Path planning for cable-driven robots

Kinematics and Robot Design Group (IRI, Barcelona)

                image shows the Hexacrane, a cable-driven hexapod
                constructed by the Kinematics and Robot design group at

IRI's Hexacrane
Cable-driven hexapods are similar to Gough-Stewart platforms, but instead of rigid limbs, they use cables to govern the load. This results in manipulators with a low weight and a high load capacity, and permits to attain larger workspaces. However, additional constraints apply: their cables can pull, but are unable to push, which obliges to keep the cable tensions positive during normal operation.

The Kinematics and Robot design group develops path planning methods allowing to operate such robots automatically
[2,3]. The techniques evolve from earlier work in [1,4] and are being tested on the Hexacrane system shown above, designed and constructed by P. Grosch. This robot adopts the particular structure of the NIST Robocrane, but the planning methods remain applicable to general hexapods. In particular, all cable anchor points could adopt different positions if desired, not requiring to be coincident in pairs. The methods also allow the planning of the motions when additional constraints apply to the platform, such as geometric, or contact constraints.

The following video shows that it is not difficult to find configurations where some cables go slack...

... whereas with a proper planning such configurations can be avoided:

The path planner in [2,3] allows to avoid such slack-cable configurations. It automatically computes "wrench-feasible" paths between two configurations, where wrench-feasible means that the cable tensions will be guaranteed to remain within predetermined bounds, for a given platform wrench subject to 6-dimensional uncertainty. This also implies that no forward singularity will be met along the path, thus permitting a full control of the platform motions at all times.

The following picture shows a slice of the six-dimensional wrench-feasible C-space, and two planning queries solved by the planner. One to connect q1 with q2, and the other to connect q3 with q4. The horizontal and vertical axes are two orientation angles of the platform. The dark gray/green areas are the regions explored by the planner in each case. More complex planning queries in the full 6-D C-space can also be solved.

Solution of a planning query in a
                            2-dimensional problem

Current work seeks to extend these techniques to the context of aerial mobile manipulation. In [5], a 6-wired aerial manipulation system called the Flycrane is proposed, and a path planner inspired in
[2] is developed for it. In this case, the control of the load is achieved by maneuvering three quadrotors independently, while keeping the cable lengths fixed. The following pictures show two planning queries solved by this planner:

The Flycrane system getting a twisted
                            part through a hole

The Flycrane getting a twisted part through a hole

The Flycrane system
                            installing a lightweight footbridge between
                            two buildings

The Flycrane installing a lightweight footbridge between two buildings

The Flycrane and its planner are the result of a collaborative effort between the CUIK and ARCAS projects.


[1] O. Bohigas, M.E. Henderson, L. Ros, J.M. Porta. "A Singularity-free Path Planner for Closed-chain Manipulators." IEEE International Conference on Robotics and Automation, ICRA (St. Paul, USA), 2012.

[2] O. Bohigas, M. Manubens, and L. Ros. "Navigating the wrench-feasible C-space of cable-driven hexapods. In Cable-Driven Parallel Robots", T. Bruckmann and A. Pott (editors) Vol. 12 of Mechanisms and Machine Science. Pages 53-68. Springer, 2012. ISBN: 978-3-642-31987-7.

[3] O. Bohigas. "Numerical Computation and Avoidance of Manipulator Singularities". Ph.D. Thesis. Technical University of Catalonia - BarcelonaTech. 2013. See Chapter 6 especially.

[4] O. Bohigas, M. E. Henderson, L. Ros, M. Manubens, and J. M. Porta. "Planning Singularity-free Paths on Closed-Chain Manipulators". IEEE Transactions on Robotics. In press.

[5] M. Manubens, D. Devaurs, L. Ros, and J. Cortés. "Motion planning for 6-D manipulation with aerial towed-cable systems" In: Robotics: Science and Systems IX. Berlin, Germany, 2013. MIT Press