plc與dcs的分析比較
dcs和plc的設計原理區(qū)別較大,plc是由繼電器控制原理發(fā)展起來的,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,并事先存入plc的用戶程序存儲器中。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。plc的cpu內(nèi)有指示程序步存儲地址的程序計數(shù)器,在程序運行過程中,執(zhí)行一步該計數(shù)器自動加1,程序從起始步(步序號為零)起依次執(zhí)行到終步(通常為end指令),然后再返回起始步循環(huán)運算。plc每完成一次循環(huán)操作所需的時間稱為一個掃描周期。不同型號的plc,循環(huán)掃描周期在1微秒到幾十微秒之間。程序計數(shù)器這樣的循環(huán)操作,這是dcs所沒有的。這也是使plc的冗余不如dcs的原因。dcs是在運算放大器的基礎上得以發(fā)展的。把所有的函數(shù)和過程變量之間的關系都做成功能塊(有的dcs系統(tǒng)稱為膨化塊)。dcs和plc的表現(xiàn)的主要差別是在開關量的邏輯解算和模擬量的運算上,即使后來兩者相互有些滲透,但是還是有區(qū)別。80年代以后,plc除邏輯運算外,控制回路用的算法功能已經(jīng)大大加強,但 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,在微秒量級,解算1k邏輯程序不到1毫秒。它把所有的輸入都當成開關量來處理,16位(也有32位的)為一個模擬量。而dcs把所有輸入都當成模擬量,1位就是開關量。解算一個邏輯是在幾百微秒至幾毫秒量級。對于plc解算一個pid運算在幾十毫秒,這與dcs的運算時間不相上下。
在接地電阻方面,對plc也許要求不高,但對dcs一定要在幾歐姆以下(通常在4歐姆以下)。模擬量隔離也是非常重要的。
相同i/o點數(shù)的系統(tǒng),用plc比用dcs,其成本要低一些(大約能省40%左右)。plc沒有操作站,它用的軟件和硬件都是通用的,所以維護成本比dcs要低很多。如果被控對象主要是設備連鎖、回路相對很少,采用plc較為合適。如果主要是模擬量控制、并且函數(shù)運算很多,好采用dcs。dcs在控制器、i/o板、通訊網(wǎng)絡等的冗余方面,一些高級運算、行業(yè)的特殊要求方面都要比plc好的多。plc由于采用通用監(jiān)控軟件,在設計企業(yè)的管理信息系統(tǒng)方面,要容易一些。
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