Passive UHF RFID Tag as a Sensor for Crack Depths

The use of ultrahigh-frequency (UHF) radio frequency identification (RFID) passive tags for defect detection is a promising application in structural health monitoring. However, it is a challenging task while most related information to tag antenna design is not available as well it suffers from the interference effect on wireless measurements. In this paper, we investigated and developed a new technique for crack depth sensing by using a passive UHF RFID tag as a sensor which interrogated by the ThingMagic M6e platform. The wireless power transfer level and the frequency sweeping are used to match between tag impedance and metal induction effect. The distance between the tag and reader is adjusted at 30 cm which can achieve high quality factor. As a result, the tag backscatter signal becomes rich with maximum peak components. The proposed technique called power peaks feature extraction (PPFE) is used to detect the artificial crack depth on the surface of the stainless steel and ferromagnetic samples. Skewness is applied on PPFE to offer a direct approximation procedure for the crack depth. A linear relationship of skewness achieves high-accuracy result with a maximum estimation error of 0.1 mm for stainless steel sample, the technique is validated and compared with the frequency-domain result, and it achieves all most the same accuracy for the stainless steel sample.