Package: Field Stabilization System

Description:  The Field Stabilization System is responsible for the high level control of the Field Stabilization APD Sensor (STRAP), which supplies statistical data on the position of the GS from the Coudé Focus, and for the control of the Field Stabilization Devices (filter wheel, and field diaphragm).

Package name: fss

Inheritance:  LCU Server Framework <<SubSystem>>lsfSERVER

Controlled HW Devices: 

• <<Device>>ndfMOTOR:lsfMOTOR
• <<Device>>afdMOTOR:lsfMOTOR
• <<Device>>apdSENSOR:lsfDEVICE
• <<Device>>apdSYNC:lsfDIGITAL

Controlled SW Devices: None

Deployment:  Sensor LCU: atfssServer.

Use Case diagram: 

Class diagram: 

Architecture: 

This Software Device is instantiated from the LCU Server Framework lsf.
The sub-class <<SW Device>>fssSERVER:lsfSERVER is composed of the following device instances:

• <<Device>>ndfWHEEL:lsfMOTOR: Neutral Density Filter
• <<Device>>afdWHEEL:lsfDEVICE: STRAP (Aperture Field Diaphragm)
• <<Device>>apdSENSOR:lsfDEVICE: STRAP Sensor (APD Head)
• <<Device>>apdSIGNAL:lsfDIGITAL: STRAP associated Digital I/O Signal.

The Software Device atfssServer provides the following Specific Commands for the control of the Neutral Density Filter (NDF), the Aperture Field Diaphragm (AFD) and APD Sensor (APD):

• SETNDF: to set the Neutral Density Filter
• SETAFD: to set the Aperture Field Diaphragm
• SETUP: to setup the APD Sensor
• OPTIMIZ:  let STRAP optimize the combinatation of afd, ndf, integration time, gate
• START,  STRTOPN,  STRTCLS,  STRTCGS: to start APD sensor acquisition in various modes
• SETGATE,  OPENGAT,  CLOSGAT: to set, open and close the input gate of the APD sensor
• STARTCH,  STOPCH,  SETCH:  to control chopping (compliant with UT M2 interface)
• ENAFS,  DISFS,  ENAFSC:  DISFSC:  to control field stabilisation (compliant with UT M2 interface)

All the commands, but SETNDF and SETAFD, interact via the command associated API functions with the strapServer SW device, running on the same LCU.

Command SETNDF

This command is a transient instance of the class: <<Control>>fssSETNDF:lsfCOMMAND_HANDLER.

It invokes the inherited methods lsfMOTOR::setPosition(char *name[]) and lsfMOTOR::getPosition().

The Neutral Density Filter is intended to be set after selection of a new Guide Star as a function of its magnitude. Only the position mode is relevant and the positions are accessible via their names (see MCM Named Positions):

• NDF #[0-5]: Neutral Density 0 to 5

The filter is forced to NDF #5 for a guide star with unknown magnitude. Possibly, the magnitude of the Guide Star might be estimated from the flux measured on the FAS image, by means of a look-up table. STRAP will automatically set the proper ND filter during its optimization process.

Command SETAFD

This command is a transient instance of the class: <<Control>>fssSETAFD:lsfCOMMAND_HANDLER.

It invokes API functions of STRAP

The Aperture Field Diaphragm is intended to be used for limiting the sky background contribution. Mainly while observing in the vicinity of the Moon, it might be set whenever the contrast needs increased. The diaphragm can be set continuously from 0" to 5".
The AFD is in normal opration set automatically by STRAP during its optimization process.
The provided commands are only for maintenance and test.

Interaction with the STRAP device

The STRAP device encompasses the control of 2 HW Devices:

• the APD Sensor: quad-cell photodiode detector
• the M6 Mirror: fast steering tip-tilt mirror

The control of both HW devices is sharing the same electronics (the STRAP VME board) and no external interaction is required in close loop.

In this section only the commands dedicated to the exclusive control of the APD head are described; the commands specific to the control of the M6 mirror are described in detail in the corresponding section (M6).

Command SETUP

This command is a transient instance of the class: <<Control>>fssSETUP:lsfCOMMAND_HANDLER.

It invokes the API function strapSetup().
The setup values are parsed according to the STRAP dictionary ESO-VLT-DIC.STRAP.

Commands START/STOP and STRT[OPN,CLS,CGS]

These 5 commands are transient instances of the classes: <<Control>>fssSTART:lsfCOMMAND_HANDLER and <<Control>>fssSTOP:lsfSTOP.

Commands SETGATE,  OPENGAT, CLOSGAT

These 3 commands are transient instances resp. of the classes: <<Control>>fssSETGATE:lsfCOMMAND_HANDLER,
<<Control>>fssOPENGAT:lsfCOMMAND_HANDLER and <<Control>>fssCLOSGAT:lsfCOMMAND_HANDLER.

This SW Device is dedicated to the function of Field Stabilization. Due to the small field of view of the APD sensor (approx. 3"), this device is operated together with the Field Acquisition System (based on a TCCD with a field of view of 27"x20") that allows perfect centering procedure of the Guide Star. The bandwidth depends only on the exposure time. The closed loop bandwidth is approx 10-20 % of the sample freq., so one can reach 100-200Hz.

Reuse from VLT TCS:  New Package

Issues to be Determined or Resolved: None

Last modified: Tue Jun 26 14:33:17 UTC 2001