Research On Fast Shutdown And Overshoot Suppression Method Of Airborne Transient Electromagnetic Transmitting Current

Helicopter borne Transient Electromagnetic Method(HTEM)is a Transient Electromagnetic Method for geological resource exploration using helicopter as flight platform.It effectively overcomes the difficult problem of ground exploration in complex terrain areas.It has the characteristics of high speed,wide exploration range and strong ability,and is widely used in the world.In recent years,with the continuous improvement of scientific and technological level,HTEM has developed rapidly in the field of geological exploration,and exploration instruments have higher requirements.Improving the quality of current waveform can effectively enhance the detection ability of instrument system.The current waveform quality is usually evaluated by current linearity,turn-off time and whether there is overshoot and oscillation of current after turn-off.The turn-off current waveform is closely related to transient response,and the difference in linearity of turnoff current will reduce the resolution of htem system;The length of turn off time determines the amount of effective data.The shorter the turn off time is,the richer the high-frequency component is,which is conducive to the early secondary field signal extraction and improve the shallow detection ability of the instrument;The early signal is closely related to shallow detection.When there is oscillation and overshoot after the current is turned off,the early signal forms a detection blind area due to the existence of oscillation,and the starting point of channel extraction moves backward,which increases the difficulty of shallow detection.In this paper,the evaluation index of turnoff current is studied in detail.The fast turn-off technology is used to improve the current linearity and shorten the current turn-off time.A set of parameter optimization method of IGBT buffer protection circuit suitable for transient electromagnetic emission system is proposed to effectively suppress the overshoot and vibration after current turn-off.The specific research contents of this paper are as follows:(1)Aiming at the waveform linearity,turn-off time,overshoot and oscillation after current turn-off,the basic model of aviation transient electromagnetic emission system is established,and the main factors affecting the turn-off current linearity and turn-off time are studied.The high-voltage clamp is used to improve the current rise and fall rate,so as to realize short-time turn-off under high current peak.According to the basic model of the circuit,the causes of overshoot and oscillation in the current off stage are analyzed.When the current off is zero,the charge and discharge of the junction capacitor and the buffer protection circuit will bring the oscillation and overshoot after the emission current is off.A simulation model is established to verify it theoretically,and a mathematical model is established to deduce the relationship between the off current and the junction capacitor and the buffer protection circuit.(2)The influencing factors of linearity and turn-off time during current turn-off are analyzed.The active constant voltage clamping method is used to improve the linearity and shorten the turn-off time.A set of active constant voltage clamp power supply with input protection function is designed and developed.The power supply can adjust the high-voltage output range of 0 ? 1350 V.the reasonable design of input and output EMI filter circuit solves the problem of large noise of modular power supply,reduces the ripple and noise of module output to less than 10 m V,and greatly improves the overall efficiency of power supply,up to 95%,improves the peak value of emission current and increases the emission magnetic moment,Provide support for deep exploration.(3)The mechanism of off current overshoot and oscillation is analyzed,and a parameter optimization method of buffer circuit is proposed.This method comprehensively considers IGBT protection,circuit quality factor and buffer circuit loss;In addition,different buffer circuits are adopted for different switching frequencies,and RCD is used to suppress the voltage spike after high-frequency IGBT is closed,so as to improve the reliability of the circuit.Under the protection conditions of switching devices,the tail oscillation suppression of transient current after switching off is realized.Finally,through the development of a full bridge inverter experimental prototype with hybrid buffer function,under the launch condition of 28 V / 45 A,the voltage spike after IGBT shutdown is suppressed from 120 V to less than 60 V,and the reverse overshoot of 4.5 V after load shutdown is realized without oscillation.It is applied to the field experiment of aviation transient electromagnetic emission system.It is observed that the waveform has no oscillation after current shutdown,the current overshoot is 0.6A,and the linearity is improved in the current shutdown stage.Based on the above research,the active constant voltage clamp is used to realize fast shutdown.Under the same peak current,the linearity is improved and the shutdown time is shortened by 27% compared with the old power supply.The parameter optimization method of buffer protection circuit is proposed to suppress overshoot and oscillation.Finally,the feasibility and accuracy of the theory are verified by indoor and outdoor experiments.This research provides a theoretical reference for the design of shallow detection and launch system in the future,It is conducive to tunnel exploration and shallow underground mineral exploration in the future.