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工(gong)業上普遍需要測量(liang)(liang)(liang)各類(lei)非電物理量(liang)(liang)(liang),例如(ru)溫(wen)度(du)、壓力、速度(du)、角(jiao)度(du)等,都需要轉(zhuan)換成(cheng)模擬量(liang)(liang)(liang)電信(xin)(xin)號(hao)(hao)才能傳輸(shu)到幾百米外的(de)控制室或(huo)顯示設備(bei)上。這種將物理量(liang)(liang)(liang)轉(zhuan)換成(cheng)電信(xin)(xin)號(hao)(hao)的(de)設備(bei)稱為變送器。工(gong)業上Z廣泛采用(yong)的(de)是用(yong)4~20mA電流來傳輸模擬量信號,這也廣泛的使用與流量計的信號傳輸上面,比如、、等等都是帶有4-20mA信號輸出的。
那么為什么需要采用4-20mA電流信號呢?采用電流信號的原因是不容易受干擾。并且電流源內阻無窮大,導線電阻串聯在回路中不影響精度,在普通雙絞線上可以傳輸數百米。上限取20mA是因為防爆的要求:20mA的電流通斷引起的火花能量不足以引燃瓦斯。下限沒有取0mA的原因是為了能檢測斷線:正常工作時不會低于4mA,當傳輸線因故障斷路,環路電流降為0。常取2mA作為斷線報警值。
電流型變送器將物理量轉換成4~20mA電流輸出,必然要有外電源為其供電。Z典型的是變送器需要兩根電源線,加上兩根電流輸出線,總共要接4根線,稱之為四線制變送器。當然,電流輸出可以與電源公用一根線(公用VCC或者GND),可節省一根線,稱之為三線制變送器。
其實大家可能注意到, 4-20mA電流本身就可以為變送器供電,如圖1C所示。變送器在電路中相當于一個特殊的負載,特殊之處在于變送器的耗電電流在4~20mA之間根據傳感器輸出而變化。顯示儀表只需要串在電路中即可。這種變送器只需外接2根線,因而被稱為兩線制變送器。工業電流環標準下限為4mA,因此只要在量程范圍內,變送器至少有4mA供電。這使得兩線制傳感器的設計成為可能。
在工業應用中,測量點一般在現場,而顯示設備或者控制設備一般都在控制室或控制柜上。兩者之間距離可能數十至數百米。按一百米距離計算,省去2根導線意味著成本降低近百元!因此在應用中兩線制傳感器必然是。
2.兩(liang)(liang)線(xian)(xian)制(zhi)變(bian)(bian)送(song)(song)(song)(song)器(qi)(qi)的(de)(de)(de)(de)(de)結構(gou)(gou)與原理(li)(li)(li)兩(liang)(liang)線(xian)(xian)制(zhi)變(bian)(bian)送(song)(song)(song)(song)器(qi)(qi)的(de)(de)(de)(de)(de)原理(li)(li)(li)是(shi)(shi)利用(yong)了(le)4~20mA信號(hao)為自(zi)身(shen)提供電(dian)(dian)(dian)(dian)(dian)能(neng)(neng)。如(ru)果變(bian)(bian)送(song)(song)(song)(song)器(qi)(qi)自(zi)身(shen)耗電(dian)(dian)(dian)(dian)(dian)大(da)于(yu)4mA,那么將不可(ke)能(neng)(neng)輸出(chu)下限(xian)4mA值。因(yin)此(ci)一(yi)(yi)般(ban)要(yao)求兩(liang)(liang)線(xian)(xian)制(zhi)變(bian)(bian)送(song)(song)(song)(song)器(qi)(qi)自(zi)身(shen)耗電(dian)(dian)(dian)(dian)(dian)(包括傳(chuan)(chuan)感(gan)器(qi)(qi)在內的(de)(de)(de)(de)(de)全部(bu)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu))不大(da)于(yu)3.5mA。這(zhe)是(shi)(shi)兩(liang)(liang)線(xian)(xian)制(zhi)變(bian)(bian)送(song)(song)(song)(song)器(qi)(qi)的(de)(de)(de)(de)(de)設(she)計根(gen)本原則之一(yi)(yi)。從整體(ti)結構(gou)(gou)上來看,兩(liang)(liang)線(xian)(xian)制(zhi)變(bian)(bian)送(song)(song)(song)(song)器(qi)(qi)由三(san)大(da)部(bu)分組成:傳(chuan)(chuan)感(gan)器(qi)(qi)、調(diao)理(li)(li)(li)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)、兩(liang)(liang)線(xian)(xian)制(zhi)V/I變(bian)(bian)換(huan)器(qi)(qi)構(gou)(gou)成。傳(chuan)(chuan)感(gan)器(qi)(qi)將溫度、壓力等物理(li)(li)(li)量轉(zhuan)化(hua)(hua)為電(dian)(dian)(dian)(dian)(dian)參量,調(diao)理(li)(li)(li)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)將傳(chuan)(chuan)感(gan)器(qi)(qi)輸出(chu)的(de)(de)(de)(de)(de)微弱或非(fei)線(xian)(xian)性(xing)的(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)信號(hao)進行放大(da)、調(diao)理(li)(li)(li)、轉(zhuan)化(hua)(hua)為線(xian)(xian)性(xing)的(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)壓輸出(chu)。兩(liang)(liang)線(xian)(xian)制(zhi)V/I變(bian)(bian)換(huan)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)根(gen)據(ju)信號(hao)調(diao)理(li)(li)(li)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)的(de)(de)(de)(de)(de)輸出(chu)控制(zhi)總體(ti)耗電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu);同時從環(huan)路(lu)(lu)(lu)(lu)(lu)(lu)上獲(huo)得電(dian)(dian)(dian)(dian)(dian)壓并穩(wen)壓,供調(diao)理(li)(li)(li)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)和傳(chuan)(chuan)感(gan)器(qi)(qi)使用(yong)。除了(le)V/I變(bian)(bian)換(huan)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)之外,電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)中每個(ge)部(bu)分都(dou)有(you)(you)其自(zi)身(shen)的(de)(de)(de)(de)(de)耗電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu),兩(liang)(liang)線(xian)(xian)制(zhi)變(bian)(bian)送(song)(song)(song)(song)器(qi)(qi)的(de)(de)(de)(de)(de)核心(xin)設(she)計思想(xiang)是(shi)(shi)將所(suo)(suo)有(you)(you)的(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)都(dou)包括在V/I變(bian)(bian)換(huan)的(de)(de)(de)(de)(de)反饋環(huan)路(lu)(lu)(lu)(lu)(lu)(lu)內。如(ru)圖,采樣電(dian)(dian)(dian)(dian)(dian)阻Rs串(chuan)聯在電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)的(de)(de)(de)(de)(de)低端,所(suo)(suo)有(you)(you)的(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)都(dou)將通過Rs流(liu)回(hui)到電(dian)(dian)(dian)(dian)(dian)源負極。從Rs上取到的(de)(de)(de)(de)(de)反饋信號(hao),包含了(le)所(suo)(suo)有(you)(you)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)的(de)(de)(de)(de)(de)耗電(dian)(dian)(dian)(dian)(dian)。在兩(liang)(liang)線(xian)(xian)制(zhi)變(bian)(bian)送(song)(song)(song)(song)器(qi)(qi)中,所(suo)(suo)有(you)(you)的(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)總功(gong)耗不能(neng)(neng)大(da)于(yu)3.5mA,因(yin)此(ci)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)的(de)(de)(de)(de)(de)低功(gong)耗成為主要(yao)的(de)(de)(de)(de)(de)設(she)計難(nan)點(dian)。下面將逐(zhu)一(yi)(yi)分析各個(ge)部(bu)分電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)的(de)(de)(de)(de)(de)原理(li)(li)(li)與設(she)計要(yao)點(dian)。
3.兩線制(zhi)(zhi)V/I變(bian)換(huan)器(qi)(qi)(qi)(qi)(qi) V/I 變(bian)換(huan)器(qi)(qi)(qi)(qi)(qi)是(shi)一(yi)種可(ke)以(yi)用電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)信號(hao)控(kong)制(zhi)(zhi)輸(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)。兩線制(zhi)(zhi)V/I變(bian)換(huan)器(qi)(qi)(qi)(qi)(qi)與(yu)一(yi)般V/I變(bian)換(huan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)不(bu)(bu)(bu)同(tong)點(dian)在(zai)(zai)(zai)(zai):電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)信號(hao)不(bu)(bu)(bu)是(shi)直接控(kong)制(zhi)(zhi)輸(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu),而是(shi)控(kong)制(zhi)(zhi)整個(ge)(ge)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)自身耗(hao)(hao)(hao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)。同(tong)時(shi),還要(yao)(yao)從(cong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)環路(lu)(lu)(lu)(lu)(lu)(lu)上(shang)(shang)提取穩(wen)定的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)為調(diao)(diao)理(li)(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)和傳感器(qi)(qi)(qi)(qi)(qi)供電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。附圖是(shi)兩線制(zhi)(zhi)V/I變(bian)換(huan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)的(de)(de)(de)基(ji)(ji)本(ben)原(yuan)理(li)(li)圖:圖中(zhong)OP1、Q1、R1、R2、Rs構(gou)(gou)成(cheng)(cheng)(cheng)了V/I變(bian)換(huan)器(qi)(qi)(qi)(qi)(qi)。分析(xi)負反(fan)(fan)饋過(guo)(guo)程:若A點(dian)因(yin)(yin)為某(mou)種原(yuan)因(yin)(yin)高于(yu)(yu)0V,則運(yun)(yun)(yun)放(fang)(fang)OP1輸(shu)(shu)出(chu)升(sheng)高,Re兩端(duan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)升(sheng)高,通(tong)(tong)(tong)過(guo)(guo)Re的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)變(bian)大(da)(da)。相(xiang)當于(yu)(yu)整體耗(hao)(hao)(hao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)變(bian)大(da)(da),通(tong)(tong)(tong)過(guo)(guo)采樣(yang)(yang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)Rs的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)也變(bian)大(da)(da),B點(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)變(bian)低(di)(負更多)。結果是(shi)通(tong)(tong)(tong)過(guo)(guo)R2將(jiang)A點(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)拉下來。反(fan)(fan)之(zhi),若A點(dian)因(yin)(yin)某(mou)種原(yuan)因(yin)(yin)低(di)于(yu)(yu)0V,也會被負反(fan)(fan)饋抬高回0V。總(zong)之(zhi),負反(fan)(fan)饋的(de)(de)(de)結果是(shi)運(yun)(yun)(yun)放(fang)(fang)OP1虛短,A點(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)=0V。下面(mian)分析(xi)Vo對總(zong)耗(hao)(hao)(hao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)(de)(de)控(kong)制(zhi)(zhi)原(yuan)理(li)(li):假設調(diao)(diao)理(li)(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)輸(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)為Vo,則流(liu)(liu)(liu)(liu)過(guo)(guo)R1的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu) I1=Vo/R1 運(yun)(yun)(yun)放(fang)(fang)輸(shu)(shu)入端(duan)不(bu)(bu)(bu)可(ke)能(neng)(neng)(neng)(neng)吸收電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu),則I1全部流(liu)(liu)(liu)(liu)過(guo)(guo)R2,那(nei)么(me)B點(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya) VB= -I1*R2 = -Vo*R2/R1 取R1=R2時(shi),有(you)VB=-Vo 電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源負和整個(ge)(ge)便送(song)器(qi)(qi)(qi)(qi)(qi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)之(zhi)間(jian)只有(you)Rs、R2兩個(ge)(ge)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu),因(yin)(yin)此(ci)(ci)所有(you)的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)都流(liu)(liu)(liu)(liu)過(guo)(guo)Rs和R2。R2上(shang)(shang)端(duan)是(shi)虛地(0V),Rs上(shang)(shang)端(duan)是(shi)GND。因(yin)(yin)此(ci)(ci)R2、Rs兩端(duan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)*一(yi)樣(yang)(yang),都等于(yu)(yu)VB 。相(xiang)當于(yu)(yu)Rs與(yu)R2并(bing)聯(lian)作(zuo)(zuo)為電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)采樣(yang)(yang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)。因(yin)(yin)此(ci)(ci)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)總(zong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu): Is=Vo/(Rs//R2)如果取R2>>Rs,Is=Vo/Rs 因(yin)(yin)此(ci)(ci),圖3中(zhong)取Rs=100歐,當調(diao)(diao)理(li)(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)輸(shu)(shu)出(chu)0.4~2V的(de)(de)(de)時(shi)候,總(zong)耗(hao)(hao)(hao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)4~20mA. 若不(bu)(bu)(bu)能(neng)(neng)(neng)(neng)滿足R2>>Rs也沒(mei)關(guan)系,Rs與(yu) R2并(bing)聯(lian)(Rs//R2)是(shi)個(ge)(ge)固(gu)定值,Is與(yu)Vo仍(reng)然是(shi)線性關(guan)系,誤差比例系數在(zai)(zai)(zai)(zai)校準時(shi)可(ke)以(yi)消(xiao)除。除了電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)正確以(yi)外,該電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)正常(chang)工(gong)作(zuo)(zuo)還需要(yao)(yao)2個(ge)(ge)條件:首(shou)先(xian)要(yao)(yao)自身耗(hao)(hao)(hao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)盡(jin)量小(xiao),省下的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)還要(yao)(yao)供給(gei)調(diao)(diao)理(li)(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)以(yi)及變(bian)送(song)器(qi)(qi)(qi)(qi)(qi)。其次(ci)要(yao)(yao)求(qiu)運(yun)(yun)(yun)放(fang)(fang)能(neng)(neng)(neng)(neng)夠單(dan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源工(gong)作(zuo)(zuo),即(ji)在(zai)(zai)(zai)(zai)沒(mei)有(you)負電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源情況(kuang)下,輸(shu)(shu)入端(duan)仍(reng)能(neng)(neng)(neng)(neng)夠接受(shou)0V輸(shu)(shu)入,并(bing)能(neng)(neng)(neng)(neng)正常(chang)工(gong)作(zuo)(zuo)。 LM358/324是(shi)Z常(chang)見也是(shi)*的(de)(de)(de)單(dan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源運(yun)(yun)(yun)放(fang)(fang),耗(hao)(hao)(hao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)400uA/每運(yun)(yun)(yun)放(fang)(fang),基(ji)(ji)本(ben)可(ke)以(yi)接受(shou)。單(dan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源供電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)時(shi),輸(shu)(shu)入端(duan)從(cong)-0.3V~Vcc-1.5V范圍內都能(neng)(neng)(neng)(neng)正常(chang)工(gong)作(zuo)(zuo)。如果換(huan)成(cheng)(cheng)(cheng)OP07等精密放(fang)(fang)大(da)(da)器(qi)(qi)(qi)(qi)(qi),因(yin)(yin)為輸(shu)(shu)入不(bu)(bu)(bu)允許低(di)至0V,在(zai)(zai)(zai)(zai)該電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)中(zhong)反(fan)(fan)而無法工(gong)作(zuo)(zuo)。 R5和U1構(gou)(gou)成(cheng)(cheng)(cheng)基(ji)(ji)準源,產生2.5V穩(wen)定的(de)(de)(de)基(ji)(ji)準電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)。LM385是(shi)低(di)成(cheng)(cheng)(cheng)本(ben)的(de)(de)(de)微(wei)功(gong)耗(hao)(hao)(hao)基(ji)(ji)準,20uA以(yi)上(shang)(shang)即(ji)可(ke)工(gong)作(zuo)(zuo),手冊上(shang)(shang)給(gei)出(chu)的(de)(de)(de)曲線在(zai)(zai)(zai)(zai)100uA附近(jin)Z平坦(tan),所以(yi)通(tong)(tong)(tong)過(guo)(guo)R5控(kong)制(zhi)(zhi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)100uA左右。OP2構(gou)(gou)成(cheng)(cheng)(cheng)一(yi)個(ge)(ge)同(tong)向(xiang)放(fang)(fang)大(da)(da)器(qi)(qi)(qi)(qi)(qi),將(jiang)基(ji)(ji)準放(fang)(fang)大(da)(da),向(xiang)調(diao)(diao)理(li)(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)及傳感器(qi)(qi)(qi)(qi)(qi)供電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。因(yin)(yin)為寬(kuan)輸(shu)(shu)入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)、低(di)功(gong)耗(hao)(hao)(hao)的(de)(de)(de)穩(wen)壓(ya)(ya)器(qi)(qi)(qi)(qi)(qi),成(cheng)(cheng)(cheng)本(ben)高;將(jiang)基(ji)(ji)準放(fang)(fang)大(da)(da)作(zuo)(zuo)為穩(wen)壓(ya)(ya)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源是(shi)一(yi)個(ge)(ge)廉價的(de)(de)(de)方(fang)案。該部分電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)也可(ke)以(yi)選擇現成(cheng)(cheng)(cheng)的(de)(de)(de)集(ji)成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu)。比如XTR115/116/105等,精度(du)和穩(wen)定性比自制(zhi)(zhi)的(de)(de)(de)好,自身功(gong)耗(hao)(hao)(hao)也更低(di)(意味著能(neng)(neng)(neng)(neng)留更多電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)給(gei)調(diao)(diao)理(li)(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)(lu),調(diao)(diao)理(li)(li)部分更容易設計(ji))。但成(cheng)(cheng)(cheng)本(ben)比上(shang)(shang)述方(fang)案高10倍以(yi)上(shang)(shang).
4.兩線制(zhi)壓(ya)(ya)力(li)變(bian)送器(qi)設計壓(ya)(ya)力(li)橋(qiao)、稱重傳(chuan)感(gan)器(qi)輸(shu)出信(xin)號微(wei)弱,都(dou)屬于(yu)mV級信(xin)號。這(zhe)一(yi)(yi)(yi)類小(xiao)信(xin)號一(yi)(yi)(yi)般(ban)都(dou)要求用差動(dong)放大器(qi)對其進行*級放大。一(yi)(yi)(yi)般(ban)選用低(di)(di)失調(diao)、低(di)(di)溫飄的(de)差動(dong)放大器(qi)。另(ling)外在兩線制(zhi)應(ying)用中(zhong),低(di)(di)功耗(hao)(hao)也是(shi)必需的(de)。AD623是(shi)常(chang)用的(de)低(di)(di)功耗(hao)(hao)精(jing)密(mi)差動(dong)放大器(qi),常(chang)用在差分輸(shu)出前級的(de)放大。 AD623失調(diao)Z大200uV,溫飄1uV/度(du)(du),在一(yi)(yi)(yi)般(ban)壓(ya)(ya)力(li)變(bian)送應(ying)用保證了精(jing)度(du)(du)足夠。 R0將0.4V疊加在AD623的(de)REF腳(5腳)上,在壓(ya)(ya)力(li)=0情況下通(tong)過(guo)調(diao)整(zheng)R0使(shi)輸(shu)出4mA,再(zai)調(diao)整(zheng)RG輸(shu)出20.00mA,完成校準。電路(lu)設計時(shi)需注意(yi),壓(ya)(ya)力(li)橋(qiao)傳(chuan)感(gan)器(qi)相當(dang)于(yu)一(yi)(yi)(yi)個(ge)千(qian)歐級的(de)電阻,耗(hao)(hao)電一(yi)(yi)(yi)般(ban)比較大。適當(dang)降低(di)(di)壓(ya)(ya)力(li)橋(qiao)的(de)激勵電壓(ya)(ya)可以減小(xiao)耗(hao)(hao)電電流。但是(shi)輸(shu)出幅度(du)(du)也隨之(zhi)下降,需要提高AD623的(de)增益。圖(tu)6給出的(de)傳(chuan)感(gan)器(qi)采用恒(heng)壓(ya)(ya)供(gong)(gong)電,實際(ji)應(ying)用中(zhong)大部分半導(dao)體壓(ya)(ya)力(li)傳(chuan)感(gan)器(qi)需要恒(heng)流供(gong)(gong)電才能獲得(de)較好的(de)溫度(du)(du)特性(xing),可以用一(yi)(yi)(yi)個(ge)運放構成恒(heng)流源為其提供(gong)(gong)激勵。
5.穩(wen)(wen)定性和(he)安全性的(de)考慮(lv)工業環(huan)境下環(huan)境惡劣且對可靠性要(yao)求高,因此兩線(xian)制變(bian)送器的(de)設(she)計(ji)上需要(yao)考慮(lv)一定的(de)保護(hu)和(he)增強(qiang)穩(wen)(wen)定性措施。
(1)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)保(bao)護。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)接(jie)反(fan)(fan)、超壓(ya)、浪(lang)涌(yong)是(shi)(shi)工(gong)業(ye)上常見的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)問題。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)接(jie)反(fan)(fan)是(shi)(shi)設備安裝接(jie)線時Z容(rong)(rong)易(yi)發生的(de)(de)(de)(de)錯誤,輸入口串一(yi)只二極管即(ji)可(ke)(ke)防(fang)止(zhi)接(jie)反(fan)(fan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)時損(sun)(sun)壞(huai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)。如(ru)果(guo)輸入端加一(yi)個全橋整流(liu)器(qi)(qi),那么即(ji)使電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)接(jie)反(fan)(fan)仍(reng)能(neng)正常工(gong)作(zuo)。為(wei)防(fang)止(zhi)雷擊(ji)、靜電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)、浪(lang)涌(yong)等能(neng)量損(sun)(sun)壞(huai)變送(song)器(qi)(qi),變送(song)器(qi)(qi)入口處可(ke)(ke)以加裝一(yi)只TVS管來(lai)吸收(shou)瞬(shun)間過(guo)(guo)壓(ya)的(de)(de)(de)(de)能(neng)量。一(yi)般(ban)TVS電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)值(zhi)取(qu)比運放極限(xian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)略低,才能(neng)起到保(bao)護作(zuo)用(yong)。如(ru)果(guo)可(ke)(ke)能(neng)遭受雷擊(ji),TVS可(ke)(ke)能(neng)吸收(shou)容(rong)(rong)量不(bu)(bu)夠,壓(ya)敏(min)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)也是(shi)(shi)必需的(de)(de)(de)(de),但是(shi)(shi)壓(ya)敏(min)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)本身漏電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)會(hui)帶來(lai)一(yi)定(ding)(ding)誤差。 2.寬電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)適應能(neng)力。一(yi)般(ban)兩線制變送(song)器(qi)(qi)都(dou)能(neng)適應大(da)范(fan)圍的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)變化(hua)(hua)而不(bu)(bu)影響精(jing)度。這(zhe)樣可(ke)(ke)以適用(yong)各類電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan),同(tong)時能(neng)夠適應大(da)的(de)(de)(de)(de)負(fu)載(zai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)。對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)Z敏(min)感的(de)(de)(de)(de)部分是(shi)(shi)基(ji)準(zhun)源(yuan)(yuan)(yuan),同(tong)時基(ji)準(zhun)源(yuan)(yuan)(yuan)也是(shi)(shi)決定(ding)(ding)精(jing)度的(de)(de)(de)(de)主(zhu)要元件。3樓圖(tu)中(zhong)基(ji)準(zhun)通過(guo)(guo)R5限(xian)流(liu),當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)變化(hua)(hua)時,R5上電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)也隨之改變,對(dui)基(ji)準(zhun)穩定(ding)(ding)性(xing)影響很(hen)大(da)。附圖(tu)中(zhong)利用(yong)恒流(liu)源(yuan)(yuan)(yuan)LM334為(wei)基(ji)準(zhun)供電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian),電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)大(da)范(fan)圍變化(hua)(hua)時,電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)基(ji)本不(bu)(bu)變,保(bao)證了(le)基(ji)準(zhun)的(de)(de)(de)(de)穩定(ding)(ding)性(xing)。 3.退(tui)藕電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)一(yi)般(ban)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)設計中(zhong),每(mei)個集(ji)成電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)(yuan)端都(dou)會(hui)有退(tui)藕電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)。在兩線制變送(song)器(qi)(qi)上電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)時,這(zhe)些電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)的(de)(de)(de)(de)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)會(hui)在瞬(shun)間導(dao)致大(da)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu),有可(ke)(ke)能(neng)會(hui)損(sun)(sun)壞(huai)遠(yuan)方儀表。因(yin)此每(mei)個退(tui)藕電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)一(yi)般(ban)不(bu)(bu)超過(guo)(guo)10nF,總退(tui)藕電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)不(bu)(bu)宜超過(guo)(guo)50nF。入口處一(yi)個10nF電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)是(shi)(shi)必需的(de)(de)(de)(de),保(bao)證長(chang)線感性(xing)負(fu)載(zai)下,電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)不(bu)(bu)震蕩。