Research On Key Technologies Of A Torsional Gravity Gradiometer Based On Electrostatic Suspension

Gravity gradient measurements has valuable applications in many fields such as geodetic survey,geological exploration,gravity aided inertial navigation and so on.To research high precision gravity gradient measurement technology,a torsional gravity gradiometer scheme based on electrostastic suspension has been designed.In the apparatus,a horizontal torsion balance,turning around a horizontal axix,is realized by electrostastic suspension.With the horizontal torsion balance,the torque caused by the vertical gravity gradient can be detected.Further more,all components of the gravity gradient tensor can be determined by a set of torque equilibrium equations which are obtained when adjusting the position of the torsion balance.Circuits for electrostatic suspension are designed,including the differential capacitance detection circuit and the high voltage amplifier circuit.The sensitivity of the differential capacitance detection circuit is 324.6 m V/p F,with a-3d B badwidth up to 26.8 k Hz.The capacitance resolution is 0.47 f F,corresponding to a displacement resolution of 1.6 nm for the electrodes in the experimental system.The high voltage amplifier circuit has an output range from 0 to 850 V.Its DC amplification gain is about 100.When the peak to peak value of its AC output is 30 V,the distortion frequency is 1.9 k Hz.The bandwidth meets the need of the electrostatic suspension system.To study the basic characteristics of horizontal torsion balances,a torsion balance system is realized by electrostatic suspension based on parallel plate electrodes.By adjusting the control voltage on eigth support electrodes,the torsion balance is controlled by feedback electrostatic forces in the z ?x? ?y? degrees of freedom.The suspension can carry 10 g in weight.Around the zero position,the motions in the three degrees decouple with each other and their closed-loop frequency characteristics are close to the ones of second-order systems.According to experiematal tests,the closed-loop torque detection sensitivity of the horizontal torsion balance is about 1.30×105 V/Nm.The restriction factors for closed-loop torque detection in the plate electrode structure are analyzed and the coupling problem between the suspension support and the torsion detection in the horizontal torsion balance is discussed.To overcome the coupling problem between the suspension support and the torsion detection in the horizontal torsion balance,a solution based on cylinder electrodes is designed.The capacitance and electrostatic force of the cylinder electrodes are analyzed and the electrostatic suspension feedback control model is established.The gravity gradient resolution of the system depends on the torque detection sensitivity of the torsion control electrodes.The high sensitivity closed-loop torque detection is experimentally verified with a vertical torsion balance and the torque sensitivity can reach 6.97×109 V/Nm.With this torque sensitivity,the system can achieve a solution of 1.9 E for gravity gradient measurement when the signal sensitivity reaches a level of 0.1 m V.It is hopeful to realize high precision gravity gradient measurements with the torsional gravity gradiometer scheme based on cylindrical electrode electrostastic suspension.