[evlatests] ACUs -- Old vs New

[Bruce Mues]

Hi All,



The NRAO/VLA specified speeds are:

AZ (40 deg/min +-5 seconds)

EL (20 deg/min +-5 sec).



The new ACU antennas are running exactly at 40 deg/min or 0.666 deg/sec in AZ and 20 deg/min or 0.333 deg/sec in EL.  Any variation in arrival time for new ACU's is the result of different executor commands (unlikely) and/or different environmental and mechanical loading, such as wind and bearing drag or brake drag.  Although the new ACU is very good at compensating for the varying loads, we can expect to see small differences in the arrival times because of them.



The old ACU's are technically speeding and should be ticketed (maintenance form pun).  Servo has historically overlooked this infraction because the old ACU speeds are at the top limit of the spec. In fact the old ACU slew speeds were intentionally set near the high limit to compensate for the long settling time that they have.  At the very top of the AZ slew speed limit spec for a 360 degree slew, you could see an early arrival time of 45 seconds over spec speed.  When performing long slews, the difference is obviously noticeable, but during normal observations the settings are helpful.  Technically, scientists should not expect to get any valid data until the lower time limit of slew speed has passed, imho.


Bruce Mues
Technical Manager II: Servo-Fiber
Work Schedule: Tu-Fr, 6:30am-4:30pm
NRAO-VLA
New Mexico, USA
P: 575-835-7417
E: bmues at nrao.edu

From: evlatests <evlatests-bounces at listmgr.nrao.edu> On Behalf Of Rick Perley via evlatests
Sent: Thursday, May 29, 2025 4:27 PM
To: evlatests <evlatests at nrao.edu>
Subject: [evlatests] ACUs -- Old vs New

I continue to troll through the 'flux density' data, taken earlier this year.

The observations include many long slews between the sources, so I can judge how long it takes the antennas to move from one source to another, when the separation is large.

It is quickly seen there is a major difference between antennas:  ea04, 15, 18 and 20 are always on source before the others.  The difference can be large - up to 30 seconds (!!!) for really big azimuth changes (meaning, about 8.5 minutes for the fast antennas, vs 9 minutes for the slower ones, for a full turn in azimuth.)

What's special about those four antennas?  - They are all 'old' ACU antennas, and indeed are the only remaining 'old' ACU antennas.  (ea05 is 'old', but was out of the array).

Question:  Is this difference expected?

The good news:  For the 'new' antennas, the arrival times on source - even for the very longest slews - are identical - within 1 second even for slews longer than 8 minutes.

Rick



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