Applications of chirp z transform and multiple modulation zoom spectrum to pulse phase thermography inspection

The fine spectrum structure is required when some key frequencies are estimated; however the implementation may be hindered due to insufficient frequency resolution. In this work, two types of transforms were introduced into Pulse Phase Thermography (PPT), the first was Chirp Z Transform (CZT), which was used for the badly sampled or short truncated data, and the other was Multiple Modulation Zoom spectrum Analysis (ZFFT), used for the long duration data. The CZT can refine the spectrum and improve the frequency resolution; consequently the subtle spectral structure of the selected local band can be observed for purchasing the precise frequency; furthermore time consumption for long point FFT calculation can be greatly alleviated by the ZFFT without worsening the resolution. The comparative test results demonstrate that the proposed CZT can obtain more accurate characteristic frequencies; in addition, the reconstructive images from the developed method are better than the ones by conventional technique, simultaneously ZFFT can save calculation time and storage space largely.

► Frequency resolution is improved by CZT. ► Time and memory can be saved by ZFFT. ► Characteristic frequencies are corrected leading to the accurate qualitative and quantitative analysis. ► ZFFT acts as a conflict resolution between calculated amount and the frequency resolution. ► There is no need to consider the experimental parameters other than the Shannon theorem.