Adaptive Compliant Foot Design for Salamander Robots

Aquatic stepping gaits in animals arguably display higher speed performance as well as energetic efficiency compared to other gaits using the limbs (i.e walking). This suggest that the foot structure and function contributes at a great extent on the propulsive force generation. This work presents the design of a salamander foot, in which the dimensions, angle range, aspect ratios and the kinematics of different salamander species were condensed in simple parameters. The foot implementation was based in the compliant SoftHand design of Pisa/IIT, in which one motor actuates the whole foot. The prototype design parameters are scaled up from the dimensions of a Tiger salamander (Ambystoma tigrinum's) foot. The results from experiments using a motion capture system to retrieve the kinematics of the foot and the force plates to measure normal forces, allow to describe when and how each of the fingers act during the whole stride, impacting the ground reaction forces (GRFs). We attempt to provide a richer understanding in locomotion schemes of salamanders featuring robust ground placement and to make robotic platforms more accurate w.r.t. biology. Qualitative comparisons between the animal and the prototype show that the robotic foot is capable to generate a GRF pattern similar to that of the animals. As additional features, the foot also shows terrain adaptability and simultaneous high resilience to hitting obstacles during operation.