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Update Energy-Drift-Correction

watts
2026-03-16 13:27:09 +01:00
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Energy-Drift-Correction.-.md
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@@ -26,3 +26,30 @@ There are a number of strategies for dealing with energy drift:
Best practice is to minimise the virtual drift by adjusting the interferometer mirrors, then minimise the real drift by rotating the endstation vacuum chamber, then using the feedback correction in Orocos to fine tune the drift correction.
# Correcting A Virtual Drift
The goal of this section is to adjust the mirrors mounted on the zone plate stage so that their surface is parallel to the movement of the stage.
Note that adjusting the interferometer mirrors to optimise the signal intensity in the interferometer sensor is not the same as aligning the mirrors mounted on the zone plate stage to eliminate virtual drift. Adjustments made to the zone plate stage mirrors will need to be compensated by adjustments to other mirrors in the system. **The required mirror adjustments might not be included in the instrument design**.
0. Check that the interferometer design provides mirror adjustments that allow corrections to mirrors mounted on the zone plate stage AND on the outer mirrors that could compensate for the change in beam angle.
1. Ensure that the drift correction in Orocos is set to zero for all axes.
2. Observe the change in the interferometric position measurement on the X-axis (ΔX) with a change in the zone plate stage position (ΔZ).
3. Calculate the required adjustment screw turns via`ΔX/ΔZ=n*p/d`, where `n` is the number of turns, `p` is the pitch of the screw thread and `d` is the distance between the adjustment screw and the center of rotation.
4. Adjust the mirror that is mounted on the zone plate stage that corresponds to the X-axis (it will have a vertical surface) and rotate it around its Y-xis by the calculated number of turns.
5. Adjust other interferometer mirrors to compensate for the change and optimise the signal level.
6. Repeat from step #2 for each axis until satisfied.
# Correcting a Real Drift (requires x-rays)
Find an easily locatable feature that is also small and doesn't change in appearance very much when changing photon energy. A metal particle close to the corner of a membrane is ideal. Symmetry is often important.
Select a pair of photon energies that will give a significant difference in focal length (i.e. movement of the ZP-Z stage).
Measure the position of the object at each energy and note the change in position.
Rotate the endstation using the Girder Mover.
Optimise the X-Y position of the Girder Mover for maximum X-ray signal.
Iterate steps 3-5 until the drift is minimised.
# Applying a Feedback Drift Correction
The zCorrectMatrix values [X,Y] are applied by Orocos in the J/K->X/Y coordinate transform. This means that all of the bad effects of a virtual drift are neutralised at the Orocos level. The only negative effect would be a (typically insignificant) shift in the coordinate system when the interferometer is reset at a significantly different photon energy from the previous reset. This occurs because the correction is calculated based on a reference point that is chosen as the position of the last "reset interferometer". Since an interferometer reset also defines a new coordinate system based on a reading of the position encoders on the coarse stages (with the fine piezo stage in a relaxed, 0 mV position) (and there will also be a coordinate shift due to previous positions being incorrect via accumulated errors in counting fringes), there is typically going to be a shift in the coordinate system anyway that will be more significant than the zCorrectMatrix shift.