The new fast wire scanner foreseen to measure small
emittance beams throughout the LHC injector chain will
have a wire travelling at a speed of up to 20 m.s-1, with a
requested wire position measurement accuracy in the
order of a few microns. The vibration of the thin carbon
wires used has been identified as one of the major error
sources on the wire position accuracy. One of the most
challenging and innovative developments in this project
has been the work to quantify the effect of wire vibrations
and fork deformation. The measurement strategy for the
former is based on the piezo resistive effect of the wire
itself, while the deflection of the fork supporting the wire
has been measured by semiconductor strain gauges.
Dynamic models of the wire and fork have been created
to predict the behaviour of the fork-wire assembly and
will be used for its optimisation. This contribution will
discuss the measurement setup and the model
development as well as their comparison. In addition it
will show that this technology can easily be implemented
in current operating devices without major modifications.Postprint (published version

Barjau Condomines, Ana

Dehning, Bernd

Effinger, Ewald

Emery, Jonathan

Guerrero, Ana

Herranz, Juan

Pereira, Carlos

UPCommons. Portal del coneixement obert de la UPC

WIRE SCANNERS AND VIBRATIONS – MODELS AND MEASUREMENTS Juan Herranz123, Bernd Dehning1, Ewald Effinger1, Jonathan Emery1, Ana Guerrero1  Carlos Pereira1, Ana Barjau2  1CERN, Geneva, Switzerland  2Universitat Politécnica de Catalunya, Barcelona, Spain  3Proactive Research and Development, Barcelona, Spain   Abstract The new fast wire scanner foreseen to measure small emittance beams throughout the LHC injector chain will have a wire travelling at a speed of up to 20 m.s-1, with a requested wire position measurement accuracy in the order of a few microns. The vibration of the thin carbon wires used has been identified as one of the major error sources on the wire position accuracy. One of the most challenging and innovative developments in this project has been the work to quantify the effect of wire vibrations and fork deformation. The measurement strategy for the former is based on the piezo resistive effect of the wire itself, while the deflection of the fork supporting the wire has been measured by semiconductor strain gauges. Dynamic models of the wire and fork have been created to predict the behaviour of the fork-wire assembly and will be used for its optimisation. This contribution will discuss the measurement setup and the model development as well as their comparison. In addition it will show that this technology can easily be implemented in current operating devices without major modifications.  INTRODUCTION A wire scanner is an electro-mechanical device which measures the transverse beam profile in a particle accelerator by means of a thin wire moving rapidly across the beam [1]. The intersection of the wire and the beam generates a cascade of secondary particles. Those particles are intercepted by a scintillator, coupled with a photomultiplier, which measures the intensity of the light generated by the crossing particles (Fig. 1). The wire position is typically measured with a precision rotary potentiometer. By synchronising the digitised potentiometer and the scintillator photomultiplier signals it is then possible to reconstruct the transverse beam profile. In order for the wire to reach a suitable speed when it crosses the beam, the actuator has to provide a motion which consists of an acceleration phase, a constant speed plateau and a deceleration phase (Fig. 2).   Figure 1: Schematic of the wire scanner instrument. The peak acceleration values will vary depending on the peak speed required, the length of the fork, the angular travel available for the complete motion and the motion pattern parameters. Figure 2: Typical scan cycle (left) and motion pattern(right). The strong initial acceleration required induces deflections and vibrations in the mechanics (shaft, fork and wire), which result in discrepancies between the true position of the wire midpoint (P) and the position measured by the angular sensor (R) (Fig. 3) [2]. Due to the variation in the acceleration, these deflections cause oscillations, thus increasing the uncertainty of the wire position even further [3].  Proceedings of IBIC2015, Melbourne, Australia TUPB052Transverse Profile MonitorsISBN 978-3-95450-176-2437 Copyright©2015CC-BY-3.0andbytherespectiveauthorsFigure 3: Typical deflections on a conceptual wire scanner instrument. For the new CERN wire scanner the specified position accuracy is 2 μm (see Table 1 for the complete list of requirements). A new wire scanner design has therefore been developed which improves the performance of the existing device,  overcoming  many  of its drawbacks [4]. However, acting only at the level of the design would not guarantee that the requirements would be met. A specific system for vibration measurement was therefore developed to allow the motion of the scanner to be optimizing for the final device.  Table 1: Requirements and Performances, New CERN Vacuum Wire Scanner Peak speed 20 m/s Fork length 156 mm Peak angular speed 128.20 rad/s Peak angular acceleration 6700 - 10000 rad/s^2 Tangential acceleration 95 – 159 g Normal acceleration 260 g Position accuracy 2 μm VIBRATION MEASUREMENT SETUP The vibration measurement system was developed using one of the existing proton synchrotron (PS) fast wire scanners. The fork of the scanner was equipped with high sensitivity semiconductor strain gauges, strategically located in different regions of the fork in order to record its dynamic deflection during the scan. Fig. 4 shows the location of the strain gauges. Gauges G4 and G8 aremainly sensitive to the twist of the shaft; gauges G2, G3, G6 and G7 are sensitive to the deflections in X direction (transversal deflections); and gauges G1 and G5 are sensitive to the deflections in the Z direction (longitudinal deflections) (Fig. 5).   Figure 4: Gauge locations on the fork.    Figure 5: Schematics of the measurable parameters. An electronic circuit based on a Wheatstone bridge and an amplifier was developed to measure the strain gauges and wire resistance variations. The amplified voltages produced were read-out using an oscilloscope. This acquisition system also records the angular position of the fork by means of the angular sensor of the actuator (Fig. 6).  Figure 6: Schematics of the data acquisition system. An accurate calibration is required in order to quantify the X and Z deflections of the fork arms and the wire elongation from the measurement of the voltage variations recorded by the acquisition system. The strain TUPB052 Proceedings of IBIC2015, Melbourne, AustraliaISBN 978-3-95450-176-2438Copyright©2015CC-BY-3.0andbytherespectiveauthorsTransverse Profile Monitorsgauges were calibrated using both experimental measurements and finite element analysis (FE) models. The calibration factors for the strain gauges and the wire can be seen in Table 2.  Table 2: Calibration Factors Sensor Factor [mm/V] Measured parameter G1 0.140 Tip longitudinal deflection G2 1.329 Tip transversal deflection G3 0.652 Tip transversal deflection G5 -0.158 Tip longitudinal deflection G6 1.329 Tip transversal deflection G7 0.652 Tip transversal deflection wire -0.350 Wire elongation MODELS A modal analysis of the fork-wire system has been performed through an FE model. The first two natural vibrational modes associated with the Z-deflection of the fork are shown in Fig. 7. The first Z-mode has a frequency of 151 Hz corresponding to the symmetrical Z-deflection of the fork tips (SZ-mode). In this case, the wire maintains a constant length.  Figure 7: Symmetric (left) and Antisymmetric (right) Z-modes of vibration obtained through the modal analysis performed with ANSYS. The second Z-mode is located at 382 Hz and corresponding to an asymmetrical Z-deflection of the tips (AZ-mode). This mode may lead to an instability of the wire generating parametrically-driven transverse oscillations. RESULTS The Fast Fourier Transform (FFT) performed on the signal of G1 and G5 (Fig. 8) clearly show the two frequencies which correspond to the first and second natural modes of the fork-wire system (~150 Hz and ~400 Hz).  Figure 9  shows the  FFT performed   on the wire signal.  the first   elongation. mode  at   ~150 Hz  does   not provoke wire  Only  the second  mode at  ~400 Hz appears, as    Figure 8: Frequency spectrum of the G1 and G5 signals. The wire length (Sm) is calculated form the wire electrical tension variations and the wire elongation calibration factor. The distance between tips (Lm) is calculate from the electrical tension variation of gauges G1, G5 and their respective calibration factors. The values of Lm and Sm show the same trend (Fig. 10).   Figure 9: Frequency spectrum of the wire elongation. The transversal gauges measurement with their respective calibration factors allows the quantification of the fork transversal displacement which is on the range 0.5 mm (Fig. 11). Wire transversal displacement estimated from the values of Sm and Lm are in the range of 10 um.  Figure 10: Comparison between wire elongation and tips separation. Motion occurs from ~ 135 to 185 ms.  The conceptual design of the PS fork, because of the flexible hinges feature, is prone to vibrate at relatively low frequencies with important amplitudes (~150 Hz and 0.6 mm for the SZ-mode, ~400 Hz and 0.1 mm for the AZ-mode), although this vibrations are in the wire longitudinal direction, which in essence would not affect Proceedings of IBIC2015, Melbourne, Australia TUPB052Transverse Profile MonitorsISBN 978-3-95450-176-2439 Copyright©2015CC-BY-3.0andbytherespectiveauthorsthe measurement accuracy, parametric vibration can appear depending on the wire initial tension. This parametric vibrations, under the inertial forces, results in significant transversal wire oscillation, which would greatly affect the accuracy of the measurement. This transversal oscillation could even cause wire to break.   Figure 11: Tip transversal displacement results from gauges G2, G3, G6 and G7. Motion occurs from ~ 135 to 185 ms.  The oscillations of the kinematic chain, which link the actuator and the fork shaft, induce traversal oscillations on the fork tips which directly affect the measurement accuracy. This oscillations and the play of this kinematic chain disrupt the actuator control system performances and consequently the accuracy of measurement. CONCLUSION A vibration measurement system has been developed and tested for use with CERN wire scanners. The results show that this system allows the vibrational behaviour of the wire and fork to be clearly visualised. A hybrid calibration procedure using experimental measurements and FE calculations has been developed in order to overcome the difficulty of calibration under a dynamic and nonlinear acceleration field. It has also been proven that measurements based on the piezo-resistive effect of the wire provide consistent information allowing the vibrational behaviour of such a system to be determined. Such system can easily be implemented in existing wire scanner devices as only an additional external signal measurement system is required. This implementation would provide valuable information about the device vibrational behaviour. This information can be combined with the profile measurement in order to apply suitable experimental correction factors to the measurement. Additionally monitoring the wire vibrations along the scanner live time, allows to evaluate the aging of the system (especially wire, fork and bearings) and prevent unexpected failures. Vibration monitoring also would serve as an indicator to modify initial motion pattern parameters according to the wear or aging of the system and therefore minimize the wire position uncertainty. Also any unexpected change on the wire configuration (like a tension variation) would be easy detected and corrective actions could be applied. For the first time the piezo resistive effect is used for wire vibrations measurements during the scan. As a general conclusion, this work suggests that the use of a rigid fork is preferable to a flexible one, to avoid the risk of generating high wire amplitudes associated with a possible parametric resonance. This should be taken into account when designing the new scanner. Future work will be focused on the implementation of the vibrational measurement system in the new wire scanner design. More accurate models will also be developed to verify the hypothesis of parametric excitation in the wire oscillations. RFERENCES [1] Proposal of PhD thesis: Minimisation of the wire position uncertainties of the new CERN vacuum wire scanner - https://edms.cern.ch/document/1398456/1  [2] Mechanical aspects important for determining the final wire position accuracy on the New Vacuum Wire Scanner.  https://edms.cern.ch/document/1311456/1  [3] CERN Vacuum Wire Scanner Mechanical Challenge https://edms.cern.ch/document/1404907/1  [4]  Design of a High-Precision Fast Wire Scanner for The SPS at CERN, N.Chritin, B.Dehning, J.Emery, J.Herranz Alvarez, M.Koujili, S.Samuelsson, J.Sirvent Blasco, R.Veness.  http://ibic12.kek.jp/prepress/papers/mopb79.pdf  TUPB052 Proceedings of IBIC2015, Melbourne, AustraliaISBN 978-3-95450-176-2440Copyright©2015CC-BY-3.0andbytherespectiveauthorsTransverse Profile Monitors

Wire scanners and vibrations: models and measurements

The new fast wire scanner foreseen to measure small emittance beams throughout the LHC injector chain will have a wire travelling at a speed of up to 20 m.s^{−1}, with a requested wire position measurement accuracy of the order of a few microns. The vibration of the thin carbon wires used has been identified as one of the major error sources on the wire position accuracy. In this project the most challenging and innovative development has been the wire vibrations measurement strategy based on the piezo resistive effect of the wire itself, while the deflection of the fork supporting the wire has been measured by semiconductor strain gauges. Dynamic models of the wire and fork have been created to predict the behaviour of the fork-wire assembly. This model, validated by the measurements, has then been used for optimisation of the wire-fork assembly. The contribution will discuss the measurement setup and the model development as well as their comparison. In addition it will show that this technology can easily be implemented in current operating devices without major modifications. For the first time the piezo resistive effect is used for wire vibrations measurements during the scan

Barjau, Ana

CERN Document Server

Wire Scanners and Vibrations - Models and Measurements

The new fast wire scanner foreseen to measure small

emittance beams throughout the LHC injector chain will

have a wire travelling at a speed of up to 20 m.s-1, with a

requested wire position measurement accuracy in the

order of a few microns. The vibration of the thin carbon

wires used has been identified as one of the major error

sources on the wire position accuracy. One of the most

challenging and innovative developments in this project

has been the work to quantify the effect of wire vibrations

and fork deformation. The measurement strategy for the

former is based on the piezo resistive effect of the wire

itself, while the deflection of the fork supporting the wire

has been measured by semiconductor strain gauges.

Dynamic models of the wire and fork have been created

to predict the behaviour of the fork-wire assembly and

will be used for its optimisation. This contribution will

discuss the measurement setup and the model

development as well as their comparison. In addition it

will show that this technology can easily be implemented

in current operating devices without major modifications

Wire scanners and vibrations: models and measurements

Abstract

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