Reconfiguration of 3D Pivoting Modular Robots

We study a new model of 3-dimensional modular self-reconfigurable robots Rhombic Dodecahedral (RD). By extending results on the 2D analog of this model we characterize the free space requirements for a pivoting move and investigate the $\textit{reconfiguration problem}$, that is, given two configurations $s$ and $t$ is there a sequence of moves that transforms $s$ into $t$? We show reconfiguration is PSPACE-hard for RD modules in a restricted pivoting model. In a more general model, we show that RD configurations are not universally reconfigurable despite the fact that their 2D analog is [Akitaya et al., SoCG 2021]. Additionally, we present a new class of RD configurations that we call $\textit{super-rigid}$. Such a configuration remains rigid even as a subset of any larger configuration, which does not exist in the 2D setting.