[mmaimcal]Re: [Almasci] Antenna Acceleration

[Tetsuo Hasegawa]

Dear Richard and all,


At 6:52 PM +0000 03.11.19, Richard Hills wrote:
>Dear Mark,
>
>(This is a response to the message before last.)
>
>Whoa!  All I was trying to check was that the resolution on the 
>transfer of data in the form of one position and one velocity per 
>48msec time interval is adequate for the most extreme motions that 
>we might contemplate.  That was the issue raise by Morita-san.  The 
>numerical tests of cubic spline interpolation simply show that it is 
>possible to reconstruct the original waveform with good fidelity.

Morita-san told me (as I understood) that the current interface 
definition is that ABM tells ACU every 48 msec the command position 
and angular velocity in AZ and EL (four values) at a time T and the 
same set of values at (T + 48 msec).  If this is wrong, please 
correct me.


My concern that I expressed at the IMCAL telecon this week is as follows:
(I have a very limited knowledge/experience in servo control, and if 
the following contains my misconception, please forgive me.  But I 
thought it important to clarify for the ALMA project.)

In general, good pointing accuracy with fast switching requires a 
very tight position loop (feedback according to the position error). 
If the actual trajectory of the antenna position is offset from the 
set of command positions during the turnover in the OTF scan, this 
position loop may try to correct this position error forcefully. 
This may cause an oscillation about the designed (and commanded) 
trajectory, and we may need to wait until this oscillation damps 
before we start another straight scan.  This effect could be 
minimized by adjusting the command trajectory so that the actual 
antenna can follow it closely, but it may be a function of slew 
speed, EL, individual antennas etc.

The turnover might be much easier done if we loosen the position loop 
(in the scanning direction) during the turnover or even during an 
entire raster (This is my intuition, but I am not confident).  If 
this is the case, an option could be to have additional information 
given from ABM to ACU; e.g., a "mode" according to which ACU can 
select an appropriate set of servo modes/parameters.

This may require a change in the antenna specification.  We certainly 
need a help by specialists.


As the ACA 12-m antennas will spend much of its life doing OTF, and 
the time required for a turnover may limit the observational 
efficiency, we are very much worried.

With best,

Tetsuo
-- 
_______________________________________________________________
Tetsuo HASEGAWA, D. Sc.   <tetsuo.hasegawa at nao.ac.jp>
ALMA-J Project Scientist
Professor, National Astronomical Observatory of Japan
2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
Phone +81-422-34-3780 / FAX +81-422-34-3764
_______________________________________________________________