Azimuth encoder1 failures
aug09
There are two azimuth encoders. Encoder 1 is on the
gregorian side and is used for pointing. Encoder 2 is on the carriage
house
side and is used to measure azimuth pending (as well as a check on the
gregorian side encoder).
When encoder 1 fails, the vertex system
automatically switches to encoder to for the pointing. This causes a
degradation in the pointing since the encoder 1 and encoder 2
values vary as a function of azimuth. This is caused by the "out of
roundness" of the azimuth encoder rack gear as well as the bending of
the azimuth arm.
After vertex system will continue to use encoder 2
for pointing until the encoder 1 error is cleared. The error can be
cleared by:
- On the vme system:
- pnt reset 1.. this will clear the error without stopping
the tracking (unless a motion failure occurs).
- pntpwrdip : this will stop the tracking, clear the fault and
then start tracking again.
- From the ocu:
- Going to the faults window, then azimuth, the pushing the clear
key.
The telescope may get a motion failure and stop
tracking during the reset if the difference between encoder 2 and
encoder 1 are large enough (the software sees a large jump in the
position and then stops). In this case pntpwrdip will try to restart
the tracking.
The plots show a
jump in the azimuth position while clearing an encoder1 fault (.ps) (.pdf):
- The telescope had been sitting at az=285, za=12 when the failure
occurred. It was still sitting there when the failure was cleared.
- Page 1:
- Top: shows the azimuth status vs time. The encoder 1 failure is
the lower red trace. It was cleared at 11:35 and again at 11:57
- Bottom: shows the azimuth position returned from the vertex
system during this time. The azimuth is jumping by about .01 degrees.
After the failure occurs, the new position stays at encoder 2 until the
failure is reset.
- Page 2: how large a pointing error can a failure introduce?
- 6 azimuth swings (-90 to 270) were done on 02aug09 (looking for
rfi).
- The top plots shows the encoder difference (encoder 1 -
encoder) in little circle azimuth degrees. The jump that occurred at
11am was at an azimuth of 285 (-80). The difference plot shows this to
be about .01 azimuth degrees which matches the jump we saw.
- bottom plot: This is the pointing error on the sky for an
encoder jump.
- the sky error is sin(za)*azencoder jump\
- black is at za=12
- red is at za=19
- the error gets up to about -65 asecs (great circle). The
encoder difference is not currently centered at 0.
Summary:
- Encoder 1 failures cause the system to use encoder 2 for pointing.
- The system remains with encoder 2 until the fault is reset.
- The pointing error can get up to about 65 Asecs (great circle)
depending on the za and the azimuth position.
- See also: enc1failures aug09.
processing: x101/090802/enc1fail.pro
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