Azimuth encoder jump 14oct03



  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:

    On 15oct03 barbara catinella noticed that the azimuth error of the calibrator source  she had done the previous two nights had jumped by -16 arcseconds. Luis murray also noted on the 15th that the azimuth encoder difference was larger than normal.
    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. I checked the azimuth encoder difference for 14oct03 and there was a jump of one of the encoders. I then looked at the first of the month for the previous 8 months and found no other jumps. The figures shows the azimuth encoder values before and during the jump (.ps)  (.pdf) (watch out.. it's 4mb!!)

   A jump of 3 teeth=192 encoder counts=99.66 arcseconds in azimuth (this is a little circle error). Barbara's calibrators were taken at 9.5 degrees za. The jump should have changed the az error by 99.66*sin(9.5)=16.4 arcseconds with the sign being things agree.

    On 16oct03 at 14:00 we moved the azimuth encoder 3 teeth moving the azimuth encoder difference from 268 to 64 encoder counts.

We need to get an index on the vertex encoders (especially the ch and dome which don't have dual encoders) so we can respond faster to these encoder jumps.  I'll also generate a daily monitor page that shows the azimuth encoder difference for the previous day and compare it to the expected values.

    processing: x101/031015/