Conference article

The design process of prosthetic feet largely depends on an iterative process of prototyping and user testing. As resources for reliable and repeatable user testing are limited, modeling and simulated testing of the design is a positive addition to this process to support further design development between prototyping. The key goal of prosthetic foot design is to mimic the function of the lost limb. A passive spring and damper system can imitate the behavior of an ankle for low level activity, e.g. walking at slow to normal speeds and relatively gentle ascents/descents. In light of this, a variety of constant stiffness prosthetic feet are available on the market that serve their users well. However, when walking at a faster pace and ascending/descending stairs, the function of the physiological ankle is more complex and the muscular activity contributes to the stride in different ways. One of the challenges in prosthetic device design is to achieve the appropriate range of stiffness of the arrangement of joints and spring elements for different tasks, as well as varying loading of the prosthetic device. This calls for an adaptive mechanism that mimics the stiffness characteristics of a physiological foot by applying real-time adaptive control that changes the stiffness reactively according to user’s needs. The goal of this paper is to define the stiffness characteristics of such a device through modeling. A finite element model was made for a well-received prosthetic foot design. The model was then validated by mechanical measurements of the actual product. We further enhanced the model to include a secondary spring/dampener element to provide added flexibility and damping of the ankle joint movement. Reactive control of the secondary element allows the simulated prosthetic foot to adapt the ankle joint to imitate the behavior of the physiological ankle during different activities and in different phases of the gait cycle.

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