Programmable Stiffness Joint

Posted on Feb 7, 2014

The `Programmable Stiffness Joint` is a device which is able to internally modify the rotational stiffness of a simple pivot joint. It is intended to serve as a proof-of-concept for future work in developing a model for determining the respective damping and stiffness components of the human ankle joint over the able-bodied gait cycle. This inform

Programmable Stiffness Joint
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ation in turn will permit prosthetists and rehabilitation engineers to design and fit prostheses better designed for the user`s gait kinetics, which in turn should produce more natural gait kinematics. The goal of our project is: To develop a working model of a device which is able to control the stiffness of a joint in such a manner as to mimic the motion of a human ankle joint. The user is able to select a stiffness level and the device will adjust itself to provide the specified level of rotational stiffness. The set-point of the joint will remain constant for all stiffnesses. This is how it works: A linear coil extension spring has a constant stiffness within its elastic extension limit. As the location of the insertions of the spring on the base and rotating member change position, the amount of spring extension experienced for a given angular change of the rotating member will vary, as will the torque produced by the spring tension. These variations are taken advantage of to produce widely different stiffnesses from a single linear extension coil spring. First, a linear spring requires a constant incremental force to achieve a given incremental change in the spring length. This is described by Hook`s Law (shown below) where F is the force applied to the spring, x is the change in length of the spring (in this case, it will only be extension), and k is the stiffness of the linear spring. Due to this relationship, the...

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