Design and implementation of wire tension measurement system for MWPCs used in the STAR iTPC upgrade

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BNL-114430-2017-JA

Design and implementation of wire tension measurement
system for MWPCs used in the STAR iTPC upgrade
X. Wang, et al

Submitted to Nuclear Instruments Methods A

October 2017

Physics Department

Brookhaven National Laboratory

U.S. Department of Energy
USDOE Office of Science (SC),
Nuclear Physics (NP) (SC-26)

Notice: This manuscript has been co-authored by employees of Brookhaven Science Associates, LLC under
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Design and implementation of wire tension measurement system for MWPCs
used in the STAR iTPC upgrade
Xu Wanga , Fuwang Shena , Shuai Wanga , Cunfeng Fenga , Changyu Lia , Peng Lua , Jim Thomasb , Qinghua Xua,∗,
Chengguang Zhua
a School of Physics and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Jinan 250100, China

arXiv:1704.04339v1 [physics.ins-det] 14 Apr 2017

b Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA94720, USA

Abstract
The STAR experiment at RHIC is planning to upgrade the Time Projection Chamber which lies at the heart of the
detector. We have designed an instrument to measure the tension of the wires in the multi-wire proportional chambers
(MWPCs) which will be used in the TPC upgrade. The wire tension measurement system causes the wires to vibrate
and then it measures the fundamental frequency of the oscillation via a laser based optical platform. The platform can
scan the entire wire plane, automatically, in a single run and obtain the wire tension on each wire with high precision.
In this paper, the details about the measurement method and the system setup will be described. In addition, the test
results for a prototype MWPC to be used in the STAR-iTPC upgrade will be presented.
Keywords: STAR; TPC; MWPCs; wire tension; FFT
1. Introduction
The STAR detector is located at the Relativistic
Heavy Ion Collider (RHIC) at Brookhaven National
Laboratory (BNL). It uses a Time Projection Chamber
(TPC) as its primary tracking device[1, 2, 3, 4, 5]. The
TPC records the tracks of particles, measures their
momenta in a 0.5 T magnetic field, and identifies the
particles by measuring their ionization energy loss
(dE/dx). Its acceptance covers a pseudo-rapidity range
of |η| < 1 over the full azimuth. Particles are identified
over a transverse momentum range from 100 MeV/c to
greater than 1 GeV/c, and momenta are measured over
a range of 100 MeV/c to 30 GeV/c.
The STAR TPC has played a central role in the
RHIC physics program for over 15 years. STAR has
decided to upgrade the inner sectors of the TPC (iTPC)
so that the pseudo-rapidity coverage of TPC will be
extended from |η| < 1 to |η| < 1.5 with improved energy
loss dE/dx measurements for particle identification and
better tracking performance. The iTPC upgrade will
require new readout electronics to match the increased
∗ Corresponding autor. Tel.: +86 531 88364515.
Address: 27 Shanda South Road, Jinan, Shandong, China, 250100
E-mail: xuqh@sdu.edu.cn

number of read-out channels in the inner sectors. The
upgrade will also replace the wire grids in the MWPC
(multi-wire proportional chamber) readout system so
they can be run at lower gain and utilize larger pads.
The new wire grids will extend the lifetime of the STAR
TPC into the next decade and the increased acceptance
of the new pad-planes will allow STAR to pursue an
enhanced physics program in the Beam Energy Scan II
program and beyond.
The iTPC upgrade project[6] will replace all 24
existing inner sectors in the STAR TPC with new, fully
instrumented, sectors. The TPC and the iTPC upgrade
use MWPCs with pad plane readout to record tracks
of ionizing particles. There are three layers of wires
above the pad plane: the anode wires (20 µm diameter
gold-plated tungsten wires), the shield wires and the
gated grid (75 µm diameter gold-plated beryllium
copper wires), which are shown in Figure 1. The
distance between the pad plane, the anode wire plane,
the shield wire plane and the gated grid are 2 mm, 2
mm, and 6 mm respectively. The pitch for the anode
wires is 4mm and 1 mm for the shield and gated grid
wires.
For the MWPC construction, the wire tension must
be strictly controlled as it is critical for the uniform gain

Preprint submitted to Nuclear Instruments and Methods in Physics Research Section A

April 17, 2017

Measured Wires
Photo Diode
Microscope
Objective

Air Jet

DAQ Pre Amp
FFT

Concave Lens

PC

Diode Laser

Figure 1: A schematic diagram showing the pad plane, anode wires,
shield and gated grid wires in the STAR MWPCs. The anode wires
pitch is 4 mm, the shield and gated grid wires pitch is 1 mm. The
distance from wires frame to pad is 2,4 and 10mm respectively.

Figure 2: A schematic diagram of the diode-laser based optical system
which was used to make the wire tension measurements.

performance of the TPC. The wires were wound first
on wire-transfer frames and the wire tension was provided by the wire winding machine. The wire tension
was then checked before a wire plane was transferred to
the side wire mounts mounted on the individual sector
strongbacks, using precision wire combs to secure the
wire pitch and relative height of each wire plane[3]. The
wire tension was verified using the wire tension measurement system described in this paper. The wire tension will be checked both on the wire frame and also on
the strongback.

Therefore, the key point for measurement is to find
a way to transform the mechanical vibration of a wire
to another signal that easily be detected. A laser based
optical platform was designed to measure the vibration
frequency of the wire as shown in Figure 2. The wires
will vibrate under excitation of an air jet. When the air
jet stops blowing, the wire will vibrate freely and the
fundamental frequency of its vibration is an indication
of the tension on the wire.

2. The wire tension measurement method
The wire is illuminated by a laser beam and the
reflected light is collected by a photodiode, which can
transform the light to analog signals. The frequency
of the mechanical vibration of wire is equal to the
reflected laser intensity fluctuation frequency.

Various methods have been developed to measure
the wire tension in the past years[7, 8, 9], which are
generally based on electromechanical excitation, for
example in a magnetic field[7] or through capacitive
coupling[8]. In both cases, it requires electrical connection to wires, which is quite time-consuming, for a
large amount of wires to be tested like our iTPC project.
Each iTPC MWPC consists of more than 1500 wires.
Here we use a laser-based optical system as described
in below to measure the vibration induced by a short
compressed gas jet, which does not need to touch the
wire and can be done in 3˜4 seconds for one wire.

Finally, the analog signals are digitized and recorded
by a data acquisition system. The digitized signals
are time domain data and they are transformed into
frequency domain by means of the Fast Fourier Transform (FFT) algorithms. Consequently, the fundamental
frequency is extracted from the frequency domain
signals and the wire tension is calculated through Eq.1
with parameters of linear density and wire length.

The tension T of a wire is related to its fundamental
vibration frequency f0 as in the following equation:
T = 4µ f02 L2 ,

(1)

Sampling and digitization of the analog signals was
done at the rate of 10 KS/s and the data length is
10 K sample points. The sampling rate is double the
maximum frequency to be detected. According to the
Nyquist theorem, the sampling rate is sufficient for accurate measurements.

where µ is the linear mass density, and L is the wire
length between two fixed ends. If the fundamental
frequency of a wire is known, then the wire tension can
be calculated through Eq.1.
2

Figure 3: The measurement bench (left) and the laser based optical platform (right).

3. System setup
Our wire tension measurement system can be used
to measure multiple wires on a wire-transfer frame
in a single run. The hardware and the software was
designed to be fully automated.

3.1. Hardware components
Besides the laser based optical platform and the air
jet, the hardware also includes a stepper motor. As
shown in Figure 3 (left), both the optical platform and
the air jet were mounted on the stepper motor platform
and can move with the motion of the platform. The
wire transfer frames were placed on a support stand
that is absolutely parallel to the moving direction of the
stepper motor. Therefore, the laser beam can scan the
wires on the frame one by one, and the tension of each
wire was obtained.

Figure 4: Block diagram of the control scheme for the wire tension
scanning system.

smaller than the wire pitch. Data acquisition and digitalization are accomplished with the fine steps that are
used to determine the focal point for the laser beam.

The laser based optical platform and air jet is shown
in Figure 3. When a measurement starts, the laser beam
focuses on the wire to be detected. To get enough intensity of reflected image, the laser spot that is focussed
on the wire must be approximately the same diameter
as the wire. The air jet is driven by a pneumatic pump
and it can blow directly upon the wire to be tested. In
addition, the time duration and physical strength of the
air jet can be adjusted to excite a suitable wire vibration.

3.2. Software framework
The software was developed using the LabVIEW
graphical programming environment on a Windows
PC. The PC hosts a PCI bus, RS-232 serial interfaces, a
motion control card (DMC2410) and a data acquisition
PCI card (NI 6230: 16-Bit, 250 kS/s). It works as
the workbench to perform instruments control and
monitoring, data acquisition and data processing as
shown in Figure 4.

To achieve automation for the laser beam focusing
system, the motor’s steps consists of coarse step and
fine steps. The travel length of the coarse step is slightly

The motion control card is responsible for the drive
and control of the stepper motor. A serial interface is
3

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Acknowledgments
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References
tion
storageuniformity
time inare
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dark.
andofdigitalization
fulfilled
a Na467Sample
the photocathode
CTTDusing
of PMTs
were
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489
[1] Zhen Cao, Chinese Physics C 34(2010) 249.
468
conveniently
tested.
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di-distance
method
was
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2500
3000
3500
4000
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490
[2] Huihai He, et al.,waveform
for the LHAASO
Collaboration in: Proceedsample point
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in
the
above
test.
Therefore,
the
PMT
need
to
be
HTML
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for the the
displaying
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results
on
469
veloped
to measure
linear dynamic
range
of PMTs
ings of 31th ICRC, 2009. (b)
in the dark box for a few hours before testing. 491
the stored
web from
the internet for remote users.
470
in the test bench. Because a one picosecond pulsed 492 [3] ZHAO Jing, et al., Chinese Physics C 37(2013) 249.
2

11Figure 6: The time domain signal acquired by the data acquisition
A graphical control interface for tension measure- 010201-8
system. The top plot (a) shows all of the sampling points acquired in
ment was developed, as shown in Figure 5. Columns
one second and the bottom figure (b) is an expanded view showing the
are used to input the parameters, such as wire length,
sample points from 2000 to 4000.
linear density, etc, and it also shows the latest results.
The graphical interface also features a set of status
LEDs to show work status for each module of the systhe tension measurements for all of the wires on one
tem.
transfer frame. The mean value for these measurements
was 50.1 grams as shown in Figure 8 (b). These results
4. Measurement results
indicate that wire winding works well and the tension
on all of the wires is uniform within an uncertainty
The wire tension was first measured on the wireof 0.8 gram(1.6%). This is well within the accepttransfer frame. There are one hundred and sixty
able range of 6% uncertainty required by iTPC upgrade.
20 µm diameter gold-plated tungsten wires on the
wire-transfer frame. The wire length is 0.899 m and the
linear mass density is 6.02 ×10−6 kg/m. The tension
was set to 50 grams during the wire winding operation.
5. Calibration and verification of the test system

The calibration of the tension measurement system
was done using one 20 µm diameter gold-plated
tungsten wire and one 75 µm diameter gold-plated
beryllium copper wire. Their lengths are the same, i.e.,
0.899m and the mass density for beryllium copper wire
is 4.06 ×10−5 kg/m. Wires with known tension were
prepared using standard weights. One end of the wire

Figure 6 shows the time domain signal measured
on one wire. Figure 7 shows the frequency domain
spectrum transformed from the time domain using
an FFT algorithm. The first peak in Figure 7 is the
fundamental frequency and the second peak is twice
the fundamental frequency, etc. The fundamental
frequency, in this case, is 160 Hz. Figure 8 (a) shows
4

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References
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[1] Zhen Cao, Chinese Physics C 34(2010) 249.

490

[2] Huihai He, et al., for the
Collaboration in: ProceedWireLHAASO
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300

350

To verify the systematic uncertainty of the tension
measurements, the tension was measured 100 times on
the same wire for the two different diameter wires. Figure 10 (a) shows the measured results for 20µm diameter wire with 50 gram tension and figure (b) shows the
results for 75 µm diameter wire with 120 gram tension.
The RMS of these measurements are 0.26g and 0.40g,
respectively. The calculation uncertainties transferred
from the errors of wire length (0.001m) and mass density (10−8 kg/m) are 0.14g and 0.27g for 20 µm and 75
µm wires respectively. The total uncertainties (0.30g
and 0.48g for wires with tension 50g and 120g respectively) are thus less than 1% of the total tension on each
wire.

50.18

Mean

30

250

was soldered on the top-side of the wire frame and the
other end was stretched with a standard weight. The
fundamental frequency was extracted and the measured
tension was compared with the standard weights. The
calibration results show that both the 20 µm and 75 µm
wires exhibit good linearity between the square of the
measured fundamental frequency and the known wire
tension as shown in Figure 9. The error bar is estimated
by repeating the measurements 100 times for each input
tension and the RMS of the measurements is taken as
the error bar, which are found to be very small (<0.8%).
The plot indicates good linearity from 10 grams to 60
grams for 20µm wires and 100-350 grams for 75 µm
wires, which covers the tension range for wires used in
the STAR iTPC upgrade(50-120grams).

hardware
used, all of the tests were independent of peo60

still outputted 476
applied; these
ion of storage
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Wire Tension (g)

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guarantee that all the 6000 PMTs of KM2A are tested

200

Figure 9: The linearity between the square of the measured frequency
and the wire tension for both 20 µm and 75 µm diameter wires. Note
that the error bar is invisible as they are smaller than the marker size.

1200

Frequency (Hz)

474

150

Wire Tension (g)

esponding position.
0

100

60

ings of 31th ICRC, 2009. (b)
[3] ZHAO Jing, et al., Chinese Physics C 37(2013) 249.

010201-8
11Figure 8: The top plot (a) shows the wire tension measured on each

wire on a wire transfer frame while the lower plot, the two lines indicate the acceptable range for iTPC upgrade (b) shows the distribution
and variation of the results.

6. Conclusion
A wire tension measurement system has been setup
using a measurement method of detecting the funda5

mental frequency of wire vibration in MWPCs for the
STAR iTPC upgrade project. The system can measure
wire tension wire by wire, automatically, with good precision and stability. The system will be used to make
wire tension measurements for all 24 MWPCs used in
the STAR iTPC upgrade.
Acknowledgments
We would like to thank the STAR collaboration for
sharing some of the hardware and their strong encouragement. We would also like to thank the STAR
iTPC upgrade collaboration members for their valuable suggestions and support. The work was supported in part by the National Natural Science Foundation of China (No. 11520101004), the Major State
Basic Research Development Program in China (No.
2014CB845400), and the Natural Science Foundation of Shandong Province, China, under Grant No.
ZR2013JQ001.

20 µ m wires
Entries
100
Mean
50.15
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52.03 ± 1.30
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time

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0

46

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Wire Tension (g)

References

(a)

1200

box at the
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he
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75 µ m wires
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Constant
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. This test

are tested

References

100
120
0.4046
62.93 ± 1.34
119.9 ± 0.0
0.3161 ± 0.0084

[1] The STAR Collaboration, The STAR Conceptual Design Report, June 15, 1992 LBLPUB-5347.
[2] The STAR Collaboration, STAR Project CDR Update, Jan. 1993
LBL-PUB-5347 Rev.
[3] H. Wieman, et al., IEEE Trans. Nucl. Sci. 44 (1997) 671.
[4] K. Ackermann, Nucl. Phys. A 661(1999) 686c.
[5] M. Anderson et al., Nucl. Instr. Meth. A 499 (2003) 659-678.
[6] Technical Design Report for the iTPC Upgrade, Star Note 0644,
https://drupal.star.bnl.gov/STAR/starnotes/public/sn0644
[7] M. Calvetti, et al.[UA1 Collaboration], Nucl. Instrum. Meth.
174 (1980) 285.
[8] P. Ciambrone et al., Nucl. Instrum. Meth. A 545 (2005) 156.
[9] S. Bhadra, S. Errede, L. Fishback, H. Keutelian and P.
Schlabach, Nucl. Instrum. Meth. A 269 (1988) 33.

30

grammable

20

ent of peo-

10

e.

0
115

116

117

118

119

120

121

122

123

124

125

Wire Tension (g)

(b)

their early
206

test work.

Sheng for

207
208

160

seful feed-

217

encouragement. We would also like to thank the STAR iTPC upgrade collaboration members for their
valuable suggestions and support. The work was supported in part by the National Natural Science Foundation of China (No. 11520101004), the Major State Basic Research Development Program in China
(No. 2014CB845400), and the Natural Science Foundation of Shandong Province, China, under Grant No.
ZR2013JQ001.

218

References

209

2A collab-

210

nsion results
dvance
the
166

211

50.18
0.7571
16.63 / 10
0.08301
24.99 ± 2.65
50.1 ± 0.1
0.8066 ± 0.0597

work was

NSF grant

212
213
214
215
216

58

Figure 10: The distribution of wire tensions measurements on calibrated
wires. Two different wires and two different tension settings
Acknowledgments
were investigated. Each wire was measured 100 times. The figure (a)
is 20µm
wire with
50 gram
setting
and (b) showsin75
We diameter
would like
to thank
thetension
STAR
collaboration
µm
wire applying
120the
gram
tension setting.
USdiameter
for sharing
some of
hardware
and their strong

60

in: Proceed-

9.

asured on each
two lines indithe distribution

219
220
221
222
223
224
225
226
227
228
229

[1] The STAR Collaboration, The STAR Conceptual Design Report, June 15, 1992 LBLPUB-5347.
[2] The STAR Collaboration, STAR Project CDR Update, Jan. 1993
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[5] M. Anderson et al., Nucl. Instr. Meth. A 499 (2003) 659-678.
[6] Technical Design Report for the iTPC Upgrade, Star Note 0644,
https://drupal.star.bnl.gov/STAR/starnotes/public/sn0644
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174(1980)285.

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