Developing a low cost dish for MeerKATWillem Esterhuyse

Subsystem manager: Antenna Structures

willem@ska.ac.za

083 447 1615

Overview

• Introduction

• Partners

• Main Specs

• Philosophy

• Schedule

• Progress

• Analyses (CFD/FEA)

• Detail Design

• Manufacturing/Construction

• Acceptance/Verification

• Life after XDM (Cost optimization and KAT7 –

composite dish)

• Conclusion

Introduction (1)

KAT Project PhasesXDM KAT7 MeerKAT

Antenna structures open tender. Equatorial mount still considered – many suppliers not interested.

Antennas open tender – proposal deadline 31 March.

Costs sensitive at this stage

Antennas open tender??

• This presentation from mechanical design

perspective

• Antenna performance/Feeds – EMSS (LJ du Toit/ Isak

Theron)

Introduction (2) - XDM15 meter PF antenna on ALT-AZ mount with feed rotator

• One-piece composite reflector that is structural

• Simple back-up structure

• No Panels to align

• Weight saving

• Concrete pedestal

• Pedestal only 30% of foundation (bedrock) volume

• Aim – Antenna for KAT that is relevant to SKA.

• Technical/Cost

• Development Subcontracted (time constraints)

• IST Dynamics (PTY) LTD

• Single line of responsibility (time, risk)

• MMS subcontracted for dish

Partners - IST

• IST Dynamics

• Prime contractor – IST exists for more than 25 years

• High tech electro-mechanical systems + software

• Military is biggest client

• ISO 9001 and ISO 14001

• www.ist.co.za, follow link to dynamics

Prototype development subcontracted to IST Dynamics.

Shared IP.

Partners - MMS

• MMS

• Subcontracted by IST

• Dish construction and design

• Exceptional experience in Composite

design and manufacture

• Existing for more than 20 years

• For more detail, see

www.mmstechno.co.za

Main Specifications and Design Goal XDM

KAT7/MeerKATProvisionalSpec Design Goal

Pointing Accuracy (deg) 0.04 0.02 25 arcsec RMS

Surface Accuracy (mm rms) 4 1.5 2 (target 1)mm

Frequency Range (MHz) 700 - 1700 Up to 8 GHz Up to 8 GHz

Wind (Operational) km/h 20 36 36

Wind (Marginal Operation) km/h 36 45 45

Wind (Drive to Stow) km/h 36 55 55

Wind (Survival) km/h 160 160 160

Azimuth Rotation speed (deg/s) 1 2 2

Elevation speed (deg/s) 0.5 1 1

Diameter 15 15 12

F/D 0.5 0.5 0.38

Lowest Natural Frequency (Hz) 3 3+ 3+

Feed Mass (kg) 200 200 Single pixel

Mount TypeAll-Az with Rotator, but

equatorial considered. Alt-Az

Philosophy

• Highest risk (technical and cost) – Reflector

• Preferably one-piece reflector (no alignment of panels)

• This requires manufacturing on site

• Tooling costs very high for steel, much lower for composites

• Composites attractive for smaller quantities (limited budget)

• Composites offer other advantages as well

• Lighter for same stiffness/strenght

• On site manufacturing easy (minimal equipment required)

• Better thermal performance

• Going to offset antennas not significant impact on reflector

• Preferably very simple backup structure (if any) –

structural reflector

• Pedestal – concrete (30% of foundation concrete)

• Revisited in cost optimization study

Schedule (KAT Antenna Structures)XDM

• Contract Signed 24 April 06 (DONE)

• PDR 20 June 06 (DONE)

• CDR 28 Aug 06 (DONE)

• FATS 12 Feb 07 (DONE)

• Installation Completed 16 May 07 (DONE)

• ATP Completed 17 July 07 (DONE)

• Documentation Completed 17 July 07 (DONE)

KAT7 & MeerKAT

• 12m Design/Cost optimization Sept 07 – Dec07 (DONE)

• KAT7 Tender Jan 07 (Progress)

• KAT7 Contract award April 07

• KAT7 Array Completed Nov 09

• MeerKAT Contract award Jan 2010

• MeerKAT Array Completed Nov 2012

ANALYSES - CFD

• Detailed CFD (Computational Fluid Dynamics) analyses

to determine pressures on dish as result of wind

loading (for use in FEA analyses)

• Results compared to published results – good correlation

• Results compared with limited experimental data – good

correlation

Analyses – FEA – Modes

Mode 1:

4.2 Hz

side to side

Mode 2:

4.4 Hz

dish “flaps”

Mode

3:

4.7 Hz

noddin

g

Analyses – FEA - displacements

LC

Dish RMS error

to focus[mm]

Actual focus point movement relative to best fit parabola focus point

Ux [mm]Focus

Uy [mm]In-plane

Uz [mm]In-plane

1 0.437 -4.39 0 -4.33

2 0.147 +3.43 0 +0.16

3 0.165 -0.3 -0.16 0

4 0.229 +3.54 +0.17 +0.05

5 0.334 +6.62 0 +1.55

6 0.416 -4.73 0 -1.94

7 0.464 +15.94 0 0

8 0.685 +19.72 0 +1.54

9 0.616 -8.30 0 +3.11

10 0.983 +5.18 0 +2.07

11 0.447 -5.68 0 -6.09

Detail Design/Manufacture - Dish

• Very simple backup structure

• Dish is structural (approx. 5 Tons)

• Symmetric, to eliminate curing distortions

• Manufacturing (see pictures later):

• Machine patterns (composite pattern material)

• Make composite moulds of the patterns

• Combine composite moulds to create dish

mould

• Innovative and cheap honeycomb structure (off

the shelf honeycomb cores very expensive).

• Vacuum Infusion process used for molding

entire dish as a unit

Detail Design/Manufacture - Dish

Layer no MaterialThickness

[mm]

1 Paint 0.05 – 0.1

2 Reflective aluminium layer 0.1 – 0.2

3 Gelcoat layer/FSP layer 0.5

4 E-glass vinylester 6

5 Quasi honeycomb structure 150

6 E-glass vinylester 6

7 Flowcoat layer/FSP layer 0.5

Typical dish lay-up

Detail Design – Drives/Controls• Azimuth axis

• 1 spring - loaded drive

• 2 deg/s, 460 degree travel

• Elevation axis

• 1 spring - loaded drive

• 1 deg/s, 0 to 95 degree elevation range

• Elevation axis is BALANCED

• Light dish (composites) makes this possible

with minimal use of counter weights

• Feed rotator axis

• 380 degree travel

Verification/Acceptance• Completed

• Achieved all design goals

• Dish tested at 1.42 and 12 GHz

• Dish accuracy around 1.7mm RMS

• (Confirmed with Theodolite and

Photogrammetry measurements)

• Improve? – pattern from Al, mold.

• Al Flame sprayed surface works well

• Pointing – 25 arcsec RMS (optically)

• Improve? – probably 2 drives

• Thermal performance – thermocouples

molded into dish front and rear to verify

analyses assumptions.

Life after XDM - Dishes

• Optimization and redesign (12m dish) effort

completed.

• Feb 08 – tender out for KAT7

• Apr 08 – KAT7 tender award

• Open tender, ie composite dish one of

options

• Dec 09 - KAT7 completion

• KAT7 prototype for MeerKAT

• Aggressive timescales

Gravity

Wind from front

Temperature, -5ºC from 20ºC

Gravity + Wind from side + Temperature, +40ºC, dish front +60ºC,

from 20ºC

Gravity

Wind from side

Gravity + Temperature, +40ºC, dish front +60ºC, from 20ºC

Frequencies

Mode 1:

4.58 Hz

Frequencies

Mode 1:

4.51 Hz

Conclusion• XDM

• Well engineered dish – not optimized for low frequencies

• Believe we have made good progress in developing a fairly

innovative dish in a short period of time from zero experience

• Major specifications were met

• Dish can be manufactured on site with little infrastructure

requirement

• Cost optimization on 12m dish completed by IST

• Tender for KAT7 currently open (closes 31 March)

• Cost sensitive at this stage

• Composite dish competes on cost

• MeerKAT choice of dish not decided yet.

• Final word – SKA Cost targets (risks, approach, site work)