Design and preliminary validation of a tactile feel-through display for virtual curvature rendering exploiting haptic invariants

The tactile Augmented Reality (t-AR) paradigm allows the delivery of controlled skin stimuli to elicit perceptual responses related to virtual haptic properties, or to manipulate without blocking the natural perception of real items. It could foster human-robot interaction, both in collaborative robotics and telerobotics, in every-day life applications. Tactile displays for t-AR employ mechanically transparent, feel-through user interfaces. In this work, we present a tactile display that can elicit the perception of concave curved surfaces, exploiting a soft and highly underactuated fabric interface, together with tendon-driven actuation. Taking inspiration from soft continuous robotics, to control the shape of the device soft continuous interface we designed a Finite Element simulator, and used it to determine optimal control sequences to produce the sensation of the desired curvature. To this aim, we heavily leveraged the theory of haptic invariants, focusing on the growth of the contact area on the fingerpad as a cue for curvature perception. We validated the system with human experiments, by asking participants to match the virtual rendered curvatures with that of real objects. Results, although preliminary, are promising, with the worst-case accuracy well above the chance level, suggesting that the system could represent a viable solution to elicit the perception of different object curvatures. Our work represents first attempt at designing t-AR systems for curvature display, laying down the foundations of a new framework for t-AR system design that exploits haptic invariant theory and integrates control techniques inspired by soft robotics.