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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