Towards Terrain Adaptablity: In Situ Transformation of Wheel-Biped Robots

Most existing bipedal robots can only move with either their wheels or feet. Even if some of them are capable of transforming between these two motions, they need to change their configuration dramatically. In order to truly combine the advantages of wheeled and footed robots, in this letter, an in situ transformation method is investigated and implemented on a wheel-biped transformable robot SR600-II. In situ transformation means a transforming process with minimum position and configuration changes. It includes two processes: foot-to-wheel (FtW) and wheel-to-foot (WtF) transformations, both of which have to pass by transition states (termed critical states) that both foot and wheel contact with floor. At critical states, the robot has to meet both constraints of wheeled and footed balances. For footed balance, the center of mass (CoM) needs to fall within the support polygon formed by the feet. In order to maintain in situ wheeled balance, the upper body posture needs to be adjusted in real-time during the transformation process to make the vertical projection of the CoM be kept between the two wheels. To achieve a smooth transformation and minimize changes in robot joints, a projection-based algorithm is proposed to adjust the robot posture to comply with both constraints. Simulations and experiments on the SR600-II prototype have validated the effectiveness of the proposed design and control strategy for wheel-biped transformable robots.