Perrin E. Schiebel

Tail use improves performance on soft substrates in models of early vertebrate land locomotors

Animal and robot experiments explore the use of a tail in aiding terrestrial locomotion. In the evolutionary transition from an aquatic to a terrestrial environment, early tetrapods faced the challenges of terrestrial locomotion on flowable substrates, such as sand and mud of variable stiffness and incline. The morphology and range of motion of appendages can be revealed in fossils; however, biological and robophysical studies of modern taxa have shown that movement on such substrates can be sensitive to small changes in appendage use. Using a biological model (the mudskipper), a physical robot model, granular drag measurements, and theoretical tools from geometric mechanics, we demonstrate how tail use can improve robustness to variable limb use and substrate conditions. We hypothesize that properly coordinated tail movements could have provided a substantial benefit for the earliest vertebrates to move on land.

Shape-Based Compliance in Locomotion

Having many degrees of freedom is both a blessing and a curse. A mechanism with a large number of degrees of freedom can better comply to and therefore better move in complex environments. Yet, possessing many degrees of freedom is only an advantage if the system is capable of coordinating them to achieve desired goals in realtime. This work supports the belief that a middle layer of abstraction between conventional planning and control is needed to enable robust locomotion of articulated systems in complex terrains. The basis for this abstraction is the notion that a system’s shape can be used to capture jointto-joint coupling and provide an intuitive set of controllable parameters that adapt the system to the environment in real time. This paper presents a generalizable framework that specifies desired shapes in terms of shape functions. We show how shape functions can be used to link low-level controllers to high-level planners in a compliant control framework that directly controls shape parameters. The resultant shape-based controllers produce behaviors that enable robots to robustly feel their way through unknown environments. This framework is applied to the control of two separate mechanisms, a snake-like and a hexapod robot.

Collisional Diffraction Emerges from Simple Control of Limbless Locomotion

Snakes can utilize obstacles to move through complex terrain, but the development of robots with similar capabilities is hindered by our understanding of how snakes manage the forces arising from interactions with heterogeneities. To discover principles of how and when to use potential obstacles, we studied a desert-dwelling snake, C. occipitalis, which uses a serpenoid template to move on homogeneous granular materials. We tested the snake in a model terrestrial terrain—a single row of vertical posts—and compared its performance with a robophysical model. Interaction with the post array resulted in reorientation of trajectories away from the initial heading. Combining trajectories from multiple trials revealed an emergent collisional diffraction pattern in the final heading. The pattern appears in both the living and robot snake. Furthermore, the pattern persisted when we changed the maximum torque output of the robot motors from 1.5 N-m to 0.38 N-m in which case local deformation of the robot from the serpenoid curve appears during interaction with the posts. This suggests the emergent collisional diffraction pattern is a general feature of these systems. We posit that open-loop control of the serpenoid template in sparse terrains is a simple and effective means to progress, but if adherence to a heading is desired more sophisticated control is needed.