In virtual reality and teleoperation scenarios, active constraints can be used to guide a user, prevent him from entering forbidden regions, and assist him in general. On a haptic interface, this implies superimposing the forces generated by the active constraints on top of the forces that are generated by the environment (real or virtual). This creates the problem of distinguishing from which of these two sources does a stimuli come from while perceiving feedback. In this paper, we present an approach that consists in adding a vibrating component on top of the forces generated by the virtual constraints, while the forces generated by the environment are kept untouched. Blind experiments in which users have to navigate through a scenario containing both active constraints and randomly positioned objects show that they manage to perceive more successfully the presence of the unexpected objects with our approach than with previously existing ones. Moreover, the penetration into the constraints is as good as with the classical approach.
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