描述
三、控制功能的選擇
該選擇包括運算功能、控制功能、通信功能、編程功能、診斷功能和處理速度等特性的選擇。
(一)運算功能
簡單PLC的運算功能包括邏輯運算、計時和計數(shù)功能;普通PLC的運算功能還包括數(shù)據(jù)移位、比較等運算功能;較復(fù)雜運算功能有代數(shù)運算、數(shù)據(jù)傳送等;大型PLC中還有模擬量的PID運算和其他高級運算功能。隨著開放系統(tǒng)的出現(xiàn),目前在PLC中都已具有通信功能,有些產(chǎn)品具有與下位機的通信,有些產(chǎn)品具有與同位機或上位機的通信,有些產(chǎn)品還具有與工廠或企業(yè)網(wǎng)進行數(shù)據(jù)通信的功能。設(shè)計選型時應(yīng)從實際應(yīng)用的要求出發(fā),合理選用所需的運算功能。大多數(shù)應(yīng)用場合,只需要邏輯運算和計時計數(shù)功能,有些應(yīng)用需要數(shù)據(jù)傳送和比較,當用于模擬量檢測和控制時,才使用代數(shù)運算,數(shù)值轉(zhuǎn)換和PID運算等。要顯示數(shù)據(jù)時需要譯碼和編碼等運算。
(二)控制功能
控制功能包括PID控制運算、前饋補償控制運算、比值控制運算等,應(yīng)根據(jù)控制要求確定。PLC主要用于順序邏輯控制,因此,大多數(shù)場合常采用單回路或多回路控制器解決模擬量的控制,有時也采用專用的智能輸入輸出單元完成所需的控制功能,提高PLC的處理速度和節(jié)省存儲器容量。例如采用PID控制單元、高速計數(shù)器、帶速度補償?shù)哪M單元、ASC碼轉(zhuǎn)換單元等。
(三)通信功能
大中型PLC系統(tǒng)應(yīng)支持多種現(xiàn)場總線和標準通信協(xié)議(如TCP/IP),需要時應(yīng)能與工廠管理網(wǎng)(TCP/IP)相連接。通信協(xié)議應(yīng)符合ISO/IEEE通信標準,應(yīng)是開放的通信網(wǎng)絡(luò)。
PLC系統(tǒng)的通信接口應(yīng)包括串行和并行通信接口(RS2232C/422A/423/485)、RIO通信口、工業(yè)以太網(wǎng)、常用DCS接口等;大中型PLC通信總線(含接口設(shè)備和電纜)應(yīng)1:1冗余配置,通信總線應(yīng)符合國際標準,通信距離應(yīng)滿足裝置實際要求。
PLC系統(tǒng)的通信網(wǎng)絡(luò)中,上級的網(wǎng)絡(luò)通信速率應(yīng)大于1Mbps,通信負荷不大于60%。PLC系統(tǒng)的通信網(wǎng)絡(luò)主要形式有下列幾種形式:1)PC為主站,多臺同型號PLC為從站,組成簡易PLC網(wǎng)絡(luò);2)1臺PLC為主站,其他同型號PLC為從站,構(gòu)成主從式PLC網(wǎng)絡(luò);3)PLC網(wǎng)絡(luò)通過特定網(wǎng)絡(luò)接口連接到大型DCS中作為DCS的子網(wǎng);4)專用PLC網(wǎng)絡(luò)(各廠商的專用PLC通信網(wǎng)絡(luò))。
為減輕CPU通信任務(wù),根據(jù)網(wǎng)絡(luò)組成的實際需要,應(yīng)選擇具有不同通信功能的(如點對點、現(xiàn)場總線、工業(yè)以太網(wǎng))通信處理器。
歐美品牌停產(chǎn)的,常用的“PLC”備品備件
我司產(chǎn)品應(yīng)用于以下領(lǐng)域:
1,《發(fā)電廠DCS監(jiān)控系統(tǒng)》
2,《智能平鋼化爐系統(tǒng)制造》
3,《PLC可編程輸送控制系統(tǒng)》
4,《DCS集散控制系統(tǒng)》
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5,《智能型消防供水控制系統(tǒng)》
6,《化工廠藥液恒流量計算機控制系統(tǒng)》
7,《電氣控制系統(tǒng)》造紙,印染生產(chǎn)線,變電站綜合自動化控制系列
本特利,英維思,伍德沃德,福克斯波羅、西屋、瑞恩、施耐德莫迪康、ABB、AB、西門子、摩托羅拉、GE發(fā)那科、安川、博世力士樂,ACSO,力士樂等各大品牌的DCS系統(tǒng)配件,機器人系統(tǒng)配件,大型伺服系統(tǒng)備件。
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