Maneuverability is among the most important aspects of mobility, and perhaps the most challenging. Steady, periodic locomotion affords parsimonious representation by models consisting of relatively simple neural and mechanical oscillators. Embodiment of these oscillators in low degree-of-freedom underactuated legged robots has produced fast, stable running, but has not recapitulated the remarkable locomotor performance of legged animals. The presence of a mobile, highly actuated spine is one feature of natural runners notably missing from both simple models of locomotion and extant high-performance legged machines. This dissertation takes the first steps toward understanding the locomotor function of such “core” actuation in the form of body bending and tail swinging through a set of experiments in animals and robots that quantify the benefits and drawbacks of an active spine in high-agility legged locomotion.
In Chapter 1, we develop a comparative framework for the design of actuated inertial appendages for planar, aerial reorientation. We first introduce the Inertial Reorientation Template, the simplest model of powered inertial reorientation behavior, and leverage its linear dynamics to reveal the design constraints linking a task with the body designs capable of completing it. We then examine three cases of more practicable inertial reorientation morphology – swinging tails, flailing limbs, and spinning wheels – and advance a notion of “anchoring” whereby a judicious choice of physical design in concert with an appropriate control policy yields a system whose closed loop dynamics are sufficiently captured by the template as to permit all further design to take place in its far simpler parameter space. This approach is effective and accurate over the diverse design spaces afforded by existing platforms, enabling performance comparison through the shared task space. We analyze examples from the literature and find advantages to each body type, but conclude that tails provide the highest potential performance for reasonable designs.
In Chapter 2, we bring the discussion back to earth, exploring whether grounded locomotion could benefit from inertial reorientation. We use a prey capture task to induce large, rapid turns in lizards and investigate the interaction between external (legged ground interaction) and internal (inertial body shape change) sources of mobility. We introduce a more detailed, horizontal plane IR tail anchor model and use it to estimate angular momentum during the animals’ maneuvers and to predict maximum reorientation due to body shape change. We find evidence that powered inertial reorientation actively aids turning, leading to much higher reorientation performance than would be expected from a rigid-bodied animal. Inertial reorientation behavior may serve multiple functions during a large terrestrial turn, providing a dependable source of rotation independent of external ground reaction forces, and reducing the need for braking forces by stabilizing orientation in task space.
In Chapter 3, we examine the wider context of legged planar maneuvering in which the lizards’ body flexibility plays a role. We use the same prey capture behavior to probe the potential benefits of the elongate, sprawled-posture body form of lizards in negotiating the task tradeoffs of a combined rotation and translation in the plane. We find evidence supporting the hypothesis that sprawled posture enhances maneuverability by permitting motion and force production in all directions without large postural shifts. We expect high-agility, maximal effort maneuvers to be constrained into stereotypy; instead, we find huge variability in gait and limb forces even as task behavior is relatively consistent. These features of lizard legged maneuverability align well with the inertial reorientation afforded by their elongate flexible bodies to provide incredible robustness to environmental perturbations.