We investigate the functional demands of bipedal running with a focus on the concept of postural stability.
Bipedalism has evolved in many lineages ranging from reptiles, avians, theropods to primates. These lineages exhibit diversified morphologies (e.g. limb segment length, mass, orientation), which yield to different locomotion characteristics. Amongst bipeds, birds demonstrate exceptional agility, locomotion efficiency and terrain traversability despite having limitations on actuation and sensory delays [Alexander, 1992, More et al 2010]. We want to understand the components required to generate such versatile motion and analyze the requirements it imposes on certain control policies.
The animal trunk is a fundamental morphological component, and it effects both locomotor kinetics and kinematics. It accounts for almost 50% of total body mass in humans, and 70-80% in birds. Higher ratios are possible in bipedal robots [ ]. Stabilizing a heavy trunk with small support base is a major challenge in bipeds. Hence, trunk orientation is integral for motion generation. We consider two two distinct animal trunk postures. Orthograde (upright) posture (such as humans), and pronogade (horizontal) posture (birds, or theropods). In order to isolate the effect of the trunk, we utilize a simplified spring loaded inverted pendulum model, extended with a rigid trunk (similar to T-SLIP, Maus et al, 2008). We focus on the effect of trunk orientations on the hip torque and ground reaction profiles for different control strategies.