Eddy Current Testing with MFLI Lock-In Amplifier

This blog 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. In the first part of the blog the experimental setup is described and in the second part the used toolset is presented.

Experimental setup

Eddy current updated

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.

formulasum

 

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

Coil schematic

 

The next section describes the used toolset which consists mainly of the MFLI and the LabOne software.

Toolset

The MFLI 5 MHz 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 (higher bandwidth offers the HF2LI lock-in amplifier). The ability of the instrument to generate up to 4 different frequencies (with MF-MD multi-demodulator option ) simultaneously and demodulate at these frequencies makes the instrument unique. This allows 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 MD multi-demodulator option.

 

parameter eddy current

 

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 describes how a lock-in amplifier from Zurich Instruments can be used as flexible eddy current measurement tool. The LabOne software can be used directly as analysis tool. The advantages of the Zurich Instruments lock-in amplifiers are that they have several tools included in one instrument which reduce the complexity of the test setup. Please contact us for further information. We would be glad to discuss your application.