Collaboration Case:  M6 Control Loop

Description:  The M6 Control Loop is triggered by a TRS timer and is responsible for sending regularly correction offsets to the main axes corresponding to the actual M6 tilt angles so as to re-center M6, thus avoiding the risk of reaching a limit. If chopping is active, it will take into account that re-centering while in the OFF (sky position) chopping phase, means to actually put M6 in the nominal chopping sky position with respect to the center.

Priority:  Critical

Performance: As fast as possible

Frequency: 0.1 Hz

Package Assignment:  M6 Subsystem.

Preconditions: 

• FSS is ONLINE
• M6 is ONLINE

Basic Course: 

• TRS timer trigger
• ATCS Get M6 Tilt Angles
  Exception Course: Command failed
• ATCS Computes (Inverse) Interaction Matrix
• ATCS converts M6 Tilt Angles to (alpha,delta) offsets
  Subflow: Chopping active and chop phase ON (object position)
• ATCS performs an Autoguider Offset to correct telescope axes of (alpha,delta)
  Exception Course: Command failed

Subflow: Chopping active and chop phase OFF (sky position)

• ATCS subtracts from the calculated (alpha,delta) offsets the component due to the fact that the M6 is currently in the OFF chopping position. The new calculated (alpha,delta) correspond to the nominal chopping position with respect to the M6 center position.
• ATCS Set M6 Tilt Angles to the new values computed in the previous step
  Exception Course: Command failed

Exception Course: Command failed

• ATCS Stops Field Stabilization
• ATCS logs an error

Postconditions: 

• M6 is recentered
• Offsets sent to main axes

Issues to be Determined or Resolved: It must be analyzed again in detail how the feedback loop to compensate with the telescope the M6 drifts due to tracking errors will be, in particular during chopping.
D.Bonaccini noticed that a 0.1Hz loop is more reasonable than the 1Hz loop currently used for the unit telescopes, since it is probably much too fast and atmosphere effects cannot be averaged out. The tuning of the loop frequency will be performed at a later stage.

It is anyway required that the calculation of the average, i.e. the diagnostic loop, is synchronized with the chopping and with the M6 control loop. Strap should provide error vectors not including the offset due to chopping. Like this no complicate calculation in AT has to be done.

Notes:

Last modified: Fri Jun 28 09:07:41 UTC 2002