In this paper the effect of force control in the fidelity of a low-force five degree-of-freedom (dof) haptic mechanism is investigated. Strict conditions that guarantee stability are presented. Our effort focuses on haptic devices, able to reproduce accurate low forces in a soft virtual environment, rather than large forces in stiff virtual environments. Open and closed loop controllers are applied to a haptic mechanism, which is a part of a training medical simulator for urological operations, and consists of a two-dof, five bar linkage and a three-dof spherical joint. The force control algorithm is described and discussed. Open and closed loop schemes are compared. Simulation and experimental results of the force control law applied to the five-dof mechanism are presented and discussed. It is shown that the use of closed loop force control law increases the haptic device fidelity and the realism of the simulation. It is also shown that in case of haptic devices used in soft virtual environments, issues like signal noise and device stiffness are more critical than maintaining system stability.
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