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Pulse-compression-thermo-tomography
Increasing the resolution of the thermographic tomography via pulse compression methods
Project duration
From: 01.10.2020 To: 31.03.2023Description
Thermography is a widely used method for product inspection and quality assurance in a wide variety of industries. The development of new fields of application is currently limited by the rather small depth range, the decreasing resolution with increasing depth and the problem that hidden structures and defects are hardly detectable. In order to reduce these deficits, new methods of component excitation and signal processing were developed and evaluated in this research project. The targeted pulse compression methods use modulated excitation signals such as chirps or binary signals and produce high temporal resolution data with a high signal-to-noise ratio after correlation of the measured data with the excitation signal.
It has been shown that by using the pulse compression method in active thermographic measurements, the quality of the results can be improved. Since this depends mainly on the shape of the signal used for heating, it should be chosen depending on the priorities of the measurement. If a measurement within a certain frequency range is desired, chirps are most suitable. Depending on the bandwidth used, depth perception can be improved by using a logarithmic chirp, although the accuracy of the measurement results may deteriorate slightly.
Measurements with heating by Barker codes show an improved ability to detect internal defects even at short signal durations. However, due to their binary structure, the measurement results require more effort to adjust the temperature responses to obtain an optimal result. Of all the waveforms shown, complementary Golay pairs exhibit the best depth detection of defects, while also producing a good SNR curve. However, two measurement acquisitions are needed for their application. Regarding the suitability of the different excitation system techniques, the use of LED lamps can be recommended with regard to a high efficiency, a good replication of the input signals as well as a good result quality. Alternatively, conventionally used halogen lamps continue to offer good applicability. Flash lamps can only be recommended for thermographic pulse compression to a limited extent.
The implementation of the research results for companies that already use thermography is very easy to accomplish, since the necessary investments will be limited in most cases to the acquisition of new control software.
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