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 failure:

    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.
        We need to retrack a source that was done prior to the jump and see what the change in the azimuth error is.
        processing: x101/020916/