Wheeled autonomous rover design and control for snow-covered terrain

• Presents an analytic approach for robot wheel design for snow-covered terrain • Introduces a new mobility factor - drawbar pull to motor torque ratio to assess the ability to create positive drawbar pull given motor torque to assess the impact of wheel dimensions. • Presents a wide-wheeled robot that retains mobility without experiencing immobilization in soft snow. • Presents an alternative to traditional differential steering for wide-wheeled robots to reduce scrubbing through control of passive front and rear axle yaw degrees-of-freedom creating symmetric and asymmetric steering modes. Two decades of research on mobility in snow-covered terrain resulting from fielding lightweight, autonomous robots that operate in snow reveal a need for design techniques that enable enhanced mobility in low cohesion snow. To facilitate such design, we present a study of the effect of wheel width and diameter on resistance torque, and we introduce a new mobility factor (drawbar pull to motor torque ratio). Both observed field performance and an analytic study rooted in terramechanics conclude that wheel width has a larger impact than wheel diameter on increasing this mobility factor, and wide wheels simultaneously decrease sinkage. However, wide wheels increase scrubbing and torque required for differential steering, which is a preferred steering method for four-wheel drive mobile robots. Thus, wide wheels generally decrease maneuverability and are not common on autonomous robots. To reduce scrubbing and steering torque requirements, we present a four-wheel-drive rover that incorporates passive front and rear axle roll and yaw degrees of freedom and uses straightforward wheel-speed feedback controllers to maintain four-wheel contact with the terrain and to constrain poses of the robot. New poses enabled by the kinematics of this “floppy” robot including crab driving and asymmetric and symmetric steering. These poses in turn allow for wide wheels and low ground pressure while enhancing maneuverability relative to differential steering alone. An optional midframe joint provides a “push-pull” drive option for negotiating otherwise immobilizing terrain.