Computing avg Platform height from 3 distomats


2014,2015 all data:

Use data 25aug14 to 11may2015 (no change in zrotation)


    We currently use 6 distomats to to measure the height of the platform. Using 6 non collinear measurements, you can measure the absolute position of a solid body in 3d space (3 x,y,z translations and 3 rotations of the body). If we loose one of the distomats, then the computations fails.

    The platform position is controlled by the 3 tiedown jacks/cables. These 3 jacks give us the capability to change the average height as well as tilt the platform. The ability to tilt the platform  was going to be used to correct for pitch,roll, focus errors in the feed positioning. After some testing it was realized that we did not have enough dynamic range in the jacks to correct this at all az,za positions (high za's  need lots of motion). So the tilting control was never used.

    So we have been using the distomats and tiedowns to only control the average height of the platform. When this height changes (with  temperature) we pull on all 3 tiedowns equally to move the platform vertically. You can think of the platform as a plane (triangle)  that is moving vertically. To measure the position of a plane you only need 3 non collinear points, so our 6 measurements are actually overkill. This assumes that the triangle does not undergo rotation as the temperature changes, or the weight gets redistributed (by az, dome motions).
  Recently some of our distomats have started to fail. They are old enough that replacements are no longer available. We've looked at replacement devices, but most of them generate too much rfi to be used (even after shielding). So we've had up to 50% of the time where we've had not measurement to control the platform height (more info).
    To solve this problem, we could  switch from trying to measure the full 3d position to just measuring the average height of the platform (with only 3 measurements).  This idea was suggested by mike nolan...

    The distomats are located about  +/- 35 degrees  from the platform corners (0,120,240).  The image below shows their positions relative to the 3 corners of the platform:


    We've got years of measurements with the 6 laser ranger distances and the computed average platform height. To compute the average platform height from 3 measurements i did the following:

2014 and 2015 using all the data.

This section  used all of the data from 2014 (181433 pnts) and  2015 (87441 pnts) that had valid 6 distomat measurements,

average platform height vs 3distomat distances.

The first plots show the average platform height plotted vs the 3 distomat average distance (.ps) (.pdf)
The 6 distomat measurements give us the 3 translational and 3 rotational values for the platform position. The z axis is vertical (up being positive).

 The rotation about the z axis of the platform.

   The 2nd plots show the zaxis rotation of the platform for 2014 and 2015 (.ps) (.pdf):
Dates when jumps occurred:

Fit residuals vs z axis rotation.

    The 3 distomat measurements use only 1 edge of each corner (there is a target on both sides of each vertex).

The next plots show the fit residuals vs the z axis rotation (.ps) (.pdf)

Histogram of fit residual

    The next plots show histograms of the fit residuals for 2014,, 2015 (.ps) (.pdf)

Dataset with no zrotation jumps

    The data was then limited to 26aug14 thru 11may15. There were no zrotation jumps during this period.
 This dataset was then fit for avgPlatHght= c0 + c1 * avgdist3    (where avgdist3 was taken from dist135 and then dist246 measurements).

    The  plots show the results from fitting the dataset with no zrotation jumps (.ps) (.pdf)

I then took the fits with no zrotation jumps and applied it to all of the 2015 data. This shows how the error increases with the zrotation jumps.

   these plots show the fit residuals for all of 2015 using the no zrotation fit (.ps) (.pdf)



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