An STM Driving Waveform Design Method Based on Active Damping for Ultrasound Mid-Air Haptics

Ultrasound mid-air haptics use an ultrasonic phased array to manipulate the ultrasonic waves’ phase, enabling the creation of foci at the desired location in space. Smooth and continuous tactile shapes can be rendered on the human hand using focal spatiotemporal modulation (STM), which requires a high spatial sampling rate and stable acoustic pressure maintenance during the switching process of the focal coordinates. However, this stability is disrupted by STM focus movement, which results in tailing oscillations at the previous focus position overlapping with the initial excitation sound pressure at the next focus position. To address this problem, this study proposes an active damping-based strategy to suppress the effect of tailing oscillations on the STM focusing performance. Also, using a genetic algorithm for parameter optimization, this study develops a mathematical model of the piezoelectric transducer (PT) to design driving waveforms with active damping. Further, the optimal number and duty cycle of inverted damping pulses are determined and used after the excitation waveform. Finally, active damping-based STM performance tests are conducted. The results demonstrate that in the STM without active damping, the tailing oscillations have a strong inhibitory or promoting effect on the oscillation start-up of the subsequent focus as the focal moving distance decreases. The active damping strategy can improve the focal temporal resolution and restore the focusing performance to a single amplitude modulation (AM) focal point focusing level. The proposed method can accurately reproduce the sound pressure amplitude of a moving focus and improve the consistency of tactile perception amplitude in the air.