Design Of Distributed Monitoring System Of Frozen Soil Temperature At Different Depths Along Qinghai-Tibet Railway

The Qinghai-Tibet Plateau is one of the key targets of China's economic development,and railway construction is a prerequisite for achieving stable development.As the Sichuan-Tibet line gradually increases in altitude,the Qinghai-Tibet Plateau has low temperature conditions for forming and preserving permafrost.The hydrothermal physical characteristics of the frozen soil make the railway of building in the frozen soil area bound to face the two major problems: frost heave and thawing,ruinning track foundation and causing serious consequences.In order to serve the needs of the construction and maintenance of the Qinghai-Tibet Plateau railway,a distributed monitoring system for frozen soil temperature at different depths was designed,aiming to monitor the soil temperature along the railway line during railway construction and operation,realize early warning of changes in frozen soil,and take protective measures.Based on the hydrothermal physical characteristics of the frozen soil and the temperature trend of the frozen soil,this paper analyzes the performance parameters such as the monitoring accuracy and maximum depth that the system needs to meet.Based on this,the non-linear characteristics of the thermistor suitable for low temperature environments were studied.The relationship of temperature and resistance for the sensor was obtained by testing point by point through experiments and piecewise fitting for monitoring and processing of temperature signal.According to the size of the area along the Qinghai-Tibet railway and the difficulty of construction,the multi-channel signal transmission technology and the multiplex channel signal transmission technology were studied.,and then a distributed 64-channel frozen soil monitoring system and a single-channel 64-point frozen soil monitoring system were designed.The contents are as follows:1?At the acquisition end of the 64-channel frozen soil temperature monitoring system,a64-channel sensor group acquisition model was established based on the multi-channel signal transmission technology.In order to eliminate lead errors and reduce the complexity of the voltage conversion circuit design,a four-wire method was introduced,and a 64-channel temperature acquisition circuit was ingeniously designed to convert the temperature resistanceof each acquisition point to a voltage value through 64 underground signal cables transmission to the ground system.2?On the receiving end of the system,a strobe circuit was designed using modules of relay and decoders to selectively receive 64 temperature and voltage signals,which can save85% of the universal interface of the single chip microcomputer and increase the processing speed.3 ? Aiming at the problem of signal distortion caused by the thermistor aging,a temperature correction method is proposed that the three groups of temperature values collected by the same sensor in a short period of time,subtracted and compared,then analyzed the probability of the distortion signaland removed the temperature point with a high probability of distortion to improve system reliability.Finally,calculated and outputed the processed temperature value.The advantages of 64-channel system are small interference between signals,high accuracy,and low coupling of acquisition channels.The disadvantages are complicated wiring.So it is suitable for high-precision monitoring environments with easy wiring.4?The single-channel 64-point frozen soil temperature monitoring system mainly uses frequency division multiplexing technology to improve the channel resource utilization,reduce the number of cables and the difficulty of wiring.Firstly,set the carrier frequency of each modulation signal and designed a 64-channel temperature signal modulation circuit to realize 64-channel modulation signal transmission in a single channel.5?At the receiving end of the single-channel system,in order to solve the problem of the output signal amplitude distortion caused by the frequency deviation of the local oscillation signal during automatic mixing,a second-order multiplying mixing circuit was designed using two multipliers and a band-pass filter,which greatly reduces the distortion of the intermediate frequency signal and improves the system accuracy.The advantages of single-channel system with 64 points are fewer signal transmission lines,and it is easy to build and maintain.The disadvantages are that there is more interference between signals,and the accuracy is slightly lower.So it is suitable for large-area frozen soil areas with low monitoring accuracy requirements.Using the high and low temperature laboratory to simulate the similar frozen soiltemperature environment,the system performance of the 64-channel and single-channel frozen soil temperature monitoring systems was tested,and compared the experimental data of the two systems.The experimental results showed that the maximum error of single channel frozen soil temperature monitoring system is 0.2°C,but has high stability,which is suitable for a wide range of preliminary monitoring.The 64 channel frozen soil temperature monitoring system has a temperature error of about 0.1 ° C,and occasionally a large error occurs,suitable for small-scale precision monitoring,all meet the actual needs.