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NIM's Magnetic Energy Balance (ME balance) relates the Planck constant to the redefinition of the kilogram using the equivalence between mechanical and electrical energies. According to the principle of ME balance, the most important parameter is the displacement of the moving coil in the uniform and radial magnetic field. This paper analyzes the requirement of uncertainty of the displacement measurement based on ME balance equation, and elaborates the details of the displacement measurement apparatus for the moving coil including three-laser heterodyne interferometer. Preliminary experimental results are also given in this paper.