Flex sensor characterization against shape and curvature changes

Resistive flex sensors were increasingly used in different areas for their interesting property to changetheir resistance when bent. In particular, they can be applied to human segment in biomedical devices toregister static and dynamic postures. In spite of their interesting properties, such as robustness, low priceand long life, they often demonstrate non-linear response and lower sensitivity at small bending angles.This paper provides investigation to improve flex sensors linearity and sensitivity to measure body jointangles with better accuracy. To this aim, an empirical model of the sheet (or surface) resistance of theactive layer, to simulate its behavior against the layer shape and size as well as the bending angle, wasprovided, to investigate whether changes of the standard rectangular shape can improve sensitivity andlinearity. In addition, to date commercial flex sensors have been characterized only against the bendingangle with a radius of curvature smaller than the device length, so limiting the application to small jointssuch as finger or knee. In order to extend the flex sensor applications, for instance, to measure the trunkposture in back disease and rehabilitation monitoring, the sensor response against a radius of curvaturegreater than the sensor length was analyzed. Finally, a new modeling technique, based on the inversemodel of the sensor characteristic, to enable fast measurements of the bending angle or the radius ofcurvature from sensor response also in real time, and fast calibration procedures, fitting the same modelto measurements with different joint size and even device, were developed.