Spherical Modular Self-reconfigurable Robots (SMSRs) have been popular in recent years. Their Self-reconfigurable nature allows them to adapt to different en-vironments and tasks, and achieve what a single module could not achieve. To collaborate with each other, relative localization between each module and assembly is crucial. Existing relative localization methods either have low accuracy, which is unsuit-able for short-distance collaborations, or are designed for fixed-shape robots, whose visual features remain static over time. This paper proposes the first visual relative localization method for SMSRs. We first detect and identify individual modules of SMSRs, and adopt visual tracking to improve the detection and identification robustness. Using an optimization-based method, tracking result is then fused with odometry to estimate the relative pose between assemblies. To deal with the non-convexity of the optimization problem, we adopt semi-definite relaxation to transform it into a convex form. The proposed method is validated and analysed in real-world experiments. The overall localization performance and the performance under time-varying configuration are evaluated. The result shows that the relative position estimation accuracy reaches 2%, and the orientation estimation accuracy reaches 6.64°, and that our method surpasses the state-of-the-art methods.

Freeform modular self-reconfigurable robot (MSRR) systems overcome traditional docking limitations, enabling rapid and continuous connections between modules in any direction. Recent advancements in freeform MSRR technology have significantly enhanced connectivity and mobility. However, limitations in connector strength and operational efficiency in existing designs restrict performance. This paper proposes a rigid freeform connector and a rigid magnetic track design to improve the connection and motion performance of the SnailBot. Each SnailBot is equipped with a multi-channel rope-driven gripper, a metal spherical shell with densely distributed circular holes on the back, and a rigid chain design conforming to the spherical surface. This combination allows each SnailBot to move precisely along the surface of a peer, facilitated by the ferromagnetic spherical shell and magnetic track. The integration of the gripper and spherical shell hole array provides robust inter-module connections in any position and orientation. The effectiveness of these designs has been validated through a series of experiments and analyses, demonstrating improved connection and motion performance in the SnailBot dual-mode connector system and expanding its potential applications and functional capabilities.

Energy sharing among modular robots enables sustainable operation of the system by maintaining energy balance among the modules. In this paper, we propose a novel energy sharing mechanism for FreeSN, a modular self-reconfigurable robot consisting of node and strut modules. Utilizing the feature that our modules are connected in a face-to-face manner, our method successfully establishes an energy sharing channel at almost any point on a sphere by placing transmission intermediaries at the interfacing face between modules, which is facilitated by the combination of brush contact and shell decomposition. Such mechanism also allows the utilization of the node module’s inner space for extra energy storage. A prototype of this energy sharing system has been implemented on FreeSN and rigorously tested. Our findings indicate that energy sharing is reliably established between modules; for strut modules positioned randomly on a node module’s surface, the probability of forming a valid connection is 56.6%. With orientation adjustment, a connection is achievable at nearly any position on the sphere, barring a few exceptional points. As a result, the operational endurance of the strut modules, which provide all the driving forces in the system, is markedly enhanced. This technique also holds potential for broader application across other freeform robotic platforms that incorporate conductive spherical surfaces for sliding connections.

Collaborative localization (CL) has garnered substantial attention in the field of robotics in recent years. Nonetheless, conventional CL algorithms have faced challenges when dealing with practical issues such as spurious sensor data and limited or discontinued observation and communication in real-world settings. This paper proposes a fault-tolerant practical estimated cross-covariance minimum variance update method (FPECMV) designed to tackle these challenges under limited connectivity. The proposed algorithm uses a CNN-based method to evaluate confidence, along with a fault isolation module to identify faults and manage spurious data in real time. The proposed fault isolation module utilizes relative measurement information that randomly occurs, without requiring high observation and communication prerequisites. Notably, the algorithm takes into account correlations among agents to maintain consistency in localization filters and attain accurate localization despite constraints posed by limited connectivity. To evaluate the performance of the proposed algorithm, experiments were conducted in a collaborative multi-robot environment with spurious sensor data and limited connectivity, using both the BULLET simulation and physical mobile robots. The experimental results indicate that the overall localization performance of the proposed algorithm is improved by 21.0% compared to the state of the art. The experiment results demonstrate the effectiveness of our algorithm in localizing group agents in challenging and intricate scenarios with limited connectivity and spurious sensor data.

Dexterous joints have attracted interest in the field of robotics. This article presents a driving-integrated spherical gear (DISG) that enables entire spherical meshing and active driving between two spherical gears, forming a dexterous multidegrees of freedom rolling contact joint. The DISG consists of a pair of spherical gears and an omnidirectional internal driver. The spherical gear shape is a combined projection of the conventional planar gear profile in the longitudinal and latitudinal directions, which can mesh and be driven over the entire sphere. An actively driving magnet and a passively following magnet are magnetically connected across the spherical gear and together form the internal driver, enabling arbitrary connection points throughout the sphere. In all configurations, one spherical gear can roll in all directions on the surface of the other. Furthermore, we analyze the kinematics of DISG and prove that the DISG-based dexterous joint has good kinematic characteristics, such as singularity-free and full-range workspace. We verify the theoretical and physical characteristics of DISG in a series of experiments on the prototype. We also compare DISG-based joints to other joint actuators and show that DISG-based joints have advantages in dexterity, motion range, compactness, and lightweight.

Rolling contact joints are attracting increasing interest in applications to robotic fingers and manipulators, due to the potential of the absence of abrasion wear, the simplification of the controller, and the enlargement of reachable configurations. This article first proposes a novel two-degree-of-freedom (DOF) spherical rolling contact (SRC) joint, with the joint model elements being formulated, includingrotation matrix,position vector, andfree modes, as with those of classic joints. As an application, a new kind of serial manipulator formed by the SRC joints is presented, and its forward and inverse kinematics are modeled. The motions of the two-DOF SRC joint and manipulator are implemented using the FreeBOT, and the control method is proposed for the FreeBOT, such that the SRC joint and manipulator realize the motion control. The kinematics and control of the two-DOF SRC joint and manipulator are validated using physics simulations and on a real manipulator formed by FreeBOTs.

Energy sharing in modular self-reconfigurable robots ensures the energy balance of the modules, thus allowing the system to work sustainably. This paper proposes an energy sharing mechanism for a novel modular self-reconfigurable robot that allows free connections among modules, termed as FreeBOT, such that each FreeBOT can share energy with peers through surface contact. Corresponding energy sharing rules are proposed to achieve an energy sharing network structure without invalid components. As alternative choices, several types of networks subjected to the above requirements are provided, which also maximize the number of FreeBOTs joining to share energy. We implement and test the prototype of the energy sharing mechanism on FreeBOT. The experimental results show that the mechanism can effectively achieve energy sharing among FreeBOTs.

This paper proposes a novel freeform strut-node structured modular self-reconfigurable robot (MSRR) called FreeSN, consisting of strut and node modules. A node module is mainly a low-carbon steel spherical shell. A strut module contains two freeform connectors, which provide strong magnetic connections and flexible spherical motions. The FreeSN system shares the benefits of freeform connection and strut-node structures. The freeform connection brings good adaptability to the environment. The triangle substructures inside the system configuration significantly improve the structural stability. The parallel execution of module motions can superpose the module capabilities and makes the system more scalable. The modules can combine these robot features by selecting the system configuration and better fit different circumstances and tasks. Four demonstrations, including assembly, obstacle crossing, transportation, and object manipulation, are designed to show the capabilities of the FreeSN system in different aspects. The results show the great performance and versatility of this MSRR system.

Modular self-reconfigurable robotic (MSRR) systems are potentially more robust and more adaptive than conventional systems. Following our previous work where we proposed a freeform MSRR module called FreeBOT, this paper presents a novel configuration detection system for FreeBOT using a magnetic sensor array. A FreeBOT module can be connected by up to 11 modules, and the proposed configuration detection system can locate a variable number of connection points accurately in real-time. By equipping FreeBOT with 24 magnetic sensors, the magnetic field density produced by magnets and steel spherical shells can be monitored. The connectable area is split into 199 non-uniform regions, including 84 uniform regions. Using a Graph Convolutional Network (GCN) based algorithm, the connection points can be located accurately under ferromagnetic environments. The system can locate a variable number of connection points for such a region division with only single connection point training data. Finally, the localization algorithm can run faster than 40 Hz on FreeBOT. With the real-time configuration detection system, the FreeBOT system has the potential to reconfigure automatically and accurately.

In this paper, the 2D robot-to-robot relative pose (position and orientation) estimation problem based on ego-motion and noisy distance measurements is considered. We address this problem using an optimization-based method, which does not require complicated numerical analysis while yields no inferior relative localization (RL) results compared to existing approaches. In particular, we start from a state-of-the-art method named square distances weighted least square (SD-WLS), and reformulate it as a non-convex quadratically constrained quadratic programming (QCQP) problem. To handle its non-convex nature, a semidefinite programming (SDP) relaxation optimization-based method is proposed, and we prove that the relaxation is tight when measurements are free from noise or just corrupted by small noise. Further, to obtain the optimal solution of the relative pose estimation problem in the sense of maximum likelihood estimation (MLE), a theoretically optimal WLS method is developed to refine the estimate from the SDP optimization. Comprehensive simulations and well-designed experiments are presented for validating the tightness of the SDP relaxation, and the effectiveness of the proposed algorithm is highlighted by comparing it to the existing approaches.

This paper proposes a novel modular selfreconfigurable robot (MSRR) "FreeBOT", which can be connected freely at any point on other robots. FreeBOT is mainly composed of two parts: a spherical ferromagnetic shell and an internal magnet. The connection between the modules is genderless and instant, since the internal magnet can freely attract other FreeBOT spherical ferromagnetic shells, and not need to be precisely aligned with the specified connector. This connection method has fewer physical constraints, so the FreeBOT system can be extended to more configurations to meet more functional requirements. FreeBOT can accomplish multiple tasks although it only has two motors: module independent movement, connector management and system reconfiguration. FreeBOT can move independently on the plane, and even climb on ferromagnetic walls; a group of FreeBOTs can traverse complex terrain. Numerous experiments have been conducted to test its function, which shows that the FreeBOT system has great potential to realize a freeform robotic system.