  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/
