Abstract
Structures could be deteriorated by aging effects of strength and stiffness[1].With the increasing of the service time,structural health monitoring for aging structures is becoming significantly important[2],therefore various sensors have been designed to detect the deformation or strain of structures.
Wired sensors for structural health monitoring use cables to supply power and transmit data.These wired sensors usually show a good performance in stability and accuracy.However,they are limited by the instrumentation time and system cost.Eliminating cables for wired sensors,passive wireless antenna sensors are easier to deploy with less cost using antennas as sensing and transmitting units.This is a promising technology for distributed sensing network over a wide area with dense deployment.
In recent years,there has been an increasing interest in the development of antenna sensors for various physical quantities.Due to the advantage of simple configuration and multimodality,patch antenna has been studied extensively.In general,the physical qualities can be obtained by analyzing resonant frequencies of the patch antenna,which would be shifted by the deformation or environment changes of the monolithic patch antenna.Yi.et al.[3]presented a wireless strain sensor based on monolithic patch antenna.The antenna sensor is attached on the surface of structures and the strain of the structure is obtained by measuring the resonant frequency shifting of the patch antenna.Huang et al.[4]demonstrated a patch antenna sensor that can detect crack propagation with submillimeter resolution by analyzing the relationship between the crack width and resonant frequency shifting in longitudinal and transverse direction.Mohammad et al.[5]proposed a patch antenna-based shear sensor.Tchafa et al.[6]proposed a patch antenna-based sensor to determine the changes in strain and temperature from the normalized antenna resonant frequency shifts.
However,the sensing unit of the onepiece antenna is designed to be stressed for deformation measurement.For these stressed antennas,the issues of incomplete strain transfer ratio,insufficient bonding strength,and randomness of crack propagation will compromise the sensor sensitivity and complicate the calibration process.To address these issues completely,we proposed several unstressed antenna sensors,which are shown as follows.
A helical antenna based passive displacement sensor is proposed[7].This antenna sensor is consisted of a normal mode helical antenna and an inserted silicon rod,which is shown in Figure 1.
Figure 1 Figures of a helical antenna-based displacement sensor
The electromagnetic field can be altered when the silicon rod dislocated,which would lead to the resonant frequencies shifting of the helical antenna.Hence,the location of the silicon rod can be certified by analyzing the resonant frequency shifting without exerting stress to the antenna.The simulation and experimental results are shown in Figure 2,which suggested a maximum effective measuring range of 7 mm with an average sensitivity of 0.616 MHz/mm.
Figure 2 Resonant frequency with respect to displacement of the sensor in each group
The resonant frequency of a patch antenna would be affected by the antenna loading.Based on this principle,the authors have proposed a crack sensor by forming a parallel plate capacitor using two microstrip lines as a sensing unit,which is shown in Figure 3[8].As the relative movement of two microstrip lines represents the deformation of the monitoring object,the sensing antenna is free of stress.
Figure 3 Crack sensor based on patch antenna fed by capacitive microstrip line
Another unstressed crack sensor was proposed by combining a monolithic patch antenna with a movable radiation patch,which is shown in Figure 4[9].The total length of the combined radiation patch would be altered by the relative movement between the patch antenna and the dielectric board,leading to a shift of resonant frequency in the sensing system.
Figure 4 Crack sensor based on patch antenna with overlapping sub-patch
Theory calculation,simulation and experimental results show an effective measuring range of 1.5 mm with a sensitivity of 120.24 MHz/mm on average,which is presented in Figure 5.
Figure 5 The relationship bet ween resonant frequency and overlapped length in theory calculation,numerical simulation,and in the experiment