Eddy Current Sensor

A non-destructing sensing solution developed at the Mechatronics and Intelligent Automation Laboratory (MIARL) under the advisory of Prof. Chun-Yeon Lin.

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Starting from Aug. 2021, I serve as a research assistant in the Department of Mechanical Engineering, National Taiwan University. My research focus revolves around non-destructive sensing facilitated by our study in eddy current/magnetic sensor.

A sensor for ferrous/non-ferrous metal.

For a metal plate, there are generally three parameters that manufacturers are concerned about. First, the material of the plate,  which would affect the electrical conductivity and magnetic permeability, and the geometry. Assuming the plate is defect-free, then we are mostly concerned with its thickness.

We developed a sweep frequency sensor that could simultaneously determine the electrical conductivity and thickness for thin non-ferrous metal plates, and we are now working towards a pulsed eddy current sensor that could be used on ferrous and non-ferrous metals alike. 

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Working Principles

Signals are sent to the excitation coil, which generates a magnetic field that would cause a fluctuation in the magnetic flux density (MFD).

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This would induce eddy current in the metal plate below the sensor, which varies depending on the material properties and geometry. The eddy current would then cause a change in MFD opposing the original change (Lenz's Law), and the combined MFD from the excitation coil and the eddy current would be picked up by an anisotropic magnetoresistive sensor.  

Analytical Solution for sensor design

The exact solutions for axially symmetric eddy-current problems were proposed by Dodd & Deeds. This is also the theoretical foundation for our study. However, the solution has a singularity at the origin, which poses difficulty in implementing in our design because of the center placement of our coil. 

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C. Dodd and W. Deeds, "Analytical Solutions to Eddy-Current Probe-Coil Problems," Journal of Applied Physics, vol. 39, pp. 2829-2838, 1968.

 

Numerical solution

The distributed current sources method provides an alternative that could bypass the singularity at the origin. 

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C. Lin, K. Lee, and B. Hao, "Distributed Current Source Method for Modeling Magnetic and Eddy-Current Fields Induced in Nonferrous Metallic Objects," IEEE/ASME Transactions on Mechatronics, vol. 23, no. 3, pp. 1038-1049, 2018, doi: 10.1109/TMECH.2017.2771763.

 
 
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Sensing system

As shown in the system block diagram, the excitation signal is generated by a PC and passes through the current amplifier before reaching the electromagnet (EM). The change in MFD caused by the EM then reaches the metal plate underneath, which induces eddy current in the plate. The MFD caused by the plate, along with the excitation MFD, was perceived by the AMR sensor, and this signal would be sent back to the PC after passing through an instrumentation amplifier. 

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Experimental Setup

With NI DAQ systems, LABVIEW, and MATLAB for signal analysis, we were able to verify our theories and also construct a reference map for the sensor. During my time as an undergraduate researcher, I conducted experiments that formed the basis for the manuscript later submitted to the IEEE/ASME Transactions on Mechatronics. (More details coming soon!)

 

Finite Element Analysis using COMSOL Multiphysics

In our study of eddy current sensors, we primarily rely on the 2D-axisymmetric model in COMSOL to carry out the FEA. 

In our study of eddy current sensors, we primarily rely on the 2D-axisymmetric model in COMSOL to carry out the FEA. The model shown on the left displays the setup. Although this exact model was not built by myself, I can construct an FEA model from start to finish in COMSOL.

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Signal: Sweep Frequency or Pulsed?

Aside from the sweep-frequency sensor developed, we are currently investigating pulsed sensors that could be more easily implemented into digital systems. 

I co-authored a conference paper titled "Time Domain Characteristics of Metal Magnetic/Eddy-current Sensor"[1] that won Silver in The 18th International Conference on Automation Technology (Automation 2021) in Kinmen, Taiwan. 

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To demonstrate our sensor's application, I made a video to complement the presentation.

 

The future of eddy current/magnetic sensor

With rising demand for automation, eddy current sensors remain a promising subject with the potential to be used in the industry or even in healthcare. More research coming soon.