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The adaptive body fitted grid generation method for moving boundaries is applied to the analysis of MEMS-based variable devices involving motion in arbitrary in-plane directions. MEMS technology is growing rapidly in the RF field, and the accurate design of RF MEMS structures that can be used for phase shifting or reconfigurable tuners requires computationally effective modeling of their transient and steady-state behavior, including the accurate analysis of their time-dependent moving boundaries. The proposed new technique is based on the finite-difference time-domain method with an adaptive implementation of grid generation. Employing this transformation, it is possible to apply the grid generation technique to the analysis of geometries with time-changing boundary conditions. A variable capacitor that consists of two metal figures that can move in arbitrary directions of the plane is analyzed as a benchmark. The numerical results that express the relationship between the velocity of the plates and the transient capacitance are shown for oblique in-plane motions and the transient effect is accurately modeled.