ScraTouch: Extending Interaction Technique Using Fingernail on Unmodified Capacitive Touch Surfaces

We present ScraTouch, an interaction technique using fingernails, as a new input modality by leveraging capacitive touch sensing. Differentiating between fingertip and fingernail touches requires only tens of milliseconds worth of shunt current data from unmodified capacitive touch surfaces, thus requires no hardware modification. ScraTouch is simple but practical technique for command invocation and mode switching. An evaluation using a point-and-select task on a touchpad showed that although the switching between the finger and nail in ScraTouch required a little more time compared with the baseline (finger touching without mode switching), in overall the operations, ScraTouch was just as fast as the baseline, and on average, 29 % faster than a long press with 500-ms threshold. We also confirmed that setting a simple threshold on the measured shunt current for recognition works robustly across users (97 % accuracy).