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This paper introduces a novel approach to control a wheeled inverted pendulum when a disturbance is applied by a human. The interaction between a human and a wheeled inverted pendulum involves a pulling or pushing force. This type of action is a severe disturbance for a wheeled inverted pendulum, as the wheeled inverted pendulum tends to maintain its initial position if there is no desired input. Thus, there are many possibilities for the wheeled inverted pendulum to be unstable as a result of interactions with humans. To solve this problem, the control algorithm of a wheeled inverted pendulum was designed to move in coordination with the external force of a human. This control algorithm is termed human-friendly motion control. It contains an optimal controller using a full-state feedback and a reduce-order disturbance observer. The disturbance torque from a human was estimated, and the estimated disturbance torque was used to generate a position reference for the human-friendly motion. This control algorithm keeps the wheeled inverted pendulum stable even when a severe disturbance is applied.