1203-CN1
1203-CN1
1203-CN1
從PLC控制系統(tǒng)與電器控制系統(tǒng)比較可知，PLC的用戶程序（軟件）代替了繼電器控制電路（硬件）。因此，對于使用者來說，可以將PLC等效成是許許多多各種各樣的“軟繼電器”和“軟接線”的集合，而用戶程序就是用“軟接線”將“軟繼電器”及其“觸點”按一定要求連接起來的“控制電路”。
為了更好的理解這種等效關系，下面通過一個例子來說明。如圖1所示為三相異步電動機單向起動運行的電器控制系統(tǒng)。其中，由輸入設備SB1、SB2、FR的觸點構(gòu)成系統(tǒng)的輸入部分，由輸出設備KM構(gòu)成系統(tǒng)的輸出部分。
圖1 三相異步電動機單向運行電器控制系統(tǒng)
a）主電路 b）控制電路
如果用PLC來控制這臺三相異步電動機，組成一個PLC控制系統(tǒng)，根據(jù)上述分析可知，系統(tǒng)主電路不變，只要將輸入設備SB1、SB2、FR的觸點與PLC的輸入端連接，輸出設備KM線圈與PLC的輸出端連接，就構(gòu)成PLC控制系統(tǒng)的輸入、輸出硬件線路。而控制部分的功能則由PLC的用戶程序來實現(xiàn)，其等效電路如圖2所示。
圖2 PLC的等效電路
圖中，輸入設備SB1、SB2、FR與PLC內(nèi)部的“軟繼電器”X0、X1、X2的“線圈”對應，由輸入設備控制相對應的“軟繼電器”的狀態(tài)，即通過這些“軟繼電器”將外部輸入設備狀態(tài)變成PLC內(nèi)部的狀態(tài)，這類“軟繼電器”稱為輸入繼電器；同理，輸出設備KM與PLC內(nèi)部的“軟繼電器”Y0對應，由“軟繼電器”Y0狀態(tài)控制對應的輸出設備KM的狀態(tài)，即通過這些“軟繼電器”將PLC內(nèi)部狀態(tài)輸出，以控制外部輸出設備，這類“軟繼電器”稱為輸出繼電器。
因此，PLC用戶程序要實現(xiàn)的是：如何用輸入繼電器X0、X1、X2來控制輸出繼電器Y0。當控制要求復雜時，程序中還要采用PLC內(nèi)部的其它類型的“軟繼電器”，如輔助繼電器、定時器、計數(shù)器等，以達到控制要求。
要注意的是，PLC等效電路中的繼電器并不是實際的物理繼電器，它實質(zhì)上是存儲器單元的狀態(tài)。單元狀態(tài)為“１”，相當于繼電器接通；單元狀態(tài)為“０”，則相當于繼電器斷開。因此，我們稱這些繼電器為“軟繼電器”。
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