A tri-state prismatic modular robotic system

Strut-type modular robots adopt an idealized truss mechanism composed of linear extensible struts (i.e., robotic modules) jointed by connector nodes. Conceptually, each strut is capable of passively rotating around its connected node, and all the struts linked by a same node share and intersect at a common center of rotation. Unfortunately, these two requirements pose difficulties in mechanically implementing such an ideal point-like compliant connector node. Meanwhile, motions of extensible struts composed of off-the-shelf prismatic actuators can be constrained in a parallel or networked robotic structure, leading to the stuck of the robotic structure and the failure of a given task. To tentatively tackle these issues, a tri-state (three-state) modular prismatic robotic system is designed with the ability to be rigid, back-drivable to external forces and to undertake self-powered motion. Two modular robotic concepts of (i) rigid node attachment and (ii) neighbor actuation are introduced and demonstrated. Specifically speaking, modular robotic structures are constituted of prismatic actuators linked by rigid connector nodes. Instead of placing revolute joints on the nodes, a passive revolute joint is in the middle of two prismatic actuators to deliver compliance. This arrangement removes many of the inherent limitations of complex connection joints and avoids the difficulty of designing and implementing point-like complaint connector nodes. In terms of neighbor actuation, modules are demonstrated that they have the ability to be self-powered or passively controlled from external forces applied by their neighboring modules. This allows robotic modules in a parallel or networked structure to change states to release physical constraints and prevent the whole structure from getting stuck. In this paper, the tri-state capability of the prismatic modules and the neighbor actuation of parallel modules are first experimentally demonstrated. Then, two physical modular robots are built and control algorithms are implemented. Experimental results validate the efficacy of the control strategy and substantiate communication and locomotion capabilities of the modular robots. Finally, a more complex robotic structure is established in a robot simulator to demonstrate that the tri-state prismatic actuators can be applied to release physical constraints of the modular robot during a locomotion task.