Skinput: Appropriating the Body as an Input Surface

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I. INTRODUCTION
Devices with significant computational power and capabilities can now be easily carried on our bodies. However, their small size typically leads to limited interaction space and consequently diminishes their usability and functionality. Since we cannot simply make buttons and screens larger without losing the primary benefit of small size, we consider alternative
approaches that enhance interactions with small mobile.A new technology called skinput technology allows us to use our hand as an interface..
Skinput is an input technology that uses bio-acoustic sensing to localize finger taps on the skin. When augmented with a pico-projector, the device can provide a direct manipulation, graphical user interface on the body. The technology was developed by Chris Harrison, Desney Tan, and Dan Morris, at Microsoft Research's Computational User Experiences Group. Skinput represents one way to decouple input from electronic devices with the aim of allowing devices to become smaller without simultaneously shrinking the surface area on which input can be performed. While other systems, like SixthSense have attempted this with computer vision, Skinput employs acoustics, which take advantage of the human body's natural sound conductive properties This allows the body to be annexed as an input surface without the need for the skin to be invasively instrumented with sensors, tracking markers, or other items. When coupled with a small projector, Skinput can simulate a menu interface like the ones used in other kinds of electronics. Tapping on different areas of the arm and hand allow users to scroll through menus and select options.Skinput could also be used without a visual interface. For instance, with an MP3 player one doesn't need a visual menu to stop, pause, play, advance to the next track or change the volume. Different areas on the arm and fingers simulate common commands for these tasks, and a user could tap them without even needing to look.


II.BIO-ACOUSTICS
When a finger taps the skin, several distinct forms of
acoustic energy are produced. Some energy is
radiated into the air as sound waves; this energy is
not captured by the Skinput system. Among the
acoustic energy transmitted through the arm, the
most readily visible are transverse waves created by
the displacement of the skin from a finger impact
When shot with a high-speed camera, these appear
as ripples, which propagate outward from the point of
contact . The amplitude of these ripples is correlated
to both the tapping force and to the volume and
compliance of soft tissues under the impact area. In
general,tapping on soft regions of the arm creates
higher amplitude transverse waves than tapping on
boney areas (e.g., wrist, palm, fingers), which have
negligible compliance.
In addition to the energy that propagates on the
surface of the arm, some energy is transmitted
inward, toward the skeleton. These longitudinal
(compressive) waves travel through the soft tissues of
the arm, exciting the bone, which is much less
deformable than the soft tissue but can respond to
mechanical excitation by rotating and translating as a
rigid body. This excitation vibrates soft tissues
surrounding the entire length of the bone, resulting in
new longitudinal waves that propagate outward to the
skin.

III.SENSING
Skinput uses an array of highly tuned vibration
sensors. Specifically cantilevered piezo films.This
sensors are capable to identify vibrations of very
small frequency. Additionally, the cantilevered
sensors were naturally insensitive to forces parallel to
the skin (e.g., shearing motions caused by stretching).
Thus, the skin stretch induced by many routine
movements (e.g., reaching for a doorknob) tends to
be attenuated. However, the sensors are highly
responsive to motion perpendicular to the skin plane
– perfect for capturing transverse surface waves and
longitudinal waves emanating from interior structures

IV.THE ARMBAND PROTOTYPE
The main device used to implement Skinput is an
armband which features two arrays of five sensing
elements. There is different set of resonant
frequencies for each sensor package . The upper
sensor package is more sensitive to lower frequency
signals, as these were more prevalent in fleshier
areas. Conversely, the lower sensor array are
sensitive to higher frequencies, in order to better
capture signals transmitted though (denser) bones.