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Exploration of Using FBG Sensor for Axle Counter in Railway Engineering

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Exploration of Using FBG Sensor for Axle Counter in Railway Engineering Exploration of Using FBG Sensor for Axle Counter in Railway Engineering K.Y. LEE, K.K. LEE, S.L. HO Department of Electrical Engineering, The Hong Kong Polytechnic University Hung Hom, Hong Kong SAR, CHINA Abstract: - Fibre Bragg Grating (FBG) sensors are finding increasing applications in smart structures. They can be embedded in civil structures such as bridges, buildings etc for the detection and monitoring of parameters such as strain, pressure and temperature. It allows assessments on the integrity of structures at various manufacturing and construction stages, and facilitates health management of many important structures in their subsequent operations. However, their application in railway engineering is rare. This paper explores the technical feasibility and potential usage of FBG sensor for axle counter application in railway engineering. In essence, an axle counter system is used for counting the number of train axles coming in and out of a section of rail track. Field tests have been carried out and concluded that FBG is having great potential for replacing conventional railway axle counter system, which is based on the principles of magnetic field disturbance technology as the wheels are passing by. FBG axle counter have lots of advantages over their conventional counterparts due to its electrical immunity to noise, multiplexing capability, compactness and more importantly, its relatively low cost of construction. Provided there is a weather proof and robust mounting methodology in the next phase, the FBG axle counter will become a strong competitor to conventional systems. Once the physical integrity of FBG systems is proven to be satisfactorily, one can extend its application to build a smart railway, particularly as part of a new generation of signaling systems. Key-Words : - Fibre Bragg Grating, Axle Counter, Train Detection & Identification 1 Introduction applications is on the railway axle counter in Railway engineers are understandably very tracks. Due to its unique construction nature, the conservative in the use of any new technology cost of FBG axle counter is relatively low when for train operations. Only proven technology compared with those of conventional with good track record will be used. Any change technologies that are used to provide the same to the conventional equipment and technical and operational information. There is methodology must be brought-in under a a good future in using FBG axle counter to technically and commercially prudent manner. operate a smart railway service. Fibre Bragg Grating (FBG) technology has been widely adopted in smart structure engineering 2 Fibre Bragg Grating Technology with good records. Its application on railway The fabrication of refractive index grating is engineering is limited at this stage, although reported by Hill et al. in 1978 [2]. Subsequently, there are applications in, for example, the Meltz et al. devise a method to controll the monitoring of the strain/stress on the train bogie fabrication of grating using UV laser [3]. frame composite structure [1]. An in-depth Basically, when a germanium-doped silica core review on the FBG characteristic revealed that it fibre is exposed to ultraviolet (UV) radiation can bring massive benefits to railway operation (with wavelength around 240 nm), it produces a by giving additional train operational permanent change in the refractive index of the information to rolling stock engineers and germanium-doped region, due to the signaling engineers in the monitoring and photosensitivity nature of the fibre and, using planning of the train traffic. One of its possible such an exposure, it is possible to obtain realize changes in the refractive index by factors as -3 2π 2π large as 10 in germanium-doped silica fibre. If 2 n = − λ eff Λ the fibre is exposed to a pair of interfering UV Β (2) beams as shown below, then in regions of constructive interference which correspond to or high UV intensity, the local refractive index will λ = 2 Λ n increase. Β eff (3) where λB is called the Bragg wavelength – that is, the wavelength that satisfies the Bragg condition. The Bragg wavelength is dependent on the effective index as well as the grating period. Fig 1. UV beams on optical fibre through a phase mask At the same time, in regions of destructive interference, where the intensity of UV light is negligible, there is no index change. Therefore, an exposure to an interference pattern will result in a periodic refractive index modulation along Fig 2. Vector diagram the length of the fibre, the period of modulation being exactly equal to the spacing between the interference fringes. 3 Bragg Condition When light is made to propagate through an UV Reflected exposed fibre with a periodically modulated Signal refractive index, it is found that, under certain conditions, the propagating light beam can be strongly coupled to a mode propagating in the Fig 3. Reflected Signal satisfying Bragg condition backward direction. This happens when the Bragg condition is satisfied, i.e. the difference in the propagation constants of the two-coupled 4 Optical Interrogation System modes equals to the spatial frequency of the The function of the optical interrogation system grating, is to detect the wavelength shift in relation to the 2π external perturbation. Due to the characteristics βg − ( − βg ) = K = of FBG, the sensor possesses an intrinsic Λ (1) self-referencing capability as the sensed where, βg is the propagation constant of the information is directly encoded in accordance to forward propagating guided mode, - βg is that of wavelength, which is an absolute parameter and the backward propagating guided mode, and Λ is independent of the light power and loss in the transmission fiber. This feature is widely the spatial period of the grating. Figure 2 shows the corresponding vector diagram acknowledged as one of the most important advantages of the FBG sensors. wherein the propagating vectors and the grating vector satisfy the condition given by (1). If neff is The wavelength-encoded characteristic enables the effective index of the mode, then equation (1) the quasi-distributed sensing of measurands, such can be written as as temperature, strain, and pressure etc., to be arbitrarily located at any point along the fiber. Based on the Bragg’s Conditions described before, it can be understood easily that by measuring the changes in λ, the changes in Λ can be calculated by assuming that neff is relatively stable with respect to ambient changes. Since Λ is sensitive to both thermal expansion due to temperature changes and Fig 4. Interrogation System strain/stress, changes in Λ actually reflects strain/stress received and therefore the Each FBG of a quasi-distributed FBG sensor corresponding force/weight applied can be array has a unique reflected wavelength. The estimated by measuring changes in λ with wavelength interval between two neighboring suitable calibration. FBGs should be greater than the maximum wavelength shift of the individual FBG that Rail beam can actually be regarded as a simple results from external perturbations. In this way, structural beam experiencing compression, the magnitude of the measurant and at each neutral and extension stratum as shown in the arbitrary sensing position along the fiber can be diagram below. tracked simultaneously. In other words, there could be many FBG sensors engraved on a single 1. Compression optical fibre. This is obviously a very significant 2. Neutral 3. Extension advantage compared with conventional electrical sensors which require an independent circuit for 1 each sensor. 2 3 It can be shown that when force is added to the sensor, the response of the reflective wavelength Fig 6. Wheel weight on rail is different as shown below. 6 Site Test Arrangement Eight FBG sensors were fabricated and glued onto the rail beam as shown in Fig 7. Fig 5. Reflective Wavelengths 5 FBG Axle Counter Exploration The weight of the train exerts significant force upon the rail to generate a high strain/stress on Fig 7. FBG sensor on Rail the rail beam. Such observation is very interesting for engineers wanting to exploit the The weight of the Electro-multiple Units intrinsic advantages of FBG sensors in railway (EMUs), i.e. the trains, would create both engineering, particularly the usage of FBG as tensile strain and stress on the rail beam as the development of a new generation of axle shown in Fig 6. The corresponding changes in Λ counters in railway engineering. thus resulted can be detected by attaching FBG sensors at positions 1 and 3 while on the neutral plane, the FBG could be used to measure the Sensor ambient temperature of the rail beam. Practically, changes in the FBG grating Λ sensed would be presented as changes in λ and hence, according to the principle, the weight of the EMU will trigger a shape change in wavelength (Δλ). Once it is captured, it will Glue on the rail Finished represent the presence of an axle that is entering into a given section of the rail track. Fig 9. FBG sensors at Tai Wai Station Eight FBG sensors were fabricated and glued Optical sensor interrogator and portable onto the rail beam at East Rail Tai Wai Station computer were used to record the data. The down line. The layout of the sensors are as optical fibre sensors were connected in series shown below in Fig 8. and fed into the input port of the interrogator as displayed below in Fig 10. The block diagram of the complete set up for measurement is shown in Fig 11. Data obtained were then transferred to the computer for recording purpose. Results are presented in terms of continuous waveform, showing the weight of the train passing all the sensors. Fig 8. FBG sensors layout at Tai Wai railway station The eight FBG sensors were located at the entrance and exit of the station.
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