The ELETTRA Fast Digital Local Orbit Feedback
C. J. Bocchetta, D. Bulfone, A. Galimberti, D. Giuressi, M. Lonza, C. Scafuri, L. Tosi, R. Visintini
Sincrotrone Trieste, SS 14 - Km 163.5, 34012 Basovizza, Trieste, Italy
Abstract
An overview is given of the ELETTRA Fast Digital Local
Orbit Feedback system. The system has been developed tostabilize the electron orbit in the Insertion Device straightsections. It uses two Photon Beam Position Monitors asdetectors and four corrector magnets to act on the electron
beam. The controller relies on a Digital Signal Processor(DSP) system based on commercial VME boards and iscompletely integrated in the ELETTRA Control System. A
powerful workbench based on Matlab has been developed
and provides complete control of the DSP from any control
room workstation. The performance of the closed loop
using a Proportional Integral Derivative (PID) controller isin good agreement with the simulations carried out withthe system model and shows an attenuation of the noise
frequency components up to 150 Hz. A newly developed
technique adopts dedicated selective filters to effectively
suppress the persistent periodical components of the beamnoise. First experiments have also been carried out with
two local feedback systems concurrently running on
different beamlines.

which implements the control algorithm. The D/Aconverters retransform the resulting output samples inanalog signals which drive the power supplies of thecorrector magnets. An Ethernet connection allows tocommunicate with the control room workstations and toremotely control the DSP operation.

1 Introduction
A third generation synchrotron light source likeELETTRA is characterized by the extraordinary brightnessof the synchrotron radiation generated by the Insertion
Devices (ID). However, the full exploitation of the photon
beam strongly relies on the stability of the accumulated
electron beam. Active feedback systems are usuallyadopted to counteract undesired beam motions.
The local orbit feedback system developed at ELETTRA
corrects the electron beam position at the photon source
point by detecting the photon beam motions with two
Photon Beam Position Monitors (phBPM) and by acting on
the electron beam by four corrector magnets to create aclosed orbit bump.
The beam motions at ELETTRA can be classified in two
main categories. The first is represented by the low
frequency components, up to some Hz; they are mainly due
to ground vibrations, thermal effects and closed orbit
distortions generated by ID gap changes. The second
consists of persistent periodic oscillations at 50 Hz and its
harmonics, derived from the mains. Different control
techniques have been adopted to counteract each type ofbeam motion.
2 System overview
The layout of the local feedback system is shown infigure 1. A couple of dedicated phBPMs and four corrector
magnets driven by separate power supplies act respectively
as sensors and actuators. The phBPM signals are sampled
by A/D converters and processed by a DSP based system

Figure 1: The Local Feedback Layout
2.1 The photon beam position monitors and corrector
magnets
The two phBPMs [1] are located in the front-end of eachbeamline. They are separated by one meter and the firstone is located nine meters from the corresponding ID
centre. The operation of the monitors is based on thephotoemission effect of four blades spaced 90° from eachother. The signals from the blades are pre-processed byhigh precision electronics which provides sub-micron
sensitivity. They are then conditioned and amplified inorder to assure a good noise rejection in the transmission
to the Controller.
The ELETTRA storage ring has 164 corrector magnets(82 horizontal and 82 vertical) and each of them is driven
by a dedicated power supply. The local feedback uses twocorrectors up-stream and two down-stream the ID for eachplane. A 4-by-2 empirically evaluated bump matrix
provides the corrector strength values which move thephoton beam to the desired positions on each phBPM
while satisfying the closed bump condition [2].
2.2 The DSP based controller
The Controller is based on the VME bus standard. Theeight analog signals coming from the two phBPM bladesare conditioned and filtered to avoid aliasing effects by ahome-developed board equipped with 4-th order low-passfilters. The same board provides also filtering andconditioning for the eight analog output signals going to
the power supply cabinet. A Pentek A/D-D/A board
features a maximum rate of 200 ksamples/s on eachindependent channel with a resolution of 16 bits. Wesample at 8 kHz. The DSP board is a Pentek 4284
equipped with one TMS320C40 processor which can run
the correction algorithms on both the vertical andhorizontal planes. An additional CPU board acts as abridge between the DSP and the Ethernet. The A/D-D/Aand the DSP boards are connected by a mezzanine bus(Modular Interface eXtension, MIX), while the VME bus
is used for the communication between the DSP and thebridge board. The programs running in the DSP are written
in "C" language. A complete development environment
and a special Ethernet communication protocol allow tocompile, download, run and debug the programs in theDSP from UNIX workstations. An interactive computer
workbench based on Matlab [3] has been developed tosupport the local feedback project [4].
3 System modelling and simulations
The dynamic behaviour of the local feedback system isdominated by the magnet/power supply transfer function.
The eddy current effect in the stainless steel vacuum
chamber has been found to be negligible in the frequency
range of interest. The overall transfer function has beenmeasured on the machine itself by exciting a corrector withsinusoidal signals at various frequencies and recording theamplitude and phase of the corresponding phBPM signals[5]. A 3-rd order polynomial model fitted to theexperimental data gives the best approximation of thefrequency response. The transfer functions of the four
magnet/power supply channels are considered identical.
The empirical model of the system has been used tosimulate the behaviour of the local feedback loop. The
simulations are particularly useful because they allow tooptimize the control parameters and try new control
techniques before their application to the real machine.
4 The standard PID controller
The first control technique adopted is a standard PID(Proportional, Integral, Derivative) controller with theaddition of a single-pole low pass filter which limits theinput signal dynamics to avoid power supply nonlinearities. The simulations allowed to tune the PID
parameters in order to optimize the closed-loop response.
The frequency response in the range 0-200 Hz has been
optimized, obtaining the following PID parameters:
Kp = 3, K i =0.01 and K d =10. The low pass filter cut-off
frequency fc has been set to 150 Hz. The behaviour of theclosed loop on the real machine is in good agreement withthe model predictions, showing a 3-dB closed-loop
bandwidth of about 150 Hz.
The 50 Hz and its lower order harmonics can be reduced
by a PID controller, but other techniques seem to be more

Figure 2: Noise spectra of a real phBPM signal with loop
open and closed using a PID optimized for the low
frequency noise components
effective for this purpose (see below). The PID will be
therefore dedicated to the lower frequency noise
components. Figure 2 shows the open/closed loop spectraof a real phBPM signal with a PID controller optimized in
the 0-40 Hz frequency range.
5 Harmonic suppression
Since the beam noise components associated to the mains(50 Hz) and its harmonics are periodic and stable, their
reduction is possible even if the frequency is higher thanthe open-loop cut-off frequency, where the phase rotationis large. A technique called "harmonic suppression" hasbeen developed for this purpose.
The harmonic suppressor scheme consists of a loop with aselective digital filter centered at the frequency to damp
(figure 3). The delay is calculated in order to achieve anoverall system open-loop rotation of 360° at the chosenfrequency. The closed-loop system behaves as a notch
filter whose depth is regulated by the programmable gain.
The selective filter is a complex-conjugate two poledigital resonator with two zeros in z = ±1; its transfer
function is:
H (z ) = G(1 − z−1−1) (1 + z )1 − (2 r cosω 0 )z−12 −2+ r z

where ω0 is the resonance frequency and r is the amplitudeof the two poles, which must be smaller than one for thestability condition. By properly setting r it is possible tochange the width of the notch which must be effective onlyat the selected frequency.
Simulation results show that, as long as the notches arecompletely separated from each other, it is possible toimplement multiple harmonic suppressors centered at
different frequencies working in parallel with a standard
PID regulator (figure 3). Figure 4 shows the phBPM signal
spectra measured on the machine where the suppressors

eliminate the 50 Hz and some selected harmonics while thePID acts on the low frequency non-periodic components.

One of the major concerns in a machine with multiple
local feedbacks is the interaction between the different
systems which can affect the efficiency of the correction oreven produce instabilities. An ideal local feedback systemdoes not perturb the orbit outside the bump. In reality,
absolute local correction is unachievable because ofdifferences in the dynamics of the four actuators, errors inthe bump matrix, etc. A number of measurements havebeen carried out to quantify the interaction between two
feedback systems and study their behaviour when both arerunning.
The four correctors of the local feedback in section S6.2
have been excited in a closed bump configuration in orderto create a sinusoidal position oscillation of the photon
beam. The maximum allowed bump amplitude has beenused to reproduce the worst case conditions. We define theleakage as the ratio per cent between the amplitudes of theoscillations measured with the phBPMs in section S2.2 andS6.2. The experiment has been repeated for different
frequencies, up to 420 Hz. The plot in figure 5 shows theleakage as a function of frequency.

Figure 3: Multiple Harmonic Suppressor scheme
The delay of each harmonic suppressor is calculated fromthe model, taking into account the closed-loop phaserotation of the system with the PID. Other tests with fiveharmonic suppressors have reduced the phBPM signal rms
by 64%.

Figure 4: Open/closed loop phBPM signal spectra with aPID regulator together with four harmonic suppressors at
50, 100, 150 and 200 Hz ( r = 0.99995 and gain (at 0) =100)
6 Interaction between feedback loops

Figure 5: Leakage between section S6.2 and S2.2 as afunction of frequency
When two feedback systems are concurrently running,mutual interactions create a coupling phenomenon which
under certain conditions of amplitude and phase can giverise to instabilities.
A dedicated measurement has been carried out toinvestigate the coupling between two active feedback loops
at the different frequencies. The measurement scheme isshown in figure 6. The loop is closed in section S2.2 and alarge phBPM sinusoidal position oscillation is artificiallycreated in order to make the feedback correct it. The
spectra of the perturbed photon beam position signal insection S2.2 are eventually measured in two conditions:loop open and loop closed in section S6.2.
The experiment has been repeated for several artificiallyinduced noise frequencies, from 20 to 320 Hz. The
comparison of the spectra reveals no perceptible
amplification showing that we are far from the instability
condition.

7 Conclusions
A DSP system has been developed to implement a local
orbit feedback at ELETTRA. It provides effective tools forharmonics.
Coupling between two local feedback systems installed indifferent sections has been evaluated: the amount ofleakage is not such to affect the feedback performance or
create instabilities.
References

Figure 6: Coupling scheme and measurement set-up
a quick characterization of the system and testing of theclosed-loop behaviour.
Different control algorithms are applied to counteract thebeam motions. Besides the standard PID controller
dedicated to the low-frequency noise components, a newtechnique called "harmonic suppression" is adopted toeliminate the periodic oscillations at 50 Hz ad its[1] A. Galimberti et al.: 'Photon Beam Position Monitors
suitable for a Local Feedback System at ELETTRA',Proc. 5th EPAC, Sitges, Spain, 1996.
[2] C. J. Bocchetta et al.: 'First Operational Results with
the ELETTRA Fast Local Feedback System', Proc.
PAC97, Vancouver, BC, Canada, 1997.
[3] 'Matlab User's Guide', The MathWorks, Inc. 24 Prime
Park Way, Natick, Mass. 01760-1500, 1994.
[4] D. Bulfone et al.: 'MADE: an Effective Workbench for
the Development of Digital Signal Processing
Applications', these Proceedings.
[5] C. J. Bocchetta et al.: 'The Design of the ELETTRA
Fast Local Feedback System', Proc. PAC97,Vancouver, BC, Canada, 1997.