Chain-based Robots

Modules aggregate as connected 1D strings of units in chain-based systems. This class of robots easily implements rolling or undulating motions as in snake robots or legged robots. However, control is much more difficult for chain-based systems than for lattice-based systems because of the continuous nature of the actuation: modules can move to any arbitrary position as opposed to a fixed number of neighbor positions in a lattice. Our previous work has not considered chain-based systems, but it is important to understand their characteristics in the interest of developing more generalized algorithms.

Polypod was the first prominent chain-based system, proposed by Yim in 1993. Polypod is made up of Segments, which are actuated 2-DOF 10-bar linkages, and Nodes, which are rigid cubes housing batteries. Multiple gaits for locomotion, including rolling, legged, and even Moonwalk gaits, were demonstrated with Polypod. Polybot succeeds Polypod, sharing the same bipartite structure. Segments in Polybot abandon the 10-bar linkage in favor of a 1-DOF rotational actuator. The latest generation of Polybot prototypes has on-board processing and CANbus (controller area network) hardware for communication. A system that uses a similar actuation design is CONRO (CONfigurable RObot). The CONRO module has two rotational degrees of freedom, one for pitch and one for yaw, and was designed with particular size and weight considerations. Considerable attention has been paid to the connection mechanism, which is a peg-in-hole connector with SMA (shape-memory alloy) latching mechanism that can be disconnected by either face. Computation is on-board each module, so unlike Polypod, CONRO has only one module type. Power can be provided externally or via batteries on later prototypes. Manually configured shapes examples are the snake and the hexapod, and the current CONRO system is designed for self-reconfiguration.

The DRAGON is a robot snake with torsion-free (constant-velocity) joints. A sophisticated connector has been developed for the DRAGON, designed for strength and tolerance for docking.

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