Study On The Design Of Crack Sensors Based On Passive RFID

In recent years,with the rapid development and improvement of infrastructure facilities,structural health monitoring（SHM）has gradually become an important research area to continuously evaluate the structural integrity of infrastructure to avoid potential economic losses and personal safety issues.At present,because of the simple structure and convenient use of radio frequency identification（RFID）tags,they have higher reliability and lower production costs.They can work in harsh environments and can be deployed on a large scale.The monitoring of cracks in building structures has broad application prospects in structural health monitoring.Therefore,this paper is devoted into a crack sensor tag based on the principle of passive chipless RFID,whose long-distance detection and location have been analyzed in detail as well.The main research contents of this thesis are shown as follows:First of all,based on the improvement of the double-voltage rectifier circuit,a novel crack sensor tag circuit was proposed.This circuit can convert the input 2.45GHz RF signal into the second harmonic signal as well as DC signal output.When the sensor is in the"broken"state,the branches of the second harmonic band-stop filter are disconnected from the sensor circuit.At same time,the sensor sends the second harmonic signal to the reader to achieve a long-distance alarm.The terminal light-emitting alarm circuit is powered to realize the position alarm.By using the simulation software ADS for design and setting up an experimental platform for physical tests of the tag circuit,the function of the tag circuit is further verified.The experimental results show that when the transmit power is 20dBm,the working distance between the reader and the sensor can reach 5.5m,and the normal working range of the sensor light-emitting alarm circuit can reach 0.8m.Secondly,in order to further improve the sensor tag to realize the simple and easy use of the tag,a dual-frequency slot antenna is designed based on the microstrip slot antenna theory.The antenna is formed by orthogonally nesting two loop-slot single-frequency antennas at 2.45 GHz and 4.9 GHz in order to reduce mutual interference between the two signals and further reduce the structure size of the antenna.Through optimization simulation,the fundamental frequency receiving antenna gain G₁^r=3dBi,the second harmonic transmitting antenna gain G₂^t=5dBi,and good impedance matching at two operating frequency points.The return loss is more than 15dB,and the isolation between the two ports can reach more than 17dB.Then,on the basis of further improvement and optimization of the sensor circuit structure and performance,it is combined with the slot antenna on a single board to achieve a complete sensor tag.The tag’s receiving and transmitting antennas are located on the front of the substrate,and the sensor circuit is located on the back of the substrate,which greatly reduces the complexity of the system.Through software simulation and experimental testing of the tag,it can be obtained that when the effective transmission power is 35 dBm,the long-term working range of the sensor tag is 7m,and the working range of the short-range light emitting positioning is 0.8m.It not only achieves the performance of the crack sensor,but also reduces the complexity of the system.Finally,the paper discusses and analyzes crack detection in multi-label scenarios,points out the problem of false alarms caused by power leakage in multi-node systems.Experimental tests show that for false alarms in multiple nodes,you can further determine whether cracks have occurred by increasing the detection distance within the normal working range.At the same time,the label’s light-emitting alarm signal not only has the positioning function but also prevents false alarm The effect can be further judged by shortening the measurement distance to see if the crack really exists and its location.These fully reflect the use value and novelty of the label.