41.1 Saturday, Jan. 5 Locomotion on heterogeneous granular substrates QIAN, F*; GOLDMAN, D/I; Georgia Tech; Georgia Tech firstname.lastname@example.org
Natural particulate substrates like deserts are often composed of collections of multi-size particles: fine sand, pebble, and boulders. While much is known about locomotion on hard ground and increasingly on homogeneous granular media like fine sand, the principles by which organisms and robots locomote over heterogeneous granular substrates are unexplored. To investigate how particle size and distribution affect speed and stability, we performed laboratory experiments in a legged robot. Our hexapedal robot (15 cm, 150 g) used an open-loop alternating tripod gait and c-shaped rigid plastic limbs (radius 1.5 cm). We filled a trackway (75 cm long, 30 cm wide) with 3 mm glass particles (“sand”) and two parallel lines of eight 2.54 cm large glass particles (“boulders”) embedded one-quarter within and separated by 10 cm. Without the boulders, for a limb frequency 3 Hz, the robot moved forward at 1.7 BL/s. Forward speed oscillated periodically in a run, and run-to-run variation in standard deviation of speed was 0.50 ± 0.04 BL/s. Locomotion across the boulder field reduced the average speed to 1.2 BL/s. Large fluctuations in speed within a run and across runs were observed (standard deviation 0.69 ± 0.25 BL/s) resulting from a diversity of foot-boulder interaction modes. Of these we identified two important modes: 1) A slipping mode, where a leg contacted and slid near the top of a boulder, causing the robot to pitch, yaw and roll, while the boulder remained still or rotated against the sand. Large fluctuations in speed were observed. 2) A forced intrusion mode, where a leg forced a boulder to penetrate vertically into the sand, yaw was comparable to movement on sand, and instantaneous speed fluctuations were smaller than in mode 1. We conclude that non-trivial interaction effects can lead to complex locomotion dynamics even for a simple locomotor.