Virtual targets on touchscreens (e.g., icons, slide bars, etc.) are notoriously challenging to reach without vision. The performance of the interaction can fortunately be improved by surface haptics, using friction modulation. However, most methods use position-dependent rendering, which forces users to be aware of the target choice. Instead, we propose using tactile feedback dependent on users’ speed, providing a viscous feeling. In this study, we compared three viscous damping conditions: positive damping, negative damping, and variable damping (viscosity was high during slow movements and low during fast movements), against a baseline condition with no tactile feedback. These viscous fields are created by changing net lateral forces based on velocity. Results indicate that, during the initial phase of movement when the finger approaches the target, various viscous feedback has an insignificant impact on targeting trajectories and movement velocity. However, positive damping and variable damping significantly influence behavior during the selection phase by reducing oscillation around the target and completion time. Questionnaire responses suggest user preference for viscous conditions and disapproval of negative viscous forces. This study provides insights into the role of viscous resistance in touchscreen interactions.

Touch is fundamental to our perception of the world and to interaction with our physical surroundings. With touch we can intuitively and effortlessly move and shape objects and control complex machines. In most modern machines, the part that interfaces with user often integrate touchscreens or touchpads, owing to their ease of use. However, these interfaces often deliver very poor tactile feedback, which limits the usability in contexts such as driving, low-light environments, and for users with visual impairments. Existing solutions like vibrotactile feedback, are poor substitute to the richness of natural touch and offer only transient sensations. Surface haptic devices offer more complexity, but since they rely on friction modulation, they require continuous movement and cannot nudge the user to arbitrary directions. These limitations highlight the need for active lateral force devices that can guide users in an arbitrary direction. This thesis introduces the Ultra loop, an active surface haptic device that generates net lateral forces using resonant traveling waves. Built around an oblong ring-shaped structure, the Ultra loop provides a large and flat interaction area with uniform force generation. Unlike existing active haptic devices, it operates at resonance, achieving a high vibration amplitude-to-input ratio, resulting in a more salient force feedback. Additionally, this thesis introduces a planar adaptation of the Ultra loop, the flat Loop, which is more compact with a height of just 5 mm, facilitating integration into consumer electronics. These devices can guide users via their sense of touch and render complex forces fields by modulating the wave amplitude and phase control as a function of the position and velocity of the user. To evaluate their effectiveness, this thesis investigates two types of rendered haptic environments: position-based elastic potential fields and velocity based viscous damping. Experimental user studies show that participants could perceive virtual 3D shapes (e.g., bumps and holes) and stepwise force fields that enhance their target-search performance. Moreover, directional cues provided by the force feedback enabled users to navigate toward a target without visual feedback, while viscous damping environments, where lateral force is a function of finger speed, reduced oscillations during selection, and improved overall targeting performance. This doctoral work systematically explores the benefits of active force feedback in touch interactions by introducing resonant traveling wave-based haptic displays and performing user studies. By advancing surface-haptic technology, this research paves the way for next-generation touch interfaces that support eye-free interaction and effortless control of complex machines.

Touchscreens and touchpads offer intuitive interfaces but provide limited tactile feedback, usually just mechanical vibrations. These devices lack continuous feedback to guide users’ fingers toward specific directions. Recent innovations in surface haptic devices, however, leverage ultrasonic traveling waves to create active lateral forces on a bare fingertip. This paper investigates the effects and design possibilities of active forces feedback in touch interactions by rendering artificial potential fields on a touchpad. Three user studies revealed that: (1) users perceived attractive and repulsive fields as bumps and holes with similar detection thresholds; (2) step-wise force fields improved targeting by 22.9% compared to friction-only methods; and (3) active force fields effectively communicated directional cues to the users. Several applications were tested, with user feedback favoring this approach for its enhanced tactile experience, added enjoyment, realism, and ease of use.

The sensation of touching virtual texture and shape can be provided to a touchscreen user by varying the friction force. Despite the saliency of the sensation, this modulated frictional force is purely passive and strictly opposes finger movement. Therefore, it is only possible to create forces along the direction of movement and this technology cannot stimulate a static fingertip or provide forces that are orthogonal to the direction of movement. The lack of orthogonal force limits the guidance to a target in an arbitrary direction and there is a need for active lateral forces to give directional cues to the fingertip. Here, we introduce a surface haptic interface that uses ultrasonic traveling waves to create an active lateral force on bare fingertips. The device is built around a ring shape cavity where two degenerate resonant modes around 40 kHz are excited with 90$^{\circ }$ phase shift. The interface provides active forces up to 0.3 N to a static bare finger uniformly over a 140×30 mm$^{2}$ surface. We report the model and design of the acoustic cavity, force measurements, and an application to create a key-click sensation. This work demonstrates a promising method for uniformly producing large lateral forces on a touch surface.