Coordinated Body Bending Improves Performance of a Salamander-like Robot

Analyzing body morphology and limb-body coordination in animals that can both swim and walk is important to understand the evolutionary transition from an aquatic to a terrestrial environment. Based on previous salamander experiments (a modern analog to early tetrapods and performed by Hutchinson's group at RVC in the UK) we built a robophysical model of a salamander and tested its performance on yielding granular media (GM) of poppy seeds. Our servo-driven robot (405 g, 38 cm long) has four limbs, a flexible body, and an active tail. Each limb has two servo motors to control up/down and fore/aft positions of limb. A joint in the middle of the body controls horizontal bending. We assessed performance of the robot by changing the body bending limit from $0^{\circ}$to $90^{\circ}$and measured body displacement and power consumption over a few limb cycles at $0^{\circ}$and $10^{\circ}$sandy slope. We fixed the angle of the legs according to body to test the effect of body bending directly. On GM, step length increased from 0 to 9.5 cm at $0^{\circ}$ and 0 to 7 cm at $10^{\circ}$slope while the average power consumption increased $50\% $. A geometric mechanics model revealed that on level GM body bending was most beneficial when phase offset $180^{\circ}$from leg movements; increasing the maximum body angular bend from $45^{\circ}$to $90^{\circ}$ led to step length increases of up to $90\% $.