Different tasks and conditions in gait call for different stiffness of prosthetic foot devices. The following work presents a case study on design modifications of a prosthetic foot, aimed at variable stiffness of the device. The objective is a proof-of-concept, achieved by simulating the modifications using finite element modeling. Design changes include the addition of a controlled damping element, connected both in parallel and series to a system of springs. The aim is to change the stiffness of the device under dynamic loading, by applying a high damping constant, approaching force coupling for the given boundary conditions. The dynamic modelling simulates mechanical test methods used to measure load response in full roll-over of prosthetic feet. Activation of the element during loading of the foot justifies the damped effect. As damping is in contrast to the main design objectives of energy return in prosthetic feet, it is considered important to quantify the dissipated energy in such an element. Our design case shows that the introduction of a damping element, with a high damping constant, can increase the overall rotational stiffness of the device by 50%. Given a large enough damping coefficient, the energy dissipation in the active element is about 20% of maximum strain energy.

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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.

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. The research is based on a finite element model of a well-received prosthetic foot design, which is validated by mechanical measurements of the actual product. We further enhance the model to include a secondary spring/dampener element. Various smart material technologies are considered in the design to provide control of flexibility and damping rate of the ankle joint movement. The 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.

The unique rheological properties of discontinuously shear thickening fluids (STF) have been employed in various engineering applications in recent years. In a commercial aspect, this has most notably been body armor and protective equipment, but also specialized smart structures in damping and force-coupling applications. The topic of this work is the application of STF in an articulating prosthetic foot for adaptable force response. Connected in series and parallel to a spring system, a STF based element can be used to affect the force transfer within the system, and thereby, influence the stiffness of the prosthetic foot dynamically over the gait cycle. The device described, prototyped and tested in this work, is a STF filled piston/cylinder design. The objective is a velocity dependent force response over the translational motion. Ranging from dampened, compliant deflection at low velocity, to a more efficient force transfer (coupling) for energy storage and return in the spring system, at higher speed. The rapid viscosity increase in the STF at a critical shear rate is used to approach a stepwise force response, thereby enabling adaptive response of the foot for different load rates. The adaptive response results in a greater range of motion with easier rollover for slow movement, for instance standing up from a seated position and adaptation to inclined surfaces, without sacrificing the energy return favorable for normal walking.