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Initial ideas for the compliant mechanisms involve either single materials, such as laser cutting the entire mechanism out of polyethylene, or multiple materials, such as using low-durometer rubber for the flexural joints, with rigid plastics serving as the links. In terms of the nitinol motors, we hope to explore using nitinol wire springs with resistive heating elements to create the main contraction force, with an opposing spring serving to relax and release the muscle once the power to the heating element is removed. Other linear actuators are also being considered at the moment.
2D fully compliant mechanisms can typically only have rockers, and not cranks, as a crank would destroy the flexural element, twisting it until it is broken. This proves a major challenge with almost all conventional mechanisms. One way of addressing this could be to use a material that only bends in one direction to allow the foot to drag in one direction and grip in the other, to use a redirecting element to convert linear motion to circular (note that this does introduce a bearing element, however it would be low speed and possibly self clearing), or to implement a crank by having a compliant linkage that is into the plane (essentially spins around like a noodle).
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