# Azimuth encoder difference

#### mar06

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).
On 09mar06 an azimuth spin was done from az=-90 to az=+270 and then back at slew rate. The encoder difference (Enc1-enc2) was sampled once a second.

The first plots show the encoder difference versus azimuth (.ps) (.pdf):

• Top plot: The encoder difference (in encoder units) versus azimuth angle. Black is the clockwise (increasing azimuth) spin while red is the CCW (counter clockwise spin). The average difference between the CW and CCW spins in 148.5 counts. This is caused by the flexing of the azimuth arm when driven in a particular direction. The Enc1 side is driven by the encoder1. The encoder 2 side is not servoed.
• Bottom plot: the same plot with the vertical scale in degrees rather than encoder counts (there are about 6935 encoder counts in a degree).
There is a positive going bump in the encoder difference at az=160 degrees and a corresponding negative going bump 180 degrees away. On 21jan00 the runout of the azimuth encoder rack gear was measured. It showed that the azimuth encoder rack gear was bowed inward (by about 1 inch) around an azimuth of 160 degrees (unfortunately the computed azimuth error was off by about a factor of 10!).
The next plots show the repeatability of the azimuth encoder rack gear error (.ps)  (.pdf):
• Top : The black line is the CW encoder difference. The Green line is the difference between the CW and CCW spins. The red line is the CCW encoder difference with the average CCW -CW difference removed. The two lines repeat pretty well.
• 2nd plot: The differences for the CW and CCW spins were averaged and then the mean was removed. The average encoder difference should be 0 since we made a complete revolution. There looks like there is a 180 degree symmetry (which you'd expect). The red plot is (-average of spins) offset by 180 degrees. The 180 degree symmetry is repeatable.
• Bottom: A third order baseline was removed from the average of the two spins (black plot) and then a gaussian was fit to the bump around az=160. Assuming that most of the bump is coming from az=160 (and not 160-180... this is supported by the runout measurement) then the fit at 160 degrees can be used to correct the pointing model around this area. The current pointing model does not fit a bump like this very well.
The data and the fit is stored in the idl save file encDifFit.sav .

processing: x101/060309/agc.pro

# Historic plots of azimuth encoder difference

 date Plots za notes 08mar06 (.ps) (.pdf) zach=8.8,zagr=10 25nov11 (.ps) (.pdf) zach=8.8,zagr=5 after encoder 2 replaced 19jan12 (.ps) (.pdf) zach=8.8, zagr=15 enc 1 coupling becomes loose during wold day 23jan12 zach=8.8, zagr=10.976 after encoder 1 coupling tightened 17jan13 (.ps) (.pdf) zach=0,zagr=15 encoder 2 replaced page 1 22jan13 zach=8.79, zagr=15 page 2 16oct19 (.ps) (.pdf) zach=8.8,zagr=8 after ch encoder moved 4 teeth, gr encoder moved 1 tooth.

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