Axiomatic Design for Axiomatic Design for Waveplate MechanismWaveplate Mechanism

I.E-655, Advanced CAD/CAM

-Krishnan V Kumar

-Rohan P Gavande

MotivationMotivation

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

Problem StatementProblem 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.

Terminology 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

Current DesignCurrent Design

½ WP

¼ WP

Full Blank

Half Blank

Supporting top frame

Supporting bottom frame

Light beam

Operational Mode 1Operational Mode 1

No Polarimetry: Full Blank only

Operational Mode 2Operational Mode 2

Linear: ½ waveplate + ¼ waveplate Blank

Operational Mode 3&4Operational Mode 3&4Circular: ½ waveplate + ¼ waveplateAll Stokes: ½ waveplate + ¼ waveplate

SpecsSpecs

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.

ConstraintsConstraints

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

Approach & MethodologyApproach & Methodologyphase Iphase 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

Approach & MethodologyApproach & Methodologyphase Iphase I

Decompose the parent level FRs to maintain independence

Map the decomposed FRs to respective decomposed DPs in physical domain

Construct the design matrix

Acclaro softwareAcclaro software

Useful tool for applying Axiomatic DesignDatabase contains theorems and corollaries

of Axiomatic DesignBuilt in examples to illustrate the design

process Most useful - Allows documentation of

comments

Working with AcclaroWorking with Acclaro

Functional RequirementsFunctional 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 45in 1 to 1.2 sec

FR 32: Rotate ½ WP in steps of 360/32FR 33: Provide accuracy of 3 arcminutes

Design Parameters (phase I)Design Parameters (phase I)

DP1: Linear motion of the top frame mounted with the ½ WP DP 11: Rail mechanismDP 12: Pneumatic cylinderDP 13: A supporting frame (box) structure in which the rail system is

mounted on the outer side

 DP 3: Gear mechanism of suitable gear ratioDP 31: MotorDP 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

Design Matrix (phase I)Design Matrix (phase I)DP1 DP2 DP3 DP4

DP11 DP12 DP13 DP21 DP22 DP23 DP31 DP32 DP33 DP41 DP42 DP43FR1 FR11 X O O O O O O O O O O O

FR12 X X O O O O O O O O O OFR13 O O X O O O O O O O O O

FR2 FR21 O O O X O O O O O O O OFR22 O O O X X O O O O O O OFR23 O O O O O X O O O O O O

FR3 FR31 O O O O O O X O O O O OFR32 O O O O O O X X O O O OFR33 O O O O O O X X X O O O

FR4 FR41 O O O O O O O O O X O OFR42 O O O O O O O O O X X OFR43 O O O O O O O O O X X X

Conclusion (phase I)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

Phase IIPhase II Customer Attributes:CA1 :Polarize light with waveplatesCA11: Compactness of structure Functional RequirementsFR1 :Waveplate MechanismFR11:Suitable configuration of waveplates FR12:Minimum travel of the mechanismFR13:FR13: The central axes of the

waveplates should be along the same line

Possible ConfigurationsPossible Configurations

½ WPFull Blank

¼ Blank¼ WP

Total z distance

½ WP

Full Blank

¼ Blank¼ WP

Total z distance

New SchematicNew Schematic½ WP Full Blank

Half Blank

Supporting top frame

Supporting bottom frame

Light beam

¼ WP

Advantages:-        

-Reduced travel of the waveplates resulting in better positioning accuracy.

-Reduction in total space occupied by the mechanism.

New FRsNew 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 45in 1 to 1.2 sec.   FR 32: Rotate ½ WP in steps of 360/32. o  FR 33: Provide accuracy of 3 arcminutes.

DPsDPs 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

New FR-DP MatrixNew FR-DP Matrix

DP3 DP4DP11 DP12 DP21 DP22DP23DP31 DP32 DP33 DP41 DP42 DP43

FR1 FR11 X O O O O O O O O O OFR12 O X O O O O O O O O O

O O O O O O O O O

FR2 FR21 O O X O O O O O O O OFR22 O O O X O O O O O O O

O O O O O O

FR3 FR31 O O O O O X O O O O OFR32 O O O O O X X O O O OFR33 O O O O O X X X O O O

FR4 FR41 O O O O O O O O X O OFR42 O O O O O O O O X X OFR43 O O O O O O O O X X X

DP1 DP2

Further AnalysisFurther 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

Salient Features of New Salient Features of New DesignDesign

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