A portable measurement system for prosthetic triaxial force transducers

A portable system was developed to process and store normal and shear stress data measured at thirteen sites at the stump-socket interface of lower-limb amputees. Forces and moments measured in the prosthetic shank were also processed and stored. Custom-designed sensors and small-size signal conditioning units described in detail elsewhere (Sanders and Daly, 1993; Sanders et al., 1995) were mounted on the prosthesis to take the measurements. The data collection unit, described in detail in this communication, was a Motorola HC16 microcontroller with appropriate peripherals and custom-written software. A 4 Mbyte PCMCIA card was used for data storage. The system collected data for approximately 5 minutes of continuous monitoring or for segmented trials of durations specified by the user. Evaluation tests showed good linearity with minimal hysteresis, though there was some 10-90 mV peak-to-peak noise on the stored data. The principal source of noise was interference between wires in the belt pack instrumentation. The noise can be reduced in the future by changing from a wire-wrapped board configuration to a printed circuit design. Use of the system on an amputee subject showed data of similar magnitude to that collected with a stationary system (cables to a computer data acquisition unit). The system win be used to collect data to improve understanding of how prosthesis design features effect interface stress distributions and also to evaluate stump-socket finite element models. Finite element models are computer-based tools that potentially will predict interface stresses for proposed socket designs and thus enhance the prosthetic design process.