Cooperative Transportation by Two 4-legged Robots

    There are many research activities on cooperative transportation by two or four wheeled robots. The wheeled robot, however, cannot move around even on a little difference in level, and it is difficult for the wheeled robots system to transport an object in real world. This study tackles with cooperative transportation by two autonomous 4-legged robots on irregular terrain where there exist steps or uneven ground. In case of legged robots, it is difficult to transport applying the same control methods that are effective in the wheeled robots. It is because that in the legged robot’s walking the robot body vibrates too seriously to sense the accurate configuration (position and orientation) of an object on the robot. It is also because the transportable load of legged robot is smaller than that of wheeled robot, and it is difficult to equip the legged robot with a large and complex mechanism to grasp an object. Hence it is important to use a simple mechanical end-effector to avoid intolerable internal force between the robot and the object.
    This study adopts the following two approaches: (A) the method to grasp an object by elastic grasping mechanism and (B) the method to put an object on a supporting part. Method (A) adopts a grasping mechanism of elastic rubber to grasp an object and sense the amount of force between two robots (Fig. 1). Each robot controls its walking velocity and walking phase by its own information of force. The control of walking velocity maintains the distance between robots. The control of walking phase makes vertical vibration synchronized with that of the other robot. Method (B) adopts a supporting part on which an object is put with a force sensor, limit-switches, and stoppers to prevent the object from falling off the supporting part (Fig. 2). Each robot senses the approximate position and orientation of an object on its own supporting part and autonomously decides its own motion to construct a rule based transportation system.
    The two robots compose a leader-follower system. The motion of each robot consists of three hierarchies: (a) the motion based on a given gait, (b) the motion of local avoidance around each foot based on contact sensors, and (c) the motion based on the object configuration. Moreover the motion that follows the given trajectory is added to the leader robot. In both method (A) and (B), there is no explicit communication between the robots, but cooperation is maintained by the motion (c) to result in transporting an object.


  1. Yasumichi Aiyama, et al.: "Cooperative Transportation by Two 4-legged Robots with Implicit Communication", 4th International Symposium on Distributed Autonomous Robotic Systems, 1998.

    Fig.1 Walking velocity control and walking phase        Fig.2 Rule based control using supporting mechanism
            control using grasping mechanism