Wireless Crack Monitoring by Stationary Phase Measurements from Coupled RFID Tags

Abstract

The possibility to wirelessly monitor the state and the evolution of cracks is of increasing interest in emerging structural health monitoring systems. A simple and effective measurement method considers the placement of two passive radio frequency identification (RFID) antennas on top of the crack, so that the crack's evolution will produce a change of the inter-antenna coupling and in turn of the phase of the backscattered field. An ad-hoc design technique, based onto the coupled-modes physics, permits to maximize the sensor's sensitivity avoiding, or at least mitigating, the read range reduction during the evolution of the displacement that is instead typical of amplitude-oriented RFID displacement sensors. The proposed idea is demonstrated by numerical and experimental examples showing the possibility of sub-millimeter resolution with low-cost devices.

Wireless Passive RFID Crack Width Sensor for Structural Health Monitoring

“…If two close RFID tags, strongly coupled with each other, are considered, it is possible to achieve a good variation of the phase of the backscattered field with respect to variation of their mutual distance [17], hence having the chance to turn the RFID couplet into a deformation sensor. However, particular care has to be taken in order to limit the variation of the useful read range of the tags during the sensing process: as a matter of fact, coupling as well as detuning can worsen the power requirements of tags (i.e.…”

“…For what concerns sensing, the phase of the tag n (n = 1, 2) of a couplet of symmetric tags can be expressed [17] as…”

“…Detuning based sensors however sacrifice read range for the sake of sensing: the read range reduction, due to tag detuning, can also be of the order of 10 dB. In [17], instead, the authors have demonstrated that phase information from RFID tags can be usefully exploited to infer deformation of adhoc designed couplet of RFID tags, only weakly affecting the read range. The coupling between two (dipole) antennas is in that case exploited, somehow similarly to the concept proposed in [12], but allowing here for compensation of the varying selfimpedance through the mutual impedance.…”

“…The present work, taking cue from [17], explores how to adapt and optimize the design of RFID passive sensors for crack monitoring in environments considered to be RFIDharsh, either because of relatively high dielectric losses (like in concrete) or because of the presence of metallic materials.…”

“…Dipoles, for instance, as the ones used in [17], are not suitable for this kind of applications because of their reknown dependence on the underneath material. On the other hand, microstrip patch antennas, thanks to their ground plane, are substantially less influenced by the underlying materials than dipole-type antennas: in fact, the ground plane helps in insulating the antenna from what is underneath, effectively making the antenna almost insensitive to the underlying material.…”