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解説■

Exploiting Low-Gap Beam Position Monitors in Orbit Stabilization
Feedback and Feed-Forward Systems at ELETTRA
D. BULFONE, V. FORCHÁI, G. GAIO and M. LONZA
Sincrotrone Trieste

Abstract
Two low-gap electron Beam Position Monitors (BPMs) equipped with digital detector electronics have been installed
either side of an Insertion Device (ID) long straight section at ELETTRA. The new BPMs have been integrated in a fast local orbit feedback system that stabilizes the electron beam orbit at the center of the ID. They also provide the basic measurements that are used by a feed-forward correction system compensating for the orbit distortion produced by an Electromagnetic Elliptical Wiggler (EEW) operated at high switching frequencies (up to 100 Hz) of the horizontal ˆeld. The
main results achieved are presented.

1.

Introduction

One of the main goals of the last generation of synchrotron light sources is the stability of the electron beam inside
the Insertion Devices (IDs). The reduced operational
coupling and emittance of the machines together with recent
progress in experimental techniques and data acquisition
rates necessitate beam stability in the sub-micron range overfrequencies up to several tens of Hz. In order to be able tomeet such speciˆcations a new generation of electron Beam
Position Monitors (BPM) is being installed at ELETTRA
and used in feedback and feed-forward systems for orbit
stabilization. Two examples are presented: a fast local orbit
feedback and a feed-forward system for the compensation of
the orbit distortion created by an Electromagnetic Elliptical
Wiggler (EEW) that works at relatively high switching frequencies of the horizontal ˆeld.

2.

Low-gap Beam Position Monitors

As part of a general ongoing upgrade program, two new
type electron BPMs1) (nominated `low-gap' BPMs) locatedeither side of the ID have been installed in the long straightsection 2 of ELETTRA. These four-button BPMs are basedon a new mechanical design of the sensor, which takes fulladvantage of the 14 mm low-gap ID chamber and is de-coupled from the rest of the vacuum chamber by bellows. Residual mechanical drifts are monitored with respect to a reference column made of carbon ˆbre and taken into account inthe ˆnal electron beam position reading. The detector electronics of a low-gap BPM consists of an analog Front-End
plus a four-channel Digital Receiver VME board that provides the digitized values of the signal for each of the BPM
buttons.

3.

The Local Orbit Feedback System

The aim of the fast Local Orbit Feedback (LOF) system is
to stabilize the electron beam position and angle at the corresponding ID center without aŠecting the rest of the orbit.
The most important noise sources considered are tempera-ture changes producing drifts on diŠerent machine equipment, closed orbit distortions generated by ID operation,mechanically and electrically driven vibrations up to sometens of Hz, which include persistent periodic oscillations derived from the mains at 50 Hz and its harmonics.
3.1 System Overview
The layout of the LOF system is shown in Fig. 1. Using thetwo low-gap BPMs and four corrector magnets as sensors
and actuators respectively, the LOF creates a local orbit
bump that corrects the position and angle of the electron
beam at the ID centre with no distortion to the rest of the orbit. The same ELETTRA standard corrector magnets normally used for orbit corrections are also employed by the fast
feedback system. The －3 dB cutoŠ frequency of the correctors is about 70 Hz, which is the main limiting factor of the
feedback performance. On the other hand, the phase delay
induced by the eddy currents generated in the stainless steel
vacuum chamber by the AC magnetic ˆeld is negligible,
resulting in one degree at 60 Hz and two degrees at 100 Hz.
The LOF processing part is hosted in a VME-based Equipment Controller. The digital samples generated at 8 kHz rateby the Digital Receiver modules are passed through the VME
bus to a PowerPC CPU board. A task running under the Linux operating system with the real-time extension RTAI2),
which provides the required deterministic response, executes
the feedback algorithm. A multi-channel DAC converter
board transforms the resulting digital output samples into
analog signals that drive the corrector magnet power supplies. The system is connected to the accelerator control system by an Ethernet link. The operation of the feedback is remotely controlled from any Matlab session running on thecontrol room UNIX workstations. In parallel to the feedback processing the PowerPC CPU board stores a large
amount of beam position data at 8 kHz that can be uploadedin Matlab for data processing and visualization.
3.2 Operational Results
The basic feedback algorithm adopted is a PID (Proportional, Integral, Derivative) controller, whose transfer func-――(C) 2003 The Japanese Society for Synchrotron Radiation Research

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Figure 1.

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Layout of the local orbit feedback system.

tion in the z-plane is:KI＋KD(1－z－1)
R(z)＝KP＋
1 － z－ 1
An additional single-pole digital low pass ˆlter limits the input signal dynamics to avoid non-linearities of the corrector
magnet power supplies.
The LOF loop has been positively closed at a rate of 8 kHz
in both vertical and horizontal planes. Fig. 2 shows the vertical beam position spectra from 0 to 100 Hz measured by a
low-gap BPM with LOF oŠ/on. The following PID
parameters have been used: KP＝3, KI＝0.01 and KD＝10.
The low pass ˆlter cut-oŠ frequency has been set to 150 Hz.
While slow orbit drifts and lower frequency componentsof the beam noise spectrum are corrected by the PID controller, a diŠerent technique called `harmonic suppression'3)
is used to eŠectively remove those speciˆc components induced by the mains. The harmonic suppression scheme (Fig.
3) consists of a loop with a selective digital ˆlter centered at
the frequency to damp. The delay is calculated in order toachieve an overall system open-loop rotation of 3609at thechosen frequency. The closed-loop system behaves as anotch ˆlter whose depth is regulated by the programmablegain. The selective ˆlter is a complex conjugate two pole digital resonator with two zeros in z＝±1. Its transfer functionis:
H(z)＝G

Figure 2. Vertical beam position spectra measured by a low-gap
BPM with LOF oŠ/on using the PID controller (KP＝3, KI＝0.01,
KD＝10).

(1－z－1)(1＋z－1)
1－(2r cos v0)z－1＋r 2z－2

where v0 is the resonance frequency, r is the amplitude of thetwo poles, which must be smaller than one for the stability
condition and G is an overall gain factor. The width of thenotch, which must be eŠective only at the selected frequency,
can be changed by properly setting r.
As long as the notches are completely separated from eachother, multiple harmonic suppressors centered at diŠerent
frequencies can be implemented and run in parallel with a

Figure 3. Block diagram of the LOF scheme showing the concurrent action of the PID controller and multiple harmonic suppressors.

standard PID regulator (Fig. 3). Moreover, as the beamnoise components associated to the mains are periodic andstable, their reduction is possible even if the frequency ishigher than the open-loop cut-oŠ frequency, where the phase

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Figure 4. Vertical beam position spectra measured by a low-gap BPM with LOF oŠ/on using the PID controller (KP＝3, KI＝0.01, KD＝10) together with one harmonic suppressor centered at 50 Hz.

Figure 5. Vertical photon beam position spectra with LOF oŠ/on
using the PID controller (KP＝3, KI＝0.01, KD＝10) together with
one harmonic suppressor centered at 50 Hz.

rotation is large. The spectra in Fig. 4 show the eŠect of the
LOF with the combined action of the PID controller and one
harmonic suppressor at 50 Hz. The rms of the vertical position signal in the 0–80 Hz range is reduced from 1.24 mm to
0.2 mm. The results have been veriˆed by measuring the spectra of the photon beam position on a downstream photon
BPM (Fig. 5).

4.

The Electromagnetic Elliptical Wiggler Feed-Forward System

An Electromagnetic Elliptical Wiggler (EEW) has been in
operation at ELETTRA since 1997 providing a source of circularly polarized radiation in the VUV/Soft X-ray region4),
with the helicity varied by modulating the horizontal magnetic ˆeld at frequencies up to 100 Hz. The EEW, which isinstalled in straight section 4, combines in one magnetic

structure periodic horizontal and vertical ˆelds. The electromagnets are powered by two Pulse Width Modulation
power supplies featuring low ripple and good stability. The
vertical ˆeld current is mono-polar (max 200 A) and generated in DC mode, while the horizontal one is bipolar (max±300 A) and can be generated both in DC and AC mode. In
designing the magnetic structure of the EEW special care wastaken to minimize deleterious eŠects on the accelerator performance that arise from magnetic ˆeld errors. Residual ˆrst
and second ˆeld integrals, resulting in closed-orbit distortion, are compensated by a feed-forward system based on external correction coils. Four fast power supplies (max ±5 A)
featuring 1 kHz bandwidth drive two pairs of horizontal/vertical air-cored coils installed at each wiggler end.
In order to improve the dynamic correction of the ˆeld integral errors when the EEW is operated in AC mode a newscheme based on the high precision measurements provided
by the two wideband low-gap BPMs has been recently developed.
4.1 System Overview
The layout of the EEW low-level control system is shownin Fig. 6. The Equipment Controller is a VME-based system
hosting a CPU board that is in charge of the basic controls
and is connected to the ELETTRA control system Ethernet
network. ADC, DAC and digital I/O VME boards manage
the I/O signals to control the two horizontal/vertical ˆeld
power supplies and the four correction coil power supplies.
The ADC and DAC boards provide 16-bit resolution andhigh stability (±10 ppm-full-scale/9C). A second CPU
board equipped with one TMS320C40 Digital Signal Processor (DSP) is dedicated to the generation of the AC
waveforms.
Two diŠerent operational modes are foreseen: DC andAC. In DC mode the settings of the EEW currents are carried out through slow ramps. A feed-forward loop runs during the ramps to compensate for the orbit distortion. The

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Figure 6.

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Layout of the EEW low-level control system.

correction values are calculated using a lookup table andcontinuously applied to the correction coil power supplies at
10 Hz data rate. The four values for each pair of horizontal/vertical wiggler settings contained in the lookup table are
empirically determined by performing an oŠ-line calibration
based on the minimization of the closed-orbit distortion
measured by the `normal' ELETTRA BPMs. The tasks incharge of the generation of the ramps and the feed-forward
loop run on the host CPU and drive the DAC boards
through the VME bus.
In AC mode the control system acts as an arbitrary
waveform generator with ˆve independent channels driving
the power supplies of the horizontal ˆeld and of the four correction coils; the vertical ˆeld current is kept constant. The
digital samples corresponding to one period of the waveform
for each of the ˆve channels are downloaded into the DSP
board via the VME bus. When the EEW is started the DSP
board generates the desired waveforms by interpolating thedownloaded samples and continuously applying the calculated values on the DAC boards at the rate of 10 kHz. Any
waveform repetition frequency from 0.01 to 100 Hz can bechosen.
4.2 Optimized Feed-Forward Procedure for the AC Operational Mode
The orbit distortion produced by the EEW working in AC
mode depends on the shape and on the repetition frequencyof the waveform used to drive the horizontal ˆeld power
supply. This is due to the limited bandwidth of the wiggler
electromagnet and to non-linearities. As a consequence thelookup table used for the correction in DC mode cannot be
employed in AC since it is determined by minimizing the orbit distortion in static conditions. Moreover, the `normal'
BPMs installed at ELETTRA do not provide the required
precision and bandwidth to measure the orbit perturbations
at the high EEW operating frequencies.
An iterative procedure employing the low-gap BPMs iscurrently used to optimize the corrections when operating in
AC mode. It assumes that measuring and suppressing the or-

bit perturbation in two machine locations where the monitors are installed is su‹cient to assure that the whole closedorbit is corrected. The betatron phase advance between thetwo monitors is about 359in the horizontal and 919in thevertical plane.
Fig. 7 shows the setup used for the optimization of the correction system. As the time relationship between the horizontal ˆeld waveform and the orbit distortion produced is fundamental to ˆnd the optimal correction values corresponding
to each point of the curve, a synchronized acquisition of the
low-gap BPMs' data is necessary. The EEW horizontal power supply provides a zero-crossing signal made of 10 ms-longpulses generated on the positive-slope side of the current
waveform. This signal is transmitted by means of a 4–20 mA
current-loop link to the low-gap BPM Equipment Controller
where it is ˆrst conditioned and then acquired by thePowerPC CPU board. A workbench based on Matlab has
been developed to support the optimization procedure fromthe control room. A set of Matlab commands allows to operate the EEW and to process the data acquired from the lowgap BPM system.
The ˆrst step of the optimization procedure is the determination of the response matrix of the two pairs of horizontal/vertical correction coils with respect to the low-gap
BPMs. The response matrix is empirically obtained byseparately exciting each correction coil and measuring thecorresponding horizontal/vertical orbit diŠerence at the monitors; the resulting 4×4 matrix accounts also for possible
cross talk between the two planes. The inverted response
matrix is used to calculate the current in the coils to correct agiven orbit error at the low-gap BPMs.
The second step is the measurement of the orbit distortion
during one period of the horizontal current waveform while
the EEW is running in AC mode. The zero-crossing pulses
are used to precisely relate each point of the horizontal ˆeld
waveform to the corresponding position errors at the lowgap BPMs. In order to ˆlter out the background noise of thebeam, which overlaps with the perturbation from the EEW,

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Figure 7.

Setup for the optimization of the AC feed-forward correction.

Figure 8. Orbit perturbation at the low-gap BPMs with feed-forward correction oŠ/on when the EEW is operated witha sinusoidal waveform at 1 Hz.

the acquired positions are averaged over many periods by thePowerPC CPU. In this way non-periodical disturbances or
periodical components at frequencies diŠerent from the
EEW switching frequency and its harmonics are attenuated.
The samples of one averaged period are passed to Matlab
where unwanted components are further reduced by lowpass ˆltering and applying notch ˆlters centered at 50 Hz andits harmonics. Interpolation is then performed in order toobtain four arrays of ˆxed length. The reference DC orbit
value is eventually subtracted from each of them to get four
data arrays representing the horizontal and vertical orbit errors measured at the two low-gap BPMs over one period.
The correction curves to compensate for the measured orbit
distortion are calculated by multiplying the four arrays

above by the inverted response matrix. These new values areadded to the pre-existing correction waveforms (zero at theˆrst iteration) and downloaded into the EEW DSP board.
This procedure is iterated until the residual error isminimized.
A Matlab script has been developed to carry out the above
procedure that is normally executed automatically.
4.3 Operational Results
Fig. 8 illustrates an example of the achieved optimization.
The EEW horizontal current is modulated between ±260 A
using a sinusoidal waveform at 1 Hz, the vertical current is
set to 160 A. The curves show the averaged horizontal/vertical orbit perturbation at the low-gap BPMs without feed-forward correction and after four iterations of the optimization

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第巻第号()Figure 9. Photon BPM's horizontal and vertical readings showing the action of the feed-forward correction system
when the EEW is modulated with a sine-wave at 1 Hz and, for comparison, with EEW stopped.

Figure 10 Vertical beam position spectra measured by a low-gap BPM showing that the harmonics of the EEW switching frequency (sine waveform at 90 Hz) are eliminated by the feed-forward correction system.

procedure, with the perturbation reduced to a negligible level. In order to verify these results in another location of thering the position of the photon beam generated by the undulator in section 6 has been measured using a photon BPM.
The plots in Fig. 9 are the horizontal and vertical positions
measured in three diŠerent cases: feed-forward correction
system inactive, active and EEW stopped.
The feed-forward correction system has been optimized
for several AC waveforms. For each combination of
waveform, frequency and vertical current a dedicated optimization is executed to ˆnd the respective correction coilarrays. Trapezoidal waveforms at 0.1 Hz with one-second
ramps between ‰at-tops and sinusoidal waveforms running
at 0.1 and 11 Hz are examples of AC operational modes
presently requested by the beam line users.

Analysis of the residual orbit distortion can be made also
in the frequency domain where the orbit noise components
due to the EEW can be easily distinguished from the background noise of the beam. Fig. 10 shows the spectra of thevertical position measured by one low-gap BPM
without/with correction. In this case the EEW is operatingwith a sinusoidal waveform at 90 Hz. The noise components
due to the modulation of the horizontal current (±260 A)
are represented by the spectral lines at 90 Hz and its multiples. The lower plot clearly shows that no residual perturbation due to the EEW is present on the beam when the feedforward correction system is working. Similar spectra can bealso measured using the photon BPMs.

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5.

第巻第号()

Conclusions

Physical Society on Experimental Physics Control Systems.

By combining sub-micron resolution and high readout rate
the new low-gap BPMs open the way to the implementationof fast orbit feedback and feed-forward systems that improve the beam stability at ELETTRA.
The ˆrst of a series of fast local orbit feedback systems has
been installed. It takes advantage of the concurrent action of
a PID controller and harmonic suppressors to respectively
correct lower frequency and mains-induced components ofthe beam noise.
A procedure to optimize the feed-forward correction system of the EEW operated in AC mode has been developedand successfully implemented. The optimized system eŠectively compensates the closed-orbit distortions allowing the
EEW to produce circularly polarized radiation with modulated helicity at frequencies up to 100 Hz without disturbing
the operation of the other beam lines.

6.

Vincenzo Forcháƒ
Sincrotrone Trieste
S.S. 14–Km. 163.5, 34012 Basovizza,
Trieste, Italy
Vincenzo Forcháƒ attended the University of Trieste (Italy) and received his
Degree in Electronic Engineering with a
specialization in Bioengineering in 2001. He is now working
at Sincrotrone Trieste in the ELETTRA Controls Group
where he is focusing on feedback systems for beam stabilization.

Giulio Gaio
Sincrotrone Trieste
S.S. 14–Km. 163.5, 34012 Basovizza,
Trieste, Italy

Acknowledgements

The authors gratefully acknowledge the valuable contribution of R. De Monte, M. Ferianis, R. Marizza and L. Pivetta.

References
1)

M. Ferianis and R. De Monte: ``The Low Gap BPM System at
ELETTRA: Commissioning Results'', Proceedings DIPAC '01,
Grenoble, France, May 2001.
2) http://www.aero.polimi.it/ Ã rtai/index.html
3) D. Bulfone et al.: ``The ELETTRA Fast Digital Local Orbit
Feedback'', Proceedings ICALEPCS '97, Beijing, China,
November 1997.
4) R. P. Walker et al.: ``Design of an Electromagnetic Elliptical
Wiggler for ELETTRA'', Proceedings PAC '97, Vancouver,
Canada, May 1997.

Daniele Bulfone*
Sincrotrone Trieste
S.S. 14–Km. 163.5, 34012 Basovizza,
Trieste, Italy
E-mail: daniele.bulfone＠elettra.trieste.it
Daniele Bulfone received his Degree inPhysics at the University of Trieste (Italy) in 1986. After working at CERN and collaborating with
the Microprocessor Laboratory of the Abdus Salam International Center for Theoretical Physics (ICTP)-Istituto
Nazionale di Fisica Nucleare (INFN) in the ˆeld of data acquisition systems for high energy physics experiments, hejoined Sincrotrone Trieste in 1988. At present he is Assistantto the Director and Head of the Controls and Diagnostics
Area. His main research interests include feedback systems
based on digital processing electronics for the cure of beam
instabilities. He is responsible of the integration of new developments and upgrades at ELETTRA and, in particular,of the multi-bunch feedback and orbit feedback projects. He
is chairman of the Interdivisional Group of the European

Giulio Gaio received his Degree in Electronic Engineering with a specializationin Control Systems in 2001. He joined
Sincrotrone Trieste to work in the ELETTRA Controls
Group during the same year and is now concentrating on
software for real-time systems.

Marco Lonza
Sincrotrone Trieste
S.S. 14–Km. 163.5, 34012 Basovizza,
Trieste, Italy
Marco Lonza received his Degree inElectronic Engineering at the University of Trieste in 1988. After a number ofworking experience including a collaboration on automatic
speech recognition at the FACE Standard–Alcatel Laboratories (Rome), he started working at Sincrotrone Trieste in the
ELETTRA Controls Group in 1992. His main interest area
includes data acquisition and real-time systems, feedback
systems and digital signal processing. Since 1997 he is Headof the ELETTRA Controls Group.

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