| Pulsed eddy current thermography uses electromagnetic pulse to thermally excite the sample,and the crack causes the surface temperature distribution change.The thermal image is obtained by thermal imager,and the crack detection is realized by the feature extraction algorithm.In order to improve the detection effect of shallow cracks on non-ferromagnetic materials,this paper designs a high-frequency induction heating power supply to reduce the skin depth of eddy current by increasing the excitation frequency and improve the temperature difference ratio between crack and non-crack areas.(1)The influence of the excitation frequency on the crack detection effect was analyzed,and the performance index of the power supply was proposed.The influence of excitation frequency on the surface crack detection of non-ferromagnetic materials was analyzed by COMSOL simulation.The design index and system scheme of induction heating power supply were proposed in combination with crack detection requirements.(2)The hardware circuit design of the high frequency induction heating power supply was analyzed and the influence of the parasitic parameters on the circuit was discussed.The full-bridge inverter and drive circuit,resonant circuit,impedance matching circuit and current sampling circuit were analyzed by theory and simulation.Finally,the performance of each module was verified by experiments.(3)The optimal inductive state of the high frequency power supply was analyzed and a full digital frequency tracking control system was designed.The working state of the series resonant inverter was analyzed,and the optimal inductive angle of the load voltage and current was obtained.To ensure the optimal operating state of the circuit,an all-digital frequency tracking technology based on the principle of direct digital frequency synthesizer was proposed.A mathematical model was established to perform numerical simulation analysis.(4)The performance of high frequency power supply and its application in crack detection system were tested.The design index of the high frequency induction heating power supply was tested and the power supply was applied to the test of stainless steel surface crack.The experimental results showed that the power supply can track the load resonance frequency within 5ms and operate stably at 400kHz/2kw.When detecting a 0.5mm deep crack on the stainless steel surface,the excitation frequency was increased from 100 kHz to 400 kHz,effectively improving the crack detection effect. |
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