Eddy Current Testing with the MFLI Lock-in Amplifier

August 5, 2015 by Marco Brunner

This blog post describes a method to detect material defects. Eddy current measurements are used for non-destructive material testing (NDT). An alternating magnetic field is penetrating a sample. The material defect is detected by a phase shift.

Experimental Setup

Eddy current measurement

In this setup, a sending and a receiving coil are used. The sending coil generates a magnetic field in the material which is inducing an eddy current. Material inhomogeneities (material defects) like air pockets can disturb the flow of the eddy current. This results in a phase change which can be measured with the receiving coil. The possibility to vary the frequency allows the user to detect material defects in different material depths. From the following formula it is visible that the penetration depth of the measurement δ depends on the material conductivity, material permeability and the frequency:

\[\delta = \frac{1}{\sqrt{\pi\cdot \sigma\cdot\mu_0\cdot\mu_r\cdot f}}\]

where

\(\delta\) = standard depth of penetration [mm]
\(\sigma\) = material electric conductivity (\(\frac{\Omega}{mm^2}\))
\(\mu_0\) = absolute permeability
\(\mu_r\) = relative permeability
\(f\) = frequency

The sending coil (red) has 11 turns and the receiving coil (blue) has 5 turns. The coil diameter is 6 mm.

coil schematic

Toolset

The 5 MHz MFLI Lock-in Amplifier is the ideal instrument for eddy current measurements. It provides a multi-sine signal generator and a current input. The Signal generator drives the sending coil up to +/-10 V with a bandwidth from DC up to 5 MHz (a higher bandwidth is offered by the HF2LI Lock-in Amplifier). The ability of the instrument to generate up to 4 different frequencies simultaneously (with the MF-MD option) and demodulate at these frequencies makes the instrument unique. This makes it possible to measure at 4 different penetration depths simultaneously and reduces the measurement time. The current input is able to measure the signals with a high phase resolution of 10 µdegree.

The LabOne user-interface visualizes the settings and signal processing path in a flow diagram. The following figure shows the graphical lock-in amplifier user-interface for one demodulator. The oscillator frequency is set to 1 MHz and the output voltage is set to 2V (red squares) with an output impedance of 50 Ohm. The signal is measured with the current input. The simultaneous measurement of the current and voltage is possible with the MF-MD option.

Eddy current parameters

The following figure shows the LabOne Plotter with the phase on the y-axis. A phase change of 20 milli-degree is detected due to an air pocket. The air pocket was created by drilling a small whole into an aluminum plate. The Plooter tool can be used for observation how the phase is changing over time. With the software trigger thresholds can be set for starting data acquistion to detect a phase change (material defect).

Phase shift small

Conclusion

This blog post describes how the MFLI from Zurich Instruments can be used as flexible eddy current measurement tool. The LabOne software can be used directly as a data analysis tool. The advantage of Zurich Instruments' lock-in amplifiers is that they have several tools included in one instrument, which reduces the complexity of the test setup.

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