Wireless Sensing in Complex Electromagnetic Media: Construction Materials and Structural Monitoring

Abstract

Abstract-In this paper, wireless sensing in the presence of complex electromagnetic media created by combinations of reinforcing bars and concrete is investigated. The wireless displacement sensing system, primarily designed for use in structural health monitoring (SHM), is composed of a comb-like nested split-ring resonator (NSRR) probe and a transceiver antenna. Although each complex medium scenario is predicted to have a detrimental effect on sensing in principle, it is demonstrated that the proposed sensor geometry is able to operate fairly well in all scenarios except one. In these scenarios that mimic real-life SHM, it is shown that this sensor exhibits a high displacement resolution of 1 µm, a good sensitivity of 7 MHz/mm in average, and a high dynamic range extending over 20 mm. For the most disruptive scenario of placing concrete immediately behind NSRR, a solution based on employing a separator behind the probe is proposed to overcome the handicaps introduced by the medium. In order to obtain a one-to-one mapping from the measured frequency shift to the displacement, a numerical fit is proposed and used. The effects of several complex medium scenarios on this fit are discussed. These results indicate that the proposed sensing scheme works well in real-life SHM applications.Index Terms-Wireless passive sensor, displacement sensor, strain sensor, nested split ring resonator (NSRR), concrete,

Multi-Point Single-Antenna Sensing Enabled by Wireless Nested Split-Ring Resonator Sensors

“…The measurement limit of the resolution can be deduced from the sensitivity of the system, which is defined as the measured frequency shift over a displacement range as follows: For example, if the minimum distinguishable frequency that can be measured by the VNA is 1 kHz, and the average sensitivity of the system in the 1-3 mm range is 12.7 MHz/mm as shown in [16] (where the d-f curve is highly linear), then the resolution limit is found as (1 kHz) / (12.7 MHz/mm) = 78.7 nm. This, of course, is a theoretical limit, and it is known that the resolution of the sensing system is 0.5 to 1 μm based on our initial experiments presented in [16] and [24]. If we wish to have a measurement range of 1 mm for each sensor in the array, then, assuming a 100 MHz antenna bandwidth, the maximum number of array elements is found as 100 MHz / 12.7 MHz/mm ≈ 8.…”

“…This is due to the capacitance change at the NSRR probe. For instance, in order to detect the relative displacement change along two points by a passive and wireless method, a variant of the comb-like NSRR probe can be utilized [16], [17], [24]. In this structure, the comb-like NSRR probe is split symmetrically into two parts that are electrically shorted by a thin jumper to form the normally continuous uppermost ring.…”

“…2. Further detailed information regarding the standard and modified comb-like NSRR probe structures can be found in [15]- [17] and [24].…”

“…Here, it is shown for the first time that several NSRR probes can be located in a 2-D configuration while different wireless readouts can be acquired concurrently from each probe. This is possible by assigning each probe in the sensor array to a different range of operating frequency through modification of their initial separation distances and/or jumper lengths [24]. In this configuration, the bandwidth allocated for each probe determines the number of array elements as well as the dynamic range of the system while the illumination pattern of the antenna is effective in the strength of the coupling.…”

An electromagnetic sensing system incorporating multiple probes and single antenna for wireless structural health monitoring

“…1 (top). The characteristics of the sensing system based on this modified NSRR probe geometry were previously studied and reported in [5]- [7] for different surrounding environments including free-space and various scenarios of complex media with concrete and rebars. The underlying concept behind the sensing system is the electromagnetic coupling formed between the modified NSRR probe and the antenna.…”