A knee-ankle-foot orthosis (KAFO) spans the knee, ankle and foot, and assists in the walking motion of those who suffer neuromuscular deficiencies. KAFOs can be classified as passive, semi-dynamic and dynamic. Passive KAFOs lock the knee joint during the whole gait cycle. Semi dynamic KAFOs lock the knee joint during the stance phase. Dynamic KAFOs attempt to reproduce normal knee motions during the whole gait cycle. Two types of dynamic KAFOs have been reported in the literature. The first one is activated by using a pneumatic system, and the second one uses a spring mechanism. Both systems are bulky and controlled through complex control systems that limit their application as assistive devices. The purpose of our research is to develop a dynamic KAFO that is actuated easily by employing shape memory materials. Such an actuation system makes the KAFO lightweight and with a great commercialization potential. The purpose of this paper is to present a conceptual design for the knee actuator of a dynamic KAFO. This actuator uses torsional shape memory rods to match the stiffness of the knee joint of the KAFO with that of a normal knee joint during the walking gait cycle. Joint stiffness is measured by the moment around the joint per degree of joint rotation. The proposed actuator includes two parts that work independently during the two phases of the gait cycle. The first part engages only during the stance phase and the other works only during the swing phase. Each part is developed by combining a superelastic (SE) rod and a rotary spring in series. The conceptual design is verified by simulation. The simulation results show that the proposed knee actuator reproduces the stiffness of the normal knee joint during the whole gait cycle. It is thus possible to develop a novel dynamic KAFO that can provide normal knee stiffness characteristics to assist individuals with quadriceps deficiency.

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