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Static and dynamic haptic feedback using time reversal and electrovibration stimulations

Abstract : The current haptic feedback in end user products provides limited tactile interactions and is less efficient than physical keyboards for typing. Most people are used to the simple tactile feedback available in smartphones. However, it is very limited, and can only convey little information: silently signaling a phone call, notifying an incoming message or acknowledging touch inputs when typing on a virtual keyboard. Although advances are made to enrich existing technologies in hand-held devices, such as more efficient actuators with broader ranges of vibrations to emulate buttons or textures, they remain limited to a single point feedback. This prevents any simultaneous multi-user scenario.This work aims to develop static and dynamic haptic feedback on large surfaces (A4 format). Interaction with screen based devices is in need of better and richer haptic feedback. Two types of feedback with complimentary performance are identified as necessary to enrich tactile interactions. Time reversal, as a static feedback technology, is studied to simulate a button press. Electrovibration, as a dynamic feedback, is investigated to simulate tactile textures or to differentiate specific areas of interaction.An analytical model based on Kirchhoff's equation for wave propagation to compute the spatial resolution of time reversal of flexural waves applied to plates is presented. Measurements on a physical system are confronted to the model's prediction. Design guidelines are elaborated and used to develop a new time reversal enabled screen with adapted drive electronics, on a 1.1 mm thick glass plate. Driving signal alternatives are investigated. Signals quantified on one bit (i.e. square type signals) with audible frequencies filtered out are found to be the most efficient in terms of amplitude generation and audible noise emission. Integration issues, such as the actuators’ distribution on the plate and their impact on focalisation point's amplitude and spatial resolution homogeneity are investigated. The effect of the fingertip pressure on the amplitude vibration is studied (6% loss of amplitude due to a 2N force applied by a fingertip on a position other than the focalisation location, and up to 37% for the same force at the focus point's location).The detection threshold measured on ten users is found to be about 10 µm and is not influenced by the force applied on the screen. While a single impact (one impulse) demonstrates the feasibility of time reversal for tactile feedback, a repetition of impacts varying in amplitude offers the possibility to generate richer haptic feedback (such as a button click). Patterns with different repetition frequencies and envelopes are compared in a user study. It appears that frequencies of 200 Hz and the smoothness of the cardinal sine envelope are found to be the best in terms of pleasantness.On the other hand, electrovibration stimulations are able to create a texture feedback by modifying the apparent friction coefficient between the fingertip and the surface. The electrostatic force generation depends on the fingertip skin's thickness. The mechanoreceptors detection threshholds are frequency dependent. A user study on the influence of the applied force on the perception threshold of tactile feedback is presented. The minimum perception thresholds are observed for 240 Hz stimulus. The effect of the applied force appears to have limited effect on the perception threshold.The combination of both stimulation approaches (time reversal and electrovibration) on a single surface will offer a rich multi-point tactile feedback, both for static buttons and dynamic textures.
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Harald Zophoniasson. Static and dynamic haptic feedback using time reversal and electrovibration stimulations. Vibrations [physics.class-ph]. Université Paris Saclay (COmUE), 2017. English. ⟨NNT : 2017SACLX035⟩. ⟨tel-02918192⟩

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