Orbit Feedback Development In SRRC
C. H. Kuo, K. T. Hsu, Jenny Chen, K. K. Lin , C. S. Chen, R. C. SahSynchrotron Radiation Research CenterNo.1 R&D Road VI, Hsinchu Science-Based Industrial Park, Hsinchu, Taiwan
Abstract
A project of global and local orbit feedback system had
been launched in the Synchrotron Radiation ResearchCenter (SRRC). In this paper, we will discuss thesuppressant results of orbit feedback experiments withvarious vibrations and orbit drift that lead to distortions in
the closed orbit and result in a larger effective emittance.
Together with the brightness reduction, beam motion
induced incident light position and angle variation candegrade the advantages of using synchrotron light.
Insertion devices are essential to produce high brilliancesynchrotron radiation, however it influences the electron
orbit and the lattice of storage ring. Feedback system isused to eliminate these undesired effects. From control
points of view, orbit feedback is an typical multiple input
multiple output (MIMO) problems. Technically, it isdifficult to implement an analog matrix operationconsisting of large amount of beam position monitors( BPMs ) and correctors. On the other hand, the flexibilityof system development is considered. Consequently, digitalprocessing was used here to implement feedback system.

were used to remove noise of electron beam position
reading, to compensate eddy current effect of vacuum
chamber, and to increase bandwidth of orbit feedback loop.
The infrastructure of digital orbit feedback system iscomposed of orbit acquisition system, gigabit fiber links,
digital signal processing hardware and software, highprecision digital-to-analog converters. The model ofstorage ring is acquired by measured method. It istransferred to control model for feedback that is based on
singular value decomposition. The control loop structure isshown in figure 1.
PID parameter[yref ]+[∆ y]Mx1

GMx1

2 Control structure
A digital orbit feedback system (DOFB) [1, 2] has beendeveloped to suppress orbit disturbances caused by lowfrequency drift and insertion devices. First, a linear
response matrix is measured by taking beam position
monitor (BPM) reading when the correctors areindividually perturbed. Then, this response matrix is used
to design a local orbit bump. The feedback controller isbased on PID algorithm [3]. Digital filtering [4] techniques[R] -1 NxM

Storage Ring[y]BPM
Mx1

1 Introduction
The insertion devices are essential to produce highbrilliance synchrotron radiation. However it influences theelectron orbit and the lattice of storage ring. Orbit feedbacksystem is used to eliminate these undesirable effects. Work
to improve beam stability had been carried out during 1997
with improvement of the orbit feedback system. NewBPM, photon BPM and data acquisition system were
installed in the storage ring. The orbit feedback system forthe storage ring of SRRC has been upgraded in terms of its
feedback bandwidth extension by increasing its dataacquisition sampling rate and compensating eddy current
effect of vacuum chamber with filter. This orbit feedback
system has been applied with the insertion devices
operation, such as U5 undulator and adjustable phase
undulator (APU). Eliminate orbit drift and low frequency
oscillation is to continue effort. In this paper, we will
discuss this new structure and the results of beam position
feedback experiments conducted on new insertion device.

Response Matrix

Data Acquisition System

Figure 1. Control block diagram.
There are two orbit feedback systems in this control loop.
One is local feedback, and other is global feedback. In
order to integrate these two feedback loops for better tuneup, bandwidth of 10 - 100 Hz is necessary. These feedback
systems are integrated with the existed control system.
BPMs data and correctors readback are updated into
control system dynamic database in the period of 100msec. Digital orbit feedback system is bounded on I/O aswell as computation. It is important to arrange the real timetask and to arbitrate computer bus properly in order tooptimize system performance.
2.1 Hardware structure
The hardware configuration of the corrector control
system in SRRC is shown in figure 2. The low layer is a
VME crate system includes a PowerPC 604e CPU boardand I/O interface cards. The CPU board consists of a
PowerPC microprocessor, 32 megabytes on-board memory,
RS-232, PMC sites and Ethernet ports. The front-end
devices are connected to this system via interfaces foranalog I/O, digital I/O etc. A PowerPC based server system
is used as the TFTP file server for downloads OS andmounted disk of network file server (NFS). All application
programs are put on server disk. These programs aredeveloped and debugged on client node to relief loading ofserver. The real-time multi-tasking kernels are embedded

in a single board computer of the VME bus. It provided asatisfactory performance, reliability, and a rich set ofsystem services. New device is easy to be created by that
only modify device table file as if on line editing. The
system can automatically boot and execute different
applications in each VME node with the same operation
system environments. The upload process will be removed
when global feedback is on. This process handles device(analog input) and send acquisition data to database when
receives the broadcast upload message from the Ethernet ineach 0.1 second. The system timing had been improved to1 ms by VME interrupt.

PBPM are sent to PowerPC with VME bus. The verticalsignals are sent to RM with PMC bus after transformation
processing of two plates. This loop is synchronized with 1ms PMC interrupt that is requested by server crate with
RM and fiber link. There is 133 Mbyte per second data
communication in PMC bus, to provide large amount andfast PBPM data transfer. All data are collected to BPM
server node. The orbit server provides fast beam position
information to be used for feedback loop. It also provides
slow orbit information for centralized database. The fastorbit information is sent via gigabit fiber linked reflective
memory to corrector and computation needs in the VME
crates.
2.3 Software structure of system
Workstation

Workstation

Workstation
PC

EthernetVME
DDB
UploadProgram

SettingServiceProgram

BPM
Reflective
MemoryBPM node

Dynamic Database

GlobalFeedback

General Reading
ProgramPower Supply
Reflective
MemoryAO cardFiber Optic

AI cardFigure 2. Hardware structure of system
Present system consists of two VME crates, i.e. orbit
server VME crate, and corrector and computation VME
crate. Within corrector and computation server, a VME bus
to ISA bus adapter is used to provide PC and VME crate
communication. The bus adapter is fit onto slot 1 of VME
crate as system controller. All programs were developedand debugged on PC and downloaded to DSP board. TheDSP board carrying TMS320C40 module handles all
signal processing, including a digital low pass filter (LPF)
and PID controller. It takes 1 ms to complete feedback
processes including operation of PID, digital low passfiltering, matrix operation, BPMs data reading fromreflective memory (RM), and corrector settings. Thecorrector setting had been upgrade to 16 bit DAC toachieve sub-urad steering resolution. All parameters can beremotely adjusted from graphical users interface of controlsystem.

Figure 3: Software diagram of system
The corrector node is supplied to corrector control with
PowerPC, 16 bits D/A card and DSP card. The structure ofapplication software is described in Fig 3. There are someprocess tasks in PowerPC. The setting process handles
corrector setting when setting command arrives fromdatabase. It spawns child tasks to process all setting
requests corresponding tasks to each incoming UPD
facility setting packet. The reading process is triggered by
the external 10 Hz clock from the network when receives
the broadcasted upload message from Ethernet, and sends
an event to wake up the data acquisition process. Theincreased I/O card is easily updated by modified
configuration table file. All acquired data are broadcasted
to Ethernet in every 0.1 s. The server of shared memory is
supplied by DDB process that is to communicate between
reading process and setting process. It also provides thedata access to extra process.

2.2 Photon beam position monitor data acquisition

3 Performance of digital orbit feedback loop

Intrinsically, the performance of feedback system islimited by BPM and PBPM resolution. The PBPM
processing electronics are based on electrometer, which iscommercial product. The PBPM data is directly acquired
from pre-amplifier of electrometer by A/D channel of
VME crate with 7 slot, which are distributed in beam line.
These crates are used as PBPM server node. It contains
that PowerPC 604, reflective memory and 16 bit A/D cardin each crate. The upper plate and low plate signals of

Performance of the orbit feedback system has been tested
with changing the gap of insertion devices and externallyapplied perturbation. The results are shown in figure 4, 5and 6. The cutoff frequency of LPF is 60 Hz, and thecombination of PID parameters are chosen to fulfill control
goals to minimize orbit changes due to various type ofperturbation. The PID parameters were not optimized yet.
It will be modified together with promoting the bandwidthof feedback. The test results shown below were based onthe parameters Kp = 0.8, Ki = 0.03 and Kd = 0.
3.1 Perturbation with U5 gap change
A 4-meters long prototype undulator with 5 cm period(U5) was newly installed at the storage ring.. Orbit waschanged due to beta beating and field error of the insertion
device. The orbit changed without and with DGFB while
adjusting the U5 gap is shown in figure 3. The difference
orbit is defined to be the orbit changed when the U5 gap
was changed from 219 mm to 100 mm and 40 mm,respectively. The displacement of orbit was much smallerwhen the digital global feedback was turned on incomparison with the case when it was off.

The observed PBPM prototype signal of U10 beam line
was drifting during the experiment, as shown in figure 6.
The reading of PBPM can be come back reference position
when DOFB was applied onto the associated feedbackloop, the photon beam position was locked to the reference
orbit. The orbit drift will be continued after feedback is off.
The spike from feedback off to on can be cleared if
adjusted PID parameters properly, in the cost of a reduced
bandwidth.
Photon BPM

exp97_1024

482

480
SL02PBPM1V (unit: um)

Orbit difference with U5 gap change and feedback on/off
0.2"gap:40mm feedback on""gap:40mm feedback off"
"gap:100mm feedback off"

m0.15

0.1Feedback off

Feedback on

Feedback off

478

476

474

dif e enc (0.0547204700-0.05

1000

2000

3000

4000

5000

6000

7000

8000

9000

O

Time (unit: 0.1 s)-0.1

Figure 6: Photon BPM with feedback on/off.
-0.15

4 Conclusion

-0.2

-0.25
010203040
BPM location in the storage ring

5060

Figure 4: Orbit difference with U5 gap change andfeedback on/off.
0.4"BPM a""BPM b""Y(outside bump)"

0.3

Local feedback on

Acknowledgements

Local feedback off

The author would like to thank C. T. Chen, C. C. Kuo
and members of the Light Source Division for theirsupport.

0.2

eam Position unit : mmA digital orbit feedback system has been developed atSRRC. The performance of this system will be improved
as the hardware is upgraded and as we gain further
operational experiences.

0.1

References
0[1] C. H. Kuo and K. T. Hsu: Local Feedback Experiment

-0.1

-0.2
050

100

150
200
250
300
Sampling interval unit: 100ms

350

400450Figure 5: Difference orbit between feedback on and off
with perturbation source: EPBM.
3.2 Perturbation from EPBM
It was notices that the orbit will be changed due to thebump leakage of elliptical polarization from bending
magnet (EPBM) [5]. The orbit changed without and with
DOFB, while EPBM is working, is indicated in figure 5.
The displacement of orbit is much smaller when the digital
local feedback is turned on in comparison with the casewhen it was off.
3.3 The PBPM of U10 beam linein the Taiwan Light Source, Proceedings of 1997
IEEE Particle Accelerator Conference, Vancouver,
1997.
[2] C. H. Kuo and K. T. Hsu: Digital Global Orbit
Feedback System Developing in SRRC, Proceedingsof 1997 IEEE Particle Accelerator Conference,Vancouver, 1997.
[3] B. C. Kuo: Automatic Control Systems, PrenticeHall , p.691, 1995.
[4] J. G. Proakis and D. G. Manolakis, Introduction To
Digital Signal Processing, Macmillan 1988.
[5] K. K. Lin: A Dynamic Local Bump System For
Producing Synchrotron Radiation With An
Alternating Elliptical Polarization, Proceedings of1997 IEEE Particle Accelerator Conference,
Vancouver, 1997.