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Southern African Large Telescope
Prime Focus Imaging Spectrograph
Preliminary Control System Design
Jeffrey W Percival
University of Wisconsin - Madison
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
1
PFIS Block Diagram
Facility
Clean
UPS
Power Suppl y
On Truss
Array Controller
On Camera
PFIS
Rutgers
Etalon
Subsystem
Twin
Fiber
Cryotiger
SAAO Detector
On Camera
Control
Room
Screens
Keyboa rds
Computer
Room
SAAO PC
Electrical Power
Pneu matics
Coolant
Control Signa ls
CCD Control and D ata
Oct 17, 2001
Computer
Room
PFIS PC
Queensgate
Cable
Igloo
Cryocoo ler
Etalon
Controll ers
RS232
SALT PFIS Preliminary Design Review
Control System
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Mechanical Requirements
• 9 unique mechanisms
• Collimator (shared by both beams)
– Slit masks
– Wave plates
• Cameras (Visible & IR)
– Focus
– Shutter (Visible beam only)
– Etalons (2)
– Gratings
– Polarizing Beam Splitter
– Filters
– Camera Articulation
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
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Mechanical Requirements
• No servoed axes
• No “real time” needs
• Shutter is
operated by
detector subsystem
• Simple motions:
• In/Out
• Rotate
• Linear
• Single-axis
• Steppers &
Pneumatics
• Low duty cycles
• COTS controllers and
drivers
Oct 17, 2001
Mecha nism
Sli t Mask
Operation
In /Out
Actuator
Ste pper
Sli t Mask
Sel ect
Ste pper
Wa ve Plate
Wa ve Plate
In /Out
Rota te 1
Pne umatic
Ste pper
Wa ve Plate
Wa ve Plate
Clam p 1
Rota te 2
Pne umatic
Ste pper
Wa ve Plate
Shu tter
Fo cus
Clam p 2
Open /Close
Li near
Pne umatic
Pne umatic
Ste pper
Eta lon 1
Eta lon 2
Grati ng
In /Out
In /Out
Sel ect
Pne umatic
Pne umatic
Ste pper
Grati ng
Grati ng
In /Out
Rota te
Pne umatic
Ste pper
Bea m Spl itte r
In /Out
Ste pper
Fi lter
Sel ect
Ste pper
Fi lter
In /Out
Ste pper
Articu lati on
Rota te
Ste pper
Articu lati on
Clam p
Pne umatic
Encode r
Pos itio n
In dex
Li mits
Pos itio n
In dex
Li mits
Bar Co de
Li mits
An gle
In dex
Li mits
An gle
In dex
Li mits
Li mits
Pos itio n
Li mits
In dex
Li mits
Li mits
Pos itio n
In dex
Li mits
Li mits
An gle
In dex
Li mits
An gle
In dex
Li mits
Pos itio n
In dex
Li mits
Pos itio n
In dex
Li mits
An gle
In dex
Li mits
Li mits
Ste ppers
Pne umatics
In cr. Enco ders
In dex Marks
Li mits
Travel
Pow er (W)
Time (s )
# Ops/Obs
Energy (J)
20 0 mm
12
20
1
24 0
35 0 mm
12
30
1
36 0
17 0 mm
90 deg
0.55
12
15
6
2
32
16 .5
23 04
10 mm
90 deg
0.55
12
1
6
32
32
17 .6
23 04
10 mm
3 mm
0.55
0.55
15
1
1
1
32
2
2
17 .6
1.1
30
30 0 mm
30 0 mm
92 mm
0.55
0.55
12
50
50
12
8
8
2
22 0
22 0
28 8
26 5 mm
45 deg
0.55
12
10
3
4
4
22
14 4
75 deg
12
3
4
14 4
15 4 mm
12
18
2
43 2
27 0 mm
12
25
4
12 00
90 deg
12
70
2
16 80
10 mm
0.55
1
4
2.2
18
14
18
18
60
SALT PFIS Preliminary Design Review
Control System
To tal Energy (J)
Typ. Obs. (s)
Avg. Pwr. (w)
96 43
36 00
2.7
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Mechanical Requirements
• Actuators
– Stepper motors: 4 + 7 (Visible) + 7 (IR) = 18
– Pneumatic Cylinders: 3 + 6 (Visible) + 5 (IR) = 14
• Encoders
– Position: 2 + 4 (Visible) + 4 (IR) = 10
– Angle: 2 + 3 (Visible) + 3 (IR) = 8
• Index Marks: 4 + 7 (Visible) + 7 (IR) = 18
• Limit Switches: 2 * (5 + 13 (Visible) + 12(IR)) = 60
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
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Mechanism Interlocks
Mechanisms have interdependencies and restrictions.
• Interlock Manager will enforce correct control
• Hardware interlocks will backstop software
Mechanism or A ctuator Motion
Restriction
Slit mask into/out of beam
Slit mask eleva tor in home positi on
Slit mask eleva tor up/do wn
Slit mask inserter in mi ddle position
Slit mask get/put
Slit mask eleva tor ali gned w it h mask bay
Etalon in/out
Camera una rticulated
Grating ou t
Grating in/out
Camera una rticulated
Etalons ou t
Grating no t rotated
Grating rotate
Etalons ou t
Grating inserted
Oct 17, 2001
Filter in/out of beam
Filter eleva tor in home posit ion
Filter eleva tor up /down
Filter inse rter in midd le positi on
Filter ge t/put
Filter eleva tor aligned wit h filt er bay
SALT PFIS Preliminary Design Review
Control System
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Control System Implementation
Control System Experience
•
•
•
•
Hubble Space Telescope High Speed Photometer flight software
WIYN 3.5m Control System: GUI, Astrometric kernel, PID loops
Sounding Rocket Star Tracker control program
HPOL Spectropolarimeter: motors, wave plates, WIYN TCS interface
Guiding Principles
• Commercial-Off-The-Shelf (COTS)
• SALT-compatible (OS, Language, hardware, documentation)
Buy, not build:
•
•
•
•
PCs: high performance, low cost
Linux or Windows
LabVIEW Software environment
National Instruments PCI-based products
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
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Control System Block Diagram
PFIS Virtual Instrument Module Block Diagra m
• Modular
• Low-level modules control
actuators
• Mid-level modules combine
actuators into mechanisms
• High-level modules apply
constraints and manage
configuration
• Simulation built in at lowest
level; allows higher levels to
be prototyped and used early
• High degree of common
functionality will speed
development
Slit mask in/out
Slit Mask
Mechan ism
Interlock
Manager
Configuration
Manager
Slit mask select
Wave plate in/out
Wave Plate
Mechan ism
Wave plate 1
Wave plate 2
Shutter
Focus slide
Etalon 1 in/out
Etalon 1
Mechan ism
Etalon 1 scan
Etalon 2 in/out
Etalon 2
Mechan ism
Etalon 2 scan
Grating selec t
Grating
Mechan ism
Grating in/out
Grating rotate
Beam splitter in/out
Filter in/out
Filter
Mechan ism
Filter selec t
Articulation
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
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Control System Software
• Modular, use LabVIEW Virtual Instrument (VI) modules
in a hierarchical model
• Simulation modes built in at actuator level
• LabVIEW interface to Telescope Control System
• LabVIEW interface to Detector Control System
• LabVIEW VI for Etalon controllers delivered as part of
Rutgers Etalon subsystem
• Emphasize parallelism: Drive down reconfiguration time
by operating mechanisms in parallel wherever possible
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
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Failure Modes
Overall strategy: No single-point failure modes
• Software is 1st line of defense
– Check for inconsistent state of sensors
– Monitor encoders during motion, check for expected
direction and speed
• Hardware interlocks are next
– Soft limits for software
– Hard limits remove power, enforce direction
• Hard stops for critical components
– Sacrifice gears and motors to protect optical elements
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
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Control System Interfaces
• PFIS interfaces
– Etalons: this interface will disappear as the etalon
assemblies are integrated into PFIS.
– Detector and controller: mechanical, electrical,
cryogenic, control, and data interfaces
– Telescope: operations interface, bi-directional flow of
commands and data
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
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Observatory-level Control
• Queue-efficient observing requires careful design
– Automated observing, driven by scripts
– Operator can not be expected to micromanage PFIS
– Instrument-centered: PFIS is not hanging off the telescope; rather,
the telescope is hanging off PFIS
– The top-level interface in SALT is the silicon surface of the CCD;
everything else is a subsystem
– PFIS reaches out for services, which are provided quickly,
deterministically, and in a closed loop
– Telescope and Detector interfaces are critical: during slit mask
peakup, the move/expose/centroid loop must be fast and
dependable
Oct 17, 2001
SALT PFIS Preliminary Design Review
Control System
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