Roombots: A hardware perspective on 3D self-reconfiguration and locomotion with a homogeneous modular robot

Highlights

• We designed, implemented, and tested the Roombots (RB) modular robots.

• We explain the RB design methodology: active connection mechanism and module.

• RB use locomotion on-grid (lattice-based environment), and off-grid locomotion.

• RB join into metamodules on-grid, and can transit from off-grid to on-grid.

• We demonstrate RB overcoming concave and convex edges, and climbing vertical walls.

Abstract

In this work we provide hands-on experience on designing and testing a self-reconfiguring modular robotic system, Roombots (RB), to be used among others for adaptive furniture. In the long term, we envision that RB can be used to create sets of furniture, such as stools, chairs and tables that can move in their environment and that change shape and functionality during the day. In this article, we present the first, incremental results towards that long term vision. We demonstrate locomotion and reconfiguration of single and metamodule RB over 3D surfaces, in a structured environment equipped with embedded connection ports. RB assemblies can move around in non-structured environments, by using rotational or wheel-like locomotion. We show a proof of concept for transferring a Roombots metamodule (two in-series coupled RB modules) from the non-structured environment back into the structured grid, by aligning the RB metamodule in an entrapment mechanism. Finally, we analyze the remaining challenges to master the full Roombots scenario, and discuss the impact on future Roombots hardware.

Introduction

We are working towards the idea of a living environment where classic roomware components are merged and enhanced by elements from robotics and information technology. We tackle this task by designing Roombots (RB), an instance of self-reconfiguring modular robots (SR-MR) which are meant to be embedded into our living environment, to build intelligent furniture and other components of our daily life. Self-reconfiguring modular robots are highly integrated, self-sustaining robotic building blocks with limited degrees of freedom (DOF). A number of them can connect to each other with active connection mechanisms (ACM), creating a robot or structure with more capabilities than a single module. Eventually, we see the task of RB to build adaptive furniture that connects into different shapes, such as stools, tables, or sofas. It should also adapt to the user’s needs, by changing shape over time, and by moving around.

Several designs of self-reconfiguring modular robotic systems have been proposed, with the goal to assemble larger structures [1], [2]. Such structures could be complex assemblies, such as parts of a space station, remote, hard-to-reach and hard-to-maintain systems such as a modular satellite, deep-sea underwater structures, or scaffolds. For the Roombots project, we are aiming to assemble furniture-like structures. For our furniture, we want to minimize the number of active robotic modules by combining active modules and passive elements—a table assembled from light-weight elements and a few Roombots modules would be by far cheaper, less heavy and structurally more sound than one made entirely from modular robots. Currently, we envision a mix of Roombots modules and passive elements, as depicted in Fig. 1. Light-weight elements of several shapes and Roombots modules would create a patchwork, held together by Roombots modules. Connector “ports” play an important role in this scenario; these are distributed around the room as passive connectors in the floor, walls, ceilings, and within the light-weight elements. The same ports are also part of Roombots modules, as its active and passive connectors.

Towards this future goal, we tackle and present experimental results for five sub-scenarios in this work. Firstly, we imagine a stock of Roombots modules on the side, and modules detaching from it. Not all the floor will be equipped with connector ports. To reach a port-equipped area (“on-grid”), we show how pair-wise connected Roombots metamodules (MM) locomote with oscillating or wheel-like rotating motion patterns over the floor (“off-grid” locomotion, Fig. 1, subtask 1). Module locomotion is controlled by a central pattern generator (CPG)  [3] implemented as a network of coupled oscillators, distributed in the different modules. CPG are decentralized and well suited to organize rhythmic and non-rhythmic motions of large numbers of modular robots.

Next, modules have to become aligned to the grid. We propose a passive mechanism based on an entrapment-like structure (Fig. 1, subtask 2): Roombots metamodules slide into a sink-like entrapment mechanism (EM). In there the MM automatically aligns, the MM’s ACM grippers connect to a EM wall port. With a couple of predefined joint moves the MM leaves the EM, and “brachiates” over the grid connection ports  [4], [5] (simulation results). Roombots modules can perform on-grid locomotion by iteratively attaching and detaching to the ports. In some cases, modules fail to attach when controlled in open-loop because of excessive bending of the structure, and necessary hardware updates for this task are discussed in Section  5.

To build a larger structure from modules and passive elements, Roombots modules will have to approach, climb, and overcome walls and planes. Those obstacles present themselves either as “concave” edges, planes, or “convex” edges. Single Roombots modules are designed and controlled to overcome concave edges (Fig. 1, subtask 3), and to switch from a horizontal plane to a vertical plane. To go upwards and switch to the next-level horizontal plane a single Roombots module has to be merged into a full Roombots metamodule, together with a Roombots “helper module” (Fig. 1, subtask 4).

Roombots are meant to assemble larger structures from passive elements. In subtask 5 we present a brief first step towards handling light-weight structures. In fact, the future Roombots hardware will require a stiffness and power upgrade before being able to handle larger elements, as depicted on the left side of the figure. For now, two Roombots modules are picking up a pair of light-weight connector plates (Fig. 1, similar to 5). For the future of this project, remaining challenges will include the tasks of picking up light-weight elements by cooperating Roombots metamodules, transporting those elements to their assembly point (Fig. 1, two metamodules lifting a light-weight element to the table top), and finally mounting and assembling everything into meaningful structures. In this work we focus at the defined five subtasks and the necessary modular robot hardware to robustly solve these scenarios. Remaining challenges (e.g. multi-metamodule handling of passive elements, RB metamodule locomotion on-grid) are identified along with this work, and are analyzed and discussed in Section  5.

The paper is structured as follows: Section  2 gives a brief overview over modular robot (MR) classifications, and significant MR implementations. In Section  3 we present the mechanical, electrical, and control design of Roombots. In Section  4, we show five hardware experiments of reconfiguration and locomotion on-grid and off-grid, in 2D and in 3D, alignment of RB from off-grid to on-grid, and handling of light-weight elements through RB modules. We discuss our results and future work in Section  5, and conclude the paper in Section  6.