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ItemWalking and running on yielding and fluidizing ground(Georgia Institute of Technology, 2012-07) Qian, Feifei ; Zhang, Tingnan ; Li, Chen ; Masarati, Pierangelo ; Birkmeyer, Paul ; Pullin, Andrew ; Hoover, Aaron ; Fearing, Ronald S. ; Golman, Daniel I.We study the detailed locomotor mechanics of a small, lightweight robot (DynaRoACH, 10 cm, 25 g) which can move on a granular substrate of closely packed 3 mm diameter glass particles at speeds up to 50 cm/s (5 body length/s), approaching the performance of small, high-performing, desert-dwelling lizards. To reveal how the robot achieves this high performance, we use high speed imaging to capture kinematics, and develop a numerical multi-body simulation of the robot coupled to an experimentally validated discrete element method (DEM) simulation of the granular media. Average forward speeds measured in both experiment and simulation agreed well, and increased non-linearly with stride frequency, reflecting a change in the mode of propulsion. At low frequencies, the robot used a quasi-static “rotary walking” mode, in which the granular material yielded as the legs penetrated and then solidified once vertical force balance was achieved. At high frequencies, duty factor decreased below 0.5 and aerial phases occurred. The propulsion mechanism was qualitatively different: the robot ran rapidly by utilizing the speed-dependent fluid-like inertial response of the material. We also used our simulation tool to vary substrate parameters that were inconvenient to vary in experiment (e.g., granular particle friction) to test performance and reveal limits of stability of the robot. Using small robots as physical models, our study reveals a mechanism by which small animals can achieve high performance on granular substrates, which in return advances the design and control of small robots in deformable terrains.
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ItemMulti-functional foot use during running in the zebra-tailed lizard (Callisaurus draconoides)(Georgia Institute of Technology, 2012-05) Li, Chen ; Hsieh, S. Tonia ; Goldman, Daniel I.A diversity of animals that run on solid, level, flat, non-slip surfaces appear to bounce on their legs; elastic elements in the limbs can store and return energy during each step. The mechanics and energetics of running in natural terrain, particularly on surfaces that can yield and flow under stress, is less understood. The zebra-tailed lizard (Callisaurus draconoides), a small desert generalist with a large, elongate, tendinous hind foot, runs rapidly across a variety of natural substrates. We use high-speed video to obtain detailed three-dimensional running kinematics on solid and granular surfaces to reveal how leg, foot and substrate mechanics contribute to its high locomotor performance. Running at ~10bodylengthss–1 (~1ms–1), the center of mass oscillates like a spring-mass system on both substrates, with only 15% reduction in stride length on the granular surface. On the solid surface, a strut-spring model of the hind limb reveals that the hind foot saves ~40% of the mechanical work needed per step, significant for the lizardʼs small size. On the granular surface, a penetration force model and hypothesized subsurface foot rotation indicates that the hind foot paddles through fluidized granular medium, and that the energy lost per step during irreversible deformation of the substrate does not differ from the reduction in the mechanical energy of the center of mass. The upper hind leg muscles must perform three times as much mechanical work on the granular surface as on the solid surface to compensate for the greater energy lost within the foot and to the substrate.
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ItemComparative studies reveal principles of movement on and within granular media(Georgia Institute of Technology, 2010-06) Ding, Yang ; Gravish, Nick ; Li, Chen ; Maladen, Ryan D. ; Mazouchova, Nicole ; Sharpe, Sarah S. ; Umbanhowar, Paul B. ; Goldman, Daniel I.Terrestrial locomotion can take place on complex substrates such as leaf litter, debris, and soil that flow or solidify in response to stress. While principles of movement in air and water are revealed through study of the hydrodynamic equations of fluid motion, discovery of principles of movement in complex terrestrial environments is less advanced in part because describing the physics of limb and body interaction with such environments remains challenging. We report progress our group has made in discovering principles of movement of organisms and models of organisms (robots) on and within granular materials (GM) like sand. We review current understanding of localized intrusion in GM relevant to foot and body interactions. We discuss the limb-ground interactions of a desert lizard, a hatchling sea turtle, and various robots and reveal that control of granular solidification can generate effective movement. We describe the sensitivity of movement on GM to gait parameters and discuss how changes in material state can strongly affect locomotor performance. We examine subsurface movement, common in desert animals like the sandfish lizard. High speed x-ray imaging resolves subsurface kinematics, while electromyography (EMG) allows muscle activation patterns to be studied. Our resistive force theory, numerical, and robotic models of sand-swimming reveal that subsurface swimming occurs in a “frictional fluid” whose properties differ from Newtonian fluids.
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ItemSensitive dependence of the motion of a legged robot on granular media(Georgia Institute of Technology, 2009-03-03) Li, Chen ; Umbanhowar, Paul B. ; Komsuoglu, Haldun ; Koditschek, Daniel E. ; Goldman, Daniel I.Legged locomotion on flowing ground (e.g., granular media) is unlike locomotion on hard ground because feet experience both solid- and fluid-like forces during surface penetration. Recent bioinspired legged robots display speed relative to body size on hard ground comparable with high-performing organisms like cockroaches but suffer significant performance loss on flowing materials like sand. In laboratory experiments, we study the performance (speed) of a small (2.3 kg) 6-legged robot, SandBot, as it runs on a bed of granular media (1-mm poppy seeds). For an alternating tripod gait on the granular bed, standard gait control parameters achieve speeds at best 2 orders of magnitude smaller than the 2 body lengths/s (≈60 cm/s) for motion on hard ground. However, empirical adjustment of these control parameters away from the hard ground settings restores good performance, yielding top speeds of 30 cm/s. Robot speed depends sensitively on the packing fraction φ and the limb frequency ω, and a dramatic transition from rotary walking to slow swimming occurs when φ becomes small enough and/or ω large enough. We propose a kinematic model of the rotary walking mode based on generic features of penetration and slip of a curved limb in granular media. The model captures the dependence of robot speed on limb frequency and the transition between walking and swimming modes but highlights the need for a deeper understanding of the physics of granular media.