Dynamic butler trajectories
During his internship at Willow Garage Tobias Kunz, a PhD student from Georgia Tech, worked on a project to enable the PR2 to quickly transport objects on a tray without them falling off. This effort in conjunction with the sushi challenge aim to give robots the ability to perfrom tasks like efficient transport of every day objects and dynamic place setting.
Dynamic trajectories are important for robots to execute delivery or butler tasks at home in a timely manner. The problem also appears in industrial robotics when objects are transported on tray-like end-effectors at high speeds. An identical problem is that of carrying an open container of liquid. Just keeping the tray horizontal is not enough to keep the object from sliding off when moving fast.
Instead, the tray needs to be tilted in a way that minimizes lateral force on the object.
Tobias implemented two different methods. The first method explicitly calculates the time-optimal trajectory for a free-floating tray along a straight line in workspace given constraints on the linear and angular accelerations of the tray. The second method considers arbitrary paths and the kinematic limitations of the robot arm. STOMP, a local trajectory optimization method, is used to iteratively minimize the lateral force acting on the object while also avoiding obstacles. The video shows the PR2 executing trajectories generated by this this method using different types of objects with different start and end locations for the tray.
Tobias’ work demonstrates the use of motion planning to enable robots to execute tasks with complex dynamic constraints. His goal is to get robots to do even more complex tasks in the future, just like humans can.
You can download and playback these dynamic butler trajectories on your own PR2 robot. The necessary instructions are available here.