現(xiàn)場(chǎng)總線網(wǎng)絡(luò):現(xiàn)場(chǎng)總線技術(shù)是當(dāng)今自動(dòng)化領(lǐng)域技術(shù)發(fā)展熱點(diǎn)之一,被譽(yù)為自動(dòng)化領(lǐng)域的計(jì)算機(jī)局域網(wǎng),它的出現(xiàn)標(biāo)志著自動(dòng)化控制技術(shù)又一個(gè)新時(shí)代的開始。現(xiàn)場(chǎng)總線是連接設(shè)置在控制現(xiàn)場(chǎng)的儀表與設(shè)置在控制室內(nèi)的控制設(shè)備的數(shù)字化、串行、多站通信的網(wǎng)絡(luò)。其關(guān)鍵標(biāo)志是能支持雙向、多節(jié)點(diǎn)、總線式的全數(shù)字通信?,F(xiàn)場(chǎng)總線技術(shù)近年來(lái)成為國(guó)際上自動(dòng)化和儀器儀表發(fā)展的熱點(diǎn),它的出現(xiàn)是傳統(tǒng)的控制系統(tǒng)結(jié)構(gòu)產(chǎn)生了革命性的變化,是自控系統(tǒng)朝著智能化、數(shù)字化、信息化、網(wǎng)絡(luò)化、分散化的方向邁進(jìn),形成新型的網(wǎng)絡(luò)集成式全分布式控制系統(tǒng)---現(xiàn)場(chǎng)總線控制系統(tǒng)FCS(Fieldbus Control System)。但是現(xiàn)在的現(xiàn)場(chǎng)總線的各種標(biāo)準(zhǔn)并行存在并且都有自己的生存領(lǐng)域,還沒(méi)有形成真正統(tǒng)一的標(biāo)準(zhǔn),關(guān)鍵是看不到什么時(shí)候能形成統(tǒng)一的標(biāo)準(zhǔn),技術(shù)也不夠成熟。另外現(xiàn)場(chǎng)總線的儀表種類還比較少可供選擇的余地小,價(jià)格也偏高,從終用戶的角度看大多還處于觀望狀態(tài),都想等到技術(shù)成熟之后在考慮,現(xiàn)在實(shí)施的少。
RS485網(wǎng)絡(luò):RS485/MODBUS是現(xiàn)在流行的一種布網(wǎng)方式,其特點(diǎn)是實(shí)施簡(jiǎn)單方便,而且現(xiàn)在支持RS485的儀表又特多,特別是在油品行業(yè)RS485/MODBUS簡(jiǎn)直是一統(tǒng)天下,現(xiàn)在的儀表商也紛紛轉(zhuǎn)而支持RS485/MODBUS,原因很簡(jiǎn)單,象原來(lái)的HART儀表想買一個(gè)轉(zhuǎn)換口非常困難而且價(jià)格昂貴,RS485的轉(zhuǎn)換接口就便宜的多而且種類繁多。至少在低端市場(chǎng)RS485/MODBUS還將是主要的組網(wǎng)方式,近兩三年內(nèi)不會(huì)改變。
ESD緊急停車系統(tǒng)
ESD是英文Emergency Shutdown Device緊急停車裝置系統(tǒng)的縮寫。這種專用的安全保護(hù)系統(tǒng)是90年代發(fā)展起來(lái)的,以它的高可靠性和靈活性而受到一致好評(píng)。 ESD緊急停車系統(tǒng)按照安全獨(dú)立原則要求,獨(dú)立于DCS集散控制系統(tǒng),其安全級(jí)別高于DCS。在正常情況下,ESD系統(tǒng)是處于靜態(tài)的,不需要人為干預(yù)。作為安全保護(hù)系統(tǒng),凌駕于生產(chǎn)過(guò)程控制之上,實(shí)時(shí)在線監(jiān)測(cè)裝置的安全性。只有當(dāng)生產(chǎn)裝置出現(xiàn)緊急情況時(shí),不需要經(jīng)過(guò)DCS系統(tǒng),而直接由ESD發(fā)出保護(hù)聯(lián)鎖信號(hào),對(duì)現(xiàn)場(chǎng)設(shè)備進(jìn)行安全保護(hù),避免危險(xiǎn)擴(kuò)散造成巨大損失。 根據(jù)有關(guān)資料,當(dāng)人在危險(xiǎn)時(shí)刻的判斷和操作往往是滯后的、不可靠的,當(dāng)操作人員面臨生命危險(xiǎn)時(shí),要在60s內(nèi)作出反應(yīng),錯(cuò)誤決策的概率高達(dá)99.9%。因此設(shè)置獨(dú)立于控制系統(tǒng)的安全聯(lián)鎖是十分有必要的,這是作好安全生產(chǎn)的重要準(zhǔn)則。該動(dòng)則動(dòng),不該動(dòng)則不動(dòng),這是ESD系統(tǒng)的一個(gè)顯著特點(diǎn)。 當(dāng)然一般安全聯(lián)鎖保護(hù)功能也可由DCS來(lái)實(shí)現(xiàn)。那么為何要獨(dú)立設(shè)置ESD系統(tǒng)呢因?yàn)橛谳^大規(guī)模的緊急停車系統(tǒng)應(yīng)按照安全獨(dú)立原則與DCS分開設(shè)置,這樣做主要有以下幾方面原因: (1)降低控制功能和安全功能同時(shí)失效的概率,當(dāng)維護(hù)DCS部分故障時(shí)也不會(huì)危及安全保護(hù)系統(tǒng); ?。?)對(duì)于大型裝置或旋轉(zhuǎn)機(jī)械設(shè)備而言,緊急停車系統(tǒng)響應(yīng)速度越快越好。這有利于保護(hù)設(shè)備,避免事故擴(kuò)大;并有利于分辨事故原因記錄。而DCS處理大量過(guò)程監(jiān)測(cè)信息,因此其響應(yīng)速度難以作得很快; ?。?)DCS系統(tǒng)是過(guò)程控制系統(tǒng),是動(dòng)態(tài)的,需要人工頻繁的干預(yù),這有可能引起人為誤動(dòng)作;而ESD是靜態(tài)的,不需要人為干預(yù),這樣設(shè)置ESD可以避免人為誤動(dòng)作。 ESD緊急停車裝置,在石化行業(yè)以及大型鋼廠及電廠中都有著廣泛的應(yīng)用。實(shí)際上它也是通過(guò)高速運(yùn)算PLC來(lái)實(shí)現(xiàn)控制的,它與PLC的本質(zhì)區(qū)別在于它的輸入輸出卡件上,應(yīng)為一切為了安全考慮,所以在硬件保護(hù)上做得較為完善,而且他要考慮到在事故狀態(tài)下,現(xiàn)場(chǎng)控制閥位及各個(gè)開關(guān)的位置。
APC AP8858NA3 Rack PDU 2G, Metered, ZeroU
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