Perception of Ultrasonic Haptic Feedback on the Hand: Localisation and Apparent Motion
Ultrasonic haptic feedback involves the creation of focused air pressure waves from an array of ultrasound transducers. These are reflected off the skin to create tactile sensations without being in direct contact with an actuator. It is potentially useful for gestural interfaces, such as those that utilise body position, hand movements or finger gestures for input, as these interfaces suffer from a lack of tactile feedback. The technique is relatively new compared to other forms of tactile feedback, such as vibration motors or pin-arrays. Consequently, there has been less controlled and rigorous research into the perception of ultrasonic haptic feedback, which is vital if it is to be used in HCI.
We help to address this by identifying the factors that influence the perception of two fundamental aspects of tactile feedback: localisation and motion across the hand. Research on ultrasonic haptics has tested the detection or differentiation of one or multiple points of feedback, the two-point visual-tactile threshold and presented interaction prototypes with limited user studies. Research is needed on what spatial or temporal parameters influence localisation and perception of motion.
This paper presents two lab experiments. The first tested localisation of static feedback on the hand to determine spatial resolution for ultrasonic haptics. The second tested the perception of motion across two axes on the hand, to identify which characteristics of feedback (distance, duration, number of stimulated positions and movement direction) elicit convincing sensations of motion.
Ultrasonic haptic feedback is a promising means of providing tactile sensations in mid-air without encumbering the user with an actuator. However, controlled and rigorous HCI research is needed to understand the basic characteristics of perception of this new feedback medium, and so how best to utilise ultrasonic haptics in an interface. This paper describes two experiments conducted into two fundamental aspects of ultrasonic haptic perception: 1) localisation of a static point and 2) the perception of motion. Understanding these would provide insight into 1) the spatial resolution of an ultrasonic interface and 2) what forms of feedback give the most convincing illusion of movement. Results show an average localisation error of 8.5mm, with higher error along the longitudinal axis. Convincing sensations of motion were produced when travelling longer distances, using longer stimulus durations and stimulating multiple points along the trajectory. Guidelines for feedback design are given.