Variable-Stiffness Tensegrity Modular Robots

The need for a new generation of robots able to safely locomote and manipulate beside or cooperatively with humans or in un-constructed environments has recently emerged as a priority of the robotics community. ln the past two decades, this challenge has been tackled by the growing field of soft robotics mostly focusing on developing completely deformable soft robotic bodies that can freely deform along any direction and comply with any unexpected or excessive external force. However, intrinsic softness can be a limitation in situations that require exerting substantial forces, to withstand bodyweights or to modulate forces applied to the environment. Inspiration for a novel mechanically hybrid approach in robotics can come from a novel model for biological structures based on the concept of tensegrity. This model rejects the idea of the vertebrate musculoskeletal system as beams, columns, and levers, and acknowledges the bones as stiff under compression elements held and stabilized in their positions by the pull of tensile components such as muscles, tendons, ligaments, and fascia. Therefore, rather than a frame supporting an amorphous soft tissue mass, the whole vertebrate body is divided into several modules stabilized by these tension components and its stiffness can vary depending on the pre-stress (i.e. tone) and stiffness of its muscles. This structural organization allows biological organisms to be more mechanically adaptive, robust, and lightweight at the same time. They can exhibit high physical compliance, apply efficient forces to the environment when needed, and, at the same time, withstand their bodyweight at different scales. In the same way, modular tensegrity structures used in robotic applications can provide a lightweight and robust framework where integrating stiff under compression struts for bodyweight support; variable stiffness cables for applying directionally larger forces when required, and soft cables to absorb energy and allow compliance. ln this thesis, tensegrity modular structures are investigated as a new approach to develop soft robots with variable stiffness capabilities. This investigation led to a set of design strategies to manufacture and actuate tensegrity modules with programmable stiffness and deformation; to mechanically and electrically connect tensegrity modules to develop untethered applications, and to implement variable-stiffness capabilities in the modules for active and passive stiffness change. Finally, preliminary results and insights are presented on using heuristic algorithms as a design method to find task-optimal morphologies, stiffness, and control of modular tensegrity robots. In this thesis modular tensegrity robots have been developed for performing locomotion and grasping tasks, however, the design methods presented can provide a guideline for designing robots for diverse applications where being lightweight, robust, and having variable-stiffness capabilities are of primary importance. We believe that the work presented and discussed in this thesis answered a gap in the current literature and most importantly, will pave the way to future research on tensegrity robots for a great variety of tasks and robotic applications.

Roombots: Design and Implementation of a Modular Robot for Reconfiguration and Locomotion

Alexander Spröwitz

In this thesis we present the design and implementation of a novel self-reconfiguring modular (SR-MR) robotic system: Roombots. We are aiming at three main applications with Roombots; locomotion through self-reconfiguration in the regular cubic 3D-lattice on structured surfaces, locomotion in non-structured environments applying central pattern generators (CPG) as the locomotion controller, and self-assembly and reconfiguration of static objects of the day-to-day environment, such as furniture. Robot assemblies from self-reconfigurable modular robots have the ability to adapt to a given task and working environment by altering their shape through a series of reconfiguration moves, and attachments and detachments between the modules. We are interested in self-reconfiguring modular robots for their shape-changing capabilities, and their distributed characteristics. We envision the following applications for Roombots: First, self-reconfiguration in a structured 3D lattice, i.e. a floor and walls equipped with connectors. Embedded connectors can provide pivot points for locomotion of SR-MR assemblies, and docking and recharging places for our adaptive furniture pieces. Second, our proposed concept for locomotion control of modular robots on non-structured ground are central pattern generators. Third, we would like to build adaptive and versatile furniture from modular robots and light-weight elements. In the following we are able to count more than 60 modular robotic systems, developed over the last two decades. However we were unable to identify an existing system which could provide us with all desired kinematic and geometric capabilities. This led us to design and implement a novel self-reconfiguring modular robotic system: Roombots. To tackle the module design we attempt to identify both meaningful design parameters from existing modular robots, and essential features for our applications. The combination of both leads to the kinematic and geometric description of the Roombots modules, and eventually to its implementation. In order to be able to assemble furniture from our Roombots units in the future, we need a reconfiguration framework which supports the specific requirements of Roombots. Metamodules made of two units attached in-series are attracted and guided by a virtual force-field, they use broadcast signals, look-up tables of collision clouds and simple assumptions about their near environment to reach their seeding positions, which are currently hand coded. For the task of locomotion in non-structured environments we propose a framework for learning to move with modular robots using central pattern generators and online optimization. The distributed implementation of CPGs offers an ideal substrate for producing locomotion patterns and for online learning, and an optimization framework for fast learning.

EPFL2010

Automatic Gait Generation in Modular Robots: to Oscillate or to Rotate? that is the question

Auke Ijspeert, Soha Pouya, Alexander Spröwitz, Jesse van den Kieboom

Modular robots offer the possibility to design robots with a high diversity of shapes and functionalities. This nice feature also brings an important challenge: namely how to design efficient locomotion gaits for arbitrary robot structures with many degrees of freedom. In this paper, we present a framework that allows one to explore and identify highly different gaits for a given arbitraryshaped modular robot. For this, we use simulated robots made of several Roombots modules that have three rotational joints each. These modules have as interesting feature that they can produce both oscillatory movements (i.e. periodic movements around a rest position) and rotational movements (i.e. with continuously increasing angle), leading to very rich locomotion patterns. Here we ask ourselves which types of movements — purely oscillatory, purely rotational, or a combination of both— lead to the fastest gaits. To address this question we designed a control architecture based on a distributed system of coupled phase oscillators that can produce synchronized rotations and oscillations in many degrees of freedom. We also designed a specific optimization algorithm that can automatically design hybrid controllers, i.e. controllers that use oscillations in some joints and rotations in others, for fast gaits. The proposed framework is verified through multiple simulations for several robot morphologies. The results show that (i) the question whether it is better to oscillate or to rotate depends on the morphology of the robot, and that in general it is best to do both, (ii) the optimization framework can successfully generate hybrid controllers that outperform purely oscillatory and purely rotational ones, and (iii) the resulting gaits are fast, innovative, and would have been hard to design by hand

Ieee Service Center, 445 Hoes Lane, Po Box 1331, Piscataway, Nj 08855-1331 Usa2010

3PAC: A Plug-and-Play System for Distributed Power Sharing and Communication in Modular Robots

Christoph Heinrich Belke, Kevin Andrew Holdcroft, Jamie Paik

In modular robotics, there is classically a compromise between coupling complexity and functionality. The mechanical and electrical designs are linked, such that adding electrical features often requires redesigning the mechanical components. However, increasing the functionality of each electrical contact adds capabilities or redundancies to existing robots without redesigning the coupling and allows for fewer contacts in new designs. This article presents a power distribution and local communication system for modular robotics, using only three electrical contacts between modules. The proposed system can be easily recreated, modeled, and outfitted into an existing modular robotic coupling to introduce power sharing, communication, or redundancy, without increasing the couplings mechanical complexity. By focusing on local power transfer, the system is scalable in number as well as coherent with modularity. We demonstrate that, through this system, distributed power sharing can improve the runtime of a simple modular robotic system by up to 62% while maintaining local communication streams. The circuit is modeled through empirical characteristics and therefore the same methods can predict the power distribution accurately in any other modular robot.