Design of Accelerator Girder System for Vibration … · 2005-08-07 · S. Sharma Ambient Ground Motion NSRRC/July 21, 2005 Mechanical Engineering Group Accelerator Systems Division

S. Sharma Ambient Ground MotionNSRRC/July 21, 2005

Mechanical Engineering GroupMechanical Engineering GroupAccelerator Systems Division

Design of Accelerator Girder System for Vibration Suppression

by

Sushil Sharma

Contributors: B. Rusthoven, V. Ravindranath and C. Doose



Outline

An overview of some storage ring girder systems

Ambient ground motion

Transmissibility and damping

Girder design (materials, geometry and alignment)

Conclusion



A girder is used for several purposes:

1. Provides common platform (strongback) to precisely mount several components.

2. Raises components to nominal beam height (acts as a spacer).

3. Simplifies installation and alignment of the components in the accelerator ring.

4. Expedites realignment (e.g. to compensate for floor settlement).

The girder, however, adds another structural element to the system. This adversely impacts beam stability due to amplification of floor motion and thermal deformations.

APS Storage Ring

APS Linac Bunch Compressor

Pedestal

Girder

Table

Girder vs. Table



Doose, Sharma [MEDSI 2002]

Wedge Jack

APS Girder System

First natural frequency is ~ 10 Hz (Rocking with high center of mass)

Higher frequencies (>23 Hz) quads motion

Vibration amplification (rms 4-50 Hz):no damping: 9damping with viscoelastic pads: 3damping with pads and shims: 1.5 (~ 35 nm)



SPring-8 Girder SystemMode n

1(H) 18.9

2(H) 21.3

3(V) 25.6

4(V) 29.5

5(H) 29.6

http://epaper.kek.jp/p01/PAPERS/TPAH117.PDF [2001]

High first natural frequency.Stiff alignment mechanism.Six support points.Vibration amplification: 1.9 (48 nm)*

Vibration of chambers (not magnets) dominates orbit fluctuations. Nakazato et al. [MEDSI 2002]

* Sharma et al., [GMV 2000]

Vertical Alignment



http://www.esrf.fr/machine/reports/sassenage02/presentations/zhang.pdf

Sharma et al. [GMV 2000]

Mode Test (Hz)

1 8.68

2 11.74

3 13.63

4 22.33

5 26.29

ESRF Girder System

The girder system has several low natural frequencies.

Vibration amplificationwithout damping: 2.2with viscoelastic damping link: 1.3 (40 nm)



Courtesy, D.J Wang, May 2005


NSRRC Girder System

First natural frequency horizontal rolling: ~ 15 Hz.Simple alignment hardware (threaded rods).Cross-talk of flow-induced vibrations in magnets and vacuum chamber.Vibration amplification:

no damping: 1.8damping with viscoelastic pads: 1.3 (92 nm)



Diamond and Boomerang girders are of similar design. Magnets are clamped to precisely machined girders surface.The magnet support (girder) shows a number of resonances in 15 Hz to 50 Hz frequency range.Without cam movers, the calculated frequencies can be quite high (f1 = 60 Hz forDiamond girder, N. Hammond, MEDSI 2002 ).Vibration amplification (rms above 4 Hz): 10

SLS Girder System

S. Redaelli et al., EPAC 2004 THPK011

Cam Mover (SLS)http://slsbd.psi.ch/pub/varia/dynal_iwbs04.pdf

Machined Top Plate



Mode-1 (67.9 Hz) Mode-2 (76.9 Hz) Mode-3 (79.8 Hz)

Mode-5 (128.8 Hz)

Mode-4 (99.0 Hz)

LCLS Girder System

Cam movers are used for beam-based alignment.

Cams are attached to the upper girder flange to minimize thermal deformations and to lower center of mass.

The first four modes correspond to girder deformations (flexure, torsion, flexure, and flexure).

Modes 5 corresponds to undulator torsion.



Ground Motion Characteristics

Seryi [2003]http://www.desy.de/~njwalker/uspas/coursemat/pp/unit_8.ppt#10


Ground motion amplitude drops sharply at higher frequencies. The first natural frequency of the support system (girder/magnets + alignment mechanism) should be as high as possible (preferably, f1 >20 Hz).



2)/(242]2)/(1[12)/(24

nnn

XYbilityTransmissi

+

+==

mkn =

damping critical offraction 2

==mkc

For vibration isolation: Systems on soft supports (/n > 2) no damping. Systems on very stiff supports (/1



http://www.minusk.com/

Seismic Isolation and Suspension Systems for Advanced LIGO

Systems on Soft Supports

http://www.ligo.org/pdf_public/techpapers_robertson.pdf[2003]

Limited to comparatively low mass.



=

==nQ

1212

maxy

n21

maxy

2maxy

Frequency

Am

plitu

de

System Viscous Damping,

Metals in elastic range < 0.01

Steel 0.001 0.002

Continuous Metal Structures 0.02 0.04

Metal Structures with Joints 0.03 0.04

Reinforced Concrete Structures 0.04 0.07 0))(1(0)()(

=++=++

xyikymxykxycym

&&&&&&

factorquality factor loss

damping critical offraction

===

Q

H. Bachmann et al., Vibration Problems in Structures, Birkhauser Verlag, Berlin, 1995.V. Adams and A Asknazi, Building Better Products with Finite Element Analysis, OnWord Press, Sata Fe, N.M., 1999.

Damping



2/1

)(2 31

+

bMML

EIf

Flexural Vibration:

Misc. Sources

* Bowden [SLAC-TN-05-028, 2002]

Comparison (for same I and L)

Girder Design - Material Properties

Al Steel Anocast Gr. Epoxy

Frequency (M>>Mb) E1/2 8.3 14 6 8

Thermal Bending /k* 0.13 0.23 0.70 0.2

Thermal Expansion 23.4 12 16.9 2.7



Anocast damps vibrations more rapidly than aluminum, cast iron, or granite, by a factor of 45, 10 and 4, respectively.

SecondsDef

lect

ion/

Forc

e

(in/

lbf)

SecondsSeconds Seconds

Girder Design - Anocast Polymer Composite

http://www.rockwellautomation.com/anorad/downloads/pdf/Anocast.pdf

http://www-project.slac.stanford.edu/lc/local/notes/tset/Mover/ mover_notes_oct12_post.pdf



Girder Design - External Viscoelastic Damping

Viscoelastic polymers have high loss factor. Viscoelastic films absorb vibrational energy by high cyclic shear deformations.Many viscoelastic materials are creep and radiation resistant. Viscoelastic damping pads (or damping links) are simple in design and can reduce rms vibration amplification to ~ 1.2

APS Damping Pad and Shim

ESRF Damping Link

http://casl.ucsd.edu/data_analysis/nomograms/isd112.htm

Materials with high structural (internal) damping may have lower thermal performance:

L (0.5m) (20m/m.C) T(0.1C) = 1m

3M ISD112

Freq

uenc

y

Temperature



Girder Design Geometry and Supports A box-type cross section is preferable for high stiffness in flexure (both directions)

and torsion. Full-length welds, gussets and plate stiffeners can significantly increase the

overall stiffness.

Unsupported length of the girder should be kept as small as possible (SPring8 girders are supported at six points).

Supports at Airy Points 3Point SupportCLS Boomerang

DiamondBoomerangSPring-8 CLS APS



Cartridge Adjuster(CEBAF)*

Wedge Jack (APS)

Motorized Jack (ESRF)

Girder Design - Alignment Mechanisms

Six-Strut System

Threaded RodThreaded Rod with Lateral Adjustments (APS)

SPring-8 Alignment

http://www-group.slac.stanford.edu/met/IWAA/TOC_S/PAPERS/KTsum02.pdf

Cam Mover (SLS)

http://accelconf.web.cern.ch/AccelConf/e00/PAPERS/WEP4A17.pdf

* Other Concepts [1994] http://www-group.slac.stanford.edu/met/IWAA/TOC_S/Papers/RRula95a.pdf



Belleville Washers

Viscoelastic Film

Locating Slot

Graphite-Epoxy

Conclusion

Low center-of-mass to reduce vibrations (and thermal deformations).- Why a beam-height of 1.4 m?

Compromise between high stiffness and ease-of-alignment:- Simple alignment mechanism.- Fasten magnets directly on precisely machined girders surface.

Reduce flow-induced vibrations and cross-talk by viscoelastic damping.Vibration mitigation should not be at the expense of thermal stability.

CLS

SPring-8

A Support Concept

No Girder !

Documents

Design of Accelerator Girder System for Vibration … · 2005-08-07 · S. Sharma Ambient Ground Motion NSRRC/July 21, 2005 Mechanical Engineering Group Accelerator Systems Division