Technology.
Surround Haptics allows to create perception of smooth tactile motion on human skin by using just a few vibrating actuators arranged on a grid. The control algorithm that underlines our technology is based on exploiting tactile illusions.
Tactile Illusions.
Tactile illusions are perceptual phenomena that arise when two or more vibrating devices or actuators are placed on human skin. These actuatores could be pager motors, speakers, soleniods, militarty-grade voice coils, or any other vibrating device. Indeed, the tactile illusions are fundamental properties of human tactile perception and do not depend on type of tactile actuators.
In Surround Haptics we mainly exploit two tactile illusions:
In Apparent Tactile Motion illusion two vibrotactile stimuli are placed on skin in close proximity and their actuation times are overlapped, as shown in figure on the right. The user would perceive not two actuators, but rather a single virtual actuator moving between physical vibrating points.
In Phantom Tactile Sensation illusion a simultaneous stimulation of two vibrating actuators placed in close proximity would create an illusory vibrating actuator located between the real actuators, as shown in figure on the right. Unlike apparent motion, the phantom illusion is static and no motion is perceived. Note that both phantom and apparent motion illusions are different from saltation or "cutaneous rabbit" illusion that has been often used before.
The Science of Illusions.
Using tactile illusions to create tactile displays technology presents a major challenge: we must be able to control illusions with a high degree of precision. For example, when using phantom sensations illusions, we have to be able to place virtual actuators in the exact location and with the exact intensity that is required by application. However, there is a very limited understanding of tactile illusions control mechanisms. To overcome this, we systematically measure the illusions' control space, construct their mathematical models, and validate them in carefully designed psychophysical experiments. The detailes of these models and illusions are presented in details in our publications.
Drawing on Skin with Tactile Brush.
We designed an algorithm that uses the tactile illusions described above to "draw" smooth moving tactile strokes on sparce actuator grids. The basic idea of the algorythm is to fill empty space between physical actuators with either illusion of motion of static phantom actuators.
The basic principles of the algorythm are shown on the figure on the right. We use apparent tactile motion illusion to produce continuous tactile motion between any two
points on the actuator grid. For example, we can draw a tactile stroke by sequentially vibrating actuators along the horizontal row.
A difficulty arises when we attempt to generate a diagonal stroke (figure b): because there is no actuator between start and end points, the user feels a gap in the middle. To solve this problem we generate a virtual actuator by using the phantom illusion and proceed to generate a tactile stroke using apparent motion, where the phantom actuator is treated just like a normal physical actuator (figure c). Our algorithm does not distinguish between virtual and physical actuators. In fact, we can produce tactile strokes consisting of virtual actuators only (see figure c).
The algorythm is universal in that it treats tactile grids as a generic content-rendering platform, independent from the specific mechanical and electrical properties of the tactile actuators used. We named the algorythm a Tactile Brush.
Hardware and Implementation.
Surround Haptics is a tactile control technolgy and can be implmented for a broad range of devices, such as chairs, vests and handheld devices. Our initial research platform was developed for a chair and is presented on the image below.
We used a 4×3 equally-spaced rectangular grid of C-2 tactors, the tactors were placed in a finely cut sheet of foam padding and glued to the back of a wooden chair. Each actuator on the grid was individually controlled by a multichannel audio card with tactile waveforms generated in a PureData sound design environment. A custom control board amplified the current and voltage of individual waveforms and sent it to the actuators. Custom application software controlled Pure Data patches over UDP protocol.
