Slack strategy for cable driven haptic interfaces

Common haptic devices induce interaction with users by transmitting direct forces, corresponding to a coupled environment. Paradoxically, because of their intrinsic mechanical limitations they struggle in cases where this last requires null forces. Cable mechanism are investigated to improve this situation thanks to their low structural inertia. However, current control approaches on these interfaces implies that all cables are always under tension. Consequently, the user is permanently linked with the mechanism and is therefore subject to friction and rotor inertia from actuators. He/She constantly feels these parasites forces even during free exploration. As an alternative, we propose to physically disconnect the user from actuators by selectively slacking some cables when no forces are required. The contact is rendered by ensuring that slack runs out right at this moment. A pilot experiment tests the feasibility of this new approach. Benefits and drawbacks on perception are explored as well as consequences on movement. Beyond the expected cancellation of vibrations and perceived inertia during the free motion, results show a large improvement on velocity performances. They also warn about a funnel effect on contact due to shortcomings of the prediction of the point of collision.