【正文】 entioned 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 magic field to control a switch, as pictured in FigureA1. When a voltage is applied to the input coil, the resulting current creates a magic field. The magic 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). 遼寧科技大學(xué)畢業(yè)設(shè)計(jì)（論文） 第 31 頁(yè) Figure A1 Simple Relay Layouts and SchematicsRelays 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 FigureA2. 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 circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on.The example in FigureA2 does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. FigureA3 shows a more plete representation of the PLC. Here there are two inputs from push buttons. We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC that will turn on a light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a puter program that the user can enter and change. Notice that both of the input push buttons are 遼寧科技大學(xué)畢業(yè)設(shè)計(jì)（論文） 第 32 頁(yè)Figure A2 A Simple Relay Controllernormally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. Do not think that the ladder logic in the PLC needs to match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types.Figure A3 A PLC Illustrated With RelaysMany relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in FigureA4 is an example of this。為 ON 時(shí)，表示機(jī)械手已經(jīng)到達(dá)指定位置，為 OFF時(shí)表示還未到達(dá)指定位置。如果小車這兩個(gè)過(guò)程都已經(jīng)完成，那么將開(kāi)始進(jìn)行后退運(yùn)動(dòng)， 表示啟動(dòng)小車后退電機(jī)，使小車向后運(yùn)行并返回初始位置。程序初始化由網(wǎng)絡(luò) 1 和網(wǎng)絡(luò) 2 完成。表 所示是 CPU312 的技術(shù)參數(shù)。這個(gè)方法使堆垛機(jī)的小車每通過(guò)一個(gè)位置就記錄一個(gè)數(shù)，一直移動(dòng)到和預(yù)定位置號(hào)一致時(shí)停止移動(dòng)。智能立體倉(cāng)庫(kù)由貨架、堆垛機(jī)、自動(dòng)控制裝置等部分組成，有自動(dòng)和手動(dòng)兩種操作方式。（6）口令保護(hù) 多級(jí)口令保護(hù)可以使用戶高度、有效地保護(hù)其技術(shù)機(jī)密，防止未經(jīng)允許的復(fù)制和修改。 （3）通訊處理器 (CP) 用于連接網(wǎng)絡(luò)和點(diǎn)對(duì)點(diǎn)連接。例如在圖表窗口已經(jīng)放置了一個(gè)元素，但是并沒(méi)有分配符號(hào)和地址的情況下，這種情形就會(huì)出現(xiàn)。這也是為什么在你編程之前要選擇 CPU。它是一個(gè)支持PLC設(shè)備和地址信息，SIEMENS 的PLC和這些 PLC支持的網(wǎng)絡(luò)設(shè)備進(jìn)行通信的方便工具。結(jié)合堆垛機(jī)的工作特性，SIMATIC S7300 是既經(jīng)濟(jì)又方便的選擇方案。隨著我國(guó)經(jīng)濟(jì)和科學(xué)技術(shù)的不斷發(fā)展，現(xiàn)代物流技術(shù)在我國(guó)也得到了很大的提高，特別是作為現(xiàn)代物流技術(shù)典范的自動(dòng)化倉(cāng)庫(kù)技術(shù)尤其令人關(guān)注。 遼寧科技大學(xué)畢業(yè)設(shè)計(jì)（論文） 第 I 頁(yè)基于 PLC 的小車定位系統(tǒng)設(shè)計(jì)畢業(yè)論文目 錄摘要 .............................................................................................................................................IABSTRACT...............................................................................................................................II目 錄 .........................................................................................................................................III1 緒論..........................................................................................................................................1 選題背景及目的...............................................................................................................1 國(guó)內(nèi)外的研究狀況...........................................................................................................1 課題的研究方法和內(nèi)容...................................................................................................32 PLC 編程軟件 STEP7 ..............................................................................................................4 SIEMENS 公司編程軟件 STEP7 的簡(jiǎn)單介紹 ...............................................................4 使用 STEP7 軟件開(kāi)發(fā)一個(gè) PLC 程序的步驟 ...............................................................4 SIEMENS 公司的 S7300 系列可編程控制器介紹 .......................................................83 控制系統(tǒng)總體方案設(shè)計(jì)以及系統(tǒng)流程圖............................................................................11 控制系統(tǒng)總體方案........................................................................................................11 小車定位控制工藝流程圖.............................................................................................144 小車定位控制 PLC 程序編制 ..............................................................................................15 PLC 選型和資源配置 ....................................................................................................15 PLC 程序設(shè)計(jì) .....................................................