  Achieving effective locomotion on diverse terrestrial substrates can require
subtle changes of limb kinematics. Biologically inspired legged robots
(physical models of organisms) have shown impressive mobility on hard ground
but suffer performance loss on unconsolidated granular materials like sand.
Because comprehensive limb-ground interaction models are lacking, optimal gaits
on complex yielding terrain have been determined empirically. To develop
predictive models for legged devices and to provide hypotheses for biological
locomotors, we systematically study the performance of SandBot, a small legged
robot, on granular media as a function of gait parameters. High performance
occurs only in a small region of parameter space. A previously introduced
kinematic model of the robot combined with a new anisotropic granular
penetration force law predicts the speed. Performance on granular media is
maximized when gait parameters minimize body acceleration and limb
interference, and utilize solidification features of granular media.
