1. Guenther Rehm, Head of Diagnostics Group
FOFB Optimisation
Guenther Rehm,
Head of Diagnostics Group
03/04/2008
FOFB Optimisation

2. Topics Problems Improvements FOFB trips due to BPM position jumps
Slow build up of excessive corrector values
Improvements
Goal to reduce absolute corrector strengths
Modal and BPM weights
Smooth stopping
03/04/2008
FOFB Optimisation

3. BPM position jumps trip FOFB
03/04/2008
FOFB Optimisation

4. TbT-PM triggered by FA glitch
03/04/2008
FOFB Optimisation

5. Sudden changes on SA channels
Manual Switches
(DSC off)
Automatic Switching
(DSC on)
03/04/2008
FOFB Optimisation

6. Slow build up of correctors
03/04/2008
FOFB Optimisation

7. Reducing unobservable modes
Put “pressure” into the feedback loop to reduce each corrector
Implemented by including a slow decay for each corrector current
Where orbit correction needs it, it will keep the corrector almost at the right value (deviation to small to observe on orbit)
Unobservable modes will decay away with a decay constant of many minutes
03/04/2008
FOFB Optimisation

8. Modal and BPM weights
In principle, Diamond has 168 EBPMs and 168 correctors each plane, so decoupled response matrices are 168x168
Square matrices could be directly inverted
However, system should also work with fewer EBPMs or correctors
Use SVD for pseudo inversion
Orbit motion can then be transformed into “mode space”
Weights can be applied to modes and BPMs, reducing measurement noise contribution.
03/04/2008
FOFB Optimisation

9. Orbit Motion in Mode SpacedB
03/04/2008
FOFB Optimisation

10. Response Matrix Eigen Values
Condition number 4400
03/04/2008
FOFB Optimisation

11. Inversion and Conditioning
Tikhonov regularization
Inversion
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FOFB Optimisation

12. Gains of individual modes
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FOFB Optimisation

13. Closed Loop Response per Mode
03/04/2008
FOFB Optimisation

14. BPM weights in each cell1
0.05
0.2
03/04/2008
FOFB Optimisation

15. Result of BPM weights
03/04/2008
FOFB Optimisation

16. Smooth Stopping of FOFB
Calculations run distributed on 24 CPUs
Synchronous stopping is essential as otherwise correction is applied on part of correctors for some time
All feedback CPUs communicate status (stopped, ready, running) through fast network
When any is stopped, all stop on next cycle
Stopping can be caused by:
command
out of limits for total corrector current (4.9 A of 5 A)
out of limits for corrector current step (0.5 A around 2 s average)
out of limits for beam position (100 um)
When stopped, trailing average is written to correctors to remove dynamic component from correction.
03/04/2008
FOFB Optimisation