Slip-ratio-coordinated control of planetary exploration robots traversing over deformable rough terrain

Wheeled exploration robots are prone to slip during locomotion on deformable rough planetary terrain, which leads to loss of velocity and extra consumption of energy. Experimental results show that the power required for driving a wheel is an increasing function of its slip ratio; further, the tractive efficiency decreases rapidly after it reaches a peak value when the slip ratio is between 0.05 and 0.2. In this study, wheel-soil interaction terramechanics, which considers the slip ratio as an important state variable, is applied to analyze the quasi-static equations of a planar robot system. The slip ratios of wheels are controllable, but the degree of freedom is the number of wheels minus 1. A generalized algorithm for distributing the slip ratios of all the wheels of a robot to optimize the energy consumption is presented. Experimental and simulation results show that the “equal slip ratio” is at least a sub-optimal solution for optimizing energy consumption. Further, a more robust control method has been developed; this methods aims to equalize the slip ratios of all the wheels while maintaining a constant body velocity on rough terrains, without solving the values of the slip ratios. This method is verified by controlling a virtual four-wheeled robot using dynamics simulations.