Encoder failures during 2007
06apr07
Sections:
Intro
08apr07 encoder 1 failure
plots:
08apr07:
azimuth summary plots (.ps) (.pdf)
08apr07:
pointing errors caused by the failure (.ps) (.pdf):
Intro
The azimuth arm has 2 encoders.
They are situated on opposite sides of the azimuth arm by the encoder rack
gear. The encoder on the gregorian dome side (az encoder 1) is used
for pointing. The encoder on the carriage house side (az encoder 2) is
used to measure the bending of the azimuth arm.
If az encoder 1 fails then the system switches to
az encoder 2. This allows continued operation with degraded pointing accuracy.
As the azimuth rotates, the azimuth arm bends slightly. The pointing is
servoed to the encoder on the dome side. The encoder on the ch side will
be off from encoder 1 by the bending amount. This changes depending
on the direction the motors are driving the azimuth.
We have been having a number of azimuth 1 encoder
failures recently. The system has continued running using az encoder
2 (and degraded pointing). Some observations are:
-
Often times the encoder 1 error looks to be a glitch (since the encoder
difference remains reasonable after the failure).
-
When a glitch occurs the system does not automatically switch back to az
encoder 1.
-
A reset command will sometimes clear the failure and switch back to az
encoder 1.
-
Occasionally we have had to reboot the vertex plc.
.
The Failures. (top)
08apr07: Az 1 encoder failure
The failure occurred at 12:33. The azimuth
was sitting at 270 degrees and the dome was at za=10 degrees.
The azimuth
summary plots (.ps) (.pdf)
show the failure:
-
Page 1: the azimuth position and the az1-az2 value for the
day. The dashed line at 12.5 is when the failure occurred. You'll
notice that after the enc1 fault, the azimuth encoder difference was still
reasonable. This means that az encoder 1 was still working after the failure.
This makes me think that the problem was a glitch and not a catastrophic
failure.
-
Page 3: The red trace at the bottom shows the enc1 fault
bit. It stayed high until around 22:24. The glitch in all the bits at 22:24
was a power dip. It looks like the power dip caused the system to reset,
removing the encoder 1 failure.
The pointing
errors caused by the failure (.ps) (.pdf):
-
The pointing model was made with encoder 1. When we switch
to encoder 2, the encoder difference becomes the pointing error. The error
on the sky is the encoder difference times the sine of the zenith angle.
The plot shows the az encoder error on the sky caused by using encoder
2. The error was present between the two dashed red lines. It got
up to about 25 arc seconds.
Processing: x101/070408/chkaz.pro
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