In 1950, Edward Mallinckrodt, a researcher at Washington University in St. Louis, accidentally discovered the phenomenon of electrovibration (also known as electrostatic vibration). He noticed that a brass electric light socket had a different texture when a light was burning than it did when the light was turned off. Along with a team of researchers, he began exploring the phenomenon in more detail by running experiments using an aluminum plate with insulating varnish. They wrote:
If the dry skin of one’s finger is moved gently over a smooth metal surface covered with a thin insulating layer, and the metal is connected to the ungrounded side of an 110-v power line, the surface has a characteristic feeling that disappears when the alternating voltage is disconnected.
This “characteristic feeling”—a sensation of friction—results from the electrostatic force produced by the alternating voltage across the electrode and insulated ground plane. As a person runs their finger over the ground plane, the movement induces an electric force field between the finger and the plane surface. Because the current alternates, the force field also alternates to attract and repel the finger. This field is ordinarily too small to be perceived by human skin directly, but movement over the surface provides a perceptible frictional force by modifying the rate at which individual cutaneous mechanoreceptors (pressure sensors in the skin) deform and send signals to the brain.
Over a half-century after Mallinckrodt’s discovery, a collaborative team of researchers from Carnegie Mellon University and Disney Research developed an algorithm for rendering 3D textures onto a touch screen using electrovibration. Nicknamed TeslaTouch, the system modifies the frequency and amplitude of an alternating voltage applied to an electrode beneath a touchscreen. By changing this voltage, TeslaTouch allows a software interface on a tablet computer to provide real-time haptic feedback by modifying the perceived friction of different parts of the screen. As the user swipes, taps, pinches, and manipulates objects on a touchscreen, the software can generate tactile effects that mimic the bumps, ridges, and textures of the surfaces of different objects.
Although Disney likely intends to use TeslaTouch as part of their interactive experiences within their theme parks and stores, the technology could eventually be incorporated into a wide range of other touchscreen surfaces. The technique has a lot of potential, but the form factor remains a primary barrier to adoption. Implementing the alternating voltage results in a much bulkier device than with an ordinary capacitive touchscreen. As the technology sees more frequent use, however, there may be technological developments that allow more smartphone and tablet manufacturers to feature electrovibration without sacrificing the compactness of their designs.
The potential uses for the technique are exciting. Electrovibration could make interactive textbooks more engaging on tablets, allowing students to explore the 3-dimensional features of an object directly on each page. Software for iOS or Android could be augmented with unique haptic feedback for button presses and swipe gestures. Games could incorporate electrovibration to add a new layer of interactivity to touch controls. Given the ubiquity of capacitive touchscreens, the addition of richer haptic feedback through electrovibration promises to enhance almost all of our interactions with software.
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