Key Technology And Experiment Of Time-domain Airborne Electromagnetic Transmitter

Nowadays, the economy of China develops more rapidly than ever before, andthe demand for mineral resources is increasing. There are rich mineral resources inhigh coverage areas such as mountains, forests, deserts, swamps and lakes. However,ground geological exploration equipment can not investigate a large area, which is thedifficulty of the current geological prospecting work. Therefore, the AEM(AirborneElectromagnetic Method) becomes the research direction.AEM is a geophysical method for investigation of the geological structure andmineral resources by aircraft with electromagnetic field as media. This paper studiesthe design and implementation of time-domain airborne electromagnetic transmitter,and validates its performance through airborne simulation and helicopter flightdetection experiments.On the basis of the research on domestic and foreign AEM system, this paperpresents the main technical indicators: bipolar trapezoidal wave output, the maximumcurrent of300A, the maximum output magnetic moment of250000Am2, fall time ofless than2.5ms. We calculate the electrical parameters of the trapezoidal wave atdifferent periods, propose a design idea of outputing trapezoidal wave in segmentcontrol of different voltage, and achieve it by PWM with the combination of high andlow frequency. We determine the feasibility of the design by simulation.The transmitter contains a control-drive module and inverter module.Control-drive module is divided into a control system, signal processing circuit anddriver circuit. Control system is based on square wave of50Hz and other frequenciesas the input sync signal, MSP430as the main chip, to produce an adjustable bridgecontrol signal and use the button to set the frequency and duty ratio parametersdisplayed by LCD. The signal processing circuit achieve functions of under-voltageand over-current protection, pulse distribution and deadtime setting. The driver circuituses a dual push-pull topology to increase the output power to ensure that multipleparallel IGBT turn on and turn off reliably. The software is written in C language, andPWM module in MCU output timing control signal. The technology of the IGBT inparallel is applied in inverter module to improve its conduction current. Absorptionprotection circuit remove the voltage spikes, control the device heat and improve system reliability. The circuit of eliminating overshoot improves the quality ofemission current, and improves the reception. EMC and thermal design improves thereliability of transmitter.Due to restrictions on the input power, the transmitter apply the technology ofreducing power consumption: Reactive power compensation is a technique withreactive exchange between bulk capacitor and load inductance to get greaterinstantaneous output power; Lossless Snubber circuit can make switching loss back tothe load for the increase in energy efficiency.After the completion of transmitter, we tested drive signal, transmitted waveform,power consumption and eliminating overshoot effect, and all met the designrequirements. We did aviation simulation experiment based on the crane swing inGoose Island of Changchun to detect ground abnormal ring. In the experimenttransmitter worked with receiving system, and was carried by crane. The experimentresults were consistent with the simulation. We did helicopter flight detectionexperiment in Tongbai County in Nanyang City, Henan Province. The experimentresults conformed to the known geological data, which verified the performance of theinstrument further. Through the analysis of the above experimental results, we makesome suggestions.