datum Command

The instrument has no means of continuously determining the exact position of its mechanisms. Instead mechanisms are moved to their demand position by a process of "dead reckoning" ie by commanding the mechanism stepper motors to move the required number of steps from some known starting point, the so-called datum position. 

Periodically (and certainly after the EPICS Controller has been power cycled) the datum position needs to be reestablished using the datum command described here. Mechanisms may be either datumed individually by specifying the required mechanism name, or in combination by specifying the 'all' argument.

Other Information:

• Currently (10/11/00) datuming is a two stage process. Mechanisms are moved at reasonable speed until the datum position is sensed. They are then 'backed off' a little to reapproach a second time at a slower speed in a more controlled manner. The process of datuming a mechanism wheel may take up to a couple of minutes (though is typically shorter). The focus mechanism and pupil imager are faster.

 
• ING currently (10/11/00) recommends that mechanisms are datumed once at the start of an observing session. Further datuming should not be necessary during a nights observing.

 

• mech_name - the name of the mechanism to be datumed. Should be one of: "fwheel1", "fwheel2", "pstop", "pimager", "focus".

 
• "all" - a string literal which indicates that all of INGRID's mechanisms should be datumed.