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 Oct 17, 2001 SALT PFIS Preliminary Design Review Control System 1

PFIS Block Diagram Oct 17, 2001 SALT PFIS Preliminary Design Review Control System 2

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 3

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 SALT PFIS Preliminary Design Review Control System 4

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 5

Mechanism Interlocks Mechanisms have interdependencies and restrictions. • Interlock Manager will enforce correct control • Hardware interlocks will backstop software Oct 17, 2001 SALT PFIS Preliminary Design Review Control System 6

Control System Implementation Control System Experience • • Hubble Space Telescope High Speed Photometer flight software WIYN 3. 5 m 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 Lab. VIEW Software environment National Instruments PCI-based products Oct 17, 2001 SALT PFIS Preliminary Design Review Control System 7

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 Oct 17, 2001 SALT PFIS Preliminary Design Review Control System 8

Control System Software • Modular, use Lab. VIEW Virtual Instrument (VI) modules in a hierarchical model • Simulation modes built in at actuator level • Lab. VIEW interface to Telescope Control System • Lab. VIEW interface to Detector Control System • Lab. VIEW 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 9

Failure Modes Overall strategy: No single-point failure modes • Software is 1 st 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 10

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 11

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 12