  This article proposes a mobile quad-arm robot: ARMS, which unifies
wheeled-legged tripedal locomotion, wheeled locomotion, and quad-arm
loco-manipulation. ARMS's four arms have different mechanisms and are partially
designed to be general-purpose arms for the hybrid locomotion and
loco-manipulation. One three-degree-of-freedom (DOF) arm has an active wheel,
which is used for wheeled-legged tripedal walking and wheeled driving with
passive wheels attached to the torso. Two three-DOF general-purpose arms are
series elastic and used for wheeled-legged tripedal walking, object grasping,
and manipulation. The upper two-DOF arm is used for manipulation only; its
position and orientation are determined by coordinating all arms. Each motor is
controlled by an angle controller and trajectory modification with angle,
angular velocity, angular acceleration, and torque constraints. ARMS was
verified with seven experiments involving joint control, wheeled-legged
locomotion, wheeled locomotion and grasping, slope locomotion, block terrain
locomotion, carrying a bag, and outdoor locomotion.
