PUMAH: Pan-Tilt Ultrasound Mid-Air Haptics for Larger Interaction Workspace in Virtual Reality

Mid-air haptic interfaces are promising tools for providing tactile feedback in Virtual Reality (VR) applications, as they do not require the user to be tethered to, hold, or wear any system or device. Currently, focused ultrasound phased arrays are the most mature solution for providing mid-air haptic feedback. They modulate the phase of an array of ultrasound emitters so as to generate focused points of oscillating high pressure, eliciting vibrotactile sensations when encountering a user's skin. While these arrays feature a reasonably large vertical workspace, they are not capable of displaying stimuli far beyond their horizontal limits, severely limiting their workspace in the lateral dimensions. In this paper, we propose an innovative low-cost solution for enlarging the workspace of focused ultrasound arrays. It features two degrees of freedom, rotating the array around the pan and tilt axes, thereby significantly increasing the usable workspace and enabling multi-directional feedback. Our hardware tests and human subject study in an ecological VR setting show a 14-fold increase in workspace volume, with focal point repositioning speeds over 0.85 m/s while delivering tactile feedback with positional accuracy below 18 mm. Finally, we propose a representative use case to exemplify the potential of our system for VR applications.