Control of passive object handling robot with free joint for reducing human assistive force

In this paper, we introduce a passive mobile robot called PRP to realize a transportation of an object in cooperation with a human which is developed based on a concept of passive robotics. The PRP (passive robot porter) consists of three omni-directional wheels with servo brakes and a controller. It can manipulate an object by controlling an external force/moment applied by a human based on the control of the servo brakes. Therefore, if the required external force and moment for realizing the manipulation of the object are become large, the burden of the human would increase and the controllability of the robot would decrease. To overcome these problems, in this paper, we propose a system in which the PRP carries the object through a free joint. By using the free joint, the orientation of the object is changed by the intentional moment applied by the human and the robot only controls the position of the object based on the servo brake control, so that the required intentional force for transporting the object could reduce compared to the previous version of the PRP without using free joint. Additionally, in this paper, we propose a motion control algorithm for the PRP with free joint and illustrate the validity of the proposed system and its control algorithm through the experiments of the object handling.