HDS03.2-W075N-HS12-0在火電廠熱工自動化領(lǐng)域,DCS和PLC是兩個完全不同而又有著千絲萬縷聯(lián)系的概念。DCS和PLC都是計算機技術(shù)與工業(yè)控制技術(shù)相結(jié)合的產(chǎn)物,火電廠主機控制系統(tǒng)用的是DCS,而PLC主要應(yīng)用在電廠輔助車間。
DCS和PLC都有操作員站提供人機交互的手段、都依靠基于計算機技術(shù)的控制器完成控制運算、都通過I/O卡件完成與一次元件和執(zhí)行裝置的數(shù)據(jù)交換、都具備稱之為網(wǎng)絡(luò)的通信系統(tǒng)。DCS和PLC如此相似,為什么會有完全不同的概念,我們在工程實踐中如何進行選擇?本文從歷史沿革、技術(shù)特點、發(fā)展方向等幾個方面作一綜述,希望能夠?qū)峁I(yè)人員有所借鑒。其中的DCS的情況以科遠的NT6000為例,力求例舉詳實闡述清晰。
1 DCS和PLC的歷史沿革及核心概念DCS為分散控制系統(tǒng)的英文(TOTALDISTRIBUTEDCONTROLSYSTEM)簡稱。指的是危險分散、數(shù)據(jù)集中。70年代中期進入市場,完成模擬量控制,HDS03.2-W075N-HS12-0代替以PID運算為主的模擬控制儀表。首先提出DCS這樣一種思想的是儀表制造廠商,當時主要應(yīng)用于化工行業(yè)。而PLC于60年代末研制成功,稱作邏輯運算的可編程序控制器(ProgrammableLogicController),簡稱PLC。主要應(yīng)用于汽車制造業(yè)。DCS和PLC的設(shè)計原理區(qū)別較大,PLC是從摸仿原繼電器控制原理發(fā)展起來的,70年代的PLC只有開關(guān)量邏輯控制。它以存儲執(zhí)行邏輯運算、順序控制、定時、計數(shù)和運算等操作的指令;并通過數(shù)字輸入和輸出操作,來控制各類機械或生產(chǎn)過程。用戶編制的控制程序表達了生產(chǎn)過程的工藝要求。將其存入PLC的用戶程序存儲器,運行時按存儲程序的內(nèi)容逐條執(zhí)行,以完成工藝流程要求的操作。
DCS是在運算放大器的基礎(chǔ)上得以發(fā)展的。把所有的函數(shù)、各過程變量之間的關(guān)系都設(shè)計成功能塊。70年代中期的DCS只有模擬量控制。
DCS和PLC控制器的主要差別是在開關(guān)量和模擬量的運算上,即使后來兩者相互有些滲透,但是仍然有區(qū)別。80年代以后,PLC除邏輯運算外,HDS03.2-W075N-HS12-0也增加了一些控制回路算法,但要完成一些復雜運算還是比較困難,PLC用梯形圖編程,模擬量的運算在編程時不太直觀,編程比較麻煩。但在解算邏輯方面,表現(xiàn)出快速的優(yōu)點。而DCS使用功能塊封裝模擬運算和邏輯運算,無論是邏輯運算還是復雜模擬運算的表達形式都非常清晰,但相對PLC來說邏輯運算的表達效率較低。
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