Edge-based Interaction on a Square Smartwatch


Researchers are proposing many approaches to overcome the usability problem of a smartwatch due to its small touchscreen. One of the promising approaches is to use touch-sensing edges to expand the control space of a smartwatch. We considered possible interaction techniques using touch-sensing edges in combination with the smartwatch touchscreen: 1) single-edge, 2) multi-edge, and 3) edge x screen (edge and touchscreen in combination).
We call these techniques square watch interaction (SWI) techniques because they are exploiting the form factor of a square smartwatch. In order to explore the design space and evaluate the usability of the SWI techniques, we implemented a square smartwatch prototype with touch-sensitive edges and conducted a series of user experiments. The experiment results showed that the SWI techniques enable precise 1D pointing and occlusion-free 2D pointing. The experiments also produced empirical data that reflect human manual skills for the edge x screen techniques. The produced empirical data will make a practical guideline for the application of the edge x screen techniques.


  • Evaluation of Edge-based Interaction on a Square Smartwatch
    Sunggeun Ahn, Jaeyeon Lee, Keunwoo Park, and Geehyuk Lee, IJHCS (accepted).





  • The SWI prototype with a linear multi-touch sensor around a watch:
  • (a) a top-down view
  • (b) a side view showing the sensor.
  • (c) the simplified schematic of the linear multi-touch sensor circuit. (Jiseong Gu and Geehyuk Lee, Touch String.)

Multi-touch sensing

    The system enable to detect four discrete touch event for the each four sides of linear touch sensor.

  • (a) a proximity image from the sensor: the sensor consist of 32 proximity sensors while each of four sides consist of 8 proximity sensors. Orange-colored circle represent touch position.
  • (b) the corresponding touch situation.

Multi-touch sensor

  • Multi-touch sensor schematics
    The sensor consists of a series of 32 infrared LEDs (8 LEDs on each side) driven by a cascade of shift registers and a series of photo-transistors connected in parallel. This is schematic file for optical multi-touch sensor around a watch.
  • Size description and Component arrangement
    This file describes how the components are arranged in the multi-touch sensor, and described size of multi-touch sensor which is fitted into LG G-watch. You can read these file using MS Visio.

Touch sensing mechanism


  • Touch state determination: Touch_Down and Touch_Up

The touch states on each of four sides are determined from the amplitude (blue-colored blocks at the above figure) of the proximity image around a peak (three blocks in the shaded region at the above) among 8 proximity sensors. If average amplitude of 3 blocks is greater than threshold value(red dotted line at the above), touch state is set to Touch_Down, if not, touch state is set to Touch_Up.

  • Detecting Finger Movement Direction: Towards, Move, Outwards

The determination of the touch state needs a hysteresis mechanism. Hysteresis mechanism means that threshold for touch state determination was changed with the derivative of a sensor output (orange-colored blocks). Using the derivative of a sensor output was helpful for more reliable detection of a touch event. The above figure shows how hysteresis mechanism is work.

In detail, hysteresis mechanism is needed to resolve following two problems: (1) touch position chattering while a finger lifts up from the sensor and (2) accidental Touch_Up occurrence while a finger moves on the sensor.

Therefore, the derivative of a sensor output (orange-colored blocks) is used to sense three directions of finger movement: (a) Towards, a finger move toward to the sensor, (b) Move, a finger moves on the sensor, and (c) Outwards, a finger move outward from the sensor. If the derivative of three sensor outputs is greater than zero, finger movement direction is Towards (a). If the derivatives are less than zero, finger movement direction is Outwards (c). Finally, if the average of the derivative of three blocks is around of zero, while the amplitude of one of around block is greater than zero and the other around the block is less than zero, finger movement direction is Move (b).

Based on the finger movement direction, the threshold for touch state determination (red dotted line) is changed to resolve forementioned problems, occurred with Move and Outwards direction. If finger movement direction is Move, the threshold set to lower value than the default, the threshold for Towards direction, to prevent unintentional occurrence of Touch_up event while a finger slides on the sensor. Meanwhile, if finger movement direction is Outwards, the threshold is set to a higher value than the default to minimize the effect of chattering while a finger lifts up from the sensor.

  • Touch position determination

The touch position also can be determined from the amplitude (blue-colored blocks at the above) of the proximity image around a peak. Touch position interpolates between the position of left-sided block and the position of right-sided block based on the difference of the amplitude between the left-sided block and right-sided block. For instance, if the amplitude of the left-sided block is same to the amplitude of the right-sided block, the touch position is set to the position of peak block.