? ? 大(da)家可能會非(fei)常熟悉RS232，RS485，CAN等(deng)(deng)工業(ye)上(shang)常用(yong)(yong)的(de)總(zong)線，他們(men)都是(shi)傳(chuan)輸(shu)數字信(xin)號(hao)的(de)方式。那(nei)么(me)，我們(men)用(yong)(yong)什么(me)方式來傳(chuan)輸(shu)模擬(ni)(ni)信(xin)號(hao)呢？工業(ye)上(shang)普遍需(xu)要測量(liang)各(ge)類非(fei)電物理量(liang)，例如溫度(du)、壓力、速度(du)、角度(du)等(deng)(deng)，這些都需(xu)要轉換成模擬(ni)(ni)量(liang)電信(xin)號(hao)才能傳(chuan)輸(shu)到幾(ji)百米外的(de)控制室(shi)或顯(xian)示設備上(shang)。工業(ye)上(shang)最廣泛采用(yong)(yong)的(de)是(shi)用(yong)(yong)4~20mA電流來傳(chuan)輸(shu)模擬(ni)(ni)量(liang)。

? ? 采用電流信號的原因是不容易受干擾，因為工業現場的噪聲電壓的幅度可能達到數V，但是噪聲的功率很弱，所以噪聲電流通常小于nA級別，因此給4－20mA傳輸帶來的誤差非常小；電流源內阻趨于無窮大，導線電阻串聯在回路中不影響精度，因此在普通雙絞線上可以傳輸數百米；由于電流源的大內阻和恒流輸出，在接收端我們只需放置一個250歐姆到地的電阻就可以獲得0－5V的電壓，低輸入阻抗的接收器的好處是nA級的輸入電流噪聲只產生非常微弱的電壓噪聲。

? ? 上限(xian)取20mA是(shi)因(yin)(yin)為(wei)(wei)防爆(bao)的(de)(de)要(yao)求：20mA的(de)(de)電(dian)(dian)(dian)流(liu)通斷(duan)引起的(de)(de)火花能(neng)量(liang)不足以(yi)(yi)引燃瓦斯。下限(xian)沒有取0mA的(de)(de)原因(yin)(yin)是(shi)為(wei)(wei)了能(neng)檢測斷(duan)線(xian)(xian)(xian)(xian)(xian)：正常(chang)工(gong)(gong)作時不會低于4mA，當傳輸(shu)線(xian)(xian)(xian)(xian)(xian)因(yin)(yin)故(gu)障(zhang)斷(duan)路，環路電(dian)(dian)(dian)流(liu)降為(wei)(wei)0。常(chang)取2mA作為(wei)(wei)斷(duan)線(xian)(xian)(xian)(xian)(xian)報警值(zhi)。電(dian)(dian)(dian)流(liu)型變(bian)(bian)(bian)送器(qi)(qi)將物理(li)量(liang)轉換成(cheng)4~20mA電(dian)(dian)(dian)流(liu)輸(shu)出，必然要(yao)有外電(dian)(dian)(dian)源為(wei)(wei)其供(gong)電(dian)(dian)(dian)。最典型的(de)(de)是(shi)變(bian)(bian)(bian)送器(qi)(qi)需(xu)(xu)要(yao)兩(liang)(liang)(liang)根電(dian)(dian)(dian)源線(xian)(xian)(xian)(xian)(xian)，加上兩(liang)(liang)(liang)根電(dian)(dian)(dian)流(liu)輸(shu)出線(xian)(xian)(xian)(xian)(xian)，總共要(yao)接4根線(xian)(xian)(xian)(xian)(xian)，稱(cheng)之為(wei)(wei)四線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)變(bian)(bian)(bian)送器(qi)(qi)。當然，電(dian)(dian)(dian)流(liu)輸(shu)出可(ke)(ke)以(yi)(yi)與電(dian)(dian)(dian)源公(gong)用一根線(xian)(xian)(xian)(xian)(xian)（公(gong)用VCC或者(zhe)GND），可(ke)(ke)節省一根線(xian)(xian)(xian)(xian)(xian)，所以(yi)(yi)現在基本上將四線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)變(bian)(bian)(bian)送器(qi)(qi)稱(cheng)之為(wei)(wei)三線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)變(bian)(bian)(bian)送器(qi)(qi)。其實大家(jia)可(ke)(ke)能(neng)注意(yi)到， 4-20mA電(dian)(dian)(dian)流(liu)本身就(jiu)可(ke)(ke)以(yi)(yi)為(wei)(wei)變(bian)(bian)(bian)送器(qi)(qi)供(gong)電(dian)(dian)(dian)，變(bian)(bian)(bian)送器(qi)(qi)在電(dian)(dian)(dian)路中相當于一個(ge)特(te)殊的(de)(de)負(fu)載(zai)，這種變(bian)(bian)(bian)送器(qi)(qi)只需(xu)(xu)外接2根線(xian)(xian)(xian)(xian)(xian)，因(yin)(yin)而被(bei)稱(cheng)為(wei)(wei)兩(liang)(liang)(liang)線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)變(bian)(bian)(bian)送器(qi)(qi)。工(gong)(gong)業電(dian)(dian)(dian)流(liu)環標準下限(xian)為(wei)(wei)4mA，因(yin)(yin)此(ci)在量(liang)程范(fan)圍內，變(bian)(bian)(bian)送器(qi)(qi)通常(chang)只有24V，4mA供(gong)電(dian)(dian)(dian)（因(yin)(yin)此(ci)，在輕負(fu)載(zai)條件下高效率(lv)的(de)(de)DC/DC電(dian)(dian)(dian)源（TPS54331,TPS54160），低功耗(hao)的(de)(de)傳感(gan)器(qi)(qi)和信號鏈產品、以(yi)(yi)及(ji)低功耗(hao)的(de)(de)處理(li)器(qi)(qi)（如(ru)MSP430）對于兩(liang)(liang)(liang)線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)的(de)(de)4-20mA收(shou)發非常(chang)重要(yao)）。這使(shi)得兩(liang)(liang)(liang)線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)傳感(gan)器(qi)(qi)的(de)(de)設計(ji)成(cheng)為(wei)(wei)可(ke)(ke)能(neng)而又(you)富有挑戰。

? ? 一般需要設計一個VI轉換器，輸入0-3.3v，輸出4mA-20mA，可采(cai)用(yong)運(yun)放(fang)LM358，供電(dian)+12v。

我們系統地來看看模擬量設備為什么都偏愛用4~20mA傳輸信號~

? ? 4-20mA. DC(1-5V.DC)信號制是國際電工委員會( IEC )過程控制系統采用的模擬信號傳輸標準。我國也采用這一國際標準信號制，儀表傳輸信號采用4-20mA.DC，接收信號采用1-5V.DC，即采用電流傳輸、電壓接收的信號系統。

? ? 一般(ban)儀器(qi)儀表的(de)信(xin)號(hao)電(dian)(dian)流都為4-20mA，指(zhi)最(zui)小電(dian)(dian)流為4mA,最(zui)大電(dian)(dian)流為20mA 。傳(chuan)輸信(xin)號(hao)時候，因為導(dao)線(xian)上也有電(dian)(dian)阻，如果用電(dian)(dian)壓(ya)傳(chuan)輸則會在導(dao)線(xian)內產生一定的(de)壓(ya)降，那接收端的(de)信(xin)號(hao)就會產生一定的(de)誤差了，所以一般(ban)使用電(dian)(dian)流信(xin)號(hao)作為變送器(qi)的(de)標準(zhun)傳(chuan)輸。

? ? 4~20mA.DC（1~5V.DC）信(xin)號(hao)制是(shi)國際電工(gong)委(wei)員會（IEC）：過程控制系(xi)(xi)統用模(mo)擬(ni)信(xin)號(hao)標準。我國從DDZ-Ⅲ型(xing)電動儀表(biao)開始采(cai)(cai)用這一國際標準信(xin)號(hao)制，儀表(biao)傳(chuan)輸信(xin)號(hao)采(cai)(cai)用4~20mA.DC，聯(lian)絡信(xin)號(hao)采(cai)(cai)用1~5V.DC，即(ji)采(cai)(cai)用電流(liu)傳(chuan)輸、電壓接收的信(xin)號(hao)系(xi)(xi)統。

4~20mA電流環工作原理：

? ? 在工業現場，用一(yi)個(ge)儀表(biao)放(fang)大器來完成信號的(de)調理并進行長線傳(chuan)輸(shu)，會(hui)產生以下問題(ti)：第一(yi)，由于傳(chuan)輸(shu)的(de)信號是電壓信號，傳(chuan)輸(shu)線會(hui)受到噪聲(sheng)的(de)干擾；第二，傳(chuan)輸(shu)線的(de)分布電阻(zu)會(hui)產生電壓降；第三(san)，在現場如何提(ti)供儀表(biao)放(fang)大器的(de)工作電壓也是個(ge)問題(ti)。

? ? 為了解決上述問題(ti)和避(bi)開相關(guan)噪聲(sheng)的(de)影響，我們(men)用電(dian)(dian)流來傳輸(shu)信號，因(yin)為電(dian)(dian)流對(dui)噪聲(sheng)并(bing)不敏感。4～20mA的(de)電(dian)(dian)流環(huan)便是用4mA表(biao)示零信號，用20mA表(biao)示信號的(de)滿刻度(du)，而低于(yu)(yu)4mA高于(yu)(yu)20mA的(de)信號用于(yu)(yu)各種故障的(de)報警(jing)。

? ? 現場(chang)儀(yi)表(biao)可實現兩線制，所(suo)謂兩線制即(ji)電(dian)源、負載串聯(lian)在(zai)一起(qi)，有(you)一公共點，而現場(chang)變送器(qi)與控制室(shi)儀(yi)表(biao)之前的信號聯(lian)絡及供電(dian)僅(jin)用兩根(gen)電(dian)線。因為信號起(qi)點電(dian)流(liu)為4mA.DC，為變送器(qi)提供了靜態工作電(dian)流(liu)，同時(shi)儀(yi)表(biao)電(dian)氣零(ling)點為4mA.DC，不與機械(xie)零(ling)點重合，這種“活零(ling)點”有(you)利(li)(li)于識別斷(duan)電(dian)和斷(duan)線等故(gu)障。而且兩線制還便于使用安全(quan)柵，利(li)(li)于安全(quan)防爆。

? ? 控制(zhi)室儀表(biao)采用電(dian)壓并聯信號傳(chuan)輸(shu)，同(tong)一(yi)個控制(zhi)系統所屬的儀表(biao)之(zhi)間有公(gong)共端，便于檢測儀表(biao)、調節儀表(biao)、計算機、報(bao)警裝(zhuang)置配用，并方便接(jie)線。

? ? 現(xian)場儀表(biao)與控制室儀表(biao)之間的聯絡信(xin)(xin)號采用(yong)4~20mA.DC的理由是：因為現(xian)場與控制室之間的距(ju)離較遠(yuan)，連接(jie)電(dian)(dian)線的電(dian)(dian)阻較大，如果(guo)用(yong)電(dian)(dian)壓信(xin)(xin)號遠(yuan)傳(chuan)，優于電(dian)(dian)線電(dian)(dian)阻與接(jie)收儀表(biao)輸入電(dian)(dian)阻的分壓，將(jiang)產生較大的誤(wu)差，而用(yong)恒流源信(xin)(xin)號作為遠(yuan)傳(chuan)，只要傳(chuan)送回路不出現(xian)分支，回路中(zhong)的電(dian)(dian)流就不會隨電(dian)(dian)線長短而改變，從而保證(zheng)了傳(chuan)送的精度。

? ? 控(kong)制(zhi)室儀(yi)表(biao)(biao)之(zhi)間的聯絡信號采用1~5V.DC理由是：為了(le)便于多臺(tai)儀(yi)表(biao)(biao)共同(tong)接收同(tong)一(yi)(yi)個信號，并有利于接線(xian)和構成各種復雜的控(kong)制(zhi)系統。如(ru)果用電流源作聯絡信號，當多臺(tai)儀(yi)表(biao)(biao)共同(tong)接收同(tong)一(yi)(yi)個信號時，它們的輸入電阻必須串聯起來，這(zhe)會(hui)(hui)使最大(da)負(fu)載(zai)(zai)電阻超(chao)過變送(song)儀(yi)表(biao)(biao)的負(fu)載(zai)(zai)能力(li)，而且(qie)各接收儀(yi)表(biao)(biao)的信號負(fu)端電位各不相同(tong)，會(hui)(hui)引入干擾，而且(qie)不能做到單一(yi)(yi)集中供電。

? ? 采(cai)用電(dian)壓源信(xin)號聯絡，與現(xian)場儀表(biao)的聯絡用的電(dian)流(liu)信(xin)號必須轉換為(wei)(wei)電(dian)壓信(xin)號，最(zui)簡單的辦法就是：在電(dian)流(liu)傳送回路(lu)中串聯一個250Ω的標準電(dian)阻(zu)，把4~20mA.DC轉換為(wei)(wei)1~5V.DC，通常由配電(dian)器來(lai)完(wan)成這(zhe)一任務。

1、首先是從現場應用的安全考慮

? ? 安(an)全(quan)重(zhong)點是以防爆(bao)安(an)全(quan)火花型儀表來考慮的(de)(de)，并以控制儀表能(neng)量為前(qian)提，把維持儀表正(zheng)常工作的(de)(de)靜態和動態功耗降低(di)到最低(di)限度。輸出4~20mA.DC標準信號的(de)(de)變(bian)送(song)器(qi)，其電(dian)(dian)(dian)(dian)源電(dian)(dian)(dian)(dian)壓通常采用24V.DC，采用直流電(dian)(dian)(dian)(dian)壓的(de)(de)主要原(yuan)因是可以不用大容(rong)(rong)量的(de)(de)電(dian)(dian)(dian)(dian)容(rong)(rong)器(qi)及(ji)電(dian)(dian)(dian)(dian)感(gan)(gan)器(qi)，就(jiu)只需(xu)考慮變(bian)送(song)器(qi)與控制室儀表連接導線的(de)(de)分(fen)(fen)布電(dian)(dian)(dian)(dian)容(rong)(rong)及(ji)電(dian)(dian)(dian)(dian)感(gan)(gan)，如2mm2 的(de)(de)導線其分(fen)(fen)布電(dian)(dian)(dian)(dian)容(rong)(rong)為0.05μ/km左右；對于單線的(de)(de)電(dian)(dian)(dian)(dian)感(gan)(gan)為0.4mH/km左右；大大低(di)于引爆(bao)氫氣的(de)(de)數(shu)值，顯然這對防爆(bao)是非常有(you)利(li)的(de)(de)。

2、傳送信號用電流源優于電壓源

? ? 因為(wei)現場(chang)與(yu)控制室之間的(de)距離較(jiao)(jiao)遠，連(lian)接電(dian)(dian)(dian)(dian)線(xian)的(de)電(dian)(dian)(dian)(dian)阻(zu)較(jiao)(jiao)大(da)時，如(ru)果(guo)用電(dian)(dian)(dian)(dian)壓(ya)源信(xin)號(hao)遠傳(chuan)，由于電(dian)(dian)(dian)(dian)線(xian)電(dian)(dian)(dian)(dian)阻(zu)與(yu)接收儀表輸入電(dian)(dian)(dian)(dian)阻(zu)的(de)分壓(ya)，將產生較(jiao)(jiao)大(da)的(de)誤(wu)差，如(ru)果(guo)用電(dian)(dian)(dian)(dian)流源信(xin)號(hao)作(zuo)為(wei)遠傳(chuan)，只要傳(chuan)送回路(lu)不(bu)出現分支，回路(lu)中的(de)電(dian)(dian)(dian)(dian)流就不(bu)會隨電(dian)(dian)(dian)(dian)線(xian)長短而(er)改變(bian)，從(cong)而(er)保(bao)證了(le)傳(chuan)送的(de)精度(du)。

3、信號最大電流選擇20mA的原因

? ? 最大電(dian)(dian)(dian)流(liu)20mA的選擇(ze)是(shi)基于安全、實用、功(gong)耗、成本的考(kao)慮。安全火花儀表只能采用低(di)電(dian)(dian)(dian)壓、低(di)電(dian)(dian)(dian)流(liu)，4~20mA電(dian)(dian)(dian)流(liu)和24V.DC對(dui)易(yi)燃氫氣也是(shi)安全的，對(dui)于24V.DC氫氣的引爆(bao)電(dian)(dian)(dian)流(liu)為200mA，遠在20mA以上，此外還要綜合考(kao)慮生產現(xian)場(chang)儀表之間的連(lian)接距(ju)離，所(suo)帶負載等因素(su)；還有功(gong)耗及成本問題，對(dui)電(dian)(dian)(dian)子(zi)元(yuan)件的要求(qiu)，供(gong)電(dian)(dian)(dian)功(gong)率(lv)的要求(qiu)等因素(su)。

4、信號起點電流選擇4mA的原因

? ? 輸出為(wei)4~20mA的(de)變送(song)器以兩(liang)(liang)線(xian)制(zhi)的(de)居多(duo)，兩(liang)(liang)線(xian)制(zhi)即電(dian)(dian)源(yuan)、負載串聯(lian)在一起(qi)(qi)，有一公(gong)共(gong)點(dian)(dian)，而(er)現場(chang)變送(song)器與(yu)控制(zhi)室儀(yi)表之間的(de)信號聯(lian)絡及(ji)供電(dian)(dian)僅用兩(liang)(liang)根電(dian)(dian)線(xian)。為(wei)什么起(qi)(qi)點(dian)(dian)信號不是(shi)0mA？這是(shi)基(ji)于兩(liang)(liang)點(dian)(dian)：一是(shi)變送(song)器電(dian)(dian)路沒有靜態(tai)工(gong)作電(dian)(dian)流將無法工(gong)作，信號起(qi)(qi)點(dian)(dian)電(dian)(dian)流4mA.DC，不與(yu)機械零點(dian)(dian)重合，這種“活(huo)零點(dian)(dian)”有利于識別斷(duan)電(dian)(dian)和斷(duan)線(xian)等故障。

? ? 采(cai)用電(dian)流信(xin)號的(de)原因是不容易受干擾、并(bing)且電(dian)流源內阻無窮大，導線電(dian)阻串聯在回路中不影響精(jing)度，在普通(tong)雙絞線上可以傳輸數百(bai)米。

? ? 采(cai)用電(dian)流信號的原(yuan)因(yin)是不容(rong)易受干擾，因(yin)為工業現(xian)場的噪(zao)聲(sheng)電(dian)壓的幅度可(ke)能達到數V，但是噪(zao)聲(sheng)的功率很(hen)弱(ruo)，所以噪(zao)聲(sheng)電(dian)流通(tong)常(chang)小(xiao)于(yu)nA級別，因(yin)此給4－20mA傳(chuan)輸帶來的誤差非常(chang)小(xiao)；電(dian)流源內(nei)阻(zu)(zu)趨于(yu)無窮大(da)，導(dao)線(xian)電(dian)阻(zu)(zu)串(chuan)聯在(zai)回(hui)路(lu)中不影響(xiang)精度，因(yin)此在(zai)普通(tong)雙絞(jiao)線(xian)上可(ke)以傳(chuan)輸數百米；由(you)于(yu)電(dian)流源的大(da)內(nei)阻(zu)(zu)和(he)恒流輸出，在(zai)接收端(duan)我們只需放置一(yi)個250歐姆到地(di)的電(dian)阻(zu)(zu)就可(ke)以獲得0－5V的電(dian)壓，低輸入阻(zu)(zu)抗的接收器的好處是nA級的輸入電(dian)流噪(zao)聲(sheng)只產生非常(chang)微弱(ruo)的電(dian)壓噪(zao)聲(sheng)。

? ? 上限(xian)(xian)取(qu)20mA是因(yin)(yin)為(wei)防(fang)爆(bao)的(de)(de)要(yao)求：20mA的(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)通(tong)斷引起的(de)(de)火花(hua)能(neng)量不足(zu)以引燃瓦斯。下限(xian)(xian)沒有(you)取(qu)0mA的(de)(de)原因(yin)(yin)是為(wei)了(le)能(neng)檢(jian)測(ce)斷線(xian)：正常工作(zuo)時不會低于(yu)4mA，當(dang)傳輸線(xian)因(yin)(yin)故障斷路(lu)(lu)，環路(lu)(lu)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)降為(wei)0。常取(qu)2mA作(zuo)為(wei)斷線(xian)報警值。電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)型變送(song)(song)(song)器(qi)(qi)將(jiang)物理量轉換成4~20mA電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)輸出，必然要(yao)有(you)外(wai)電(dian)(dian)(dian)(dian)(dian)源為(wei)其(qi)(qi)供電(dian)(dian)(dian)(dian)(dian)。最典型的(de)(de)是變送(song)(song)(song)器(qi)(qi)需(xu)要(yao)兩(liang)根電(dian)(dian)(dian)(dian)(dian)源線(xian)，加上兩(liang)根電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)輸出線(xian)，總(zong)共要(yao)接4根線(xian)，稱之為(wei)四線(xian)制(zhi)(zhi)變送(song)(song)(song)器(qi)(qi)。當(dang)然，電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)輸出可(ke)以與電(dian)(dian)(dian)(dian)(dian)源公(gong)用一根線(xian)公(gong)用VCC或者GND，可(ke)節省一根線(xian)，稱之為(wei)三(san)線(xian)制(zhi)(zhi)變送(song)(song)(song)器(qi)(qi)。其(qi)(qi)實大家(jia)可(ke)能(neng)注意到，4-20mA電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)本身(shen)就可(ke)以為(wei)變送(song)(song)(song)器(qi)(qi)供電(dian)(dian)(dian)(dian)(dian)。變送(song)(song)(song)器(qi)(qi)在(zai)(zai)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)中(zhong)(zhong)相當(dang)于(yu)一個特殊(shu)的(de)(de)負載，特殊(shu)之處在(zai)(zai)于(yu)變送(song)(song)(song)器(qi)(qi)的(de)(de)耗電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)在(zai)(zai)4~20mA之間根據傳感器(qi)(qi)輸出而變化。顯示儀表只(zhi)需(xu)要(yao)串(chuan)在(zai)(zai)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)中(zhong)(zhong)即可(ke)。這種(zhong)變送(song)(song)(song)器(qi)(qi)只(zhi)需(xu)外(wai)接2根線(xian)，因(yin)(yin)而被稱為(wei)兩(liang)線(xian)制(zhi)(zhi)變送(song)(song)(song)器(qi)(qi)。工業(ye)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)環標準下限(xian)(xian)為(wei)4mA，因(yin)(yin)此只(zhi)要(yao)在(zai)(zai)量程(cheng)范(fan)圍內，變送(song)(song)(song)器(qi)(qi)至少有(you)4mA供電(dian)(dian)(dian)(dian)(dian)。

? ? 因此、4-20mA的(de)信(xin)號(hao)輸(shu)出一(yi)般不容易受干擾(rao)而且(qie)安全(quan)可(ke)靠(kao)、所(suo)以工業上普遍(bian)使用的(de)都是二(er)線制(zhi)4-20mA的(de)電源輸(shu)出信(xin)號(hao)。但為了能更好的(de)處(chu)理(li)傳感器的(de)信(xin)號(hao)、目(mu)前(qian)還有更多其它形式的(de)輸(shu)出信(xin)號(hao)：3.33MV/V；2MV/V；0-5V; 0-10V等。

另附一張4到20mA轉電壓信號的簡單電路圖：