Transmitting Waveform Control Technology For Transient Electromagnetic Method Based On Inductive Load

Transient Electromagnetic Method(TEM) is a kind of magnetic source electromagnetic detection method in time domain based on the principle of electromagnetic induction. TEM has the characteristics of small topographic influence, strong penetrating power, high detection density and high working efficiency etc. So far, TEM has been widely used in engineering geological exploration, groundwater and geothermal survey, disaster water advanced detection etc.Compared with abroad, the theoretical study of the TEM in China started late, equipment research and development is still in its infancy. There are only a few institutions obtained certain research results. Such as ATTEM from Jilin University, WTEM from Chongqing University and Chongqing research institute and CUGTEM200 X developed by China University of Geosciences. Judgment of TEM transmitting system is an important factor in the overall performance of the transmitting current waveform quality and stability of the system performance. Current TEM stimulating waveform has square wave, trapezoidal wave, sine wave and triangle wave. Through waveform excitation response, the effect of square wave current is best. But in a complex environment, such as mines and tunnels, because of the limitation of instrument device by detecting the location space, the size of transmitting space coil relative to surface detection applications is much smaller. Thus, small coil of number of turns is often used, it has strong inductance, resistance characteristics that cannot be ignored, in the case large current and low voltage, traditional TEM transmitting system would appear waveform distortion, difficult to shut off the current, adverse effects such as final oscillations trailing, seriously affecting the quality of the transmitting waveforms. Perfect stimulating current waveform should be short, shut off the linear decline cut-off time and no overshoot and oscillation phenomenon, current rise rapidly trapezoidal wave. This paper mainly studies on TEM transmitting current waveform control technology. So as to realize the ideal of TEM transmitting stimulating current waveform.In nearly 10 years, TEM is popular both in China and abroad, and increasingly expanded application fields, new technology and new system appear constantly. In terms of the instrument, due to the continuous development of modern power electronic technology, some practical instrument has been developed. But in the use of more small coil turns large inductive load, the transmitting current waveform distortion is larger and it is very difficult to control waveform. Currently, there are three kinds of TEM transmitting current control mode: the first is a “free control”, which uses power on both ends of the coil load resistor in parallel; the second is the use of passive components, such as PROTEM47, PROTEM67 and passive constant voltage clamping circuit from Chongqing University; the third is the constant voltage clamping circuit by the Chinese Academy of Sciences. This paper proposed TEM waveform control technology based on the inductive load, not only solves the short shut-off time and shut off linearly, and transmitting current waveform shut off without a backward overshoot and oscillation, transmitting current of plus or minus period high consistency, current frontier increase rapidly, transmitting peak current is adjustable.System with FPGA(Field-Programmable Gate Array) as the core control unit, the transmitting system select frequency, duty ratio and absorption time parameters by the keyboard. And display the measured transmitting peak current and turn-off time in the 12864 LCD screen. Control circuit board of the transmitting system produce driving signal for H bridge circuit. Between the control circuit board and drive circuit, drive circuit and H bridge circuit using opto-couplers isolation, makes the strong and weak electrical signals in isolation to improve stability and security of the system. The transmitting system is equipped with a power supply module to provide adjustable dc voltage for transmitting main road, in the case of the load to determine to achieve the adjust ability of transmitting peak current. The design of the main road is improved and based on the “ The constant voltage clamping high spend turn off circuit” put forward by Dr Haitao Zhao from the Chinese Academy of Sciences. And the transmitting system developed into a precision instrument by precisely control. This paper analyzes the selection principle of absorbing the resistor of the damping absorption circuit, the improvement of RCD snubber circuit and the relationship between the rising time and turn off time according to the law of conservation of energy from the angle of energy loss. Especially considering the influence of the equivalent resistance of constant voltage clamping circuit and the capacity of the RCD snubber circuit. The control of TEM transmitting waveform by the transmitting system improved the quality of the current waveform and the current waveform, also provides perfect data to the results of TEM exploration.In order to verify the performance of the transmitting system, this paper finally gives the contrast experiments between the indoor and field experiments of the existing transmitting system. Through the precision control of transmitting waveforms, the transmitting system performs well, it can emit high quality of the current waveform, which verifies the feasibility of the instrument system.