Abstract: To improve the motion stability of the underactuated spherical robot at the low velocity, a spherical robot equipped with a high-rate flywheel is presented. The mechanical structure of the robot is mainly composed of a shell, an inner case, a long-axis motor, a short-axis motor and a rotating flywheel. The simplified model and structural parameters are given. Based on the motion analysis, the dynamic model of the robot is derived by the constrained Lagrangian method, and the dynamic model is simplified. The model analysis results show that the gyroscopic effects of high-rate rotating flywheel can reduce the influence of the external disturbing torques, thereafter the yaw angle stability is enhanced when the robot moves at the low velocity. The linear displacement experiments are carry out on the plastic ground, and compared with the flywheel stopping, when the flywheel rotates at the high-rate, the yaw angle deviation and yaw angular velocity fluctuation of the robot decrease during the movement, and the center of the robot deviated from the straight decreases at the end of the movement. The experiments result verifies the effectiveness of the design.

Key words: Dynamics model, Flywheel, Motion stability, Spherical robot