Cooperative Robot Manipulators for Parcel picking and placing

Abstract

In this thesis, a control scheme for lifting parcels using two robot manipulators is presented. The robots do not have a rigid grasp on the object. Instead, they use friction to lift the parcels. First, a controller calculates the desired force to make sure the parcels do not slip. The required force, as well as a trajectory are then sent to a hybrid force position controller, which controls the force the robot is pushing with in the direction perpendicular to the contact surface, while controlling the position in all other directions. The control scheme thus controls both robots separately given a desired force and position. That way the controller a lot simpler compared to closed kinematic chain controllers, which aim to control the entire system as a single robot. Since the parcels weight is unknown for an actual system where the robots pick and place parcels, the controller is extended with an adaptive part, that estimates the weight of the parcel. The estimated weight can than be used to improve the feedfordward controller, making the system less reliant on feedback. The estimator works by calculating the difference between the predicted forward dynamics and the actual dynamics of the system. It is shown that the estimator yields good results for estimating the mass of parcels. Furthermore, the trajectory tracking property of the system is improved when using the estimated weight for feedforward control.