An oscillating current flowing through a coil produces an oscillating magnetic field. When an electrically conducting material like a metal is brought close to the coil, the oscillating magnetic field produces eddy currents in the metallic material. The strength of the eddy current depends on the electrical conductivity of the material, the distance between the coil and the material, and the frequency of the excitation of the coil. The eddy currents in the electrical conductor produce a magnetic field opposing the magnetic field generated by the coil. The electrical impedance of the coil, placed in close proximity to a metal, is altered due to the eddy currents in the metal. Measurement of the change in the impedance is a method to determine the electrical conductivity of the metal.

The design of the new Electro-Mechanical Eddy Current System probe. Similar to the MFM, the design relies on time-varying magnetic fields produced by the moving magnet that create eddy currents in a conductive sample.
The presence of a defect or crack under the electromagnetic coil dramatically changes the electrical impedance compared with the same material without a defect. Measurement of the difference between the two impedances has become the basis of the development of eddy current nondestructive evaluation (NDE) of electrically conductive materials. It is possible to produce an image of the local electrical conductivity and magnetic property variations by mapping the impedance data acquired from scans of a conductive sample.

To improve the spatial resolution beyond the coil diameter, modifications to the atomic force microscope (AFM) were developed in the last decade. The magnetic force microscope (MFM) was used to generate and detect eddy currents. A magnetic tip cantilever of a MFM equipped with a piezoelectric element was brought close to an electrical conductor and was vibrated. A vibrating magnetic tip generates eddy currents in the material under the tip. The magnetic field of the eddy currents in the material opposes the motion of the magnetic tip causing damping of the cantilever. Changes in the amplitude of the motion of the cantilever are measured and mapped to obtain an eddy current image of the sample.

While eddy current imaging can be performed with low macroscopic resolution with electromagnetic coils, extremely high resolution in the tens of nanometers range has been achieved with modified AFM. This leaves a significantly large gap of spatial resolution of hundreds of nanometers to hundreds of micrometers. In this work, a new probe design based on the MFM is presented, and is shown to be a promising method to bridge this gap.

The Electo-Mechanical Eddy Current System (EMECS) probe design relies on time-varying magnetic fields produced by the moving magnet that creates eddy currents in a conductive sample. These currents create opposing magnetic fields that damp the motion of the magnet. It is expected that the spatial resolution of the probe will be on the order of the diameter of the magnetic tip. In this probe design, the magnet is attached to the membrane of an electret microphone. A large coil is used as an external excitation coil that moves the magnet.

The motion of the magnet is detected as the output of the microphone, and changes in the amplitude correspond to changes in the magnetic field at the magnet. The membrane and back plate are connected to a local pre-amp that greatly increases the signal-to-noise ratio. A lockin amplifier was used to measure the DC values of amplitude change. This allowed for very fine bit resolution in the amplitude range of interest. In addition, the lock-in amplifier allows characterization of the phase change along with the amplitude change.

A probe based on this design was constructed. As an initial study, the magnet was chosen to have cylindrical geometry. The magnetic fields from the magnet were much higher than those seen in the middle of the coil at the operating frequency of 1 kHz. This, along with model validation, shows that the effects of the external magnetic field are negligible compared to the large magnetic fields in the vicinity of the magnetic tip.

Several numerical models were used for validation of different portions of the study. A finite element model of the coil alone was developed to verify that magnetic field measurements in the middle of the coil were in agreement to theory. Predicted magnetic field values from theory were 2.8G, while the field was measured at 3.3G, which validates that the field estimates are at least in the correct range.

The preliminary results from the experiments show an increase in spatial resolution, but there are some issues. First, the expected resolution of the probe is on the order of the diameter of the magnet, whereas the actual resolution appears to be greater than 3× this resolution. When designing the probe, it was assumed that the magnetic field was parallel to the velocity of the magnet. This assumption is not necessarily true, however, due to eddy current generation in the sample from the external excitation coil. In an unflawed specimen, this term should still be zero, but the presence of the flaw would create magnetic field components that are perpendicular to the velocity of the magnet, which would result in seeing the notch much further out than expected. Also, the asymmetries in the data can be explained by this, and the fact that the magnet is not exactly in the center of the probe, nor is it a perfect dipole. The reason this did not affect the MFM designs is the coil used to excite the MFM magnet was on the opposite side of the sample.

This work was done by M.R. Cherry, J. Welter, and M.P. Blodgett of the Air Force Research Laboratory; and S. Sathish and R. Reibel of the University of Dayton Research Institute. AFRL-0211


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Development of High-Resolution Eddy Current Imaging Using an Electromechanical Sensor

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This article first appeared in the June, 2012 issue of Defense Tech Briefs Magazine.

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