Singularity-Invariant Transformations in Stewart-Gough Platforms: Theory and Applications

Júlia Borràs Sol

Barcelona. Spain

29/01/2009

Singularity-Invariant Transformationsin Stewart-Gough Platforms:

Theory and Applications

Thesis Project

Thesis Advisor : Federico Thomas Tutor : Raúl Suárez

Contents

1. Introduction

2. State of the art

i. Kinematics

ii. Classifications

iii. Singularities

iv. Workspace and dexterity

3. Objectives and Contributions

i. General Objectives

ii. Achieved Objectives/Publications

4. Work scheduling & Publications

5. Conclusions

In general, its forward kinematics and singularity analysis are difficult

Introduction

Stewart-Gough Platform is a parallel manipulator consisting of a moving rigid platform and a rigid base connected with six extensible legs.

State of the art

[1] D. Stewart,“A platform with 6 degrees of freedom,” Proc. of The Institution of Mechanical Engineers, Vol.180, No. 15, pp. 371-386, 1965/66.

[2] V.E. Gough and S.G. Whitehall,“Universal tyre test machine,” Proc. 9th Int. Tech. Congr. FISITA., Instn Mech. Engrs, pp. 117, 1962.

[20] B. Dasguptaa and T.S. Mruthyunjayab,“The Stewart platform manipulator: a review,” Mechanism and Machine Theory, Vol. 35, pp.15-40, 2000.

Since 1990’s, there has been a steady increase in research interest in the Stewart-Gough platform.

Parallel manipulators (respect Serial Manipulators):-Greater rigidity.-Superior positioning capabilities. -Much better load/mass ratio

(load is distributed on the legs).

Stewart-Gough Platform: prototypes

li

qi

pi

i.Kinematics

Forward kinematics problem: Given the lengths of the 6 legs, find the position and orientation of the mobile platform.

Inverse kinematics problem: Given the position and orientation of the mobile platform, find the legs lengths.Easy. You have the coordinates of all the points in the same reference.

Compute the lengths with the Euclidean distance formula.

No closed-form solution in general. Six sphere equations system must be solved. Intersection theory is used to prove that the maximum number of solutions is 40.

Several configurations are possible for a given set of lengths.

Configuration Space Workspace

FKIK

[10] F. Ronga and T. Vust, “Stewart platforms without computer?”,Preprint, Université de Genève, 1992[9] D. Lazard, “Stewart platforms and Grobner basis,” Proceedings of Advances in Robotics Kinematics, pp. 136-142, 1992.

[11] M. Raghavan, “The Stewart platform of general geometry has 40 configurations,” Journal of mechanical design, Vol. 115, No. 2, pp. 277-282, 1993.

Combinatorial representation using graphsPossible names:

3-3 3 platform points, 3 base points3 / 2 / 1 tripod / 2 leg group / single leg

Each graph represents the combinatorial class of manipulators with the same structure

[14] J.C Faugère and D. Lazard, “Combinatorial classes of Parallel Manipulators”, Mechanism and Machine Theory, 1995

ii. Classifications

Classification based on

Rigid Subcomponents

[23] X. Kong and C.M. Gosselin, “Classification of 6-SPS Parallel Manipulators According to Their Components,” Proc. ASME Des. Eng. Tech. Conf., 2000.

Point-Line Point-Plane Line-Line Line-Plane

Examples of one of the classes presented in [23]

Platforms in a class have -neither the same forward kinematics -nor the same singularity structure .

Existence of rigid subcomponents simplifies its analysis.

iii.Singularities

SINGULARITIES

[5] J-P. Merlet, “Singular Configurations of Parallel Manipulators and Grassmann Geometry,” The International Journal of Robotics Research, Vol. 8, No. 5, pp. 45-56,1989.

Plücker Coordinates of the leg lines

Type I Type II

[6] C. Gosselin and J. Angeles, “Singularity Analysis of Closed-Loop Kinematic Chains,” IEEE Transactions on Robotics and Automation, Vol. 6, No. 3, pp. 281-290, 1990.[24] B. Mayer St-Onge and C. Gosselin, “Singularity Analysis and Representation of the General Gough- Stewart Platform,” The International Journal of Robotics Research, Vol. 19, No. 3, pp. 271-288, 2000.

Architectural singularitiesType III

[7] O. Ma and J. Angeles, “Architecture Singularities of Platform Manipulators,” Proc. IEEE Intl. Conf. on Robotics and Automation, Vol. 2, pp. 1542-1547, 1991.

With the prismatic joints locked, architecturally singular manipulators exhibit a self-motion.

When legs are forming a linear complex independently of the leg lengths.

Architectural singularities are directly related with projective invariants.

When the attachments fulfill some metric relation

[22] M.L. Husty and A. Karger, “Architecture Singular Parallel Manipulators and Their Self-Motions,” in Advances in Robot Kinematics, J. Lenarcic and M. M. Stanisic (eds.), Kluwer Academic Publishers, pp. 355-364, 2000.

[33] A. Karger, “Architecture Singular Planar Parallel Manipulators,” Mechanism and Machine Theory, Vol. 38, pp. 1149-1164, 2003.

Architectural singularitiesType III

iv.Workspace and dexterity

Manipulability ellipsoid

Condition number

Well-conditioned workspace Distance to singularities

[39] J-P. Merlet, “Jacobian, Manipulability, Condition Number, and Accuracy of Parallel Robots,” Journal of Mechanical Design, Vol. 128, pp. 199-206, 2006.

Manipulability index

Error amplifications

Accuracy/dexterity of the robot

Closeness to a singularity

Criteria for optimal design

Strong coupling between position and orientation.

Must take into account singularity analysis and leg collision.

Problems:

All indexes depend on the chosen norm and units, and they all rely on the Jacobian matrix.

1. Introduction

2. State of the art

i. Kinematics

ii. Classifications

iii. Singularities

iv. Workspace and dexterity


i. General Objectives

ii. Achieved Objectives/Publications


5. Conclusions

Contents

i.Objectives

In general, moving the attachments modifies the singularity locus in an unexpected way.

When the platforms contains rigid subassemblies, we can rearrange the legs so that the singularity locus is modified in a controlled way.

Key points: - kinematics of the components are not modified.- the resultant Jacobian determinant is not modified, up

to scalar multiplication.

Singularity-invariant transformations derived from rigid components:

Delta-transform Line-Plane transform

Does any singularity-invariant Plane-Plane transformation exist?

Objectives: Applications of the SI transformations

* Generate families of Stewart-Gough platforms with the same singularity locus.

* Classify and characterize architectural singularities through singularity-invariant transforms.

* Provide a tool to optimize in a design process -the dexterity or-avoid possible collisions between its legs

without modifying the singularities.

* Decouple the two problems of- location of the singularities in a given workspace- improving the dexterity in a given region of the configuration space.

* Design reconfigurable platforms whose attachments can be modified statically or dynamically.

-If a reconfiguration is done with a singularity-invariant transformation, the control of the platform is not increased by the fact of being reconfigurable.

* Make contributions in the study of mathematical tools to deal with singularities:- Grassmann-Cayley algebra.- Geometric Algebra

* Avoid multiple spherical joints.

ii. Achieved objectives

Point-Line transformation:

J. Borràs, F. Thomas and C.Torras, "On Delta-transforms,'' submitted to the IEEE Transactions on Robotics (second revision).

J. Borràs, F. Thomas and C. Torras, "Analyzing the Singularities of 6-SPS Parallel Robots Using Virtual Legs,'' Proc. II International Workshop on Fundamental Issues and Future Research Directions for Parallel Mechanisms and Manipulators, pp. 145-150, 2008.

J. Borràs, F. Thomas and C. Torras, "Architecture Singularities in Flagged Parallel Manipulators,'' IEEE International Conference on Robotics and Automation, pp. 3844-3850, 2008.

Line-Plane transformation:

J. Borràs and F. Thomas, "Kinematics of the Line-Plane Subassembly in Stewart Platforms,'' accepted to the IEEE International Conference on Robotics and Automation, 2009.

J. Borràs, F. Thomas, E. Ottaviano and M. Ceccarelli, "A Reconfigurable 5-DOF 5-SPU Parallel Platform,'' submitted to IEEE/ASME International Conference on Reconfigurable Mechanisms and Robots.

Contributions on Grassmann-Cayley algebra dealing with singularities:

J. Borràs, F. Thomas and C. Torras, "Straightening-Free Algorithm for the Singularity Analysis of Stewart-Gough Platforms with Collinear/Coplanar Attachments," submitted to Computational Kinematics.

Point-Line Transformation

m n

The length of the new leg can be found uniquely


We got the first singularity invariant transformation!

Applying sequentially

Applying sequentially several Delta-transforms, we obtain complex transformations.

Applying simultaneously

Can this factor be zero?

Applying simultaneously

ArchitecturalSingularityCondition

Line-Plane Component


SINGULARITIES OF THE LINE-PLANE COMPONENT:

(x1,y1,z1)

(x2,y2,z2)

(x3,y3,z3)

(x4,y4,z4)

(x5,y5,z5)

Let us consider the hypersurface in


The same coefficients !!


We got the second singularity invariant transformation!!

Reconfigurable robot


Contents

1. Introduction

2. State of the art

I. Kinematics

II. Classifications

III. Singularities

IV. Workspace and dexterity


I. General Objectives

II. Achieved Objectives/Publications


5. Conclusions

Work Scheduling

2007

J. Borràs, F. Thomas and C. Torras, "Straightening-Free Algorithm for the Singularity Analysis of Stewart-Gough Platforms with Collinear/Coplanar Attachments," submitted to Computational Kinematics.


J. Borràs, F. Thomas and C.Torras, "On Delta-transforms,'' submitted to the IEEE Transactions on Robotics (second revision).

J. Borràs and F. Thomas, "Kinematics of the Line-Plane Subassembly in Stewart Platforms,'' IEEE International Conference on Robotics and Automation, 2009.

J. Borràs and R. Di Gregorio, "Polynomial Solution to the Position Analysis of Two Assur Kinematic Chains with Four Loops and the Same Topology,'' Journal of Mechanisms and Robotics, to appear in 2009.

J. Borràs, F. Thomas and C. Torras, "Analyzing the Singularities of 6-SPS Parallel Robots Using Virtual Legs,'' Proc. II International Workshop on Fundamental Issues and Future Research Directions for Parallel Mechanisms and Manipulators , pp. 145-150, 2008.

J. Borràs and R. Di Gregorio, "Direct Position Analysis of a Large Family of Spherical and Planar Parallel Manipulators with Four Loops,'' Proc. II International Workshop on Fundamental Issues and Future Research Directions for Parallel Mechanisms and Manipulators, pp. 19-29, 2008.

J. Borràs, F. Thomas and C. Torras, "Architecture Singularities in Flagged Parallel Manipulators,'' IEEE International Conference on Robotics and Automation, pp. 3844-3850, 2008.

J. Borràs, E. Ottaviano, M. Ceccarelli, and F. Thomas, "Optimal Design of a 6-DOF 4-4 Parallel Manipulator with Uncoupled Singularities,'' Revista de la Asociación Española de Ingeniería Mecánica, Año 16, Vol. 2, pp. 1047-1052, 2008.

Publications

Conclusions

We think that it is a new and original approach to singularity analysis.

Singularity-invariant transformations can be useful to:

Classify manipulators with respect to singularities

Design reconfigurable robots

Provide a tool to designers to optimize workspace or dexterity.

Increase the number of known manipulators with closed-form kinematics

Characterize architectural singularities with full geometrical meaning.

Thank you!!

Singularity-Invariant Transformations in Stewart-Gough Platforms: Theory and Applications

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