Baochen Sun

Detection of rail corrugation based on fiber laser accelerometers

Efficient inspection methods are necessary for detection of rail corrugation to improve the safety and ride quality of railway operations. This paper presents a novel fiber optic technology for detection of rail corrugation based on fiber laser accelerometers (FLAs), tailored to the measurement of surface damage on rail structures. The principle of detection of rail corrugation using double integration of axle-box acceleration is presented. Then we present the theoretical model and test results of FLAs which are installed on the bogie to detect the vertical axle-box acceleration of the train. Characteristics of high sensitivity and large dynamic range are achieved when using fiber optic interferometric demodulation. A flexible inertial algorithm based on double integration and the wavelet denoising method is proposed to accurately estimate the rail corrugation. A field test is carried out on the Datong–Qinhuangdao Railway in north China. The test results are compared with the results of a rail inspection car, which shows that the fiber laser sensing system has a good performance in monitoring rail corrugation.

Applications of fiber optic sensors in concrete structural health monitoring

The research of fiber optic extrinsic Fabry-Perot interferometer (EFPI) sensors and their applications in concrete structural health monitoring are presented in this paper. Different types of fiber optic EFPI sensors are designed and fabricated. Experiments are carried out to test the performance of the sensors. The results show that the sensors have good linearity and stability. The applications of the fiber optic EFPI sensors in concrete structural health monitoring are also introduced. Ten fiber optic sensors are embedded into one section of the Liaohe Bridge in Qinghuangdao-Shenyang Railway. Field test demonstrates that the results of fiber optic sensors agree well with conventional strain gauges.

Monitoring system for a high steep slope based on optical fiber sensing technology

Landslide is typical geo-hazards which causes serious threat to the human lives and infrastructures. Its necessary to develop a monitoring system to avoid landslide disasters. In this paper, a real time monitoring system based on optical fiber sensing technology was designed to monitor the health status of slope. Three types of optical fiber sensors, including FBG strain gauge embedded in anchor, FBG inclinometer, and FBG soil-pressure sensor, were demonstrated. A light switch was introduced to expand the demodulation equipment to 16 channels, which enlarges the capacity of more than 200 FBG sensors. This remote and real-time monitoring system can be used not only for landslide monitoring but also for large scale structure health monitoring.

A novel fiber optic Fabry-Perot temperature sensor

Fiber optic Fabry-Perot sensor has been shown to possess high sensitivity among phase-modulated fiber optic sensors. In the last decade, they have found numerous applications in industry, military, and civil engineering. But they are not easy to manufacture. To avoid the difficult in fabricating the temperature sensor by embedding the gauge in to temperature-sensitive materials, a novel Fabry-Perot sensor is designed and fabricated. The most influential factors of the thermal sensitivity of the sensor are analyzed, which gives the guidelines in design of different sensitivity sensors. Experiments were carried out to verify the theoretical analysis. And the results show that the sensors have good performance and by changing the gauge length the sensitivity of the sensor can be easily controlled.

Fiber optical accelerometer based on 45 degrees Fabry-Perot cavity

The paper proposes an accelerometer construction based on 45-degrees Fabry-Perot (F-P) interferometer cavity. The uniform intensity cantilever consists of a mass block in the middle and a 45-degrees F-P cavity fixed inside the mass. The mass block can oscillate freely when the vibrating sensor is subject to the vibration and the F-P cavity length is changing. The G-lens end face and total reflective film make up the two reflective films of the F-P cavity, and the reflectivity are 4% and 90% respectively. In the F-P cavity, a 45-degrees mirror fixed in the middle of the G-lens and total reflective film. The mirror can change the transmission of the light and increase the optical path difference. The total reflective film fixed in the steel tube and the G-lens fixed in the fine tuning bolt. The bolt can fine adjust the F-P cavity in sensor encapsulating. The sensor structure lead to the optical loss in the airborne and tilted mirror, besides the distance of F-P gap in steel tube and the optical coupling efficiency can’t work out accurately, so we did a series deterministic test before encapsulating, for example the selection of the structures, the diameter of the optical fibers and the diameter of the reflective films. At last, 9/125 μm optical fiber, 1.4 mm total reflective film and the structure of total reflective film out of steel tube were used for the accelerometer. The sensitivity can reach 0.042 rad/g and the resonant frequency of the accelerometer is 400 Hz.

Distributed fiber optic crack sensor for concrete structures

In this paper a novel distributed fiber optic crack sensor with polymer encapsulation based on optical time domain reflectometer (OTDR) is presented. The sensor head includes two layers of polymer and a bare fiber embedded in them. The crack induced deformation in the sensor head will result in the bend loss in the fiber. Utilizing the OTDR measurement, the position and the size of the crack can be determined. Two types of fiber arrangement are introduced in this paper. Experiment was carried out to evaluate the performance of the distributed fiber optic crack sensor. The primary results show that the fiber optic crack sensor has good linearity between the bend loss and the crack open size. The crack size resolution is better than 1mm.

FBG sensor network in Qinghai-Tibet railway monitoring system

A fiber Bragg grating (FBG) sensors network in the roadbed temperature monitoring system in Qinghai-Tibet railway is presented. This sensor network is composed of FBG sensor chains embedded in the roadbed, slave optical cables, work stations which include the FBG sensor interrogator and the optical router, master optical cables, and center workstation. We give the primary experimental results of this sensor network in monitoring the roadbed temperature of the railway at DK0985+000 near Chumar River. The temperature data was collected after a month when the temperature field in the monitoring hole became stable. This experiment had been carried out for more than one year. The FBG sensor network has successfully measured the temperature in the roadbed with an accuracy of 0.1 centigrade. Furthermore, the experimental result shows the stability and reliability of the FBG sensor network in the harsh environment of Qinghai-Tibet plateau. The prospect of the FBG sensor network in monitoring other parameters in the Qinghai-Tibet plateau is also presented.

Research on integrated fibre Bragg grating/extrinsic Fabry-Perot sensor

This paper reports on a novel integrated FBG/EFPI sensor. This sensor consists of an in-fiber Bragg grating (FBG) and an optical fiber based extrinsic Fabry-Perot interferometer (EFPI): FBG to measure the strain and EFPI to measure the temperature. The accuracy of the sensor in measuring temperature and strain is estimated to be plusmn0.1degC in a range from 20degC to 70degC and plusmn2.5muepsiv from - 1000muepsiv to +1000muepsiv, respectively

Distributed feedback fiber laser acoustic emission sensor for concrete structure health monitoring

This paper introduces a highly-sensitive fiber optical acoustic emission (AE) sensor and a parameter analysis method aiming at concrete structure health monitoring. Distributed feedback fiber-laser (DFB-FL), which is encapsulated to have a high acoustic sensitivity, is used for sensor unit of the AE sensor. The AE signal of concrete beam in different work stages, based on the four-point bending experiment of the concrete beam, is picked up, and the relationship between the concrete beam work stages and the AE parameter is found. The results indicate that DFB-FLAES can be used as sensitive transducers for recording acoustic events and forecasting the imminent failure of the concrete beam.

Study on demodulation algorithm of fiber optic Fabry-Perot sensors based on spectrum adjusting

This paper aims at proposing a modified algorithm based on spectrum adjusting. The theoretical analysis of the error between the real light source spectrum and the Gaussian spectrum is presented. The hardware of the demodulation system is introduced, including the coupler, collimation lens, volume phase grating, focus lens, CCD array, A/D card, and PC. The modified algorithm will adjust the interferometric spectrum of the sensor by getting rid of the effect of the light source spectrum. Experiment was carried out to test the performance of the demodulation system. It can be found from the result that the algorithm has improved the accuracy of the demodulation system significantly. The demodulation accuracy for the strain sensor is better than 0.5 microstrain.