Research On Spherical Capacitive Displacement Sensing Technology

The changes in surface deformation and mass migration that occur during the conception and occurrence of earthquakes can be reflected by the gravity field,and superconducting gravimeters play an important role in earthquake monitoring as excellent gravimetric instruments.As a gravimetric instrument with perfect application of superconductivity principle,superconducting gravimeter uses superconducting suspension system instead of mechanical spring in traditional spring-type gravimeter,which makes its working stability far more than other types of instruments.It is also due to this advantage that superconducting gravimeter is widely used in solid tide monitoring,hydrology and water quality crustal deformation.The aim of this paper is to analyse the spherical capacitance sensor in a superconducting gravimeter to understand the effect of its structural parameters on the spherical capacitance.A sigma-delta modulator circuit is designed for the detection of spherical capacitance signals to bridge the gap in research on spherical capacitance sensors in superconducting gravimeters in China.The spherical capacitance sensor system in the superconducting gravimeter is analysed,and the principle of the spherical capacitance sensor for gravity measurement is introduced.Based on the spherical capacitance sensor in the GWR superconducting gravimeter,a model of the spherical capacitance sensor analysed in this paper is established and the specific structural parameters of the spherical capacitance sensor are given.The theoretical calculation of the spherical capacitance sensor analysed in this paper is carried out in a spherical coordinate system.Based on its special curved orthogonal structure,a mathematical approach using a combination of differential and Taylor methods is proposed for the calculation of spherical capacitance.Neglecting the effect of edge effects,formulae are derived for the calculation of individual spherical capacitance as well as for differential spherical capacitance.The variation in linearity and sensitivity of the differential spherical capacitance is then analysed from different aspects.A fourth-order mash structure sigma delta modulator for differential spherical capacitance signal detection is designed.The oversampling and noise shaping techniques of the modulator are introduced.The maximum signal-to-noise ratio of the modulator with different orders,quantizer bits and oversampling rate is deduced.The ideal sigma delta modulator is simulated.Considering the influence of integrator overload,the modulator coefficients are scaled and optimized.The non ideal factors in the design of time circuit are analyzed,and the modulator under non ideal conditions is simulated and analyzed.The signal-to-noise ratio is 127.5d B and the effective bits are 20.89 bit.