- Email: [email protected]
Amorphous magnetostrictive wires used in delay lines for sensing applications
Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
Amorphous magnetostrictive wires used in delay lines for sensing applications E. Hristoforou* Laboratory of Physical Metallurgy, Department of Mining and Metallurgical Engineering, Zografou Campus, National Technical University of Athens, 9 Heroon Polytechniou Strees, Athens 15780, Greece
Abstract In this paper we give a review on the use of amorphous magnetostrictive wires in delay lines for sensing applications. Initially, we demonstrate the engineering model of the operation of magnetostrictive delay lines (MDL), illustrating the micro-strain generation, propagation and detection. Accordingly, we present the developed sensing elements based on this technique. The sensing elements are based on the parameters affecting the operation of the MDL, which are the ambient field, the interrogating electromagnetic field and the mechanical action on the magnetic element. Finally, we discuss on the development of a new magnetostrictive device, which incorporate the excitation and sensing means and can be used in sensing applications. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.50; 75.80 Keywords: Magnetic materials; Magnetostriction; Sensors
1. Introduction Sensors and transducers have an increasing interest in the society, industry and academia, because of their importance in many technological applications [1]. All modern vehicles and transport means use a vast variety of sensors and transducers. The operation of all medical instruments is also based on sensors and transducers. Industry is also employing more and more transducers for the monitoring and control of production lines. These are just a few but important examples illustrating that the argument of modern engineering is based on sensors is not very far from the truth. *Tel.: +301-7722178; fax: +301-7722119. E-mail address: [email protected] (E. Hristoforou).
Performing a literature survey, one can find many ways of dividing sensors in categories. In fact, we can see such categorization in three ways. The first tells us what a sensor can measure. The most significant or broad categorization due to this principle is physical and chemical sensors. The second is the physical phenomenon and the material where the operation of the sensor is based on. The main categories here are conducting, semiconducting, dielectric, magnetic and superconducting sensors. The last one is where the sensor can be used, where the main categories are industrial, transport, automotive, medical, military, domestic and environmental sensors. Magnetic materials and phenomena are not absent in the sensor technology. In fact, magnetic sensors play a significant role in physical
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 5 6 3 - 2
388
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
measurements used in all kinds of applications [2–5]. The most often used magnetic phenomena in today’s magnetic sensor technology are the magneto-resistance [6–8], the magneto-impedance [9–11], the magnetostriction [12–15], the electromagnetic induction and the Hall effect. Behind all these effects there is one single parameter governing the operation of these sensors, which is the magnitude of the magnetization field applied on the magnetic sensor core. Changing the design of the sensor core material and arrangement of materials, we generate various kinds of devices able to measure different physical sizes for different applications. The properties of the magnetic materials used for sensor cores govern in most cases the properties of the sensor. Recent advances in magnetic material technology enhanced the position of the magnetic sensors in the global market. One of these advances is the development of amorphous magnetic wires. They mainly have soft magnetic properties, which can be used for the design of very competent sensing elements [16–19]. In our research we used a particular technique based on the magnetostriction effect, the magnetostrictive delay line (MDL) technique, in order to design and develop mechanical sensors, measuring position and displacement, stress and force and magnetic field [20]. These sensors have found applications in industrial, automotive and military applications. The most recent application is their use for fast non-destructive testing and evaluation (NDT&E) of magnetic surfaces. The use of amorphous magnetostrictive wires enhanced the operation and the properties of some families of our sensors, allowing a more sensitive and linear sensor response. In this paper, we firstly illustrate a new approach for the engineering modeling of the MDL operation, which can help in the corresponding design of the MDL sensors. Then, we demonstrate the most recent results in the response of our sensors, using amorphous magnetostrictive wires. Finally, we present a new kind of magnetoelastic material in the form of wire, which can be used for distribution field measurements and more especially for NDT&E.
2. Modeling of magnetostrictive delay lines In order to present our MDL modeling, we start with the simplest possible arrangement, shown in Fig. 1. Concerning this setup, the transient field Hðx; tÞ; along the length of the MDL, transmitted via the coil is 1 Hðx; tÞ ¼ f ðxÞIðtÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi IðtÞ: a 2 þ x2 The transient current I is assumed to be a rising ramp from 0 to I0 for 0oton1 T; a steady-state current I0 for n1 Toton2 T and a falling ramp from I0 to 0 for n2 Toton3 T; where T is the time increment and ni integer numbers. Having applied the field Hðx; tÞ; the rotation of the magnetic moments of the outer cell of the magnetic domains of either the positive or the negative magnetostrictive wire towards the direction of Hðx; tÞ; results in micro-strain generation. We have experimentally found that such transient microstrain has an almost un-hysteretic behavior due to the small hysteresis of the B2H loop of the wire. We also assume that the micro-strain is given by 2
lðHÞ ¼ ls ð1 ecH Þ; c > 0: The micro-strains lðx; tÞ caused by the rising transient current are summed up as a group of MDL
Vo Ie
MDL
Ie
Fig. 1. The simplest MDL arrangement: a one-turn coil for micro-strain generation in a magnetostrictive wire. The field experienced by each infinitesimal area of the surface of the magnetostrictive wire is equal to the field applied on a planar surface of the same material, by a straight current conductor, provided that the gap between planar surface—straight conductor equals the distance between exciting coil and magnetostrictive wire.
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
strains Grðx; tÞ Grðx; tÞ ¼ n1 X 2 2 2 ls ð1 ecðnI0 =n1 Þ =a þðxðn3 þn2 þn1 nÞTvÞ Þ Þ n¼1
with v the longitudinal sound velocity of the MDL. The micro-strains lðx; tÞ caused by the steady-state transient current are summed up as a group of strains Gsðx; tÞ; which follows Grðx; tÞ: Gsðx; tÞ ¼ n2 X 2 2 2 ls ð1 ecðI0 Þ =ða þðxðn3 þn2 nÞTvÞ Þ Þ: n¼n1
Fig. 2. Micro-strain generation and propagation in the case of long excitation pulse: (a) Superimposed micro-strains, (b) propagating elastic pulse and (c) corresponding flux change within the magnetostrictive wire in the region of elastic pulse propagation.
389
Finally, the micro-strains lðx; tÞ caused by the falling transient current are summed up as a group of strains Gfðx; tÞ; which follows Gsðx; tÞ: Gfðx; tÞ ¼ n3 X 2 2 2 ls ð1 ecððnn3 ÞI0 =n3 Þ =ða þðxðn3 nÞTvÞ Þ Þ: n¼n2
These groups of micro-strains propagate along the magnetostrictive wire as an elastic pulse. This elastic pulse causes a flux change along the volume of the material it propagates, which is can be detected by a search coil, set around the wire. Suppose that the pulsed transient current is short, then the change of the flux in the wire gives a single-pulsed voltage output, as illustrated in Fig. 2. If the pulsed current has a long steady state, then the first derivative of the elastic pulse, which corresponds to the pulsed voltage output breaks into two pulses, opposite in signs which are detected by the search coil, as illustrated in Fig. 3. We performed numerical calculations of the elastic pulse groups and their first derivatives. We
Fig. 3. Micro-strain generation and propagation in the case of long excitation pulse: (a) Superimposed micro-strains, (b) propagating elastic pulse and (c) corresponding flux change within the magnetostrictive wire in the region of elastic pulse propagation.
390
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
As said in Introduction, we have developed position, load and field sensors based on the MDL technique. We have tested amorphous magnetostrictive wires in almost all kinds of MDL sensors and we hereinafter illustrate the most significant and not-announced results. In Fig. 4(a), a position sensor is illustrated, where the active core is an Nd–Fe–B permanent magnet and the sensing core is an Fe–Si–B wire annealed in 3801C for 15 min in Ar atmosphere. The active core is moving parallel to the MDL, and the sensor response, illustrated in Fig. 4(b) is the maximum of the three neighbor search coils. From this results, the sensitivity of the sensor has been found to be 1–2 mm/V, thus suggesting a competitive sensor with respect to the state of the
art. Provided that a micro-processor controlled circuit is to be used the system accuracy can reach levels of 10 mm/m. In Fig. 5(a), a force/torque sensor is illustrated, where the sensing core is an Fe–Si–B wire prepared in the same conditions as the previous one. The tensile stress or the torsion is applied along the length of the wire and the sensor response is illustrated in Fig. 5(b). The monotonic and unhysteretic response of the arrangement promises that under the proper housing of the sensor, we can have a competitive load cell or torque meter. In Fig. 6(a), a distribution field sensor is illustrated, which is able to detect the position and the amplitude of a local field spike. The wire used is an Fe–Co–Si–B negative magnetostriction wire, which has been annealed under 3801C for 20 min in Ar atmosphere. The response of the sensor under DC field is given in Fig. 6(b). This sensor has been designed for non-destructive testing and evaluation of magnetic surfaces and is able to measure the position and size of cracks of a surface with only to perpendicular translations on the magnetic surface. Each localized crack causes magnetic field perturbation, which in turn causes discrete elastic pulses in the MDL. The
Fig. 4. A displacement sensor: (a) The sensing principle and (b) the response of the sensor.
Fig. 5. A force/torque sensor: (a) The sensing principle and (b) the response of the sensor.
have also experimentally tested the amorphous wire using the arrangement of Fig. 1 and we have had an agreement of 97% between the modeling calculations and the experimental results. This fact suggests that we can continue our modeling for the MDL sensor arrangements.
3. Sensor applications
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
391
Fig. 6. A field sensor: (a) The sensing principle and (b) the response of the sensor.
delay time and the amplitude of these voltage outputs suggests the position and size, respectively, of the corresponding cracks.
4. A new magneto-elastic element Using the classic MDL arrangement we faced difficulties concerning the resolution in elastic micro-strain generation. In fact, using coils for elastic pulse generation, we could not obtain discrete elastic pulses closer than 2 times the diameter of the coil. This performance cannot be acceptable in some cases like non-destructive evaluation. Having the motivation of such resolution we have conceived and developed the device illustrated in Fig. 7(a). According to this arrangement, a conducting cylinder is used as the substrate of the new magneto-elastic device, on which we deposit a thin tube of magnetostrictive material. Then, passing pulsed current through the conductor, the magnetostrictive thin tube is excited circumferentially, thus resulting in a circumferential micro-strain along the whole
Fig. 7. The new magneto-elastic device: (a) The device and (b) the operation of the device. Transmitting no current through the conductor results in no net magnetization. Transmitting current larger than a critical value Id able to move domain walls, results in magnetization of the wire. Transmitting pulsed current close to the saturation levels, we have micro-strain generation.
length of the tube. Provided that the material has undergone proper tailoring in order to become magneto-elastically uniform and in the absence of magnetic anomalies along its length, the only propagating micro-strains are those originated at the ends of the magnetostrictive tube. Then, in the presence of magnetic anomalies like field spikes, i.e. due to the presence of cracks on a magnetic surface under test, the magneto-elastic symmetry is broken resulting in discrete elastic pulses propagating along the material. These elastic pulses do not suffer from dispersion problems like in the case of the sensor in Fig. 5(a).
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E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
A magneto-elastic element has been developed, using 1 mm Cu cylinder as the conducting substrate. An special electro-deposition facility has been developed, with the Cu cylinder used as the cathode and a stainless-steel tube surrounding the Cu tube as the input. Using FeCl3 solution, and pulsed current electro-deposition, we obtained a 1 mm thickness Fe tube around the Cu cylinder. After the deposition process we stress-current annealed the magnetostrictive film under 0.3 A DC current, 500 MPa stress for 10 min. In fact, this very first trial of such magneto-elastic arrangement demonstrated magneto-elastic response. A series of experimental work is under way, in order to perform systematic studies on this new arrangement, aided by our modeling.
References [1] E.O. Doebelin, Measurement Systems: Applications and Design, 4th Edition, McGraw-Hill, New York, 1990. ! [2] J.M. Barandiaran, J. Gutierrez, C. Gomez-Polo, New sensors based on the magnetoelastic resonance of metallic glasses, Sensors Actuators A 81 (2000) 154. [3] A. Hernando, M. Vasquez, J.M. Barandiaran, Metalic glasses and sensing applications, J. Phys. E 21 (1988) 1129. [4] T. Meydan, Application of amorphous materials to sensors, J. Magn. Magn. Mater. 133 (1994) 525. [5] P. Ripka, G. V!ertesy, Sensors based on soft magnetic materials, J. Magn. Magn. Mater. 215–216 (2000) 795. [6] R.L. White, Giant magnetoresistance materials and their potential as read head sensors, IEEE Trans. Magn. 30 (1994) 346.
[7] J.M. Daughton, GMR applications, J. Magn. Magn. Mater. 192 (1999) 334. [8] R.E. Fontana Jr., Process complexity of magnetoresistive sensors: a review, IEEE Trans. Magn. 31 (1995) 2579. [9] L.V. Panina, K. Mohri, T. Uchiyama, Giant magnetoimpedance (GMI) in amorphous wire, single layer film and sandwich film, Physica A 241 (1997) 429. [10] M. V!azquez, M. Knobel, M.L. S!anchez, R. Valenzuela, A.P. Zhukov, Giant magnetoimpedance effect in soft magnetic wires for sensor applications, Sensors Actuators A 59 (1997) 20. [11] L. Brunetti, P. Tiberto, F. Vinai, H. Chiriac, Highfrequency giant magneto-impedance in joule-heated Cobased amorphous ribbons and wires, Mater. Sci. Eng. A 304–306 (2001) 961. [12] D.S. Nyce, Magnetostriction: one way to monitor linear position, Hydraul. Pneum. 53 (2000) 35. [13] H. Chiriac, C.S. Marinescu, New position sensor based on ultra-acoustic standing waves in FeSiB amorphous wires, Sensors Actuators A 81 (2000) 174. [14] E. Quandt, Giant magnetostrictive thin film materials and applications, J. Alloys Compounds 258 (1997) 126. [15] L. Lanotte, R. Germano, Application of magnetoelastic waves for sensors of displacement, Sensors Actuators A 59 (1997) 337. [16] M. Vazquez, A. Hernando, Soft magnetic wire for sensor applications, J. Phys. D 29 (1996) 939. [17] F.B. Humphrey, Applications of amorphous wire, Mater. Sci. Eng. A 179–80 (1994) 66. [18] P.T. Squire, D. Atkinson, M.R.J. Gibbs, S. Atalay, Amorphous wires and their applications, J. Magn. Magn. Mater. 132 (1994) 10. ! ari, M. Neagu, [19] H. Chiriac, C.S. Marinescu, T.-A. Ov! Sensor applications of amorphous glass-covered wires, Sensors Actuators A 76 (1999) 208. [20] E. Hristoforou, Magnetostrictive delay lines and their applications, Sensors Actuators A 59 (1997) 183.
Amorphous magnetostrictive wires used in delay lines for sensing applications E. Hristoforou* Laboratory of Physical Metallurgy, Department of Mining and Metallurgical Engineering, Zografou Campus, National Technical University of Athens, 9 Heroon Polytechniou Strees, Athens 15780, Greece
Abstract In this paper we give a review on the use of amorphous magnetostrictive wires in delay lines for sensing applications. Initially, we demonstrate the engineering model of the operation of magnetostrictive delay lines (MDL), illustrating the micro-strain generation, propagation and detection. Accordingly, we present the developed sensing elements based on this technique. The sensing elements are based on the parameters affecting the operation of the MDL, which are the ambient field, the interrogating electromagnetic field and the mechanical action on the magnetic element. Finally, we discuss on the development of a new magnetostrictive device, which incorporate the excitation and sensing means and can be used in sensing applications. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.50; 75.80 Keywords: Magnetic materials; Magnetostriction; Sensors
1. Introduction Sensors and transducers have an increasing interest in the society, industry and academia, because of their importance in many technological applications [1]. All modern vehicles and transport means use a vast variety of sensors and transducers. The operation of all medical instruments is also based on sensors and transducers. Industry is also employing more and more transducers for the monitoring and control of production lines. These are just a few but important examples illustrating that the argument of modern engineering is based on sensors is not very far from the truth. *Tel.: +301-7722178; fax: +301-7722119. E-mail address: [email protected] (E. Hristoforou).
Performing a literature survey, one can find many ways of dividing sensors in categories. In fact, we can see such categorization in three ways. The first tells us what a sensor can measure. The most significant or broad categorization due to this principle is physical and chemical sensors. The second is the physical phenomenon and the material where the operation of the sensor is based on. The main categories here are conducting, semiconducting, dielectric, magnetic and superconducting sensors. The last one is where the sensor can be used, where the main categories are industrial, transport, automotive, medical, military, domestic and environmental sensors. Magnetic materials and phenomena are not absent in the sensor technology. In fact, magnetic sensors play a significant role in physical
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 5 6 3 - 2
388
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
measurements used in all kinds of applications [2–5]. The most often used magnetic phenomena in today’s magnetic sensor technology are the magneto-resistance [6–8], the magneto-impedance [9–11], the magnetostriction [12–15], the electromagnetic induction and the Hall effect. Behind all these effects there is one single parameter governing the operation of these sensors, which is the magnitude of the magnetization field applied on the magnetic sensor core. Changing the design of the sensor core material and arrangement of materials, we generate various kinds of devices able to measure different physical sizes for different applications. The properties of the magnetic materials used for sensor cores govern in most cases the properties of the sensor. Recent advances in magnetic material technology enhanced the position of the magnetic sensors in the global market. One of these advances is the development of amorphous magnetic wires. They mainly have soft magnetic properties, which can be used for the design of very competent sensing elements [16–19]. In our research we used a particular technique based on the magnetostriction effect, the magnetostrictive delay line (MDL) technique, in order to design and develop mechanical sensors, measuring position and displacement, stress and force and magnetic field [20]. These sensors have found applications in industrial, automotive and military applications. The most recent application is their use for fast non-destructive testing and evaluation (NDT&E) of magnetic surfaces. The use of amorphous magnetostrictive wires enhanced the operation and the properties of some families of our sensors, allowing a more sensitive and linear sensor response. In this paper, we firstly illustrate a new approach for the engineering modeling of the MDL operation, which can help in the corresponding design of the MDL sensors. Then, we demonstrate the most recent results in the response of our sensors, using amorphous magnetostrictive wires. Finally, we present a new kind of magnetoelastic material in the form of wire, which can be used for distribution field measurements and more especially for NDT&E.
2. Modeling of magnetostrictive delay lines In order to present our MDL modeling, we start with the simplest possible arrangement, shown in Fig. 1. Concerning this setup, the transient field Hðx; tÞ; along the length of the MDL, transmitted via the coil is 1 Hðx; tÞ ¼ f ðxÞIðtÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi IðtÞ: a 2 þ x2 The transient current I is assumed to be a rising ramp from 0 to I0 for 0oton1 T; a steady-state current I0 for n1 Toton2 T and a falling ramp from I0 to 0 for n2 Toton3 T; where T is the time increment and ni integer numbers. Having applied the field Hðx; tÞ; the rotation of the magnetic moments of the outer cell of the magnetic domains of either the positive or the negative magnetostrictive wire towards the direction of Hðx; tÞ; results in micro-strain generation. We have experimentally found that such transient microstrain has an almost un-hysteretic behavior due to the small hysteresis of the B2H loop of the wire. We also assume that the micro-strain is given by 2
lðHÞ ¼ ls ð1 ecH Þ; c > 0: The micro-strains lðx; tÞ caused by the rising transient current are summed up as a group of MDL
Vo Ie
MDL
Ie
Fig. 1. The simplest MDL arrangement: a one-turn coil for micro-strain generation in a magnetostrictive wire. The field experienced by each infinitesimal area of the surface of the magnetostrictive wire is equal to the field applied on a planar surface of the same material, by a straight current conductor, provided that the gap between planar surface—straight conductor equals the distance between exciting coil and magnetostrictive wire.
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
strains Grðx; tÞ Grðx; tÞ ¼ n1 X 2 2 2 ls ð1 ecðnI0 =n1 Þ =a þðxðn3 þn2 þn1 nÞTvÞ Þ Þ n¼1
with v the longitudinal sound velocity of the MDL. The micro-strains lðx; tÞ caused by the steady-state transient current are summed up as a group of strains Gsðx; tÞ; which follows Grðx; tÞ: Gsðx; tÞ ¼ n2 X 2 2 2 ls ð1 ecðI0 Þ =ða þðxðn3 þn2 nÞTvÞ Þ Þ: n¼n1
Fig. 2. Micro-strain generation and propagation in the case of long excitation pulse: (a) Superimposed micro-strains, (b) propagating elastic pulse and (c) corresponding flux change within the magnetostrictive wire in the region of elastic pulse propagation.
389
Finally, the micro-strains lðx; tÞ caused by the falling transient current are summed up as a group of strains Gfðx; tÞ; which follows Gsðx; tÞ: Gfðx; tÞ ¼ n3 X 2 2 2 ls ð1 ecððnn3 ÞI0 =n3 Þ =ða þðxðn3 nÞTvÞ Þ Þ: n¼n2
These groups of micro-strains propagate along the magnetostrictive wire as an elastic pulse. This elastic pulse causes a flux change along the volume of the material it propagates, which is can be detected by a search coil, set around the wire. Suppose that the pulsed transient current is short, then the change of the flux in the wire gives a single-pulsed voltage output, as illustrated in Fig. 2. If the pulsed current has a long steady state, then the first derivative of the elastic pulse, which corresponds to the pulsed voltage output breaks into two pulses, opposite in signs which are detected by the search coil, as illustrated in Fig. 3. We performed numerical calculations of the elastic pulse groups and their first derivatives. We
Fig. 3. Micro-strain generation and propagation in the case of long excitation pulse: (a) Superimposed micro-strains, (b) propagating elastic pulse and (c) corresponding flux change within the magnetostrictive wire in the region of elastic pulse propagation.
390
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
As said in Introduction, we have developed position, load and field sensors based on the MDL technique. We have tested amorphous magnetostrictive wires in almost all kinds of MDL sensors and we hereinafter illustrate the most significant and not-announced results. In Fig. 4(a), a position sensor is illustrated, where the active core is an Nd–Fe–B permanent magnet and the sensing core is an Fe–Si–B wire annealed in 3801C for 15 min in Ar atmosphere. The active core is moving parallel to the MDL, and the sensor response, illustrated in Fig. 4(b) is the maximum of the three neighbor search coils. From this results, the sensitivity of the sensor has been found to be 1–2 mm/V, thus suggesting a competitive sensor with respect to the state of the
art. Provided that a micro-processor controlled circuit is to be used the system accuracy can reach levels of 10 mm/m. In Fig. 5(a), a force/torque sensor is illustrated, where the sensing core is an Fe–Si–B wire prepared in the same conditions as the previous one. The tensile stress or the torsion is applied along the length of the wire and the sensor response is illustrated in Fig. 5(b). The monotonic and unhysteretic response of the arrangement promises that under the proper housing of the sensor, we can have a competitive load cell or torque meter. In Fig. 6(a), a distribution field sensor is illustrated, which is able to detect the position and the amplitude of a local field spike. The wire used is an Fe–Co–Si–B negative magnetostriction wire, which has been annealed under 3801C for 20 min in Ar atmosphere. The response of the sensor under DC field is given in Fig. 6(b). This sensor has been designed for non-destructive testing and evaluation of magnetic surfaces and is able to measure the position and size of cracks of a surface with only to perpendicular translations on the magnetic surface. Each localized crack causes magnetic field perturbation, which in turn causes discrete elastic pulses in the MDL. The
Fig. 4. A displacement sensor: (a) The sensing principle and (b) the response of the sensor.
Fig. 5. A force/torque sensor: (a) The sensing principle and (b) the response of the sensor.
have also experimentally tested the amorphous wire using the arrangement of Fig. 1 and we have had an agreement of 97% between the modeling calculations and the experimental results. This fact suggests that we can continue our modeling for the MDL sensor arrangements.
3. Sensor applications
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
391
Fig. 6. A field sensor: (a) The sensing principle and (b) the response of the sensor.
delay time and the amplitude of these voltage outputs suggests the position and size, respectively, of the corresponding cracks.
4. A new magneto-elastic element Using the classic MDL arrangement we faced difficulties concerning the resolution in elastic micro-strain generation. In fact, using coils for elastic pulse generation, we could not obtain discrete elastic pulses closer than 2 times the diameter of the coil. This performance cannot be acceptable in some cases like non-destructive evaluation. Having the motivation of such resolution we have conceived and developed the device illustrated in Fig. 7(a). According to this arrangement, a conducting cylinder is used as the substrate of the new magneto-elastic device, on which we deposit a thin tube of magnetostrictive material. Then, passing pulsed current through the conductor, the magnetostrictive thin tube is excited circumferentially, thus resulting in a circumferential micro-strain along the whole
Fig. 7. The new magneto-elastic device: (a) The device and (b) the operation of the device. Transmitting no current through the conductor results in no net magnetization. Transmitting current larger than a critical value Id able to move domain walls, results in magnetization of the wire. Transmitting pulsed current close to the saturation levels, we have micro-strain generation.
length of the tube. Provided that the material has undergone proper tailoring in order to become magneto-elastically uniform and in the absence of magnetic anomalies along its length, the only propagating micro-strains are those originated at the ends of the magnetostrictive tube. Then, in the presence of magnetic anomalies like field spikes, i.e. due to the presence of cracks on a magnetic surface under test, the magneto-elastic symmetry is broken resulting in discrete elastic pulses propagating along the material. These elastic pulses do not suffer from dispersion problems like in the case of the sensor in Fig. 5(a).
392
E. Hristoforou / Journal of Magnetism and Magnetic Materials 249 (2002) 387–392
A magneto-elastic element has been developed, using 1 mm Cu cylinder as the conducting substrate. An special electro-deposition facility has been developed, with the Cu cylinder used as the cathode and a stainless-steel tube surrounding the Cu tube as the input. Using FeCl3 solution, and pulsed current electro-deposition, we obtained a 1 mm thickness Fe tube around the Cu cylinder. After the deposition process we stress-current annealed the magnetostrictive film under 0.3 A DC current, 500 MPa stress for 10 min. In fact, this very first trial of such magneto-elastic arrangement demonstrated magneto-elastic response. A series of experimental work is under way, in order to perform systematic studies on this new arrangement, aided by our modeling.
References [1] E.O. Doebelin, Measurement Systems: Applications and Design, 4th Edition, McGraw-Hill, New York, 1990. ! [2] J.M. Barandiaran, J. Gutierrez, C. Gomez-Polo, New sensors based on the magnetoelastic resonance of metallic glasses, Sensors Actuators A 81 (2000) 154. [3] A. Hernando, M. Vasquez, J.M. Barandiaran, Metalic glasses and sensing applications, J. Phys. E 21 (1988) 1129. [4] T. Meydan, Application of amorphous materials to sensors, J. Magn. Magn. Mater. 133 (1994) 525. [5] P. Ripka, G. V!ertesy, Sensors based on soft magnetic materials, J. Magn. Magn. Mater. 215–216 (2000) 795. [6] R.L. White, Giant magnetoresistance materials and their potential as read head sensors, IEEE Trans. Magn. 30 (1994) 346.
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