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  關(guān)于PLC，我是個(gè)初學(xué)者，今天在此對(duì)自己前一段時(shí)間的學(xué)習(xí)做個(gè)總結(jié)，以備后用。由于本人水平有限，本文一定還存在不少錯(cuò)誤和不妥之處，敬請(qǐng)大家批評(píng)指正。
     本文所有程序，均在Step7Template.mwp中編寫，CPU類型選西門子 S7—200系列226 CN。
     先說程序，測低速(每分鐘1000轉(zhuǎn)以下) ：   LD    I0.0
                                                                         LD    M20.1
                                                                        CTU   C0,+5000    //設(shè)定增計(jì)數(shù)器上限
                                                                        
                                                                        LD    SM0.0
                                                                         A     M20.0
                                                                       TON   T37,+30      //立即接通，延時(shí)三秒斷開

                                                                        LD    SM0.0
                                                                         O     M20.0#p#分頁標(biāo)題#e#
                                                                         AN    T37
                                                                         =     M20.0         //設(shè)定T37定時(shí)器復(fù)位信號(hào)

                                                                        LD    M20.0
                                                                        LD    M20.1
                                                                       CTU   C1,+20     //設(shè)定增計(jì)數(shù)器C1上限

                                                                       LD      C1
                                                                      MOVW  C0,VW200     
                                                                        =     M20.2

                                                                      LD    M20.2

                                                                      =     M20.1       //設(shè)定增計(jì)數(shù)器C1的復(fù)位信號(hào)
外圍電路下面介紹，先講程序。由I0.0作輸入口，T37延時(shí)三秒后給C1一個(gè)信號(hào)，C1計(jì)1，然后T37被復(fù)位，再延時(shí)，3秒后C1計(jì)2，…
直到C1計(jì)到20。20個(gè)三秒就是一分鐘 ，期間I0.0口的脈沖信號(hào)由C0計(jì)數(shù)器計(jì)數(shù)，計(jì)滿一分鐘就把結(jié)果移到VW200中。脈沖信號(hào)由外圍電路得到。
       以上程序能測一千以下的轉(zhuǎn)速，實(shí)驗(yàn)時(shí)上限是一千四百多吧。由于該程序用的是低速計(jì)數(shù)器，轉(zhuǎn)速較高時(shí)，受PLC時(shí)鐘周期影響，在一分鐘時(shí)轉(zhuǎn)速還未記好，C0就被清零，所以會(huì)有上限。接下來我們用PLC內(nèi)部不受時(shí)鐘周期影響的高速計(jì)數(shù)器來測高速（一千轉(zhuǎn)以上）：
                                                           LD     SM0.1
                                                          CALL   SBR_0               //調(diào)用高速計(jì)數(shù)器初始化子程序
 
                                                          LD     SM0.0
                                                           A      M20.0
                                                          TON   T37,+100           //設(shè)定計(jì)時(shí)器值，延時(shí)10秒
                                                      
                                                         LD    SM0.0
                                                          O     M20.0

                                                         AN    T37
                                                          =     M20.0                   //立即接通延時(shí)十秒斷開

                                                         LD     T37                       
                                                         MOVD   HC0, VD100               //I0.0為高速計(jì)數(shù)器HC0輸入口，計(jì)數(shù)結(jié)果移入VD100
                                                         MOVD   VD100, VD200
                                                         MUL    +6, VD200                  //計(jì)數(shù)結(jié)果乘以6，放入VD200
                                                        LD     T37
                                                        CALL   SBR_0                      //T37計(jì)時(shí)到，調(diào)用高速計(jì)數(shù)器初始化子程序
 
                                                           SBR_0                              //高速計(jì)數(shù)器初始化子程序#p#分頁標(biāo)題#e#
                                                    
                                                       LD     SM0.0
                                                       MOVB   16#F8, SMB37                //設(shè)置控制位：增計(jì)數(shù)；已使能；
                                                       MOVD   +0, SMD38                   //裝載 CV
                                                       MOVD   +0, SMD42                   //裝載 PV
                                                       HDEF   0, 0
                                                       HSC    0
       以上程序測速范圍為1000轉(zhuǎn)以上，實(shí)驗(yàn)時(shí)測得最高為2500+，因?qū)嶒?yàn)條件有限，上限未知。有人會(huì)有疑問：你怎么采用的是測10秒，然后將計(jì)數(shù)結(jié)果乘以6當(dāng)做一分鐘的轉(zhuǎn)速，而不直接測一分鐘的轉(zhuǎn)速呢？ 因?yàn)檗D(zhuǎn)速結(jié)果最后要用數(shù)碼管顯示出來，對(duì)觀察者來說，10秒以后顯示與一分鐘后顯示相比，前者更好一些。最后我們采用的是6乘以10的策略。
      最后來說說外圍電路吧。外圍電路把轉(zhuǎn)速轉(zhuǎn)換成脈沖信號(hào)輸入PLC，上面兩段程序用的都是I0.0口。信號(hào)的轉(zhuǎn)換和采集用霍爾傳感器，
    
接法如圖：VCC接24V，GND接電源負(fù)極，A接信號(hào)輸入端I0.0，A端和24V間接電阻。接好后將霍爾元件平的一面朝被測物體固定好，如一個(gè)輪子，在輪子面上霍爾對(duì)應(yīng)位置安裝霍爾磁體。測速原理：輪子每轉(zhuǎn)一圈，磁體和霍爾元件接觸一次（其實(shí)是接近），它們接觸時(shí)帶來A端電壓的降低，由此給PLC一個(gè)脈沖信號(hào)。磁體和霍爾之間間距3至5mm。安裝時(shí)注意磁體的正反面。
      好了，測轉(zhuǎn)速就先說這么多。經(jīng)過以上這些，測得的數(shù)據(jù)只是放在PLC的內(nèi)存里，我們可以在軟件里監(jiān)測PLC運(yùn)行情況，看到這些數(shù)據(jù)。是不是有點(diǎn)麻煩，有沒有更好的方法能看到這些數(shù)據(jù)？當(dāng)然，可以用LED數(shù)碼管來把數(shù)據(jù)顯示出來。