We developed MELEW-3, a quadruped-wheeled robot developed to combine the strengths of legged and wheeled locomotion systems. Unlike conventional quadruped-wheeled robots, MELEW-3 features legs with four degrees of freedom (DoFs) and an additional active wheel on each leg, totaling 20 DoFs. The leg employs a parallel mechanism with spherical joints that allows movement in three translational directions (forward/backward, left/right, and up/down) and rotation around the yaw axis. This configuration enables not only stable legged locomotion but also agile wheeled motion, such as slalom driving.

Each leg is powered by AK80-9 brushless DC motors, supported by the MD80 motor controller, enabling precise position control even during multiple rotations. The active wheels are directly driven and compactly designed, integrating the same type of motor to simplify system architecture. With a size of 500 × 690 × 580 mm and a mass of 28 kg, the robot is currently powered externally. The control system is built around a distributed architecture using three PCs: one for motion planning, one for sensor data processing, and one for motor control. These systems communicate via Robot Operating System (ROS), allowing for flexible and modular development.

A series of experiments demonstrated the robot’s basic capabilities. In leg flexion/extension tests, MELEW-3 could track target positions under a load of 23.5 kg, though some deviation due to actuator stiffness was observed. During wheeled motion tests, the robot exhibited stable acceleration and deceleration by maintaining its center of gravity within the support polygon. Notably, MELEW-3 successfully performed slalom maneuvers, which are challenging for typical 3-DoF legged-wheeled robots, thanks to its ability to control the wheel’s steering angle via the additional yaw DoF. Moreover, coordinated locomotion combining legged gaits and wheel rotation showed that the robot could move faster than by legged walking alone.

In conclusion, MELEW-3 demonstrates the effectiveness of integrating a 4-DoF leg structure with active wheels, enabling versatile mobility across complex terrains. Future work will focus on implementing autonomous navigation by enhancing state estimation and environmental recognition capabilities.