5.4: A Textile Based Capacitive Pressure Sensor

INTRODUCTION The sense of touch plays an important role in everyday life. It provides the body with early contact to surrounding objects. By touching an object, we can tell if it is sharp or smooth; by holding it, we can estimate its weight. Skin is the most extended sensor of the entire human body. The haptic interface gives the first feedback to object manipulation; without sensing the pressure exerted over the skin surface, it becomes quite impossible to measure hand strength in order to pick up an object or simply stroke it gently. Recently, with the development of wearable computing, the need for a wide extended pressure sensor has increased. Such sensors have to be elastic and possibly extendable to cover a three-dimensional surface, robust to work in harsh environment and reliable to supply accurate measurements. Finally, they must be produced at low-cost as to not affect the overall product cost. It should be evident how a wearable computing concept is challenging. It requires new solutions that enable electronic components to fit into clothes. Different architectures have been proposed to resolve the contrasting needs for unobtrusive devices with the computing power supplied by more traditional electronics. A major issue of this new perspective is researching new materials to use as a support for electronics. A fabric substrate is very appealing: it is elastic and extendable [2], supported by a well known technology and produced at low-cost. Some smart pressure-sensors interfacing with a flexible substrate have been developed, but they are either based on electro-optical fabric ([3]) which is not suitable for the low cost market, or they need expensive and cumbersome PCB electronics ([4]). This paper introduces an approach for decoding the pressure exerted over a broad piece of fabric by means of capacitive sensing. The device described produces an image of the pressure field over the sensing surface, providing both information on the position of the area touched and on the pressure exerted on it.