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    User name jmaxwell

    Log entry time 02:03:58 on March 9,2012

    Entry number 364900


    keyword=[Target] Target Position Test Results

    A hot spot in the beam with 0.5V slow raster amplitude made us think perhaps the target is not where we think it is. During the day we centered the larger hole target, and used that encoder value to recalculate all the other target position values. The worry was that the relative positions of these other positions, calculated using stepper motor steps, might be off enough to cause a noticable difference all the way down at the carbon disk.

    First, I opened the slow raster amplitude up to 1.5V, which should correspond to roughly 2cm, with the fast raster off. Using the slow raster ADC plots, I saw little in the way of a hot spot in the raster at the nominal position calculated today.

    Then, I moved the target to halfway between the carbon disk and the crosshair for a few short runs. This allowed me to center the aluminum ladder strut in the slow raster picture. See the first screen grab. The optical encoder value for this shot was 16381, and the steps were 1600000.

    Next, I went back to the nominal value of the large hole, to be sure I would see the same, centered picture which I saw this afternoon. The second grab shows this. You can see the hole is pretty well center. The optical encoder read 41892, very close to this afternoon's values.

    With these two values, 16381 and 41892 corresponding to a distance on the ladder of 3.22 inches, this means an encoder tick is about 1.2 *10^-4 inches.

    We'll rehash these numbers tomorrow, but right now the values in the table and in PDP are pretty close. It seemed the hot spot we saw at first may have been a left-right issue, not an up-down one. The raster image posted to the halog at least leads me to believe this, anyway. For the rest of the night we should use the existing values and the 1.4V raster amplitude or less and pay close attention to the slow raster pictures to see if we're crashing into anything. Step the target position in increments of 10000 steps to move away from hot spots.

    Some runs from this test and their motor steps and optical encoder values:

    2962: 1600000, 16381

    2963: 1650000, 16891

    2964: 4091425, 41892

    FIGURE 1

    FIGURE 2