Application of Pressure Transmitter in Rail Transit Structure Monitoring

Abstract: The construction activities in the rail transit control and protection area are becoming more and more frequent, and higher requirements are put forward for the monitoring of the safety protection of the rail transit structure. Combined with an automatic monitoring project, the pressure transmitter used in the industrial field was used to carry out automatic monitoring of rail transit structure settlement. The system design was introduced in detail, and the error source and measurement accuracy were analyzed. The monitoring accuracy of the settlement monitoring system based on the pressure transmitter is equivalent to that of the traditional static level. It can effectively overcome the shortcomings of the traditional static level, such as small measuring range, large volume and difficult installation. When monitoring the section with large longitudinal slope, the advantage obvious.

1 Introduction
        Along with the rapid development of the economy, the urbanization process has accelerated markedly, and the scale and speed of urban construction and urban transformation are expanding. The construction and operation within the scope of urban rail transit control and protection areas have emerged in large numbers, which inevitably disturbs the existing rail transit structure, which easily leads to imbalance of structural stress, causing local horizontal displacement, settlement, stretching, and Compression, shearing, bending, torsion and other deformations cause tunnel leakage, boundary changes, settlement of the track bed, and geometric changes of the track. If the structural deformation exceeds the control value, it will seriously threaten the operational safety of rail transit and cause huge losses to people's lives and property. Therefore, it is particularly important to monitor the urban rail transit structure affected by the construction of external operations to ensure the safety of its operations, and to feedback and guide the construction of external operations.

        Because many of the spatial relationships between external operations and rail transit structures in the rail transit control protection zone are mostly up or down, the structural deformation is mainly reflected in the structural elevation changes, and the static level is used to monitor the monitoring points of the rail transit structure. The change of subsidence has been applied in cities such as Beijing, Shanghai and Guangzhou, and has achieved good results [1~3]. At present, the static level gauge that directly measures the liquid level using the float is most widely used in the automatic monitoring of the settlement of domestic rail transit structures. Due to the principle of directly measuring the change of the liquid level, the range of such static level is generally small, depending on the height of the container, generally not more than 100 mm. The installation accuracy of the equipment is relatively high, and the initial elevation is basically the same when installing. With this type of static level, it is necessary to lay out sections in sections where the slope of the line is designed to vary greatly, and a longitudinal cascade static level is set between the level gauges [4]. The system design and installation are difficult and increase. The cost of monitoring the system.

2 settlement monitoring system based on pressure transmitter
2.1 Pressure transmitter principle
        Pressure transmitters, which are widely used in the industrial field, can be used to measure the liquid level, density and pressure of liquids, gases or vapors. The working principle [5] is that the pressure difference from the double-sided pressure guiding tube acts directly on the transmission. The two-side isolation diaphragm of the sensor is transmitted to the measuring component through the sealing liquid in the diaphragm, and the measuring component converts the measured pressure signal into a corresponding electrical signal and transmits it to the converter, and becomes a standard after being processed by amplification or the like. Electrical signal output.

        The measured value of the pressure transmitter is the pressure of the water head. The sensor itself does not need a liquid storage tank. Take the Yokogawa EJA110 differential pressure transmitter as an example. Its specification is 125mm long × 110mm wide × 197mm high. Its specifications and traditional static level Compared to the greatly reduced. The pressure transmitter measures a large range. The Yokogawa EJA110 differential pressure transmitter has a range of 50mm to 10000mm (H2O) with a measurement accuracy of ±0.075% and a resolution of ±0.01%. When the maximum height difference between pressure transmitters is less than 10cm, the measurement accuracy can reach ±0.1mm, which is equivalent to the accuracy of the traditional static level. When the height difference is 1m, the measurement accuracy is ±0.75mm and the resolution is ±0.1mm, which can make up for the shortcomings of the traditional static level instrument, making the design and installation of the settlement monitoring system more convenient. Sections with large variations in slope design are more advantageous. Pressure transmitters are industrial grade applications, and their long-term stability can reach ±0.1% in 5 years. It can be operated continuously for 5 years without adjusting the zero point. The influence of temperature and static pressure is very small. Under high temperature and high pressure environment Can maintain high stability.

2.2 system design
        The settlement monitoring system based on pressure transmitter is mainly composed of four parts: on-site monitoring equipment and sensor subsystem, data acquisition and transmission subsystem, monitoring center and user subsystem. The monitoring equipment and sensor subsystem are mainly hardware devices such as pressure transmitters and collectors arranged at the monitoring site; the main function of the data acquisition and transmission subsystem is to control the collector, collect corresponding data periodically and transmit it to the monitoring center. Into the data, and execute the relevant commands; the monitoring center is the control center, data storage center, data processing, analysis and evaluation center, monitoring and warning release center of the entire monitoring system; the user subsystem realizes to display various data to the user in real time according to the requirements. And accept user control and input to the system.

        At the field acquisition end, the pressure transmitter measures the process pressure of the fluid, so a separate reservoir is required to ensure the pressure of the entire system. A liquid storage tank is arranged at one end of the system, and is connected with the high pressure end of each pressure transmitter by a communication tube to ensure that the height of the liquid surface in the liquid storage tank is higher than the height of the center position of all pressure transmitters. When the monitoring section is long, in order to ensure the pressure of the whole system, the liquid storage tank can be separately set at both ends of the system. The installation height difference between each monitoring point can be selected according to the measurement accuracy. To ensure the monitoring accuracy is better than ±1.0mm, the maximum height difference between the monitoring points should not be greater than 1m.

The liquid filled in the system is generally antifreeze. If pure water is used as the medium, preservatives should be added to the water to prevent the liquid from deteriorating. Pour the antifreeze into the reservoir and then open the pressure transmitter exhaust valve exhaust from the farthest end until the sensor monitoring data is stable. The density of the filling liquid is measured at the liquid storage tank by a densitometer, and as a parameter input, the height difference of each monitoring point relative to the liquid level of the liquid storage tank is obtained.

3 engineering case
        In the automatic monitoring project of the impact of the demolition of a building on the structure of the rail transit tunnel, in order to timely grasp the impact on the tunnel structure during the demolition process, the settlement monitoring system based on the pressure transmitter was used to monitor the real-time settlement of the tunnel. Stress monitoring sensors are also deployed. The rail transit tunnel within the affected area is divided into two sections: single-hole two-way and two single-hole single-line sections. The length of the affected area is about 160m, the maximum slope design of the line is 42.5‰, and the maximum height difference of the section tunnel is 7. 5m, it is very difficult to design and install with traditional static level.

        A total of 5 monitoring sections are arranged in the project, the section spacing is about 50m, the pressure transmitters are installed in the left shoulder portion of the tunnel, and the reference point pressure transmitter and liquid storage tank are set in the stable area on the north side of the line. The maximum height difference is about 220mm, the pressure transmitter adopts Yokogawa EJA110A type, the collector adopts Kekang BGK-Micro-40 measuring unit, and the data sampling interval is 10min.

4 monitoring data analysis
4.1 Error source analysis
        The use of pressure transmitters for rail transit structure settlement monitoring is similar to traditional static leveling, and is also affected by a variety of error sources, which are mainly affected by temperature changes, environmental pressure, train vibration, and fill fluid quality.

(1) Temperature influence
        The density of the liquid filled in the system changes with temperature. The change of liquid density also changes the volume of the liquid. As the temperature increases, the height change of the water column also accelerates [6]. The real-time monitoring data of the temperature of each section over 3 months was counted, except that the temperature difference between section 1 and the other 4 sections was within ±0.5 °C, and the temperature difference of the other sections was within ±0.25 °C. The No. 1 section is close to the light rail of the light rail platform. It is affected by the start and stop of the platform and the train. The temperature of the section is slightly different from other sections. Since the temperature difference between the sections in the tunnel is small, the influence of temperature on the height difference is negligible. If the monitoring system is installed outdoors, the temperature difference may be caused by the difference between occlusion and sunshine, and the monitoring results should be corrected according to the temperature measured by each sensor.

(2) Influence of air pressure and gravity
        The difference between air pressure and gravity will affect the height of the liquid level. Since the pressure transmitters are basically installed on the same elevation surface, and in a relatively closed environment of the tunnel, it can be considered as a small range. Gravity acceleration g and air pressure remain unchanged [7, 8].

However, due to the flow of air in the tunnel caused by the operation of the rail transit train, the change of the air pressure in the tunnel will cause some fluctuations in the measurement results. However, if the train interval is long or during the outage period, the pressure change will gradually stabilize. , the impact on the monitoring results is small. When the monitoring accuracy is high, the low-pressure end of all monitoring point transmitters can be connected by air tubes to form a closed system, which can significantly reduce external environmental interference.

(3) Train impact
        The effects of vibration generated by the rail transit train passing through the monitoring area at a certain speed, the electromagnetic interference of the power cable to the electromagnetic interference of the pressure transmitter, and the air pressure caused by the air flow will all affect the monitoring results. Taking this project as an example, during a certain day of monitoring, the monitoring personnel record the approximate time of the train passing the sensor and then compare it with the time series of the sensor monitoring data. When the train passes the monitoring point, increasing the load of the tunnel at this part will cause the monitoring point to sink. Since the part of the train passing through the project is the reference point, the sinking of the reference point will cause the monitoring point to rise. It can be seen that the time of the train operation through the sensor is mostly consistent with the time when the sensor data is hopped. The jump size ranges from 0mm to 2mm, and the settlement increases.
Pressure transmitter monitoring data and train passing time comparison
(4) Bubble influence in the filling liquid
        During the monitoring process, it was found that the settlement of No. 4 monitoring section was basically consistent with the reduction of the reference point liquid level. Since the pressure transmitter has no unexhausted air in the capsule, the positive pressure of the transmitter is always affected by the accumulated gas pressure, and this pressure is constantly changing, causing the deviation of the monitoring value and the whole system. The pressure has a certain correlation. Under the influence of the external environment, the liquid dissolved in the system will continuously precipitate and accumulate in the sensor to cause measurement error. In order to eliminate the influence of bubbles in the system, one can choose a liquid with stable properties such as silicone oil as the medium; the other is to periodically vent the pressure transmitter to make the sensor monitor data stable.

4.2 Monitoring accuracy analysis
        It can be clearly seen from the process curve monitored on a certain day that the monitoring curve of the pressure transmitter is very stable during the nighttime shutdown from 0:00 to 5:00. Due to train operation, electromagnetic interference, noise and abnormality during the day, the monitoring data during operation needs to be identified and filtered.
One day system monitoring process curve
        The statistical analysis of the monitoring results during the train outage can truly reflect the monitoring accuracy of the pressure transmitter. It can be seen from Table 1 that the monitoring accuracy of the pressure transmitter is better than 0.1 mm during the stop of the night train. The daily monitoring results are best taken as the average value of the nighttime period as the monitoring result value of the day. It can be seen from Table 2 that the accuracy of the monitoring data during the tunnel operation period is filtered, and the monitoring accuracy of the pressure transmitter in the tunnel operation period reaches sub-millimeter. The real-time settlement can also be achieved by filtering and smoothing the monitoring data. The accuracy of the monitoring.
Standard deviation of monitoring pressure transmitters during train outage
4. 3 Comparison of monitoring results
        In order to verify the reliability of the pressure transmitter for settlement monitoring, prisms were installed on the tunnel lining near the pressure transmitters of No. 2, No. 3 and No. 5 sections of the project, and the measurement robot was used for monitoring to change the monitoring results and pressure. The results of the transmitter monitoring were compared and the results are shown in Table 3. It can be seen that the average difference between the measurement robot and the pressure transmitter is -0.5 mm. It can be said that the monitoring results of the two methods are basically consistent, and the monitoring accuracy is within the sub-millimeter level.
Difference between monitoring robot and pressure transmitter monitoring results
5 Conclusion
        (1) The pressure transmitter is used to monitor the settlement of the rail transit structure. The monitoring accuracy is comparable to that of the traditional static level. It can effectively overcome the shortcomings of the traditional static level gauge and the difficulty of installation.
Larger sections and the need to monitor the upper part of the tunnel arch have a greater advantage than traditional static level.
        (2) The pressure transmitter is used for automatic monitoring of structural settlement. The main source of error is the effect of no air bubbles, electromagnetic interference and train vibration in the system. Later, fiber grating can be used to transmit signals to reduce electromagnetic interference.
        (3) The system collector is scanned on a channel-by-channel basis, so the acquisition time of each sensor is not strictly synchronized. For the settlement monitoring system, the high difference of the liquid level relative to the reference point at the same time is required, which also has a certain influence on the measurement accuracy of the height difference. In order to synchronize the data as much as possible, all pressure transmitters can be connected to adjacent channels, but the strict synchronization of the data requires further study.
        (4) The data quality of the whole settlement monitoring system is closely related to the installation quality. Whether the data is stable or not depends on continuous observation for a period of time. To ensure the reliability and continuity of the data, it is recommended to use other manual monitoring methods to verify and supplement each other. .