Foot angle determination using conductive polymer sensors

Abstract

A study was carried out to assess the possibility of monitoring of joint angles, foot posture and foot motion through the use of conductive polymer sensors. The sensors are composed of a carbon polymer coating on an elastic fabric and they behave like strain gauges. A mechanically driven hinge was used to simulate joint motion by generating an angle change between its wings. A sensor strip was clamped longitudinally across the hinge in order for it to stretch when the angle between the wings increases. An electrogoniometer was used to monitor the angle spanned by the two wings of the hinge. Series of simultaneous measurements of angle and resistance were conducted at different speeds. Results indicate that a unique rate of change of voltage can be assigned to a specific angular velocity. This idea allows the tabulation of a database of voltages and time derivatives of voltages with corresponding angles and angular velocities. Angular velocity information was obtained by computing the derivatives of sensor output voltages in real time and comparing both voltage and their time derivatives to values in the database, with linear interpolation used as necessary. Angular displacement was then obtained by numerically integrating velocity information. Three carbon sensors were then applied on socks and were placed on different locations of maximum strain on the foot. Wireless data transmission was added in order to enable unhindered foot motion for future applications.