KnuckleGuide: Mid-Air Haptic Guidance System Targeting Dorsal Hand Using Airborne Ultrasound

Ultrasound-based mid-air haptic guidance has gained significant attention as a contact-free solution for conveying direction information at a minimal distraction with high flexibility and robust environmental adaptability, particularly valuable when other modalities are limited or unavailable. Current research predominantly targets the palmar side of the hand, which becomes inaccessible for perceiving ultrasound-based haptic cues during critical interaction moments such as surface exploration or object gripping. This paper presents KnuckleGuide, a mid-air ultrasonic haptic navigation system focused at the dorsum hand while leaving the palmar side free for physical exploration. We propose two ultrasonic guidance strategies: a four-direction strategy (left, right, up, down) and an eight-direction strategy (including diagonals) and evaluate them through a user study with 16 participants performing surface navigation tasks. The effectiveness of the system was validated in three experiments, underscoring the feasibility of dorsum-targeted mid-air haptics for guiding surface exploration, offering new possibilities in assistive technologies and eye-free navigation.