Adaptive control of a robotic manipulator for soft catching of a falling object

Abstract

This paper presents a controller design of a robotic manipulator for soft catching of a falling object. If a robotic system is able to catch a falling object softly, there will be many applications expected in human activities such as industry, welfare, nursing, housework and office work, because this ability allows a human operator or another robot system to move an object to the catching robot without any transportation systems such as an conveyor or a mobile structure. First, this paper considers a nonlinear decoupling control of a robotic manipulator. Next, a controller design is presented for catching a falling object with a small impact force. This controller consists of two parts: a position tracking controller that tracks a desired trajectory before contact between the object and the robot end-effector, and a force controller that is triggered after the contact. We employ a position-based impedance controller so that the entire control system can be constructed as a position-based controller. In order to achieve the soft catching, precise motion control is required to achieve the same velocity of the robot end-effector with a falling object when they are in contact. Hence, we employ an adaptive controller that consists of a feedback controller to compensate for disturbance such as friction and a feed forward controller to improve the tracking performance to the desired trajectory by adjusting controller parameters in real time. Experimental results with a falling raw egg demonstrate the effectiveness of the proposed approach.