Use Case:  Set Nasmyth Wheel

Description:  Set Nasmyth wheel to required position
This action is dedicated to optical alignment procedures and is therefore not foreseen to be used during normal operation
where the Nasmyth wheel shall be set to the specific position allowing the telescope beam to reach M9.
The allowed positions are:

• Flat Retro-Reflecting Mirror
• Free Hole
• 2 positions for Alignment Tools (optical fibres or other dedicated devices)

Role(s)/Actor(s): 
Primary: Maintenance
Secondary: Nasmyth sub-system

Priority: Major

Performance: 
less than 10 seconds for motion to a predefined position;
less than 1 second for chopping on Alignment Tool positions.

Frequency: During optical alignment procedures only, typ. before and after alignment procedures.

Package Assignment:  Nasmyth Focus Device Subsystem.

Preconditions: 

• Nasmyth Wheel is ONLINE

Basic Course: Motion to a predefined position

• Send command SETPATH <FREE|RETRO|TOOL1|TOOL2> to set the Nasmyth wheel to given predefined position
  Exception Course: Command failed

• ATCS gets actual Nasmyth Wheel position Exception Course: Command failed Subflow: Move to different wheel position

• ATCS sends command to Nasmyth Wheel device

  Interface:  IfNasmythWheel.Motor

  Exception Course: Command failed
• ATCS returns OK reply

Subflow: Move to different wheel position

• ATCS switch off Nasmyth Beacons
  Exception Course: Command failed
  Postcondition: Nasmyth Beacons switched off

Alternate Course: Chopping on Alignment Tool position

• Send command SETPATH <TOOL1|TOOL2>,<stroke> to chop the Nasmyth wheel of a given stroke off the specified Tool position
  Exception Course: Command failed
• ATCS sends command to Nasmyth sub-system

  Interface:  IfNasmythWheel.Motor

  Exception Course: |Chopping stroke| > 15mm
  Exception Course: Command failed
• ATCS returns OK reply

Exception Course: Command failed

• ATCS returns ERROR reply
  Postcondition: Nasmyth wheel not moved

Exception Course: |Chopping stroke| > 15mm

• ATCS returns ERROR reply
  Postcondition: Nasmyth wheel not moved

Postconditions: 

• Nasmyth wheel at given position
• Nasmyth beacons switched off

Issues to be Determined or Resolved: 

Notes:

• The chopping stroke is to be given in mm. This corresponds to an angular offset of:
  offset = atan(stroke/wheelRadius) radians.
  where wheelRadius = 60mm (see FISBA document VLT-TRE-FIS-151100-032).
• The command GETPATH returns the name of the wheel position and the stroke value when applicable

Last modified: Fri Nov 23 10:41:30 UTC 2001