Design And Research Of Emission Acquisition Unit For Mid And Low Frequency Seabed In-situ Acoustic Measurement System

Seabed sediment acoustic characteristics of the related research at the bottom of the ocean resources exploration and development,the military Marine environment protection,underwater acoustic communications,submarine engineering investigation,underwater target detection,etc all have important significance.In situ acoustic measurement,as a research method to directly reflect the real acoustic characteristics of the in-situ environment of seabed sediments,has achieved rapid development in China in recent years.The acoustic characteristics detection ability in high frequency band has reached the level of foreign countries,but there is still a lack of acoustic characteristics measurement equipment in low and middle frequency band（1-10 k Hz）in China.Acoustic emission acquisition is one of the key technologies of seabed in-situ acoustic measurement equipment,which directly determines the accuracy of acoustic measurement results and frequency detection range.Based on this,the design and development of low and medium frequency acoustic emission acquisition system,the main work is as follows:1.Design the overall architecture of acoustic emission acquisition system.According to the application scenario of the measuring device,the structure of the low-frequency acoustic emission acquisition system in seabed sediments was determined,including FPGA main control core board,peripheral baseboard and transmitting and receiving module,and the performance and technical indicators of the acoustic emission acquisition system were drawn up.In addition,the structure of the mid and low frequency acoustic transducer connected with the acoustic emission acquisition system is described.2.Develop the hardware circuit design of the medium and low frequency acoustic emission acquisition system.Main control chip selection,power supply circuit,SDRAM cache circuit,AD analog-to-digital conversion circuit,DA analog-to-digital conversion circuit,network port communication circuit,SD card storage circuit and transmitting and receiving module design and debugging.3.Develop acoustic emission acquisition system software.The software is divided into upper computer and lower computer.The upper computer software includes serial port and network port communication program,acoustic emission collection parameter setting program,waveform display program,save or open file program,etc.The FPGA master control system software of the lower computer includes transmitting specified waveform signal program,collecting signal program,caching and storing data program,communication program,etc.4.Conduct performance tests of low and medium frequency acoustic emission acquisition system and low and medium frequency acoustic transducer.The simulation results show that the amplitude of DA transmitting analog signal is ±4V,and the range of AD receiving analog signal is ±1V,which is consistent with design expectation.The background noise intensity of the two AD acquisition channels is 1.83% and 2.37%,respectively.The signal consistency of the two AD acquisition channels is good,and the amplitude consistency error is less than 0.939%.5.Carry out experiments to verify the effectiveness of the medium and low frequency acoustic emission acquisition system.The experiment is divided into oscilloscope emission frequency test,acoustic characteristics test based on anechoic pool,in-situ acoustic characteristics test based on the ocean.The maximum absolute error of sinusoidal pulse with excitation frequency of 1-10 k Hz is 0.05 Hz.The comparison between the acoustic characteristics of the anechoic pool and the calibrated sound velocity in the water shows that the standard deviation of the measured sound velocity in the 2-10 k Hz band is 3.23m/s,and the relative standard error is within 0.3%.The average measured sound velocity value of sandy sediment in 1-3k Hz band is 1651.78m/s,which is within the normal sound velocity range of sandy sediment.