Feedforward and Feedback Working Together
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At this point the poor reader may be somewhat confused. Not only do
both feedback and fast feedforward control the same phase shifters, but the
beam energy that feedback measures is being constantly adjusted by
feedforward. How do they keep it all straight and avoid confusing each
other?
First consider the sharing of the sector 17 and 18 phase shifters.
This is handled with special feedforward hardware. In simple terms, it
has two inputs: the amount of energy feedback wants the two sectors to
contribute and the amount of energy fast feedforward wants to
contribute. It effectively adds these two requests and calculates the
necessary phase settings to obtain that total energy gain.
The two systems share the sectors (rather than using two separate pairs
of sectors) for two reasons.
-
Only two phase shifters are needed instead of four.
-
The two systems can share energy headroom. That is the sharing avoids the case
where feedback complains that it needs to increase the energy but can't
because its two sectors are contributing their maximum possible amount
(both phases set to the BNS phase) while at the same time the
feedforward sectors have several hundred MeV of spare energy because
at the moment the beam intensities are not at their maximum values.
To understand how feedback and fast feedforward avoid confusing each
other it is best to consider a few examples.
-
Assume the klystrons' energy gains remain constant but the positron
intensity suddenly drops (because the flux concentrator tripped off).
Fast feedforward will detect the intensity change, calculate the
expected change in beam loading, and adjust the phase shifters before
the low intensity pulse is accelerated. Assuming that all the fast
feedforward gains are set correctly, the low intensity pulse will have
exactly the right energy. Feedback will see no energy change and hence
will do nothing.
-
Now take the previous case but assume that the feedforward gains are
not set exactly right so it makes a 10% error in its calculation of
the beam loading. In this case feedback will see a small energy (only
10% of what it would have been if fast feedforward did not exist) and
will adjust its outputs to correct it. This takes several beam pulses.
-
Assume the beam intensities remain constant but Klystron 5-1 trips
off. The beam intensity will not change hence fast feedforward will not
change anything. Feedback will read its BPMs, detect the energy
change, and adjust its outputs to correct it. This will take several
beam pulses.
Next: Feedforward Details
Up: Overview
Previous: Feedforward
SLAC Controls Software
Fri Nov 4 11:34:56 PST 1994