Unified modeling of longitudinal slip and longitudinal skid for grouser-wheeled rovers without slip ratio

When planetary rovers traverse across extensive megaripple deposits, some planetary rover’s wheels climbing up a slope will experience longitudinal slip, while others moving down a slope simultaneously will suffer longitudinal skid. It is important to establish a unified model of longitudinal slip and longitudinal skid for online control optimization and terrain mechanical parameters estimation. However, when the wheel running state changes from longitudinal skid to longitudinal slip, the equivalent shear deformation modulus of a same terrain estimated using traditional terramechanics models is different, while the estimated modified sinkage exponent of a same terrain suffers an unexpected break. It is difficult to establish a unified model with traditional terramechanics theories. A unified model without slip ratio for smooth wheels was first established using a switching function, and named as smooth unified model. The relative error of the drawbar pull estimated using the smooth unified model is less than 19.7 % compared with the experimental data. And a unified terramechanics model without slip ratio for grouser wheels was established by introducing the grouser effect coefficient and equivalent radius to the smooth unified model, and named as grouser unified model. The equivalent terrain mechanical parameters estimated using the grouser unified model change continuously without break, and the relative error of the estimated drawbar pull can be captured within 22.37 % compared with the experimental data. Finally, field experiments were conducted to validate the performance of the grouser unified model.