Normal and shear interface stresses in lower-limb prosthetics

Instrumentation for measuring normal stresses and biaxial shear stresses at the residual-limb/prosthetic-socket interface during ambulation is presented. Three-directional force transducers were designed using strain-gage technology. With an amplifier gain of 4000, shear and normal sensitivities are 0.132 V/kPa and 0.0259 V/kPa, respectively. Nonlinearity, crosstalk, noise, and drift are reported to be within acceptable levels. Transducers were calibrated in the shear direction by static loading in the sensing surface plane. At a gain of 4000, the sensitivity was 0.132 V/kPa. Nonlinearity was less than the quantization error of the data acquisition system. Crosstalk between shear directions was measured to be less than 1% full-scale. Normal direction calibration with or without a 2.38-mm-thick silicone rubber sheet between a metal loading plate and the Pelite surface showed a sensitivity of 0.0259 V/kPa at a gain of 4000. Nonlinearity was less than 2% full-scale. The principal source of noise was from cable motion during ambulation. The estimated error introduction was 5%. Drift and electronic noise were minimal