6th International Symposium on Advances in Robot Kinematics, Salzburg, Austria, 1998. DOUBLE CIRCULAR-TRIANGULAR SIX-DEGREES-OF- FREEDOM PARALLEL ROBOT V. BRODSKY, D. GLOZMAN AND M. SHOHAM Department of Mechanical Engineering Technion-Israel Institute of Technology Haifa, 32000 Israel E-mail: shoham@tx.technion.ac.il Abstract This paper describes a new structure of a six-DOF parallel robot. First, a known planar three-DOF double-triangular structure is modified by replacing the stationary triangle with a circle. It increases the work envelope considerably especially when rotational motions are required. The ability for unlimited rotational motion allows extending the structure into six-DOF by using two sets of stationary circles and moveable triangles. Each set can actuate the moving triangle in a planar three-DOF motion and hence actuate a line connecting the centers of the movable triangles in four-DOF. The robot's end-effector is attached to a link along this line while rotation about and translation along this line are obtained by the additional rotational DOF of the movable triangles. The solution of the direct kinematics of this six-DOF manipulator is given in a closed- form and it is shown that at most, four different solutions exist. 1. Introduction The kinematic structure of most contemporary robots is an open kinematic chain structure (known also as serial manipulators). Only relatively few commercial robots are composed of a closed kinematic chain (parallel) structure. However, the increasing interest in parallel robots points to the potential embedded in this structure which has not yet been fully exploited. The advantages of parallel robots as compared to serial ones are: • higher payload-to-weight ratio since the payload is carried by several links in parallel, • higher accuracy due to non-cumulative joint error, • higher structural rigidity, since the load is usually carried by several links in parallel and in some structures in compression-traction mode only, • location of motors at or close to the base, • simpler solution of the inverse kinematics equations. Conversely, they suffer from smaller work volume, singular configurations and a more complicated direct kinematic solution (which is usually not required for control purposes). Examples of different structures of parallel manipulators are given, for example, in [Hunt, 1983; Innocenti and Parenti-Castelli, 1994; Lin et al., 1992; merlet, 1994; Pierrot et al., 1991; Tahmasebi and Tsai, 1995; Ben-Horin and Shoham, 1996; Waldron et al., 1989]. A comprehensive atlas of parallel robots was composed by Merlet and can be found in the web site [http:// www.inria.fr/ prisme/ personnel/ merlet/ merlet_eng.html].