雷達物位計的供電方(fang)式有(you)兩種(zhong)(zhong)，一(yi)(yi)種(zhong)(zhong)是二(er)線(xian)制另一(yi)(yi)種(zhong)(zhong)是四線(xian)制。儀表(biao)的接線(xian)問題也是一(yi)(yi)個比較重要的環節，這部分工作做好是保(bao)證(zheng)儀表(biao)正常、準確、穩(wen)定測量的一(yi)(yi)個保(bao)障因素。今(jin)天我(wo)們就來解析(xi)一(yi)(yi)些二(er)線(xian)制和(he)四線(xian)制雷達物位計的接線(xian)知識。

所有(you)早期(qi)的(de)雷達物位計都是(shi)四線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)的(de)，電(dian)(dian)源(yuan)線(xian)(xian)(xian)(xian)(xian)和輸(shu)出信(xin)號(hao)線(xian)(xian)(xian)(xian)(xian)是(shi)獨(du)立的(de)。隨著低功耗、高靈(ling)敏度、快(kuai)速(su)電(dian)(dian)子元器件(jian)的(de)快(kuai)速(su)發展，雷達物位計實現了二(er)(er)(er)線(xian)(xian)(xian)(xian)(xian)供電(dian)(dian)，供電(dian)(dian)和信(xin)號(hao)輸(shu)出兩根(gen)線(xian)(xian)(xian)(xian)(xian)。二(er)(er)(er)線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)雷達物位計的(de)推出，使其能夠替代(dai)更多的(de)傳統(tong)物位計。因為(wei)二(er)(er)(er)線(xian)(xian)(xian)(xian)(xian)制(zhi)(zhi)儀(yi)表(biao)的(de)滿載工作電(dian)(dian)流必(bi)須在4mA以內，所以響(xiang)應會比較慢。在某些情況下，仍然需(xu)要4線(xian)(xian)(xian)(xian)(xian)系(xi)統(tong)，有(you)時需(xu)要220VAC電(dian)(dian)源(yuan)，也需(xu)要四線(xian)(xian)(xian)(xian)(xian)系(xi)統(tong)。

（圖(tu)源網絡(luo)）

二線(xian)(xian)(xian)制(zhi)雷(lei)達物位計(ji)(ji)(ji)使(shi)用相(xiang)同(tong)的(de)(de)(de)(de)(de)兩根線(xian)(xian)(xian)進(jin)行(xing)供(gong)電(dian)(dian)(dian)、信號(hao)輸(shu)出(chu)和(he)(he)通信。一(yi)(yi)般采用24VDC供(gong)電(dian)(dian)(dian)，模(mo)擬量(liang)輸(shu)出(chu)采用4～20mADC電(dian)(dian)(dian)流信號(hao)，現場(chang)總(zong)線(xian)(xian)(xian)通訊輸(shu)出(chu)可(ke)選擇ProfibusPA、FF等(deng)不(bu)同(tong)總(zong)線(xian)(xian)(xian)類型。在(zai)回(hui)路(lu)(lu)中施加一(yi)(yi)個恒定的(de)(de)(de)(de)(de)直流電(dian)(dian)(dian)壓(ya)(ya)(ya)，然后儀(yi)(yi)(yi)表根據儀(yi)(yi)(yi)表讀數(shu)或(huo)在(zai)回(hui)路(lu)(lu)中選擇的(de)(de)(de)(de)(de)現場(chang)總(zong)線(xian)(xian)(xian)通信電(dian)(dian)(dian)源產生4～20mADC的(de)(de)(de)(de)(de)電(dian)(dian)(dian)流。回(hui)路(lu)(lu)的(de)(de)(de)(de)(de)24VDC電(dian)(dian)(dian)源可(ke)以(yi)獨立也(ye)可(ke)以(yi)合并，和(he)(he)PLC電(dian)(dian)(dian)源通道。其他(ta)設(she)備(bei)(bei)，如遠程顯示(shi)器(qi)(qi)(qi)或(huo)記錄器(qi)(qi)(qi)，也(ye)可(ke)串聯(lian)在(zai)同(tong)一(yi)(yi)電(dian)(dian)(dian)路(lu)(lu)中，但這些設(she)備(bei)(bei)會降低儀(yi)(yi)(yi)器(qi)(qi)(qi)可(ke)用的(de)(de)(de)(de)(de)工(gong)作電(dian)(dian)(dian)壓(ya)(ya)(ya)。雷(lei)達物位計(ji)(ji)(ji)對(dui)最低工(gong)作電(dian)(dian)(dian)壓(ya)(ya)(ya)和(he)(he)最大回(hui)路(lu)(lu)電(dian)(dian)(dian)阻都(dou)有(you)(you)要(yao)求，回(hui)路(lu)(lu)的(de)(de)(de)(de)(de)設(she)計(ji)(ji)(ji)必須考慮回(hui)路(lu)(lu)中的(de)(de)(de)(de)(de)所有(you)(you)設(she)備(bei)(bei)和(he)(he)導線(xian)(xian)(xian)總(zong)長度，確保回(hui)路(lu)(lu)總(zong)電(dian)(dian)(dian)阻不(bu)超過(guo)規范要(yao)求，如果超過(guo)，儀(yi)(yi)(yi)器(qi)(qi)(qi)將無(wu)法(fa)工(gong)作。二線(xian)(xian)(xian)制(zhi)儀(yi)(yi)(yi)表的(de)(de)(de)(de)(de)功耗(hao)很小。最低點可(ke)在(zai)50mW以(yi)下，最高點可(ke)消耗(hao)0.6w，具體取(qu)決于指(zhi)示(shi)物位計(ji)(ji)(ji)的(de)(de)(de)(de)(de)電(dian)(dian)(dian)流值，所以(yi)儀(yi)(yi)(yi)器(qi)(qi)(qi)的(de)(de)(de)(de)(de)設(she)計(ji)(ji)(ji)必須滿足最小可(ke)用功率，始終保持強大的(de)(de)(de)(de)(de)信號(hao)和(he)(he)給可(ke)以(yi)接(jie)(jie)受的(de)(de)(de)(de)(de)響(xiang)應速(su)度。二線(xian)(xian)(xian)回(hui)路(lu)(lu)供(gong)電(dian)(dian)(dian)儀(yi)(yi)(yi)器(qi)(qi)(qi)與四(si)線(xian)(xian)(xian)儀(yi)(yi)(yi)器(qi)(qi)(qi)相(xiang)比具有(you)(you)許多優勢，包括僅使(shi)用一(yi)(yi)對(dui)屏蔽(bi)雙絞線(xian)(xian)(xian)供(gong)電(dian)(dian)(dian)、發送數(shu)據以(yi)測定物位，基本的(de)(de)(de)(de)(de)筒倉系統也(ye)可(ke)以(yi)使(shi)用同(tong)一(yi)(yi)系列24VDC電(dian)(dian)(dian)源與簡單的(de)(de)(de)(de)(de)遠程顯示(shi)器(qi)(qi)(qi)串聯(lian)連接(jie)(jie)。

安裝(zhuang)四線儀表時，要(yao)用幾根(gen)線加電(dian)線保護管(guan)或其他(ta)作為輸入(ru)輸出(chu)的(de)開關量輸出(chu)信號，安裝(zhuang)成本相(xiang)對高(gao)一(yi)點。但是，對于(yu)精度(du)(du)要(yao)求非常高(gao)的(de)場(chang)合，雷達物(wu)位計(ji)往(wang)往(wang)依(yi)賴于(yu)壓力和溫度(du)(du)參數的(de)輸入(ru)，這需要(yao)額外的(de)電(dian)源(yuan)，而(er)這在二線制系統中(zhong)是無法(fa)提(ti)供(gong)的(de)。隨著(zhu)通信要(yao)求，比如RS485或其他(ta)高(gao)速數字數據，需要(yao)更(geng)大(da)的(de)功率，這是二線制儀器(qi)無法(fa)提(ti)供(gong)的(de)。