# Enhanced Nondestructive Testing Using Pulsed Eddy Current and Electromagnetic Ultrasonic Techniques
## 5 Test of Detection Ability of Composite Detection Probe  
To verify the detection capability of the composite detector, an experimental platform was constructed, as illustrated in Figure 11.  
Electromagnetic ultrasonic detection requires an excitation signal of higher frequency and larger voltage, whereas the excitation signal for pulsed eddy current detection is of lower frequency. Accordingly, this study utilized the TMS320F28335 DSP (Texas Instruments, Dallas, TX, USA) processor as the signal generator and a full-bridge inverter circuit as the power amplifier. The MOSFETs within the full-bridge inverter circuit were controlled via a PC program, enabling the application of excitation signals at varying frequencies to the excitation coil. The configuration of the full-bridge inverter circuit is depicted in Figure 12.  
In this setup, different frequency pulse excitations can be applied to the load by synchronously toggling the 1,3 MOS and 2,4 MOS transistors. The ideal excitation waveform is shown in Figure 13.  
The test material primarily consisted of red copper with a thickness of 10 mm. Mahining was applied to introduce cuboid defects into the upper surface of the material, measuring 50 mm in length, 5 mm in width, and 1 mm in depth, distributed from 2 mm to 9 mm from left to right, as shown in Figure 14.  
Figure 11: Composite detection experimental platform. Figure 12: Full-bridge inverter circuit. Initially, defects on the upper surface of the material were detected using an excitation signal of 1 kHz, with the pulsed eddy current signal results presented in Figure 15. Figure 15a shows the result without any defects, while Figure 15b compares different results with the defect-free result. The peak value of the detection signal increases with the defect depth, indicating significant changes. This demonstrates that defects ranging from 1 mm to 9 mm in depth on the upper surface of the material can be effectively identified by the pulsed eddy current detection signal. The material was then flipped over, positioning the defects beneath the surface, and the pulsed eddy current detection signal was acquired at various defect locations using the composite detection probe, as illustrated in Figure 16. Figure 16b,c show magnified images of the two peaks. Figure 16: Eddy current testing signal for different depths of surface defects: (**a**) induced voltage under different defect depth of lower surface; (**b**) magnified image of the first voltage peak; (**c**) the second voltage peak magnified picture;. Figure 13: Ideal output waveform for a full-bridge inverter circuit. Figure 12.0 Eddy current detection signals at different depths of defects on the upper surface: (**a**) detection signal without defect; (**b**) induced voltage under different defect depth of lower surface; (**c**) relationship between peak value of induced voltage and defect depth of upper surface. Figure 16 reveals that the peak value of the composite probe does not significantly change when detecting defects on the lower surface of the material. This is attributed to the need for a thinner coil in the composite probe to reduce the lift distance of the permanent magnet. Consequently, compared to traditional pulsed eddy current probes, the compound probe used in this study features fewer coil turns and lower inductance, resulting in relatively stable peak values during the detection of defects on the lower surface. To address this issue, an RLC circuit was constructed for the detection coil. The equivalent circuit is shown in Figure 12.1  
As depicted in Figure 17, once the pulse excitation stabilizes, the detection coil begins to discharge. In this scenario, the voltage across capacitor C is expressed as follows:  
\[\left\{ \begin{array}{l}u_{c}=A_{1}e^{T_{1}t}+A_{2}e^{T_{2}t}\\ T_{1}=-\frac{R}{2L}-\frac{\sqrt{(RC)^{2}-4 LC}}{2 LC}\\ T_{2}=-\frac{R}{2L}+\frac{\sqrt{(RC)^{2}-4 LC}}{2 LC}\end{array}