Development of a portable and fast wire tension measurement system for MWPC construction

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Submitted to ‘Chinese Physics C’

arXiv:1603.02986v2 [physics.ins-det] 25 Apr 2016

Development of a portable and fast wire tension measurement system
for MWPC’s construction *
Jing-Hui Pan1,2,3 Chang-Li Ma2,3;1) Xue-Yu Gong1;2) Zhi-Jia Sun2,3,4;3)
Yan-Feng Wang2,3 Chen-Yan Yin1 Lei Gong5
1 University of South ChinaHengyang 421001, China
2 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
3 Dongguan Neutron Science Center, Dongguan 523803, China
4 State Key Laboratory of Particle Detection and Electronics, Beijing 100049, China
5 Fujian Fuqing Nuclear Power Co., Ltd, China National Nuclear Corporation, Fuqing 350318, China

Abstract: In a multi-wire proportional chamber detector(MWPC), the anode and signal wires must maintain
suitable tensions, which is very important for the detector’s stable and perfect performance. As a result, wire tension
control and measurement is essential in MWPC’s construction. The thermal neutron detector of multi-functional
reflectometer at China Spallation Neutron Source is designed using a high pressure 3 He MWPC detector, and in the
construction of the detector, we developed a wire tension measurement system. This system is accurate, portable
and time-saving. With it, the wires’ tension on a anode wire plane has been tested, the measurement results show
that the wire tension control techniques used in detector manufacture is reliable.
Key words: MWPC, wire tension, tension measurement, CSNS
PACS: 29.30.Aj, 29.40.Cs, 29.85.Ca

1

Introduction

shown as formula (1):

China Spallation Neutron Source(CSNS) is under
construction, and three neutron scattering instruments
are built at the same time, belong which, the multifunctional reflectometer employs a high pressure 3 He
gas multi-wire proportional chamber(MWPC) as its neutron detector[1]. This detector is designed using an 8
atm 3 He/C3 H8 (80/20) mixture as working gas and goldplated tungsten as anode and signal wires. The sensitive
area of the detector is designed to be 200mm×200mm,
and the neutron space resolution is expected less than
2mm, besides the counting rate can reach to 107 /cm2 s[2].
As the influence of electrostatic force and gravity, the anode and signal wires will have position offset, as a result
the performance of the detector, including magnification,
position resolution, etc, will be affected[3]. To ensure the
wires’ position deviation as small as possibleit is very
important to keep the wires with proper tensions, so we
have to measure and control the wire tension in the detector construction. The principle of the wire tension
measurement is based on the relationship between the
tension and its vibration inherent frequency, which is

T = 1 × 10−6 · ρ(2Lf0 )2 .

(1)

Where T (N) is the wire tension, f0 (Hz) is the wire
vibration fundamental frequency, ρ(mg/m) is the linear
mass density, and L(m)is the wire length.
Based on the principle, a lot of equipment was designed, and in general they can be sorted into two classes.
In the first class, it inputs different periodic driving force
to a wire, and make the wire resonance, and then by determining the resonance to measure the wire inherent
frequency. Because of driving force generator and resonance determination module, these equipment is complicated and the measurement is time consuming[4–10].
In the other class, a short-time driving force was used to
make the wire vibrate, and then measuring the signal of
the vibration to analysis the wire inherent frequency[11–
15]. In these equipment, the wire inherent frequency is
measured directly, so the measurement is quicker and
more accurate, but they need a powerful signal processing system.
In this paper, we present a simple, quick and accurate equipment by adopting the wires’ vibration method

∗ Supported by National Natural Science Foundation of China (A050506), State Key Laboratory of Particle Detection and Electronics
and Key Laboratory of China Academy of Engineering Physics (Y490KF40HD)
1) E-mail: machangli@ihep.ac.cn
2) E-mail: gongxueyu@126.com
3) E-mail: sunzj@ihep.ac.cn
c 2013 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of
Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd

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2

Description of tension measurement
system

According to Faraday’s lawwhen a wire with current
(I) is placed in a magnetic field (B), it will experience a
force (F), and the force is equal to I·L×B. Meanwhile,
when a wire is vibrating in a magnetic field with velocity ν, electric potential is induced along the wire. The
potential is equal to L · ν×B. Based on this principle,
we designed a simply equipped wire tension measurement system: two electromagnets are used to generate
an adjustable magnetic field, and the wire which will be
measured is placed between them. A mono-stable trigger
mode is used to produce narrow pulses, then the pulses
are amplified by a power amplifier and used to stimulate
the wire to vibrate in the magnetic field. To make sure
the wire generates largest vibration, the pules’ widths are
controlled to about one fourth of the expected wire inherent vibration period. Then the vibration signal, which
is an attenuate sine wave in theory, is amplified by a operational amplifier module. At last, the vibration signal
is recorded and analyzed by a digital oscilloscope or a
computer controlled data acquisition board, and the vibration frequency is measured. A schematic drawing of
the system is shown in Fig.1.

It makes the system adapt to measure wires with inherent vibration frequency no less than 62.5Hz. The power
amplifier in the system has a 3A rated current and can
amplify the a TTL pulse to 15V level. In the operational
amplifier module, a 500Ω resistor is connected at input
terminal and a 2MΩ potentiometer is used as feedback
resistor, so the voltage amplification factor can get up
to 4000. For flexibility, we design two options to analyze vibration signals. One method is using a digital
oscilloscope to record the signal and do Fourier transform, and the other alternative is using a 12 bits ADC
data acquisition board to converts the analog signals to
digital signals and transmits them to computers, then a
LabVIEW program is used to display and analyze the
signals. To suppress direct current interference, 50µF
capacitors are used among different function modules.
1.2

magnetic field (×103G)

for measuring the wires’ tension in MWPC construction.

1
0.8
0.6
0.4
0.2
0

-40

-20

0

20

40

distance from the center magnetic field (mm)

Fig. 2. The magnetic field distribution between
two magnets along the measured wire.

Fig. 1. Schematic block diagram of the wire tension measurement system.

As shown in Fig.1, two solenoidal electromagnets
with 22mm diameter magnetic poles, which are operated
with a 30V DC power supply, are placed in the same direction with about 10mm distance. When the power supply’s voltage is set to 30V, the magnetic field between the
magnets along the wire’s direction is shown in Fig. 2. If
the current in a wire is 1A, the force applied to the wire
is about 3.24 × 10−3 N. The mono-stable trigger, which
is connected with a 1µF capacitor and a 10KΩ variable
resistor and operated with a 5V DC power, can generate TTL pulse with a maximum width of 4ms, which
is calculated by the formula, τ = K · R · C, (K = 0.4,
which is from mono-stable trigger chip product manual).

Fig. 3. The upper waveform is a wire’s vibration
signal and the lower one is the corresponding spectrum of the Fourier transform which is analyzed
by a digital oscilloscope.

A vibration signal is captured by oscilloscope and
its frequency spectrum are shown in Fig.3. In this picture, the wire’s diameter, length and applied tension are
25µm, 240mm and 40g respectively, and the operation
voltage of the electromagnets is set at 30V. The highest
peak at about 420Hz on the frequency spectrum is the

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fundamental harmonic, while the peaks at about 1260Hz
is the third harmonic of the wire. As this, measuring a
wire’s frequency with the system takes only a few seconds. At the same time, this system records a half first
harmonic signal[10].
For flexibility, an ADC data acquisition board and
a LabVIEW computer program are designed as an alternative to a digital oscilloscope. The data acquisition
board has a sample rate of 80KHz and can convert ±8V
analog signals into digital ones. With a USB2.0 con-

nector digital signals in the board are transmitted to a
personal computer. Using a LabVIEW signal analysis
program, the measurement digital accuracy can reach
to 1Hz, which is on the same order of the measurement
error. Only taking a low-voltage power supply and a
personal computer, the system can be used at any places
we need. Fig.4 are the front panel and the flow diagram
of its corresponding graphical program in the LabVIEW
platform respectively.

Start

Input trigger
pulse signal

Signal
acquisition

Fourier
transform

Vibration signal

Frequencydomain signal

End

Fig. 4. The LabVIEW program of the wire tension measurement system (left: front panel, right: flow diagram of
the graphical program).

800

measured frequency (Hz)

counts/(1.5Hz)

30
25
20
15
10
5
0

700
600
500
400
300
200
100

280

290

300

310

320

0
0

330

2

4

6

8

10

12

1/2

measured fequency (Hz)

sqrt(tension) (g )

Fig. 5. The distribution of repeated measurements
of a single wire with 20g tension.

Fig. 6. The measured frequencies v.s. the wire tensions. X axis is the wire tension’s square root and
Y axis is the measured wire vibration frequencies.

Table 1.

The calculated and measured frequencies and their errors of some wires.

Tension(g)
calculated frequency(Hz)
measured frequency(Hz)
Tension(g)
calculated frequency(Hz)
measured frequency(Hz)

20
302±14
301.5±1.7
80
605±28
601.8±1.3

30
370±17
371.4±1.3
90
642±30
645.9±1.2

40
428±20
426.3±1.1
100
676±31
670.5±1.7

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50
478±22
480.5±0.6
110
709±33
700.8±0.8

60
524±24
522.4±2.5
120
741±34
735.6±1.2

70
566±26
561.5±1.2
130
771±36
764.9±1.8

Submitted to ‘Chinese Physics C’

(a)

(b)

(c)

(d)

Fig. 7. (a),(b),(c),(d) are vibration signals and corresponding frequency spectra when the electromagnets’ voltage
is set at 10, 15, 20, and 30V respectively.

3
3.1

Performance of the measurement system

can distribute different wire tensions perfectly.

Accuracy of the system

As shown in Ref[11], by enhancing magnetic field intensity, we can increase the amplitudes of wires’ vibration signals and signal to noise ratio. So we set the electromagnets’ voltage at different values for checking the
system performance. Fig.7 (a)-(d) are the vibration signals and their frequency waveforms of wire with 25um
diameter, 240mm long and 80g tension when the electromagnet’s voltage is set at 10, 15, 20, 30V respectively.
As expected, the signal amplitude and the signal to noise
ratio increased obviously with the magnetic field’s enhancement. Fig.8 is shown the signal amplitudes and
measured frequencies with different magnetic intensities.
Fig.9 shows that the measurement errors. Obviously, for
getting better results, the system should set the magnetic
field as strong as possible.
The effect of trigger pulse width on the wire vibration
signal is tested too, as expected, it is similar to that of
magnetic intensity. The dependence of the vibration signal amplitude on the trigger pulse amplitude are shown
in Fig.10.

For testing the accuracy of the wire tension measurement system, we measured 240mm long and 25µm diameter wires with 20, 30, 40. . . 130g tension 40 times respectively. The linear mass density is ρ=9.3±0.8mg/m,
which is provided by the wire’s supplier. Fig.5 is the
measured frequency results of a 20g tension wire, and
the measured frequencies of wires with other tension values have similar distribution. The measured average
frequencies and their standard deviations are listed in
Tab.1, which are marked as “measured frequency”. In
the measurement, the electromagnet voltage was set to
25V. Based on formula (1), we calculated the inherent
frequency of every wire, and the results are shown in
Tab.1 too. In Tab.1, the calculation error is from linear
density uncertainty of the wires. As shown in Tab.1, the
measured results are very close to the calculated values.
We draw the results in Fig.6 and fit the results with a
line. As shown in the picture, the measurement system

3.2

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Signal-to-noise ratio V.S. magnetic intensity

Submitted to ‘Chinese Physics C’

570

measured frequency (Hz)

signal amplitude (V)

1.6
1.4
1.2
1
0.8
0.6

568
566
564
562
560
558
556
554
552

0.4

14

16

18

20

22

24

26

28

550

30

14

16

electromagnet voltages (V)

20

22

24

26

28

30

electromagnet voltages (V)

The signal amplitude and the average measured frequency v.s. voltages of the electromagnet.

is from 250mm to 450mm. In these measurements, the
electromagnets’ voltage is 30V. Fig.11 is the results, as
displayed, the system can get correct frequencies for different length wires. As the results shown, wire’s tension of MWPC with sensitive area from 200mm×200mm
to 450mm×450mm can be measured by the system correctly.

0.6
0.55
0.5
0.45
0.4
0.35
0.3
0.25

500
0.2

measured frequency (Hz)

measured frequency error (%)

Fig. 8.

18

0.15
14

16

18

20

22

24

26

28

30

electromagnet voltages (V)

Fig. 9. The frequency error at different voltages of
the electromagnet.

400

300

200

100

signal amplitude (V)

1.7
0
0

1.6

1

1.5

2

2.5

3

3.5

4

-1

1/L (m )
1.5

Fig. 11. The average measured frequency v.s. the
wire length’s reciprocal.

1.4
1.3
1.2

3.4

1.1

In the MWPC neutron detector of MR in CSNS,
many 25µm, 206mm long wires with 50g tension are
welded on a PCB board parallel with 2mm intervals to
make an anode wire plane. To make sure the detector
have steady and uniform amplification factor, the tension’s deviation is controlled to no larger than 10%. Using the system, we measured the wires’ tension. The
results are shown in Fig.12 and the standard deviation
is about 3.5%. This mean the wire tension control techniques in the wire plane manufacture is reliable and accurate.

9

10

11

12

13

14

15

trigger pluse amplitude (V)

Fig. 10. The trigger pulse amplitude v.s. the amplitude of the wire signal.

3.3

0.5

Wire length effect

For testing the performance the system in measuring
different size MWPC, we measured some 25µm diameter wires with 50g tension, and the length of the wires

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The wires’ tension of an anode wire plane

Submitted to ‘Chinese Physics C’

80

(a)

35

number of wires/(0.4g)

tension (g)

70
60
50
40
30
20
0

(b)

30
25
20
15
10
5

20

40

60

80

100

120

140

160

180

200

0

220

wire position (mm)

42

44

46

48

50

52

54

56

58

60

tension (g)

Fig. 12. The measured wire tensions of an anode wire plane. (a) X axis is the wires’ position in millimeter; (b)
measured results distribution.

4

Discussion

For the construction of MWPC neutron detectors
in CSNS, we developed a portable, accurate and timesaving wire tension measurement system. With a lowvoltage power supply and a digital oscilloscope or a personal computer, the system can measure a wire’s tension
in a few seconds and the error is about 3Hz. Compared
with the wire’s diameter and tension uncertainties, the
measurement error is very small and can be neglected.
We also use the system testing all wires’ tension in an
anode wire pane of a MWPC, the results shown the wire

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Science Foundation of China, State Key Laboratory of
Particle Detection and Electronics and Key Laboratory
of China Academy of Engineering Physics. The design of
the system consults the paper A Device for Quick and Reliable Measurement of Wire Tension in Princeton/BaBar
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