Research On Distance Measurement Of Falling Body’s Optical Center And Mass Center For Absolute Gravimeter

The falling body is the key component of the free-fall optical interference absolute gravimeter,which has great impact on the measurement accuracy.With the development of the absolute gravimeter,various measurement methods for the distance between the optical center and the center of mass in falling body have emerged.The torsional pendulum method is the most accurate measurement method,and the distance between the optical center and the center of mass is calculated by the displacement and angle generated by the falling body’s torsion.Therefore,the measurement scheme is improved on the basis of the torsional pendulum method.And the four-way differential laser interferometer is designed to increase the displacement measurement accuracy.A new processing algorithm for the displacement and angle signal is proposed,so that the measurement accuracy of the entire device is improved.The main work content of the paper is as follows:(1)The monofilament single pendant motion model is established according to the torsional pendulum method and the motion modalities of falling body are analyzed.Then we get the frequency of each motion mode and derive the projection equation in the z direction during the torsion.The single-frequency laser interferometer and the photoelectric autocollimator are used as the measurement scheme of displacement and angular,respectively.The torsion wire is produced and fixed with the falling body.The initial excitation is applied by rotating guidance to reverse the falling body.The displacement and angular data generated by the simulation of the imitation fall torsion,and the calculation of the distance between the optical and the center of mass is achieved by fitting and constructing algorithm to obtain simulation results.(2)The optical path of the four-channel differential laser interferometer is analyzed,and the phase error,unequal error,and DC error in the measurement process are analyzed.The error compensation scheme and high precision segmentation algorithm are finally determined.The laser interferometer is subjected and a high-precision piezoelectric displacement station is used to test the interferometer,which indicates that the resolution at 0.3 to 0.5 Hz is better than 0.1nm.(3)In the Lab VIEW environment,the write and debugging of the displacement data acquisition program is completed,and the angle acquisition program is introduced.We fit the angle signal and extract the initial torsion angle,damping coefficient,and angular frequencies.The CEEMD algorithm is selected to make a baseline drift correction pre-processing,and finally the torsional single frequency item in the aquary signal is combined by the genetic algorithm.(4)The complete measurement device for distance between the optical center and the mass of center is built,and distances along the x and z direction between the optical center and the center of mass are carried out in 10 sets of repetitive measurements.We calculate their A-class uncertainties.Then we verify the scheme of genetic algorithm fitting and excluding the torsional base frequency component.The A-class uncertainty of distance along the z direction decrease from 20.83μm to3.93μm.The rotating counterweight nut is adjusted in the z direction,resulting in 5sets of distance measurement data along the z direction between the optical center and the center of mass,the maximum error is 2.46μm.