Modular reconfigurable robots are touted for their flexibility, as their bodies can assume a wide range of shapes. A particular challenge is to make them move efficiently in 3D without compromising the scalability of the system. This paper proposes decentralized and fully reactive controllers for pose control of 3D modular reconfigurable robots. The robots operate in liquid environments and move by routing fluid through themselves. Each module uses only two bits of sensory information per face. Additionally, the modules can use up to five bits of information that are exchanged via shared power lines. We prove that robots of convex shapes are guaranteed to reach a goal object with a preferred orientation. Using computer simulations of Modular Hydraulic Propulsion robots, all controllers are assessed for different environments, system sizes, and noise, and their performances compared against a centralized controller. Given the simplicity of the solutions, modules could be realized at scales below a millimeter-cube, where robots of high spatial resolution could perform accurate movements in 3D liquid environments
- decentralized control,
- mobile robots,
- decentralized pose control,
- modular reconfigurable robots,
- liquid environments,
- centralized controller,
- modular hydraulic propulsion robots,
- fully reactive controllers,
- decentralized controllers
Modular reconfigurable robots can take different shapes and therefore cope with a variety of tasks and conditions , . Proposed applications include search and rescue , an inspection of underwater environments , and the construction of temporary structures . In these applications, the ability of robots to control their pose is crucial. A robot may need to navigate a narrow passage or inspect or manipulate objects . Whereas these abilities have been demonstrated with individual modules, realizing them at the level of a large ensemble remains a challenge. In addition, through miniaturization, the modular resolution of robots of a given size can be increased, enabling them to perform more accurate movements. Miniaturized modular robots could also be used in novel applications, such as micro-medicine. However, this requires the modules to have exceedingly low hardware resources, making the design of controllers a challenge. To engender scalability, we seek modular systems that are decentralized and use simplistic hardware and software. Current solutions for pose control are either centralized or require the use of complex sensors or controllers. Tactically Expandable Marine Platform  and Roboat  modules are capable of 3 degrees of freedom (DoF) motion and can self-reconfigure into temporary 2D structures on the surface of the water. However, control is centralized and requires an external camera or GPS to obtain the pose of each module. The AMOUR robot  is capable of 6 DoF motion underwater, yet can only reconfigure in 1D. Control is centralized but works with any thruster configuration. ModSquad  is an aerial system capable of 4 DoF motion. While each module computes its own control inputs, the desired pose is provided by a central planner. The Distributed Flight Array (DFA) ,  is another aerial system capable of decentralized pose control. It requires the use of external sensing to determine its horizontal position and yaw angle. Both ModQuad and DFA are limited to a 2D reconfiguration space. The Modular Hydraulic Propulsion (MHP) concept  proposes a robot with a cubic lattice capable of 6 DoF motion in a liquid environment. MHP robots are modular networks that propel by routing fluid through themselves. In previous work, a decentralized 2 DoF motion controller was proposed for translation towards a goal. The physical MHP platform  is limited to 2D.
In this paper, we propose decentralized 5 DoF pose controllers for convex-shaped MHP robots that are fully autonomous. The controllers solve the problem of approaching a goal with a preferred orientation. They use only simple binary pumps and sensors and require no run-time memory. To the best of our knowledge, they are the first solutions achieving 5 DoF motion of modular robots in a fully autonomous, reactive, and decentralized way. The simplicity of the solutions allows future miniaturization of the system, whereas the lattice structure gives rise to a large reconfiguration space
In this paper, we proposed a set of fully reactive and decentralized controllers for modular reconfigurable robots that perform pose control in liquid environments. The control strategies enable a convex-shaped robot to reach the goal with a preferred face. The robot uses simple binary sensors and pumps. Additionally, the strategy allows the modules to access up to 5 bits of information through shared power lines. We formally proved that one of the controllers is guaranteed to succeed in both 2D and 3D environments. We evaluated the performance of the proposed 3D controllers in computer simulation studies. The 3D-0SP controller performed robustly in high-drag environments, whereas the other controllers performed well in all environments. All controllers coped well with actuation noise, however, only the 3D-0SP controller performed robustly with respect to sensor noise. The performance of 3D-0SP and 3D-2SP controllers was sensitive to the initial goal distance. The performance of the 3D-2SP and 3D-5SP controllers scaled well, but we predict it will degenerate for very high modular resolutions. Future work will validate the controllers on the physical 2D MHP platform and consider the problem of pose control of robots of non-convex shapes. Moreover, monitoring tasks, such as the tracking of a dynamic goal , could be considered.
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FULL Paper PDF file:Decentralized Pose Control of Modular Reconfigurable Robots Operating in Liquid Environments
Decentralized Pose Control of Modular Reconfigurable Robots Operating in Liquid Environments
2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Macau, China, 2019, pp. 4855-4861,
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Professor Siavosh Kaviani was born in 1961 in Tehran. He had a professorship. He holds a Ph.D. in Software Engineering from the QL University of Software Development Methodology and an honorary Ph.D. from the University of Chelsea.