漏磁(ci)場有兩種拾取方法，既可以(yi)測(ce)(ce)量漏磁(ci)感應(ying)強度(du)的(de)絕(jue)對值，也可以(yi)測(ce)(ce)量漏磁(ci)感應(ying)強度(du)的(de)梯度(du)值。
磁(ci)場傳(chuan)(chuan)感(gan)(gan)器的(de)作用(yong)是(shi)將磁(ci)場轉換為電信(xin)號。按原理(li)可分為體效(xiao)(xiao)應(ying)元(yuan)(yuan)件(jian)、面效(xiao)(xiao)應(ying)元(yuan)(yuan)件(jian)、P-N節注(zhu)入和(he)表面復合效(xiao)(xiao)應(ying)元(yuan)(yuan)件(jian)、量子效(xiao)(xiao)應(ying)元(yuan)(yuan)件(jian)、磁(ci)致伸縮效(xiao)(xiao)應(ying)元(yuan)(yuan)件(jian)和(he)光纖磁(ci)傳(chuan)(chuan)感(gan)(gan)器等(deng)。磁(ci)場傳(chuan)(chuan)感(gan)(gan)器都是(shi)建(jian)立在(zai)各種效(xiao)(xiao)應(ying)和(he)物理(li)現象(xiang)的(de)基礎(chu)之上的(de)，表3-1給(gei)出了不同種類磁(ci)場傳(chuan)(chuan)感(gan)(gan)器的(de)測量范(fan)圍(wei)，它們的(de)敏感(gan)(gan)范(fan)圍(wei)差異較大。在(zai)具體應(ying)用(yong)過程(cheng)中，需要根據(ju)測量對(dui)象(xiang)的(de)特點(dian)來選擇適合的(de)傳(chuan)(chuan)感(gan)(gan)器。
在不銹鋼管(guan)漏磁檢測中，常使用的有下列幾種磁敏傳感器。
1. 各向異性磁阻(zu)傳感器
各(ge)(ge)向(xiang)(xiang)異性(xing)磁(ci)(ci)阻(zu)傳感器(qi) AMR（Anisotropic Magneto-Resistive sensors）由(you)沉(chen)積(ji)在(zai)硅片上的坡(po)莫合金（Ni80Fe20）薄膜形(xing)(xing)成電(dian)(dian)(dian)(dian)阻(zu)，沉(chen)積(ji)時(shi)外加磁(ci)(ci)場，形(xing)(xing)成易(yi)磁(ci)(ci)化軸(zhou)方(fang)向(xiang)(xiang)。易(yi)磁(ci)(ci)化軸(zhou)方(fang)向(xiang)(xiang)是指各(ge)(ge)向(xiang)(xiang)異性(xing)的磁(ci)(ci)體能獲得(de)最佳磁(ci)(ci)性(xing)能的方(fang)向(xiang)(xiang)，也就是無外界磁(ci)(ci)干(gan)擾時(shi)磁(ci)(ci)疇整齊排列的方(fang)向(xiang)(xiang)。鐵磁(ci)(ci)材料的電(dian)(dian)(dian)(dian)阻(zu)與(yu)電(dian)(dian)(dian)(dian)流(liu)(liu)和磁(ci)(ci)化方(fang)向(xiang)(xiang)的夾(jia)角有(you)關，電(dian)(dian)(dian)(dian)流(liu)(liu)與(yu)磁(ci)(ci)化方(fang)向(xiang)(xiang)平(ping)行時(shi)電(dian)(dian)(dian)(dian)阻(zu)R最大，電(dian)(dian)(dian)(dian)流(liu)(liu)與(yu)磁(ci)(ci)化方(fang)向(xiang)(xiang)垂直時(shi)電(dian)(dian)(dian)(dian)阻(zu)Rmin最小，電(dian)(dian)(dian)(dian)流(liu)(liu)與(yu)磁(ci)(ci)化方(fang)向(xiang)(xiang)成0角時(shi)，電(dian)(dian)(dian)(dian)阻(zu)可表(biao)示為
R=Rmin+(Rmax-Rmin)cos2θ (3-2)
在磁(ci)(ci)阻(zu)傳感器中，為了消(xiao)除溫度等外(wai)(wai)界因素對(dui)輸出(chu)的(de)(de)影響，一(yi)般由4個(ge)相(xiang)同的(de)(de)磁(ci)(ci)阻(zu)元(yuan)件構(gou)成惠斯通電橋。理論分析與(yu)實(shi)踐(jian)表明(ming)，采用45°偏置磁(ci)(ci)場，當沿與(yu)易磁(ci)(ci)化軸垂(chui)直的(de)(de)方向施(shi)加外(wai)(wai)磁(ci)(ci)場，且外(wai)(wai)磁(ci)(ci)場強(qiang)度不太大時，電橋輸出(chu)與(yu)外(wai)(wai)加磁(ci)(ci)場強(qiang)度呈(cheng)線性(xing)關(guan)系。
2. 磁通門
磁(ci)通門傳感器(qi)(qi)又稱為磁(ci)飽和(he)式磁(ci)敏傳感器(qi)(qi)，它是利用某些高磁(ci)導(dao)率的(de)(de)軟磁(ci)性(xing)材(cai)料(liao)（如坡(po)莫(mo)合金）做磁(ci)心，以(yi)其在(zai)交(jiao)直(zhi)流磁(ci)場作用下的(de)(de)磁(ci)飽和(he)特性(xing)以(yi)及法拉第(di)電(dian)磁(ci)感應原理研制的(de)(de)磁(ci)場測量(liang)裝置。
這種磁(ci)(ci)(ci)敏傳(chuan)感器(qi)的(de)最大特點是適(shi)合測量(liang)零磁(ci)(ci)(ci)場(chang)附近的(de)弱(ruo)磁(ci)(ci)(ci)場(chang)。傳(chuan)感器(qi)體積小，重(zhong)量(liang)輕，功(gong)耗低，不受磁(ci)(ci)(ci)場(chang)梯度影(ying)響(xiang)，測量(liang)的(de)靈敏度可(ke)達0.01nT，并且(qie)可(ke)以和磁(ci)(ci)(ci)秤(cheng)混合使用(yong)。該裝置已普(pu)遍應(ying)用(yong)于航空(kong)、地面、測井等方面的(de)磁(ci)(ci)(ci)法勘探工作中。在軍事上，也可(ke)用(yong)于尋找地下武器(qi)（炮彈、地雷(lei)等）和反潛。還可(ke)用(yong)于預(yu)報天然(ran)地震及空(kong)間磁(ci)(ci)(ci)測等。
3. 巨磁阻元件
物(wu)質在(zai)一定磁(ci)(ci)場(chang)(chang)作(zuo)用下電(dian)(dian)(dian)阻(zu)發(fa)(fa)生改變的(de)現象(xiang)，稱(cheng)為磁(ci)(ci)阻(zu)效(xiao)應(ying)。磁(ci)(ci)性(xing)金屬和合金材(cai)料一般都(dou)有(you)這種現象(xiang)。一般情況下，物(wu)質的(de)電(dian)(dian)(dian)阻(zu)率在(zai)磁(ci)(ci)場(chang)(chang)中僅發(fa)(fa)生微小(xiao)的(de)變化，但(dan)在(zai)某種條件(jian)下，電(dian)(dian)(dian)阻(zu)變化的(de)幅度相當大，比通常情況下高十余倍，稱(cheng)為巨磁(ci)(ci)阻(zu)效(xiao)應(ying)（GMR）。這種效(xiao)應(ying)來自(zi)(zi)于(yu)載(zai)流(liu)電(dian)(dian)(dian)子的(de)不同(tong)自(zi)(zi)旋(xuan)狀態與磁(ci)(ci)場(chang)(chang)的(de)作(zuo)用不同(tong)，因而導致電(dian)(dian)(dian)阻(zu)值的(de)變化。GMR是一個量(liang)子力學效(xiao)應(ying)，它是在(zai)層(ceng)狀的(de)磁(ci)(ci)性(xing)薄膜結(jie)(jie)構中觀察到的(de)，這種結(jie)(jie)構由鐵(tie)(tie)磁(ci)(ci)材(cai)料和非磁(ci)(ci)材(cai)料薄層(ceng)交替疊合而成。當鐵(tie)(tie)磁(ci)(ci)層(ceng)的(de)磁(ci)(ci)矩(ju)相互(hu)平行時，載(zai)流(liu)子與自(zi)(zi)旋(xuan)有(you)關的(de)散射最(zui)小(xiao)，材(cai)料有(you)最(zui)小(xiao)的(de)電(dian)(dian)(dian)阻(zu)。當鐵(tie)(tie)磁(ci)(ci)層(ceng)的(de)磁(ci)(ci)矩(ju)為反向平行時，與自(zi)(zi)旋(xuan)有(you)關的(de)散射最(zui)強，材(cai)料的(de)電(dian)(dian)(dian)阻(zu)最(zui)大。
構成GMR磁(ci)(ci)(ci)(ci)頭和(he)(he)傳(chuan)(chuan)(chuan)感(gan)(gan)器的(de)(de)(de)核心元件是(shi)自旋閥（spin valve）元件。它(ta)的(de)(de)(de)基(ji)本結構是(shi)由釘(ding)扎磁(ci)(ci)(ci)(ci)性(xing)層(ceng)(ceng)(ceng)(ceng)（如(ru)(ru)Co）、Cu間隔(ge)層(ceng)(ceng)(ceng)(ceng)和(he)(he)自由磁(ci)(ci)(ci)(ci)性(xing)層(ceng)(ceng)(ceng)(ceng)（如(ru)(ru)NiFe等易磁(ci)(ci)(ci)(ci)化層(ceng)(ceng)(ceng)(ceng)）組成的(de)(de)(de)多(duo)層(ceng)(ceng)(ceng)(ceng)膜。由于(yu)釘(ding)扎磁(ci)(ci)(ci)(ci)性(xing)層(ceng)(ceng)(ceng)(ceng)的(de)(de)(de)磁(ci)(ci)(ci)(ci)矩(ju)與自由磁(ci)(ci)(ci)(ci)性(xing)層(ceng)(ceng)(ceng)(ceng)的(de)(de)(de)磁(ci)(ci)(ci)(ci)矩(ju)之間的(de)(de)(de)夾(jia)角發生(sheng)變(bian)化會導致SV-GMR元件的(de)(de)(de)電(dian)阻(zu)值(zhi)改(gai)變(bian)，進而使(shi)輸(shu)出電(dian)流(liu)發生(sheng)變(bian)化。運用(yong)(yong)SV-GMR元件的(de)(de)(de)磁(ci)(ci)(ci)(ci)傳(chuan)(chuan)(chuan)感(gan)(gan)器，其檢(jian)(jian)測靈敏(min)度比(bi)使(shi)用(yong)(yong)MR元件的(de)(de)(de)高(gao)幾個數量級(ji)，更(geng)容易集成化，封裝尺寸更(geng)小，可靠(kao)性(xing)更(geng)高(gao)。它(ta)不僅可以取代以前的(de)(de)(de)MR傳(chuan)(chuan)(chuan)感(gan)(gan)器，還可以制成傳(chuan)(chuan)(chuan)感(gan)(gan)器陣列，實現智(zhi)能化，用(yong)(yong)來表述通(tong)行車輛、飛機(ji)機(ji)翼(yi)、建筑防護裝置(zhi)或(huo)管道系(xi)統中(zhong)隱蔽缺(que)陷(xian)的(de)(de)(de)特征，跟蹤地磁(ci)(ci)(ci)(ci)場的(de)(de)(de)異常現象等。當前，GMR傳(chuan)(chuan)(chuan)感(gan)(gan)器已在液壓氣缸位置(zhi)傳(chuan)(chuan)(chuan)感(gan)(gan)、真假(jia)紙幣(bi)識別、軸承編碼、電(dian)流(liu)檢(jian)(jian)測與控制、旋轉位置(zhi)檢(jian)(jian)測、車輛通(tong)行情況(kuang)檢(jian)(jian)測等領域得到(dao)應用(yong)(yong)。
4. 霍爾元件(jian)
霍爾元件(jian)在漏(lou)磁檢(jian)測(ce)中(zhong)應用較為廣泛。霍爾元件(jian)是由半導(dao)體(ti)材料制(zhi)成的一種晶(jing)(jing)體(ti)。當給晶(jing)(jing)體(ti)材料通以(yi)電(dian)流并置于磁場之(zhi)中(zhong)時，在晶(jing)(jing)體(ti)的兩面就會(hui)產生電(dian)壓，電(dian)壓的大(da)小與(yu)磁場強(qiang)度成正比關系。
固體(ti)導電(dian)材料(liao)幾乎可以使(shi)電(dian)子(zi)暢通無阻地流過，就像傳(chuan)統的臺球模型演(yan)示的那樣，晶體(ti)點陣上的離子(zi)不會使(shi)傳(chuan)導電(dian)子(zi)發生(sheng)折射。當(dang)電(dian)流由(you)晶體(ti)的一(yi)端輸入時，電(dian)子(zi)或者(zhe)相(xiang)互之間發生(sheng)折射，或者(zhe)向著晶體(ti)的另一(yi)端折射。
根(gen)據固體物理理論可知(zhi)，晶體上的電(dian)壓Vh為： Vh=RhIBz/b (3-3)
式中，1為(wei)(wei)所使用的電流；Bz為(wei)(wei)磁(ci)場強(qiang)度(du)在垂(chui)直于(yu)電流方(fang)向上的分量；b為(wei)(wei)晶體在磁(ci)場方(fang)向上的厚度(du)；Rh為(wei)(wei)霍爾系(xi)數。
一(yi)般情況下，如果晶體與磁(ci)場B之(zhi)間成一(yi)定夾角(jiao)，則 B2=Beosθ。
由金(jin)(jin)屬制成的霍爾元(yuan)(yuan)件并不是(shi)最好的，因為(wei)(wei)金(jin)(jin)屬的霍爾系數(shu)都(dou)很低(di)。根據霍爾元(yuan)(yuan)件工作原理，霍爾系數(shu)越(yue)大，霍爾電壓也就越(yue)高。因此，在制作霍爾元(yuan)(yuan)件時，一般(ban)選用(yong)元(yuan)(yuan)素周期表中第II和第IV族元(yuan)(yuan)素混(hun)合制作，而(er)且其對溫度的變化也最不敏感。此區域的元(yuan)(yuan)素，載(zai)流子(zi)一般(ban)為(wei)(wei)空位(wei)而(er)不是(shi)電子(zi)。
5. 感應(ying)線圈
感應(ying)線(xian)圈(quan)是鋼管漏磁(ci)檢測中(zhong)應(ying)用最為廣(guang)泛(fan)的(de)磁(ci)敏傳感器(qi)，主要有水(shui)平和垂(chui)直(zhi)線(xian)圈(quan)兩種(zhong)布(bu)置方式(shi)，如圖(tu)3-2所(suo)示。根(gen)據(ju)提離效應(ying)和法(fa)拉第電(dian)磁(ci)感應(ying)定(ding)律(lv)，為了使檢測信號(hao)與缺(que)陷特(te)征之間具有良好的(de)對應(ying)關系，感應(ying)線(xian)圈(quan)提離距(ju)離以(yi)及掃查速度(du)應(ying)盡量保持恒定(ding)。
水平線(xian)圈(quan)(quan)以速度(du)v穿越缺陷上部漏(lou)磁場時(shi)所產生(sheng)的感(gan)應電(dian)(dian)(dian)(dian)動(dong)(dong)(dong)勢(shi)應為(wei)線(xian)圈(quan)(quan)前沿和尾(wei)部感(gan)應電(dian)(dian)(dian)(dian)動(dong)(dong)(dong)勢(shi)之差。設(she)線(xian)圈(quan)(quan)長度(du)為(wei)l、寬(kuan)度(du)為(wei)2w、提離(li)值為(wei)h1、匝數(shu)為(wei)，線(xian)圈(quan)(quan)前沿產生(sheng)電(dian)(dian)(dian)(dian)動(dong)(dong)(dong)勢(shi)為(wei)SueR，線(xian)圈(quan)(quan)尾(wei)部產生(sheng)電(dian)(dian)(dian)(dian)動(dong)(dong)(dong)勢(shi)為(wei)eL，線(xian)圈(quan)(quan)產生(sheng)感(gan)應電(dian)(dian)(dian)(dian)動(dong)(dong)(dong)勢(shi)為(wei)Δe，根據法拉第(di)電(dian)(dian)(dian)(dian)磁感(gan)應定律可(ke)得
此外，從圖3-3中(zhong)可(ke)以看(kan)出(chu)，水平線(xian)(xian)(xian)圈(quan)輸(shu)出(chu)感(gan)(gan)應電(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)本質為處于同一提離(li)高度(du)的前后導(dao)線(xian)(xian)(xian)在同一時(shi)刻的電(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)差動(dong)(dong)輸(shu)出(chu)。因此，感(gan)(gan)應線(xian)(xian)(xian)圈(quan)電(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)輸(shu)出(chu)與線(xian)(xian)(xian)圈(quan)寬度(du)有關(guan)，并(bing)存在最(zui)佳寬度(du)使(shi)得線(xian)(xian)(xian)圈(quan)輸(shu)出(chu)最(zui)大感(gan)(gan)應電(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)。此時(shi)，線(xian)(xian)(xian)圈(quan)運動(dong)(dong)至缺(que)(que)陷中(zhong)間位(wei)置，并(bing)且前沿(yan)產生正向極(ji)(ji)值(zhi)電(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)而(er)尾部產生反(fan)向極(ji)(ji)值(zhi)電(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)，經過差動(dong)(dong)后可(ke)獲(huo)取(qu)最(zui)高感(gan)(gan)應電(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)輸(shu)出(chu)。根據式（3-11），當(dang)x=0時(shi)，可(ke)獲(huo)得感(gan)(gan)應線(xian)(xian)(xian)圈(quan)位(wei)于缺(que)(que)陷中(zhong)間位(wei)置時(shi)電(dian)(dian)(dian)(dian)動(dong)(dong)勢(shi)(shi)Δeo與線(xian)(xian)(xian)圈(quan)寬度(du)參數w的關(guan)系式Δeo(w)，即
同樣，設置缺(que)陷寬度(du)2b為(wei)0.5mm，深度(du)d為(wei)0.75mm以及感(gan)(gan)應線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)提離高度(du)h1為(wei)0.25mm，根(gen)據(ju)式（3-13）可獲得最佳(jia)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)寬度(du)參(can)(can)數wo為(wei)0.3253mm。根(gen)據(ju)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)最佳(jia)寬度(du)參(can)(can)數重(zhong)新計算感(gan)(gan)應線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)前沿、尾(wei)部(bu)以及整體輸出(chu)(chu)感(gan)(gan)應電動勢曲線(xian)(xian)(xian)(xian)(xian)(xian)(xian)，如圖3-4所示。從圖中(zhong)可以看出(chu)(chu)，當線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)移動到(dao)(dao)缺(que)陷正上方時，線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)前沿感(gan)(gan)應電動勢輸出(chu)(chu)極小值(zhi)而尾(wei)部(bu)輸出(chu)(chu)極大值(zhi)，經(jing)差動后(hou)水平線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)輸出(chu)(chu)電動勢達(da)到(dao)(dao)最大值(zhi)。檢(jian)(jian)測線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)的(de)(de)最優寬度(du)參(can)(can)數與缺(que)陷尺寸和傳感(gan)(gan)器提離值(zhi)有關。在實(shi)際(ji)生產(chan)過程中(zhong)，可根(gen)據(ju)鋼管軋制(zhi)過程中(zhong)產(chan)生的(de)(de)自然缺(que)陷特征(zheng)對檢(jian)(jian)測線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈(quan)寬度(du)進行優化(hua)設計，以達(da)到(dao)(dao)最佳(jia)的(de)(de)檢(jian)(jian)測效果。
下面進(jin)一步(bu)討(tao)論垂直(zhi)線圈漏(lou)磁信號輸出特性(xing)。
如圖3-5所示(shi)，垂直線(xian)圈以速度，穿(chuan)越缺陷上部漏磁場時所產生(sheng)的電(dian)動(dong)勢(shi)輸出應(ying)為(wei)線(xian)圈頂(ding)部和底部感應(ying)電(dian)動(dong)勢(shi)之差(cha)。設線(xian)圈長度為(wei)l、匝(za)數為(wei)、寬度為(wei)2w、中心(xin)提離(li)值為(wei)，線(xian)圈頂(ding)部產生(sheng)電(dian)動(dong)勢(shi)為(wei)er，線(xian)圈底部產生(sheng)電(dian)動(dong)勢(shi)為(wei)eB，線(xian)圈產生(sheng)整體感應(ying)電(dian)動(dong)勢(shi)為(wei)Δe，根據法拉第(di)電(dian)磁感應(ying)定律(lv)可得
從圖3-5中(zhong)可以看出(chu)(chu)，eт、eB和(he)e三者波形相似，垂(chui)直線(xian)(xian)(xian)圈輸(shu)出(chu)(chu)感應(ying)電動(dong)勢(shi)本(ben)質(zhi)為上下兩根導線(xian)(xian)(xian)在(zai)同一(yi)時(shi)刻的電動(dong)勢(shi)差動(dong)輸(shu)出(chu)(chu)。在(zai)缺陷中(zhong)心位(wei)置，垂(chui)直線(xian)(xian)(xian)圈感應(ying)電動(dong)勢(shi)輸(shu)出(chu)(chu)為零(ling)，而(er)在(zai)缺陷兩端附近(jin)感應(ying)電動(dong)勢(shi)具有(you)最大輸(shu)出(chu)(chu)值。垂(chui)直線(xian)(xian)(xian)圈頂部和(he)底部距離越大，整體感應(ying)電動(dong)勢(shi)輸(shu)出(chu)(chu)越大。因此，在(zai)條件允許的情況下，垂(chui)直線(xian)(xian)(xian)圈應(ying)盡量(liang)貼近(jin)鋼(gang)管(guan)表面并可通(tong)過(guo)增大線(xian)(xian)(xian)圈的寬(kuan)度(du)(du)來提高電動(dong)勢(shi)輸(shu)出(chu)(chu)。但在(zai)設計線(xian)(xian)(xian)圈寬(kuan)度(du)(du)時(shi)必須考慮背景(jing)噪(zao)(zao)聲的影(ying)響，垂(chui)直線(xian)(xian)(xian)圈寬(kuan)度(du)(du)越大，線(xian)(xian)(xian)圈包含的背景(jing)噪(zao)(zao)聲越多，從而(er)會(hui)降低缺陷漏磁信號的信噪(zao)(zao)比。
