The integration of magneto-resistive sensors into eddy current probes can significantly expand the capabilities of conventional eddy current nondestructive evaluation techniques. The room temperature solid-state sensors have typical bandwidths in the megahertz range and resolutions of tens of microgauss. The low frequency sensitivity of magneto-resistive sensors has been capitalized upon in previous research to fabricate very low frequency eddy current sensors for deep flaw detection in multilayer conductors. In this work a modified probe design is presented to expand the capabilities of the device. The new probe design incorporates a dual induction source enabling operation from low frequency deep flaw detection to high frequency high resolution near surface material characterization. Applications of the probe for the detection of localized near surface conductivity anomalies are presented. Finite element modeling of the probe is shown to be in good agreement with experimental measurements

Simpson, John

Wincheski, Russell A.

NASA Technical Reports Server

 
 
 
 
 
 
DEVELOPMENT AND APPLICATION OF WIDE BANDWIDTH 
MAGNETO-RESISTIVE SENSOR BASED EDDY CURRENT PROBE 
 
 
Buzz Wincheski1 and John Simpson2  
 
 
1NASA Langley Research Center, Hampton, VA 23681 
2Lockheed Martin Space Operations, Hampton, VA 23681 
 
 
 
ABSTRACT. The integration of magneto-resistive sensors into eddy current probes can significantly 
expand the capabilities of conventional eddy current nondestructive evaluation techniques.   The room 
temperature solid-state sensors have typical bandwidths in the megahertz range and resolutions of tens of 
microgauss.  The low frequency sensitivity of magneto-resistive sensors has been capitalized upon in 
previous research to fabricate very low frequency eddy current sensors for deep flaw detection in 
multilayer conductors.  In this work a modified probe design is presented to expand the capabilities of 
the device.  The new probe design incorporates a dual induction source enabling operation from low 
frequency deep flaw detection to high frequency high resolution near surface material characterization.  
Applications of the probe for the detection of localized near surface conductivity anomalies are 
presented. Finite element modeling of the probe is shown to be in good agreement with experimental 
measurements. 
 
 
 
Keywords: Eddy Current, Magneto-resistive, high frequency, high resolution 
PACS: 81.70.Ex, 81.40.Np, 81.05.Bx 
 
 
 
INTRODUCTION 
 
The use of magneto-resistive sensors in the fabrication of novel eddy current probes 
has been performed by several groups over the last decade [1-5].  The focus of these efforts 
has been the development of sensors that can operate at very low frequencies without a loss 
of sensitivity to the magnetic field, leading to the development of systems with flaw 
detection capabilities at centimeter-scale flaw depths [1-2].  In this work the high frequency 
capabilities and small spatial size of typical magneto-resistive sensors is explored for the 
development of high resolution near surface material characterization.  Commercial, off-
the-shelf, anisotropic magneto-resistive sensors are employed as the sensing element in the 
probe.  The sensors operation from DC to megahertz frequencies enables the configuration 
of a wide bandwidth eddy current probe.  Optimization of the probe for both deep flaw 
detection and near surface material characterization is performed using a dual induction 
source tailored for operation in each of the two regimes.   
 
https://ntrs.nasa.gov/search.jsp?R=20100042300 2019-08-30T13:48:19+00:00Z
 MAGNETO-RESISTIVE FLUX FOCUSING EDDY CURRENT PROBE 
 
The design of the wide bandwidth magneto-resistive sensor was conceived based 
upon adding high resolution near surface material characterization capabilities to the 
magneto-resistive flux focusing eddy current probe [1,5].  A schematic diagram of the 
probe, developed for deep flaw detection in multi-layer structures, is shown in figure 1.  
The unique probe design incorporates a driver pickup configuration in which a mu-metal 
flux-focusing lens is used to minimize direct coupling between the drive coil and the 
magnetic field sensor.  When placed above a material under test, the alternating magnetic 
field of the magnetizing coil induces a current flow that diffuses into the test object.  The 
probe is designed such that the magnetic field along the centerline of the probe decays 
slowly with depth into the material under test, enabling deep flaw detection in multi-layer 
structures.  The spatial resolution of the probe, however, is limited by the relatively large 
diameter of the induction source and the diffusive properties of the low frequency induced 
currents into the part under test. 
 
 
DESIGN OF WIDE BANDWIDTH MAGNETO-RESISTIVE SENSOR BASED 
EDDY CURRENT PROBE 
 
In designing a magneto-resistive probe capable of achieving high spatial resolution it 
is noted that the limiting factors on the spatial resolution of the previous design, discussed 
above, are solely attributable to the induction source.  The AMR sensing element itself is 
quite small, with  X-ray imaging of the sensor chip showing the surface area of the thin 
film AMR bridge to be approximately 0.7 mm^2, and the wide bandwidth of the sensors 
FIGURE 1.  Magneto-resistive flux focusing eddy current probe.   
Differential Output Amplifier 
Sensor Supply 
Voltage 
Voltmeter 
Current 
Source 
Feedback 
Source 
Magnetizing Coil 
(6mm radius 
typical) 
Honeywell HMC1001 
AMR sensor 
Flux Focusing 
Len 
enable operation from DC up to 5 MHz [6].   Design changes for high spatial resolution 
must therefore focus on the induction source.   
Figure 2 displays the incorporated design change to the induction source of the 
magneto-resistive sensor based eddy current probe to enable both high resolution, near 
surface imaging and deep flaw detection capabilities.  A second induction source 
specifically for high frequency, high spatial resolution imaging has been added to the probe 
design.  The induction source consists of a single strand copper wire (#38 Gauge, 100 
micron diameter) placed directly across the face of the probe.  The wire is positioned to 
produce maximum coupling between the field lines of the wire and the AMR sensor bridge.  
The sensitive axis of the AMR bridge is aligned with the centerline of the probe.  The wire 
is recessed into the surface of the probe (and the AMR sensor chip) to avoid damage to the 
wire during inspections.  Low frequency deep flaw detection capabilities are maintained 
with the original large diameter multi-turn coil wound on a mu-metal lens.  The large 
difference in operating frequency of the coils enables simultaneous operation of the probe 
for both high frequency, high resolution imaging of surface features and low frequency, 
deep flaw detection.  Typical operating frequencies in the two regimes are 1 MHz and 500 
Hz respectively. 
 
FINITE ELEMENT MODELING RESULTS 
 
Three dimensional finite element modeling for a simplified inspection geometry was 
performed to test the feasibility of the probe design shown in fig. 2.  All finite element 
modeling results were acquired using the COMSOL multi-physics AC/DC package.  As 
shown in fig. 3, the model consists of a single strand induction source above a conducting 
plate with a 500 micron diameter x 250 micron deep flat-bottom-hole.  The complete 
discretized volume is 100 mm^3 and consists of roughly 300000 elements.  An external 
current density of 2e6 A/m^2 at a frequency of 1 MHz was placed in the induction wire, 
equivalent to 16 mA total current in the 100 micron diameter wire.  
The modeling data was processed to extract the magnetic field in the vertical 
direction (normal to the surface of the part under test) at the sensor location for various 
hole to induction wire distances, with the position of the sensor held constant relative to the 
induction wire.  In all runs the induction wire was placed parallel to the y axis through the 
point x=0, z=120 microns.  The hole to induction wire distance was changed by displacing 
the center of the hole along the x axis at y=0.  The measurable magnetic field at each hole 
FIGURE 2.  Design of AMR Eddy Current Probe for deep inspection capabilities with high spatial 
resolution for surface breaking flaws. 
 
One strand, #38 gauge 
induction source 
Large diameter 
multi-turn coil on 
mu-metal lens 
FIGURE 3.  Finite element model and results for simplified inspection geometry. 
to wire distance was calculated by extracting the field values along a line parallel to the 
induction source and displaced in the (x,z) plane a distance equivalent to the targeted 
spacing between the induction wire and AMR sensor in the eddy current probe.  The 
difference in the field between the y=0 position (aligned with the hole center) and the 
y=2mm position (null location ‘far’ from hole) was taken as the change in the magnetic 
field at the sensor location due to the flat-bottom-hole.  These results are displayed in the 
chart in Fig. 3.  Overlaid on the chart is a cross sectional view of the geometry 
corresponding to the induction wire location centered on the flat bottom hole.  The sensor 
location and sensitive axis is represented on the graph by the double arrow just above and 
to the left of the induction wire.  The line graph displays the field change as the sensor and 
induction wire are scanned across the flat-bottom-hole.  A peak field change of 
approximately 0.4 A/m (5 mOe) is predicted, more than two orders of magnitude above the 
guaranteed resolution of the sensors [6].  The line graph shows an asymmetry about the 
center of the hole due to the fixed location of the sensor to the left hand side of the 
induction wire.  
 
EXPERIMENTAL RESULTS 
 
Experimental evaluation of the high frequency capabilities of the wide bandwidth 
magneto-resistive sensor based eddy current probe was performed using a reference 
standard fabricated to the specifications shown in Fig. 4. The reference standard was 
constructed from a ¼” thick aluminum 2024 plate.  A series of drills were machined into 
the part at varying diameter, depth, and center to center distances in order to evaluate the 
surface breaking flaw detection capabilities and the spatial resolution of the probe.  The 
smallest drill size, depth, and center to center distances were .0135”, .01”, and .025” 
respectively.   
x 
y z 
FIGURE 4.  Design of test standard for experimental evaluation of wide bandwidth magneto-resistive sensor 
based eddy current probe. 
The wide bandwidth magneto-resistive sensor based eddy current probe was 
constructed based upon the design shown in Fig. 2. The probe face diameter is 10mm, 
excluding the mu-metal lens and large diameter multi-turn coil.  For the high frequency 
evaluation, the sensor and induction wire were covered with a mu-metal shield with outside 
diameter of 13.5 mm and height of 19 mm.  Although the large diameter multi-turn coil 
was not included in this round of testing, it would be wound directly on this mu-metal lens 
as shown in Fig. 2. 
The eddy current probe was driven at a frequency of 1 MHz while the output of the 
AMR sensor was monitored using a lock-in amplifier with a band pass filter referenced to 
the drive signal.  Data were acquired by digitizing the amplitude and phase outputs of the 
lock-in amplifier and storing these values as a function of position as the part was scanned 
under motion control.  Before beginning a scan, the outputs of the lock-in amplifier were 
zeroed on an unflawed portion of the sample using the built in auto-offset function of the 
device.  In order to enable high spatial resolution the time constant of the lock-in amplifier 
was reduced to 20 mS and the scan rate of the probe over the part was set at 3 mm/second.  
During a scan line data were acquired every 40 mS, twice as long as the output filter time 
constant, producing a grid spacing of 120 microns.  
Fig. 5 displays two C-scans for the inspection of the calibration standard of Fig. 4 
with the wide bandwidth magneto-resistive sensor based eddy current probe operating at 1 
MHz.  The plotted data is the normalized amplitude of the AMR sensor which is 
proportional to the increase in the magnetic field amplitude in the z direction (out of plane) 
as compared to an unflawed area on the sample.  The amplitude data in all C-scans are 
plotted in logarithmic scale and cover six orders of magnitude. The direction of the 
induction wire is rotated between the two C-scans of Fig. 5.  In Fig. 5a the induction wire is 
parallel to the short edge of the sample where as for Fig. 5b it is parallel to the long edge of 
the sample.  It is clear from the figures that spatial resolution of the probe is reduced along 
the axis parallel to the induction wire, as the point-like indications become skewed in this 
direction.  Scans in both directions clearly isolate the solitary .0135” diameter x .01” deep 
drill (the inset line plot shows the magnetic field change across this defect for the parallel 
orientation C-Scan).  Drills spaced closer than approximately 0.05”, however, cannot be 
FIGURE 5.  Experimental results for scan of calibration sample of Fig. 3 with Wide Bandwidth Magneto-
Resistive Sensor Based Eddy Current Probe where induction wire is positioned parallel to (a) short edge of 
sample and (b) long edge of sample.  Inset line scan shows response across isolated .0135” diameter drill. 
clearly individually identified when the induction wire direction is parallel to the 
displacement vector between the drill centers.  When the induction wire direction is 
perpendicular to this displacement vector, even the most closely spaced holes, with 0.025” 
center to center spacing, are clearly isolated. 
As the direction of the induction wire is known a-priori, it is a simple matter to reduce 
the spread of the data in this direction through a deconvolution of the data with the point 
spread function of the drive source. The point spread function of the source can be 
determined by examining the probe response to the isolated .0135” drill in a calibration 
scan.  Taking this as a point defect, the spread of the C-scan data is then the point spread 
function of the induction source.  In processing the scan data for the part under test, the 
predetermined point spread function is rotated to coincide with the direction of the 
induction wire during the scan and then deconvolved with the acquired image data.  Fig. 6a 
displays the results of this processing for a scan in which the induction wire is placed 
approximately 45 degrees to each edge of the sample.  The skew of the indications along 
the direction of the induction wire has been greatly reduced and all indications are 
individually resolved.  For comparison with a conventional eddy current probe, Fig 6b 
displays the vertical channel C-scan acquired with a 1 MHz shielded 1/8” diameter eddy 
current pencil probe.  Here the relatively large diameter of the conventional probe results in 
significant overlap of the response from the individual drills.  There is also very little 
change in the spatial area of the response for the various diameter drills.  Only the 
magnitude of the probe response is directly related to the flaw diameter.  In contrast, the 
high frequency component of the wide bandwidth magneto-resistive sensor based eddy 
current probe produces much more localized indications with both the spatial area and 
magnitude proportional to the drill diameter.  
  
 
(a) 
(b) 
FIGURE 6.  Experimental results for scan of calibration sample of Fig. 4 with (a) wide bandwidth magneto-
resistive sensor based eddy current probe and (b) conventional shielded pencil probe.  
 
SUMMARY 
 
 In this work a new eddy current probe design has been presented.  The probe 
configuration includes two induction sources with a single pickup sensor. The use of a 
magneto-resistive device as the magnetic field sensor enables efficient monitoring of 
magnetic fields at frequencies from DC to megahertz.  While the low frequencies 
capabilities of magneto-resisitive sensor based eddy current probes have been presented in 
the past [1-5], the current design adds capabilities for high frequency high spatial resolution 
of surface breaking defects. Along with the probe design, finite element modeling and 
experimental verification of the high frequency operation of the device has been presented.  
The results show a significant increase in the resolvability of small and closely spaced 
surface anomalies as compared to a conventional eddy current pencil probe, and pave the 
way for fabrication and testing of a wide bandwidth probe with dual induction sources for 
simultaneous low frequency deep flaw detection and high frequency high resolution near 
surface material characterization.   
               
REFERENCES 
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(a) 
(b) 
2. R. Sikora, T. Chady, S. Gratkowski, M. Komorowski, and K. Stawicki,  “Eddy Current 
Testing of Thick Aluminum Plates with Hidden Cracks,” in Review of progress in 
quantitative NDE, Vol. 22,pp. 427-434, 2003.  
3. R. Rempt, “Using Magnetoresistive Sensors for a Sensitivity Quantum Leap in Aircraft 
Nondestructive Evaluation (NDE),” Proc. of SPIE,Vol. 5770, pp 32-45,  2005. 
4. A. Jander, C. Smith, and R. Schneider, ‘Magnetoresistive Sensors for Nondestructive 
Evaluation,” Presented at 10th SPIE Conference 5770, NVE, 2005. 
5. R. Wincheski, M. Namkung, and J. Simpson, “Magnetoresistive Flux Focusing Eddy 
Current Flaw Detection,” U.S. Patent Number 6,888,346 B2, 2005. 
6. Honeywell Sensor Products, 1- and 2-axis Magnetic Sensors, MHC1001/1002 
HMC1021/1022, www.magneticsensors.com, 900248 Rev. B. 
 


Development and Application of Wide Bandwidth Magneto-Resistive Sensor Based Eddy Current Probe

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