[mmaimcal]Re: [Almasci] Antenna Acceleration

[Richard Hills]

Dear Tetsuo,

I believe your understanding of the messages is correct.

I very much agree that a simple servo loop working on position error would 
certainly get into trouble trying to follow a trajectory like this.  My 
simple-minded understanding of the way round this (which I hinted at in my 
earlier message) is that the servo should look at the actual velocity, 
acceleration and "jerk" required, as well as the position, and work out what 
current it should apply to the motors to achieve this.  It won't get it 
perfectly right but it should be close, because of imprefections in the system 
and the things you point out like differences between antennas, etc.  The 
position servo then has to deal with these imperfections.  We did this at a 
simple level on JCMT (there we were just using "feed-forward" of the velocity 
but not the acceleration) and it works reasonably well when properly tuned up.
We found this was better than having two modes - one for rapid motions and one 
for e.g. tracking a given object.

I also agree that if there is a problem with executing fast scans then we 
should certainly look at the definition of the interface again.  As things 
stand we only tell the telescope what we want it to do about 1/20th of a 
second ahead of time.  Now clearly the higher level software knows what it is 
planning for much longer than this.  At least a few seconds ahead and 
typically several minutes.  If it would help the servo to know in advance then 
it ought to be possible to tell it, but obviously this will complicate the 
interface.

I note also that if one is doing a repetative pattern, which we very often 
will be, then in principle the servo could learn from what happened on e.g. 
one turn-around how to improve its performance on the next attempt.

What I don't know is what has actually been implemented, or what performance 
is either predicted or achieved.  I think we don't have the right circulation 
on this mail discussion and we need to bring in someone who does know about 
this topic - presumably Steffano and/or Jeff Magnum should be asked who is the 
right person.  The trouble is that they are all rather busy at the moment.

In response to Robert Laing's question - I am sure that simulations have been 
done and that enough must be known to be able to generate a simple generic 
model, but again I don't know who has their finger on this.

Best Richard