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