A fast and simple method for the measurement of the mechanical tension of wires of Multi Wires Proportional Chambers (MWPCs) is described. The system is based on commercial components and does not require any electrical connection to the wires or electric or magnetic field. It has been developed for the quality control of MWPCs of the Muon Detector of the LHCb experiment in construction at CERN. The system allows a measurement of the wire tension with a precision better than 0.5% within 3-4 seconds per wir

Baldini, W

Carboni, G

Chiozzi, S

De Capua, S

Evangelisti, F

Germani, S

Graziani, G

Kachtchouk, A

Landi, L

Lenti, M

Lenzi, M

Passaleva, G

Savrié, M

CERN Document Server

Note LHCb-Muon2007-12018 August 2007A Laser Based Instrument for MWPC WireTension MeasurementW. Baldini a, S. Chiozzi a, F. Evangelisti a, S. Germani a,L. Landi a, M. Savrie´ a, G. Graziani b, M. Lenti b, . M. Lenzi b,G. Passaleva b, G. Carboni c, S. de Capua c,d, A. Kachtchouk daINFN-Sezione di Ferrara and Universita` di FerrarabINFN-Sezione di Firenze and Universita` di FirenzecINFN-Sezione di Roma Tor Vergata and Universita` di Roma Tor VergatadCERNAbstractA fast and simple method for the measurement of the mechanical tension of wiresof Multi Wires Proportional Chambers (MWPCs) is described. The system is basedon commercial components and does not require any electrical connection to thewires or electric or magnetic field. It has been developed for the quality control ofMWPCs of the Muon Detector of the LHCb experiment in construction at CERN.The system allows a measurement of the wire tension with a precision better than0.5% within ∼ 3− 4 seconds per wireKey words: MWPC, wire tension, quality controlPACS: 29.40Cs, 29.40Gx1 IntroductionThe LHCb Muon System [1–3] is made of 1368 Multi Wire Proportional Cham-bers (MWPC) with anode wire or cathode pad readout to provide spatialx− y information. Almost all chambers 1 have four sensitive layers, consistingof 5 mm thick gas volumes, with the wire plane placed symmetrically in themiddle of the volume, with a 2 mm pitch. The wires have a diameter of 30µmand are made of gold–plated tungsten . According to the chamber size, thenumber of wires in a layer varies from 200 to 760 and the wire length variesfrom 20 to 46 cm. To ensure chamber stability, the mechanical tension of thewires must be must in the range 50 g < T < 90 g , and must therefore bemeasured for all the wires. To deal with the huge number of wires, an auto-matic method that can perform a fast and precise measurement of the tensionhas been developed. Various methods to measure the wire tension have beendeveloped in the past years [4–9]. They are generally based on an electrome-chanical excitation of the wires obtained, for example by injecting a currentpulse through the wire surrounded by a magnetic field [4] or on electrostaticexcitation through capacitive coupling [8]. In both cases it is necessary toelectrically connect the wires to an external circuit, that is a somehow haz-ardous and time consuming operation. Moreover in some cases the tension isobtained by finding a resonance condition by scanning through several exci-tation frequencies [7] which is again unpractical when a large number of wireshas to be measured. In this paper we describe an alternative method wherethe wire fundamental frequency is excited by a very short mechanical hit andthe induced vibration is detected by a laser–based optical system.2 Description of the systemA wire of length l fixed at the two ends and stretched with a mechanicaltension T vibrates with a fundamental frequency f0 which is related to thefundamental frequency of the oscillating wire by the equation:T = 4µl2f02 (1)where µ is the wire linear mass density.The setup that has been developed, based on the above principle, is sketchedin Fig.1.1 In a small subset there are only two layers2WireLaserLenssoundcardto PC Photodiodesolenoid Mylar striphammerFig. 1. Schematic description of the setup.The light beam from a commercial 3 mW, 635 nm laser diode 2 is directed ontothe wire, parallel to the wire length. The light reflected by the gold–plated wiresurface forms a ring image on a plane perpendicular to the wire itself. Thelight is collected by a photodiode 3 having a sensitive area of 3.6× 3.6 mm2 anda spectral response in the range 350-1100 nm. The wire vibration is detectedthrough the corresponding light intensity modulation on the photodiode activearea. Due to the available lever arm, a wire displacement of a few diameters canbe easily detected. To increase the amount of collected light, a lens focusingthe reflected light on the photodiode surface is used. In order to reject thelight reflected from the cathode surface underneath the wire, the photodiodeis placed out of the laser–wire plane, at an angle of about 30◦. The signalfrom the photodiode is amplified by an Hi-Fi audio preamplifier and thensent to a standard PC’s soundcard, where it is digitized in a 16 bit format,with a sampling frequency of 11 kHz. The wire fundamental frequency is thenfound by applying a standard Fast Fourier Transform (FFT) algorithm to therecorded signal and a subsequent peak search algorithm.3 Choice of the wire excitation methodSeveral wire excitation methods has been tested during the development phase:electromechanical, pneumatic, and mechanical. The electromechanical excita-tion, obtained by injecting a sinusoidal current signal into the wire in a mag-2 Model RS 213-3584. The laser diode package includes a focusing lens with a focusrange 30 mm–∞ and can provide a beam size of 20× 60µm2, matching well thewire diameter of 30µm .3 Model THORLABS DET110.3netic field, was used only to test the optical imaging of wire vibrations, as itrequires unpractical electrical connections to the wires. A direct mechanicalexcitation was then attempted: a) by directing a burst of compressed gas to-wards the wire; b) by hitting the wire itself with a piano–like hammer made ofan elastic Mylar strip moved by a commercial push–type DC–pulsed latchingsolenoid (see Fig.1). The hammer excitation gives by far the largest signal am-plitude, yielding output signals large enough to make the system essentiallyinsensitive to parasitic vibrations or artificial light background (f = 100 Hz).A typical signal and its FFT is shown in Fig.2, where a high narrow peakcorresponding to f0 is clearly visible with essentially no background.Fig. 2. Output signal of the photodiode obtained with the mechanical excitationmethod. Left) recorded output waveform; right) FFT spectrum of the same wave-form. The highest peak corresponds to the wire fundamental frequency while thesmaller ones correspond to the artificial light background.4 Calibration and resultsThe system has been checked for linearity and reproducibility by a calibrationprocedure. Two sets of measurements were taken by stretching two wires ofdifferent length (20 cm and 46 cm, to cover the whole range of length of thewires in the LHCb muon system) with a set of calibrated weights. The resultsof the calibration are shown in Fig. 3(a) where the measured frequencies arefit with Eq. 1.An excellent agreement is observed between the measurements and the theory,for the whole range of MWPCs wire lengths and a reasonably large range oftensions. The experimental error can be estimated by repeating the measure-ment on the same wire. As shown in Fig.3(b) a measurement error σT /T ofabout 0.2% is obtained.The final setup, used for routine automatic tests, is shown in Fig. 4. The laser–photodiode–hammer system has been mounted on a motor controlled support(see fig. 4) which automatically moves over each wire and performs the tensionmeasurement.4(a) (b)Fig. 3. (a) Calibration curves (Frequency vs. square root of Tension) for two wirelengths: L = 46 cm (squares) and L = 20 cm (circles). The solid lines are the resultsof a fit with Eq. 1. (b) Distribution of measurements on a single wire.LASER DIODE WIRESPHOTODIODEMYLAR STRIPHAMMERFig. 4. The setup implemented for LHCb chamber measurements. Left: table withmotor controlled bridge. Right: detail of the laser-photodetector system.Due to the tolerance in the wire positioning (up to 100µm), a scan of eachwire was implemented (from −400µm to +400µm with respect to the expectedposition) in order to get a reliable and stable measurement over all the wiresof the chamber under test. This implies that the measuring time, in principlevery low, becomes about 3 − 4 seconds per wire, allowing the measurementsof the 760 wires of the largest MWPCs within 40–50 minutes. An example ofthe results of the measurements performed on the LHCb MWPCs is shown inFig. 5. In the top plot, the solid line ( at T = 50 g) shows the minimum wiretension acceptable for the LHCb chambers while the dashed line (at T = 90 g)indicates the safety level before the wire elastic limit 4 The measurements aremade on chambers with 612 wires 26.5 cm long. The spread of the measured4 The elastic limit of the wire used in LHCb MWPCs is 140 g .5SysEntries  612Mean    308.1RMS     176.3Wire position0 100 200 300 400 500 600Wire Tension (g)405060708090100Measured Tension vs Wire Number. Panel 00614Wire Tension (g)50 55 60 65 70 75 80 85 90 95 100020406080100120140160Measured Tension vs Wire Number. Panel 00614 Sys1Entries  612Mean    69.58RMS     1.736 / ndf 2χ  7.662 / 9Prob   0.5685Constant  7.0± 140.6 Mean      0.07± 70.08 Sigma     0.050± 1.736 Fig. 5. Example of wire tension distribution as measured with the system describedin the text. The solid and the dashed lines indicate the acceptance interval set forthe LHCb wire chambers.values (σT /T ∼ 2%) is due to the wire–by–wire variation of the tension intro-duced by the wiring system, while the measurement error is about one orderof magnitude smaller.5 ConclusionsA fast and reliable method to measure the mechanical tension of multiwireproportional chambers wires has been described. The system is based on com-mercial components and does not require any electrical connection to thewires or any magnetic or electric field. The measurements performed on theLHCb MWPCs show that this method is linear and reproducible over thedesired range of wire lengths and tensions, with an accuracy of about 0.2%.The method is being extensively exploited for the quality control of the LHCbMWPCs, showing excellent robustness and reliability.AcknowledgmentsThe authors would like to thank the technical staff of the Ferrara, Firenzeand Roma Tor Vergata I.N.F.N. Sections, for their support, which made therealization of the system possible. Special thanks go to S. Atutov and L.Tomassetti, of the Ferrara Trap–Rad group, for their precious suggestions.6References[1] LHCB Collaboration, Muon System Technical Design Report, CERN-LHCC-2001-010[2] LHCB Collaboration, Addendum to the Muon System Technical Design Report,CERN-LHCC-2003-002[3] LHCB Collaboration, Second Addendum to the Technical Design Report,CERN-LHCC-2005-012[4] M. Calvetti, G. Piano Mortari, A. Placci and M. Rijssenbeek [UA1Collaboration], Nucl. Instrum. Meth. 174 (1980) 285.[5] B. Koene and L. Linssen, Nucl. Instrum. Meth. 190 (1981) 511.[6] Y. Asano et al., Nucl. Instrum. Meth. A 254 (1987) 35.[7] S. Bhadra, S. Errede, L. Fishback, H. Keutelian and P. Schlabach, Nucl.Instrum. Meth. A 269 (1988) 33.[8] P. Ciambrone et al., Nucl. Instrum. Meth. A 545 (2005) 156.[9] M.Hosack et al., Nucl. Instrum. Meth. A 556 (2006) 115.7

A Laser Based Instrument for MWPC Wire Tension Measurement

A Laser Based Instrument for MWPC Wire Tension Measurement

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