The VLTI system foresees two generations of fringe sensor: FINITO and PRIMA FSUs. The former is dedicated to H band; it controls the internal OPD with a temporal modulation with an external reference OPD. The latter, working with the ABCD model and in K band, is based on the introduction of known phase offsets for the interferometric signal (spatial phase modulation) and on the measurement of the corresponding combined power. Simulation models for both FSUs are developed with Matlab. Instrumental parameters, i.e. phase, transmission, visibility, are tabulated for ease of maintenance and to speed execution time. For the use of siderostats, due to fast turbulence, the need for intensity calibration arises. Assuming slow intensity variations with respect to phase variations, different algorithms can apply, yielding to numerical control of perturbations as a function of model parameters.