Robophysical Investigation of Root Circumnutation through Heterogeneous Environment

Circumnutation, a cyclic endogenous circular pattern exhibited by the tip of a growing root, occurs in a diversity of plants, but its function is not fully understood. To investigate the hypotheses that such motion facilitates substrate penetration and exploration, we built a planar soft robot [Hawkes et al. 2017], which grows from the tip like a plant root and can bend in 2D space by oscillating inflation of the series pneumatic artificial muscles (sPAMs) arranged on the two sides. Existing work observed force reduction effects from circumnutation in homogeneous granular material [Dottore et al. 2016]. Here we demonstrated that tip oscillation aids the robot to penetrate heterogeneous environment, e.g. hard obstacles, by growing the robotic root into a lattice of rough cylinders (d=8cm) distributed uniformly on a board (120X120cm^2). Systematic variation of initial robot positions revealed that the non-oscillating tip strategy led to an increased probability to become pinned to obstacles and unable to grow more than 23.8±19.7cm; the oscillating tip penetrated the lattice significantly further, 55.2±24.9cm. The results show that without complicated control and sensing mechanism, oscillatory movement of a growing structure enables robust navigation in a heterogeneous environment.