Research On Non-sensitive Degree Of Freedom Feedback Control For Superconducting Gravity Gradiometer

Superconducting gravity gradiometer(SGG)is a relative gravity gradient measuring instrument based on superconducting magnetic levitation principle.It has the advantages of low inherent noise and good stability,and is expected to be used in the next generation of high resolution airborne gravity gradiometer.In dynamic environment,the cross-coupling noise introduced by the horizontal line movement of the platform is one of the dominant noises limiting the instrument resolution.Due to the lack of mature and effective suppression means,the noise becomes a technical bottleneck in the practical application of SGG.Cross-coupling noise is mainly introduced by the horizontal line movement of the platform through the deviation of the sensitive axis orientation of the two levitated test masses.Therefore,the sensitive axis alignment of the two test masses must be adjusted.The premise of adjustment is that the orientation deviation is measured under the condition of stable orientation of the sensitive axis.However,due to the insufficient stiffness of the levitated test mass in the non-sensitive degrees of freedom(DOF),its sensitive axis is in a real-time changing state,which will not only introduce crosscoupling noise,but also limit the measurement accuracy of the orientation deviation of the sensitive axis.Therefore,the motion of the test mass along the non-sensitive DOF must be constrained,and for the full maglev test mass,the feedback control technique is the most suitable method.The feedback control can reduce the displacement of the test mass along the non-sensitive DOF,which is equivalent to increasing the horizontal stiffness,so as to suppress the noise introduced into the vertical detection due to the horizontal motion of the test mass.For this reason,the research of non-sensitive DOF feedback control technology of test mass is carried out in this paper,which lays a technical foundation for cross-coupling noise suppression.The motion model of the test mass along the non-sensitive DOF belongs to a 4-DOF coupled motion system.Firstly,the basic dynamic model is established and the coupling effect in the model is analyzed.A control strategy based on decoupling control is proposed,and a superconducting circuit suitable for decoupling is designed on this basis.Combined with three-dimensional electromagnetic field numerical simulation,the dynamic characteristics of non-sensitive DOF for the test mass are analyzed.Then,taking the closed-loop system with one degree of freedom as an example,this paper completed the analysis and design of the transfer function of each link of the control system,and combined with Simulink simulation,established a four-DOF control model including each coupling effects,and analyzed the influence of different ways of coupling on the closed-loop system.On this basis,the coupling effect is investigated experimentally.These include: measuring the effective inductive coupling coefficient and magnetic coupling coefficient between superconducting circuits to provide optimization direction for circuit fabrication;at the same time,a corresponding attitude adjustment method is proposed for the coupling caused by the levitation attitude of the test mass and verified by experiments.Finally,the feasibility of the superconducting decoupling circuit is verified by the modal test of the system.In order to meet the hardware platform required by the control experiment,this paper also designed and built a digital control system based on digital signal processor(DSP)and field programmable gate array(FPGA),completed the hardware test and software algorithm verification,and provided the equipment foundation for the low-temperature experiment.On the other hand,the superconducting quantum interferometer(SQUID),as the core sensor in the closed-loop system,is limited by the complex external electromagnetic environment and has the problem of “lost lock”.Therefore,in terms of electromagnetic compatibility,two filtering systems are designed in this paper to improve the electromagnetic environment of SQUID.Firstly,the signal entering the experimental Dewar is filtered by the design of feed-through filter.Then based on the principle of eddy current loss,a transformer with lowpass filtering property is designed to filter signals entering SQUID.Simulation and experiment show the feasibility of filtering system designFinally,on the basis of the verified the above unit technology,combining with the SGG probe,completed the integration testing of the control system,implement four DOF control effect of the test mass.the results showed that the test mass along the non-sensitive DOF can inhibit more than an order of magnitude of the displacement,can be controlled in the nm level.Then,the two test masses in the SGG are controlled simultaneously,and the suppression performance of cross-coupled noise is preliminarily evaluated.The results show that with the control of non-sensitive DOF,the time-domain noise attenuation of the sensitive axial differential output of the SGG is nearly three times,and the control technique has been effectively applied in the SGG.