[evlatests] Results of ea21 Pointing tests (part 1)

[Gregory Maglathlin]

Rick,
Thank you for the quantitative information.  I will talk to this data 
(referenced pointing mode)  first as it is
the easiest for me to assimilate.

First, the reduced overshoot in the new ACU is due to position 
pre-processing that is performed on the
commands provided by the MIB.   The position (and velocity) loops have 
limiting integrators which will
saturate if allowed.  The position pre-processor of the new ACU is 
designed to turn any step function (which
of course cannot be realized) into an S-curve which does not tax the 
integrator in the system preventing
saturation and hence overshoot.  It is good to see that this is working.

Second, I am assuming that the 2 Hz oscillation is the fundamental 
resonance of the antenna structure.
Controlling the acceleration allowed in the position pre-processor has 
reduced the excitement of this
frequency.  There are a few methods to further reduce this oscillation.  
Two come to mind immediately.

First, reduce the allowed acceleration in the pre-processor.  This is 
very simple and can be done by changing
  a parameter in the configuration display at the ACU.  This will cause 
each axis to get to the desired position a little more slowly
but should reduce the oscillation at the end.  We should be able to find 
a sweet spot here, which may be antenna
dependent.  Antenna dependency is not a problem as the ACU is designed 
to have an antenna specific configuration.

Second, I have always been intrigued by a paper that Ken introduced me 
to (GBT Memo 203) when we first met years ago
regarding impulses at the beginning of a move counteracted by impulses  
through the move to
counteract any oscillations.  This would be tricky to  implement 
(especially considering the ACU/MIB interaction)
but warrants further thought.  The fact that the ACU is running both 
velocity and position loops at 1 KHz may make
this a realizable option.

Of course, I will study this data some more and move on to Part 2.

Best regards and Happy New Year,
Greg