Design And Application Of Very Low Frequency Piezoelectric Sensor With Ultra-low Noise

There are many low-frequency vibration phenomena in nature,such as earthquake on land,debris flow and landslide.In addition to the wind and waves in the ocean and so on caused by middle and high frequency sound waves,there are also low frequency or very low frequency sound waves caused by tide,ocean waves and so on,fluctuation frequency can reach below 0.1Hz.These fluctuations contain a wealth of ocean information,and the detection of these low frequency or very low frequency sound waves is of great help to the development of Marine resources and Marine experiments.However,ordinary piezoelectric sensors have some defects such as low sensitivity and signal-to-noise ratio when measuring VLF signals.In order to detect a kind of very low frequency signals in tidal phenomena,a low noise very low frequency piezoelectric sensor is designed and developed.It has not only good low frequency response characteristics but also low low frequency noise,and can measure vibration signal as low as 0.01 Hz.(1)The amplifying circuit of low noise very low frequency piezoelectric sensor is designed.According to the selected impedance characteristics of the piezoelectric element and the lower cut-off frequency of the sensor,a preamplifier circuit based on discrete JFET amplifier is designed.Based on the classical En-In noise model,the amplifier noise is analyzed and calculated.The influence of the main noise sources and piezoelectric elements on the amplifier noise is analyzed in detail.The key components in the circuit are selected.The theoretical performance of amplifying circuit is analyzed by using circuit simulation software and noise calculation results.(2)The amplifying circuit and very low frequency piezoelectric sensor are tested.After testing,the working frequency range of the amplifier circuit is 0.008Hz～100Hz.The measured noise and theoretical noise of the amplifier circuit shorting and equivalent capacitance are compared,then it is concluded that piezoelectric ceramic elements can increase the noise of voltage amplifier in low frequency band.The working frequency range of the piezoelectric sensor is 0.01Hz～10Hz,the sensitivity reached-168db@0.1Hz,and the spectrum level of equivalent noise is 150 d B@0.1Hz,which are measured by hydrostatic pressure method in the anechoic tank.In the frequency range below 10 Hz,compare the noise spectrum between the piezoelectric sensor and the low-frequency Marine environmental noise,and the results show that the noise of piezoelectric sensor is lower than that of Marine environment in the same frequency band.(3)The very low frequency piezoelectric sensor was used to measure the instantaneous tidal bore in the Qiantang River.The time-frequency analysis results show that most of the energy distribution of the Qianjiang tides is below 10 Hz,and the energy change in the VLF portion can be measured approximately 3 minutes before the arrival of the tidal bore by the low-frequency characteristics of the sensor.The tide level measurement time difference is within 30 s,and the sensor can realize dynamic measurement of similar low frequency fluctuation in Qianjiang tidal zone.The tidal level and instantaneous flow were measured dynamically,and the measured results were in agreement with the predicted trend of the Qianjiang tidal.