  Passive tumbling uses natural forces like gravity for efficient travel. But
without an active means of control, passive tumblers must rely entirely on
external forces. Northeastern University's COBRA is a snake robot that can
morph into a ring, which employs passive tumbling to traverse down slopes.
However, due to its articulated joints, it is also capable of dynamically
altering its posture to manipulate the dynamics of the tumbling locomotion for
active steering. This paper presents a modelling and control strategy based on
collocation optimization for real-time steering of COBRA's tumbling locomotion.
We validate our approach using Matlab simulations.
