Study Of Test Masses Balance In Superconducting Gravity Gradiometer By Electropolishing Method

As one of the most effective exploration tools, airborne gravity gradiometer has being developed world wildly. Due to the low intrinsic noise, superconducting gravity gradiometer(SGG) has been considered as the most promise instrument for high-resolution exploration. However, the high level of vibration noise of the aircraft always prevent the airborne SGG from approaching its intrinsic resolution. The actual resolution is dominated by Common-mode rejection(CMR). To obtain a high CMR rate, the parameters of two accelerators of the gradiometer should be precisely matched. Among the parameters, the match of test mass is extremely important, since it directly determine the natural frequency and the transfer function of the accelerators. In this thesis, an effective technology has been developed to match the two niobium test mass for SGG.Based on the working principles of SGG, the requirements for test-mass matching has been first analyzed. It has been found that, to obtain a resolution better than 1E Hz for airborne surveys, the mass should be matched within a difference of 1ppm, and the thickness of the clapboards should be matched to less than 10?m. Additionally, all the surface of the test mass should be polished.By comparing several technologies possible to homogeneously remove mass from the complexly shaped test mass, electropolishing has finally been chosen for mass matching, thickness matching and surface polishing. To optimizing the technical parameters, the influence of electrolyte temperature, cell voltage and the spacing between the electrode and the polished surface have been investigated by measuring the polarization curves. Finally, a set of parameter is determined for test mass matching, i.e.10? for electrolyte temperature, 7V for cell voltage, and 10 mm for the spacing between the electrode.To match the complex-shaped test mass, special hangers have been designed respectively for thickness matching, mass matching and surface polishing. During polishing, the total charge number is precisely controlled by computer so that the precise mass can be removed for the test-mass with.A pair of niobium test-mass have been matched with mass difference less than 0.5ppm and thickness difference less than 2?m. The matched test masses have been successfully used in the SGG.