A comparison of braking strategies for elastic joint robots

It has recently been shown that intrinsically elastic robots are capable of outperforming rigid robots in terms of peak velocity by making systematic use of energy storage and release. Certainly, high link side velocities are beneficial for performance, however, they also increase the probability of self damage or human injury in case of a collision. To ensure the physical integrity of both human and robot, it is therefore crucial to avoid potentially dangerous collisions and react in a compliant manner if unwanted contact has occurred or may occur unforeseeable. In this paper, we consider the most intuitive collision anticipation and pre-reaction scheme, namely stopping an elastic robot, if possible in minimum time. For 1-DOF elastic joints with limited elastic deflection we extend existing model-based and model-free controllers and compare their performance. Furthermore, we analyze the braking trajectory that is achieved with the different strategies. The 1-DOF solution is extended to the double pendulum case, where we show that feasible estimates for maximum and final position can be obtained at the very first instant of braking.