The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

STRETCHED WIRE METHOD FOR MAGNET ALIGNMENTWITH MEDIUM FREQUENCY CURRENT
Ryuhei Sugahara, Yasunobu Ohsawa, Norihito Ohuchi and Masato Yoshida
Accelerator Laboratory, High Energy Accelerator Organization
Tsukuba, Ibaraki, Japan

Abstract
The stretched wire method is one of magnet alignmenttechniques. In alignment of the straight section, thisstretched wire method is simple and even better inprecision than modern instruments such as a laser
tracker[1]. There are two ways to detect wire position. One
is capacitance measuring method using semi-conductive
wires, and the other is to pickup electric signals using
metallic wires.
We are investigating simple and cheap hand-madestretched wire technique, in which we just stretch a thinmetallic wire and feed alternating current of medium
frequency. Signals are picked up by handmade pickup
coils. Our aiming alignment precision is 0.1 mm.
Frequency dependence of resolution and precision isreported as well as effect of an ion plate placed nearby.

In the following, frequency dependence of resolution
and precision, and effect of an ion plate placed nearby arereported.

EXPERIMENTAL SETUP
Fig.1 shows the setup of the experiment.

INTRODUCTION
The stretched wire method is one of magnet alignment
techniques. There are two ways to detect wire position.
One is to stretch a semi-conductive wire, and detectcapacitance between two electrodes, between which thewire passes through. Change of the capacitance isconverted to wire position. This kind of instrument has
good precision of several tens of micro-meters, and canbe purchased commercially. But the price is comparatively expensive.
The other way is to stretch a thin metallic wire and feed
alternating current of some frequency to the wire. Signal
is picked up by electrodes between which the wire passes
through. Higher frequency gives us better resolution andbetter precision. But high frequency current requires coaxial structure in the stretched wire. By this reason, thistechnique is hard to be used in magnet alignment, but
used only in monitoring the change of position.
We are investigating a magnet alignment method, inwhich we stretched a thin metallic wire without coaxialstructure, and feed alternating current of some mediumfrequency. With higher frequency, better resolution andbetter precision can be obtained, but it becomes harder fora long wire to propagate current. With lower frequency,
situation is vice versa. Signals are picked up by handmade
pickup coils. This system can be made cheaply. Ouraiming precision is 0.1 mm, which is enough for usual
magnet alignment in the accelerator.

Fig.1: Schematic drawing for the experimental setup
A brass wire, which had diameter of 0.3 mm and lengthof 100 m, was stretched. A brass wire was selected due toits low volume resistivity of 7.0×10-8 Ωm and large
tensile strength of 52-77 kg/mm2. A 0.8 m long part of thewire was stretched strongly by loading weight of 2 kg.
One end of this part is fixed to the base of the wire
support rod and the other end is connected to the weight
through pulley. The fixed end is connected to the power
supply through a shunt resistor. The rest part of the wire
was stretched rather loosely between two plastics fences
standing 10 m apart each other. One end is connected tothe power supply, and the other end to one end of the 0.8m wire part. Total length of the wire is about 100 m.
In pickup coils, copper wire of 0.4 mm diameter was
wound 300 turns around bobbins of 82.83 mm circumference. Two pickup coils are fixed 40 mm apart eachother on a movable support. Position of the support in thedirection perpendicular to the wire is measured by amicro-meter.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

A wave former, AFG 310 of Sony-Tektronix, producesalternating current with arbitrary frequency, which is fed
to a power amplifier, BSW 60-5 of Takasago Ltd. Bothends of the wire are connected to the output terminals of
the power amplifier. The current in the wire is observedby measuring voltage, V1, between both ends of a shunt
resistor of 0.11 Ω. Output from pickup coils is observedby measuring voltage, V2 and V3, between both ends ofresistors of 10 Ω, which terminate pickup coils. Voltage ismeasured by a digital multimeter 2000 of Keithley, andsometimes observed by a digital oscilloscope, TDS 380Pof Sony-Tektronix.

EXPERIMENTAL RESULTS
Resolution
Observing the multimeter display, we found thestability of output is ±1 µV (the least digit). This value
can be converted to corresponding space resolution using
a calibration constant described in the following.
Obtained space resolution is shown in Fig. 2 as a functionof frequency of alternating current fed to the wire.

Fig.3: Difference between output from two pickup coils,
V2-V3, (blue triangles), and deviations of each data pointfrom linear fitting (red circles); (a) for the frequency
100Hz and (b) for 1kHz.
ion plate placed 50 mm above. Higher frequency gives
better standard deviation. The standard deviation is better
than 5 µm for the frequency higher than 500Hz. But
improvement with frequency higher than 500Hz is small.
Fig.2: Resolution for L=100mPrecision
Fig. 3 shows some example of analysis results of data.
Difference between output from two pickup coils (V2V3), and deviations of each data points from a fitted line
are shown as a function of the position (X) of the coilsupport. Fig. 3(a) is for the frequency 100Hz ofalternative current fed to the wire, and (b) for 1kHz.
Measurement was executed for the frequency, 100Hz,200Hz, 500Hz and 1kHz. Each plot was fitted to thelinear form Y = M0 + M1⋅X. The same measurement wasexecuted with an ion plate, 4 mm thick and 910×1090
mm2 wide, placed above the pickup coils at various height.
Standard deviation in linear fitting is shown in Fig.4 asa function of frequency, where the blue solid triangles
show results with the ion plate placed as far as 700 mm
above the wire, and solid red circles show results with theFig.4: Standard deviations in linear fitting. Blue triangles
show those without an ion plate, and read circles with anion plate placed 50 mm above the wire.
An ion plate placed above the pickup coils does notdegrade the standard deviation much.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Fig.5 shows the variation of zero-cross point as afunction of wire current frequency. The zero-cross point,
X0, is defined as X-position where the output from two
pickup coils are balanced.
X0 = -M0/M1
This variation might be caused by ion material used in thebase part of the coil support.

Fig.5: Variation of zero cross point as a function of wire
current frequency. Definition of markers is the same tothat in Fig.4.

Effect of ion plate
Change of coil output was measured with various heightof an ion plate. Fig. 6 shows the change of (V2-V3),
which is converted to the spatial unit by the formula
X = (Y – M0)/M1,
where Y=V2-V3, and M0 and M1 are coefficients inlinear fitting. Wire current frequency was changed from100Hz to 1kHz. X-position of the coil support was fixed.
If the height of an ion plate is larger than 200 mm, change
is less than 0.07 mm.

Fig.6: Change of X-position calculated from (V2-V3)
with variation of height of an ion plate. Four curves show
those for wire current frequency of 100Hz (circles),
200Hz (squares), 500Hz (triangles) and 1kHz (reversedtriangles), respectively.

SUMMARY
Cheap stretched wire technique without coaxial
structure was studied as a magnet alignment method with
precision of 0.1 mm. A thin brass wire of 0.3 mm
diameter was stretched over distance of 100 m.
Alternating current of various frequency from 100Hz to
1kHz was fed to the wire, and the frequency dependenceof the pickup signal was measured, because it isconceivable that high frequency alternating current might
be hard to propagate long non-coaxial cable. And it is also
conceivable that the system night be affected by ion
material, such as magnets, placed nearby. So the effect ofan ion plate was also studied.
Attenuation of alternating current due to high frequency
is only about 10% in case of 1kHz for the wire length of100m. The resolution is about 24, 12 and 6 µm for 100Hz,200Hz and 500Hz, respectively. Frequency higher than
500Hz does not improve resolution much. Change ofzero-cross point of about 0.2-0.3 mm is observed when
the frequency is changed from 100Hz to 1kHz. Although
this change might be caused by ion material in the pickup
coil support, further investigation is required. Dependenceof X-position, calculated from (V2-V3), to height of anion plate is less than 60 µm for the height larger than 200mm.
It is found from this experiment that our aiming
precision 0.1 mm is achievable with a stretched wiretechnique of non-coaxial structure for the length of 100 m

ACKNOWLEDGEMENT
We thank to Professor Kamiya, the director general ofAccelerator Laboratory, for his budgetary support andkind encouragement to this experiment.

REFERENCES[1] K. Egawa et. al., “Magnet system for the KEKB main ring”,
Nucl. Instrum. & Methods in Phys. Research A 499 (2003)
P24-44.