This paper presents the design and analysis of a reduced degree-of-freedom Robotic Modular Leg (RML) mechanism used to construct a quadruped robot. This mechanism enables the robot to perform forward and steering locomotion with fewer actuators than conventional quadruped robots. The RML is composed of a double four-bar mechanism that maintains foot orientation parallel to the base and decouples actuation for simplified control, reduced weight and lower cost of the overall robotic system. A passive suspension system in the foot enables a stable four-point contact support polygon on uneven terrain. Foot trajectories are generated and synchronized using a trot and modified creeping gait to maintain a constant robot body height, horizontal body orientation, and provide the ability to move forward and steer. The locomotion principle and performance of the mechanism are analyzed using multi-body dynamic simulations of a virtual quadruped and experimental results of an integrated RML prototype.

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