Heads or Tails? Cranio-Caudal Mass Distribution for Robust Locomotion with Biorobotic Appendages Composed of 3D-Printed Soft Materials

The addition of external mass onto an organism can be used to examine the salient features of inherent locomotion dynamics. In this biorobotics study general principles of systems in motion are explored experimentally to gain insight on observed biodiversity in body plans and prevalent cranio-caudal mass distributions. Head and tail mass can make up approximately 20% of total body mass in lizards. To focus on the effect of differential loading of the ‘head’ and the ‘tail’ we designed an experiment using weights of 10% total body mass connected to the front and rear at varying distances to simulate biological mass distribution. Additive manufacturing techniques with compliant materials were utilized to make the biomimetic limbs. Obstacle traversal performance was evaluated over 126 trials in a variety of Moment of Inertia (MOI) configurations, recording pitch angles. Results showed that a forward-biased MOI appears useful for regaining contact in the front wheels during obstacle negotiation, while large passive tails can have a destabilising effect in some configurations. In our robophysical model, we explore both wheeled and legged locomotion (‘whegs’), and additionally examine damping the motion of the chassis by utilizing soft non-pneumatic tires (‘tweels’) which reduce body oscillations that arise from locomotion on irregular terrain.