Axiomatic Design for Waveplate Mechanism

1. Axiomatic Design for Waveplate Mechanism I.E-655, Advanced CAD/CAM -Krishnan V Kumar -Rohan P Gavande

2. Motivation • The mechanism is an important constituent of the South Africa Large Telescope (SALT) • Interesting system dynamics to address: - relative linear motions and rotations • Simple functionality and complex in terms of accuracy, repeatability, and control

3. Problem Statement • Analysis of the existing design of the Waveplate mechanism in the PFIS structure using Acclaro software, to determine the scope of improvement and • Implementation of principles of Axiomatic Design to generate a new design plan.

4. Terminology • Waveplate – An optical surface used to polarize light • Blank – Another optical surface, does not polarize the light • The Waveplate mechanism consists of combinations of waveplates and blanks, used to study the light beam

5. Supporting top frame ½ WP Supporting bottom frame ¼ WP Full Blank Half Blank Light beam Current Design

6. Operational Mode 1 • No Polarimetry: Full Blank only

7. Operational Mode 2 • Linear: ½ waveplate + ¼ waveplate Blank

8. Operational Mode 3&4 • Circular: ½ waveplate + ¼ waveplate • All Stokes: ½ waveplate + ¼ waveplate

9. Specs • The maximum insertion mode changeover time is 6 sec • The ½ and ¼ waveplates should be able to rotate about their own axes through 45 in 1 – 1.2 secs • Motion of waveplates should be possible in both directions. • Rotational positioning of both the waveplates must be repeatable to the same angular position in steps of 360/32 to 3 arc minutes.

10. Constraints • The maximum heat and power generated should not exceed 50W • The z envelope of the mechanism should not exceed 55 mm • The gap width between the top and bottom frame is 5 mm

11. Approach & Methodologyphase I • Understand the functionality of every component in the current design (system dynamics) • Formulate the parent level FRs from the given specs • Assign DPs present in the design satisfy the FRs

12. Approach & Methodologyphase I • Decompose the parent level FRs to maintain independence • Map the decomposed FRs to respective decomposed DPs in physical domain • Construct the design matrix

13. Acclaro software • Useful tool for applying Axiomatic Design • Database contains theorems and corollaries of Axiomatic Design • Built in examples to illustrate the design process • Most useful - Allows documentation of comments

14. Working with Acclaro

15. Functional Requirements FR 1:Align the central axis of top waveplate (1/2 WP) with the beam of light FR 11: Guide way for motion of top frame FR 12: Positioning of the waveplate in the frame in less than 6 sec FR 13: Provide support for guiding and positioning mechanism FR 3:Rotate ½ WP to observe polarization at different angles FR 31: Provide drive for the driving gear such that ½ WP be able to rotate through 45in 1 to 1.2 sec FR 32: Rotate ½ WP in steps of 360/32 FR 33: Provide accuracy of 3 arcminutes

16. Design Parameters (phase I) DP1:Linear motion of the top frame mounted with the ½ WP DP 11: Rail mechanism DP 12: Pneumatic cylinder DP 13: A supporting frame (box) structure in which the rail system is mounted on the outer side DP 3:Gear mechanism of suitable gear ratio DP 31: Motor DP 32: Indexing mechanism. Slots are provided on the waveplate holder at 360/32 and an indexing detent (rod) is provided that drives in and out of these slots DP 33: Anti-backlash mechanism

17. Design Matrix (phase I)

18. Conclusion (phase I) The Design Matrix displays a decoupled design • The rail mechanism affects both FR11(Align) and FR12(Guide) • The Motor DP31 affects FR31(Rotation) FR32(Indexing) and FR33(Accuracy) Solution: • Change DPs or reduce coefficient of X

19. Phase II • Customer Attributes: CA1 :Polarize light with waveplates CA11: Compactness of structure • Functional Requirements FR1 :Waveplate Mechanism FR11:Suitable configuration of waveplates FR12:Minimum travel of the mechanism FR13:FR13: The central axes of the waveplates should be along the same line

20. Possible Configurations

21. Supporting top frame Full Blank ½ WP Supporting bottom frame ¼ WP Half Blank Light beam New Schematic Advantages:-         -Reduced travel of the waveplates resulting in better positioning accuracy. -Reduction in total space occupied by the mechanism.

22. New FRs • FR 1: Align the central axis of top waveplate (1/2 WP) with the beam of light FR 11: Positioning of the waveplate in the frame in less than 6 sec.   FR 12: Provide support for positioning mechanism • FR 3: Rotate ½ WP to observe polarization at different angles. o  FR 31: Provide drive for the driving gear such that ½ WP be able to rotate through 45in 1 to 1.2 sec.   FR 32: Rotate ½ WP in steps of 360/32. o  FR 33: Provide accuracy of 3 arcminutes.

23. DPs • DP1: Linear motion of the top frame mounted with the ½ WP -DP 11: Lead Screw Actuator. -DP 12: End Supports • DP 3: Gear mechanism of suitable gear ratio • -DP 31: Motor -DP 32: Indexing mechanism. Slots are provided on the waveplate holder at 360/32 and an indexing detent (rod) is provided that drives in and out of these slots. o    -DP 33: Anti-backlash mechanism

24. New FR-DP Matrix

25. Further Analysis • Determine the coefficient of X to reduce the impact of DPs • The coefficient of X can be reduced based on the configuration of anti-backlash mechanism and the indexing mechanism

26. Salient Features of New Design • Uncoupled design for two FRs • Reduction in overall weight of the mechanism • Only one set of rails is needed • DP12 and DP to be integrated since functional independence can be maintained and same material can be used