MODUR: A Modular Dual-reconfigurable Robot

Technical Summary: MODUR – A Modular Dual-Reconfigurable Robot

Problem Statement
Modular Self-Reconfigurable Robot (MSRR) systems offer enhanced adaptability and robustness by altering topological relationships between modules. However, existing MSRR architectures, such as spherical or cubic lattice-based systems (e.g., Roombot, SMORES), suffer from two primary limitations:

1. Coupled Connector Motion: In many designs, connectors are grouped such that moving one child module mechanically couples and restricts the movement of adjacent child modules.
2. Limited Adjacent Position Migration: Current modules often lack the ability to easily transport child modules to adjacent topological positions (e.g., moving a child from a parent to a sibling or grandparent) without complex disassembly sequences.

These constraints hinder efficient 3D continuous self-reconfiguration and limit the operational versatility of the robotic system.

Methodology
The paper proposes MODUR, a novel MSRR that integrates the concept of Reconfigurable Mechanisms (RM) directly into the module design. The core methodology involves:

• Mechanical Architecture: The MODUR module is designed as a Spherical Reconfigurable Parallel Mechanism (SRPM). It consists of four hermaphrodite connectors (acting as vertices) and six Scissor Linkage Groups (SLGs) arranged along the edges of a tetrahedron.

  • Connectors: These feature a compact (33 mm diameter), genderless design combining magnetic pre-alignment and a screw-in hook mechanism for rigid locking.
  • Actuation: The SLGs function as a six-bar Remote Center Motion (RCM) mechanism, acting as prismatic (P) pairs. They are driven by lead screw motors, allowing the connectors to expand within a range of 60° to 180°.
  • Dual-Reconfigurability: The system enables high-level self-reconfiguration (changing topological connections between modules) and low-level reconfiguration (changing the module's own shape via SLG actuation).

• Kinematic Modeling and Workspace Analysis:

  • A kinematic model was established treating the module as a sphere with four points (one input, three outputs).
  • The workspace of the output connectors was comprehensively analyzed under three constraints: angular limits (60°–180°), SLG interference at connectors, and non-intersection of opposite SLGs.
  • The analysis revealed that when adjacent connectors are unconnected, a single connector can access approximately 35.4% of the sphere's surface area, facilitating significant mobility.

• Motion Planning: Two basic motions were defined based on the workspace analysis:

  1. Rotation: Changing the orientation of a child module relative to the parent without disconnecting.
  2. Transition: Moving a child module from one parent to another (or to an adjacent position) by coordinating the expansion/contraction of SLGs and connector locking/unlocking.

• Control Strategy:

  • The prototype utilizes 12 motors (6 for connector locking, 6 for SLG actuation), creating a redundant actuation system (6 motors for 5 degrees of freedom).
  • A Hierarchical Approximation PID (HAPID) control method was developed to manage redundant forces, preventing singularities and minimizing internal force conflicts caused by unfolding angle errors.
  • Control is divided into "local control" (single connector manipulation for speed) and "global control" (coordinated manipulation for precision).

Key Contributions

1. Novel MSRR Design: Introduction of MODUR, which decouples the motion of child modules, allowing independent movement and overcoming the coupled motion limitations of previous MSRRs.
2. Adjacent Position Migration: The design enables modules to transport child modules to adjacent topological positions through shape reconfiguration, a capability previously difficult to achieve in lattice-based systems.
3. Workspace Analysis: A theoretical analysis of the connector workspace considering interdependent constraints, providing the foundation for defining basic motion primitives (rotation and transition).
4. Experimental Validation: Successful hardware implementation and testing of a functional prototype, validating the kinematic model, control strategy, and reconfiguration capabilities.

Results

• Prototype Implementation: A 3D-printed prototype (PLA) was constructed using Arduino Mega2560, 12 N20 geared motors, and AS5600 angle sensors.
• Trajectory Tracking: Experiments tracking a helical trajectory demonstrated that the redundant actuation with HAPID control maintains position deviations within approximately 1°.
• Redundancy Verification: Comparative experiments showed that the redundant drive system significantly outperforms non-redundant configurations, which suffered from gravity-induced leaning and inaccurate end positions.
• 3D Reconfiguration: Physical experiments successfully demonstrated the "Transition" motion, where a module (MC) moved from a parent (MP) to a new target (MR), forming a triangular connection and then disconnecting from the original parent.

Significance and Claims
The paper claims that MODUR represents a significant step forward in MSRR design by bridging the gap between modular robotics and reconfigurable mechanisms. By decoupling connector motions and enabling adjacent position migration, MODUR lays the theoretical and practical groundwork for 3D continuous self-reconfiguration. The authors state that this capability is the "cornerstone" for creating more versatile robotic systems capable of complex tasks in varied environments.

The authors remain modest regarding current limitations, noting that the prototype relies on external power and decentralized electronics due to integration challenges. Future work is identified as focusing on compact electronic integration, exploring multi-module manipulation tasks (e.g., grasping), and developing a motion planning framework specifically for 3D reconfiguration.