| With the rapid development of Chinese economics, society and urbanization, and the urban underground structure and environmental pollution problems are drawing more and more attention. Engineering geology and environmental assessment are mostly involved in the shallow subsurface investigation. Since the difference of urban underground structure and environmental pollution are reflected in the abnormal of magnetism and electricity, meanwhile the detection of engineering geology and environmental problems are mostly belonged to shallow surface detection. Therefore, it is feasible to track and investigate the abnormal information of shallow subsurface by using electromagnetic apparatus to study the electromagnetic field distributed rule with the change of space and time, finding abnormal body underground, solving the environmental problems of geological engineering and so on. The multi-frequency electromagnetic induction detection system works in multi-frequency principle in a test. Since the electromagnetic wave has the ability of penetrability, meanwhile the electromagnetic waves of different frequencies achieve different probing depths, Multi-frequency electromagnetic induction detection technology can be used to investigate targets at different depth underground.This paper describes the principle of multi-frequency electromagnetic induction detection system, proposes a set of rational design which refers to the foreign advanced technology of related instruments. The system bases on DSP technology which achieves multi-frequency current emissions, obtains primary electromagnetic field and the secondary separately, collects data by dual channels and processes them real time. Multi-frequency electromagnetic induction detection system is divided into three parts: current transmitting part, signal acquisition part and software processing part. The current transmitting part sends multi-frequency sine signals coded by DSP TMS320C6713 to control the power switch of the H-bridge after the isolation circuit and the power amplification, converting the DC voltage signal into the multi-frequency sinusoidal current signal which generate primary magnetic field through the transmitting coil. The current in the object underground induced by the primary field produces the secondary field. The signal acquisition part obtains the two fields at the same time through the receiving coil and reference coil (to offset the primary field), then send them into A/D data acquisition circuit after matching amplifier, anti-aliasing filter. The software processing part collects the primary and the secondary magnetic field's signal and process them by the main control unit DSP TMS320C6713, and then send the encoded results of each frequency to MSP430F169 MCU. MSP430F169 transfers the data to the U disk, and PC LabVIEW software generates xls decimal form from U disk data, facilitating the following software processing.The transmitting part is one of the most important part of the multi-frequency electromagnetic induction detection system, the function of this part converts the DC signal into multi-frequency sinusoidal current signal which generate primary magnetic field surrounding the transmitting coil. The current in the object underground induced by the primary field produces the secondary field. Since the secondary field response is directly related to the identification of abnormal body, the stability of transmitter determines the accuracy of detection results. Multi-frequency sinusoidal signal transmission part process the multi-frequency sinusoidal signal into the control sequence at the same length, which bass on the recursive algorithm though voltage full-bridge inverter circuit. After the electric isolation circuit it is converted into bipolar PWM waveform which use to control the H-bridge turn on and off, loading the coil to charge and discharge constantly, therefore we can obtain the frequency components with the amplitude and the phase expected.In signal acquisition part, since the transmitting current is small, the secondary electromagnetic field excited by the primary electromagnetic field is weak, and the received signal is also the multi-frequency signal, meanwhile the primary magnetic field signal and the secondary magnetic field signal will be received by coils at the same time. Because the secondary magnetic field signal is very weak, only some millionth the primary magnetic field signal (ppm orders of magnitude), the secondary magnetic field signal is completely submerged in the total field (the primary field + the secondary field) from the receiving coils. In order to eliminate the impact of the primary field, the system adds another receiver coil which called reference coil. Adjusting the structure and connection of the receiver coil and reference coil, so that they can meet the conditions to eliminate a field, and then the primary magnetic field and the secondary magnetic field signal separated by two channels. The two signal paths will do the task receiving the two signals under the control of DSP real time through impedance matching circuit, preamplifier circuit and anti-aliasing filter. Software processing part is consist of the master control unit programming of TMS320C6713, microcontroller programming of MSP430F169 and PC programming software based on LabVIEW, the whole system works through the three parts. Slave System TMS320C6713 completed the entire work process controlling system, specifically the following: bipolar PWM control sequence algorithm's generation, data sending through the McBSP displacement, data acquisition and processing real time, data communication with the MSP430F169. Finally the MSP430F169 MCU controls the U disk's read-write module for the data storage. PC LabVIEW software converts the data stored in the U disk by reading the binary data inside, into a usable format decimal data xls format file, facilitating to the following mapping software by surfer.Papers distributed that the multi-frequency electromagnetic induction detector received wave transmitting test and data receiving test. In model experiments text verified the reliability of the system, and had some results. Finally, the thesis of the work are summarized, in order to improve the system design, presented its follow-up study of design and some of the recommendations. |
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