Team AVR - Control, Vision and Robotics Lab

Tensegrities and cable prestressed systems

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The context of this project is the design of robotic assistants for MRI-guided procedures. In percutaneous interventionnal radiology, an MRI scanner is used to guide diagnosis or treatment procedures such as biopsies or cryoablations performed with a needle. In this case, the introduction of a robotic assistant can be considered to improve the precision and ease the needle manipulation.

The design of such devices in this environment is however tedious due to the different MRI-related constraints such as compacness, lightness and MR-compatibility. Moreover, the interaction of the device with the patient and/or the surgeon should be properly managed for safety reasons.

In this project, we investigate the use of tensegrity mechanisms, and cable prestessed systems more generally in order to develop variable stiffness devices that are beneficial for the robot/patient interaction. These systems are compact, lightweight and can be remotely actuated through cables that are beneficial for the MR-compatibility of the system. Variable stiffness feature can also be obtained through a variation of the prestress within the elastic cables.

Analysis, design and control of these novel systems however consitute scientific issues. This project aims at tackling these issues and propose novel robotic solutions within the application field of interventionnal radiology [1].

Design and control of variable stiffness tensegrity mechanisms

One of our main focus is the design and control of a variable stiffness tensegrity mechanism for the design of a robotic needle holder.

Such implementation was made possible by prior works on the behavior analysis of these systems, such as their modeling [6], the determination of their workspace [3] and the development of dedicated control strategies for variable stiffness cable-driven devices [2,4,7]. A tensegrity mechanism is composed of bars that are kept in compression by tensionned elastic cables. Their actuation then allow for both the reconfiguration and the stiffness modulation within the system. A prototype of a planar tensegrity mechanism was developped and experimentally validated within a joint work with the LIRMM in Montpellier (LIRMM - UMR 5506, CNRS - University of Montpellier) in the framework of the CAMI Labex ANR project (CAMI Labex - ANR-11-LABX-0004).

Tensegrity-based needle-holder and experimental setup on a planar variable stiffness tensegrity mechanism.

The enclosed video is providing experimental results on the trajectory tracking and stiffness variation obtained with the prototype.

Design of variable stiffness joints using cable prestressed systems

Another focus of the project is the design of variable stiffness joints using cable prestressed systems.

Prior works on the synthesis of such systems were conducted in order to automatically generate suitable arrangements for variable stiffness joint. Using such method, we designed a variable stiffness spherical joint dedicated to needle manipulation in percutaneous interventionnal radiology [5]. 3D-printed prototype of such device was designed. In this case, pneumatic actuation of the prestress combined with multimaterial manufacturing leads to large stiffness variations as assessed by its experimental validation.

Overview of the variable stiffness spherical joint.

The enclosed video is providing more details about the particular geometry, the mobilities and the experimental evaluation of the spherical joint.



  • The 3D-printer is funded by the Investissements d’Avenir program (CAMI Labex & Equipex ROBOTEX) under references ANR-10-EQPX-44 and ANR-11-LABX-0004.
  • PhD grant from CAMI Labex and Région Alsace (Q. Boehler, 2013-2016)
  • Aviesan France Life Imaging infrastructure (FLI)

More information...

  1. Q. Boehler, Analyse, conception et commande de mécanismes de tenségrité et systèmes précontraints. Application à la robotique dans l'IRM, PhD Thesis, University of Strasbourg, 2016.
  2. Q. Boehler, S. Abdelaziz, M. Vedrines, P. Poignet, P. Renaud, From modeling to control of a variable stiffness device based on a cable-driven tensegrity mechanism, Mechanism and Machine Theory, vol. 107, pages 1-12, 2017.
  3. Q. Boehler, Isabelle Charpentier, M. Vedrines, P. Renaud. Definition and Computation of Tensegrity Mechanism Workspace, Journal of Mechanisms and Robotics, vol. 7, no. 4, page 044502, 2015.
  4. Q. Boehler, S. Abdelaziz, M. Vedrines, P. Poignet, P. Renaud, Towards the Control of Tensegrity Mechanisms for Variable Stiffness Applications: A Case Study In New Trends in Mechanism and Machine Science, Proceedings of the 6th European Conference on Mechanism Science (EUCOMES), pages 163--171, Nantes, France, August 2016.
  5. Q. Boehler, M. Vedrines, S. Abdelaziz, P. Poignet, P. Renaud, Design and evaluation of a novel variable stiffness spherical joint with application to MR-compatible robot design In IEEE International Conference on Robotics and Automation (ICRA), Stockholm, Sweden, May 2016.
  6. Q. Boehler, M. Vedrines, S. Abdelaziz, P. Poignet, P. Renaud, Influence of Spring Characteristics on the Behavior of Tensegrity Mechanisms In Advances in Robot Kinematics, pages 161--169, Ljubljana, Slovenia, June 2014.
  7. Q. Boehler, A. Zompas, S. Abdelaziz, M. Vedrines, P. Poignet, P. Renaud, Experiments on a variable stiffness tensegrity mechanism for an MR-compatible needle holder In 5th Joint Workshop on New Technologies for Computer/Robot Assisted Surgery (CRAS), Brussels, Belgium, September 2015.