Oct 17, 2001 SALT PFIS Preliminary Design ReviewControl System

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Southern African Large TelescopePrime Focus Imaging Spectrograph

Preliminary Control System DesignJeffrey W Percival

University of Wisconsin - Madison

Oct 17, 2001 SALT PFIS Preliminary Design ReviewControl System

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PFIS Block Diagram

PFIS RutgersEtalon

Subsystem

SAAO DetectorOn Camera

ControlRoom

ScreensKeyboards

ComputerRoom

PFIS PC

IglooCryocoolerEtalonControllers

Electrical PowerPneumaticsCoolantControl SignalsCCD Control and Data

Array ControllerOn Camera

Power SupplyOn Truss

QueensgateCable

Facility

RS232

Cryotiger

CleanUPS

TwinFiber

ComputerRoom

SAAO PC

Oct 17, 2001 SALT PFIS Preliminary Design ReviewControl 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 ReviewControl 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

Mechanism Operation Actuator Encoder Travel Power (W) Time (s) # Ops/Obs Energy (J)Slit Mask In/Out Stepper Position 200 mm 12 20 1 240

IndexLimits

Slit Mask Select Stepper Position 350 mm 12 30 1 360IndexLimitsBar Code

Wave Plate In/Out Pneumatic Limits 170 mm 0.55 15 2 16.5Wave Plate Rotate 1 Stepper Angle 90 deg 12 6 32 2304

IndexWave Plate Clamp 1 Pneumatic Limits 10 mm 0.55 1 32 17.6Wave Plate Rotate 2 Stepper Angle 90 deg 12 6 32 2304

IndexWave Plate Clamp 2 Pneumatic Limits 10 mm 0.55 1 32 17.6Shutter Open/Close Pneumatic Limits 0.55 1 2 1.1Focus Linear Stepper Position 3 mm 15 1 2 30

LimitsIndex

Etalon 1 In/Out Pneumatic Limits 300 mm 0.55 50 8 220Etalon 2 In/Out Pneumatic Limits 300 mm 0.55 50 8 220Grating Select Stepper Position 92 mm 12 12 2 288

IndexLimits

Grating In/Out Pneumatic Limits 265 mm 0.55 10 4 22Grating Rotate Stepper Angle 45 deg 12 3 4 144

IndexLimits

Beam Splitter In/Out Stepper Angle 75 deg 12 3 4 144IndexLimits

Filter Select Stepper Position 154 mm 12 18 2 432IndexLimits

Filter In/Out Stepper Position 270 mm 12 25 4 1200IndexLimits

Articulation Rotate Stepper Angle 90 deg 12 70 2 1680IndexLimits

Articulation Clamp Pneumatic Limits 10 mm 0.55 1 4 2.2

Steppers 18 9643Pneumatics 14 3600Incr. Encoders 18 2.7Index Marks 18Limits 60

Total Energy (J)Typ. Obs. (s)Avg. Pwr. (w)

Oct 17, 2001 SALT PFIS Preliminary Design ReviewControl System

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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 ReviewControl System

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Mechanism Interlocks

Mechanism or Actuator Motion Restriction

Slit mask into/out of beam Slit mask elevator in home position

Slit mask elevator up/down Slit mask inserter in middle position

Slit mask get/put Slit mask elevator aligned with mask bay

Etalon in/out Camera unarticulated

Grating out

Grating in/out Camera unarticulated

Etalons out

Grating not rotated

Grating rotate Etalons out

Grating inserted

Filter in/out of beam Filter elevator in home position

Filter elevator up/down Filter inserter in middle position

Filter get/put Filter elevator aligned with filter bay

Mechanisms have interdependencies and restrictions.• Interlock Manager will enforce correct control• Hardware interlocks will backstop software

Oct 17, 2001 SALT PFIS Preliminary Design ReviewControl 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 ReviewControl System

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Control System Block Diagram• 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

PFIS Virtual Instrument Module Block Diagram

Slit mask select

Filter in/out

Beam splitter in/out

Grating rotate

Grating in/out

Etalon 2 in/out

Etalon 1 in/out

Grating select

Shutter

Wave plate in/out

Wave plate 1

Wave plate 2

Focus slide

Slit mask in/out

Filter select

Articulation

Slit MaskMechanism

Wave PlateMechanism

GratingMechanism

FilterMechanism

InterlockManager

ConfigurationManager

Etalon 1 scan

Etalon 2 scan

Etalon 1Mechanism

Etalon 2Mechanism

Oct 17, 2001 SALT PFIS Preliminary Design ReviewControl 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 ReviewControl 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 ReviewControl 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 ReviewControl 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