基于西門子plc1200的液壓式壓力機(jī)控制系統(tǒng)設(shè)計(jì)含CAD圖

基于西門子plc1200的液壓式壓力機(jī)控制系統(tǒng)設(shè)計(jì)含CAD圖,基于,西門子,plc1200,液壓式,壓力機(jī),控制系統(tǒng),設(shè)計(jì),cad 外文出處： Sensors?and?Actuators?A?125(2006)?186－191 1．外文資料翻譯譯文（約3000漢字）： 用于裂紋檢測(cè)和表征的脈沖磁通泄漏技術(shù) 阿里索菲安*，桂云田，Sofiane扎伊 哈德斯大學(xué)計(jì)算和工程學(xué)院，Queensgate，Hudders實(shí)驗(yàn)室HD1 3DH，英國(guó) 2004年12月21日收到; 2005年6月16日修訂; 于2005年7月15日接受 2005年8月29日在線 摘要：磁通泄漏（MFL）技術(shù)已經(jīng)廣泛用于通過(guò)施加磁化的非侵入性檢查鋼裝置。在缺陷可能發(fā)生在被檢查的結(jié)構(gòu)的近表面和遠(yuǎn)表面的情況下，當(dāng)前的MFL技術(shù)不能確定它們的大致尺寸。因此，可能必須包括額外的換能器以提供所需的額外信息。本文提出了一種稱為脈沖磁通泄漏（PMFL）的新方法，用于裂紋檢測(cè)和表征。介紹了探頭設(shè)計(jì)和方法。通過(guò)理論模擬和實(shí)驗(yàn)研究了時(shí)頻域中的信號(hào)特征。結(jié)果表明，該技術(shù)可以潛在地提供關(guān)于缺陷的附加信息。最后，建議潛在的應(yīng)用程序。 ?2005 Elsevier B.V.保留所有權(quán)利。 關(guān)鍵詞：脈沖磁場(chǎng); NDT＆E;缺陷檢測(cè)和表征 1.介紹 磁漏技術(shù)廣泛應(yīng)用于管道和油箱地板檢查[1-7]。這種技術(shù)需要被測(cè)試樣品的磁化。磁化產(chǎn)生在一定方向上在試樣中流動(dòng)的磁通量，其理想地垂直于待檢測(cè)的裂紋的軸。任何缺陷的存在將實(shí)現(xiàn)為試樣中的通量的磁導(dǎo)率的突然變化。有缺陷的部件的磁導(dǎo)率通常低于無(wú)瑕疵的部件，提供高的通量阻力，并迫使它采取不同的路線。在其他路徑磁飽和的情況下，一些通量離開樣品到周圍空間，暫時(shí)導(dǎo)致通量“泄漏”。這種泄漏可以通過(guò)位于試樣表面附近的磁傳感器容易地檢測(cè)到。影響漏磁通分布的缺陷參數(shù)是缺陷深度與管壁厚度，長(zhǎng)度，寬度，邊緣處的銳度和最大深度處的銳度的比率[2]。在實(shí)踐中， 磁化裝置通常是永磁體或電磁體[8,9]。對(duì)于直流檢測(cè)，霍爾器件，磁阻和SQUID [10]可用于測(cè)量漏磁場(chǎng)。對(duì)于交流測(cè)量，拾波線圈是另一種選擇。 MFL技術(shù)的優(yōu)點(diǎn)是其簡(jiǎn)單性和低成本。與渦流技術(shù)相比，該技術(shù)對(duì)磁性材料的磁性能的變化更加魯棒，渦流技術(shù)也屬于電磁NDT技術(shù)。像許多電磁技術(shù)一樣，MFL也是非接觸的，這是在線動(dòng)態(tài)檢查的非常有用的特征。然而，與渦流不同，MFL僅適用于磁性材料。 在許多應(yīng)用中期望改進(jìn)NDT技術(shù)的精度，例如管道檢查，其中缺陷檢測(cè)和表征的良好精度可以幫助減少不必要的昂貴的管道替換。為了滿足這一要求，本文提出了脈沖MFL技術(shù)。與其他MFL技術(shù)相比，該技術(shù)可能提供關(guān)于結(jié)構(gòu)缺陷的更豐富的信息。我們對(duì)這個(gè)提出的技術(shù)的研究提出在本文。在下面的章節(jié)中，報(bào)告了PMFL的模擬和實(shí)驗(yàn)，然后得出結(jié)論和進(jìn)一步的工作。 2.脈沖MFL 直流MFL技術(shù)提供關(guān)于在位置和尺寸方面檢測(cè)到的缺陷的有限信息。通常，必須確保僅存在一種類型的缺陷，并且這些缺陷只能發(fā)生在被檢查結(jié)構(gòu)的一側(cè)，以允許精確推斷缺陷尺寸，因?yàn)樵摷夹g(shù)僅依賴于一個(gè)測(cè)量特征，即磁場(chǎng)泄漏強(qiáng)度。在一些實(shí)施方式中，它們與其他模態(tài)的傳感器互補(bǔ)，以允許區(qū)分近表面和遠(yuǎn)表面缺陷。對(duì)于交流MFL，檢查通常僅對(duì)樣品的一側(cè)敏感，取決于所選擇的激發(fā)頻率。使用脈沖MFL，探頭以方波驅(qū)動(dòng)，豐富的頻率組件可以提供來(lái)自不同深度的信息，由于皮膚效應(yīng)。預(yù)期我們可以檢查較厚的樣品的遠(yuǎn)側(cè)缺陷，同時(shí)對(duì)近表面缺陷具有良好的靈敏度。此外，我們還應(yīng)該有額外的信息，如缺陷的位置和大小。脈沖MFL系統(tǒng)的發(fā)展是我們對(duì)脈沖渦流NDT系統(tǒng)工作的延伸[11,12]。 為了探索該技術(shù)的潛力，使用U形鐵氧體磁軛設(shè)計(jì)和構(gòu)建探針。圖。如圖1所示： 圖。 （a）探針和測(cè)試充注的示意圖和（b）脈沖MFL探針布局和尺寸。 探頭的尺寸（mm）。來(lái)自霍尼韋爾的SS490系列[13]的霍爾器件傳感器已經(jīng)被選擇，并被定位在軛的極之間的中間，以測(cè)量垂直于樣品表面的磁場(chǎng)密度?；魻杺鞲衅鞯撵`敏度為3.125 mV / G。線圈的線圈纏繞在磁軛的頂部水平部分周圍，并以矩形波形驅(qū)動(dòng)以激勵(lì)。在操作期間，控制激勵(lì)電流以避免鐵氧體磁軛磁飽和。使用100kHz采樣率的14位數(shù)字化進(jìn)行數(shù)據(jù)采集。 3.模擬 有限元模型（FEM）已廣泛用于電磁無(wú)損檢測(cè)技術(shù)的研究，包括MFL [4]。在這項(xiàng)工作中，一個(gè)稱為FEMLAB的FEM包用于研究表面和子表面裂紋對(duì)磁場(chǎng)的影響，并預(yù)測(cè)系統(tǒng)輸出。該包使用有限差分法執(zhí)行我們所需的瞬態(tài)分析。圖。圖2示出了在模擬中使用的網(wǎng)格模型。圍繞插槽創(chuàng)建更精細(xì)的網(wǎng)格，以提供更準(zhǔn)確的結(jié)果。在模擬中使用的所有槽的寬度為1mm。在本文中，表面槽的深度是指從槽的表面到底部末端的槽的長(zhǎng)度，并且在模并且在模擬中，其從1mm變化到3mm。子表面槽的深度是樣品的頂表面與槽的頂部之間的距離，其在樣品的底表面上總是具有開口。子表面槽位于模擬中表面下0.5和1 mm處。圖。圖3示出了在每個(gè)時(shí)隙的右手側(cè)邊緣上方計(jì)算的正常磁場(chǎng)密度。圖。圖3a和b分別顯示了表面和子表面槽的結(jié)果。在圖中，時(shí)間= 0是激勵(lì)脈沖開始上升時(shí)。它由信號(hào)的形狀表示 圖。 3.有限元模擬結(jié)果：（a）表面槽和（b）表面下裂紋 （符號(hào)用于識(shí)別，而不是實(shí)際數(shù)據(jù)點(diǎn)）。 圖。 4.不同興奮的比較 技術(shù)可以潛在地通過(guò)使用信號(hào)的時(shí)間信息來(lái)區(qū)分所檢測(cè)的時(shí)隙的深度以及缺陷的位置。 圖。 圖4示出了計(jì)算的法向磁場(chǎng)相對(duì)于具有不同激勵(lì)波形的表面槽的到中心長(zhǎng)軸的距離x的曲線圖。 表面槽的深度為3 mm。 曲 線顯示，瞬態(tài)的性能略好于10 kHz頻率激勵(lì)，顯著優(yōu)于直流和低頻激勵(lì)。 4.實(shí)驗(yàn)結(jié)果 實(shí)驗(yàn)設(shè)計(jì)給我們一些初步結(jié)果，說(shuō)明該技術(shù)的能力。樣品具有深度從1至9mm變化的表面槽和具有位置深度范圍的子表面槽 1.5至7mm。槽的寬度約為1mm。 線圈由脈沖寬度為40ms的方波驅(qū)動(dòng)。本節(jié)中所示的信號(hào)曲線是：初始實(shí)驗(yàn)結(jié)果表明，最大信號(hào)峰值幅度是根據(jù)槽的中心長(zhǎng)軸的不同正常距離獲得的，這取決于槽是在頂表面上還是在底表面上樣品。當(dāng)槽位于表面下方時(shí)，由于場(chǎng)擴(kuò)散，測(cè)量場(chǎng)更加擴(kuò)展。因此，正峰到負(fù)峰距離大于槽寬度。這通過(guò)圖1中繪制的實(shí)驗(yàn)結(jié)果來(lái)說(shuō)明。槽的寬度約為3mm。表面槽的深度為3mm，埋入槽位于表面下1mm。樣品的厚度為10mm。通過(guò)在狹縫上以1mm步長(zhǎng)手動(dòng)掃描探針獲得結(jié)果，并且x = 0是狹縫的中心主軸。每次進(jìn)行測(cè)量時(shí)，探針都不移動(dòng)。對(duì)于曲線獲取信號(hào)幅度。已知使用MFL技術(shù)，磁場(chǎng)的極性 圖。 5.子表面和表面槽的掃描結(jié)果 圖。 6.表面槽深為1，2和3毫米的結(jié)果; 激勵(lì)脈沖的上升沿在時(shí)間= 1ms開始（符號(hào)用于識(shí)別，而不是實(shí)際數(shù)據(jù)點(diǎn)）。 如果傳感器在裂紋上掃描則更改。曲線顯示，使用1mm掃描步長(zhǎng)，對(duì)于表面和子表面槽，正峰和負(fù)峰之間的距離分別為4和8mm。 從圖中可以看出。如圖5所示，信號(hào)的幅度隨著探頭相對(duì)于槽軸的相對(duì)位置而變化。從現(xiàn)在開始，當(dāng)探頭定位成測(cè)量最高信號(hào)幅度時(shí)，獲得所使用的輸出信號(hào)。正峰值是假設(shè)由于對(duì)稱性，負(fù)峰值具有相同的絕對(duì)振幅。圖。圖6顯示了來(lái)自具有不同深度的表面槽的結(jié)果信號(hào)。其示出了該技術(shù)能夠通過(guò)使用信號(hào)的幅度來(lái)區(qū)分所檢查的時(shí)隙的不同深度，只要時(shí)隙的位置是已知的。應(yīng)當(dāng)注意，實(shí)驗(yàn)信號(hào)輸出相對(duì)于時(shí)間的所有曲線都被布置成使得激勵(lì)脈沖的上升沿與時(shí)間= 1ms一致。 圖。圖7示出了對(duì)于表面和子表面槽獲得的信號(hào)的比較。它清楚地示出了子表面的信號(hào)具有不同的特性，其中它最初緩慢增加并且在某個(gè)時(shí)間點(diǎn)之后以更快的速率增加。換句話說(shuō)，子表面槽信號(hào)的拐點(diǎn)發(fā)生在表面槽信號(hào)的拐點(diǎn)之后。通過(guò)獲取信號(hào)的一階導(dǎo)數(shù)可以更清楚地看到這些。 實(shí)驗(yàn)上發(fā)現(xiàn)，探針不能檢測(cè)到表面下1mm的子表面槽。為了證明區(qū)分表面和子表面的能力以及能夠區(qū)分子表面不連續(xù)性的位置深度，使用具有水平長(zhǎng)度為93mm的較大軛的另一探針。結(jié)果示于圖1。如圖8所示，這表明次表面縫隙信號(hào)的拐點(diǎn)再次出現(xiàn)在表面縫隙信號(hào)之后。表面信號(hào)的拐點(diǎn)出現(xiàn)在. 圖。 7.表面和子表面槽信號(hào); 激勵(lì)脈沖的上升沿在時(shí)間= 1ms開始（符號(hào)用于識(shí)別，而不是實(shí)際數(shù)據(jù)點(diǎn)）。 大致相同的時(shí)間，而較深位置的子表面槽的拐點(diǎn)發(fā)生在稍后的時(shí)間，比在更靠近表面的子表面槽的情況下。 所有這些結(jié)果表明，拐點(diǎn)可以用于區(qū)分檢測(cè)到的時(shí)隙的深度位置。 圖。 圖9示出了信號(hào)的頻率分析。 曲線支持我們的陳述，即在頻率分析中表示為相位的時(shí)間信息對(duì)于表征缺陷是有用的。 低于50Hz的低頻分量似乎不僅區(qū)分表面和子表面之間，而且區(qū)分表面下面的子表面槽的距離。 位置鑒別似乎也可以使用200 Hz左右的頻率實(shí)現(xiàn)。 因此，很清楚，槽的距離表面的距離的確定. 面對(duì)槽位于，可以方便地使用脈沖MFL技術(shù)實(shí)現(xiàn)。 圖。 8.使用較大軛的探頭的表面和子表面槽信號(hào); 激勵(lì)脈沖的上升沿在時(shí)間= 1ms開始（符號(hào)用于識(shí)別，而不是實(shí)際數(shù)據(jù)點(diǎn)）。 圖。 9.表面和子表面信號(hào)的頻率分析：（a）幅度，（b）相位（符號(hào)用于識(shí)別，而不是實(shí)際數(shù)據(jù)點(diǎn)）。 5。結(jié)論 已經(jīng)提出和研究了脈沖MFL技術(shù)的變體。數(shù)值分析和實(shí)驗(yàn)研究表明，通過(guò)使用時(shí)頻域中的特征，包括拐點(diǎn)的到達(dá)時(shí)間，信號(hào)幅度和頻率分量的相位變化，PMFL顯然具有缺陷位置和尺寸的優(yōu)點(diǎn)。已經(jīng)表明，該技術(shù)除了給出裂紋的相對(duì)深度之外，還能夠辨別裂紋的位置。探頭設(shè)計(jì)應(yīng)根據(jù)手頭的應(yīng)用進(jìn)行調(diào)整，因?yàn)榇跑椀某叽鐩Q定了穿透深度，通常較大的磁軛提供更深的穿透。掃描結(jié)果還顯示出在軛極之間利用磁傳感器的線性陣列的潛力，以更好地理解樣品條件。模擬結(jié)果表明，瞬態(tài)或脈沖MFL執(zhí)行表面和子表面裂紋檢查的最佳總體。 PMFL的優(yōu)點(diǎn)使其可以潛在地適用于鐵磁材料的許多應(yīng)用，包括檢測(cè)鐵磁金屬帶中的裂紋，其中缺陷可以存在于兩側(cè)，而存取限于帶的一側(cè)。對(duì)于未來(lái)的工作，將研究PMFL用于使用特征融合技術(shù)的腐蝕檢測(cè)/表征的能力[14,15]。該模擬技術(shù)也將被改進(jìn)以進(jìn)一步探索該技術(shù)的潛力。 確認(rèn) 作者感謝EPSRC部分資助該項(xiàng)目。 參考文獻(xiàn) [1] B.P.C. 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Sophian, Defect classification using a new feature for pulsed eddy current sensors, NDT&E Int. 38 (1) (2005) 77–82. 傳記 Ali Sophian博士于1998年獲得BE（榮譽(yù)）學(xué)士學(xué)位，2004年獲得英國(guó)哈德斯菲爾德大學(xué)的電子工程學(xué)博士學(xué)位。他目前是哈德斯菲爾德大學(xué)計(jì)算與工程學(xué)院的研究員。他的研究興趣包括渦流NDT系統(tǒng)設(shè)計(jì)，信號(hào)處理和嵌入系統(tǒng)。自從他開始了由TWI有限公司共同發(fā)起的博士項(xiàng)目以來(lái)，他一直致力于電磁無(wú)損檢測(cè)系統(tǒng)。 桂云田博士于1985年和1988年分別在四川大學(xué)（中國(guó)成都）獲得計(jì)量學(xué)與儀器學(xué)士學(xué)位和精密工程碩士學(xué)位。在四川大學(xué)擔(dān)任副教授3年后，他于1995年在英國(guó)攻讀博士學(xué)位，并于1998年在德比大學(xué)獲得學(xué)位。他目前是計(jì)算機(jī)學(xué)院的讀者，工程，英國(guó)哈德斯菲爾德大學(xué)和四川大學(xué)客座教授。他在傳感器，智能儀表，非破壞性測(cè)試，數(shù)字信號(hào)處理，計(jì)算機(jī)視覺(jué)和微機(jī)電領(lǐng)域擁有多元化和積極的研究 系統(tǒng)（MEMS），由EPSRC，皇家學(xué)會(huì)，皇家工程學(xué)院和世界級(jí)工業(yè)資助。他在上述領(lǐng)域出版了100多本英文或中文的書籍和論文，并成功地監(jiān)督了幾位研究員，如博士，博士和碩士研究生。他是IEEE的高級(jí)成員，也是國(guó)際期刊和會(huì)議的定期審閱者。 So fi ane Zairi博士于1996年獲得了Monastir大學(xué)應(yīng)用物理學(xué)和半導(dǎo)體理學(xué)學(xué)士學(xué)位（榮譽(yù)）。他于1997年在法國(guó)獲得利昂里昂INSA的微電子學(xué)碩士學(xué)位。然后，他在2001年在法國(guó)里昂的中央法學(xué)院獲得了集成傳感器的博士學(xué)位。后來(lái)，他加入了在英國(guó)格拉斯哥的斯特拉斯克萊德大學(xué)的一個(gè)文后的項(xiàng)目。這個(gè)由SHEFC理事會(huì)資助的項(xiàng)目，其中制造了涉及MOEMS和RF MEMS領(lǐng)域的微系統(tǒng)的多功能單元庫(kù)。 Zairi博士對(duì)嵌入式微系統(tǒng)領(lǐng)域和使用HDL和基于FEM的分析方法的軟件建模表現(xiàn)出極大的興趣。他目前是哈德斯菲爾德大學(xué)計(jì)算與工程學(xué)院的研究助理。  2．外文資料原文（與課題相關(guān)，至少1萬(wàn)印刷符號(hào)以上）： Pulsed magnetic flux leakage techniques for crack detection and characterisation Ali Sophian ?, Gui Yun Tian, Sofiane Zairi School of Computing and Engineering, University of Hudders?eld, Queensgate, Hudders?eld HD1 3DH, UK Received 21 December 2004; received in revised form 16 June 2005; accepted 15 July 2005 Available online 29 August 2005 Abstract Magnetic flux leakage (MFL) techniques have been widely used for non-intrusively inspecting steel installations by applying magnetization. In the situations where defects may take place on the near and far surfaces of the structure under inspection, current MFL techniques are unable to determine their approximate size. Consequently, an extra transducer may have to be included to provide the extra information required. This paper presents a new approach termed as pulsed magnetic flux leakage (PMFL) for crack detection and characterisation. The probe design and method are introduced. The signal features in time–frequency domains are investigated through theoretical simulations and experiments. The results show that the technique can potentially provide additional information about the defects. Lastly, potential applications are suggested. ? 2005 Elsevier B.V. All rights reserved. Keywords: Magnetic flux leakage; Pulsed magnetic field; NDT&E; Defect detection and characterisation 1. Introduction Magnetic flux leakage techniques are widely used for pipe and tank floor inspection [1–7]. This technique requires magnetisation of the specimen under test. The magnetisation generates magnetic flux flowing in the specimen in a certain direction, which is ideally perpendicular to the axis of the crack to be detected. The presence of any flaws will imple- ment as an abrupt change of magnetic permeability to the flux in the specimen. The permeability of the flawed part is gen- erally lower than flawless parts, providing high resistance to the flux and forcing it to take a different route. In cases where the other routes are magnetically saturated, some flux leaves the specimen to the surrounding space temporarily causing flux ‘leakage’. This leakage is readily detectable by a magnetic sensor located in the proximity of the specimen surface. The defect parameters that affect the distribution of the leakage flux are the ratio of depth of the defect to the thickness of the pipe wall, length, width, sharpness at the edges and sharpness at the maximum depth [2]. In practice,the magnetisation device is usually a permanent magnet or an electromagnet [8,9]. For dc inspection, Hall devices, mag- netoresistives and SQUIDs [10] can be used to measure the leakage field. For ac measurements, pick-up coils are another alternative. The advantage of MFL techniques is its simplicity and low cost. The technique is more robust to the variation of magnetic properties in magnetic materials compared to eddy current techniques, which belong to electromagnetic NDT techniques as well. Like many electromagnetic techniques, MFL is also non-contact, which is a very useful feature for on-line dynamic inspection. Unlike eddy currents, however, MFL only works with magnetic materials. Improvement in accuracy in NDT techniques is desired in many applications, such as pipe inspection where good accuracy in defect detection and characterisation can help reduce unnecessary costly pipe replacements. To meet this requirement, pulsed MFL technique is proposed in this paper. The technique potentially offers richer information about structural defects compared to the other MFL techniques. Our study on this proposed technique is presented in this paper. In the following sections, simulation and experiments on PMFL are reported, followed by conclusions and further work. 2. Pulsed MFL The dc MFL technique provides limited information on the defects detected in terms of location and sizing. Gen- erally, it has to be ensured that only one type of defect is present and these defects can only happen on one side of the inspected structure to allow accurate inference of the defect size, because the technique only relies on one mea- surement feature, i.e. the magnetic field leakage intensity. In some implementations they are complemented with sen- sors of other modality to allow discrimination of near and far surface defects. With ac MFL, the inspection is generally sensitive to only one side of the sample depending on the excitation frequency chosen. With pulsed MFL, the probe is driven with a square waveform and the rich frequency com- ponents can provide information from different depths due to the skin effects. It is expected that we could inspect thicker samples for far side defects and at the same time has good sen- sitivity for near surface defects. In addition, we should also have additional information such as the location and size of defects. The development of the pulsed MFL system is an extension of our work on pulsed eddy current NDT systems [11,12]. To explore the potential of the technique, a probe was designed and built using a U-shaped ferrite yoke. Fig. 1 shows the dimensions of the probe in mm. A Hall device sensor from Honeywell’s SS490 family [13] has been chosen and is positioned halfway between the yoke’s poles to measure the magnetic field density normal to the sample surface. The Hall sensor has a sensitivity of 3.125 mV/G. A coil of wire is wound around the top horizontal part of the yoke and driven with a rectangular waveform for excitation. During operation, the excitation current is controlled to avoid the ferrite yoke getting magnetically saturated. Data acquisition is performed using a 14-bit digitisation at 100 kHz sampling rate. Fig. 1. (a) Illustration of the probe and a test ample and (b) pulsed MFL probe layout and dimensions. 3. Simulation Finite element modelling (FEM) has been widely used for the study of electromagnetic NDT techniques, including MFL [4]. In this work, a FEM package called FEMLAB is used to study the effects of surface and sub-surface cracks on the magnetic field and to predict the system outputs. The package uses the finite difference method to perform tran- sient analysis that is required for our purpose. Fig. 2 shows the meshed model used in the simulation. Finer meshes are created surrounding the slot to give more accurate results. The width of all the slots used in the simulation is 1 mm. In this article, the depth of the surface slots refers to the length of the slot from the surface to the bottom tip of the slot and in the simulation it is varied from 1 to 3 mm. The depth of sub-surface slots is the distance between the top surface of the sample to the top of the slots that always have an opening on the bottom surface of the sample. The sub-surface slots are located 0.5 and 1 mm below the surface in the simulation. Fig. 3 shows the calculated normal magnetic field density above the right hand side edge of the each slot. Fig. 3a and b show the results from surface and sub-surface slots respec- tively. In the figures, time = 0 is when the excitation pulse starts rising. It is shown by the shapes of the signals that the technique can potentially discriminate the depths of the slots detected and also the position of the defect by using temporal information of the signals. Fig. 4 shows the plots of the calculated normal magnetic field against the distance x to the central major axis of a sur- face slot with different excitation waveforms. The surface slot’s depth is 3 mm. The plots show that the transient per- forms slightly better than the 10 kHz frequency excitation and significantly better than both the dc and the low frequency excitations. Fig. 3. FEM simulation results: (a) surface slots and (b) sub-surface cracks (symbols are for identification, not actual data points). Fig. 4. Comparison of different excitations. 4. Experimental results The experiments are designed to give us some initial results that illustrate the capabilities of the technique. The samples have surface slots with depths varying from 1 to 9 mm and sub-surface slots with location depths ranging from 0.5 to 7 mm. The width of the slots is approximately 1 mm. The coil is driven with a square waveform with a pulse width of 40 ms. The plots of signals shown in this section are: initial experiment results show that the highest signal peak ampli- tudes are obtained at different normal distances from the slot’s central major axis depending whether the slot is on the top surface or on the bottom surface of the sample. When the slot is located below the surface, the measured field is more spread out due to field dispersion. Therefore, the positive peak to negative peak distance is larger than the slot width. This is illustrated by experimental results plotted in Fig. 5. The slots’ width is approximately 3 mm. The depth of the surface slot is 3 mm and the buried slot is located 1 mm below the surface. The thickness of the sample is 10 mm. The results were obtained by manually scanning the probe over the slots with a 1 mm step and x = 0 being the central major axis of the slots. The probe is unmoved every time a measurement is taken. The signal amplitudes are taken for the plot. It is known that with MFL techniques, the polarity of the magnetic field Fig. 5. Scanning results of sub-surface and surface slots. Fig. 6. Results with surface slots with depths of 1, 2 and 3 mm; the rising edge of the excitation pulse initiates at time=1 ms (symbols are for identification, not actual data points). changes if the sensor is scanned over a crack. The plots show that with the 1 mm scanning step used, the distances between the positive and negative peaks are 4 and 8 mm for the surface and sub-surface slots, respectively. As can be seen in Fig. 5, the amplitude of the signal varies with the relative position of the probe to the slot axis. From now on, the output signals used are obtained when the probe is such positioned that the highest signal amplitude is mea- sured. The positive peaks are taken with the assumption that the negative peaks have the same absolute amplitudes due to symmetry. Fig. 6 shows the resulting signals from sur- face slots with different depths. It shows that the technique is able to differentiate different depths of the inspected slots by using the amplitudes of the signals, provided that the location of the slot is known. It should be noted that all plots of the experimental signal output against time have been arranged so that the rising edge of the excitation pulse coincides with time = 1 ms. Fig. 7 shows the comparison of the signals obtained for both surface and sub-surface slots. It clearly shows that the signal of the sub-surface has a different characteristic where it initially increases slowly and after some point in time increases at a faster rate. In other words, the inflexion points of sub-surface slot signals happen later than those of surface slot signals. These can be more clearly seen by taking the first derivative of the signals. Experimentally it was found that the probe could not detect sub-surface slots 1 mm below the surface. To demonstrate the ability to discriminate surface and sub-surface and also to be able to discriminate the location depths of the sub-surface discontinuities, another probe with a bigger yoke having a horizontal length of 93 mm is used. The results are shown in Fig. 8, which demonstrates that again the inflexion points of the sub-surface slot signals happen later than the surface slot signals. The inflexion points for surface signals occurs at Fig. 7. Surface and sub-surface slot signals; the rising edge of the excitation pulse initiates at time=1 ms (symbols are for identification, not actual data points). approximately the same time, while the inflexion point of the deeper-located sub-surface slot happen at a later time than in the case of a sub-surface slot that is closer to the surface. All these results indicate that the inflexion point can be used to discriminate the depth location of a detected slot. Fig. 9 shows the frequency analysis of the signals. The plots support our statement that the temporal information, which is represented as phase in the frequency analysis, is useful for characterising the defects. The low frequency com- ponents, below 50 Hz, seem to discriminate not only between surface and sub-surface but also discriminate the distance of the sub-surface slots below the surface. Location discrimina- tion seems to also be achievable using the frequencies around 200 Hz. It is, therefore, clear that determ