Research Of Gravity Gradiometer Protype Based On Flexure-strip Suspension

Gravity gradiometers are widely used in the fields of inertial navigation,gravity field recovery,resource geological exploration and so on.Gravity gradiometers are divided into two main categories from the measurement principle:one is based on the differential accelerometer measurement mode,and the other is based on the torque balance measurement mode.The former has a fast response but the complexity of the entire system is high.The output is the differential result of two accelerometers in the accelerometer pairs,which needs high requirements on the dynamic range of the accelerometer and the common mode rejection of the accelerometer pairs.The latter directly obtains the differential result of the torque,so there is no problem of reading the two outputs and then performing the difference.However,the response time of the system is too long to satisfy the application requirements of rapid measurement of many measurement points in field resource exploration.Addressing the problem that the response time of traditional gravity gradiometer based on torque measurement mode is too long,we have proposed a torque type gravity gradiometer based on flexure-strip suspension.This gradiometer can well satisfy the requirements of field gravity gradient measurement.In order to achieve the hige sensitivity and the quick response time of the system at the same time,although mechanical sensitivity of torsion pendulum is appropriate sacrificed,the whole sensitivity can be compensated by highly sensitive measurement and control circuits.In recent years,with the rapid development of the electronic technology,the sensitivity of the circuit measurement is getting higher,so the high mechanical sensitivity of the traditional twisted wire is no longer necessary,and the loss of mechanical sensitivity can be compensated by the circuit measurement.This presents a historical opportunity to compromise the contradiction between sensitivity and response time.For the goal of resource exploration,the magnitude of the abnormal value of gravity gradient caused by underground resources is calculated.The detection range and resolution target of the instrument are determined.The fundamental principle of the torque type gravity gradiometer is introduced and the main noises of the system is given.In order to satisfy the resolution requirements of resource exploration,the internal noise of the system and the sesimic noise are modeled and analyzed based on the gradient measurement scheme.The effects of mechanical sensitivity and torsional pendulum assembly accuracy on internal noise and sesimic noise noise are given,which provide theoretical basis for system parameter design.For the requirements of gravity gradient measurement,the symmetry requirements of torsion pendulum and electrodes are analyzed,and the mechanical and circuit parameters are designed.The influences of mechanical thermal noise and capacitive transducer noise are reduced by optimizing the stiffness of the flesure-strip.The influence of sesimic noise is reduced less than the internal noise of the instrument by putting forward requirements of assembly accuracy,and optimizing the distance between centroid and rotation point.On the basis of typical design parameters,the modal simulation and the closed-loop simulation are performed on the system to verify the feasibility of the design parameters.In order to verify the functions and performance of the gravity gradiometer,the processing and testing of the unit are completed,and the prototype is built.The mechanical thermal noise,the capacitive transducer noise,and the readout noise are tested respectively.The sensitivity coefficient of the actuator is calibrated,and the relationship between feedback voltage and input torque is given.The closed-loop control is realized,and the control results show that the stabilization time is 20 s,which is significantly improved compared with the stability time of the E(?)tv(?)s torsion with the scale of 40 min.The noise floor test shows that the resolution of each unit satisfies the resolution target of2E/Hz^1/2 at 0.1Hz,and the whole perfomance level is 2?20E/Hz^1/2 at 0.1Hz,which is mainly limited by the seismic noise.The variation of the gradient component W_xx-W_zzinduced by moving a Pb brick is measured,and the possible error sources are analyzed.The measurement results are consistent with the theoretical value within the limit of error.This type of gravity gradiometer can be furtherly improved due to its high potential resolution and simple principle,which provides a feasible implementation approach for the full tensor measurement of gravity gradient in the future.