Parallel robotics

Constantly higher needs in the domain of machine industry are bringing us to develop new technology, other than conventional serial robots. The parallel-kinematic machine outperform today’s common serial machine on several points like stiffness and dynamics.

The mission of the Parallel Robotics Group is to develop new dedicated parallel-kinematic machines in all their aspects, reaching from the architecture to the final calibration. The Parallel Robotics Group has a long tradition in our laboratory. It can present important successes like the DELTA robot, which was sold over 4000 times, and the newly developped HITA STT machine-tool.

Parallel-kinematic technology can be applied in any domain where you need multiple axes and outstanding performances.

Kinematics

The following designs have all developed at the lab LSRO (former name of the lab). These are only some kinematics out of a big collection but they represent typical tendencies of the actual research.

Delta

/webdav/site/lsro/shared/PRD/kinematics/kinematics_delta.jpg The Delta is one of the most known and best sold parallel kinematics. It has been sold over 4000 times in the world in different variants, mostly for packing tasks.

This robot has been developped at the Laboratoire de Système Robotique and has 3 degrees of freedom in translation. It is based on 3 parallelograms and can be actuated in different ways reaching from linear actuators to rotational actuators liked shown in the figure.

The shown demonstrator achieves accelerations around 50g and is therefor one of the fastest robots. The guaranteed precision is 0.1 mm which is sufficient for its targeted pick and place operations. All these characteristics are given for a workspace of 600 mm diameter and 220 height.

This robot, developed by Prof. Clavel, won the ABB Golden Robot Award 1999.

Clavel, R.: “DELTA, a fast robot with parallel geometry“, 18th International Symposium on Industrial Robots, p.91-100, IFS Publications, 1988

Clavel, R., Pham, P., Lorent, B. Le Gall, B., Bouri, M.,   “New Variants of Delta Robots and Double-Tilt Platform for Assembly“, Invited paper at the International Colloquium of the Collaborative Research Center SFB 562, Braunschweig 2008.

MinAngle

/webdav/site/lsro/shared/PRD/kinematics/kinematics_minangle.jpg

The MinAngle is a 3 degrees of freedom (dof) parallel kinematics based on 3 identical kinematical chains which produce movements in Rx, Ry and z. All joints of the kinematic chains are made out of flexible joints.

The model has been designed to constitute the left hand of a machine tool that needs to orient the workpieces in a very precise manner and with high rotation amplitudes. The simulation model has joints designed for ±8° leading to an output angle of ±15° on both rotational dofs and a stiffness of about 107 N/m in all directions (length of a leg: 180mm; section: 70mm x 25mm).

Another advantage of the Orion MinAngle kinematics is the absence of spherical joints. The whole kinematics is built up from simple and well-known 1 dof pivots. Every kinematic chain can be machined monolithically, thereby avoiding any assembly inaccuracies and contact surfaces with low stiffness. The 3 base points have to be moved in a nearly linear vertical movement which results in a theoretically infinite stroke in z. These 3 points can be actuated in any imaginable way, depending on the specification requirements of the final device (e.g. flexure-based linear guideway with direct-drive linear motor, or linear guideway with ballscrew and servomotor).

These kinematics allow to reach very high rotational amplitudes for a flexure based robot. Additionally to this a very high precision and repeatability of the movement can be achieved since the flexible joint does not have any backlash, friction or hysteresis.

The rotation occurs around a RCM (Remote Center of Motion) which is placed exactly at the output of the robot.

The measurements show a first eigenfrequency around 400Hz.

Le Gall, B.,  Clavel, R and Bouri, M., “Ultra High Precision Robotics: A Potential Attractive Area Of Interest For Mm And IFTOMM“, Book Chapter, Technology Developments: the Role of Mechanism and Machine Science and IFToMM, Springer Edition. 2011.

Pham, P.; Regamey, Y.-J. ; Fracheboud, M. ; Clavel, R. “Orion MinAngle: A flexure-based, double-tilting parallel-kinematics for ultra-high precision applications requiring high angles of rotation” Proceedings of the 36th International Symposium on Robotics ISR 2005, December 2005, Tokyo, Japan

Hita STT

/webdav/site/lsro/shared/PRD/kinematics/kinematics_hita.jpg

The Hita STT (Stiffness Tracking Technology) [3] is a hybrid 5-axis machine tool based on a 4-axis parallel kinematics and a serial turntable. The special architecture of the parallel kinematics allows a singularity-free rotation of 120°. Combined with the turntable it therefore makes 5-side machining possible.

The concept has been realized and shows extraordinary performances:

  • 5 g acceleration
  • 30kg mobile mass
  • 1st mechanical eigenfrequency at 120Hz
  • minimal stiffness 1.2 10^7 N/m

An industrial prototype has been manufactured and calibrated. As enumerated above its characteristics are very satisfying and they confirm the whole concept.

Thurneysen, M ; Clavel, R ; Bouri, M ;Frayssinet, H ; Giovanola, J ; Schnyder, M ; Jeannerat, D,  “Hita-STT, a new parallel five-axis machine tool, “Parallel Kinematic Machines in Research and Practice“, vol. 24, p. 529-544, Chemnitz, 2004.

Thurneysen, M., Schnyder,, M., Clavel, R., Giovanola , J.,  “A New Parallel Kinematics for High-Speed Machine Tools – Hita STT“, Development Methods and Application Experience of Parallel Kinematics, Proceedings of the 3rd Chemnitz Parallel Kinematics Seminar, pp. 533-562, Chemnitz, Germany, 2002

Alpha5

/webdav/site/lsro/shared/PRD/kinematics/kinematics_alpha5.jpg

The Alpha5 kinematics were developed at the Laboratoire de Systèmes Robotique (LSRO) at the EPFL in Lausanne. It is a totally parallel kinematics possessing 5 degrees of freedom with the ability to reach the ± 90° in amplitude on both, the A and B axis. For nowadays parallel kinematics this is a considerably high amplitude of rotation on both axes. This ability has been until now one of the big advantages of serial machines compared to parallel machines.

These kinematics are controlled using a field bus architecture, namely the Profibus, and a flexible control software that is developed at the LSRO.

The architecture of the control as well as the control software prove to be a fast and efficient manner for controlling this parallel robot. Thanks to the modularity of the control software it has been possible to rapidly adapt it for this new parallel kinematics and to the new fieldbus hardware.
An impressive demonstration of the robot’s capabilities has finally been accomplished, where the robot paints the logo of the Swiss Federal Institute of Technology on a ping-pong ball using all 5 axes.

Pham, P.; Bouri, M.; Thurneysen, M.; Clavel, R. “Profibus PC based Motion Control with Application to a new 5 Axes Parallel Kinematics“. Proceedings of the 35th International Symposium on Robotics ISR 2004, Paris, France