• robot locomotion
• service robots
• wheel-legged robot
• Gazebo

This work presents the study and design of a new lower body, based on a hybrid wheel-legged locomotion system, of ALTER-EGO, a two-wheeled robot with an anthropomorphic upper body designed for physical interaction. The purpose is to overcome ALTER-EGO's limits and also maintain advantages due to the two-wheeled configuration. Starting from the proposed idea for the robot’s lower body, the mathematical system model is defined, the robot dynamic is studied, and implemented in Simulink. By the maximum joint torques analysis, optimal values of lower body links' length and the robot's associated critical configurations are determined. Finally, a control system that ensures the two-wheeled robot balancing and the transition from the two-wheeled to the four-wheeled locomotion mode with the following achievement of the robot's desired posture is implemented and simulated using ROS and Gazebo.