In this paper, we present a diabolo model that can be used for training agents in simulation to play diabolo, as well as running it on a real dual robot arm system. We first derive an analytical model of the diabolo-string system and compare its accuracy using data recorded via motion capture, which we release as a public dataset of skilled play with diabolos of different dynamics. We show that our model outperforms a deep-learning-based predictor, both in terms of precision and physically consistent behavior. Next, we describe a method based on optimal control to generate robot trajectories that produce the desired diabolo trajectory, as well as a system to transform higher-level actions into robot motions. Finally, we test our method on a real robot system playing the diabolo, and throw it to and catch it from a human player.

Robotic contact juggling is a challenging task in which robots must control the movement of a ball rapidly and indirectly without holding it while keeping the ball in and sometimes out of contact with the robot’s body. In this work, we address the problem of learning such robotic contact juggling from trial and error via model-based reinforcement learning (MBRL). The key insight is that complex robot-ball interactions of the contact juggling actually consist of a small set of simple dynamics that each corresponds to a distinct interaction "primitive" such as touching and releasing the ball. Accordingly, we develop a tailored MBRL method that incrementally fits a set of simple dynamics models to the movements of a robot and a ball while also learning a switching model that can select a proper dynamics model depending on the current state and action. The learned model can then be used in an MBRL framework to seek optimal juggling control. We demonstrated the effectiveness of our approach on a simulator of contact juggling performed by a robotic arm.