Shaping Resistive Bend Sensors to Enhance Readout Linearity

Resistive bend sensors have been increasingly used in different areas for their interesting property to change their resistance when bent. They can be employed in those systems where a joint rotation has to be measured, in particular biomedical systems, to measure human joint static and dynamic postures. In spite of their interesting properties, such as robustness, low price, and long life, the commercial bend sensors have a response which is not actually linear, as an electronic device to measure bend angles should be, to recover human posture without distortion. In this work, different interfaces for sensor device readout were analyzed and compared from the output response linearity point of view. In order to obtain a sensor characteristic as closer as possible to the ideal linear one, a way to calculate the sensor characteristic with a generalized resistive strip contour, starting from an empiric sheet resistance model, was developed, in order to find what is the more suitable nonuniform geometry. 1. Introduction In order to measure human body kinematics, it is convenient to adopt sensors, which can measure bending angles with good precision despite a low cost. Commercial bend sensors are usually made of a few micrometer thick resistive material deposited onto a thicker plastic insulating substrate. The resistive strip is screen printed with a special carbon ink, to be applied on virtually any custom shape and size film [1]. Normally, however, as well as the overall sensor, it has a rectangular geometry. The overall thickness is anyway negligible compared to the total largeness and lengthiness. The ink’s resistance value changes with the deflection due to an applied external force. All sensor materials, however, must be able to bend repeatedly without failure for the sensor to work. This type of sensors are available on the market (Images SI Inc. [2], Flexpoint Sensor Systems Inc. [3]). They can be applied to body joint as electronic goniometers, to realize goniometric sock for rotation assessment of body segments in human posture recognition [4–8]. The device sizes can be fitted to each type of joints. From a characterization point of view, the model which takes into account the mechanical aspect of the sensor predicts a linear behavior of the electric resistive variation with the bending angle [9]. Nevertheless, the sensor resistance has increasing derivatives, especially for small angles, which result in a nonlinear characteristic, as provided by the sensor electrical characterization. However, if the sensor readout is represented by the voltage