The Research On Key Technology Of Linear And Slow Switch-off TEM Transmitting System

In recent years,due to the development of social society and technology,detecting the underground structures and mineral resources has become one of the research hotspots gradually.Of all the methods,time-domain transient electromagnetic method is the most reliable and widely used method.In the field tests,the falling edge of the transmitting current waveform decays to e-exponential due to the inductive nature of the transmitting coil.The waveform of the induced voltage is too large to be received and hard to be rejected.At the same time,since the switch-off period is too short,the ability of the transmitting system to detect the deep layer of the underground structure is restricted greatly.To increase the detecting depth of transient electromagnetic method system,decrease the top value of the induced voltage and make it easier to reject the primary field,we should make the transmitting current switch-off linearly,and prolong the switch-off period of the transmitting current.However,there is no equipment that can prolong the switch-off period of the transmitting current.The methods of optimizing the linearity mainly focus on high voltage clamping fast switch off.It is impossible for those equipment to be used to detect the great depth in field tests.To solve the problems above,we designed the linear slow switch-off transmitting system.The design is committed as follows:1.By selecting the switching elements with great power,we design the transmitting system to fulfill the demand of 60 A transmitting current based on the traditional inverting bridge structure.At the same time,we design oscillation suppression circuit,high efficiency drive circuit and BD/GPS synchronous circuit.In this way,basic functions of the transmitting circuit with great power are fulfilled.The oscillation current of the transmitting circuit is suppressed.The quality of the transmitting current is optimized.2.We studied the method of optimizing the linearity of the transmitting current.By adjusting the voltage of the voltage maintaining circuit,we make it possible for the transmitting current to switch off linearly and optimizing the linearity of the transmitting current.According to the drawback of the multi-C serial structure,we designed the voltage-sharing protecting circuit.In this way,the safety coefficient of the linearity-optimizing circuit is greatly promoted.Many simulation tests ate committed using Multisim software,which lead us to the right direction of upgrading the circuit.By building the linearity-optimizing circuit and recording the debugging result of the test,we make it clear that the scheme of the linearity-optimizing circuit is feasible.3.By designing the slow switch-off circuit,we prolong the switch-off period.In detail,we design a new type of continuous current circuit.It may prolong the switch-off period of transmitting current without affecting the rising time.Lots of experiments are committed through Multisim platform and actual circuit structure.The result of the experiments shows that the slow switch-off circuit is able to prolong the switch-off period without affecting the rising time.When the current is 20 A,the switch-off period can be prolonged from 488?s to 2.21 ms.However,the linearity is only 59.1%.4.We adapt the linearity optimizing circuit to lift the linearity of the slow switch-off circuit.After building the structure of the circuit,the result of the experiment shows that linear slow switch-off circuit can not only optimize the linearity,but also prolong the switch-off time.Many experiments are committed with different transmitting coils at different current level.5.According to the result of the primary field calculation,the analysis of the frequency spectrum and the result of the SQUID-TEM field test,we know that the linear slow switch-off system can decrease the induced voltage of the primary field to 50% of the formal value.The shape of the waveform resembles to square waveform.In the frequency spectrum of linear slow switch-off system,the value of low frequency part is more than the formal value.The off-time of the center magnetic field is 1.56 ms.Compared with the formal off-time 460?s,the time extension is 1.1ms.Losing lock phenomenon appears no more.The results of the experiments indicate that the linear slow switch-off system can optimize the current waveform and lengthen the depth of detecting field.The SQUID-TEM system can function steadily.