Page 267 - Handbook of Biomechatronics

Lower-Limb Prosthetics 261

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Fig. 13 The Ossur Re-Flex Shock consists of a composite spring in front which pro-
vides optimal shock absorption leading to reduced impact on the body. (From www.
ossur.com.)

The static testing involved slowly loading and unloading the pylon while
measuring the linear displacement of the mechanism and the applied force.
The stiffness was derived from the force-displacement curves of the data.
While the stiffness for the Re-Flex was found to be relatively linear, the stiff-
ness for the AirStance and Stratus were found to be nonlinear. All three
pylons showed hysteresis in their force-displacement curves, indicative of
energy loss during the load-unload cycle. Dynamic testing involved measur-
ing the response of the pylons to a step of load and unload applied force. The
Re-Flex and the Stratus behaved as underdamped second-order systems
with overshoot and some small oscillations, whereas the AirStance was over-
damped for light loads and underdamped for the heavy ones. The total dis-
placement of the AirStance to the step inputs was one order of magnitude
less than that of the other two units, attributable to the much greater stiffness
of the AirStance pylon.