Ultraloop: Active Lateral Force Feedback Using Resonant Traveling Waves

The sensation of touching virtual texture and shape can be provided to a touchscreen user by varying the friction force. Despite the saliency of the sensation, this modulated frictional force is purely passive and strictly opposes finger movement. Therefore, it is only possible to create forces along the direction of movement and this technology cannot stimulate a static fingertip or provide forces that are orthogonal to the direction of movement. The lack of orthogonal force limits the guidance to a target in an arbitrary direction and there is a need for active lateral forces to give directional cues to the fingertip. Here, we introduce a surface haptic interface that uses ultrasonic traveling waves to create an active lateral force on bare fingertips. The device is built around a ring shape cavity where two degenerate resonant modes around 40 kHz are excited with 90$<^>{\circ }$ phase shift. The interface provides active forces up to 0.3 N to a static bare finger uniformly over a 140x30 mm$<^>{2}$ surface. We report the model and design of the acoustic cavity, force measurements, and an application to create a key-click sensation. This work demonstrates a promising method for uniformly producing large lateral forces on a touch surface.