Fast Feedback System for CEBAF Page: 3 of 6
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Presently developed orbit and energy locks will operate at a 60 Hz rate, which is the frequency of the beam synchro-
nization pulse. The currently available low-level data acquisition software for the BPMs can provide position signals at a
maximum rate of 60 Hz, which limits the rate at which the feedback loops can operate. There are plans to streamline and
upgrade the low-level data acquisition software for some of the BPMs so that they can provide beam position updates in
the vicinity of lkHz rate. In order to run the energy lock loops at higher rates, a reflective memory network will have to be
set up between the measurement IOCs and the RF IOCs, since Ethernet cannot be used as a reliable means of communi-
cating the correction signal to RF IOCs at rates greater than 240 Hz.
The feedback loop software is divided in two parts: EPICS [3] software and VxWorks [4] software. EPICS software
manages the Graphical User Interface (GUI) for the feedback loop, triggers the appropriate modules of VxWorks software
as required and monitors the status of the loop. The tasks of measurement, computation of correction signal and actuation
are done using VxWorks software. The timing for this software is synchronized from the beam synchronization pulse sig-
nal. Figure 3 shows a schematic diagram of the software layout for an energy lock loop. The organization of the VxWorks
software is described in reference [2].EPICS part
VxWorks
part
ActuationUser Interface
RF IOC Diagnostics IOC
F y c e~ ak - - - J L - - - J
Synchejsk2 Synche_taski
rActuator_set, init am
L - - - J L - - J
Clk Ticki
read_GSET emet
L _ _ J r BPMread
________L - - - - J
Compute settings
L - --- J
F writeGSET
Cik Tickld
L - --- IFigure 3: Schematic Software Layout for Energy Lock Loop
III. MODELLING OF SYSTEM
The objective of using fast energy and orbit locks is to lock the beam energy and beam orbit at a desired location in
the accelerator. Thus, the quantities of interest at the desired lock location are[X, X, Y, Y', AE/E] , namely the position and
angle of trajectory of beam in X and Y planes and the energy variation. The variations in these quantities need to be esti-
mated from BPM measurements in the presence of sensor noise and a corrective action needs to be taken using actuators
such as corrector magnets and RF vernier cavities in the presence of process noise. Thus, modelling of the system in the
state space formalism of digital control theory [5] and design of an optimal state estimator which estimates
[X, X', Y, Y', AE/E] and an optimal controller provide a suitable solution. The description of the system in state space for-
malism is given byx(k + 1) = Ax(k)+ Bu(k)+w(k)
y(k) = Cx(k)+Du(k)+v(k)(1)
(2)x(k) is the state vector which contains the attributes of the system that are dynamically significant, A is the system dy-
Measurement
Computation
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Chowdhary, M. P.; Krafft, G. A.; Shoaee, H. & Watson, W. A., III. Fast Feedback System for CEBAF, article, October 1, 1995; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc716530/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.