This paper discusses two improvements to localized tactile feedback on a transparent surface using Time Reversal Mirror (TRM) of ultrasonic guided waves. A mock-up device comprising a glass plate of 150 × 200 × 1 mm3 with a 12-element sparse piezoelectric actuator array bonded at the boundary of the plate is constructed together with driving electronics. The transfer function from each actuator to an arbitrary focus point is first recorded using a laser doppler vibrometer. The re-emission waveforms are then generated by convolving the time-reversed transfer function with a target signal to control the shape of the vibration at the focus point. The re-emitted waveforms are compressed to binary signals to improve the peak displacement of the focused wave field. The mock-up device obtains a peak amplitude of 19 μm on the plate with a peak-to-peak driving voltage of 150 V. Two improvements to the tactile feedback using ultrasonic TRM are described. First, the undesired conversion of ultrasonic vibration to audible spectra resulting from the coupling of the fingertip and the plate surface is investigated. The authors show that despite the inevitable frequency shift, the tone of the audible leakage can be controlled by the repetition period of the TRM. Second, the tactile feeling of a single-point focus of ultrasonic guided wave is acute and hard to detect due to its small resolution cell compared to the size of a fingertip. The authors propose a dual-point focus scheme, the distance of which is less than that of two-points discrimination, to enlarge the perceivable region without changing the spectra of the re-emission signals.
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