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Name Designation Affiliation Signature & Date
Authored by:
A. Krebs Mechanical
Engineer EMSS
Antennas
Approved by:
A. Taylor SPF Band 345 PM
Oxford University
I.P. Theron SPF Lead Engineer
EMSS Antennas
SINGLE PIXEL FEED VACUUM SERVICE
DESIGN DOCUMENT
Document number ........................................................................ SKA-TEL-DSH-0000092 Revision ........................................................................................................................... 2 Author .................................................................................................................. A. Krebs ................................. (with L.D. Mc Nally, I.J. Liebenberg, J. van Staden and J. Conradie) Date ................................................................................................................. 2018-10-08 Status ................................................................................................................... Released
Alexander Krebs (Oct 12, 2018)
ATaylor (Oct 12, 2018)ATaylor
Isak Theron (Oct 12, 2018)Isak Theron
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DOCUMENT HISTORY Revision Date Of Issue Engineering Change
Number Comments
A 2016-09-30 - First draft prepared for DDR
1 2016-11-30 - Released after DDR feedback
1A 2017-11-03 - Draft prepared for CDR
2 2018-10-08 CN0038, CN0057 Released with CDR feedback and vacuum pump station
re-design.
DOCUMENT SOFTWARE Package Version Filename
Word processor MS Word Word 2013 SKA-TEL-DSH-0000092_Rev2_SPFVacDesignDoc.docx
Block diagrams
Other
ORGANISATION DETAILS Name SKA Organisation
Registered Address Jodrell Bank Observatory
Lower Withington
Macclesfield
Cheshire
SK11 9DL
United Kingdom
Registered in England & Wales Company Number: 07881918
Fax. +44 (0)161 306 9600
Website www.skatelescope.org
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TABLE OF CONTENTS
ABBREVIATIONS............................................................................................. 6
1 SCOPE ........................................................................................................ 7
1.1 Introduction ............................................................................................................................ 7
1.2 Scope of the Document .......................................................................................................... 7
2 DOCUMENTS................................................................................................ 8
2.1 Applicable Documents ............................................................................................................ 8
2.2 Reference Documents ............................................................................................................. 8
3 DESIGN DESCRIPTION ................................................................................... 10
3.1 Context .................................................................................................................................. 10
3.1.1 High Level Context Diagram .......................................................................................... 10
3.1.2 External Interface Definitions ....................................................................................... 11
3.2 Functional Architecture ........................................................................................................ 12
3.3 Product Breakdown Structure .............................................................................................. 12
3.4 Design Driving Requirements ................................................................................................ 13
3.5 Internal Interfaces ................................................................................................................. 14
4 MAJOR COMPONENT DESIGN ........................................................................ 16
4.1 Vacuum Pump Trade-off Study ............................................................................................. 17
4.1.1 Rotary Vane Pump ........................................................................................................ 17
4.1.2 Turbo Molecular Pump ................................................................................................. 18
4.1.3 Scroll Pump ................................................................................................................... 19
4.1.4 Vacuum Pump Down Select .......................................................................................... 20
4.1.5 Testing ........................................................................................................................... 22
4.1.6 Selected Vacuum COTS Pump ....................................................................................... 24
4.2 Vacuum Pump Assembly ....................................................................................................... 26
4.2.1 Pump Vacuum Module Assembly ................................................................................. 26
4.2.2 Vacuum Pump Enclosure .............................................................................................. 28
4.2.3 Vacuum Pump Controller Assembly ............................................................................. 29
4.3 Vacuum Lines ........................................................................................................................ 38
4.3.1 Vacuum Manifold .......................................................................................................... 39
4.3.2 Vacuum Hoses ............................................................................................................... 40
5 POWER REQUIREMENTS................................................................................ 42
6 MAINTENANCE AND LOGISTICS ....................................................................... 42
6.1 Vacuum Pump Assembly ....................................................................................................... 42
6.1.1 Vacuum Pump ............................................................................................................... 42
6.1.2 Vacuum Pump Enclosure .............................................................................................. 43
6.1.3 Vacuum Controller Assembly ........................................................................................ 43
6.2 Vacuum Lines ........................................................................................................................ 43
6.3 Vacuum Manifold .................................................................................................................. 43
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7 SAFETY ..................................................................................................... 43
8 CONCLUSION ............................................................................................. 44
LIST OF FIGURES
Figure 1: SPF vacuum service context diagram..................................................................................... 10
Figure 2: SPF vacuum external interfaces. ............................................................................................ 11
Figure 3: SPF vacuum service functional breakdown. .......................................................................... 12
Figure 4: SPF vacuum service PBS. ........................................................................................................ 12
Figure 5: Internal interfaces of the SPF vacuum service. ...................................................................... 14
Figure 6: MeerKAT dish structure. ........................................................................................................ 16
Figure 7: SKA indexer rotation angle. ................................................................................................... 17
Figure 8: Leybold SC15D scroll pump orientation dependency. ........................................................... 23
Figure 9: Leybold Scrollvac 15 Plus COTS scroll pump. ......................................................................... 25
Figure 10: Leybold Scrollvac 15 Plus dimensions .................................................................................. 25
Figure 11: Leybold Scrollvac datasheet................................................................................................. 26
Figure 12: SKA pump base. ................................................................................................................... 26
Figure 13: Pump vacuum module assembly. ........................................................................................ 27
Figure 14: Fail-safe vibration mount for pump vacuum module assembly. ......................................... 27
Figure 15: Vacuum pump enclosure. .................................................................................................... 28
Figure 16: Vacuum pump enclosure with side panel removed ............................................................ 28
Figure 17: Vacuum controller assembly................................................................................................ 30
Figure 18: Block diagram of vacuum pump controller assembly. ......................................................... 30
Figure 19: Motor starter components. ................................................................................................. 31
Figure 20: DG M TT 275 surge protection. ............................................................................................ 31
Figure 21: UPD24TP series solid-state relay. ........................................................................................ 32
Figure 22: Trip class 10 for 193-EECP .................................................................................................... 34
Figure 23: Solid-state pass-through overload relay. ............................................................................. 34
Figure 24: Fibre transmitter circuit. ...................................................................................................... 35
Figure 25: Fibre receiver circuit. ........................................................................................................... 36
Figure 26: Power supply circuit. ............................................................................................................ 36
Figure 27: NTC thermistor package. ..................................................................................................... 36
Figure 28: Logic circuit for SSR control. ................................................................................................ 37
Figure 29: Initialisation circuit. .............................................................................................................. 38
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Figure 30: Monostable trigger and output waveforms. ....................................................................... 38
Figure 31: ISO-KF seal with internal and external centring ring. .......................................................... 39
Figure 32: Vacuum manifold and vacuum hoses for two cryogenic feeds and one spare port. .......... 39
Figure 33: Stauff pipe clamp assembly. ................................................................................................ 40
Figure 34: Pump vacuum hose. ............................................................................................................. 40
Figure 35: Section view of a welded vacuum hose. .............................................................................. 41
LIST OF TABLES
Table 1: SPF vacuum service external interfaces .................................................................................. 11
Table 2: SPF vacuum service internal interfaces ................................................................................... 15
Table 3: Rotary vane pump options ...................................................................................................... 18
Table 4: Turbo molecular pump options............................................................................................... 19
Table 5: Scroll vacuum pump options. .................................................................................................. 20
Table 6: Pump type comparison ........................................................................................................... 21
Table 7: Orientation test results ........................................................................................................... 22
Table 8: Stabilised temperature, cool down duration and rise time at different vacuum levels ......... 24
Table 9: DG M TT 275 specifications ..................................................................................................... 31
Table 10: D53TP25D-10 specifications .................................................................................................. 33
Table 11: 193-EECP specifications ........................................................................................................ 35
Table 12: Leybold Scrollvac Plus maintenance plan ............................................................................. 42
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ABBREVIATIONS
ALMA Atacama Large Millimeter Array
COTS Commercial Off the Shelf
FLA Full Load Amperage
GM Gifford-McMahon
HARTRAO Hartebeesthoek Radio Astronomy Observatory
I2C Inter-Integrated Circuit
LNA Low Noise Amplifier
LRU Line Replaceable Unit
MeerKAT Karoo Array Telescope
MOV Metal Oxide Varistor
NPT National Pipe Thread
NRAO National Radio Astronomy Observatory
OMT Orthogonal Mode Transducer
PCB Printed Circuit Board
PCBA Printed Circuit Board Assembly
PDR Preliminary Design Review
PTC Positive Temperature Coefficient
PSU Power Supply Unit
RF Radio Frequency
RFI Radio Frequency Interference
RMS Root Mean Square
SCR Silicon-Controlled Rectifiers
SKA Square Kilometre Array
SKA-MID Square Kilometre Array Mid-Frequency Instrument (350 MHz to 14 GHz)
SPF Single Pixel Feed
SPFC Single Pixel Feed Controller
SPFHe Single Pixel Feed Helium service
SPFVac Single Pixel Feed Vacuum service
SSR Solid State Relay
TBC To Be Confirmed
TBD To Be Determined
TVS Transient Voltage Suppressor
VLA Very Large Array (in New Mexico, USA)
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1 SCOPE
1.1 Introduction
This document is the design report for the SKA single pixel feed vacuum service (SPFVac), item number 317-060000 as specified in [AD2], which forms part of the single pixel feed sub-element of the SKA-MID dish element [AD1].
1.2 Scope of the Document
This design document, based on the associated development specification, provides and justifies performance, interface, environmental, safety, logistic support, special design, construction and reasoning leading to design decisions and requirements which are inputs to the engineering and development of the item.
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2 DOCUMENTS
2.1 Applicable Documents
The following documents are applicable to the extent stated herein. In the event of conflict between the contents of the applicable documents and this document, the applicable documents shall take precedence.
[AD1] I. P. Theron, et al., “Single Pixel Feed (SPF) Sub-Element Design Document”, SKA-TEL-DSH-0000020, Rev. 6, 2018-07-17.
[AD2] L.D. Mc Nally, “Single Pixel Feed Vacuum Service Development Specification”, SKA-TEL-DSH-0000091-001, Rev. 1, 2016-11-30.
[AD3] G. Smit, “SKA1 Dishes Element Power Budget”, SKA-TEL-DSH-0000041, Rev. 2, 2017-03-03.
[AD4] T. Kusel, “MeerKAT Environmental Requirements”, M0000-0000V1-06 TM, Rev. B, 2010-11-18.
[AD5] T.J. Steyn, “SPF Controller to SPF Vacuum ICD”, SKA-TEL-DSH-0000096, Rev. 1, 2016-11-30.
2.2 Reference Documents
The following documents are referenced in this document. In the event of conflict between the contents of the referenced documents and this document, this document shall take precedence.
Adixen PASCAL series 2010SD, last accessed 09/11/2017, http://www.nanophys.kth.se/nanophys/facilities/nfl/aja/manuals-pdf/ADIXEN_PASCAL_MECHANICAL_PUMPS.pdf
Edwards EM Series (Small) E2M1.5, last accessed 09/11/2017, http://www.lesker.com/newweb/vacuum_pumps/pdf/manuals/e2m0.7 to e2m1.5 manual.pdf
ULVAC GLD Series GLD-136, last accessed 09/11/2017, https://www.lesker.com/newweb/vacuum_pumps/pdf/manuals/ulvac-instruction-manual_gld-137a-202a.pdf
Oerlikon Leybold HV Standard – Trivac B Series D4B, last accessed 09/11/2017, http://www.vacuumpumpsupply.com/content/D4B and D8B manual.pdf
Pfeiffer HiPace 80 Turbo Operating Instructions, last accessed 09/11/2017, https://www.lesker.com/newweb/vacuum_pumps/pdf/manuals/hicube80tc110manual.pdfhttp://www.lesker.com/newweb/vacuum_pumps/pdf/manuals/pfeiffer hicube classic.pdf
Leybold Vacuum Turbovac, last accessed 09/11/2017, http://www.idealvac.com/files/brochures/Leybold_Turbovac_Operating_ Instructions.pdf
Turbo Molecular Pump [UTM-FH/FW Series], last accessed 09/11/2017, http://www.ulvac.com/userfiles/files/Vacuum Pumps/Turbo Molecular Pump UTM-FH, UTM-FW.pdf
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Varian Triscroll 300 Series, last accessed 29/09/2016, http://www.agilent.com/en-us/products/vacuum-technologies/primary-medium-vacuum-pumps/dry-scroll-pumps/triscroll-300
Varian SH-110, last accessed 09/11/2017, http://www.agilent.com/en-us/products/vacuum-technologies/primary-medium-vacuum-pumps/dry-scroll-pumps/sh-110
Oerlikon Leybold ScrollVac SC15D, last accessed 29/09/2016, http://idealvac.com/files/brochures/Leybold_SC_5D_15D_30D_Scroll_Pump_ Brochure.pdf
Advantages of Three Phase Power over Single Phase Power, last accessed 29/09/2016, http://www.electrotechnik.net/2010/11/advantages-of-three-phase-power-over.html
Leybold Vacuum, TRIVAC 50 product information, last accessed 09/11/2017, https://www.ajvs.com/library/Leybold_TurboVac_TMP_50_Brochure.pdf
Edwards nXDS Scroll Pump Instruction Manual, last accessed 09/11/2017, http://www.idealvac.com/files/manuals/Edwards_nXDS_ScrollPumpManual.pdf
Leybold Dry Compressing Vacuum Pumps, last accessed 09/11/2017, https://www.leyboldproducts.uk/media/pdf/f6/29/1b/CP_020_Dry_Pumps_EN58b7f51a685e9.pdf
Edwards a clear edge nXDS dry scroll pump, last accessed: 09/11/2017, https://www.lesker.com/newweb/vacuum_pumps/pdf/brochures/nxds_ brochure.pdf
Leybold ONLINE SHOP GLOBAL, SCROLLVAC 15 plus, last accessed 13/08/2018, https://www.leyboldproducts.com/products/dry-compressing-vacuum-pumps/scrollvac-scrollvac-plus/pumps/2709/scrollvac-15-plus?number
Edwards, nEXT85 TURBOMOLECULAR PUMP, last accessed 13/08/2018, https://www.edwardsvacuum.com/uploadedFiles/Content/Pages/Products/Edwards_nEXT85_DataSheet.pdf
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3 DESIGN DESCRIPTION
The single pixel feed (SPF) vacuum service encapsulates the active process of generating a starting vacuum for the cryogenic feed packages installed on the SKA dish. The design includes all components required to perform this function. It includes environmental protection for the active components as well as start/stop control, monitoring and localised failsafe checks. Included in the design is the distribution of vacuum ports such that a maximum of 3 feed positions can be serviced.
3.1 Context
The SPF sub-element consists of: 3 separate single pixel feed packages (band 1, band 2 and band 345), the SPF helium service (SPFHe), the SPF controller (SPFC) and the SPF vacuum service (SPFVac).
The SPF vacuum service, depicted in Figure 1, is located on the feed indexer and has the main function of evacuating the cryogenically cooled feeds to a sufficiently low pressure to allow operation of the Gifford-McMahon cryocoolers or to provide a roughing vacuum for a turbo pump mounted on the feed package. At the point where maintenance of the vacuum is taken over by cryopumping of the cold surfaces, the SPF vacuum service is switched off and is only used again if a newly installed feed requires evacuation, or an operational feed needs to be regenerated. Figure 1 shows the SPF vacuum service in the context of the other dish elements, as well as the interface types present between them.
Figure 1: SPF vacuum service context diagram.
SKA SPF Sub-element
(SPF)
SPF
Vacuum Service
(SPFVac)
Dish Structure
Sub-element
(DS)
En
viro
nm
en
t
Dish Infrastructure
Sub-element (DI)
Fibre Network
(Connection to SPFC)
Fib
re C
on
ne
ctio
ns
SPF
Controller
(SPFC)
Control and
Monitoring Messages
SPF Band 345
Vacuum
Power Mechanical
SPF Band 2
SPF Band 1
Vacuum
Vacuum
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The external interfaces for the SPF vacuum service are shown in Figure 2 and Table 1.
Figure 2: SPF vacuum external interfaces.
Table 1: SPF vacuum service external interfaces
Interface Number
External To
Interface Type Component A Component B
I.M.DS_SPF.04 SPF Conductive SPFVac Dish structure
I.M.DS_SPF.10 SPF Mechanical SPFVac Dish structure
I.M.DI_SPF.01 SPF Fibre SPFVac Dish fibre network
SPF IIF04 SPFVac Monitoring and Control via Fibre SPFVac SPFC
SPF IIF06 SPFVac Vacuum SPFVac Reserve
SPF IIF07 SPFVac Vacuum SPFVac SPF band 2
SPF IIF08 SPFVac Vacuum SPFVac SPF band 345
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3.2 Functional Architecture
The functional architecture required to enable the SPF vacuum service to execute its function is shown in Figure 3.
SPF Vacuum Service
Provide interface between vacuum pump and feeds
(Vacuum Manifold & Lines)
Generate Vacuum(Vacuum Pump)
Report Data to SPFC(Vacuum Pump Controller)
Figure 3: SPF vacuum service functional breakdown.
3.3 Product Breakdown Structure
The physical hardware required to perform each function is depicted in Figure 4. The reasoning behind each of the specific hardware choices is addressed in section 4.
Figure 4: SPF vacuum service PBS.
SKADC LEVEL 5Sub- Element Major Compos (MC)
SPF Level 1____________SPF Lead
SPF Level 2Components of MCs *LRU____________
SPF MC Leads
SPF Level 3+SRU and lower *LRU____________
SPF MC Leads
Vac Pump (COTS)
(061001)
Vac Pump Controller
Assy.(061002)*
Vac Manifold(317-063000)*
SPF Vacuum(317-060000)
Vac PumpEnclosure(061003)*
Vac Pump Assy(317-061000)
Vacuum Hoses(317-062000)
Pump Vacuum Module Assy.
(061004)*
Controller PCBAssy (061006)
Vac Manifold HoseAssy. (062001)*
Vac Feed Hoses Assy. (062002)*
Misc.
Controller Mech.(061012,13)
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3.4 Design Driving Requirements
The SKA dish design positions the feed packages on the feed indexer which faces the sub reflector and in turn is suspended away from the main reflector together with the sub-reflector by means of the Arm structure. This is similar to the MeerKAT design (see Figure 6). In order to keep the pressure-drop over the piping as low as possible and to avoid routing the vacuum lines through wraps at points of rotation/translation, it is decided to locate the vacuum service with the feeds on the feed indexer. This resolves pressure-drop issues, but limits the type of pump that can be used due to:
• Uncertainty about resonant frequencies of the mounting structure at the time of writing, a pump with high vibration was seen as a risk.
• Its position on the antenna positioner, the selected pump should be able to endure changes in antenna positioning and orientation while operational.
• RFI limits as this is the location with the most severe restrictions, the pump may not contain digital electronics or brushes on the motors.
In addition to performance considerations, the design of the vacuum pump assembly is strongly driven by requirements based on cost, modularity, ease of assembly and ease of testing. The following design guidelines form the background for the detail design of all sub-systems and components:
• As far as possible, COTS hardware shall be used.
• Designs shall be as modular as possible, requiring the minimum matching of subcomponents to function correctly.
• The design shall maximise hand and tool access during assembly and integration.
• Close / fine tolerances shall be avoided, unless strictly necessary for the operation of a subsystem or component.
• The use of small (<M3) or non-metric fasteners shall be avoided, unless strictly necessary for the operation of a subsystem or component.
• Where possible, component material and manufacturing costs shall be minimised by
o avoiding unnecessarily large raw material sizes,
o using easily obtainable materials and
o using standard production techniques and processes.
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3.5 Internal Interfaces
The internal interfaces for the SPF vacuum service are shown in Figure 5 and Table 2.
Figure 5: Internal interfaces of the SPF vacuum service.
Mechanical
Key:
Power
SPFV Components
Vacuum Pump Assembly
Pump vacuum module
Motor
Vacuum Pump Enclosure
Vacuum HosesSPFV.IIF.12
Vacuum Pump Controller
SPFV.IIF.8
SPFV.IIF.9
Vacuum inlet hose(COTS)
SPFV.IIF.1
SPFV.IIF.3
SPFV.IIF.6
SPFV.IIF.5
SPF Vacuum
(Major Components)
Vacuum Feed Hose (x3)
Vacuum Manifold
Vacuum Manifold Hose
SPFV.IIF.10
SPFV.IIF.11
Cooling fan
Enclosure mounted purge valve
SPFV.IIF.13SPFV.IIF.14
SPFV.IIF.15
Signal
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Table 2: SPF vacuum service internal interfaces
Interface nr. Type of interface Component 1 Component 2
SPFVac.IIF.1 Vacuum Pump vacuum module Vacuum inlet hose (COTS)
SPFVac.IIF.3 Mechanical Pump vacuum module Vacuum pump enclosure
SPFVac.IIF.5 Vacuum Vacuum inlet hose (COTS) Enclosure mounted purge
valve
SPFVac.IIF.6 Mechanical Enclosure mounted purge
valve Vacuum pump enclosure
SPFVac.IIF.8 Mechanical Vacuum pump enclosure Vacuum pump controller
SPFVac.IIF.9 Conductive Vacuum pump controller Pump vacuum module
(Motor)
SPFVac.IIF.10 Vacuum Enclosure mounted purge
valve Vacuum manifold hose
SPFVac.IIF.11 Vacuum Vacuum manifold hose Vacuum manifold
SPFVac.IIF.12 Vacuum Vacuum manifold Vacuum feed hose
SPFVac.IIF.13 Conductive Vacuum pump controller Enclosure mounted purge
valve
SPFVac.IIF.14 Conductive Vacuum pump controller Pump vacuum module
(Cooling fan)
SPFVac.IIF.15 Signal Vacuum pump controller Pump vacuum module
(Temperature)
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4 MAJOR COMPONENT DESIGN
The SPF vacuum service components are located on the feed indexer of the dish structure. The SKA dish structure is similar to the MeerKAT dish structure shown in Figure 6. This identifies the positions of some of the main positioner components as well as the location of the elevation and indexer wraps. The design and location of the vacuum service components is discussed in the subsequent text.
Figure 6: MeerKAT dish structure.
The SPF vacuum service consists of the vacuum pump assembly (comprising the pump vacuum module assembly, vacuum pump enclosure and vacuum pump controller assembly) and the vacuum lines (comprising the vacuum manifold and vacuum hoses). This breakdown is depicted in Figure 4.
Different vacuum pumps were investigated to determine the most suitable vacuum pump for the use on the SKA dish. The design and reasoning behind component selection for each of the components is addressed in the sections below.
Boom Structure
Feed Indexer
Elevation Wrap
Pedestal
Indexer Wrap
Main Reflector
Sub Reflector
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4.1 Vacuum Pump Trade-off Study
Various vacuum pump technologies exist of which the following pumps were considered in a trade-off study:
• Rotary vane pump
• Turbo molecular pump
• Scroll pump
Other vacuum pump technologies not considered further include the diaphragm, screw, diffusion, ion and mechanical booster vacuum pumps as they do not suit the SKA SPF vacuum service requirements as mentioned in section 3.4. Each of the aforementioned technologies offers advantages and disadvantages as discussed in the subsequent text. Summaries of the different technologies and specific products are included in tabular form.
Rotary vane pumps are successfully used in radio astronomy installations. These installations include the VLA and MeerKAT. The SKA dish design is in many respects similar to the MeerKAT dish design, and a rotary vane option is again investigated for implementation.
Rotary vane pumps can reach ultimate pressures of < 5x10-2 mbar without difficulty and are typically less expensive than other pump technologies. Other advantages include robust design, long maintenance intervals and little to no effect on the pump while being moved during operation. The main disadvantage of rotary vane pumps for use in cryogenic applications is the use of oil lubrication, posing the inherent risk of oil contamination of the feed cryostat. Due to the oil sump of the rotary vane pump, it is orientation dependant. In the case of MeerKAT, the rotary vane pump is positioned in such a way to allow all indexer and antenna orientations without the risk of oil spilling from the sump. This cannot be done on the SKA dish due to more extreme indexer rotation angles. MeerKAT has a rotation angle of +/- 120° where SKA at the time of writing has a rotation angle of +/- 200°, see Figure 7.
Figure 7: SKA indexer rotation angle.
+/- 200°
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This problem can be addressed by mounting the pump on a gravitational swing below the feed indexer. However, doing so will introduce new design challenges and risks such as the frequent bending of the vacuum lines and an increase in vacuum line length. If a very strong argument for this type of pump can be made, the indexer design can be modified to reduce the rotation angle at a cost of adding balancing weight.
Rotary vane pumps have a typical operating ambient temperature range of +12°C to +40°C mainly due to the viscosity of the oil becoming too high at temperatures below +12°C to be able to safely start the vacuum pump. This is an issue as the average temperature on site between May and August is less than +12°C [AD4] meaning pump availability will be limited during this period, or not at all available at times.
Table 3 lists a number of the most suitable rotary vane pump options from different suppliers. The Leybold HV standard Trivac series D8B rotary vane pump, was identified as most suitable. It has the sufficiently low ultimate pressure, power consumption and mass, as well as an adequate pumping speed.
Table 3: Rotary vane pump options
Parameters Unit Adixen PASCAL series 2010SD
[RD1]
Edwards EM Series (Small) E2M1.5 [RD2]
ULVAC GLD-137AA
[RD3]
Leybold HV Standard –
Trivac Series D8B [RD4]
Weight kg 26 10 26 21.2
Size (H x D x W)
mm 240 x 437 x 164 238 x 323 x 142 240 x 486 x 170 265 x 462 x 162
Ultimate pressure mbar 2 x 10-3 1.5 x 10-3 6.7 x 10-3 < 2 x 10-3
Power usage W 450 160 400 370
Power requirements
- 342V to 460V
3-phase 50 Hz
200V to 230V 1-phase
50 Hz
380V to 415V 3-phase
50 Hz
380V to 420V 3-phase
50 Hz
Pumping speed l/s 2.7 0.5 2.25 2.7
Temperature range
°C 12 to 45 12 to 40 7 to 40 12 to 40
The use of a turbo molecular pump for the SKA vacuum service would allow a very low pressure to be reached prior to starting the cryocooler. A low start-up vacuum level is beneficial, as less gas is initially frozen out on the cryocooled surfaces, which in theory extends the duration that the feed can maintain cryopumping to provide good thermal isolation and keep a constant temperature on the 2nd stage of the cryocooler.
The SKA indexer design positions the vacuum service just behind the feeds on the indexer. With control electronics and large currents typically required to drive these pumps, RFI is an inherent risk. The option of moving high risk RFI components into an RFI shielded compartment located in the pedestal is a possible solution. Turbo pumps require a primary pump (backing pump) as well as digital control electronics to allow proper functioning which increase the power consumption, mass and volume of the vacuum pump assembly. Turbo molecular pumps have very high rotational speeds and in effect can act like a gyroscope. This was seen as a risk in that the pump can be damaged during emergency stop conditions. It has been confirmed by both Leybold and Edwards that turbo pumps are
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not orientation dependant and can be moved while it is operating, granted the motion is smooth. Turbo molecular pumps have also been successfully used on radio telescopes such as ALMA. Table 4 lists a number of the most suitable turbo molecular pump options from different suppliers. The Leybold Turbovac pump was identified as the most suitable option for use on the SKA. It is the lightest and smallest, has sufficient ultimate pressure and pumping speed and uses the least power. The backing pumps required for use with the turbo molecular pump are not discussed in this section, however they are suitable for use with dry vacuum technologies and the maintenance, power consumption and environmental requirements of these backing pumps will need to be taken into account if such a system is deemed to be a viable option.
Table 4: Turbo molecular pump options.
Parameters Unit Pfeiffer HiPace
80 [RD5]
Leybold Vacuum Turbovac 50
[RD6], [RD12]
ULVAC UTM-350FH [RD7]
Edwards nEXT85 [RD17]
Weight kg 2.4 2.0 17 3.0
Size (H x D x W)
mm 149 x 138 x 114 166 x 93 x 93 276 x 251 x 185 149 x 117 x 115
Ultimate pressure mbar 1 x 10-8 5 x 10-8 1 x 10-10 5 x 10-9
Power usage (max)
W 110 45 600 80
Power requirements
- 90V to 265V
1-phase 50 Hz
220V 1-phase
50 Hz
200V to 240V 1-phase
50 Hz
220V 1-phase
50 Hz
Pumping speed (N2)
l/s 67 28 330 84
Temperature Range
°C 5 to 35 Up to 55
(low end not specified)
0 to 40
With forced air up to 40 (low
end not specified)
Scroll pump technology allows oil free vacuum creation to levels of 1 x 10-2 mbar. A scroll vacuum pump uses two interleaving scrolls; one of the scrolls is fixed, while the other orbits eccentrically without rotating, to displace gas and create a vacuum. Despite the vacuum level not being as low as some of the other technologies, this pump technology was investigated further to determine its suitability for use in cryogenic applications. A noncomplex, robust dry vacuum system is seen to be largely suitable for the SKA indexer design.
Table 5 lists the most suitable scroll pump options and suppliers. The ScrollVac 15 Plus pump was selected as the most suitable option for use on SKA. The pump is available in a single-phase as well as three-phase option - 3-phase is preferred. The Scrollvac 15 Plus has a sufficiently low ultimate pressure and sufficient pumping speed to satisfy the requirements. Although it does not outperform the Edwards nXDS15i in terms of ultimate pressure and pumping speed, the Scrollvac 15 Plus is preferred. A major driving factor against the nXDS15i is the single-phase supply only option which at 300W may induce phase imbalance on the dish power network. Further the nXDS15i is designed with a large amount of control electronics which will likely need to be removed or bypassed due to strict RFI restrictions on the indexer. This pump is thus not seen as suitable for SKA implementation.
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The Scrollvac 15 Plus is heavier and bulkier than the ScrollVac SC15D (which is identical to the Anest Iwata ISP-250E scroll pump which was initially selected), but it achieves a lower pressure using less power. More importantly, since it uses an external fan as opposed to a blade on the pump axis, it is much better protected against dust and it forces the air in one direction which simplifies the housing.
Table 5: Scroll vacuum pump options.
Parameters Unit Leybold
Scrollvac 15 Plus [RD16]
Varian SH-110 [RD9]
Leybold ScrollVac SC15D
[RD10]
Edwards nXDS15i [RD13]
[RD15]
Weight kg 26 19.5 23 25.2
Size
(H x D x W) mm 325 x 430 x 282 257 x 384 x 276 336 x 370 x 252 302 x 432 x 282
Ultimate pressure
mbar 9 x 10-3 6.6 x 10-2 1.6 x 10-2 7 x 10-3
Power usage W 300 190 400 300
Power requirements
-
380V to 415V
3-phase, 50Hz
200V to 240V 1-phase, 50Hz
230V 1-phase 50Hz
380V to 415V
3-phase 50Hz 200V to 230V
1-phase 50Hz
200V to 240V
1-phase
50Hz
Cooling method - Forced air Air Air Air
Pumping speed l/s 3.4 1.5 4.2 4.2
Temperature range
°C +5 to +40 +5 to +40 +5 to +40 +5 to +40
To determine the most suitable vacuum solution for use on SKA, the most suitable vacuum pumps stated in the previous sections for each pump type were compared using the following parameters:
1. Orientation dependency
2. Pumping speed
3. Can pump move during operation
4. Ultimate pressure
5. Cost
6. Maintenance interval
7. Mass
8. Power usage
9. Power requirements
10. Cooling method
11. RFI implications
12. Lubrication requirements
13. Size
14. Other components required (e.g. backing pump)
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Table 6 summarises the most suitable vacuum pump option from each vacuum pump type considered. It gives typical values for various pump parameters of the evaluated models in each of the categories.
Table 6: Pump type comparison
Parameters Unit Leybold HV
Standard – Trivac Series D8B
Leybold Vacuum Turbovac 50 (with
DIVAC 2.2L backing pump [RD14])
Leybold ScrollVac 15 Plus
Pumping speed l/s 2.7 33 3.4
Ultimate pressure mbar < 2 x 10-3 5 x 10-8 9 x 10-3
Mass kg 21.2 2
(+12.6 backing pump) 26
Power usage (max)
W 370 45
(+245 backing pump) 300
Power requirements - 380V 3-phase, 50 Hz 220 V 1-phase, 50 Hz 380 V 3-phase, 50 Hz
Size (H x D x W)
mm 265 x 462 x 162
Custom enclosure for Turbovac 50: 166 x 93 x 93 DIVAC 2.2L:
166 x 226 x 341
325 x 430 x 282
Orientation dependency - Yes* No
(as long as dry vacuum backing pump is used)
No
Can pump move during operation
- Yes Yes Yes
Maintenance interval - 6 months – 1 year 1 year 1 year
Maintenance complexity
- Replace oil Replace diaphragm
seals on backing pump Replace tip seals
Cooling method - Air Air Forced air
RFI enclosure required - No Yes No
Lubrication required - Yes No No
Backing pump required - No Yes No
The Leybold HV Standard – Trivac Series D8B, was seen as unsuitable due to the pump using oil for lubrication and its orientation dependency. Rotary vane pump technology has been successfully used on MeerKAT and the VLA, however the advantages thereof are not sufficient to warrant redesigning the indexer to keep the pump within the required orientation. The addition of a gimbal platform would increase complexity and negate the advantages over the remaining pump technologies considered. The start-up temperature of +12°C is also seen as unfavourable. For these reasons, the rotary vane pump will not be considered further.
The Leybold Vacuum Turbovac 50 with DIVAC 2.2L backing pump provides a number of advantages. Turbo molecular pumps can reach much lower ultimate pressures in a short time, induce little vibration, do not use oil, and have the lowest weight and energy consumption. The turbo pump that was compared also has a wider allowable ambient temperature range, specifically at the lower temperatures. The pump can be installed in any orientation without detrimental effect to the pump
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itself. Pump movement while in operation has been demonstrated at HARTRAO where the pump is located inside the apex room. The turbo molecular pump’s less desirable characteristics include high cost, its maintenance requirements, and the high risk of RFI emissions. In addition, placing the pump station inside the pedestal shielded compartment increases the length and complexity of the vacuum lines and manifold design with the vacuum level at the feed package vacuum port sufficiently worse than what the pump is physically capable of doing at its inlet flange.
Advantages offered by the scroll pump include being cheaper and less complicated in design compared with a turbo molecular pump. It is robust in design, can be installed in any position to comply with all antenna positioner orientations, does not generate a problematic level of RFI, and can be moved while operating. The disadvantage of using the scroll pump is that it has the weakest ultimate pressure of all the vacuum technologies considered.
This section presents some tests that verify that the lower vacuum levels provided by a turbo pump is not an absolute requirement to operate the SPF cryostats.
Scroll Pump Orientation Dependency
To confirm that the scroll pump will deliver constant vacuum levels in any orientation, the following worst-case orientations given in Table 7 were tested. Angles noted are in relation to the elevation axis of the antenna and the rotation axis of the feed indexer. Development tests were performed on the Leybold SC15D scroll pump. Similar testing will be performed for the Leybold Scrollvac 15 Plus during qualification testing.
The scroll pump, with pressure sensor connected directly to its inlet port, was evacuated from atmospheric pressure. A pressure reading was taken after 80 minutes at the pump inlet port. These results are given in Table 7 and plotted in Figure 8.
Table 7: Orientation test results
Test no. Elevation angle Rotational angle Ultimate pressure after 80 minutes
(mbar)
1 0 0 9.32x10-3
2 -15 0 9.89x10-3
3 +45 0 9.96x10-3
4 +45 +45 1.16x10-2
5 +45 +90 1.06x10-2
6 +90 0 1.09x10-2
7 +90 +45 9.99x10-3
8 +90 +90 1.00x10-2
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Figure 8: Leybold SC15D scroll pump orientation dependency.
It is noted that the pressure versus pump-down time curves are similar for all tested orientations.
From these tests, it is confirmed that the scroll pump technology is not orientation dependent. The pump can be installed in any orientation with insignificant change to the ultimate pressure. The pump will, however, have to be tested in the different orientations for a long period of time to determine the wear on the seals and bearings of the scroll pump to better determine maintenance intervals.
Cryocooler Start-up Pressure
The main requirement of the vacuum system for SKA is the evacuation of the feeds to a sufficiently low pressure for successful cryocooler start-up. This vacuum level has been previously defined as 5x10-2 mbar and has been successfully used as such on the VLA and on MeerKAT. Turbo molecular pumps reach pressures below 1x10-4 mbar with ease. However, considering the large RFI risks and the accompanying costs of these systems, alternatives such as scroll pumps are considered as an alternative. Higher end scroll pumps can reach vacuum levels of 1x10-2 mbar.
The aim of the subsequent investigation is to determine whether a cryocooler can be started successfully at typical scroll pump pressures. This work package details the test approach to determine the cryocooler start-up response at different pressure levels.
The test objectives include the following:
1. Determine to what extent the cryocooler first- and second-stage stabilised temperatures are affected when starting the cryocooler at different vacuum levels.
2. Determine the maximum vacuum level at which the cryocooler can be started.
3. Determine whether the scroll vacuum pump can evacuate the cryostat to the vacuum level determined in 2 and determine the pump down time to reach that vacuum level.
The cryocooler was started at 7 different cryostat pressures. The cool down time between 287 K and 45 K, and the warm up time between when cooler was switched off and 200 K was recorded. The results are given in Table 8.
0.001
0.01
0.1
1
10
100
1000
10000
0 10 20 30 40 50 60 70 80Pre
ssu
re [
mb
ar]
Time [min]
Leybold SC15D Scroll Pump Orienation Dependancy
(1) 0_0deg
(2) -15_0deg
(3) 45_0deg
(4) 45_45deg
(5) 45_90deg
(6) 90_0deg
(7) 90_45deg
(8) 90_90deg
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Table 8: Stabilised temperature, cool down duration and rise time at different vacuum levels
Start Pressure (mbar) 0.5
mbar 0.15 mbar
0.1 mbar
0.08 mbar
0.05 mbar
0.035 mbar
2e-5 mbar
1st Stage Temp Minimum (K)
37.5 37.5 37.7 38.2 39.3 37.6 37.8
2nd Stage Temp Minimum (K)
9.5 9.3 9.0 9.3 9.6 9.6 9.4
Radiation Shield Temp Minimum (K)
41.5 41.3 41.6 42.2 43.5 41.5 41.2
1st Stage Cooldown Time 287K- 45K (s)
4103 3979 4043 4552 4272 3803 3828
2nd Stage Cooldown Time 287K - 15K (s)
2969 2900 2909 3114 2945 2749 2652
Cooler Run Time (s) 198207 170989 266836 12240 5326 86955 7650
1st Stage Rise Time to 200K (s)
3663 3814 3786 4107 4495 5137 7259
2nd Stage Rise Time to 200K (s)
7175 7561 7481 8718 9424 10834 13874
The data shows that the first- and second-stage absolute temperatures were unaffected. Cooldown times of the cooler stages increased by approximately 7% (comparing worst-case and best-case start-up vacuum levels. The effect of a degraded vacuum level on temperature rise duration is evident for vacuum levels worse than 0.08 mbar.
The results obtained during the tests described in this document indicate that the impact on cryocooler performance at higher cryostat start-up pressures is low. Cryocooler start-up vacuum levels of <0.08 mbar is proposed for SKA. Start-up vacuum levels are easily attained by scroll pump technology in any orientation (see Table 7). The recommended start up pressure for the cryostat is easily obtainable for the Scrollvac 15 Plus scroll vacuum pump. The results from these tests confirm that scroll pump technology is a suitable dry vacuum solution for use on SPF vacuum services.
The commercial off-the-shelf (COTS) Leybold Scrollvac 15 Plus scroll pump shown in Figure 9 is selected for SKA. Scroll pump technology is chosen over the turbo molecular pump technology as it reaches a sufficiently low ultimate pressure to start the cryocooler in any orientation. In the event of the scroll pump acting as a roughing pump for a feed local turbo pump, the vacuum levels fall well within the typical roughing vacuum required for small turbo pumps. The vacuum system design is thus also less complex, it poses less of an RFI risk, it is easily serviceable and it is also significantly more cost effective than a turbo-molecular pump system. The user manual for the pump can be viewed in [RD16]. Pump dimensions are shown in Figure 10, note that the single phase and 3-phase pump have the same dimensions, and Figure 11 shows the pump specifications. The 3-phase Scrollvac 15 Plus is chosen as there is minimal loss in performance compared with the 1-phase motor. Three-phase motors are smaller for the same power rating whereas single-phase motors are more prone to vibration and rely on capacitors that need maintenance over long operational periods. Another advantage of 3-phase over 1-phase is that it does not add to the 3-phase current imbalance at DSH level. The 3-phase option is thus more economical, efficient and convenient.
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Figure 9: Leybold Scrollvac 15 Plus COTS scroll pump.
Figure 10: Leybold Scrollvac 15 Plus dimensions (single phase version with identical dimensions shown).
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Figure 11: Leybold Scrollvac datasheet.
4.2 Vacuum Pump Assembly
Typically, COTS vacuum pumps are not designed for use in an outdoor environment. Due to the intended mounting position of the SPF vacuum system on the feed indexer, the design of a vacuum pump enclosure to protect the pump is required. Although not its primary function, the vacuum pump enclosure may provide additional RFI shielding. Together, the pump vacuum module assembly, vacuum pump enclosure and vacuum pump controller assembly form the vacuum pump assembly. Refer to Figure 4 for SPF vacuum context.
The pump vacuum module assembly consists of the vacuum pump, mounting plate and power harness assembly. These components are discussed in the sections below. In addition, but not physically attached to the pump vacuum module, the purge valve forms part of the pump vacuum module as an attaching part of the next higher assembly, the vacuum pump assembly.
As discussed in section 4.1.6, the Leybold 15 Plus scroll vacuum pump is selected for SKA. The scroll pump is by design intended for mounting with 4 x M6 fasteners. For pump maintenance purposes, it must be possible to remove the pump from the vacuum pump enclosure after assembly on the feed indexer. As the standard pump does not allow for this, a machined aluminium base with fixing holes located for ease of access is designed to attach to the pump in a shop environment. The pump base for the Leybold 15 Plus is shown in Figure 12.
Figure 12: SKA pump base.
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Leybold recommends the use of a purge valve on the pump inlet that allows the scroll pump to vent to atmosphere remotely. When the normally closed valve is opened, air is drawn through the scrolls for a few minutes before a vacuum is drawn on the cryostat. This is to overcome the challenge encountered with scroll pump technology where prolonged pumping or storage may result in moisture being trapped in the scrolls. This hinders the pump from reaching its designed ultimate pressure. Simply allowing the pump to draw atmospheric air for a few minutes allows the moisture to migrate out of the scrolls and out through the exhaust thus purging the pump of condensate.
Figure 13 shows the pump vacuum module assembly with the enclosure mounted purge valve assembly positioned as it would be on the enclosure wall. The purge valve is a normally closed valve that will only open for a few minutes before a cryostat is evacuated. The air filters are installed to ensure no dust gets into the scroll pump or vacuum manifold during the purging process.
Figure 13: Pump vacuum module assembly.
In order to minimise pump vibration on the indexer structure, the pump vacuum module assembly and vacuum pump enclosure are independently mounted to the indexer by means of vibration mounts. The vibration mounts used for the pump vacuum module are chosen to be fail-safe. The selected vibration mounts are for use in compression and shear applications making it suitable for use on the rotary vain pump which will see varying changes in orientation.
Figure 14: Fail-safe vibration mount for pump vacuum module assembly.
Pump vacuum module
Vacuum inlet hose (COTS)
Enclosure mounted purge valve
Power harness
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The vacuum pump enclosure consists of a stainless steel sheet metal enclosure protecting the pump vacuum module assembly and vacuum pump controller assembly against the external environment. The vacuum pump enclosure design is shown in Figure 15.
Figure 15: Vacuum pump enclosure.
Figure 16 shows the inside of the enclosure with a section view from the left face. The enclosure is divided into two compartments, an inlet and an exhaust compartment. This is done to generate a directional flow of ambient air over the pump aiding in cooling. The air intake is beneath the enclosure facing the indexer platform which will minimize the dust and water intake through the enclosure.
Figure 16: Vacuum pump enclosure with side panel removed (left: control interface, right: vacuum interface)
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During initial testing of the vacuum pump assembly it was noticed that the MeerKAT design for the vacuum pump enclosure will no longer be suitable for the selected scroll pump. The heat generated by scroll pumps is substantially more than that of the rotary vane pump used on MeerKAT for which convective cooling was sufficient.
Development testing has shown that directional air flow inside the vacuum pump’s enclosure lowers the ambient air to a level that Leybold is confident the pump will not see any premature failure. Considering here also the low active operational hours of the vacuum pump and regular maintenance intervals.
Electrical contact between all the panels and sieves are ensured by the manufacturing processes with no additional grounding straps being required. The enclosure exterior and interior will be epoxy coated white. To ensure electrical conduction between the lid and enclosure base, the surface of the lid and mating surfaces on the enclosure base and lid will remain uncoated. Quarter turn captive DZUS fasteners are used to ensure easy removal of the enclosure lid without the risk of losing fasteners during on dish maintenance tasks.
If necessary, the vacuum pump can be removed and replaced by removing the enclosure lid, unfastening the nuts holding down the pump and disconnecting the internal vacuum hose. The vacuum pump can be removed without needing to remove the vacuum controller assembly. Care was taken to select components posing no RFI risk. Similar fans are used on the helium compressor which satisfied the RFI requirements.
The vacuum enclosure offers some protection from direct rain, wind-blown dust and sunlight. The enclosure makes use of a 0.3mm wire mesh with a 0.4mm2 aperture which has been used successfully on the MeerKAT vacuum enclosure as an insect guard. Dust and water which may enter through the mesh is not expected to harm the pump. The Scrollvac 15 Plus has a completely enclosed motor with bearings for scroll motion completely enclosed within the housing. The IP rating of the fitted fan is unknown, however suitable alternative COTS fans have been identified and will be requested as standard in the event of failure during qualification.
The vacuum pump enclosure lid is held in place by 8 quarter-turn captive DZUS fasteners. DZUS fasteners are preferred in this instance due to the regular maintenance requirement on the pump vacuum module components which requires the scheduled opening and closing of the enclosure. Captive quarter-turn fasteners are easy to use and will remain attached to the enclosure lid when loosened. Ejector springs will help raise the fastener above the mating surface of the retainer spring.
The vacuum pump controller assembly receives external power supplied by the antenna positioner and relays the power to the pump vacuum module assembly when commanded by the SPFC.
Mechanical Design
The vacuum pump controller assembly is an LRU that must allow easy replacement when required. The vacuum pump enclosure provides the required mechanical strength to mount the vacuum controller assembly to the wall of the vacuum pump enclosure and also allows for a grounding connection (4 x M6 holes) to the vacuum pump enclosure.
A machined enclosure is designed to optimise the available space and that required for the internal electrical components. The vacuum pump controller assembly is mounted inside the vacuum pump enclosure base, to the right side of the pump vacuum module seen from behind the indexer and also shown in Figure 16. To install/remove the controller, the vacuum pump enclosure cover will first be removed. The vacuum controller assembly is shown in Figure 17.
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Figure 17: Vacuum controller assembly (left: pump vacuum module interface, right: control interface).
Electrical Design
Figure 18 illustrates how the control circuit interfaces with the motor and motor starter circuit. The analogue control circuit power is taken upstream from the pump fuse. In the event of a blown fuse, the controller will still be powered and the overload protection will indicate this error and the motor will be switched off. The solid-state relay (SSR) is controlled by a latching logic circuit that uses the overload protection and temperature sensor as error inputs. The temperature sensor is fitted to the vacuum pump body to monitor the motor’s temperature. The manufacturer stipulates that the pump should not be operated for extended periods of time at ambient temperatures below +5 °C and above +40 °C. Experimentally it has been determined that this corresponds to a body temperature of +5 °C and 85 °C. The ready signal indicates when the motor’s ambient temperature is within this range.
Figure 18: Block diagram of vacuum pump controller assembly.
Unshielded CavityValve Open/Closed
Offline/Online/Ready/Running
Pump Start/Stop
Pump Vacuum Module
Motor(0.3 kW)
5V PSU
Fibre Connector
Solid-state Relay
Vacuum Pump Controller PCB
OverloadProtection
Fuse
Control & Monitoring
TxFrm230V:6V
/2
/2
/2
/2
L1 L2 L3 N
Temperature Sensor
/2
Valve
Fuse
Fan
Fuse
Conn(12 pin)
Fuse
Surge Protection
3 Phase(5-pin)
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4.2.3.2.1 Motor Starter Components
The solid-state motor starter components are illustrated in Figure 19.
Surge
Protection
Solid-StateRelay
Overload Relay
VacuumPump
3- PhaseSupply Semi-
conductor Fuse
Figure 19: Motor starter components.
4.2.3.2.2 Surge Protection
The DG M TT 275 from Dehnguard with its heavy-duty zinc oxide varistors serves as a surge and lightning protection device. This part is prewired for a TT1 earthing system and consists of a base part with plug-in protection modules. The plug-in modules allow for easy exchange without the need for tools. All the protective modules have a visual operating state/fault indication. A summary of the surge protection specifications is given in Table 9.
Figure 20: DG M TT 275 surge protection.
Table 9: DG M TT 275 specifications
Parameter Value
SPD according to EN 61643-11 Type 2
SPD according to EN 61643-1 Class II
Nominal discharge current (8/20 us) [In] 20 kA
1 This refers to the earthing arrangements where the first letter indicates the connection between earth and the power supply, and the second letter indicates the connection between earth and the load being supplied. TT implies a direct connection to earth at both ends.
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Max. discharge current (8/20 us) [Imax] 40 kA
Lightning impulse current (10/350 μs) [N-PE] [Iimp] 12 kA
Voltage protection level [L-N] ≤ 1.25 kV
Voltage protection level [L-N] at 5 Ka ≤ 1 kV
Voltage protection level [N-PE] ≤ 1.5 kV
Follow current extinguishing capability [N-PE] 100 Arms
Response time [L-N] [tA] ≤ 25 ns
Response time [N-PE] [tA] ≤ 100 ns
Short circuit withstand capability at max. mains-side overcurrent protection
50 kArms
4.2.3.2.3 Semiconductor Fuse
The purpose of these fuses is to protect the SSR against short circuits and over currents. The solid-state switching elements in the SSR have very short thermal time constants. Extreme current levels and surges caused by load or line faults may cause permanent failure of the switching elements. Standard circuit breakers and fuses cannot react fast enough to prevent the fault current from exceeding the maximum levels that the switching elements can withstand. A Midget fuse with part number LA60Q15-2 from Littelfuse is used. It is rated for 15 A 600 VAC and has an I2t 2 of 180 A2t.
4.2.3.2.4 Solid-State Relay
The UPD24TP series SSR from Crydom is used to switch power to the motor. Two back-to-back random-fire Silicon-Controlled Rectifiers (SCR)/Thyristors, as opposed to a Triac, are used as the output switching element, providing added reliability in commercial and heavy industrial applications. An internal resistor and capacitor snubber circuit provides additional dV/dt attenuation, and a transient voltage suppressor (TVS) eliminates the need for external overvoltage protection. A summary of the SSR is given in Table 10.
Figure 21: UPD24TP series solid-state relay.
2 I2t is the thermal energy required to melt a specific fuse element. Tests to determine the I2t rating is performed with a fault current of at least 10x the rated current with a time constant of less than 50 µs in a DC test circuit.
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Table 10: D53TP25D-10 specifications
Parameter Value
Control voltage range 4.5-8 Vdc
Operating voltage (per channel) 24 – 280 Vrms
Transient overvoltage 600 Vpk
Max. load current per phase 15 A
Max. surge current (16.6 ms) 239/250 Apk
Max. I2t for fusing (8.3 ms) 285/259 A2s
The forward voltage drop of the output SCRs is approximately 1 V. At normal load currents, the power loss can be estimated at 1.25 W for every 1 Arms. SSRs mounted directly onto a panel (chassis) can work up to 5 Arms without requiring a heatsink. The full load ampere (FLA) of the vacuum pump is 1.3 A, meaning that the SSR does not require a heatsink.
4.2.3.2.5 Overload Relay
The motor overload relay from Allan Bradley used is the 193E1 Plus Pass-Thru with part number 193-EECP. This device has no terminals, i.e. the conductors are fed through the device and the current is sensed by means of current transformers. This overload relay has a selectable Trip Class between 10, 15, 20 or 30 and a selectable tripping current between 1.0 and 5.0 A. In the event of an overcurrent or phase imbalance, the latching relays are actuated and sensed by the control circuitry, which then disables the SSR. Automatic reset of the overload relay is enabled.
Measurements showed that the full load is 1.3 A and the locked rotor current is 1.6 A, up to 6 times greater than the typical case. The adjustable tripping current is set to 1.3 A. The trip rating is 120 % of the dial setting. On the time axis 100 s intercepts the trip curve at about a quarter multiple which is about 1.3 x 1.2 x 1.25 = 1.95 A below the recommended circuit breaker rating of 2 A. Thus, in the event of an overcurrent of 1.95 A, the motor would be switched off after approximately 100 s.
A summary of the motor overload relay specifications is given in Table 11.
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Figure 22: Trip class 10 for 193-EECP (solid line represents cold start).
Figure 23: Solid-state pass-through overload relay.
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Table 11: 193-EECP specifications
Parameter Value
Rated current 1.0 to 5.0 A
NEMA operating voltage 600 V
IEC operating voltage 690/1000 V
Overload trip Electronic
Trip class (adjustable) 10, 15, 20, 30
Reset type Automatic (120 s) and manual
Phase loss 3 s
4.2.3.2.6 Control and Monitoring PCBA
The control and monitoring PCBA consists of a power supply circuit, a latching logic circuit, two fibre optic transmitters, two fibre optic receivers, an 85 µA current source to drive the temperature sensor and an onboard solid-state relay to control the vacuum valve.
Fibre optic interface
The logic coding/truth table of the fibre transmitters are described in [AD5]. A Huber+Suhner ODC-4 receptacle and pigtail terminated in SC connectors provides the optical interface to the fibre connection from the SPFC (refer to SKA-TEL-DSH-0000066). On the PCBA, the fibre receivers that are used are Avago HFBR-24E2Z receivers and the fibre transmitters are Avago HFBR-14E4Z.
The start/stop serve as input to the on/off logic circuit controlling the pump’s SSR. The running signal becomes active when the SSR is switched on. The ready signal is driven by the temperature measuring circuit described further in this section. The online signal is driven by the PCBA supply power.
The circuit that drives each optic fibre transmitter is displayed in Figure 24 and the optic fibre receiver circuit is displayed in Figure 25.
Figure 24: Fibre transmitter circuit.
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Figure 25: Fibre receiver circuit.
Power Supply
In order to minimise RFI, the power supply is a linear design consisting of a chassis-mount transformer, overcurrent protection, overvoltage protection, a rectifying circuit and a low dropout regulator. The maximum power requirement of the PCBA is 1.25 W (250 mA at 5 V). The transformer is a 25 VA, 230 VAC input, 6 VAC output device. The regulator is an adjustable, low dropout LT1965 regulator set at 5.0 V. Overcurrent protection is implemented with a slow blow fuse (PN 5ST 100-R) on the transformer primary and a PCB mount thermistor before the rectification diodes. Overvoltage protection is provided with a clamping circuit immediately after the rectification diodes.
Figure 26: Power supply circuit.
Temperature Sensor
The vacuum pump temperature is monitored directly on the pump surface to indicate whether it is safe to run the pump or not. The minimum temperature at which the pump may be started is specified in the Leybold vacuum pump manual [RD16] as +5 °C. It is also specified that the pump should not be run in an ambient temperature of +40 °C or higher for extended periods of time. Experiments during development have shown that the pump surface temperature near the motor can safely reach >90°C. The same experiments have shown that the pump surface will reach approximately 85°C when the ambient air is recorded as 40°C. Thus, a temperature range is set from +5°C to +85°C as the safe zone for the pump to operate and match the specified requirements.
The negative temperature coefficient (NTC) thermistor from Vishay BC Components (PN NTCALUG01A103J) is an inexpensive alternative to the PT-1000 sensor that was used to monitor the pump temperature in MeerKAT. See Figure 27. The sensor is biased with an 85 µA current source, generating the safe zone upper and lower temperature limits of 0.091 V and 2.267 V respectively.
Figure 27: NTC thermistor package.
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The safe zone limits are monitored using the TPS3700 window comparator from Texas Instruments. The window comparator has a threshold of 0.4 V. In order to monitor the upper limit of 0.091 V a 0.5 V reference (ADR130B) is connected to the buffering instrumentation amplifier’s (AD8226) reference pin. The resultant voltage range of 2.767 V to 0.591 V is fed into the window comparator by means of voltage division. Any voltage outside this range, i.e. below 4 °C or above 85 °C will cause the TPS3700’s output to go low. The output of the TPS3700 is buffered and fed into the fibre transmit logic circuit and start/stop logic circuit.
On/off logic latching circuit
The latching circuit is shown in Figure 28 and Figure 29. The start/stop control signal is a constant high/low signal respectively and is used to switch the motor on and off by controlling the SSR. On power-up, the S/R latches’ states are undefined. The DS1233 reset device from Maxim is used to disable the S/R latches (CD4043BPWR) for 350 ms. Disabling disconnects the state outputs from the Q outputs. Pull-down resistors (not shown) on the Q outputs ensures that the switches are open. During this time, an initialisation signal places the latches in a known start-up state. The start-up state is defined as S1 close and S2 open. These switches are P-MOSFETs contained in a dual load switch device from Texas Instruments (PN TPS22960). A start signal will propagate through S1 and set FF2, thereby closing S2 and starting the motor. A stop signal resets FF2 which opens S2 and stops the motor. In the event of an error or the pump becoming not-ready, both FFs are reset, opening both S1 and S2. A stop signal is required to reset FF1.
A 10 ms initialisation pulse is generated by the LMC555 mono-stable device. The ADM6322 supervisor device from Analog Devices is used to generate the trigger input for the LMC555. On power-up, the ADM6322’s active-high reset pin is asserted for 140 ms when Vcc >= 4.63 V, after which the output returns to zero. This creates the trigger input to the LMC555. The waveforms of the initialisation circuit are shown in Figure 30.
Vacuum valve control
The control signal is a constant high/low signal and is used to switch the power to the valve on and off respectively by controlling the onboard SSR. There is also an onboard fuse to protect the SSR.
Figure 28: Logic circuit for SSR control.
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Figure 29: Initialisation circuit.
Figure 30: Monostable trigger and output waveforms.
4.3 Vacuum Lines
Conflat vacuum fittings are selected as the standard for permanent vacuum connections also exposed to the external environment. Conflat fittings are seen as more robust, leak tight to higher vacuum levels, and have longer lifespans than the alternative ISO-KF fittings. The sealing surfaces of conflat fittings are not exposed to the environment as is the case with ISO-KF seals which are prone to UV damage over time. The fact that conflat fittings require tools and some effort to remove is seen as an advantage as it negates the possibility of unintentional loosening of the fittings. However, ISO-KF25 fittings are used on the vacuum pump station inlet and feed cryostat vacuum interface due to the difficulty of installing conflat fittings at these locations and the fact that both these locations may be disconnected for routine maintenance. ISO-KF seals with external centring ring will be used as these have a stainless steel ring covering the seal to protect it from UV damage (see Figure 31).
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Figure 31: ISO-KF seal with internal and external centring ring.
The vacuum manifold body consists of stainless steel tube of 1” outer diameter. The main manifold is bent in a circular shape to accommodate the placement of the three feeds on the indexer (see Figure 32). The manifold will use the same design as was successfully used on MeerKAT.
Three separate stainless steel tubes of ¾" OD with DN16CF interfaces are TIG welded directly onto the 1" tube. The outlet ports correspond to the respective feed cryostat vacuum interfaces as positioned by the feed mounting pedestals on the indexer.
Figure 32: Vacuum manifold and vacuum hoses for two cryogenic feeds and one spare port.
The vacuum hose for a feed is only installed when the feed is installed on the indexer. When a feed is not installed for long periods the vacuum hose must be removed and a conflat blanking plate must be placed on the manifold.
Vacuum manifold
Vacuum feed hose
Vacuum manifold hose
Spare vacuum connection
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Mounting
The vacuum manifold is rigidly mounted to the feed indexer platform using Stauff pipe clamp assemblies (see Figure 33). These clamps are spaced along the 1" tube to ensure proper fastening of the manifold during all antenna positioner orientations. It also allows for easy manifold replacement when required.
Figure 33: Stauff pipe clamp assembly.
The vacuum feed hose is similar in construction to the pump vacuum hose and consists of convoluted stainless steel flexible tubing with a ¾” internal diameter. The manifold end is fitted with a DN16CF interface where the vacuum pump assembly end, or similarly the feed cryostat end, is fitted with an ISO-KF25 interface. These hoses are manufactured items and re use the identical tube and hose adaptors before terminating in their respective end interfaces. See Figure 34 for an illustration of the pump vacuum hose. The assembly technique is illustrated in Figure 35.
Figure 34: Pump vacuum hose.
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Figure 35: Section view of a welded vacuum hose.
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5 POWER REQUIREMENTS
All power to the vacuum pump assembly is supplied by the power supply unit, and the requirements for this is discussed in [AD3].
6 MAINTENANCE AND LOGISTICS
This section addresses maintenance and logistics of the SPF vacuum service components. Feedback from users of the components/subsystems is included as background.
6.1 Vacuum Pump Assembly
The vacuum pump assembly, as presented in the PBS in Figure 4, consists of the pump vacuum module, vacuum pump controller assembly and vacuum pump enclosure assembly. These items are maintained individually and are line replaceable units.
The Scrollvac 15 Plus user manual lists 3 typical levels of maintenance tasks and checks, these are listed in Table 12.
Table 12: Leybold Scrollvac Plus maintenance plan
Frequency (Months)
Maintenance task
12 • Inspect and clean the inlet strainer
• Inspect and clean the external fan cover (if required)
30 • Check pump performance and replace tip seals (if required)
60 • Replace pump bearings
• Electrical safety check
From a maintenance task perspective, the 12-month inspection can be performed while the pump is installed on the dish. Similarly, the 30-month inspection can be performed while the pump vacuum module is installed on the dish, however should the performance no longer meet the minimum requirements for vacuum generation, the pump vacuum module needs to be removed and tip seals replaced in a workshop environment. All 60-month services will need to be performed in a dedicated workshop environment.
An alternative, low risk, approach to the 30-month maintenance plan is to schedule this as a run to failure maintenance routine. Typical system checks will indicate if a vacuum pump is no longer capable of reaching the specified vacuum level and can then be scheduled for replacement. Due to the minimal active hours of the vacuum pump over its lifetime it is unlikely that the pump will have tip seal failure after 30 months. The typical scroll pump is designed for prolonged continuous operation which is not the case for the SKA project.
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To service the pump vacuum module, it will be removed from the antenna structure as described in section 4.2 and taken to the maintenance facility for inspection and servicing. During this time, a replacement unit will be installed on the feed indexer to allow for minimal down time.
The pump vacuum module, has a mass of approximately 26 kg, and is supplied with a lifting eye bolt for handling with a crane for lifting on and off the feed indexer. Its ergonomic shape also allows for carrying it short distances by its protruding surfaces without risk of damage.
The pump will be transported in its transport container when moved from the antenna positioner to the maintenance facility.
It is recommended to also inspect the purge valve air filters during the routine inspections and test the purge valve functionality on a regular basis. The purge valve can be inspected via routine system checks.
The vacuum pump enclosure does not have any serviceable items and should only require replacement if structural damage is noticed during the scheduled pump vacuum module maintenance.
No maintenance is required on the vacuum controller assembly after installation. The vacuum controller assembly is an LRU item and will simply be swopped out if faulty. Maintenance tasks will be performed at a dedicated workshop.
6.2 Vacuum Lines
The vacuum lines, as presented in the PBS in Figure 4, consists of the vacuum manifold hose and the vacuum feed hose. These items are identical in construction and are maintained only in the event of a leak. System checks will make this event clear and require a technician to remove the faulty component and replace it with a new component. Trouble shooting will identify the source of the leak and the course of action depending on the severity of the damage.
6.3 Vacuum Manifold
No maintenance is foreseen on the vacuum manifold. The vacuum manifold, weighing 3kg will be fitted to the receiver indexer without extra lifting equipment. Leaks are more likely to occur at the manifold interfaces where a seal replacement should resolve the problem. If a leak is noticed, the manifold will be replaced with a replacement unit. Trouble shooting will identify the source of the leak and the course of action depending on the severity of the damage. Transportation of the manifold will be done with an appropriate vehicle.
7 SAFETY
Unsupervised or unwarranted opening of an operating vacuum pump assembly, or one that has not been properly shut down, carries a risk of equipment damage and injury. Appropriate warning labels and markings as specified in [AD1] will be attached to the vacuum pump assembly equipment.
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8 CONCLUSION
This document supplies detail on the design of the vacuum service for the SPF.
The selected COTS items to be used for system implementation are identified and the remaining design decisions are stipulated. Where modifications to these items may be required, this is indicated.
The design of the vacuum service presented in this document is deemed suitable for its application in the SKA SPF.
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