Collaboration Case:  Chopping Control Loop

Description:  The chopping control loop is triggered by a timer and is responsible for coordinating the chopping cycles.
A chopping cycle consists in 2 phases ON and OFF during which M6 is:

• ON : aligned on the GS (star phase) and
• OFF: off the star (sky phase).

Performance: 

Frequency: 0.1Hz to 5Hz.
Chopping requirements have to be carefully analysed and the requirements for the STAP closed loop imposes limits on the chopping frequency:

• Currently the goal is to reach 5Hz chopping frequency by using just the ATs.
• Typically:
  • 5 to 8 STRAP loop cycles are necessary to close the loop
  • with a 13th mag star on ATs, 5 ms integration time are necessary. Integration time is 2+(Mv-10) ms.
• Typically then about 30 to 40 ms are necessary to close the loop
• Remain less than 170 ms per cycle for science purposes
• Stroke on sky +-6", ON/OFF Ratio 0.2 to 5.0, settling time less than 5ms (see VLT-ICD-ESO-1500-1826 Interface Control Document between VLTI and its Instruments)

Package Assignment:  Field Stabilization Subsystem.

Preconditions: 

• ATCS is ONLINE
• M6 is ONLINE
• FSS Strap is ONLINE
• 2 TRS timers are setup such that they correspond to the ON and OFF cycle
• Chopping configuration parameters are valid and available

Basic Course: 

• TRS timer trigger
  
• ATCS check for stop chopping condition

Exception Course: Command failed

• ATCS Stops Field Stabilization
• ATCS logs the error
  Postcondition: Chopping is stopped

Postconditions: 

• M6 Chopping cycles are performed

Issues to be Determined or Resolved: 

ACTION MCO JFA:

• Measure the time needed to receive and handle the order to change between object and sky, including the mirror settling time. It must be possible to settle a stable chopping loop, with a defined jittering and with sinchronized opening and closing of the loop. Instruments using chopping will not receive any control signal when the mirror chops, but will trust the absolute times calculated using the chopping parameters and measured using the time bus.

Notes:

Last modified: Fri Mar 1 13:51:44 UTC 2002