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In this paper, a dynamic analytical model to investigate a mass sensing MEMS device using electrostatic actuation and mode localization is developed. The sensor is composed of two cantilevers with different lengths, connected together with a weak mechanical spring. Such systems lead the vibration modes to be localized when a perturbation is introduced, a phenomenon that can be used to raise the sensitivity of the sensor by orders of magnitude, compared to mass detection based on frequency shift. The major problem raised is that the system has to be perfectly symmetric before the mass is added, which is usually not the case because of the manufacturing tolerances. Here we start with a system initially and intentionally unbalanced (a short beam and a long beam). A DC voltage is then applied on the short beam to dynamically rebalance the system, because the DC voltage leads to spring softening effect. The dynamic behavior of such sensor can be modeled by a two degrees of freedom mass-spring system. An electrostatic force is applied on the degree of freedom corresponding to the short beam. This electrostatic force implies a bias voltage to balance the system and an AC voltage to excite the system. We developed the electrostatic force by using a third order Taylor series expansion on the static displacement to take into account the influence of the static displacement on the dynamic behavior of the system. The static displacement is calculated using a modal projection method with one mode projected on the Euler-Bernoulli equation for a cantilever. The eigenfrequencies and eigenmodes of the system can then be computed as a function of the perturbation and the results are compared to the FEM simulations using Comsol Multiphysics.