58.6 Thursday, Jan. 5 Towards a terramechanics for legged locomotion on granular media LI, Chen*; GOLDMAN, Daniel I.; Georgia Tech; Georgia Tech email@example.com
During locomotion on sand, animals and robot models interact with granular substrates to generate thrust and lift. While resistance forces on simple shapes like discs and plates during intrusion along vertical or horizontal trajectories are well studied, no general model yet exists to predict resistance forces for intrusion along complex trajectories during footsteps. Recently a granular resistive force theory (RFT) was used to model forces on an intruder moving in the horizontal plane at a fixed depth, e.g. a sand-swimming lizard. The RFT divides the intruder into small elements each generating forces that are assumed independent. Summation of the element forces predicts net thrust and drag. To begin to create a terramechanics for legged locomotion on granular media, we extend the RFT to intrusion in the sagittal plane. We measure the lift and drag on a small plate (3.8 x 2.5 x 0.6 cm3) moving in granular media (1 mm diameter poppy seeds, 0.3 mm and 3 mm glass particles) of controlled compaction as a function of depth, angle of attack, and direction of motion. Both lift and drag increase with depth and depend sensitively on angle of attack and direction of motion at given depth. Lift, but not drag, is an order of magnitude larger for intrusion into than out of the media due to symmetry breaking by gravity. For a model c-shaped limb rotating about a fixed axle, integration of plate forces captures the net lift and thrust measured in experiments. The RFT predicts that reversal of the c-shaped limb results in a smaller maximal lift with significant negative lift (suction) during the late phase of rotation, which is confirmed by experiments. In accord with difference in lift, on poppy seeds a small bio-inspired legged robot (15 cm, 80 g) walks 50% faster at any frequency with c-shaped limbs than with reversed c-shaped limbs.