Design And Implementation Of Emission Current Measurement System Of Magnetic Source Electromagnetic Method Transmitter

At present,in the field of emission current measurement of magnetic source airbone electromagnetic method transmitter,because of its large output current,and the modulation mode is PWM,so there will be interference in the rising section and flat top section of trapezoidal wave.Therefore,how to obtain the transmitted waveform accurately is one of the most urgent problems to be solved.The purpose of this paper is to obtain the output current waveform of high power transmitter,the output current measurement system of high power transmitter has been designed.And the current shunt voltage sampler,data acquisition system and the denoising method of PWM noise have been studied.The design of high current shunt sampler is mainly making a current shunt resistance which could obtain the high power transmitter output current accurately,the order of magnitude of the inductance of the resistance must be nH and the load impedance must be much less than the transmitter.The structure of the coaxial sleeve type shunt sampler has been adopted,the structure has the advantages of small resistance,small inductance and strong anti-interference ability.The finite element method is used to model the coaxial sleeve resistance,the inductance of the structure is calculated theoretically.The inductor of different sizes of the resistance has been calculated by using electromagnetic field finite element simulation software Ansoft Maxwell.A set of data acquisition system is designed by using the graphical programming software LabVIEW.The system can display and collect the voltage data of the sampling shunt in real time by using the NI-USB4432 acquisition card.It is of great significance to denoise the PWM noise for obtaining the final transmitter waveform.In this paper,the lifting wavelet transform is used to reduce the amount of calculation,at the same time,it can also make the denoising process in the time domain to reduce distortion.Threshold selection method based on Stein unbiased likelihood estimation is used to obtain threshold value.The method can compensate the shortcomings of the standard lifting wavelet transform.Accordingly,it can not only reduce the distortion,but also to improve the effect of denoising purposes in the full frequency range.This paper measures the inductive impedance of the shunt sample by using the LCR meter.The measurement results show that the resistance of the sampling resistor is 1.5 m? and the inductance is 45.64 nH,fully meet the measurement requirements.An actual test experiment has been carried out by using the shunt to measure output current of high power transmitter.The test results show that,compared with the straight type sampling resistor structure,distortion is improved by one order of magnitude.The Matlab algorithm is used to design the denoising algorithm mentioned above,the PWM noise of the rising segment and the flat top section is basically filtered out with low distortion,meets the project requirements.