In future lunar/planetary exploration missions, wheeled rovers will be promising robots to perform wide-area explorations. Wheeled robots, however, are prone to slip in soft terrain: therefore, securing mobility will be a critical issue. This paper first describes a soil-wheel interaction model which is simply found from Coulomb's friction law. The model enables slip-ratio estimation from a combination of wheel load and traction. Second, a simple control methodology, wheel load distribution control, for maintaining vehicle mobility is discussed. Moreover, a six-wheeled robot which implements the control system was developed. The robot has six wheels and six vertical suspensions with built-in linear actuators. Load cells are installed between each wheel and suspension to monitor the wheel load, and positions of wheels are individually controlled by moving the suspensions up and/or down so as to keep the desired wheel loads. In this study, a simple algorithm for distributing equivalent load to every wheel was implemented. A simulation and a series of experiments in an outdoor environment were performed and the effectiveness of the control system was examined.
