ARCHI: a new redundant parallel mechanism-modeling, control and first results

After Gough [1] and Stewart [2] in the 50's and 60's introducing the idea of ‘hexapods’ (6-dof) [3][4], Clavel and his Delta structure [5] in the late 80's opened a new era with machines able to reach extremely high performances, used first for pick-and-place operations, and more recently for 3 or 5-axis machining. Considering the machining of complex shape objects, the solution often proposed by parallel mechanisms is dramatically different from the solution in use in industry. In fact, industrial machines are based on a 5dof serial chain whose weak point is the ‘head’, i.e. the last two rotating joints; on the other hand, most parallelmechanism-based machines rely on a 6-dof arrangement. Different designs have proven the efficiency of parallel mechanisms in this type of applications [6][7]. Approaches using less actuators (that is, five actuators for five machining axis) have been proposed recently, but they suffer from the same limitation than fully parallel chains: a limited tilting angle. Hybrid serial/parallel or parallel/serial structures (Tricept [8], Eclipse [9], DS Technologies Sprint) are solutions for 5-axis machining: the parallel/serial arrangement of Tricept offers a good dynamic behavior thanks to its parallel sub-part, but it still suffers from the limitations of a serial ‘head’; Sprint architecture guarantees a good behavior of the ‘head’ but is limited in terms of tilting angle. In this paper, a solution based on the principle of Motion-Sharing where both the tool and the work-piece are moved, and where at least a part of the motion is due to parallel chains is studied. A new parallel redundant structure, ARCHI, has been designed as a part of a 5-axis equipment for machining. Its basic design and models are presented, and control strategies are discussed.