍛煉。向老師求教，向同學(xué)們問詢，在網(wǎng)絡(luò)上面搜索答案，到圖書館借書，這都是我這次能一次又一次克服這些困難所使用的方法。在此，我鄭重的向在此過程中給予我?guī)椭睦蠋熀屯瑢W(xué)們表達(dá)深深的謝意，同時(shí)也感謝學(xué)校能夠給我們提供這么好的實(shí)驗(yàn)條件，能讓我們不僅僅是在紙張上完成自己的課程設(shè)計(jì)，更是讓我們能親眼看見自己辛苦幾個(gè)月的設(shè)計(jì)成果。當(dāng)然，各位指導(dǎo)老師和同一組的同學(xué)也對我做了很多的指導(dǎo)和幫助，這對我能按時(shí)按要求完成整個(gè)系統(tǒng)的設(shè)計(jì)有著很大的作用。最終，在通過各方面的努力下，該電鍍生產(chǎn)線控制系統(tǒng)的設(shè)計(jì)基本完成，在設(shè)計(jì)當(dāng)中我充分考慮到了操作的方便性，工藝的簡單性。在整個(gè)系統(tǒng)的設(shè)計(jì)中PLC的編程是整個(gè)設(shè)計(jì)最主要的一部分，我的編程是采用梯形圖編程語言，所有的編寫步驟和編寫要點(diǎn)是嚴(yán)格按照設(shè)計(jì)要求來完成的，而且我編寫程序之前對電鍍生產(chǎn)