quadEM Positions XY

[Luka Debenjak via Tech-talk <tech-talk@aps.anl.gov>]

Hello everyone,

This will be my first email to the Tech-Talk.
I am testing the Quad Electrometer AH501B with quadEM driver and would like to understand how the numbers for X and Y positions are calculated. According to the manual [1] the position is calculated as: 

Position = Diff/Sum * PositionScale - PositionOffset.

But if I calculate the position manually from given Diff and Sum it does not match to what I get from the corresponding PV ($(P)$(R)Pos[X,Y]:MeanValue_RBV). Even in the example screenshot from the quadEM driver manual [2] the X and Y positions do not correspond to the calculation above, e.g. Position X: 2800.5 != -860.2/3181.4 * 12330.0 - 3606.7 = −6940.5.

I am either doing something wrong or is the formula for Position incomplete. I would like to understand this, because we would like to use this Quad Electrometer to measure the beam position, so we need to understand the meaning of the X and Y Positions and how they are calculated.

Best regards,
Luka Debenjak

[1] http://cars9.uchicago.edu/software/epics/quadEMDoc.html
[2] http://cars9.uchicago.edu/software/epics/quadEM.png