Figure 14 Principle of electrovibration. Redrawn from Giraud et al. (2013)3 with permission of IEEE.
With the advent of devices that provide no tactile feedback in situations where we were used to experiencing tactile cues, such as typing on a keyboard, a new term emerged to address this shortcoming. Surface haptics refers to the creation of virtual haptic effects on physical surfaces, such as direct-touch user interfaces. This chapter reviews the technologies that have been used to create variable friction displays and describes the challenges associated with modulating the friction experienced on the fingertip as it moves across a flat surface. Incorporating such tactile feedback in flat screen devices is critical to their use as effective human interfaces.
On the now ubiquitous tablets and smartphones that have touch screens, tapping a button or a key on the screen is easily executed without the need for an intervening interface like a physical keyboard or computer mouse. The absence of any haptic feedback on these devices is often problematic because the success of the action cannot be perceived immediately, and because features presented visually cannot be felt. Many of these touch screens also support gestures, making it easy, for example, to navigate through material quickly by swiping the hand across the surface and then focus on particular content by extending the index finger and thumb to zoom into a region of interest.
A new class of displays has emerged to address the limitations of flat-screen devices. They operate by controlling the friction force or lateral resistance to motion between the fingertips and the flat surface of the display. Such devices are often referred to as variable friction displays. The interest in incorporating these effects into existing technology is driven in part by the relative ease with which variable friction displays can be integrated into existing tablet and smart-phone screens. The objective of these friction devices is to create the illusion of texture or surface features by varying the lateral forces experienced on the fingertip as it moves across the flat screen. This means that the display can both visually and haptically present features, such as a slider that resists movement and is then released as it is unlocked, or a textured button that can “grasp” a finger to indicate that it has been selected. In contrast to some of the other display technologies described in chapter 5, surface haptic displays are usually co-located with visual displays.
Two main technologies are being explored to create variable friction surface haptics: ultrasonic vibrations and electrovibration, both of which can be used to modulate the perceived friction between the fingertip and the surface of the touch screen. The effects of these two technologies differ in that ultrasonic vibration reduces the perceived friction on a surface, whereas electrovibration enhances it; both require sensing the position of the finger on the display surface. Ultrasonic vibration devices use piezoelectric actuators to vibrate a surface at ultrasonic frequencies (e.g., ~30 kHz) thereby reducing the contact time of the finger on the surface, and in so doing reducing friction. The ultrasonic vibration itself is not perceived, but the decrease in friction between the finger and surface as the amplitude of the vibration increases (~ ±3 µm) is perceptible. Measurements made of the friction force under these conditions show that the friction experienced by the finger moving across the surface can be reduced by up to 95%.
Two mechanisms have been proposed to account for the reduction in friction. First, there is intermittent contact between the finger and the vibrating screen as the finger moves out of phase with the surface of the plate in a bouncing motion; and second, the finger bounces against the squeeze film of pressurized air trapped between the finger and the glass plate. Images taken of the fingertip in contact with the vibrating glass plate indicate that squeeze film levitation is essential to the reduction in friction. This work has also highlighted the importance of the microstructure of the fingertip skin, in particular the asperities, in determining the dynamics of friction—the skin appears to bounce on the surface but does not completely detach from it.1
Electrovibration, or electrostatic friction modulation, was first described in the 1950s after the accidental discovery that moving a dry finger over a conductive surface covered with a thin insulating layer excited by a 100 V signal created a rubbery feeling.2 This sensation results from the frictional shear force created by the electrostatic attraction between the conductive surface and the finger, as illustrated in figure 14. By controlling the amplitude and frequency of the voltage applied to the conductive surface, different textures can be generated. This technology should be distinguished from the electrotactile cutaneous displays described in chapter 5, in which current passing through the skin electrically stimulates the underlying nerve fibers.
Figure 14 Principle of electrovibration. Redrawn from Giraud et al. (2013)3 with permission of IEEE.
Electrovibration has several properties that make it attractive for consumer electronic devices, namely quiet operation, relative ease of implementation with existing touch-screen devices, and the capability of using it on both flat and curved surfaces. Such devices have been developed commercially using self-capacitive touch-screen panels as the display technology. During the past decade, TeslaTouch (Disney Research) and E-Sense Technology (Senseg) used such touch screens, with a transparent electrode on a clear substrate, to electrically induce attractive forces between a moving finger and the surface of the display.4 Using modulation of the attractive force, a range of tactile sensations were generated, including textures and edges. Specific textural properties can be presented by varying the amplitude and frequency of the signals exciting the display surface. It is important to remember that these effects are perceptible only when the fingers are moving on the screen, and that the same tactile signal is delivered to the entire surface, so different features cannot be produced simultaneously on the display surface. It is possible to design and manufacture systems wherein different areas of the display are independently controlled, which would permit the presentation of more complex tactile patterns. However, these have proven to be very complex systems to control.
All these surface haptics technologies modulate the friction felt by the finger as it moves across a glass surface, although the actual friction experienced by users can vary significantly. The nature of the surface, the moisture content of the skin in contact with the display, and the thickness of the stratum corneum (the outer layer of the skin) can all influence the electrostatic forces on the finger and hence the perceived friction. Moisture content has a substantial effect on friction because it softens the stratum corneum so that it conforms more to the surface, thereby increasing the area of contact. It is possible to couple ultrasonic vibrations with electrovibration to enhance the range of sensations that can be presented to a user; the combination allows both increases and decreases in friction to be displayed on a surface.5 When this is implemented in a single device, users perceive a friction continuum rather than two distinctly operating effects.
Surface haptics displays have a broad range of applications due to the pervasiveness of flat-screen displays and the need to enhance their tactile communication capabilities. The importance of conveying texture tactually cannot be overestimated in interactive surfaces that are manually explored. For this reason, rendering high-fidelity textures has been the objective of much recent research in this field. A wide array of applications of surface haptics are being explored, from providing navigation cues to the visually impaired, to offering a medium for expressing emotion when communicating virtually,6 to enhancing educational interactions by engaging the haptic sense in conjunction with visual input.