Meeting Abstract
Substrate dynamics can have a significant impact on terrestrial locomotion behaviors; for example, movement strategies that perform well on hard ground can lead to failure on yielding substrates like granular media. We performed a robophysical experiment to investigate how granular media dynamics can be manipulated to modify performance in a simple behavior, jumping. Using recent models of granular bulk reaction forces during rapid intrusions [Aguilar & Goldman, Nature Physics, 2015] and a numerical optimal control algorithm, we control a jumping robot to leap from a bed of loose packed poppy seeds to a commanded apex using minimal motor work. The optimal controller was able to exploit the terrain dynamics to jump to commanded heights; the patterns of robot self-deformation differed from those which produced similar heights on hard ground. For a subset of optimized behaviors, jumps overshot the commanded/predicted apex height by ~40%. In particular, this excessive jump height occurred when the intruding foot briefly (~40 ms) halted its penetration before resuming its downward thrust. As a result, we hypothesized the presence of additional un-modeled terrain dynamics which activate when the intruder pauses. In testing this hypothesis, intrusion force measurements confirmed the production of anomalous resistive force after a paused penetration. Back-scattered laser speckle measurements revealed that, after a paused intrusion, grain motion decayed on a similar timescale (10 to 100 ms) to the required pause time of the anomalous jumps. The findings of this mutualistic interaction between soft matter physics and optimal robotic control underscores how seemingly subtle substrate physics may potentially incentivize exotic locomotion behaviors in robots and organisms alike.
