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Southern African Large Telescope Prime Focus Imaging Spectrograph

Southern African Large Telescope Prime Focus Imaging Spectrograph. Preliminary Control System Design Jeffrey W Percival University of Wisconsin - Madison. PFIS Block Diagram. Mechanical Requirements. 9 unique mechanisms Collimator (shared by both beams) Slit masks Wave plates

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Southern African Large Telescope Prime Focus Imaging Spectrograph

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  1. Southern African Large TelescopePrime Focus Imaging Spectrograph Preliminary Control System Design Jeffrey W Percival University of Wisconsin - Madison SALT PFIS Preliminary Design Review Control System

  2. PFIS Block Diagram SALT PFIS Preliminary Design Review Control System

  3. 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 SALT PFIS Preliminary Design Review Control System

  4. 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 SALT PFIS Preliminary Design Review Control System

  5. 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 SALT PFIS Preliminary Design Review Control System

  6. Mechanism Interlocks • Mechanisms have interdependencies and restrictions. • Interlock Manager will enforce correct control • Hardware interlocks will backstop software SALT PFIS Preliminary Design Review Control System

  7. 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 SALT PFIS Preliminary Design Review Control System

  8. 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 SALT PFIS Preliminary Design Review Control System

  9. 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 SALT PFIS Preliminary Design Review Control System

  10. 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 SALT PFIS Preliminary Design Review Control System

  11. 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 SALT PFIS Preliminary Design Review Control System

  12. 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 SALT PFIS Preliminary Design Review Control System

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