Locomotion Control Functions for the Active Chassis of the MMX Rover

JAXA's Martian Moons eXploration Mission (MMX) includes the delivery of an exploration rover to the Mars moon Phobos in 2026, engineered by the Centre National d'Études Spatiales (CNES) and the German Aerospace Center (DLR). On Phobos, the gravity is about two thousand times lower than on Earth, which is why it is also called a milli-g environment. While the actual surface of Phobos is largely unknown, it is agreed within mission team that areas covered with regolith are to be expected. The design of the rover includes four actuated legs, with one rotational degree of freedom (DOF) each, and four individually driven, non-steerable wheels. The first task of the rover after landing on the surface of Phobos will be to deploy itself from its cruise configuration and to stand up on its wheels. Due to the rover dimensions, a full rotation of the legs and a wheel slip compensation are essential for this task. During operations, the locomotion system needs to provide drive and steer, as well as point turn abilities. Furthermore, the solar panel sun pointing, as well as the adjustment of scientific instruments, require that the rover aligns its body orientation and alters its body-to-ground distance. To satisfy all these requirements to the locomotion system of the MMX rover, appropriate locomotion control functions were developed. For driving curves and performing point turns, the skid steering method is applied and an “inching” locomotion mode for especially soft and steep terrain is adapted. In this inching locomotion, the front and rear wheel pair move alternately while the rover body moves up and down, which leads to enhanced traction performance compared to conventional driving in soft sand. This implies lower wheel slippage and sinkage resulting in a higher safety of the full rover system. The body orientation function, which is based on a kinematic control as well, provides a well coordinated movement and zero longitudinal slippage of the wheels during its execution. In this paper, a detailed description of the control algorithms is given and results from lab tests are presented and discussed. In a successful mission, these locomotion control functions will be the first ones actuating a wheeled mobile robot in milli-g environment.