Usually, force control algorithms developed for medical applications use a basic elastic model to represent the interaction between the robot and soft tissue. It is known that soft tissues are complex structures with viscoelastic properties as well as nonheterogeneity and hysteresis behavior. This work intends to improve the performance and the robustness of a force control scheme using a more realistic interaction model. The proposed force control is a model based algorithm using a Kelvin-Boltzmann viscoelastic model as the interaction model.