Current passive prosthetic ankles are lighter, simpler, and less expensive than powered prosthetic ankles. These current passive designs, however, do not provide adequate torque at the instant when it is needed to propel the body forward. This paper presents a novel 2 degree of freedom (DOF) passive compliant prosthetic ankle that uses a network of conventional springs. One DOF allows the lower leg component to compress when the weight of the amputee is applied during walking. The second DOF allows rotation about the prosthetic ankle joint. The force generated along the leg during walking is converted into ankle torque used to propel the body forward during push-off. An optimization routine is used to select the stiffness values and connection locations of the springs used in the compliant mechanism. The optimization yields a design that generates a torque-deflection profile that is very similar to that of a natural ankle. The mechanism demonstrates apparent active behavior (negative spring constant) at the ankle during push-off without using active components.

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