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Research Papers

A Controller Avoiding Dynamic Model Degeneracy of Parallel Robots During Singularity Crossing

[+] Author and Article Information
Damien Six

LS2N, UMR CNRS 6004,
Nantes 44321, France;
Ecole Centrale Nantes,
Nantes 44321, France
e-mail: Damien.Six@ls2n.fr

Sébastien Briot

LS2N, UMR CNRS 6004,
Nantes 44321, France
e-mail: Sebastien.Briot@ls2n.fr

Abdelhamid Chriette

LS2N, UMR CNRS 6004,
Nantes 44321, France;
Ecole Centrale Nantes,
Nantes 44321, France
e-mail: Abdelhamid.Chriette@ls2n.fr

Philippe Martinet

LS2N, UMR CNRS 6004,
Nantes 44321, France;
Ecole Centrale Nantes,
Nantes 44321, France
e-mail: Philippe.Martinet@ls2n.fr

1Corresponding author.

Manuscript received February 20, 2017; final manuscript received May 30, 2017; published online August 7, 2017. Assoc. Editor: Venkat Krovi.

J. Mechanisms Robotics 9(5), 051008 (Aug 07, 2017) (8 pages) Paper No: JMR-17-1046; doi: 10.1115/1.4037256 History: Received February 20, 2017; Revised May 30, 2017

Parallel robots present singular configurations that divide the operational workspace into several aspects. It was proven that type 2 and leg passive joint twist system (LPJTS) singularities can be crossed with a trajectory respecting a given dynamic criterion. However, the practical implementation of a controller able to track such trajectories is up to now limited to restrictive cases of type 2 singularities crossing. Analyzing the structure of the inverse dynamic model, this paper proposes a global solution allowing the tracking of trajectories respecting the general criterion for any singularity that leads to potential issues of dynamic model degeneracy. The tracking is operated in the robot joint space. Experimental results on a five-bar mechanism showed the controller ability to successfully cross type 2 singularities.

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Figures

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Fig. 1

Example of a five-bar mechanism in a type 2 singularity configuration. The vector x˙s represents the direction of the uncontrolled platform motion. f1 and f2 are the forces that can be generated through actuation on the platform. In this configuration, the resultant force that can be applied by the system on the platform is collinear to f1 and f2. Two actuators are used to generate an effort along one direction; an overconstraint is consequently generated in the mechanism.

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Fig. 2

Computed-torque controller in joint space

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Fig. 3

Five-bar mechanism and parametrization scheme

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Fig. 4

Tracked trajectory in Cartesian space (to scale)

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Fig. 5

Input torque applied, σ and σs values, and tracking error on a singularity crossing trajectory. σ = σs = 0 means that the robot is close to a singularity and the wrench projection in the controller is active.

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