Observing Operations | Reviews | Survey Management

Sloan Digital Sky Survey
Review of Observing Systems and Survey Operations

Interlocks For the 2.5 Meter SDSS Telescope
J. Anderson Jr.
April 10, 2000


    The purpose of the interlock system is to lower the risk of personnel injury and to protect the telescope, instruments, and  related equipment near the telescope. The function of the interlock system is to monitor telescope behavior and inhibit unsafe actions. The initial requirements were developed from Steve Brackerís hazard analysis and input from collaborating institutions. The interlock system requirements were reviewed and accepted at the November 1996 SDSS Installation Task Force meeting, then further reviewed and modified at the April 1997 SDSS Controls & Interlocks Review held near Fermilab. The current performance specifications for the Telescope Motion Control Interlock System can be found in the document "Technical Requirements and Performance Specifications for the 2.5-Meter Telescope", available on the web at http://tdpc01.fnal.gov/sdss/project.htm under Requirements and Specifications.

    Some of the key system elements included a modular system design, easy to repair or replace components, the ability to quickly bypass a failed component, and a redundant design topology. The redundant design topology has been implemented with a two level design philosophy. The Low Level, or Level 2,  Interlocks are critical motion control interlocks implemented completely in hardware. The High Level, or Level 1, Interlocks are complex interlocks implemented with an industry standard programmable logic controller and networked I/O sub-system. To assist the observers in determining the state of the interlock system a graphical display system has been developed in TCL/TK by Dennis Nicklaus and later upgraded by Robert Lupton. This allows the observers to check the real time interlock system status from any X-Terminal. The graphical display system, along with logic diagrams that are located in hard-copy form in a binder near the telescope, allow the observers to quickly identify problems and view the state of the system.

Current State
    The motion control interlock system was final tested on December 11-13, 1999.  System testing was accomplished by causing a known system violation and monitoring for the correct response. The final test results can be found in the document "SDSS Telescope Motion Interlock System Final Test Report", available on the web at http://tdpc01.fnal.gov/sdss/project.htm  under Documentation.

    Motor capstan slip detection and telescope velocity limits remain to be installed and tested. The design is complete and printed circuit boards are being manufactured. Design testing at Fermilab is expected to begin in early May with installation at APO tentatively scheduled for mid-June. The hooks into the interlock system are complete and have been tested.

    Building motion interlocks have been implemented within the interlock system but have not been wired to the building. The connection of these interlocks is pending a lightning protection review.

    Instrument change interlocks have been installed to support manual instrument change operations. The hardware necessary for automatic instrument change operations has been installed. The configuration of the interlocks for automatic instrument change operations is expected by summer.

Long Term Robustness and Maintenance
    The majority of the components used in the interlock system for the 2.5m telescope have been chosen from the components used in the personnel protection systems at Fermilab. Long term robustness and reliability is not expected to be a problem for the duration of the survey. The required maintenance is anticipated to be routine system testing and validation at 6 to 12 month intervals.

Review of Observing Systems and Survey Operations
Apache Point Observatory
April 25-27, 2000

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