A Master of Science Thesis in Mechatronics Submitted by Hossein Sadjadi Entitled, "Morphology for Planar Hexagonal Modular Self-Reconfigurable Robotic Systems," March 2009. Available are both Soft and Hard Copies of the Thesis.
This thesis primarily covers the design and implementation of a planar hexagonal Modular Self-Reconfigurable Robotic System (MSRRS) along with the construction of its reconfiguration path planner and control algorithm. Both platform and algorithm are designed based on a multilayer approach where each layer is dedicated to perform a specific task; in other words, the design itself is considered to be modular. In the first part a universal module is carefully designed to maintain certain criteria that seem to be in line with the common goals of this promising field including homogeneity, cost-effectiveness, fast actuation and quick and strong connections. In the second part, a reconfiguration path planner and a control algorithm is developed to determine the required sequence of individual module movements that transforms the shape of the system from an arbitrary initial configuration to a desired goal configuration in an optimal manner while enforcing several constraints and taking into account the kinematic model of the system. The shape of the physical platform was inspired form natural structures such as bees' nest and crystal molecules, where homogeneous hexagonal modules are capable of forming variety of structures. Electromagnets installed on six sides serve as the required actuating force providing fast and cost effective motion for the module. In this case, each module is not able to perform any motion alone; however, a combination of two or more modules makes the motion possible. Moreover, pull type solenoids located on six corners of the module provide quick and strong inter-module connections. Although the implemented working prototype is both large and restricted to a planar geometry, it is designed such that its hardware and software can be scaled up in the number of units and down in unit size; similarly, the platform has the potential to be extended for 3D applications. The software infrastructure of this platform is designed in a way that different hierarchies for distributed control and communication can be implemented. The path planner is designed to minimize the number of module movements during reconfiguration while enforcing collision avoidance and connectivity constraints. The algorithm is based on a hierarchical multilayer approach, where upper layers decompose the problem into sub-problems solvable by lower layers. The core of the algorithm relies on a heuristic function and a Markov Decision Process (MDP) optimization to generate a centralized near-optimal reconfiguration path planner and a control algorithm for a lattice, homogenous, rigid, planar hexagonal MSRRS. In this approach the connectivity test and MDP formulation require a centralized stage, yet the scalability issues required to move towards a truly decentralized approach are discussed in this thesis as well. Among several novel approaches incorporated in this system, multilayer nature of both hardware and software design provides openness, flexibility and ease of modification or adaptation for other platforms. In this approach each layer is dedicated to perform a specific task and can be modified or enhanced separately while keeping the remaining layers untouched.