(Georgia Institute of Technology, 2015-08-18)
Karsai, Andras
The purpose of this investigation is to identify and calculate the forces that occur on an impulsive object as it intrudes into granular media. The system analyzed is a computational model of a sinusoidally actuated spring-mass system jumping on a bed of granular material. Various types of ground reaction forces on the robot's foot are investigated and their parameters are systematically varied to compare to experimental data taken from the real-world jumping robot system. Different types and combinations of ground reaction forces are investigated since a single force type was found to be insufficient to fully explain the experimental system's dynamics. The mechanics of this setup are modeled as a set of ordinary differential equations, which are computationally solved to determine the jumping mechanics. Maximal jump heights are calculated across a wide variety jumping motions and granular media densities with different types of ground reaction force laws.The relations that are investigated include a depth-dependent spring-like force, a velocity-squared-dependent force, and an added-mass force. The results of finding a well-fitting combination of force laws across many jump types and volume fractions can be used to imply a valid comprehensive force law for impulsive motion on a granular surface. The anticipated outcome is that there exists such a comprehensive force law, but each force type's contribution will vary as a function of volume fraction. Finding optimal jumping motions using this comprehensive law may lead to better implementations of impulsive commands in fields such as robotics and biomechanics where granular material is involved.
(Georgia Institute of Technology, 2015-06-30)
Mcinroe, Benjamin
In the evolutionary transition from an aquatic to a terrestrial environment, early walkers adapted to the challenges of locomotion on complex, flowable substrates (e.g. sand and mud). Our previous biological and robotic studies have demonstrated that locomotion on such substrates is sensitive to both limb morphology and kinematics. Although reconstructions of early vertebrate skeletal morphologies exist, the kinematic strategies required for successful locomotion by these organisms have not yet been explored. To gain insight into how early walkers contended with complex substrates, we developed a robotic model with appendage morphology inspired by a model analog organism, the mudskipper. We tested mudskippers and the robot on different substrates, including rigid ground and dry granular media, varying incline angle. The mudskippers moved effectively on all level substrates using a fin-driven gait. But as incline angle increased, the animals used their tails in concert with their fins to generate propulsion. Adding an actuated tail to the robot improved robustness, making possible locomotion on otherwise inaccessible inclines. With these discoveries, we are elucidating a minimal template that may have allowed the early walkers to adapt to locomotion on land.