{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,16]],"date-time":"2026-02-16T16:50:47Z","timestamp":1771260647757,"version":"3.50.1"},"reference-count":29,"publisher":"MDPI AG","issue":"19","license":[{"start":{"date-parts":[[2022,10,3]],"date-time":"2022-10-03T00:00:00Z","timestamp":1664755200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100001809","name":"National Natural Science Foundation of China","doi-asserted-by":"publisher","award":["52275550"],"award-info":[{"award-number":["52275550"]}],"id":[{"id":"10.13039\/501100001809","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Metal sorting is the first step in scrap metal recycling. The traditional magnetic separation method can classify ferromagnetic metals, but it is not applicable to some nonmagnetic metals with higher value. To address this situation, we propose an eddy current testing (ECT) technology-based method for classifying nonmagnetic metals. In this study, a triple-coil electromagnetic sensor, which works as two coil pairs, is tested. By analyzing the physical model of the sensor, a feature related to the conductivity of the sample under test is obtained as the difference in the tangent of the impedance changes in the two coil pairs. Additionally, we derive a linear relationship between this feature and the lift-off height, which is verified experimentally and will help to solve the classification error caused by the variation in the lift-off height. In addition, we find that the excitation frequency does not affect this linear feature. Moreover, in this study, the spectrum scanning method is converted into a single-frequency measurement, and the time consumption is greatly reduced, which improves the efficiency of the real-time metal classification system.<\/jats:p>","DOI":"10.3390\/s22197511","type":"journal-article","created":{"date-parts":[[2022,10,10]],"date-time":"2022-10-10T05:12:21Z","timestamp":1665378741000},"page":"7511","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":13,"title":["Linear Characteristics of the Differences in Phase Tangents of Triple-Coil Electromagnetic Sensors and Their Application in Nonmagnetic Metal Classification"],"prefix":"10.3390","volume":"22","author":[{"given":"Dong","family":"Wang","sequence":"first","affiliation":[{"name":"School of Instrument and Electronics, North University of China, Taiyuan 030051, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6553-1978","authenticated-orcid":false,"given":"Zhijie","family":"Zhang","sequence":"additional","affiliation":[{"name":"School of Instrument and Electronics, North University of China, Taiyuan 030051, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5927-3052","authenticated-orcid":false,"given":"Wuliang","family":"Yin","sequence":"additional","affiliation":[{"name":"School of Electrical and Electronic Engineering, University of Manchester, Manchester M60 1QD, UK"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Haoze","family":"Chen","sequence":"additional","affiliation":[{"name":"School of Instrument and Electronics, North University of China, Taiyuan 030051, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Huidong","family":"Ma","sequence":"additional","affiliation":[{"name":"School of Instrument and Electronics, North University of China, Taiyuan 030051, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2660-3654","authenticated-orcid":false,"given":"Guangyu","family":"Zhou","sequence":"additional","affiliation":[{"name":"School of Instrument and Electronics, North University of China, Taiyuan 030051, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Yuchen","family":"Zhang","sequence":"additional","affiliation":[{"name":"School of Instrument and Electronics, North University of China, Taiyuan 030051, China"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2022,10,3]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"105294","DOI":"10.1016\/j.resconrec.2020.105294","article-title":"The influence of end-of-life regulation on vehicle material circularity: A comparison of Europe, Japan, Australia and the US","volume":"168","author":"Soo","year":"2021","journal-title":"Resour. Conserv. Recycl."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"269","DOI":"10.1016\/j.procir.2016.11.222","article-title":"Comparative Study of End-of-Life Vehicle Recycling in Australia and Belgium","volume":"61","author":"Soo","year":"2017","journal-title":"Procedia CIRP"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"489","DOI":"10.1016\/j.wasman.2010.09.015","article-title":"End-of-Life Vehicles management: Italian material and energy recovery efficiency","volume":"31","author":"Santini","year":"2011","journal-title":"Waste Manag."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"6376","DOI":"10.1109\/JSEN.2020.3038203","article-title":"Pulsed Eddy Current-Based Method for Electromagnetic Parameters of Ferromagnetic Materials","volume":"21","author":"Chen","year":"2020","journal-title":"IEEE Sensors J."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"33","DOI":"10.1016\/j.ndteint.2015.06.005","article-title":"Electrical conductivity measurement of ferromagnetic metallic materials using pulsed eddy current method","volume":"75","author":"Chen","year":"2015","journal-title":"NDT E Int."},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Zhang, R., Wang, J., Liu, S., Ma, M., Fang, H., Cheng, J., and Zhang, D. (2022). Non-Destructive Testing of Carbon Fibre Reinforced Plastics (CFRP) Using a Dual Transmitter-Receiver Differential Eddy Current Test Probe. Sensors, 22.","DOI":"10.3390\/s22186761"},{"key":"ref_7","first-page":"8424","article-title":"Thickness measurement of metallic plates with finite planar dimension using eddy current method","volume":"69","author":"Huang","year":"2020","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"198301","DOI":"10.1109\/ACCESS.2020.3035333","article-title":"Measurements of Thickness for Metallic Plates with Co-Axial Holes Using a Novel Analytical Method with the Modified Integration Range","volume":"8","author":"Yin","year":"2020","journal-title":"IEEE Access"},{"key":"ref_9","first-page":"1","article-title":"Measurement of Magnetic Plates at a Few Hertz with Two Concentric Coils and Thickness Estimation Using Mutual Inductance","volume":"70","author":"Cheng","year":"2021","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"102571","DOI":"10.1016\/j.ndteint.2021.102571","article-title":"A comparison of non-linear optimisation algorithms for recovering the conductivity depth profile of an electrically conductive block using eddy current inspection","volume":"125","author":"Hampton","year":"2021","journal-title":"NDT E Int."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"4111","DOI":"10.1109\/TII.2018.2885406","article-title":"Determination of the Magnetic Permeability, Electrical Conductivity, and Thickness of Ferrite Metallic Plates Using a Multifrequency Electromagnetic Sensing System","volume":"15","author":"Lu","year":"2018","journal-title":"IEEE Trans. Ind. Inform."},{"key":"ref_12","doi-asserted-by":"crossref","unstructured":"Wang, H., Huang, J., Liu, L., Qin, S., and Fu, Z. (2022). A Novel Pulsed Eddy Current Criterion for Non-Ferromagnetic Metal Thickness Quantifications under Large Liftoff. Sensors, 22.","DOI":"10.3390\/s22020614"},{"key":"ref_13","doi-asserted-by":"crossref","unstructured":"Zhang, W., Bu, J., Li, D., Zhang, K., and Zhou, M. (2022). Coupling Interference between Eddy Current Sensors for the Radial Displacement Measurement of a Cylindrical Target. Sensors, 22.","DOI":"10.3390\/s22124375"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"102706","DOI":"10.1016\/j.ndteint.2022.102706","article-title":"Multi-frequency imaging with non-linear calibration of magnetoresistance sensors for surface and buried defects inspection","volume":"132","author":"Wang","year":"2022","journal-title":"NDT E Int."},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Santos, A.C., Barrancos, A., Rosado, L.S., and Janeiro, F.M. (2022, January 1). Low-cost multi-frequency eddy current coating thickness measurement system. Proceedings of the 2022 International Young Engineers Forum (YEF-ECE), Caparica\/Lisbon, Portugal.","DOI":"10.1109\/YEF-ECE55092.2022.9850131"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"11239","DOI":"10.1016\/j.aej.2022.04.028","article-title":"Pulsed Eddy Current signal processing using wavelet scattering and Gaussian process regression for fast and accurate ferromagnetic material thickness measurement","volume":"61","author":"Sudirman","year":"2022","journal-title":"Alex. Eng. J."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Zhou, X., Ruan, Y., Mou, X., Yuan, Y., and He, Y. (2022). A Design of Electromagnetic Velocity Sensor with High Sensitivity Based on Dual-Magnet Structure. Sensors, 22.","DOI":"10.3390\/s22186925"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"4369","DOI":"10.1016\/j.measurement.2013.06.050","article-title":"GMR array uniform eddy current probe for defect detection in conductive specimens","volume":"46","author":"Postolache","year":"2013","journal-title":"Measurement"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"2773","DOI":"10.1109\/TIM.2016.2600918","article-title":"A Novel Compensation Algorithm for Thickness Measurement Immune to Lift-Off Variations Using Eddy Current Method","volume":"65","author":"Lu","year":"2016","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"2246","DOI":"10.1016\/j.measurement.2012.04.025","article-title":"Liftoff insensitive thickness measurement of aluminum plates using harmonic eddy current excitation and a GMR sensor","volume":"45","author":"Ramos","year":"2012","journal-title":"Measurement"},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Du, Y., Zhang, Z., Yin, W., Zhu, S., Chen, Z., and Xu, H. (2020). Conductivity Classification of Non-Magnetic Tilting Metals by Eddy Current Sensors. Sensors, 20.","DOI":"10.3390\/s20092608"},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"151125","DOI":"10.1109\/ACCESS.2020.3017693","article-title":"A Novel Conductivity Classification Technique for Non-Magnetic Tilting Metals by Eddy Current Sensors","volume":"8","author":"Du","year":"2020","journal-title":"IEEE Access"},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"59949","DOI":"10.1109\/ACCESS.2021.3073693","article-title":"Sloping-Invariance for Nonferrous Metallic Slabs at Multiple Frequencies by Eddy Current Sensors","volume":"9","author":"Du","year":"2021","journal-title":"IEEE Access"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"135334","DOI":"10.1109\/ACCESS.2021.3116247","article-title":"A Novel Conductivity Classification Technique for Nonmagnetic Metal Immune to Tilt Variations Using Eddy Current Testing","volume":"9","author":"Liu","year":"2021","journal-title":"IEEE Access"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"9352","DOI":"10.1109\/JSEN.2018.2870676","article-title":"Novel Noncontact Eddy Current Measurement of Electrical Conductivity","volume":"18","author":"Wang","year":"2018","journal-title":"IEEE Sensors J."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"164","DOI":"10.1109\/TIM.2015.2479106","article-title":"A Novel Triple-Coil Electromagnetic Sensor for Thickness Measurement Immune to Lift-Off Variations","volume":"65","author":"Yin","year":"2015","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"2829","DOI":"10.1063\/1.1656680","article-title":"Analytical solutions to eddy-current probe-coil problems","volume":"39","author":"Dodd","year":"1968","journal-title":"J. Appl. Phys."},{"key":"ref_28","unstructured":"Fengzhang, R. (2006). Fundamentals of Materials Physics, Machinery Industry Press."},{"key":"ref_29","unstructured":"Zhiyong, G., Xiehe, S., and Xianglong, M. (2015). Physical Properties of Materials and Their Analytical Test Methods, Harbin Institute of Technology Press."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/22\/19\/7511\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T00:46:01Z","timestamp":1760143561000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/22\/19\/7511"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2022,10,3]]},"references-count":29,"journal-issue":{"issue":"19","published-online":{"date-parts":[[2022,10]]}},"alternative-id":["s22197511"],"URL":"https:\/\/doi.org\/10.3390\/s22197511","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2022,10,3]]}}}