Realization of a piezoresistive felt sensor for pressure measurements on lower limb prosthesis

Abstract

Felts with piezoresistive properties are made by mixing conductive fibres with wool or synthetic fibres. They are sensible to strain which modifies their thickness. The modification of felt’s thickness, in fact, changes the conductive fibres interaction and therefore the resistance. Figure 1 shows the sensor used for pressure measures in lower limb amputees: it’s a U-shaped piece of conductive felt, whose arms are covered with conductive paint in order to create electrodes; contacts are made with snap fasteners and clips. This transducer can be connected to a portable device (fig. 2), controlled by a Digital Signal Processor, able to acquire signals from sensor conditioning circuits and to store data into a flash memory. An off-line processing reads data from the memory, elaborates them and plots relative time diagrams by a program purposely developed in LabVIEW environment. With this system we acquired weight distribution information on a crutched patient (a transfemural amputee) during the rehabilitation at Centro Protesi INAIL in Vigoroso di Budrio (Bologna, Italy). The sensor was positioned between ischium and prosthesis . The subject was asked to make some movements for about 80 seconds