1070 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 5, MAY 2011 Nondestructive Inspection Using Rotating Magnetic Field Eddy-Current Probe Junjun Xin, Naiguang Lei, Lalita Udpa, Fellow, IEEE, and Satish S. Udpa, Fellow, IEEE Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824 USA Rotating magnetic field eddy-current (RoFEC) probe for nondestructive evaluation of steam generator tubes in a nuclear power plant offers an alternate method that has compact configuration and higher speed compared to traditional bobbin coil, rotating probe coils, and array probes. This paper investigates the feasibility of the proposed RoFEC eddy-current probe which is composed of three windings excited by three-phase ac current and does not require mechanical rotation of probe. Results of finite-element modeling using reduced magnetic vector potential (RMVP) formulation are presented for modeling the inspection of ferromagnetic and nonferromagnetic tubes. Design parameters of the excitation coils and GMR pick-up sensor are optimized by means of a parametric study. Index Terms—Eddy-current testing, nondestructive testing. I. INTRODUCTION E DDY-CURRENT inspection has proved to be a fast and effective nondestructive technique to detect and size flaws that occur in metallic tubes, such as steam generator tubes. Conventionally, eddy-current probes such as bobbin coil probes cannot detect circumferentially oriented defects; rotating coil probes can obtain an eddy-current C-Scan of tube so that the circumferential defects are detected but the inspection using rotating probe coils scanning along a helical path takes too much time. Also, the complex mechanical rotation affects the data quality and reliability of the probe [1]. Array probes are capable of determining circumferential location of defects on the tube and have high inspection speed, but need complicated excitation and data analysis system [2]. Rotating magnetic field probe, generating rotating field electronically without me- chanical movement, can combine the merits of previous three types of probes and avoid their disadvantages. M. Enokizono presented a rotational magnetic flux sensor, which is consisting of two pairs of exciting coils perpendicular to each other and one three-axis search coil centered between exciting coils [3]. However, the probe is designed for planar test samples and also just sensitive to backside cracks of ferromagnetic plate. T.E. Capobianco proposed a probe composed of orthogonal excitation coils to generate rotating magnetic field (RMF) and pancake pickup coil [4]. R. Grimberg developed many kinds of RMF probes for different applications involving steel wire and ferromagnetic tube test samples [5], [6]. This paper presents a novel geometry of excitation coils to generate rotating magnetic fields with a pickup sensor array made of giant magnetoresistance (GMR) elements. Magnetore- sistive sensors are characterized by high sensitivity to fields along the easy axis of the element. The RoFEC probe is smaller, simpler to build, and can be used for nonferromagnetic and fer- romagnetic tube defect inspection. The paper is organized as fol- lows. Section II describes the theoretical principles of rotating magnetic field eddy-current probe with excitation and recep- tion coils. The finite element modeling and formulation using reduced magnetic vector potential formulation are introduced Manuscript received May 26, 2010; revised August 02, 2010; accepted Oc- tober 02, 2010. Date of current version April 22, 2011. Corresponding author: L. Udpa (e-mail: udpal@egr.msu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2108996 Fig. 1. Magnetic flux density of rotating field eddy-current probe. in Section III. In Section IV, the modeling of nonlinear mate- rial properties, using an iterative procedure is discussed. Finally, Section V provides the simulation results for different defect and sensor parameters. Optimization with respect to probe design parameters is also investigated in the final part by conducting a systematic parametric study. II. ROTATING MAGNETIC FIELD EDDY-CURRENT PROBE The proposed RoFEC probe works in a send-receive mode with excitation and reception coils. The excitation coil con- sists of three identical windings located on same axes physi- cally 120 degrees apart and driven by three-phase alternating current source with adjustable amplitude, phase and frequency. An array of GMR sensors or a bobbin coil is used as a pickup sensor. The magnetic flux densities and associated with the three windings are perpendicular to the plane of each winding, as shown in Fig. 1. The horizontal and vertical compo- nents and of resultant magnetic flux density synthesize a constant amplitude magnetic flux density M with the phase changing at the same frequency as excitation current source. The magnetic field is mainly radial and induces eddy currents in the tube wall, which flow circularly around the radial axis. Consequently the probe is sensitive to the defects in the tube wall of all orientations. The response signal due to defects can be picked up by an array of GMR sensors or a bobbin coil located at an appropriate location along the outside of source windings. III. FINITE ELEMENT MODELING AND SIMULATION A three-dimensional finite element model for the probe and tube geometry is developed using Reduced Magnetic Vector Po- 0018-9464/$26.00 © 2011 IEEE