Study On The Measurement Of Common Mode Rejection Ratio And Suppression Of Common-mode Noise For Superconducting Gravity Gradiometer

Superconducting gravty gradiometer（SGG）has been considered as the most potential substitute with better resolution for the rotating accelerometer gravity gradiometer（RAGG）in the field of airborne exploration.Though SGG presents much lower noise level than RAGG under laboratory quite condition,it has not been able to challenge RAGG in airborne measurements.It is vibration noise of the aircraft that couples into the output signal and seriously degrade the instrument resolution.Therefore,suppressing the vibration-introduced noise is of key importance to push the SGG to airborne applications.TheΓ_zz-type SGG measures the vertical in-line component of gravity gradient tensor.Through various types of mismatch,the random vibration in vertical direction of the airborne platform will couple into the output signal as so-called SGG common-mode noise,while the vibrations at other degrees of freedom will result in the cross-coupled noise.This dissertation focuses on the suppression of SGG common-mode noise.It starts with the development of a vertical vibrator as an inset of the liquid-helium Dewar,with which the common-mode rejection ratio（CMRR）of the SGG canbe precisely measured.Based on the vibrator,a method to improve the SGG’s CMRR has been investigated.For this method,the superconducting levitating currents are finely adjusted to compensate the SGG mismatches.The common-mode noise and the cross-coupled noise are mixed and coexist in the SGG output signal.However,they enter the SGG through different path and the methods to suppress them are consequently differ from each other.Therefore,as a necessary supporting condition for any endeavor to suppress the common-mode noise,the SGG’s CMRR should be precisely measured independence of the cross-coupled noise.To do it,a vertical vibrator is needed to provide the SGG sensor with a vertical acceleration modulation.Such a vibrator has been first developed in this work.The working principle of the vibrator is governed by the forced vibration of spring-mass oscillator.The vibrator is built based on the liquid-helium Dewar.The SGG sensor acts the mass block of the oscillator,and the oscillator is drived vertically by a voice-coil actuator working at room temperature,so that the SGG sensor can obtain a vertical alternating acceleration.Since the CMRR measurement precision strongly depends on the amplitude of the accompanying motion at other degrees of freedom,a number of measures have been adopted to suppress the accompancying motion.Firstly,one spiral spring and two spirally slotted flat flexures are utilized to construct the oscillator,aiming to increase the oscillator’s stiffness except that at the degree of freedom of vertical linear motion.Secondly,the oscillator consisits of a number of precision adjustment mechanisms,with which the accompanying driving force or torque can be minimized for the unwanted degrees of freedom.To reduce the waveform distortion of the agitated acceleration,the driving current including the high-order harmonics is synthesized according to calculations for the voice-coil actuator.The performance of the oscillator has been tested.It can provide the SGG sensor with an alternating common-mode acceleration,whose peak-to-peak amplitude is up to 0.1 m/s².The the waveform distortion of moduation acceleration is measured to be 1.2%.The amplitude ratio of the accompanying horizontal vibration to the agitated vertical vibration is as low as 0.42%@1Hz,and the ratio of the accompanying angle vibration to the agitated vertical vibration is 3.7×10^-3rad/m@1Hz.With these performances,it is estimated for a typical SGG that the CMRR can measured up to 1×10⁵.Making use of the developed vibrator,aΓ_zz-SGG sensor was integrated with Dewar and its initial CMRR has been measured to be 7.8×10³.With the measured CMRR as criterion,a systemic investigation has been carried to suppress the common-mode noise.In this work,the superconducting levitating current is elaborately adjusted to counter the effects introduced by mismatches.Finally,the matching degree of both the stiffness of the superconducting spring and the scale factor of displacement detection has been significantly improved,and the SGG CMRR increases to 1.6×10⁵.