SPAR-Leg: Serial-Parallel Anthropomorphic Robotic Leg

This paper aims at a preliminary research on designing a hybrid mechanism inspired by human leg’s musculoskeletal architecture and locomotion capabilities. Complexity of the workspace solution in serial-parallel mechanisms limits the design approach. In order to have an estimation from the human leg geometry and reachable workspace, a dataset is collected from the human leg of 10 females and 10 males. In addition to the measurements done on the dimensions of this population, the reachable workspace of their leg is observed using MediaPipe pose estimation model. This is done by capturing full movement of the human leg for each person in front of two cameras. On the other hand, the musculoskeletal system of the human leg reveals that two serially attached parallel mechanisms are well-suited for the design. The upper mechanism, which is the thigh mechanism, includes 6 UPS links as the actuators, and a US constraint link to avoid elongation in the leg. Similarly, 3 UPS and a S constraint link form the lower mechanism. Two mathematical models based on geometric approaches are derived for the mechanisms. In order to reach a workspace fitted to that of human leg, an optimization process is performed on the mechanism. Due to the complexity of the whole model’s workspace, the optimization process is done on the upper mechanism to reach its maximum workspace. Based on the ratios between thigh and calf dimensions, the lower mechanism’s design parameters are obtained, and its workspace is verified to be optimal. Afterwards, a simple geometric calculation of the whole mechanism is done and compared with the results of workspace observation using MediaPipe. A brief simulation is done on the upper mechanism in two different movements in order to select suitable actuators based on the required forces. Finally, forward kinematics problem of the mechanism is solved and validated by performing simulations.