Research Of The Multi-turn Coil Fast Turn-off TEM Transmitter Based On The Segmental Discharge Technology

Airborne electromagnetic method is a geophysical detection method based on electromagnetic field.The detection system has the characteristics of large detection area,fast speed,high reliability,deep detection depth and high accuracy,which is widely applied in geological exploration.In the detection system,the load coil of the transmitter is very sensitive,leading to the waveform of the output current taking a long time to turn off,which results there are some blind area of the system in the shallow detection,and this is difficult to meet the detection requirements.In view of this situation,the experts at home and abroad proposed different solutions,this paper summarizes the former research results and the existing problems,then puts forward a fast turn-off transient electromagnetic transmitter provided with multistage load and segmental discharge with the function of constant pressure clamp and energy feedback.This design makes the output current has many good features,including the advantage of high stability,high linearity,short turn off time,high positive and negative consistency.In this paper,the author selects trapezoid waves as the emission waveforms after comparing square waves,trapezoid waves,sine waves and so on.According to the constant voltage clamp theory,the way of using clamp capacitor discharge can shorten the current off time and the energy absorption loop is adopted to improve the waveform quality.Through comparing the different kinds of absorption circuit,a lossless absorption circuit is designed with energy feedback switch and the feedback inductance,using clamp voltage comparator circuit,hall sensor acquisition circuit and hysteresis control current limiting circuit to protect the circuit.The model of multistage coil load is adopted,through establishing and solving the mathematical model,the influence of coil parameters on the falling edge of waveform is analyzed,and the reasonable circuit parameters are designed.By means of the toolbox of Simulink in MATLAB software,the author tests and simulates the circuit,and the simulation waveform is obtained and the rationality of the design is verified.The FPGA is used as the main controller of the control system,according to the flow chart,we carry out the logic design building the peripheral circuit and draw the circuit board based on the PCB schematic diagram.The acquisition module of the system includes the voltage sampling,the current acquisition and the comparator design.The driving module is designed to contain the driving circuit and the driving power,which has the good compatibility and the isolation protection function.According to the simulation results and the circuit parameters,we set up the experimental device and carry out the test experiment,then we obtain the output waveform results.The comparison of the output current waveform between the multi-section load discharge mode and the traditional discharge mode proves that the turn-off time of the waveform is shortened through the segmented discharge.Under the segmented discharge mode,the comparison of output current waveforms with different clamp voltages shows the turn-off time of the waveform is shorter,moreover,the actual waveform has a good agreement with the theoretical simulation value,and the effect of the waveform is better when we use the higher clamping voltage.The transmitting system and receiving system are jointly tested and the improved transmitting system is compared with the reception results of the traditional shutdown mode.It is verified that the time and the maximum value of the received signal have been substantially improved,which conforms to the design requirements.Finally,the full text is summarized and the suggestions for the improvement are put forward.