Design of Railway Signal Centralized Monitoring System Based on Fiber Bragg Grating Sensing Technolo

Analysis of fiber Bragg grating technology and configuration software

The fiber grating is formed by the periodic change of refraction in the fiber core caused by ultraviolet exposure technology. The periodic structure of refractive index distribution in fiber Bragg grating leads to the reflection of light with a specific wavelength, thus forming the reflection spectrum of fiber Bragg grating. The wavelength of reflected light is sensitive to temperature, stress and strain. When the ambient temperature, stress or strain changes, the peak wavelength of fiber Bragg grating reflected light shifts. The temperature, stress and strain can be sensed by measuring the wavelength shift.

The configuration software is "configuration monitoring software". "Configure" means "configuration", "setting" and "setting", which means that users complete their required software functions in a simple way similar to "building blocks" without writing computer programs, that is, the so-called "configuration". "Supervisory control" refers to the monitoring, control and management of automatic equipment or processes through computer signals.

1.1 theoretical basis

The period of fiber grating is in the order of hundreds of nanometers. When the light of a broadband light source is incident into the fiber grating, the periodic structure of refractive index distribution leads to the reflection of light of a specific wavelength. The wavelength of the reflected light (i.e. resonant wavelength) is determined by Bragg formula:

Fiber Bragg grating stress sensor is usually attached to an elastomer and encapsulated at the same time. When the elastomer is under pressure, the fiber Bragg grating and the elastomer will strain together, resulting in the resonant wavelength drift of the fiber Bragg grating. The drift amount is given by equation (2):

As can be seen from equation (3), if the drift of the resonant wavelength of the fiber grating and the change of its working temperature are known, the strain of the fiber grating can be calculated.

1.2 physical logic model

Fiber Bragg grating sensing output is the amount of wavelength change. Special wavelength demodulation equipment must be used for wavelength identification, and then sent to the computer for analysis, processing and alarm.

Figure 1 shows the stress measurement system composed of fiber Bragg grating stress sensor and wavelength demodulator.

The stress measurement system is mainly composed of broadband light source, fiber Bragg grating stress sensor, wavelength demodulator based on adjustable F-P filter, computer and software analysis and processing system. The figure shows multiple fiber Bragg grating stress sensors distributed at equal intervals. These fiber Bragg gratings are usually connected in series. The broadband optical signal sent by the broadband light source is transmitted to the series connected sensing grating through the isolator and 3 dB coupler. After the wavelength selection of these fiber gratings, a group of narrow-band light with different wavelengths is reflected, and the reflected light is received by the wavelength demodulator through the 3 dB coupler again. These wavelengths are identified by the wavelength demodulator to obtain a group of stress sensing information. When the external stress of the sensor changes, the wavelength change, that is, the stress change, is monitored by the grating demodulator, and then input to the computer for data analysis and processing. Finally, the pressure distribution of the sensor is obtained, so as to judge whether there will be dangerous effects such as collapse and displacement, and play the role of alarm.

The wavelength demodulator consists of adjustable F-P filter, sawtooth scanning voltage generator, jitter signal generator, signal mixer and LP low-pass filter. The working principle of the wavelength demodulator is: the wavelength coded pressure sensing signal is input to the tunable F-P filter. When the sawtooth wave drives the F-P filter to make its transmission peak coincide with the reflection peak of the fiber Bragg grating, the reflection peak position of the fiber Bragg grating can be measured according to the relationship between the driving voltage of the F-P filter and the transmission wavelength. However, the transmission spectrum output by F-P filter is the convolution of grating reflection spectrum and F-P filter transmission spectrum, which will increase the bandwidth and reduce the resolution. Therefore, a small dithering voltage is added to the scanning voltage, and its output is mixed and low-pass filter to measure the dithering frequency. When the signal is zero, the measured data is the reflection peak wavelength of the grating.

1.3 configuration software

Configuration software is any combination of modularization. The software includes system configuration software platform and application software platform. The operating system can be window NT, Linux, UNIX, etc. TCP / IP protocol is adopted for network transmission. Based on the system software, the supporting platform of application software, namely distributed real-time running environment, is established. It includes distributed system management and monitoring, real-time database based on memory, real-time database configuration tool, three-dimensional graphics configuration tool based on OpenGL, multi-level and multi protocol network communication structure, etc. Based on the configuration platform, the secondary development application software platform is fully established on the basis of open international industrial standards, and needs to be open. Considering the development of the system, the commercial database is adopted, and the open design of the application system needs to have multi-level characteristics, so as to provide a solid foundation for multi-directional and multi-level continuous expansion of system functions.

The hierarchical structure of the configuration software platform is divided into four parts: operating system (optional windows, Linux, UNIX, etc.), real-time running environment (including real-time database, real-time database configuration tool, relational database and multi-level / multi protocol network interface), graphic configuration, database maintenance and SCADA application functions.

2 system scheme

2.1 system logic function

The railway on-line monitoring system includes two subsystems: data measurement system and data management and analysis. The data measurement system is composed of sensor subsystem and data acquisition and transmission subsystem; The data management and analysis system includes monitoring data management subsystem and data analysis and processing subsystem. The system logic function is shown in Figure 2.

The sensor subsystem consists of sensors distributed on site, mainly including optical fiber temperature, strain, pressure, displacement and vibration sensors. The optical fiber sensor analyzer can be configured with instruments in different channels according to the number of on-site monitoring points. The sensors are mainly distributed on the power switch cabinet, cable joint, transformer, transformer and other power transmission and transformation equipment of traction substation for on-line temperature measurement and automatic alarm. They are also distributed in civil engineering structures such as bridges, railway tracks and railway tunnels for safety monitoring of stress and deformation structures.

The data acquisition subsystem is completed by the fiber Bragg grating analyzer. The fiber Bragg grating analyzer is connected to the field sensor. The instrument analyzes the periodic structure of refractive index distribution in the fiber Bragg grating, resulting in the reflection of light of a specific wavelength, so as to form the reflection spectrum of the fiber Bragg grating. The wavelength of reflected light is sensitive to temperature, stress and strain. When the ambient temperature, stress or strain changes, the peak wavelength of fiber Bragg grating reflected light shifts. The temperature, stress and strain can be sensed by measuring the wavelength shift. The sensed data is transmitted to the data processing and control system through the data transmission system.

The data transmission system is the communication channel between the fiber Bragg grating analyzer and the monitoring system. In places with wired transmission, optical fiber (such as traction substation or section post) can be considered, and the channel is stable and reliable; In places without conditions (such as tunnels, bridges, etc.), wireless mode can be adopted. Although the quality of the channel will be worse, the on-line monitoring system can still meet the requirements.

For the data processing and control system, in order to ensure the real-time and stability of the system, the configuration software is adopted. The system has the functions of real-time data acquisition, data out of limit processing, historical data adoption, accident sampling, accident recall, etc. the real-time system is equipped with a decision support system to help the dispatcher make a rapid and accurate judgment on the site situation.

2.2 system structure

The overall system structure is shown in Figure 3.

(1) Several fiber Bragg grating sensors are installed inside each tested equipment to collect field information;

(2) The main control room or main control box shall be set at the relatively centralized monitoring site to place the fiber Bragg grating analyzer and communication equipment to analyze the sensing information data of temperature, stress and strain, early warning report and data transmission;

(3) The master station dispatching system collects the data of fiber Bragg grating analyzer distributed in each site (rail, railway bridge, railway tunnel, railway traction substation, etc.) through different transmission channels (optical fiber, microwave, carrier, GPRS, etc.) according to a certain transmission protocol, and the dispatching system carries out alarm, sampling and other processing of real-time field data, The decision support system assists the dispatcher to put forward the prediction scheme.

2.3 temperature measurement system of traction transformer

Taking the traction transformer temperature measurement system as an example, the physical structure and field installation of the system are described in detail, as shown in Figure 4.

Function description of each part of the system:

(1) Fiber Bragg grating temperature sensor: it is arranged in the high-voltage switchgear to collect temperature signals;

(2) Transmission system: transmit the temperature sensor signal to the fiber Bragg grating demodulator in the control room;

(3) Fiber Bragg grating signal demodulation system: demodulate the temperature signal and provide real-time information of field temperature;

(4) System software: provide software support for the operation of the whole system.

In conclusion, fiber Bragg grating sensing technology has high application value in railway safety protection. Practice has proved that the on-line monitoring system is safer and faster than the traditional railway safety inspection, and will bring greater economic benefits and better social effects. In particular, a variety of communication technologies, configuration software and decision support system are used to provide safety guarantee for dispatcher's decision-making.