Design of touch-sensitive surface with arbitrary shape based on time-domain reflectometry using injket printing

Touch sensors are one of the most fundamental input devices of many different applications these days. However, most dominant 2D touch sensing technologies are based on a grid of connections and sensors, which are thick and cumbersome. Thus flexibility and form factor do matter. In this paper, we propose an algorithm to design a flexible two-dimensional touch-sensitive surface using inkjet printing technology. Due to its flexibility and conformability, it is possible to turn everyday objects and surfaces into touch sensors. Different from existing touch sensing technologies like resistive based and capacitive based sensing method, we detect touch motion using Time-domain Reflectometery (TDR), which is originally used to locate faulty in electric cable. An existing work on TDR based touch sensing method suggested implementing Hilbert space-filling curves´ pattern to improve sensing accuracy in all directions. But pure Hilbert curves are restricted to only square shape with size of 2ⁿ unit length. In this paper, we tackled this problem by introducing an algorithm to automatically generate appropriate quasi-Hilbert Space-filling Curve (SFC) for any shape of surface. Our algorithm could fill any arbitrary surface with high coverage curve while retain advantages of Hilbert SFCs. We also utilize conductive inkjet printing with silver nanoparticle ink to rapidly prototype thin and flexible surfaces on paper substrate.