Tactile Stimulation by Repetitive Lateral Movement of Midair Ultrasound Focus

Abstract

We report a new vibrotactile modulation method of midair ultrasound focus, namely, lateral modulation (LM), in which the focus quickly moves along a small cyclic trajectory and provides stronger and clearer vibrotactile stimuli than those by the conventional amplitude modulation (AM) method. Midair ultrasound haptics has an essential technical advantage of offering remote, non-contact, and pinpoint tactile stimuli on device-free bare skin. On the other hand, lack of clarity in the presented vibrotactile sensation has often been pointed out, and until now, an AM focus has been valid only on glabrous skin. Our main scientific contribution of the article is to verify the LM method, with the following experimental findings newly obtained. We confirmed that with the same maximum output amplitude of the ultrasound phased arrays, LM stimuli with circular focal trajectories were sensed stronger than AM stimuli by glabrous skin and hairy skin in a modulation frequency of 10-200 Hz. We also found that the detection threshold in glabrous skin mainly depended on the focal speed, whereas the tendency in hairy skin was different from that. With these results, we discuss a basis of perceptional mechanism that responds to LM stimuli, along with practical aspects of potential applications.

Discriminating Between Intensities and Velocities of Mid-Air Haptic Patterns

“…Using our estimated Weber fraction, one could decrease the reference MAH velocity by 10 levels and still remain in this range. Similarly, previous studies reported a perceptual threshold for MAH intensities around 30% of the maximum output pressure [10,20]. Using our estimated Weber fraction, one could decrease the reference intensity by 9 levels and still remain above this threshold.…”

Comparing Lateral Modulation and Amplitude Modulation in Phantom Sensation

“…1-D). LM at 20 Hz is more strongly perceived than AM at the same frequency [6]. Moreover, at 20 Hz, the sensitivity of RAII is lower than that of RAI and SAI [3].…”

“…The stimulus area of LM was larger than that of AM because of the movement of the stimulus points. The perceived intensities of LM and AM differ for the same driving power [6]. In this experiment, we did not equalize the perceived intensities but maximized it in both cases to make the participants clearly perceive the stimuli.…”

“…Lateral modulation: LM is the ultrasound stimulus method that moves an ultrasound focus periodically and laterally several millimeters while maintaining the intensity constant [6] (Fig. 1-D).…”

“…In this study, we examined the conditions to elicit PhS using ultrasound radiation pressure that can control the stimulus quantitatively with high reproducibility and controllability. In particular, we compared PhS elicited by lateral modulation (LM) [6] and amplitude modulation (AM) [4] presented by ultrasound. LM is the stimulus in which an ultrasound focus is moved periodically and laterally by several millimeters.…”