Optimal Feed-Forward Control for Robotic Transportation of Solid and Liquid Materials via Nonprehensile Grasp

TL;DR

This paper introduces a feedforward control method for robotic transportation of objects on trays, preventing sloshing and slipping during movement, and can be integrated into existing systems for improved handling of solid and liquid materials.

Contribution

It presents a novel feedforward control approach that combines smoothing and re-orientation to enhance robotic object transportation without firm grasping, applicable to both known and unknown reference signals.

Findings

Effective in suppressing sloshing and slipping during transport.

Compatible with teleoperation and point-to-point motions.

Generates minimum-time trajectories respecting physical constraints.

Abstract

In everyday life, we often find that we can maintain an object's equilibrium on a tray by adjusting its orientation. Building upon this observation and extending the method we previously proposed to suppress sloshing in a moving vessel, this paper presents a feedforward control approach for transporting objects with a robot that are not firmly grasped but simply placed on a tray. The proposed approach combines smoothing actions and end-effector re-orientation to prevent object sliding. It can be integrated into existing robotic systems as a plug-in element between the reference trajectory generator and the robot control. To demonstrate the effectiveness of the proposed methods, particularly when dealing with unknown reference signals, we embed them in a direct teleoperation scheme. In this scheme, the user commands the robot carrying the tray by simply moving their hand in free space, with the hand's 3D position detected by a motion capture system. Furthermore, in the case of point-to-point motions, the same feedforward control, when fed with step inputs representing the desired goal position, dynamically generates the minimum-time reference trajectory that complies with velocity and acceleration constraints, thus avoiding sloshing and slipping. More information and accompanying videos can be found at https://sites.google.com/view/robotwaiter/