This paper proposes a control method for locomotion mode transformation of a mobile robot with wheel-arms. The proposed method aims at transformation from a four-wheeled mode for high speed mobility to an inverted pendulum mode, which has advantages of high viewing position and small turning radius. Since the initial state of the system is far away from the target equilibrium point of the wheeled inverted pendulum system, we use a nonlinear controller based on sliding mode control. In contrast that the previous transformation methods cannot control the robot velocity until the robot body is lifted up, the proposed method can take into account the robot velocity from the beginning of the transformation, which enables to complete the transformation in a smaller space. To analyze the asymptotic stability of the control system on the sliding surface, we derive an invariant set in which the system state converges to the origin without going out. Furthermore, the effectiveness of the proposed method is demonstrated in both simulations and real robot experiments.

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