SkinTrack turns lower arm into touchpad

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Technology developed in the US promises to turn a person’s lower arm into a touchpad, an advance that could make small screens on high-tech wearables more user-friendly.

Called SkinTrack and developed by the Human-Computer Interaction Institute’s Future Interfaces Group at Carnegie Mellon University, the new system allows for continuous touch tracking on the hands and arms. The university further claims that it can detect touches at discrete locations on the skin, enabling functionality similar to buttons or slider controls.

Previous “skin to screen” approaches have employed flexible overlays, interactive textiles and projector/camera combinations. SkinTrack requires only that the user wear a ring, which propagates a low-energy, high-frequency signal through the skin when the finger touches or nears the skin surface.

“The great thing about SkinTrack is that it’s not obtrusive; watches and rings are items that people already wear every day,” said Yang Zhang, a first-year Ph.D. student in HCII. He will present details of the technology May 10 at CHI 2016, the Association for Computing Machinery’s Conference on Human Factors in Computing, in San Jose, California.

“A major problem with smartwatches and other digital jewelry is that their screens are so tiny,” said Gierad Laput, a Ph.D. student in HCII and part of the research team. “Not only is the interaction area small, but your finger actually blocks much of the screen when you’re using it. Input tends to be pretty basic, confined to a few buttons or some directional swipes.”

“SkinTrack makes it possible to move interactions from the screen onto the arm, providing much larger interface,” said Chris Harrison, assistant professor in the HCII and adviser to the research.

Carnegie Mellon University's SkinTrack enables users to turn their skin into a touchpad for controlling smartwatches. User wears only a signal-emitting ring, which propagates electromagnetic waves in the skin that can be localized with sensors worn on the wrist. (Future Interfaces Group, Carnegie Mellon University)
Carnegie Mellon University's SkinTrack enables users to turn their skin into a touchpad for controlling smartwatches.(Future Interfaces Group, Carnegie Mellon University)

The user wears a ring that produces a high-frequency electrical signal. When the finger gets near to the skin or touches the skin, that signal propagates through the skin.

By using electrodes integrated into the watch’s strap, it’s possible to pinpoint the source of those electromagnetic waves because the phase of the waves will vary. Electrodes corresponding to the 12 o’clock and 6 o’clock positions on the watch, for instance, can detect phase differences that can determine the position of the finger along the width of the arm; electrodes at the 3 o’clock and 9 o’clock positions can determine the finger’s position along the length of the arm.

The researchers found that they could determine when the finger was touching the skin with 99 per cent accuracy and they could resolve the location of the touches with an average error of 7.6mm. That is said to compare well with other on-body finger-tracking systems and approaches touchscreen-like accuracy.

The researchers showed that SkinTrack could be used as a game controller, to scroll through lists on the smartwatch, to zoom in and out of onscreen maps, and to draw. A number pad application enabled users to use the back of the hand as a dial pad for the onscreen number pad; hovering a finger over the hand acts as a cursor, highlighting numbers on the screen to aid in targeting touch points.

The system does, however, have limitations: keeping the ring powered up is a challenge and signals tend to change as the device is worn for long periods, thanks to factors such as sweat and hydration and the fact the body is in constant motion.