02. Signal-to-Noise Evaluation of Pancake Coil Probes

This study compares detection capabilities of a pancake impedance coil with and eddy current probe composed of a transmit-receive coil pair with laterally spaced transmit and receive coils. When scanning material, ET probes always exhibit small changes in proximity ("Lift-off") between the coil and surface of the material being inspected. This study shows the superior signal-to-noise characteristics of a Transmit-Receive Coil Probe with laterally spaced transmit and receive coils as compared with a conventional pancake impedance coil probe.

The Picture below shows a Pancake Impedance ET Probe Coil under consideration

The Picture Below shows a Surface Transmit-Receive Eddy Current Probe

Input Data is as Follows:

Coil Diameter = 3.2 mm (0.127")

Coil Thickness = 0.5 mm (0.02")

Base Lift-off of the Coils = 1 mm

Center-to-Center Separation of Transmit and Receive Coils (TR Probe) = 7 mm

Plate Thickness = 1.1 mm

Plate Conductivity = 1 MS/m

Relative Magnetic Permeability of the Plate = 1

Test Frequency = 250 kHz

For Lift-off Signal (Noise) Calculations, Change in Lift-off = 0.1 and 0.2 mm

For Flaw Signal Calculations, Rectangular Flaw (Crack) is 50% through-wall and 2 mm long

Results of Flaw Signal Calculations as Compared to Signal from 0.2 mm changes in Lift-off for the Impedance and Transmit Receive Coil Probes are shown below:

The Results show that the Transmit-Receive Probe has a much stronger flaw signal (by about 400%) as compared to noise from variations in probe lift-off as compared with the Impedance Coil Probe