Locating the Leg Joints Using Magneto-Inertial Sensors for Adjusting the Segmental Lengths of a Lower-limb Exoskeleton

Fitting a subject to a lower-limb exoskeleton requires precise estimation between the distances of the leg joints to avoid injury. The current method manually identifies landmarks on the leg and measures the distance between them, but the process could take up to thirty minutes, and needs the work of two people for accuracy. According to literature, the location of a spheroidal joint, like that of the hip, can be automatically estimated using MIMUs and rigid body kinematic constraints to an accuracy of millimeters. To validate those conclusions, we replicated the algorithm used with three different experiments, a mechanical setup, and a test for both a human knee and hip. Our mechanical setup replicated a hinge joint by movement of attached links that created a fixed and planar circular path with a commercial MIMU (OPAL, by APDM, INC) rigidly attached to one of the links. For the leg experiments we attempted to replicate the planar movement from the mechanical setup. At least four trials were performed for each test with the standard deviation and average error of each test used as metrics to evaluate performance. For similar distances, the standard deviation and average error increases for both the knee and the hip experiments when compared to the mechanical setup, possibly due to leg movements outside of the reference plane. Future work should focus on algorithms to compensate for the non-planar movements of the leg as well as better techniques for the leg to replicate circular movement like the experimental setup.