| Structural health monitoring technology is of great significance to the management and maintenance of large engineering structures.Traditional strain monitoring equipments have the disadvantages of wired connection,high cost and complex operation and maintenance.Various wireless strain sensors developed in recent years have the disadvantages of relatively complex circuit modules and high cost.As a new automatic identification technology,radio frequency identification(RFID)realizes the wireless communication between RFID tag and RFID reader by relying on the spatial coupling of radio signal.It has the advantages of non-contact information transmission,low cost and energy consumption and convenient operation.In this paper,RFID technology is applied to strain wireless measurement in two ways.The first way is to combine the RFID tag which can collect data with the full-bridge strain gauge,and use the wireless communication between RFID tag and the reader to realize strain wireless measurement.The second way is to take the RFID tag antenna itself as a strain sensing unit and indirectly measure the strain by using the linear relationship between the antenna resonant frequency drift and the strain.Based on the above two methods,this paper carries out the research and test of structural wireless strain sensor based on RFID technology,and studies the modelling and prediction algorithm of strain measurement data.The specific research contents of this paper are as follows:(1)The RFID tag circuit strain sensor is designed by combining the semi-active RFID tag which can collect data with the full-bridge strain gauge,and the wireless communication between the RFID tag and the reader is used to realize the wireless measurement of strain.The adopted RFID chip model is CC2530,which belongs to semi-active UHF-RFID chip,and has two models of dormant state and active state.The analog-to-digital converter in the chip can collect millivolt voltage signals.The RFID tag circuit strain sensor is formed by leading out the power supply and signal acquisition pin of RFID tag,and connecting them with the excitation pin and signal output pin of full-bridge strain gauge respectively.The sensor has the advantages of tiny volume and low power consumption.Several groups of comparative experiments of measuring strain are conducted to verify the measurement accuracy.The strain measurements on6061 aluminum alloy plate show that the RMSE of the measurement between RFID wireless strain sensor and wired strain acquisition system is 22 με.In order to remove the limitation that RFID tag must have its own battery,the passive RFID tag circuit strain sensor is designed.The designed rectifier antenna and voltage stabilizing circuit are embedded into the RFID tag,and the tag is activated by electromagnetic wave energy.The strain measurement test of the passive RFID tag circuit strain sensor is carried out.The measurement of the passive RFID tag circuit strain sensor is close to that of the wired strain acquisition system.(2)Using the linear relationship between resonant frequency drift and strain of microstrip patch antenna,the RFID antenna strain sensor is designed by taking RFID tag antenna itself as strain sensing unit.The finite element model of the side-feed microstrip patch antenna is established in the antenna simulation software HFSS.By the simulation of the stress-deformation state of the antenna,it is found that there is a linear relationship between the resonant frequency drift and the strain of the microstrip patch antenna.The simulation results are proved by the test experiment of the side-feed microstrip patch antenna.On this basis,the RFID tag antenna strain sensor is designed.The size parameters of the microstrip patch antenna are optimized through HFSS simulation to realize impedance matching with the passive RFID chip at the resonant frequency.The surface slotting technology is used to miniaturize the antenna.The length dimension of the RFID tag antenna strain sensor is reduced by 40%.The strain sensing performance of the antenna under deformation is simulated in HFSS,and the microstrip patch antenna is manufactured and tested.The microstrip patch antenna is pasted on the tensile specimen and stretched by the universal testing machine.The test result shows that there is a good linear relationship between the resonant frequency drift and the strain of the RFID tag antenna strain sensor,and the linear correlation coefficient is 0.97.(3)For the strain measurement prediction of RFID strain sensor,this paper carries out the research on the modelling and prediction algorithm suitable for strain monitoring data,applying the advanced algorithm,i.e,Variational heteroscedastic Gaussian Process(VHGP)to strain modelling and prediction,and writing the code for strain prediction.The strain measurement of RFID tag circuit strain sensor is predicted by using VHGP,and the prediction is in good agreement with the measurement of RFID tag circuit strain sensor.Further,the large-scale structural strain monitoring data is used as a case to verify the prediction performance of the VHGP.The results show that the modelling and prediction performance of VHGP is better than that of another heteroscedasticity Gaussian Process,i.e.,maximum likelihood Gaussian Process(MLHGP),and the prediction accuracy of strain data is improved on average of 51.9%.In order to further improve the prediction accuracy,this paper proposes the component variational heteroscedastic Gaussian Process(CVHGP),establishing a Bayesian dynamic linear model to separate and extract the different components of the strain data,then using VHGP to predict the components of each strain components,and finally superimposing the prediction results.This paper carries prediction of strain measurement of RFID tag circuit strain sensor by CVHGP,and the prediction accuracy is improved by 57.2% than that of VHGP.The results show that CVHGP has a good prediction performance on the periodically changing strain data.This paper has 144 figures,23 tables and 80 references. |
