Some applications require a vandal resistant user interface to an embedded computer system. A plate of steel with shock resistant sensors is as robust as one can imagine.

The principle of this tablet like sensor is rather simple: a rigid rectangular plate has four sensors under it, one in each corner. When the user presses a point in the visible area of the plate, the pressure is divided unevenly among the corner sensors and the position of the pressing can be calculated from the measured pressure values. The calculation formula is rather simple but practice showed that it does not give accurate results right away. Therefore we needed to derive a correction formula which significantly improved the localisation accuracy of the pressings. The final location error with the prototype in the picture is on the average 1.8 mm.

A similar input device, brand named Infini Touch was developed independently by QSI Corporation, USA. Their web pages and their US patent application US 2006/0279553 A1 describe more details about the principles of this technology.

This study was conducted in co-operation with VTT Technical Research Centre of Finland, Department of Automation Science and Engineering in Tampere University of Technology and the TAUCHI research group in Tampere University.

Links
Emfit Ltd. EMFI sensors
Our paper about the tablet in IEEE Explore

Designed by Alpo Värri 25.8.2008