Cordless Position Sensor Based on the Magnetostrictive Delay Line Principle

Abstract

In this paper a cordless position sensor based on the Magnetostrictive Delay Line
principle is presented. The working principle and the response of the sensor to coaxial and parallel
moving magnetic field are analyzed. The output is selected to be the differential voltage of pairs of
successive search coils. The measurements suggest monotonic dependence of the output of the
sensor with respect to the permanent magnet’s position.

Full text

Cordless position sensor based on the Magnetostrictive Delay Line
principle
Georgios Kokkinis
1,a
1
Laboratory of Physical Metallurgy, Department of Mining and Metallurgical Engineering, National
Technical University of Athens, Athens 15780, Greece
a
gkokkinis@metal.ntua.gr
Keywords: Cordless position sensor, magnetostrictive delay lines, amorphous wires
Abstract. In this paper a cordless position sensor based on the Magnetostrictive Delay Line
principle is presented. The working principle and the response of the sensor to coaxial and parallel
moving magnetic field are analyzed. The output is selected to be the differential voltage of pairs of
successive search coils. The measurements suggest monotonic dependence of the output of the
sensor with respect to the permanent magnet’s position.
Introduction
Magnetic materials are important in the design and development of sensors [1]. The
magnetostrictive delay line (MDL) technique has been implemented for developing various types of
mechanical sensors [2,3]. Various types of position sesnors have been proposed based on the MDL
technique, involving measurement of moving permanent magnet [4], conductor [5] and soft magnet
[6]. One of the first attempts for sensitive measurement of the displacing permanent magnet has
been described in [7], while industrial possible products may be depicted in [8,9].
Since we are interested in making sensor prototypes with high accuracy and low cost in
electronics and mechanics we used the basic MDL setup. The output of the sensor is set to be the
differential voltage of pairs of successive search coils. The simplicity of the arrangement meets
these specifications, while maintaining the advantage of cordless operation and improving the
sensitivity and uncertainty of sensors. There are several modeling techniques and reverse
engineering methods that can be used. For a survey on these techniques see [12-23].
Sensing Arrangement
The sensor is illustrated in Fig. 1. The sensing element is a magnetostrictive delay line (MDL) in the
form of ribbon or wire. A short excitation coil is set around the one end of the MDL. An array of
short, single layer coils (3), connected in series and named hereinafter “search coils”, is set around
the MDL along its length, used as the sensor output. A moving hard magnet, able to be displaced
parallel to the sensing material, is the active core of the sensor. Without any loss of the generality, in
our specific application the moving magnet was the end part of the hydraulic piston, having a
magnetic pole orientation parallel to the length of the MDL. Details of such an arrangement can be
found in [7]. The length of the sensor can vary by changing the number of search coils, thus
allowing a variable sensor length, in a relatively inexpensive production technique.
The sensor operates as follows: Pulsed current is transmitted through the excitation coil. Then,
the pulsed magnetic field along the length of the MDL generates an elastic pulse, propagating along
the MDL length. As the elastic pulse propagates along the length of the MDL, it causes changes of
magnetic flux at the intersections of MDL and search coils, thus inducing a voltage pulse train with
pulse intervals corresponding to the distance between consequent coils. In the absence of the
moving magnet and low ambient field along the array of the short search coils, these voltage pulses
are small in amplitude. In the presence of the moving magnet the ambient biasing field at the MDL
intersection changes, resulting in a corresponding change of the pulsed voltage output.
Key Engineering Materials Vol. 495 (2012) pp 220-224
Online available since 2011/Nov/15 at www.scientific.net
© (2012) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/KEM.495.220
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Fig. 1: Schematic illustration of the position sensor
Experimental setup and measurements
The length of all coils was 1mm, with 100 turns. The 0.125mm Fe77.5Si7.5B15, as cast, amorphous
wire was used as soft magnetic material. The distance between two subsequent search coils was set
to 5cm, in order to get discrete pulses in the output, their total number was four. The pulsed current
conducted through the excitation coil had the following features: I = 4A, f = 1 KHz, duty cycle =
0.3%. The permanent magnet’s field, in one case, was simulated by solenoid with the following
characteristics: 130 turns, 1cm length. Through the solenoid was conducted DC current generating a
constant magnetic field of H = 1300 A/m in the solenoid. For the rest of the measurements an Nd-
Fe-B permanent magnet was used. The magnet was placed parallel to the MDL at a distance of
10mm and was moved manually by using a linear spacer of 10nm sensitivity. Consequently, we
determined the dependence of the magnetic field component along the MDL axis upon the position
of the moving magnet. For this reason we used a two dimensional Hall magnetic field sensor with
sensitivity of 100 nT and uncertainty of 10 mT. This dependence is illustrated in Fig. 2.
As has been already stated the output of the sensing arrangement is the differential voltage of
pairs of successive search coils.
1 2 3 4 1
...
out out out out out n out nout
V V V V V V V
−
= − + − + + − . (1)
Where n the number of search coils and V
nout
the voltage output of the n
th
coil.
The response of the sensor with respect to the movement of the solenoid is exhibited in fig. 3 (the
point of origin has been set on the first search coil). Finally the response of the sensor to the moving
permanent magnet is shown in fig. 4.
Horizontally movable permanent magnet
Key Engineering Materials Vol. 495 221

0 10 20 30 40 50 60 70 80 90 100
5
10
15
20
25
30
35
Magnet position (mm)
Output of a s ingle search
coil (mV)
-50 -25 0 25 50
-1200
-1000
-800
-600
-400
-200
0
200
400
Magnet position (mm)
Field along the MDL
axis(A/m)
-60 -40 -20 0 20 40 60
0
500
1000
1500
2000
2500
3000
3500
4000
Magnet position (mm)
M a g n it u d e o f
P la n ar f ie ld ( A /m )
Fig. 2. Dependence of the magnetic field component along the MDL axis upon the position of
the moving magnet
-25 0 50 100 150 170
-15
-10
-5
0
5
10
15
20
25
30
Magnet Position (mm)
D i f fe r e n ti a l o u t p u t o f t w o
p a ir s o f s e a r c h c o i ls (m V )
Fig. 3. Sensor’s response with moving solenoid placed parallel at a distance of y=10mm:a)
pulsed voltage output of a single coil with respect to the position of the solenoid, b) differential
pulsed voltage output of four subsequent coils with respect to the position of the solenoid.
-50 -25 0 25 50
0
5
10
15
20
25
30
35
40
Magnet Position (mm)
Ne gativ e pe ak o f sin gle
se arch coi l out put ( mV)
-50 -25 0 25 50
0
2
4
6
8
10
12
14
16
18
20
Magnet Position (mm)
Positiv e peak of single
searc h coil out put (mV)
-50 -25 0 25 50
5
10
15
20
25
30
35
40
Magnet Position (mm)
O utp ut of sin gle
s ea rch co il (m V)
-40 0 50 100 150 180
-40
-30
-20
-10
0
10
20
30
40
50
Magnet Position (mm)
Diffe rential outp ut of tw o
pairs of search coils (mV )
Fig. 4. Sensor’s response with moving magnet placed parallel at a distance of y=10mm:a)
Negative peak of the output of a single search coil with respect to the position of the magnet, b)
Positive peak of the output of a single search coil, with respect to the position of the magnet, c)
pulsed voltage output of a single coil with respect to the position of the magnet, d) differential
pulsed voltage output of four subsequent coils with respect to the position of the magnet.
-50 -25 0 25 50
-4000
-3000
-2000
-1000
0
1000
2000
3000
4000
Magnet Position (mm)
Field Perpendicular
to the MDL axis (A/m)
222 Materials and Applications for Sensors and Transducers

Analysis
Fig. 3(a) demonstrates independent changes of signal’s magnitude of each search coil response
while fig. 3(b) suggests a monotonic, nearly linear, dependence between the position of the solenoid
and the differential output of the sensor. Thus the position of the solenoid can be determined
through a simple signal processing algorithm.
Fig 2(a) and 4(a) exhibit a resemblance between the curve of the perpendicular magnetic field to
the MDL axis and negative peak of the output of a single search coil with respect to the position of
the magnet. Similarly it appears to be a connection between the magnitude of the field, in the same
plane as the sensor and the positive peak of the output of a single search coil, with respect to the
position of the magnet, as seen in fig 2(c) and 4(b). Further research is on the way to prove and
model the previous statements.
Fig 4(d) suggests a partially monotonic dependence between the position of the permanent
magnet and the differential output of the sensor. A monotonic and linear response can be obtained
with further modeling of the sensor’s parameters, mainly the permanent magnet’s field magnitude
and distance from the MDL.
The response of the sensor, according to fig. 3(c) and 4(d), is more linear at the presence of a
weaker field, as in the case of the moving solenoid.
Problems. Non-uniformity. Non-uniformity is defined as the fluctuation of the uniformity
function Vout(x). Non-uniformity has been analytically discussed in the past, concluding that the
basic tailoring process is stress–current annealing and a normalization process [10], which
eliminates the local stresses induced during production of the material, as well as the effect of
misaligned domains.
Bias field effect. This is defined as the dependence of the MDL voltage output on the dc field
component along its axis. The presence of unexpected ambient fields considerably modifies the
MDL response. There are only two ways to avoid such effect: one is to magnetically shield the
transducer and the other is to measure and take into account the stray field presence.
Reflections and after-effects. Elimination of reflections and after-effects improves the sensitivity
and resolution of MDLs. A reduction of reflections can be obtained by high compliance termination
of the magnetostrictive elements as well as by proper geometrical design of the MDL set-up.
Conclusion
The physics of the sensing arrangement has been specified and the measurements of the proposed
differential output have been presented. The measurement results provide strong evidence for the
feasibility of a position sensor, based on the suggested MDL arrangement. In order to use such
sensors at industrial scale, some problems should be solved to optimize the sensor characteristics,
namely the linearity and repeatability of the sensor response. Regarding the problem of the sensor
linearity, one has to tailor the λ(H) function [11] to obtain a linear response of the voltage output
with respect to the applied field.
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224 Materials and Applications for Sensors and Transducers

Materials and Applications for Sensors and Transducers
10.4028/www.scientific.net/KEM.495
Cordless Position Sensor Based on the Magnetostrictive Delay Line Principle
10.4028/www.scientific.net/KEM.495.220
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