Azimuth encoder jump 15sep02
There are two encoders mounted 180 degrees apart on the azimuth
arm. These absolute encoders have a gear on the end of their shaft
that is mated to the the azimuth encoder rack gear via a spring.
The encoder on the dome side is used for pointing. The encoder on the carriage
house side is used to measure any bending in the azimuth arm. This is done
by measuring the relative change in the two encoders as the arm swings.
The telescope stopped tracking on15sep02 at about 22
hours with an azimuth bending error. The difference between the two encoders
got larger than an allowable value. This can happen if:
One side of the azimuth stops while the other continues moving (there are
motors on both sides).
One or both encoders lose contact with the encoder rack gear. The azimuth
continues to move but the encoder does not turn.
The vertex motor data is logged once a second to a file.
This allowed us to go back and look at what was happening during the failure.The
figure shows the azimuth
motion before and during the jump (.ps) (.pdf).
The *'s are the 1 second samples.
Tracking 2 sources after the jump gave a little circle
azimuth offset of 1446 asecs . This differs from 1430 by 16 little circle
asecs. The sources were tracked at 12 to 15 deg za. The great circle error
difference is 16/(sin(13.5)=3.7 arc seconds.
The Top figure is the azimuth position versus time. The azimuth is moving
at slew rate near az=123 deg. At 21.861 hours the azimuth decelerates and
The middle plot shows the encoder difference AzGr-AzCh in degrees. It shows
a jump at 21.861 hours.
The bottom plot has the velocity of the azimuth arm measured from the amplifier/motor
resolver in black. This is independent of the encoder. The green line is
the velocity computed using the encoder positions measured once a second.
There is a dip at 21.861 hours. If this was a real jump in motion then
the amplifier velocity should also have jumped.
Figure 2 is a blowup of the AzGr - AzCh encoders. The red lines are linear
fits to the data before and after the jump. From these fits the jump was
.3973 degrees or 1430.2 arc seconds.
We need to retrack a source that was done prior
to the jump and see what the change in the azimuth error is.