Eddy current pulsed thermography (ECPT) has been widely used in the field of non-destructive testing due to its safety, non-contact detection, high spatial resolution and intuitive results. Inductive excitation source is an important component of ECPT and provides high-frequency alternating current to drive the excitation coil. However, a resonant frequency distortion phenomenon exists in the excitation source during the detection process, which seriously affects the output power of the excitation source and the sample detection effect. This paper presents a fast resonant frequency tracking loop for full bridge series resonant inverter which is used to search the resonance frequency in real time through direct digital synthesizer (DDS) and all-digital phase-locked loop. Theoretical analysis and simulation are presented to explain the working principle of the loop. Then, an experimental prototype is manufactured which serves as an excitation source for the ECPT experimental system. Compared with traditional excitation sources, the prototype does not need a water-cooled device and the tracking speed can be adjusted by modifying the parameters of DDS. Finally, experiments have been conducted on both artificial slot of 45# steel and natural cracks of rail and stainless steel to investigate the influence of resonant frequency tracking speed on the crack detection. The results revealed that reducing the resonant frequency tracking time can efficiently improve defect detectability and the manufactured prototype showed more application potential. This article is part of the theme issue ‘Advanced electromagnetic non-destructive evaluation and smart monitoring’.

中文翻译：

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涡流脉冲热成像无损检测的谐振频率跟踪模式

涡流脉冲热成像（ECPT）以其安全、非接触检测、高空间分辨率和直观结果等优点在无损检测领域得到了广泛的应用。电感励磁源是ECPT的重要组成部分，提供高频交流电来驱动励磁线圈。但在检测过程中激发源存在谐振频率失真现象，严重影响激发源的输出功率和样品检测效果。本文提出了一种用于全桥串联谐振逆变器的快速谐振频率跟踪回路，通过直接数字合成器（DDS）和全数字锁相环实时搜索谐振频率。给出了理论分析和仿真来解释回路的工作原理。然后，制造了一个实验原型，作为 ECPT 实验系统的激发源。与传统激励源相比，样机不需要水冷装置，跟踪速度可以通过修改DDS的参数来调整。最后，对45#钢的人工开槽和钢轨和不锈钢的自然裂纹进行了实验，研究了谐振频率跟踪速度对裂纹检测的影响。结果表明，减少谐振频率跟踪时间可以有效提高缺陷可检测性，制造的原型显示出更大的应用潜力。