Azimuth bending errors dec07
The telescope was brought back
online after the painting project in early dec07. During this period we
had a few azimuth bending limit errors. These errors occur when the
difference between the two azimuth encoders (dome side enc1, ch side
enc2) get larger than about 800 encoder counts. This can happen if the
azimuth arm is being bent for some reason. It can also happen if one of
the encoders disengages from the encoder rack gear. The errors occurred
on 09dec07 and 10dec07.
09dec07 az bending error (.ps) (.pdf): The two events occurred at
5:42 and 7:20 am.
10dec07: az bending error (.ps) (.pdf): This occurred around 11pm .
- Page1: The first 2 plots show the az (dome side) and za (dome)
for the two events (the dashed green lines show the events). The bottom
plot is the difference in the two encoders (enc1-enc2) vs hour (the
units are encoder counts).
- Page2: The first event at 5:42.
- Top: A blowup of the azimuth position (dome side in degrees) at
- middle: encoder difference at the failure.
- Bottom: The values of the two encoders at the failure.
Black in encoder1 (dome side) and red is encoder 2 (ch side).
- Both encoders continued to move during the failure so they
remained in contact with the rack gear.
- enc2 did not move as fast as the enc1. This error looks like
a true bending error.
- Page3: 2nd event at 7:42 am. When the encoder was inspected, the
encoder gear was not engaged in the encoder rack gear. Pushing on the
spring loaded encoder mount engaged the encoder in the rack gear.
- Top az position vs hr. The azimuth was increasing clockwise.
- middle: encoder difference (enc1-enc2) vs hour.
- bottom: enc1 and enc2 positions.
- encoder 2 (ch side) stopped moving at the failure. It was no
longer engaged in the rack gear. The encoder
rack gear is not perfectly round (.ps) (.pdf) . Looking at the az encoder
difference plots, the bending failure at az=327 degrees is a spot where
the enc1-enc2 is going negative. The enc2 side of the rack gear is
bowing out (enc2 is moving along a longer arc than enc1). Something
prevented the encoder mount from pushing out far enough to remain in
contact with the rack gear.
- The azimuth was moving counter clockwise at slew rate with the
dome at 10 degrees.
- The top plot shows the azimuth position. The bottom plot shows
the encoder difference.
- The red enc2 (red, ch side) stopped moving. When the encoder was
inspected it had disengaged from the rack gear by about 1/2 inches.
- This is not a spot where the rack gear curvature changes rapidly.
- The az enc2 (on the ch side) has come out of the rack gear in two
spots (az=327 and az=273). The rack gear bows out at 327 but not so
much at 273.
- After the az=327 failure, the encoder cover was found to
stop free motion of the encoder. After fixing this we still had the
az=273 failure the next day. We need to disassemble the encoder mount
and clean it.
- The error at az=53 was a true bending. When we drove the azimuth
back and forth over this region, there was a vibration in the platform.
It was coming from the sides of the azimuth wheels rubbing against the
azimuth track. This can cause a bending error (especially if we are
reversing directions at this location). The azimuth rails no longer
have any grease on the top or sides of the rails.We need to
regrease them to prevent these errors.