Feedback Details



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Feedback Details

This section can be skipped by the casual reader.

In most regards the scavenger energy feedback loop is just like most of the other feedback loops. It reads BPMs, calculates states (in this case the energy, positions and angles of the scavenger beam), and then calculates the actuator settings needed to correct the states back to their desired setpoints.

It is that last step - the calculation of the actuator settings - where this loop is non-standard. Basically, the general part of feedback assumes everything is linear. As mentioned above, to calculate phase settings corresponding to a desired energy change requires several arc-tangents which are definitely not linear.

This is handled in a fairly straight-forward way. We have defined a pseudo-actuator known as TOTEGAIN or LI1718 ENERGY GAIN. Its value is calculated by the standard feedback software and indicates the total energy (in GeV) feedback wants sectors 17 and 18 to contribute. If feedback measures that the energy is too low by 0.05 GeV, it will increase this actuator setting by that amount.

After the standard software calculates TOTEGAIN, a routine specially written for the scavenger energy loop is called. This routine calculates the phase shifter settings needed to to get the requested TOTEGAIN. In doing this it takes the following information into account.

  1. The total energy gains available from sector 17 and from sector 18 (at zero phase).
  2. The subbooster phase settings for each of the two sectors.
  3. The desired BNS phase for each of the two sectors (this is in the LCON secondary of the SBST primary in the database). It attempts to keep the contribution of the two sectors to the energy spread constant and equal to what it would be if both of them had phases equal to LCON.
  4. The amount of energy headroom it should leave for fast feedforward.
  5. The energy scale factor for fast feedforward. This is the total amount of energy feedforward could ever request no matter how high the beam intensity. The present value of this constant is 2.0 GeV and it means that when the feedforward sum signal (the one which goes into the quadratic calculation) is equal to 1.0 volt (its maximum possible value) then 2.0 GeV should be added to the energy contribution of sectors 17 and 18.
For you database fans, all this information is kept in the FPRM primary of the EP01 database. It is updated and sent to the micro when the loop is turned on.

This special routine not only calculates the phases needed to get the TOTEGAIN energy, it does similar calculations for energy gains about 1 and 2 GeV more than that. As mentioned before, it fits quadratics to the curves of phase as a function of total energy gain. The resulting quadratic coefficients get written out to the real actuators: AMPLs 2-6 in EP01. The DACs of these AMPLs are inside the fast feedforward hardware and it uses them to calculate the actual phase settings.



next up previous contents
Next: Summary Display for Up: Scavenger Energy Loop Previous: Feedforward Details



SLAC Controls Software
Fri Nov 4 11:34:56 PST 1994