一個或若干PLC與PC機聯(lián)出系統(tǒng),PC機起到原編程器及
人機界面操作站的作用,這20世紀(jì)90年代的新潮流,這樣為系統(tǒng)集成帶來了商機,同時編程軟件和人機界面軟件(監(jiān)控軟件或稱
組態(tài)軟件)及軟件接口(或稱驅(qū)動軟件)也得到了發(fā)展。
PLC廠家在原來CPU模板上提供物理層RS232/422/485接口的基礎(chǔ),逐漸增加了各種通訊接口,而且提供完整的通訊網(wǎng)絡(luò)。由于數(shù)據(jù)通訊技術(shù)發(fā)展很快,用戶對開放性要求很強烈,現(xiàn)場總線技術(shù)及以太網(wǎng)技術(shù)也同步發(fā)展,所以PLC構(gòu)成的PCS系統(tǒng)比DCS的開放性所處的現(xiàn)狀稍好一些。
羅克韋爾AB公司已形成了
多層結(jié)構(gòu)體系,即Ether Net、Control Net、Device Net及Asi等現(xiàn)場總線(原DH+網(wǎng)也可兼容)。西門子公司在Profibus-DP通訊網(wǎng)絡(luò)及Profibus-FMS網(wǎng)絡(luò)以外,提出了S7 Routing網(wǎng)絡(luò),即Profibus-DP和Industrial Enternet兩層結(jié)構(gòu)。網(wǎng)絡(luò)還在發(fā)展,我國應(yīng)已積極的姿態(tài)投入其中。
2001年我國機械工業(yè)成為工業(yè)發(fā)展新亮點,總產(chǎn)值比上年增長17.15%,汽車產(chǎn)量為世界前10位,機床產(chǎn)量為世界第5位。機械工業(yè)利潤增長33.35%,占整個工業(yè)新增利潤六成多。出口同樣出現(xiàn)可喜的增長。機械工業(yè)提出要實施網(wǎng)絡(luò)化,對這一點,PLC從業(yè)人員應(yīng)有清醒的認(rèn)識,應(yīng)對網(wǎng)絡(luò)化的開放性、
網(wǎng)絡(luò)構(gòu)成的性能/價格比和網(wǎng)絡(luò)的可靠性、安全性、先進性上特別下功夫。
PLC控制器本身的硬件采用積木式結(jié)構(gòu),各廠家產(chǎn)品結(jié)構(gòu)大同小異。以日本歐姆龍C200HE為例,為總線模板框式結(jié)構(gòu),基本框架(CPU母板)上裝有CPU模板,其它槽位裝有I/O模板;如果I/O模板多時,可由CPU母板經(jīng)I/O擴展電纜連接I/O擴展母板,在其上裝I/O模板;另一種方法是配備遠程I/O從站等。這些都說明了PLC廠家將硬件各部件均向用戶開發(fā),便于用戶選用,配置成規(guī)模不等的PLC,而且這種硬件配置的開放性,為制造商、分銷商(代理商)、系統(tǒng)集成商、最終用戶帶來很多方便,為營銷供應(yīng)鏈帶來很大便利,這是一大成功經(jīng)驗。
PLC內(nèi)的I/O模板,除一般的DI/DO、AD/DA模板外,還發(fā)展了一系列特殊功能的I/O模板,這為PLC用于各行各業(yè)打開了出路,如用于條形碼識別的ASCII/BASIC模板,用于反饋控制的PID模板,用于運行控制、機械加工的高速計數(shù)模板、單軸位置控制模板、雙軸位置控制模板、凸輪定位器模板、射頻識別接口模板等,這在以后還會有很大發(fā)展。另外在輸入、輸出的相關(guān)元件、強干擾場合的輸入、輸出電隔離、地隔離等方面也會更加完善。
PLC中的CPU與存儲器配合,完成控制功能。它與DCS系統(tǒng)處理溫度、壓力、流量等參數(shù)的系統(tǒng)不同,采用快速的巡回掃描周期,一般為0.1~0.2s,更快的則選用50ms或更小的掃描周期。它是一個數(shù)字采樣控制系統(tǒng)。
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