On the Design of a Fast Parallel Robot Based on Its Dynamic Model

1 Motivation, Problem Statement, Related Work Most pick-and-place applications involved four-degree-of-freedom robots; indeed, packaging, picking, packing and palletizing tasks required three translations and one rotation around a vertical axis and such motions, named Schoenflies motions [1] or Scara motions, have to be realized in the shortest possible cycle time: the need for 4dof, Scara-like, robot mechanisms able to reach high speed (typically more than 4 m/s) and high acceleration (typically more than 100 m/s) come directly from such applications. To date, two families of mechanism architectures have been intensively considered, studied, and used in numerous plants: the simple and efficient RR-R-P serial chain which is the base of all Scara robots, and the parallel Delta robot [2] which was specifically developed by Clavel at EPFL at the end of 80s to tackle pick-and-place tasks. In order to reach high accelerations, this machine was designed with all actuators fixed on its base which minimizes moving parts masses. However, the rotational motion of this robot is obtained using a central R-U-P-U-R chain (R: Revolute, U: Universal, P: Prismatic, bold representing the actuated joint) which could suffer from a lack of stiffness at workspace boundaries. In addition, this telescopic arm has a short service life. Apart from those two famous robot categories, other lower mobility parallel mechanisms able to realize Scara motions have been developed in the recent years. For example, Angeles [3] proposed a four-dof parallel mechanism. In addition, EPFL developed Kanuk and Manta robots [4], and the machine tool HITA STT which has been modified to pick-and place manipulator [5]. However, it is worth noting that these innovative robots have been designed by mostly (only?) resorting to kinematics considerations, such as optimizing their architecture with respect to kinematic manipulability or “isotropy”. Obviously, such considerations are of tremendous importance and no one can ignore them, but we would like in this paper to show, on a practical example, how dynamics can affect the design of a fast parallel robot in the view of optimizing its behaviour for pick-and-place operations.