Proprioceptive control of an over-actuated hexapod robot in unstructured terrain

Legged robots such as hexapods have the potential to traverse unstructured terrain. This paper introduces a novel hexapod robot (Weaver) using a hierarchical controller, with the ability to efficiently traverse uneven and inclined terrain. The robot has five joints per leg and 30 degrees of freedom overall. The two redundant joints improve the locomotion of the robot by controlling the body pose and the leg orientation with respect to the ground. The impedance controller in Cartesian space reacts to unstructured terrain and thus achieves self-stabilizing behavior without prior profiling of the terrain through exteroceptive sensing. Instead of adding force sensors, the force at the foot tip is calculated by processing the current signals of the actuators. This work experimentally evaluates Weaver with the proposed controller and demonstrates that it can effectively traverse challenging terrains and high gradient slopes, reduce angular movements of the body by more than 55% and reduce the cost of transport (up to 50% on uneven terrain and by 85% on a slope with 20 °). The controller also enables Weaver to walk up inclines of up to 30 °, and remain statically stable on inclines up to 50 °. Furthermore, we present a new metric for legged robot stability performance along with a method for proprioceptive terrain characterization.