Seite - 5 - in Proceedings of the OAGM&ARW Joint Workshop - Vision, Automation and Robotics

A framework for cellular robots with tetrahedral structure Michael Pieber and Johannes Gerstmayr Abstract—An adaptive tetrahedral element (ATE) has been designed, which can attach to and detach from other ATEs along their deformable faces. The goal is to obtain any configuration or shape autonomously. The tetrahedrons edges represents six actuators and each ATE has its own micro- controller, battery and wireless transceiver module. Several connected ATEs are forming an adaptive robot with tetrahedral structure (ARTS) which is intended to represent any geometric form with a piecewise flat surface. Contrary to existing cellular and tetrahedral robots ARTS combines the advantages of self- reconfigurable modular robots and tetrahedral robots which have the ability to change their shape. I. INTRODUCTION Self-reconfigurable robots with the ability to represent arbitrary shapes leads to an enormous number of real-world applications. Such applications are feasible within the self- assembly of large scaffolds, using ATEs with an overall size of one meter. Adaptive structures are needed e.g. for the growing complexity of current architectural design. In the mid-range size of ATEs, using centimeters for each actuator, the possibility to represent any 3D geometry could be used for rapid-prototyping and for the visualization of 3D structures in business and education. II. RELATED WORK Ahmadzadeh et al. [1] identified and cited 94 modular robots. Most of these are arrays of kinematically-constrained simple robots with few degrees of freedom [5], [3], [8], [2], [7]. These robots can attach to and detach from each other manually or automatically mostly with a mechanically [5] or magnetic [8], [2] connection mechanism. The combination of self-reconfiguration robots with the ability to represent arbitrary shapes are presented recently in [6]. The connection mechanism along the deformable faces of the ATEs are patented [4] by the authors of the present paper. III. ARTS – A TETRAHEDRAL ROBOT ARTS is a modular robotic system which is based on adaptive tetrahedral elements (ATEs). The single ATEs can be understood as cells of a larger structure, similar to cells in biology. Each ATE can deform and has six degrees of freedom resp. actuators. In a continuum mechanics interpre- tation, an ATE can undergo any kind of stretch or shear deformation. The deformation of the single ATEs gives the robotic system are large amount of variability. Michael Pieber and Johannes Gerstmayr are with the Institute of Mechatronic, University of Inns- bruck, 6020 Innsbruck, Austria {michael.pieber, johannes.gerstmayr}@uibk.ac.at Fig. 1. Three tetrahedral elements attach along their deformable faces to an adaptive robot with tetrahedral structure. The elements standing in initial position an a plate. First the element on the left side attach to the middle ATE. In the next step both ATEs are connecting to the third ATE on the right side. Each ATE itself is a mechatronic system, which includes the actuators, four double-spherical joints, 3 pairs of con- nectors at each of the four faces, a control and power unit, a wireless connection and a battery, see Fig. 2. In the current design, most parts are manufactured using a high- end 3D printer ’ProJet 3500 HD’ from 3D Systems, with the material VisiJet M3-X. In comparison to conventional cubic or spheric modular robots, the tetrahedral structure leads to a light-weight design. Furthermore, the ATEs can connect and change the overall shape of the structure, see Fig. 1, and finally shall have the possibility to move ATEs along the surface by deformation of surrounding ATEs. As a challenge of the design, there are restrictions for the elongation of each actuator, which leads to severe limitations of the motion space of each cell. This also limits the angles of the edges at the spherical joints, being boundaries to the geometrical design. The system of ATEs, from which we currently have built four fully functional elements, is used in a way, that they are always either positioned at a fixed space on a ground plate, or they are connected to one or several other ATEs, compare Fig. 1. The unique design is based on the connection at the faces, rather than the nodes. This avoids any restrictions within the connection of several tetrahedral elements, as known from other tetrahedral robots, see the references provided above. The advantage of tetrahedral robots is the convenient computation of the movement of the structure, which can be understood as a deformable mesh. The mesh – similar to a finite element mesh – can be modeled to be elastic with certain geometric limitations, which can be 5