Analysis of an Actuated Two Segment Leg Model of Locomotion

Rao, Nikhil V. M.S.M.E, Purdue University, December 2013. Analysis of an Actuated Two Segment Leg Model of Locomotion. Major Professor: Dr. Justin E. Seipel, School of Mechanical Engineering. Research studies on dynamic models of legged locomotion have generally focused on telescoping-type leg models. Such telescoping spring loaded inverted pendulum (SLIP) models have been able to accurately predict observed center of mass (CoM) trajectories. There have been comparatively fewer studies on dynamics of locomotion with segmented legs. Some earlier studies on the dynamics due to leg segmentation appear straightforward. For example, a simple model with the only joint moment being due to a passive sprung knee has been shown to behave similarly to a telescoping spring-mass model. However, in real-life animal locomotion, there are multiple jointmoments acting at the hip, knee and the ankle joints. The joint-moments could act together to cause the whole body dynamics to diverge from those of the canonical telescoping spring mass model. The focus of this thesis is to understand the combined effect when hip and knee moments act together. In particular, this work will analyze a lumped mass two segment leg model with a hip-torque actuation and a passive sprung knee. Such mechanisms also represent real life scenarios such as motions of above knee amputees with a passive knee prosthetic. A key finding is that the governing equations for the two-segment (knee) version have a distinct structure when compared to the telescoping version of SLIP. The two-segment model with a hip-moment and a knee spring influences forces acting on the mass center in a more complex way and, unlike the telescoping leg, leads to an active force along the leg. The effect of this active force on the whole body dynamics can have an overall similar outcome to that of a spring component acting along the leg, and so is capable of replacing the spring at the knee. This result also