Effect of Tire Design and Steering Mode on Robotic Mobility in Barren Terrain

Abstract

ARobotic tasks call for a range of steering activity: one extreme is highway driving with negligible turning for hundreds of kilometers; another is forklift handling, which calls for agile turning. Steady state turning of a wheeled vehicle on natural terrain with slow but capable locomotors characteristic of planetary robotic vehicles is the scope of this research. Two tire designs were developed, implemented and evaluated aboard the Nomad robot, enabling a comparative study of their effect on mobility and steering. Rigid tires, utilized on desert terrain, are relevant to planetary exploration where elastomeric tires are inappropriate. Pneumatic tires, specialized for Antarctic terrain, achieved performance advantages on ice. The research presented here investigates the collateral issues of steering and mobility for the two tire designs. Experiments involve a single robot that can exhibit both skid and explicit steering while driving in steady state circles on gravel terrain. Skid steering is accomplished through the creation of a differential velocity between the inner and outer wheels. In explicit steering, a change in the heading of the wheels causes a modification to the vehicle heading. Power draw, individual wheel torque, and position data have been gathered for the purpose of quantifying performance. The experimental results show that power and torque for skid and explicit turning degenerate to equal values at infinite radius (straight driving). As the turn radius decreases from that of straight driving to that of a point turn, greater power and torque are necessary as larger slip angles are induced. In the limiting case of a point turn, with both rigid and pneumatic tires, the power for skid steering is on the order of double that of an explicit point turn. For explicit steering, with turns greater than a radius of 4 m, the pneumatic tires exhibit a lower power draw than rigid tires.