Experimental Physics and
Industrial Control System

Mark Rivers via Tech-talk <[email protected]> · Tue, 26 Feb 2019 15:12:37 +0000

Hi Luka,

> 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 believe the documentation is correct.  Here is the code from drvQuadEM.cpp:

 doubleData[QESumAll] = doubleData[QECurrent1] + doubleData[QECurrent2] +
                           doubleData[QECurrent3] + doubleData[QECurrent4];
    if (geometry == QEGeometrySquare) {
        doubleData[QESumX]   = doubleData[QESumAll];
        doubleData[QESumY]   = doubleData[QESumAll];
        doubleData[QEDiffX]  = (doubleData[QECurrent2] + doubleData[QECurrent3]) -
                               (doubleData[QECurrent1] + doubleData[QECurrent4]);
        doubleData[QEDiffY]  = (doubleData[QECurrent1] + doubleData[QECurrent2]) -
                               (doubleData[QECurrent3] + doubleData[QECurrent4]);
    }
    else {
        doubleData[QESumX]   = doubleData[QECurrent1] + doubleData[QECurrent2];
        doubleData[QESumY]   = doubleData[QECurrent3] + doubleData[QECurrent4];
        doubleData[QEDiffX]  = doubleData[QECurrent2] - doubleData[QECurrent1];
        doubleData[QEDiffY]  = doubleData[QECurrent4] - doubleData[QECurrent3];
    }
    denom = doubleData[QESumX];
    if (denom == 0.) denom = 1.;
    doubleData[QEPositionX] = (positionScale[0] * doubleData[QEDiffX] / denom) -  positionOffset[0];
    denom = doubleData[QESumY];
    if (denom == 0.) denom = 1.;
    doubleData[QEPositionY] = (positionScale[1] * doubleData[QEDiffY] / denom) -  positionOffset[1];

I believe the discrepency you are observing is due to the way that averaging is done.  There are 2 ways that the averaging could be done:

1) AveragePosition = AverageDiff/AverageSum * PositionScale - PositionOffset

2) AveragePosition = average(Diff/Sum * PositionScale - PositionOffet)

The driver uses method 2, i.e. it computes the position for each reading, and then computes the average of these over the AveragingTime.

These 2 methods do not yield the same answer.  In the screen shot you can see that the AveragePositionX is 2800.5 but the standard deviation when computing the average is 400409, i.e. it is huge compared to the mean.

If you set the sampling time and averaging time to be the same, so that NumAverage=1 then methods 1 and 2 should be identical and if you do the computation manually you will get what you expect.

Mark

________________________________
From: [email protected] <[email protected]> on behalf of Luka Debenjak via Tech-talk <[email protected]>
Sent: Tuesday, February 26, 2019 6:48 AM
To: [email protected]
Subject: quadEM Positions XY

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

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System