This powerpoint was presented as part of a workshop on Eddy Current Testing for Conservation at the annual metting of the AIC on May 19, 2009 Los Angeles, CA.

1. Innovations in Eddy Current
Technology
by Curtis Desselles
Blythe McCarthy - Freer Gallery

2. Valuable information about cultural materials
is not always visible to the naked eye.
Tattoodjj, “Statue of Prometheus”, June 8, 2008,
online image, Flickr. Rockefeller Center, NY.
Opacity, “statue of Bastet, gilded bronze, Late
Period”, March 8, 2009, online image, flickr.

3. Northwestern State University & NCPTT are
continuing research into use of eddy current testing
on cultural heritage.
NCPTT, Photograph courtesy of Jason Church

4. Without nondestructive testing, cultural information
will be lost.
Corroded Penny - www.wildbell.com/.../ 2008/05/img_9834.jpg

5. By making affordable eddy current instruments,
preservationists can have access to the technology.
Eddy Current Testing, Freer Gallery

6. Affordable eddy current instruments can be used to
analyze cultural materials.
Eddy Current Testing, Freer Gallery

7. Eddy Currents have been used in a variety of
nondestructive applications.
Adam Hoke, an aviation maintenance technician with the U.S. Coast
Guard, inspects the belly of a Boeing 737 using an eddy current
scanner. (Photo by Randy Montoya) Sandia Labs.

8. Eddy currents were discovered in 1851 by Foucault.
Léon Foucault (1819-1868), Wikipedia.

9. Applications of eddy currents include detection of
cracks, discontinuities, flaws, and thicknesses.
Measuring paint thickness - NDT Resource Center

10. Testing for Cracks - NDT Resource Center

11. Voltage
 is a measurement of the energy
contained within an electric
field, or an electric circuit, at a
given point, measured in volts

12. Current
 a flow of electricity through a
conductor, measured in
amperes

13. Resistance
 a material's opposition to the
flow of electric current,
measured in ohms

14. Capacitance
 a capacitor is an electronic
component used for storing
charge and energy, measured
in farads

15. Inductance
 a measure of the reaction of
electrical components
(especially coils) to changes in
current flow by creating a
magnetic field and inducing a
voltage. Its unit is the henry

16. Conductance
 a measure of a material's ability
to conduct electric charge; the
reciprocal of the resistance,
measures in Siemens

17. Impedance
 the total effective resistance in
an electric circuit to the flow of
an alternating current

18. Reactance
 opposition to the flow of
alternating current caused by
the inductance and capacitance
in a circuit rather than by
resistance.

19. What are eddy currents?
Eddy Currents - NDT Resource Center

20. Eddy currents are analogous to a flowing stream.
EDDIES IN THE OYASHIO CURRENT, G. K. Vallis, Princeton University

21. Similarly, eddy currents can be created in metals.
Drawing Courtesy of Curtis Desselles

22. Here are the steps to construct an
affordable eddy current analyzer.
Chichen Itza - Travelblog.org

23. Eddy currents can be created in the follow way in
cultural materials.
• Create an alternating current (AC) in the form of a
square or sine wave (excite circuit).
• Excite a series LC circuit (coil and capacitor) who’s
resonant frequency matches that of the excite circuit.
• Bring the probe(s) (coil) near the metal to be tested.
• Convert the voltage across the probe(s) to direct
current (DC) using a diode.
• Divide and store the voltage to a capacitor.
• Measure the voltage in the capacitor using an
analog to digital converter (ADC) ADC0831.

24. A Complete Eddy Current
Analyzer Circuit

25. Magnet wire is used to constructed for use as
probes.
Magnet Wire RadioShack® - $5.99

26. Coil Construction - 30 Gauge Magnet Wire
Photographs Courtesy of Curtis Desselles and Jason Church

27. Determine the inductance value with an LCR meter.
LCR Meter - Photograph Courtesy of Curtis Desselles

28. Use a square or sine wave to excite the LC circuit
LC Circuit - Drawing Courtesy of Curtis Desselles

29. Circuitry is built to excite the LC circuit to resonance
Propeller Demo Board - Parallax, Inc.

30. The frequency of the square or sine wave must
match the resonant frequency of the LC circuit.
L = Inductance in Henries
C = Capacitance in Farads

31. Once at resonance, the voltage and current across
the coil is at its maximum.
Drawing Courtesy of Curtis Desselles

32. The AC voltage is converted to DC using a diode
and it is stored in a capacitor.

33. We then convert the analog signal to a digital signal
using an ADC.
ADC0831 - Parallax Inc.
ADC PBasic code - Curtis Desselles

34. Using Microsoft Visual Basic an algorithm was
created to convert the data to a bitmap.

35. Once the eddy current analyzer is built and
calibrated, metal artifacts were tested.
Reference Samples - Freer Gallery

36. Four Techniques used in Eddy Current Analysis
• Identification of defects in metals.
• Identification and conductivity.
• Thickness determination on gilded or coated surfaces.
• Hallmark recovery on metal objects.
Photographs Courtesy of Jason Church

37. Four Techniques used in Eddy Current Analysis
“Identification of defects in metals”
• Eddy Current Parameter
●
Phase lag is the shift in time between the eddy current
response from a disruption on the surface and a disruption
at some distance below the surface.
Drawing Courtesy of Curtis Desselles

38. Four Techniques used in Eddy Current Analysis
“Identification of defects in metals”
• Phase Lag
●
Three defects at different depths and frequencies
R B G
Drawing Courtesy of Curtis Desselles - 50 KHz

39. Four Techniques used in Eddy Current Analysis
“Identification of defects in metals”
• Phase Lag
●
Three defects at different depths and frequencies
A B G
Drawing Courtesy of Curtis Desselles - 300 KHz

40. Four Techniques used in Eddy Current Analysis
“Identification of defects in metals”
• Methodology
●
Set frequency to 150 KHz
●
Bring probe within 1 mm from test surface
●
Scan object from left to right in a slow but steady
manner
●
Results appear on laptop screen
●
High frequency
●
Lower depth penetration and higher resolution
●
Low frequency
●
Greater depth penetration and lower resolution

41. Four Techniques used in Eddy Current Analysis
“Identification of defects in metals”
Case Study
Aluminum (1mm) - 158.5 KHz Screen Capture - 158.5 KHz

42. Four Techniques used in Eddy Current Analysis
“Identification and Conductivity”
• Eddy Current Parameter
●
Lift-off - This technique usually involves
zeroing out a probe (coil) in air. Then placing
the probe in contact with the sample surface.
For nonmagnetic materials, the change in
impedance of the coil can be correlated
directly to the conductivity of the material.
●
Resistance v. inductive reactance

43. XL
R
Screen capture of id measurements
Black is baseline conductivity

44. Four Techniques used in Eddy Current Analysis
“Identification and Conductivity”
• Lift-Off
●
Zero probe
●
Touch probe to sample
●
Lift off in a slow and steady
manner
●
Touch probe to sample
●
Results will appear on screen
Photograph Courtesy of Jason Church

45. “Identification and Conductivity”

46. Four Techniques used in Eddy Current Analysis
“Thickness Measurements”
• Eddy Current Parameter
• Lift-Off
●
Zero probe
●
Touch probe to sample
●
Lift off (1 mm) in a slow and steady manner
●
Scan left to right
●
Results will appear on screen

47. Four Techniques used in Eddy Current Analysis
“Thickness Measurements”
Lift-Off
Drawing Courtesy of Curtis Desselles

48. Four Techniques used in Eddy Current Analysis
“Recovery of Hallmarks”
• Eddy Current Parameter
●
Phase lag is the shift in time between the
eddy current response from a disruption on
the surface and a disruption at some distance
below the surface.
●
Touch probe to sample
●
Scan left to right
●
Repeat scanning of entire object
●
Results will appear on screen

49. Four Techniques used in Eddy Current Analysis
“Recovery of Hallmarks”
Lead Bale Seal, Los Adaes,
Williamson Museum, NSU
Scan of handle area

50. Building the Instrument

51. How do we get from this…

52. …to this

53.  Pre-drill Holes to Later Insert Electrical
Components

54.  Copy Circuit
Design to
Copper
Circuit Board
 Use Sharpie
Marker for
Layout

55. Completed Circuit Diagram

56.  Etch Copper Board in Ferric Chloride
Sharpie Ink Acts as a Resist

57. Periodic Agitation Speeds the Process

58. Solder on the Components

59. 560 pf Capacitor
↓
↑
RCA Coil Inlet

60. Rectifier Diode
↓
↑
.1 µf Capacitor

61. 741 IC 8pin Chip 100 mega ohm Resistor
↓↓ ×
↑
1 mega ohm Resistor

62. ADC0831 Analog to Digital Converter
↓
↑
Wires to Serial Port

63. Coil Construction - 30 Gauge Magnet Wire

64. Coil Construction - 30 Gauge Magnet Wire
Iron Ferrite Core
↓
RCA Output
↓

65. Future Directions
 iPhone Applications
 Tomographic Layers
 Database of Standard Parameters
 Better Resolution

66. iPhone Applications

67. Eddy Current Imaging

68. Database of Standard
Parameters

69. Conclusion
• These case studies show the potential of the affordable
eddy current instrument.
• As cultural resources age and decay, valuable information
can be lost.
• This research allow for affordable eddy current
instruments to be applied to cultural materials.
Cultural Material - Los Adaes - Photographs courtesy of Curtis Desselles

70. Acknowledgements
• Northwestern State University of Louisiana (NSU)
• National Center for Preservation Technology and Training
(NCPTT)
• Dr. Mary Striegel
• Dr. Hiram Gregory
• Dr. ElizaBeth Guin
• Jason Church
• Houren Zhu