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Tensile stress distribution sensors based on amorphous alloys
Journal of Magnetism and Magnetic Materials 119 (1993) 247-253 North-Holland
Tensile stress distribution sensors based on amorphous alloys E. H r i s t o f o r o u Institute of Materials Science, NCSR "Demokritos", Aghia Paraskevi, Athens 153 10, Greece
and R . E . Reilly Electronic and Electrical Engineering Departement, King's College London, Strand WC2R 2LS, London, UK Received 30 May 1991; in revised form 27 March 1992
In this paper, we report experimental results on the response of tensile stress sensors based on the magnetostrictive delay line technique, operating u n d e r pulsed field excitation. Their operation is based on the change of the magnetic circuit due to the change of the relative permeability of an a m o r p h o u s ribbon when tensile stress is applied on it. They are low compliance sensors and can be used in cases where large displacement of the active core is not desirable.
I. Introduction
We have been interested in developing and using low compliance force sensors, especially for use in arrays, since they have potential uses in a number of applications for measuring stress, force or pressure [1]. In the past, while a number of this kind of sensing devices have been proposed, the oldest example, controlling a large part of the force sensor market, is the strain gauge, in either the form of a single gauge element or of a Wheatstone bridge structure [2]. These sensors offer a fairly linear response and a very low compliance, although their sensitivity is not satisfactory, about 0.1-4%. An evolution of this type of sensors is the silicon diaphragm [3,4]: They are based on the principle that the resistance of a conductor changes when tensile stress is applied on it. Correspondence to: Dr. E. Hristoforou, Institute of Materials Science, N C S R "Demokritos", Aghia Paraskevi, A t h e n s 153 10, Greece. Tel.: +30-1-6533706; telefax: +30-1-6519430.
Low compliance magnetic force and torque sensors have been presented in the past [5-7], based on the change of the relative permeability of an active core, when tensile stress is applied on it. Cost of manufacturing of these magnetic sensors can be kept low, although their nonlinear response, presence of hysteresis in ac applications [8] and change of their response due to ambient magnetic field make them less attractive than strain gauges in terms of performance. Additionally, the existing low compliance force sensors cannot easily be formed into sensing arrays. In this large, although specific, area of applications, the final cost of an array sensor depends linearly on the number of required sensing points. Such a high cost of an array sensor leads us to conceive and develop a new family of low compliance force sensors, which can easily be formed in arrays. For this purpose, we used the magnetostrictive delay line (MDL) technique under pulsed magnetic field operation as it is shown in fig. 1. A pulsed magnetic field under appropriate magnetic bias conditions applied along the length of the MDL,
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
248
E. Hristoforou, R.E. ReiUy / Tensile stress distribution sensors
--~le~ ?
~
(a) Vo(t)
3L., e 3L.,
let
time
netic core. Such a change can be caused either by displacing a magnetically soft active core, or by changing its permeability due to an applied tensile stress on it. It is mentioned that permeability of a soft magnetic ribbon changes with the applied tensile stress along its length [5]. This way, the delay lines are used for sensing the change of the magnetic circuit as well as for serialising information. A detailed description of the sensors is given in the next section. In all these devices, the applied pulsed and bias magnetic fields are kept fixed, obtaining thus changes in the induced pulsed voltage output at the receiving coil, due only to changes in the magnetic permeability of the active cores. The delay line always remains free of stresses. In this paper, we also present a two dimensional force sensor and the main guidelines for manufacturing and calibrating a discrete or an array sensing device.
(c)
2. Description of sensors
Fig. 1. Description of delay line arrangement and operation. (a) Delay line arrangement; (b) serialisation of acoustic pulses and (c) matrix of conductor-delay lines intersection. 1. Pulsed current conductor. 2. Magnetostrictive amorphous wire delay line. 3. Receiving coil.
results in an acoustic pulse propagating along the M D L [9-11], which may be detected by a receiving coil, as shown in fig. l(a). A n u m b e r of discrete acoustic pulses can be serialized in one delay line, which are the pulsed current conductor-MLD intersections, if the acoustic stress points of origin (PO) are well apart to avoid mutual interference (fig. l(b)). Using an array of m M D L s orthogonal to an array of n straight and parallel pulsed current conductors, one can create rn x n acoustic stress points of origin, as shown in fig. l(c). These acoustic pulses mainly depend on the magnetic circuit at the PO and the receiving coil. The magnetic circuit at the PO can change by changing the magnetic coupling between the M D L and a neighbouring soft mag-
This group of sensors is based on the elastomagnetic properties of the ironrich Metglas alloys. A m o r p h o u s alloys having a chemical composition similar to Metglas 2605SC and 2605S2, m a d e by a rapid solidification process, are magnetostrictive so that they can be used as M D L s and have a relative permeability dependent on the tensile stress, applied on their surface. Such tensile stress alters the magnetic characteristics such as the magnetic permeability. Both these properties of the Metglas 2605SC alloy are used as the fundamental idea behind all the proposed sensors of this group, since its magnetoelastic characteristics are improved compared with others. The principle of operation of this family of sensors is illustrated in fig. 2(a). Metglas 2605SC is used as an M D L and active core. Pulsed current is transmitted through the conductor, orthogonal to the MDL. If the active core is unstressed, then it operates as a magnetic screen between conductor and MDL. Only a fraction of the applied pulsed magnetic field penetrates to the MDL, so that the resultant acoustic signal
249
E. Hristoforou, R.E. Reilly / Tensile stress distribution sensors
% 2
a
i
a
t
~
j
Vo m
g
1=2.27 A
i
a
1=4.52 A l=6.18 A
--a
------o----• ----o--
1=9d)9 A 1=11.35 A 1=1362A
(,()
~'ll
20
~
IO
{) .
IIX)
,.......~-''''~ ,.,...,.a __.-.-a - ' ' ' m
~11J
0 b
-----o-31)
211
40
""~-* I ~ • -----a-x
le=l.3 A 1e=2.2 A Ie--4.5 A 1e=6.8 A 1e=9.1 A 1e=13.6 A 1e=20.5 A
b
o
>
~.
NI/mn~ )
Fig. 3. Balance structure sensor. (a) Arrangement of the sensor. (1) Pulsed current conductor, (2) delay line, (3) receiving coil and (4) active core. (b) Response of the sensor of fig. 6, under various values of pulsed current.
60
AplJlicd SII'C~,S(Nl/Illlll21
Fig. 2. Single force element, (a) Layout of the sensor. 1. Pulsed current conductor. 2. Delay line. 3. Receiving coil. 4. Active core. (b) Response of the sensor of (a), under various values of applied pulsed current Ie.
and consequently the detected output is a minimum. If tensile stress is applied, as described previously, the permeability of the active core decreases, the magnetic coupling between the two soft magnetic materials alters and hence more magnetic field penetrates to the MDL, resulting thus in an increase of the amplitude of the detected voltage output, V0. The dependence of V0 on the applied tensile stress is illustrated in fig. 2(b). Any changes of the response of the sensor is due to applied stress and not due to any arbitrary movement. The Metglas delay line is unstressed and fixed for all measurements. In all the sensors of this group, the amplitude of the detected pulsed voltage V0 is the output, the applied tensile stress being the input. A n improved version of the principal idea of fig. 2(a) is given in fig. 3(a). According to this arrangement, Metglas ribbons and pulsed current conductors are above and below the MDL. Pulsed current is transmitted in the same direction in both conductors. One of the cores remains free of
stresses, the other being the active one. Applying no stress on the active core, the symmetry of the magnetic circuit results in no acoustic stress at the PO and consequently, zero voltage output. Tensile stress along the active core results in the creation of a propagating acoustic pulse. The characteristic curves of the sensor of fig. 3(a) is given in fig. 3(b) [9]. Fig. 4 shows a new arrangement of this family of sensors. The balanced structure used in fig. 3(a) is also used here. If a pulsed current I e is transmitted in the same direction in the two conductors, then, in the absence of the core S, there is zero magnetic flux in the delay line and consequently zero voltage output is detected. When core S is positioned at a fixed distance close to the delay line, then the magnetic circuit changes and an acoustic pulse results. Observing the magnetic circuit of this arrangement at the PO, it can be seen that applying tensile stress on the core S results in a decrease of the coupling
Fig. 4. A new balance structure sensor. (1) Pulsed current conductor, (2) delay line, (3) receiving coil and (4) active core.
250
E. Hristoforou, R.E. Reilly / Tensile stress distribution sensors
between core and line, so that the magnetic flux in the line decreases due to the balance structure. So, increasing the stress on the core results in a decrease in voltage output at the receiving coil. One would expect to measure voltage output from a maximum value down to zero corresponding to maximum tensile stress on the active core, which means that its relative permeability effectively becomes equal to one.
30"
g g
• []
20"
•
Ie=13.63A Ie=ll.3A Ie=9.1A Ie=6.1A Ie=4.5A Ie=2.2A
10"
0 10
20
30
40
3. Response of the sensor
3.1. Experimental set-up Fig. 5 shows the basic experimental set-up for getting the response of the sensor of fig. 4. An optic bench was used as basis for this experiment. The delay line was positioned on a 2 m m thick, 2 cm wide and 30 cm long support made of Perspex. The 800 turn, 5 m m long detecting coil made from 0.03 m m enamelled wire was at one end of this support (not shown in fig. 5). This support was well fixed on the optic bench. A 50 cm long and 5 m m wide Metglas 2605SC ribbon, was used as magnetic core S. We applied tensile stress by using a torque motor, and we determined its magnitude by using a load cell, as given in ref. [11].
3.2. Experimental results
60
Fig. 6. R e s p o n s e of the sensor of fig. 4, u n d e r various values of applied current E e.
values of exciting pulsed current I e was measured and fig. 6 shows the response of the sensor of fig. 4. All these results were taken by increasing and decreasing the applied tensile stress on the Metglas ribbon and the reported output was the same for both cases, indicating the absence of hysteresis. It was observed that fig. 6 fits an exponential curve, so that Vo(o-) can be given by: V0(o" )
=
(1)
V0max e - c ~ ,
where V0max is the maximum value of V0 and c---3.5 m m Z / N . In the same way as before, the normalized function V0o(tr), with respect to V0ma~, is given by: V0n(O" ) = V 0 ( o - ) / / V 0 m a x =
According to the experimental set-up described previously, the dependence of V0 on the applied tensile stress on the ribbon under various
50
Tensilestress(N/ram2)
e -¢~.
(2)
From the results of fig. 3(b), it is observed that the response of the sensor of fig. 3(a) fits an exponential equation of the form: V0(o" ) = V0max(1 -
e-C'~),
(3)
where V0maxis the maximum value of V0. Coefficient c is as given in eqs. (1) and (2), since the response of the sensor of fig. 3(a) is exactly the opposite as for the sensor of fig. 4. So, its normalized function with V0max is given as follows: Von(Or ) : Fig. 5. Experimental set-up to get the response of the sensor of fig. 4. 1. T o r q u e motor. 2. Load cell. 3. Active core. 4. Optic bench. 5. Pulsed current contuctors. 6. Delay line.
V0(o-)//Vomax =
1 - e -c'~.
(4)
The significance of the normalised function will be illustrated in the discussion, since it will prove helpful for sensor calibration.
251
E. Hristoforou, R.E. Reilly / Tensile stress distribution sensors
4. A two-dimensional sensing element
So, the tensile stresses applied on the two cores at the two sides, o-~ and o-b are given by:
A n idea to the construction of a two or three dimensional force sensor is p r o p o s e d here. T h e basic d i a g r a m of a two dimensional force sensor based on the sensor o f fig. 4 is p r e s e n t e d in fig. 7. T h e two active cores are orthogonal to each other. A two-dimensional force F applied on it can be analysed in two forces F x and Fy, their direction being as illustrated in fig. 7. Force F x causes a force F 1 and consequently a tensile stress s on the core. F 1 is given by: F 1 = Fx sin a.
(5)
Force Fy is adding and subtracting a force F 2 at the two parts of the core. W i t h o u t any loss of the generality, it is assumed that for the case of fig. 7, F 2 is a d d e d and subtracted to the F 1, at the right and left active cores, respectively. So, F z is given by: F 2 = Fy cos a.
(in N / m m 2) (10)
for the case o f a 5 m m wide and 25 ~ m thick active core. T h e s e two stresses cause two different outputs in the two c o r r e s p o n d i n g delay lines. By using the principle o f the structure of fig. 4, eq. (7) gives V0~,(o-) and V0bn((r) for the left and right part o f the core, respectively, as follows: Voan
=
exp(--CO'a) and Vob. = exp(--c~rb).
F a = F 1 + F 2 -- F x sin a + Fy cos a
(7)
and cos a.
(8)
(11)
So, F a and F b can be calculated by knowing the normalized values Voa, and Vobn. F a and F b are given by: F a = - l n ( V o a n ) / 4 c = - In( Voa/Voamax)/4c, Fb = -- In( V0b. ) / 4 C = -- In( Vob/VObma x ) / 4 C .
(6)
So, the left and right parts o f the core withstand two forces F a and F b, respectively, which are given by:
F b = F 1 - F 2 - - F ~ sin a - F y
o-a = F a / 0 . 1 2 5 and ~rb = F J 0 . 1 2 5
(12) So, F x and Fy can be calculated:
Fx = (Fa +
Fb)/2
= --ln(VoaVob/Vo~maxVobma,,)/8C, Fy= ( F a - F b ) / 2 =
(13)
--ln(VoaVO~mJVo~Vo.max)/SC.
But, a = 45 °, t h e n eqs. (10) and (11) b e c o m e : F, = 0 . 7 0 7 ( F x + Fy) and F b = 0 . 7 0 7 ( F x - F y ) . (9)
Fig. 7. Basic diagram of a two-dimensional discrete force sensor. 1. Delay line. 2. Pulsed current conductor. 3. Point of applied force F. 4. Active core.
5. Discussion T h e reason that we p r e f e r e d to use unannealed ribbons for both delay lines and active cores instead o f a n n e a l e d ones runs as follows: A l t h o u g h the m a g n e t o m e c h a n i c a l coupling factor of the as-cast used ribbons b e c o m e s about 0.75 after annealing instead o f about 0.30 of the same ribbon in the as-cast condition, the brittleness of the a n n e a l e d material is m u c h higher than the a n n e a l e d one. So, we p r e f e r r e d to use as-cast ribbons as active cores in o r d e r to increase the life time o f the p r o p o s e d sensing devices, O n the o t h e r hand, use of a n n e a l e d delay line, in increasing the m a g n e t o m e c h a n i c a l coupling factor decreases the sensitivity o f the tensile stress sensor,
252
E. Hristoforou, R.E. Reilly / Tensile stress distribution sensors
since the presence of the active core becomes less important for this case. This means that the ratio between the pulsed magnetic flux into the delay line, under zero and maximum tensile stress is higher when the delay line is in the as-cast condition, than in the annealed one. Nevertheless, although our material is in the as-cast form, the ratio between maximum and minimum response of the sensor of fig. 2 (which is equivalent to a single strain-gauge element) is greater than 0.15%, in comparison to the gain of a single strain gauge, which is 0.1-4%. The problem of nonuniform and hence nonreproducible response of these sensors, which is increased if delay lines are made of a wider ribbon by chemical cutting and mechanical polishing, has been faced as follows: We measured the response of the sensor by changing the position of the sensing point, using a number of ribbons, always tested as cast. Our results showed that the response of the sensor was not the same for any measurement. But we did observe that this response after being normalized with the maximum output of the sensor was identical for all cases, measurements with the experimental accuracy (12-bit A / D converter). So, use of table and results in repeatable readings, for at least 12-bit A / D converter accuracy. Thus, the sensor of fig. 4 appears to be the more promising one, in terms of potential for industrial manufacture, since its maximum response corresponds to zero applied field, allowing thus the calibration of the sensor before any use. The method of manufacturing the sensors of figs. 2, 3, and 4 is described next. Fig. 8(a) shows the arrangement of a single sensing point corresponding to the arrangement of fig. 4. The steps of manufacturing the sensors are as follows: - In the first place, the M D L support should be made. As the delay line must be always unstressed, three layers of epoxy glass, the medium one having two parallel epoxy glass ribbons in order to create an air channel after connection, in which the M D L is laid, are connected together by heating under pressure. The two outer epoxy glass layers are made of etched printed circuit board in order to make the pulsed current conductors. This way, the delay line is set in the
5
--
Vo(t)
I
Vo(t)
I
Fig. 8. Sensor arrangement. (a) Descrete tensile stress sensor; (b) one-dimensional sensing array. 1. Delay line. 2. Pulsed current conductors. 3. Delay line support. 4. Receiving coil. 5. Epoxy glass ribbons, used as supports of the active core. 6. Copper for soldering the active core. 7. Active core.
channel of the epoxy glass support, so that one can be sure that it is not touched by any means. - After making the M D L support, two long epoxy glass ribbons are connected on top of the channel, in order to be parallel to the delay lines. The epoxy glass ribbons have copper strips on their surface. Such arrangement is made by etching epoxy glass printed circuit board. The active cores of the sensors are to be soldered on the copper strips as follows: Long Metglas ribbons are prestressed under a small tensile stress, e.g. 10 N / m m 2, above the copper strips and are soldered, so that an applied force on them does not cause any displacement. Positioning the prestressed Metglas ribbon on top of the copper structure creates discrete active cores: each Metglas section between two long bars, withstands the same amount of tensile stress (equal to the initially applied one) and any locally applied force F causes no stress or movement at the neighbouring Metglas sections. Such a structure could be repeated in one or two dimensions, by following
E. Hristoforou, R.E. ReiUy / Tensilestress distribution sensors the latter described method as shown in fig. 8(b): One M D L (or M D L array) is positioned in the three-layer epoxy glass support, each M D L being isolated from the next one by one epoxy glass ribbon. Another array of Metglas ribbons is prestressed and soldered on the previously mentioned copper strips of the epoxy glass ribbons. If the length of the part of the strip between the two bars is d and the Young's modulus of the Metglas K, then the force along the core F 1 is given by: F = 2Fl(x 2 + dx)/(d/2
+ x ) and F 1 = Kx,
253
Further work is also under way to detect the response of fig. 4 under compression. Such resuits will be usefull for the case of the two-dimensional sensor of fig. 7.
Acknowledgements Acknowledgements are due to the Arthritis and Rheumatism Council for funding this project. Acknowledgements are also due to Dr. E. Devlin, for helpful discussions about these devices.
(14) where x is the elongation of the core. But, since x is very small, eq. (17) becomes: F = 4Fld ( F 1 / K ) / ( d + 2 F a / K ) .
(15)
Hence, eqs. (2) and (15) could give the value of the applied force F if the value of V0m~x is known.
6. Conclusions In this paper we have presented new results of a family of sensors, which is based on the change of the permeability of soft magnetic materials. The new results show an exponential decay due to the applied stress, allowing calibration of the sensor before any use due to the normalization process. We have also proposed a two-dimensional force sensor based on this idea. Work is under way to test various kinds of delay lines and active cores, used as cast and after annealing.
References [1] E.O. Doebelin, Measurement System: Applications and Design, Fourth Ed. (McGraw-Hill, New York, 1990). [2] M. Dean and R.D. Douglas, Semiconductors and Conventional Strain Gauge (Academic Press, New York, 1962). [3] J. Knutti, Sensor '88 Conf. (May 1988) Nuremberg, Germany. [4] M. Shimazo et al, Proc. 3rd Sensor Symp. Japan (1983) p. 309. [5] R. Boll and H. Warlimont, IEEE Trans. Magn. MAG-17 (1981) 3053. [6] K. Mohri and E. Sudoh, IEEE Trans. Magn. MAG-17 (1981) 3379. [7] I. Sasada, A. Hiroike and K. Harada, Intermag '84, paper BE-03, Hamburg (1984). [8] T. Meydan and K.J. Overshott, J. Appl. Phys. 53 (1982) 8383. [9] E. Hristoforou and R.E. Reilly, IEEE Trans. Magn. MAG-26 (1990) 1563. [10] R.E. Reilly, US Patent no. 4924711 (15 May 1991). [11] E. Hristoforou, Ph.D. Thesis, University of London (1991).
Tensile stress distribution sensors based on amorphous alloys E. H r i s t o f o r o u Institute of Materials Science, NCSR "Demokritos", Aghia Paraskevi, Athens 153 10, Greece
and R . E . Reilly Electronic and Electrical Engineering Departement, King's College London, Strand WC2R 2LS, London, UK Received 30 May 1991; in revised form 27 March 1992
In this paper, we report experimental results on the response of tensile stress sensors based on the magnetostrictive delay line technique, operating u n d e r pulsed field excitation. Their operation is based on the change of the magnetic circuit due to the change of the relative permeability of an a m o r p h o u s ribbon when tensile stress is applied on it. They are low compliance sensors and can be used in cases where large displacement of the active core is not desirable.
I. Introduction
We have been interested in developing and using low compliance force sensors, especially for use in arrays, since they have potential uses in a number of applications for measuring stress, force or pressure [1]. In the past, while a number of this kind of sensing devices have been proposed, the oldest example, controlling a large part of the force sensor market, is the strain gauge, in either the form of a single gauge element or of a Wheatstone bridge structure [2]. These sensors offer a fairly linear response and a very low compliance, although their sensitivity is not satisfactory, about 0.1-4%. An evolution of this type of sensors is the silicon diaphragm [3,4]: They are based on the principle that the resistance of a conductor changes when tensile stress is applied on it. Correspondence to: Dr. E. Hristoforou, Institute of Materials Science, N C S R "Demokritos", Aghia Paraskevi, A t h e n s 153 10, Greece. Tel.: +30-1-6533706; telefax: +30-1-6519430.
Low compliance magnetic force and torque sensors have been presented in the past [5-7], based on the change of the relative permeability of an active core, when tensile stress is applied on it. Cost of manufacturing of these magnetic sensors can be kept low, although their nonlinear response, presence of hysteresis in ac applications [8] and change of their response due to ambient magnetic field make them less attractive than strain gauges in terms of performance. Additionally, the existing low compliance force sensors cannot easily be formed into sensing arrays. In this large, although specific, area of applications, the final cost of an array sensor depends linearly on the number of required sensing points. Such a high cost of an array sensor leads us to conceive and develop a new family of low compliance force sensors, which can easily be formed in arrays. For this purpose, we used the magnetostrictive delay line (MDL) technique under pulsed magnetic field operation as it is shown in fig. 1. A pulsed magnetic field under appropriate magnetic bias conditions applied along the length of the MDL,
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
248
E. Hristoforou, R.E. ReiUy / Tensile stress distribution sensors
--~le~ ?
~
(a) Vo(t)
3L., e 3L.,
let
time
netic core. Such a change can be caused either by displacing a magnetically soft active core, or by changing its permeability due to an applied tensile stress on it. It is mentioned that permeability of a soft magnetic ribbon changes with the applied tensile stress along its length [5]. This way, the delay lines are used for sensing the change of the magnetic circuit as well as for serialising information. A detailed description of the sensors is given in the next section. In all these devices, the applied pulsed and bias magnetic fields are kept fixed, obtaining thus changes in the induced pulsed voltage output at the receiving coil, due only to changes in the magnetic permeability of the active cores. The delay line always remains free of stresses. In this paper, we also present a two dimensional force sensor and the main guidelines for manufacturing and calibrating a discrete or an array sensing device.
(c)
2. Description of sensors
Fig. 1. Description of delay line arrangement and operation. (a) Delay line arrangement; (b) serialisation of acoustic pulses and (c) matrix of conductor-delay lines intersection. 1. Pulsed current conductor. 2. Magnetostrictive amorphous wire delay line. 3. Receiving coil.
results in an acoustic pulse propagating along the M D L [9-11], which may be detected by a receiving coil, as shown in fig. l(a). A n u m b e r of discrete acoustic pulses can be serialized in one delay line, which are the pulsed current conductor-MLD intersections, if the acoustic stress points of origin (PO) are well apart to avoid mutual interference (fig. l(b)). Using an array of m M D L s orthogonal to an array of n straight and parallel pulsed current conductors, one can create rn x n acoustic stress points of origin, as shown in fig. l(c). These acoustic pulses mainly depend on the magnetic circuit at the PO and the receiving coil. The magnetic circuit at the PO can change by changing the magnetic coupling between the M D L and a neighbouring soft mag-
This group of sensors is based on the elastomagnetic properties of the ironrich Metglas alloys. A m o r p h o u s alloys having a chemical composition similar to Metglas 2605SC and 2605S2, m a d e by a rapid solidification process, are magnetostrictive so that they can be used as M D L s and have a relative permeability dependent on the tensile stress, applied on their surface. Such tensile stress alters the magnetic characteristics such as the magnetic permeability. Both these properties of the Metglas 2605SC alloy are used as the fundamental idea behind all the proposed sensors of this group, since its magnetoelastic characteristics are improved compared with others. The principle of operation of this family of sensors is illustrated in fig. 2(a). Metglas 2605SC is used as an M D L and active core. Pulsed current is transmitted through the conductor, orthogonal to the MDL. If the active core is unstressed, then it operates as a magnetic screen between conductor and MDL. Only a fraction of the applied pulsed magnetic field penetrates to the MDL, so that the resultant acoustic signal
249
E. Hristoforou, R.E. Reilly / Tensile stress distribution sensors
% 2
a
i
a
t
~
j
Vo m
g
1=2.27 A
i
a
1=4.52 A l=6.18 A
--a
------o----• ----o--
1=9d)9 A 1=11.35 A 1=1362A
(,()
~'ll
20
~
IO
{) .
IIX)
,.......~-''''~ ,.,...,.a __.-.-a - ' ' ' m
~11J
0 b
-----o-31)
211
40
""~-* I ~ • -----a-x
le=l.3 A 1e=2.2 A Ie--4.5 A 1e=6.8 A 1e=9.1 A 1e=13.6 A 1e=20.5 A
b
o
>
~.
NI/mn~ )
Fig. 3. Balance structure sensor. (a) Arrangement of the sensor. (1) Pulsed current conductor, (2) delay line, (3) receiving coil and (4) active core. (b) Response of the sensor of fig. 6, under various values of pulsed current.
60
AplJlicd SII'C~,S(Nl/Illlll21
Fig. 2. Single force element, (a) Layout of the sensor. 1. Pulsed current conductor. 2. Delay line. 3. Receiving coil. 4. Active core. (b) Response of the sensor of (a), under various values of applied pulsed current Ie.
and consequently the detected output is a minimum. If tensile stress is applied, as described previously, the permeability of the active core decreases, the magnetic coupling between the two soft magnetic materials alters and hence more magnetic field penetrates to the MDL, resulting thus in an increase of the amplitude of the detected voltage output, V0. The dependence of V0 on the applied tensile stress is illustrated in fig. 2(b). Any changes of the response of the sensor is due to applied stress and not due to any arbitrary movement. The Metglas delay line is unstressed and fixed for all measurements. In all the sensors of this group, the amplitude of the detected pulsed voltage V0 is the output, the applied tensile stress being the input. A n improved version of the principal idea of fig. 2(a) is given in fig. 3(a). According to this arrangement, Metglas ribbons and pulsed current conductors are above and below the MDL. Pulsed current is transmitted in the same direction in both conductors. One of the cores remains free of
stresses, the other being the active one. Applying no stress on the active core, the symmetry of the magnetic circuit results in no acoustic stress at the PO and consequently, zero voltage output. Tensile stress along the active core results in the creation of a propagating acoustic pulse. The characteristic curves of the sensor of fig. 3(a) is given in fig. 3(b) [9]. Fig. 4 shows a new arrangement of this family of sensors. The balanced structure used in fig. 3(a) is also used here. If a pulsed current I e is transmitted in the same direction in the two conductors, then, in the absence of the core S, there is zero magnetic flux in the delay line and consequently zero voltage output is detected. When core S is positioned at a fixed distance close to the delay line, then the magnetic circuit changes and an acoustic pulse results. Observing the magnetic circuit of this arrangement at the PO, it can be seen that applying tensile stress on the core S results in a decrease of the coupling
Fig. 4. A new balance structure sensor. (1) Pulsed current conductor, (2) delay line, (3) receiving coil and (4) active core.
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E. Hristoforou, R.E. Reilly / Tensile stress distribution sensors
between core and line, so that the magnetic flux in the line decreases due to the balance structure. So, increasing the stress on the core results in a decrease in voltage output at the receiving coil. One would expect to measure voltage output from a maximum value down to zero corresponding to maximum tensile stress on the active core, which means that its relative permeability effectively becomes equal to one.
30"
g g
• []
20"
•
Ie=13.63A Ie=ll.3A Ie=9.1A Ie=6.1A Ie=4.5A Ie=2.2A
10"
0 10
20
30
40
3. Response of the sensor
3.1. Experimental set-up Fig. 5 shows the basic experimental set-up for getting the response of the sensor of fig. 4. An optic bench was used as basis for this experiment. The delay line was positioned on a 2 m m thick, 2 cm wide and 30 cm long support made of Perspex. The 800 turn, 5 m m long detecting coil made from 0.03 m m enamelled wire was at one end of this support (not shown in fig. 5). This support was well fixed on the optic bench. A 50 cm long and 5 m m wide Metglas 2605SC ribbon, was used as magnetic core S. We applied tensile stress by using a torque motor, and we determined its magnitude by using a load cell, as given in ref. [11].
3.2. Experimental results
60
Fig. 6. R e s p o n s e of the sensor of fig. 4, u n d e r various values of applied current E e.
values of exciting pulsed current I e was measured and fig. 6 shows the response of the sensor of fig. 4. All these results were taken by increasing and decreasing the applied tensile stress on the Metglas ribbon and the reported output was the same for both cases, indicating the absence of hysteresis. It was observed that fig. 6 fits an exponential curve, so that Vo(o-) can be given by: V0(o" )
=
(1)
V0max e - c ~ ,
where V0max is the maximum value of V0 and c---3.5 m m Z / N . In the same way as before, the normalized function V0o(tr), with respect to V0ma~, is given by: V0n(O" ) = V 0 ( o - ) / / V 0 m a x =
According to the experimental set-up described previously, the dependence of V0 on the applied tensile stress on the ribbon under various
50
Tensilestress(N/ram2)
e -¢~.
(2)
From the results of fig. 3(b), it is observed that the response of the sensor of fig. 3(a) fits an exponential equation of the form: V0(o" ) = V0max(1 -
e-C'~),
(3)
where V0maxis the maximum value of V0. Coefficient c is as given in eqs. (1) and (2), since the response of the sensor of fig. 3(a) is exactly the opposite as for the sensor of fig. 4. So, its normalized function with V0max is given as follows: Von(Or ) : Fig. 5. Experimental set-up to get the response of the sensor of fig. 4. 1. T o r q u e motor. 2. Load cell. 3. Active core. 4. Optic bench. 5. Pulsed current contuctors. 6. Delay line.
V0(o-)//Vomax =
1 - e -c'~.
(4)
The significance of the normalised function will be illustrated in the discussion, since it will prove helpful for sensor calibration.
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E. Hristoforou, R.E. Reilly / Tensile stress distribution sensors
4. A two-dimensional sensing element
So, the tensile stresses applied on the two cores at the two sides, o-~ and o-b are given by:
A n idea to the construction of a two or three dimensional force sensor is p r o p o s e d here. T h e basic d i a g r a m of a two dimensional force sensor based on the sensor o f fig. 4 is p r e s e n t e d in fig. 7. T h e two active cores are orthogonal to each other. A two-dimensional force F applied on it can be analysed in two forces F x and Fy, their direction being as illustrated in fig. 7. Force F x causes a force F 1 and consequently a tensile stress s on the core. F 1 is given by: F 1 = Fx sin a.
(5)
Force Fy is adding and subtracting a force F 2 at the two parts of the core. W i t h o u t any loss of the generality, it is assumed that for the case of fig. 7, F 2 is a d d e d and subtracted to the F 1, at the right and left active cores, respectively. So, F z is given by: F 2 = Fy cos a.
(in N / m m 2) (10)
for the case o f a 5 m m wide and 25 ~ m thick active core. T h e s e two stresses cause two different outputs in the two c o r r e s p o n d i n g delay lines. By using the principle o f the structure of fig. 4, eq. (7) gives V0~,(o-) and V0bn((r) for the left and right part o f the core, respectively, as follows: Voan
=
exp(--CO'a) and Vob. = exp(--c~rb).
F a = F 1 + F 2 -- F x sin a + Fy cos a
(7)
and cos a.
(8)
(11)
So, F a and F b can be calculated by knowing the normalized values Voa, and Vobn. F a and F b are given by: F a = - l n ( V o a n ) / 4 c = - In( Voa/Voamax)/4c, Fb = -- In( V0b. ) / 4 C = -- In( Vob/VObma x ) / 4 C .
(6)
So, the left and right parts o f the core withstand two forces F a and F b, respectively, which are given by:
F b = F 1 - F 2 - - F ~ sin a - F y
o-a = F a / 0 . 1 2 5 and ~rb = F J 0 . 1 2 5
(12) So, F x and Fy can be calculated:
Fx = (Fa +
Fb)/2
= --ln(VoaVob/Vo~maxVobma,,)/8C, Fy= ( F a - F b ) / 2 =
(13)
--ln(VoaVO~mJVo~Vo.max)/SC.
But, a = 45 °, t h e n eqs. (10) and (11) b e c o m e : F, = 0 . 7 0 7 ( F x + Fy) and F b = 0 . 7 0 7 ( F x - F y ) . (9)
Fig. 7. Basic diagram of a two-dimensional discrete force sensor. 1. Delay line. 2. Pulsed current conductor. 3. Point of applied force F. 4. Active core.
5. Discussion T h e reason that we p r e f e r e d to use unannealed ribbons for both delay lines and active cores instead o f a n n e a l e d ones runs as follows: A l t h o u g h the m a g n e t o m e c h a n i c a l coupling factor of the as-cast used ribbons b e c o m e s about 0.75 after annealing instead o f about 0.30 of the same ribbon in the as-cast condition, the brittleness of the a n n e a l e d material is m u c h higher than the a n n e a l e d one. So, we p r e f e r r e d to use as-cast ribbons as active cores in o r d e r to increase the life time o f the p r o p o s e d sensing devices, O n the o t h e r hand, use of a n n e a l e d delay line, in increasing the m a g n e t o m e c h a n i c a l coupling factor decreases the sensitivity o f the tensile stress sensor,
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E. Hristoforou, R.E. Reilly / Tensile stress distribution sensors
since the presence of the active core becomes less important for this case. This means that the ratio between the pulsed magnetic flux into the delay line, under zero and maximum tensile stress is higher when the delay line is in the as-cast condition, than in the annealed one. Nevertheless, although our material is in the as-cast form, the ratio between maximum and minimum response of the sensor of fig. 2 (which is equivalent to a single strain-gauge element) is greater than 0.15%, in comparison to the gain of a single strain gauge, which is 0.1-4%. The problem of nonuniform and hence nonreproducible response of these sensors, which is increased if delay lines are made of a wider ribbon by chemical cutting and mechanical polishing, has been faced as follows: We measured the response of the sensor by changing the position of the sensing point, using a number of ribbons, always tested as cast. Our results showed that the response of the sensor was not the same for any measurement. But we did observe that this response after being normalized with the maximum output of the sensor was identical for all cases, measurements with the experimental accuracy (12-bit A / D converter). So, use of table and results in repeatable readings, for at least 12-bit A / D converter accuracy. Thus, the sensor of fig. 4 appears to be the more promising one, in terms of potential for industrial manufacture, since its maximum response corresponds to zero applied field, allowing thus the calibration of the sensor before any use. The method of manufacturing the sensors of figs. 2, 3, and 4 is described next. Fig. 8(a) shows the arrangement of a single sensing point corresponding to the arrangement of fig. 4. The steps of manufacturing the sensors are as follows: - In the first place, the M D L support should be made. As the delay line must be always unstressed, three layers of epoxy glass, the medium one having two parallel epoxy glass ribbons in order to create an air channel after connection, in which the M D L is laid, are connected together by heating under pressure. The two outer epoxy glass layers are made of etched printed circuit board in order to make the pulsed current conductors. This way, the delay line is set in the
5
--
Vo(t)
I
Vo(t)
I
Fig. 8. Sensor arrangement. (a) Descrete tensile stress sensor; (b) one-dimensional sensing array. 1. Delay line. 2. Pulsed current conductors. 3. Delay line support. 4. Receiving coil. 5. Epoxy glass ribbons, used as supports of the active core. 6. Copper for soldering the active core. 7. Active core.
channel of the epoxy glass support, so that one can be sure that it is not touched by any means. - After making the M D L support, two long epoxy glass ribbons are connected on top of the channel, in order to be parallel to the delay lines. The epoxy glass ribbons have copper strips on their surface. Such arrangement is made by etching epoxy glass printed circuit board. The active cores of the sensors are to be soldered on the copper strips as follows: Long Metglas ribbons are prestressed under a small tensile stress, e.g. 10 N / m m 2, above the copper strips and are soldered, so that an applied force on them does not cause any displacement. Positioning the prestressed Metglas ribbon on top of the copper structure creates discrete active cores: each Metglas section between two long bars, withstands the same amount of tensile stress (equal to the initially applied one) and any locally applied force F causes no stress or movement at the neighbouring Metglas sections. Such a structure could be repeated in one or two dimensions, by following
E. Hristoforou, R.E. ReiUy / Tensilestress distribution sensors the latter described method as shown in fig. 8(b): One M D L (or M D L array) is positioned in the three-layer epoxy glass support, each M D L being isolated from the next one by one epoxy glass ribbon. Another array of Metglas ribbons is prestressed and soldered on the previously mentioned copper strips of the epoxy glass ribbons. If the length of the part of the strip between the two bars is d and the Young's modulus of the Metglas K, then the force along the core F 1 is given by: F = 2Fl(x 2 + dx)/(d/2
+ x ) and F 1 = Kx,
253
Further work is also under way to detect the response of fig. 4 under compression. Such resuits will be usefull for the case of the two-dimensional sensor of fig. 7.
Acknowledgements Acknowledgements are due to the Arthritis and Rheumatism Council for funding this project. Acknowledgements are also due to Dr. E. Devlin, for helpful discussions about these devices.
(14) where x is the elongation of the core. But, since x is very small, eq. (17) becomes: F = 4Fld ( F 1 / K ) / ( d + 2 F a / K ) .
(15)
Hence, eqs. (2) and (15) could give the value of the applied force F if the value of V0m~x is known.
6. Conclusions In this paper we have presented new results of a family of sensors, which is based on the change of the permeability of soft magnetic materials. The new results show an exponential decay due to the applied stress, allowing calibration of the sensor before any use due to the normalization process. We have also proposed a two-dimensional force sensor based on this idea. Work is under way to test various kinds of delay lines and active cores, used as cast and after annealing.
References [1] E.O. Doebelin, Measurement System: Applications and Design, Fourth Ed. (McGraw-Hill, New York, 1990). [2] M. Dean and R.D. Douglas, Semiconductors and Conventional Strain Gauge (Academic Press, New York, 1962). [3] J. Knutti, Sensor '88 Conf. (May 1988) Nuremberg, Germany. [4] M. Shimazo et al, Proc. 3rd Sensor Symp. Japan (1983) p. 309. [5] R. Boll and H. Warlimont, IEEE Trans. Magn. MAG-17 (1981) 3053. [6] K. Mohri and E. Sudoh, IEEE Trans. Magn. MAG-17 (1981) 3379. [7] I. Sasada, A. Hiroike and K. Harada, Intermag '84, paper BE-03, Hamburg (1984). [8] T. Meydan and K.J. Overshott, J. Appl. Phys. 53 (1982) 8383. [9] E. Hristoforou and R.E. Reilly, IEEE Trans. Magn. MAG-26 (1990) 1563. [10] R.E. Reilly, US Patent no. 4924711 (15 May 1991). [11] E. Hristoforou, Ph.D. Thesis, University of London (1991).