ABB 3BSE013230R1              

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從PLC的軟件程序來(lái)考慮提高控制系統(tǒng)的可靠性

    為了提高PLC控制系統(tǒng)工作的可靠性，可以專門設(shè)置一個(gè)定時(shí)器，作為監(jiān)控程序部分，對(duì)系統(tǒng)的運(yùn)行狀態(tài)進(jìn)行檢測(cè)。若程序運(yùn)行能正常結(jié)束，則該定時(shí)器就立即被清零；若程序運(yùn)行發(fā)生故障，如出現(xiàn)死循環(huán)等，該定時(shí)器在設(shè)定的時(shí)間到就無(wú)法清零，此時(shí)PLC發(fā)出報(bào)警信號(hào)。在設(shè)計(jì)應(yīng)用程序時(shí)，使用這種方法來(lái)實(shí)現(xiàn)對(duì)系統(tǒng)各部分運(yùn)行狀態(tài)的監(jiān)控。如果用PLC來(lái)控制某一對(duì)象時(shí)，編制程序時(shí)可定義一個(gè)定時(shí)器來(lái)對(duì)這一對(duì)象的運(yùn)行狀態(tài)進(jìn)行監(jiān)視：該定時(shí)器的設(shè)定時(shí)間即為這一對(duì)象工作所需的最大時(shí)間；當(dāng)啟動(dòng)該對(duì)象運(yùn)行時(shí)，同時(shí)也啟動(dòng)該定時(shí)器。若該對(duì)象的運(yùn)行程序在規(guī)定的時(shí)間結(jié)束工作，發(fā)出一個(gè)工作完成信號(hào)，使該定時(shí)器清零，說(shuō)明這一對(duì)象的運(yùn)行程序正常；否則，屬運(yùn)行不正常，發(fā)出報(bào)警信號(hào)或停機(jī)信號(hào)。監(jiān)控程序的梯形圖如圖7所示。圖7中定時(shí)器T1為檢測(cè)元件，X001為控制對(duì)象動(dòng)作信號(hào)，X002為動(dòng)作完成信號(hào)，M2為報(bào)警或停機(jī)信號(hào)。假設(shè)被控對(duì)象的運(yùn)行程序完成一次循環(huán)需要50s，則定時(shí)器K值可取510（T1為100ms定時(shí)器）。當(dāng)X001=1時(shí)，被控對(duì)象運(yùn)行開始，T1開始計(jì)時(shí)；如在規(guī)定的時(shí)間內(nèi)被控對(duì)象的運(yùn)行程序能正常結(jié)束，則X002動(dòng)作，M1復(fù)位，定時(shí)器T1被清零，等待下一次循環(huán)的開始；若在規(guī)定時(shí)間沒有發(fā)出被控對(duì)象運(yùn)行完成的動(dòng)作信號(hào)，則判斷為故障，T1的觸點(diǎn)閉合，接通M2發(fā)出報(bào)警信號(hào)或停機(jī)信號(hào)。

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