描述
對于一個自動化工程(特別是中大規(guī)??刂葡到y(tǒng))來講,選擇網(wǎng)絡(luò)是很重要的,甚至有人提出了“網(wǎng)絡(luò)就是控制器”的概念。首先,網(wǎng)絡(luò)必須是開放的,以方便不同設(shè)備的集成及未來系統(tǒng)規(guī)模的擴(kuò)展;其次,針對不同網(wǎng)絡(luò)層次的傳輸性能要求來選擇網(wǎng)絡(luò)的形式,這必須在較深入地了解該網(wǎng)絡(luò)標(biāo)準(zhǔn)的協(xié)議、機(jī)制的前提下進(jìn)行;另外,綜合考慮系統(tǒng)成本、設(shè)備兼容性、現(xiàn)場環(huán)境適用性等具體問題,確定不同層次所使用的網(wǎng)絡(luò)標(biāo)準(zhǔn)。一個實時系統(tǒng)的性能可從時間、可靠性和應(yīng)用對象三個方面來衡量。
掌握plc掃描原理與其它控制設(shè)備比較,PLC最重要的特征是“掃描”。PLC上電后,自動重復(fù)執(zhí)行程序掃描和I/O掃描,即輸入采樣、用戶程序執(zhí)行和輸出刷新三個階段。完成上述三個階段稱作一個掃描周期。在輸入采樣階段,PLC以掃描方式依次地讀入所有輸入狀態(tài)和數(shù)據(jù),并將它們存入I/O映象區(qū)中相應(yīng)的單元內(nèi),輸入采樣結(jié)束后,轉(zhuǎn)入用戶程序執(zhí)行和輸出刷新階段,即使輸入狀態(tài)和數(shù)據(jù)發(fā)生變化,I/O映象區(qū)中的相應(yīng)單元的狀態(tài)和數(shù)據(jù)也不會改變。在用戶程序執(zhí)行階段,PLC按由上而下、先左后右的順序依次地掃描程序(梯形圖),根據(jù)邏輯運算的結(jié)果,刷新RAM存儲區(qū)或I/O映象區(qū)對應(yīng)單元的狀態(tài)。在輸出刷新階段,根據(jù)I/O映象區(qū)內(nèi)對應(yīng)的狀態(tài)和數(shù)據(jù)刷新所有的輸出鎖存電路,再經(jīng)輸出電路驅(qū)動相應(yīng)的外設(shè),這時,才是PLC的真正輸出??梢姡捎肞LC程序控制的過程和結(jié)果與繼電器邏輯回路是有區(qū)別的,特別是涉及到梯級的次序、脈沖信號的捕捉等,與PLC的掃描原理是密切相關(guān)的。實踐中,大量的程序問題均源于此,常常會出現(xiàn)不可思議的結(jié)果。因此,設(shè)計PLC程序,必須精通PLC的基本原理。
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主營DCS系統(tǒng)配件
機(jī)器人系統(tǒng)配件
大型伺服系統(tǒng)配件
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