A Fiber Bragg Grating Displacement Sensor For Bidirectional Measurement And Its Application

With rapid development of economic and technological,urban metro rail construction has advanced by leaps and bounds,gradually becoming an important player in public transport systems.In recent years,underground tunnel collapses have been happening and it is necessary to monitor the tunnel structure online in real time to ensure its safety during operation.Displacement is an important indicator to reflect the safety condition of the underground tunnel,due to the existence of electromagnetic interference inside the underground tunnel and the complex environment of humidity,displacement sensors are required to have excellent performance such as signal antielectromagnetic interference,traditional electromagnetic signal sensor adaptability is difficult to meet.Fiber Bragg Grating sensing technology is resistant to electromagnetic interference,electrically insulated,corrosion resistant,small in size,highly sensitive,can be transmitted over long distances,and can realize distributed measurement advantages.It is suitable for long-term online monitoring in complex environments such as metro tunnels.This paper is based on the actual monitoring needs of the project to develop a suitable for bi-directional measurement and the creep-resistant optical fiber Bragg grating displacement sensor,and successfully applied to engineering practice.The main work is as follows:(1)A monitoring solution based on optical fibre grating displacement sensing technology is proposed based on the shortcomings of existing metro tunnel monitoring technology.The study focuses on the current research status of optical fibre grating displacement sensors in recent years,and introduces the structural components of the sensors according to their classification,while outlining the principles of optical fibre grating sensing technology,as well as the multiplexing technology and temperature compensation methods of optical fibre gratings.(2)The design of a fiber-optic grating displacement sensor was completed to target the fatigue and failure of existing sensors in measuring positive and negative bidirectional displacements of elastic elements(cantilever beams)due to constant stresses.The double "cantilever beam + wedge slider" structure is designed to alternate the deformation of the two cantilever beams during measurement,solving the temperature-strain cross-sensitivity problem and improving the creep resistance of the sensor.A variable thickness cantilever beam is used to achieve strain concentration and increase measurement sensitivity.The free end of the cantilever beam is designed with a circular surface in contact with the wedge slider surface to reduce frictional losses and achieve stable displacement transfer.The cantilever beam is theoretically derived from its surface strain based on the structural dimensions of the cantilever beam using knowledge of the mechanics of materials,and the sensor measurement principle is theoretically derived in conjunction with the fibre optic grating sensing principle.(3)In order to test the performance parameters of the optical fiber grating displacement sensor for engineering applications,a comprehensive performance test experiment was carried out,including calibration test experiment,temperature compensation characteristics test,creep performance test and accuracy test experiment.Through the experimental data analysis,the sensitivity of the sensor within the range of 50 mm is 29.369pm/mm,the repeatability error is 1.28%,the hysteresis error is0.872%,the measurement sensitivity is high and the error is small,can meet the engineering monitoring needs.(4)Combined with the practical engineering,eight fibre-optic grating displacement sensors were prepared to form an FBG monitoring network to achieve distributed measurement,which was successfully applied to the safety monitoring of dehiscence deformation in a city Metro tunnel.The deformation monitoring is mainly carried out in four aspects: relative settlement of the roadbed and lining,differential settlement of the roadbed,expansion joints of the roadbed and annular joints of the pipe sheet.By analysing the graphs of the displacement changes,it was found that the maximum deformation was 1 mm among the eight monitoring points and the deformation state of the measured structure was stable,indicating that the sensor has good measurement performance and is suitable for long-term structural health monitoring.