Hierarchical control in sea star inspired locomotion*

There is a growing effort to understand decentralized control mechanisms, particularly in application to robotic systems with distributed sensors and actuators. Sea stars, being equipped with hundreds of tube feet, are an ideal model system for studying decentralized sensing and actuation. The activity of the tube feet is orchestrated by a nerve net that is distributed throughout the body; there is no central brain. We developed mathematical models of the biomechanics of the tube feet and the sea star body. We then formulated hierarchical control laws that capture salient features of the sea star nervous system. Namely, at the component level, the individual tube feet follow a state-dependent feedback controller. At the system level, a directionality command is communicated to all tube feet. We studied the locomotion gaits afforded by this control model. We find that these minimally-coupled tube feet coordinate to generate robust forward locomotion on different terrains. Our model also predicts different gait transitions consistent with our experiments performed on Protoreaster nodosus. These findings offer a new paradigm for walking using soft actuators, with potential applications to autonomous robotic systems.