Design And Application Of Subgrade Collapse Detection System Based On Transient Electromagnetic Method

With the rapid development of cities and changes of urban roads,the road safety has become the focus of urban development.In recent years,the road collapse accidents have occurred frequently throughout the country,which has become a major hidden danger threatening the safety of people's lives and property.Whether the abnormal geological area under the road can be detected in time is the key means to prevent road collapse accidents.Instruments commonly used in the field of road detection are ground penetrating radar,high-density electrical meter,seismic wave detector,etc.Due to the complexity of urban road geology,the detection depth of ground penetrating radar is shallow,which can not meet the requirements of deep detection of urban underground space construction.However,the high-density electrical meter needs to penetrate the detection electrodes into the ground,which cannot be constructed because of the hardening of road cement.Seismic wave detectors need to impose explosive sources on the ground to detect.Road closures can affect traffic,cause vibration and noise,are not safe,and are inefficient in construction.In order to solve the problem that the above geophysical methods are applied to road detection,a road collapse detection instrument based on transient electromagnetic method is presented to solve the problem of road collapse detection.In this paper,a theoretical model of roadbed collapse detection based on transient electromagnetic method is proposed.For the reason of road collapse accident,a method of road collapse detection based on transient electromagnetic method is designed,and ANSYS MAXWELL finite element simulation is carried out to verify the feasibility and validity of the scheme.The hardware system design of the instrument is developed from two aspects:transmission system and acquisition system.The transmission system design is divided into power module design and transient electromagnetic transmission module design.The power supply is in the form of digital power and analog power isolation.The STM32F429 microprocessor is used as the main control chip.using the principle of weak power control,the signal is sent by the main control chip,and mission current generated in H-bridge circuit by isolated driver SI8233.The current passes through the emitter coil to form an exciting magnetic field,which diffuses the electromagnetic eddy current to the geologic body.After the current is cut off,the surveyed geologic body will induce a secondary magnetic field.The second field signal will be received by the receiving probe.The signal will be amplified in place after adjustment and sent to the acquisition system.It will be collected and converted by ADS1278 analog-to-digital conversion chip.Transfer to STM32 for data processing,and finally transfer the data to the three-proof Android tablet through the wireless communication module.The design of system software is introduced from two aspects: embedded software program and upper computer application software program.Based on the hardware structure framework,software design should follow the principle from bottom to top,and design corresponding embedded software program according to the module division of hardware.The main program of MCU is developed by MDK Keil u Vision5 and STM32 Cube Mx,and it has joined the RT-Thread real-time operating system.It uses multi-threaded scheduling to complete the functions of receiving upper-computer instructions,sending impulse excitation,collecting data,wireless transmission,etc.It designs communication protocol for the wireless transmission of data and instructions to ensure the reliability of transmission.The upper computer application software is designed and developed by the QT collapse platform software based on C++.The software functional requirements design is divided according to the hardware structure,mainly including the project module,the collection module and the setup module.Because the signal of transient electromagnetic is very weak,and it collected by the traditional transient electromagnetic instrument is transmitted to the acquisition circuit through the cable.Before acquisition,the signal will be amplified,and the noise will be amplified accordingly.In order to reduce noise interference,a preprocessing module is added to the local end of the receiving coil to amplify and filter the signal directly from the source.In order to solve the checksum test of transient electromagnetic instrument,a transient electromagnetic signal simulator is designed,which can be used for test collection before the use of transient electromagnetic instrument,as well as fast positioning in case of acquisition failure,to improve work efficiency.To solve the problem of weak detection ability of shallow layer,a method called "silhouette" is proposed to eliminate the interference of primary field and enhance the resolution of shallow layer.Finally,the instrument is systematically tested for hardware and software,and the working status of hardware and software is evaluated respectively.The test results show that both hardware and software of the instrument have reached the overall design requirements.In the field road detection experiment,the visualization result map is generated by collection and analysis,it has verified and compared with the actual road conditions,and concluded that the instrument detection effect is significant.In the case of complex road detection and a large number of strong magnetic interference sources,some errors may occur in the detection data.Later studies hope to enhance the anti-interference of the instrument and ensure the stability of the instrument detection.In addition,on the software side,i want to add the real-time map marking function,which can manage the detected and undetected roads,display the abnormal roads that may appear on the map,and then dock with the platform of municipal security departments to form a "joint measurement and prevention" mechanism to ensure the safe operation of urban roads.