Sensors and Actuators A 113 (2004) 270–276

The possibility of utilizing the high permeability magnetic materials in construction of magnetoelastic stress and force sensors Adam Bie´nkowski a,∗ , Roman Szewczyk b a

Institute of Metrology and Measuring Systems, Warsaw University of Technology, Sw. A. Boboli 8, 20-525 Warsaw, Poland b Industrial Research Institute for Automation and Measurements, Al. Jerozolimskie 202, 02-486 Warsaw, Poland Received 1 August 2003; received in revised form 5 January 2004; accepted 9 January 2004 Available online 20 February 2004

Abstract The paper presents new possibilities in the field of construction of the magnetoelastic stress and force sensors. Two methods of applying mechanical stress to the core of the magnetoelastic sensing elements were presented. The first one is suitable for bulk magnetic materials, such as ferrites. The second method can be utilized in a case of both ribbon and bulk ring cores. Such ring-shaped cores can be made of amorphous alloys as well as ferrites. Both these methods enable achieving uniform distribution of stresses in the samples with closed magnetic circuit. The experiment was performed on high permeability Mn–Zn ferrite and Co-rich amorphous alloy. The results confirm, that magnetic properties of both ferrite and amorphous alloys change significantly under compressive stress from the external forces. The relative changes of the permeability exceed 100% for stresses up to 25 MPa. Paper presents also functional characteristics of the magnetoelastic sensor which operate in the resonant circuit configuration. Output frequency signal from the sensor operating in this configuration is suitable for further digital processing. © 2004 Elsevier B.V. All rights reserved. PACS: 75.50 Kj; 75.50 Gg; 75.80 +q Keywords: Magnetoelastic sensors; Ferrites; Amorphous alloys

1. Introduction Magnetoelastic sensors based on the classical crystalline materials (such as Fe–Ni–Co alloys) were successfully used in the field of industrial applications [1]. But possibilities of a new applications of these, classical materials were significantly depleted. On the other hand, process of development of new magnetic materials for cores of inductive components, such as high permeability ferrites and amorphous alloys, can give fresh impetus [2,3] for construction of magnetoelastic sensors for mechatronic applications. For this reason, it is necessary to find new solution for magnetoelastic stress sensors, both in the field of sensing core materials as well as methods of applying stresses to the sensing elements. Due to their mechanical and magnetic properties both ferrites as well as amorphous alloys seems to be suitable for construction of force and stress sensors [4,5]. For this reason results of the comparative investigation of magnetoelastic ∗

Corresponding author. Tel.: +48-22-660-8551; fax: +48-22-849-0395. E-mail address: [email protected] (A. Bie´nkowski). 0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.01.010

properties of these materials was carried out. Results of this experiment may be helpful for constructors of magnetoelastic sensors, who have to chose proper sensing core material. On the other hand, in the process of the development of the material for the magnetoelastic sensing element the physical aspects of the magnetoelastic Villari effect must be taken into consideration.

2. Physical aspects of the magnetoelastic Villari effect Magnetoelastic Villari effect is connected with the changing of the total free energy of the magnetic material under the influence of stresses caused by external forces. The total free energy E of a magnetized sample may be presented as a sum of the individual free energies [6]: E = EH + ED + ER + Eσ + EW ,

(1)

where EH is the energy of the magnetizing field H, ED is the energy of demagnetization of the sample, ER is the random anisotropy energy, Eσ is the stress-induced anisotropy energy

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(magnetoelastic energy) and EW is exchange energy. The stress induced anisotropy energy Eσ is given by [7]: Eσ = 23 λs σ sin2 φ = Kσ sin2 φ,

(2)

where λs is the saturation magnetostriction and φ is the angle between magnetization Ms and the direction of the stress σ. From practical point of view, the magnetoelastic effect can be observed as changes of the flux density B as well as permeability µa (for given magnetizing field Hm ) under influence of mechanical stresses σ. The higher values of these changes can be observed in a case when participation of magnetoelastic energy in the total free energy of material is more significant. For this reason, material with as low as possible magnetocrystaline anisotropy should be used in construction of the magnetoelastic sensing elements [8]. Moreover, due to minimization of the demagnetization energy, samples with closed magnetic circuit should be used. For positive sign of λs σ factor, the magnetoelastic energy Eσ is minimal when Ms and σ are parallel and increases to a maximum of (3/2)λs σ when Ms and σ are at right angles. If this quantity is negative, the minimum of energy occurs when Ms and σ are at right angles. So the way in which a material responds to stress depends only on the sign of the product λs and σ (where compressive stress is noted as −σ, and tensile stress as +σ). The axis of stress is an easy axis if λs σ is positive. If λs σ is negative the stress axis is the hard axis and the plane perpendicular to the stress axis is an easy plane of magnetization [9]. Presented physical aspects of the magnetoelastic Villari effect determine the main requirements for both magnetic material for the sensing element as well as a construction of the force converter which enable applying of the mechanical stresses to the magnetoelastic sensing element. For this reason, the physical aspects of magnetoelastic Villari effect must be taken into consideration by constructors of the magnetoelastic sensors.

3. Method of applying stresses to the core of magnetoelastic sensor The method of applying the mechanical stresses should enable achieving of the uniform distribution of stresses in the magnetoelastic sensing element. If the distribution of stresses is non-uniform the highly stressed areas are present in the core. For this reason the range of stresses in which magnetoelastic sensor can operate is significantly limited [10]. Moreover, sensing core should have closed magnetic circuit. If the magnetic circuit is open, the demagnetization energy occur in the total free energy of the material. This demagnetization energy causes lower participation of the magnetoelastic energy in the total free energy. As a result, demagnetization energy causes significant decrease of the stress sensitivity of the magnetoelastic sensor.

Fig. 1. Method of applying the compressive stress to the frame-shaped magnetoelastic sensing element made of bulk magnetic material (such as ferrite).

3.1. Frame-shaped core method In a case of sensing elements made of bulk magnetic materials, such as ferrites, the frame-shaped core method can be used [11]. The general idea of applying of the compressive force F to the frame-shaped sensing element is presented in Fig. 1. The frame-shaped core provides the closed magnetic circuit and enables compressive force to be applied. Due to the special nonmagnetic backings, the compressive stresses in the core’s columns could be applied in the range up to 60 MPa. In order to determine the distribution of stresses in the core, photo-stress investigations were carried out [11]. These results confirm that in the core’s columns the distribution of stresses is uniform. On the frame-shaped core (presented in Fig. 1) both magnetizing and detecting winding were made. These winding enable measurements of the changes of the magnetic hysteresis loop B(H) under the influence of the compressive stresses −σ. Changes of the magnetic permeability µa under stresses −σ, as well as magnetoelastic characteristic B(−σ H ) can be also measured with this method. 3.2. Ring-shaped cores method In the ring-shaped cores method the compressive force F is applied to the ring-shaped sensing element perpendicularly to the magnetizing field H, as it is presented in Fig. 2. In such a case uniform, compressive stress σ can be reached on the all length of the magnetic circuit of the sensing element [12]. Moreover, commercially available, ring-shaped cores can be used as the magnetoelastic stress and force sensors. The special device for applying stresses to the ring core [13] is presented in Fig. 3. Base backings (1) allow a ring

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4. Sensing cores materials

Fig. 2. General idea of the method of applying the compressive stress to the ring-shaped stress sensor.

Fig. 3. Device for applying the compressive stress to the ring-shaped sensing element (patent pending P-345758). (1) Base backing, (2) nonmagnetic cylindrical backing, (2a) grooves for windings, (3) magnetoelastic sensing element.

core (3) to be subjected of the compressive force F. Due to the special, nonmagnetic cylindrical backing (2) the distribution of stresses in the core is uniform. Measuring and magnetizing windings are placed in grooves (2a) at the cylindrical backings (2). In this method of applying stresses both of bulk material rings and ribbon ring cores can be investigated. Because of uniform distribution of stresses in the sample the local high stresses are absent. For this reason also a brittleness materials can be investigated. Presented method creates the new possibilities in the field of both construction of magnetoelastic stress and force sensors, as well as investigation of the influence of stresses on the functional properties of the cores of the inductive components of mechatronic devices.

400

Mn0.51Zn0.44Fe2.05O4

As it was mentioned above as a cores of the magnetoelastic sensors high permeability materials should be used. For this reason the investigation was carried out on the high permeability Mn–Zn ferrite as well as Co-based amorphous alloy. Due to their advantages as high electric resistivity, high compressive stress strength and rust resistance the ferrites are useful material for magnetoelastic sensing element [5]. Low magnetocrystalline anisotropy of high permeability of the Mn–Zn ferrites (such as Mn0.51 Zn0.44 Fe2.05 O4 ) make them especially suitable for construction of the magnetoelastic stress sensors. On the other hand, due to their unique mechanic properties, amorphous alloys (such as Co68 Fe4 B13 Si13.5 Mo1.5 ) are also material suitable for construction of the cores of stress sensors [4]. Moreover, amorphous alloys do not have crystalline structure and as a result they do not explicit crystalline anisotropy. For this reason amorphous materials have very high permeability and creates very promising possibilities in the construction of magnetoelastic force and stress sensors.

5. Results 5.1. Sensors with cores based on the high permeability ferrites For construction of the model sensor based on the high permeability ferrite frame-shaped core was used. The influence of compressive stresses σ on the magnetic hysteresis loop B(H) of the frame-shape sensing element is presented in Fig. 4. The compressive stresses σ produce the decrease

B (mT)

σ = 0 MPa -4 MPa -10 MPa

300 200

-25 MPa -45 MPa - 60 MPa

100 0 -80

-60

-40

-20

0

20

40

60

80

-100 H (A/m) -200 -300 -400

Fig. 4. Influence of compressive stress σ on the magnetic hysteresis loop of the Mn0.51 Zn0.44 Fe2.05 O4 ferrite (frame-shaped sensing element).

273 1.2

1

µ(σ )/µ(σ = 0)

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Mn0.51Zn0.44Fe2.05O4 0.8

0.6 Hm = 80 A/m 8 A/m

40 A/m 12 A/m

24 A/m

0.4

16 A/m

0.2

0 -60

-50

-40

-30

-20

-10

0

compressive stress σ (MPa)

Fig. 5. The influence of compressive stress σ on the relative permeability µ(σ)/µ(σ = 0) of the Mn0.51 Zn0.44 Fe2.05 O4 ferrite, for constant values of magnetizing field Hm .

This phenomenon is caused by the low value of magnetizing energy for small magnetizing field Hm . As a result, in a case of low magnetizing field, participation of magnetoelastic energy in total free energy of the sensing element is more significant. Presented changes of relative permeability µ(σ)/µ(σ = 0) as a function of the compressive stress (Fig. 5) describe

Hm = 20 A/m

1.7 Co68Fe4B13Si13.5Mo1.5 in as-quenched state

25 A/m

1.6 1.5

µa(σ )/µa( σ=0)

of the flux density B and considerable change of the form of the hysteresis loops. In Fig. 5 the influence of the compressive stress σ on the value of relative magnetic permeability µa (σ)/µa (σ = 0) is presented fore different value of magnetizing field Hm . The most significant decreasing of permeability (up to 90%) were observed for the lowest value of magnetizing field Hm .

1.4 50 A/m

1.3

75 A/m 125 A/m

1.2

250 A/m

1.1 1 0.9 -25

-20

-15

-10

-5

0

compressive stress σ (MPa)

Fig. 6. The influence of compressive stress σ on the relative permeability µ(σ)/µ(σ = 0) of the of the Co68 Fe4 B13 Si13.5 Mo1.5 amorphous as-quenched alloy, for constant values of magnetizing field Hm .

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Hm = 1.2 A/m Co68Fe4B13Si13.5Mo1.5 in annealed state

2

µa(σ )/(µa(σ =0))

2.5

1.5 A/m

2 A/m

1.5

3 A/m 5 A/m 1

10 A/m

0.5 -25

-20

-15

-10

-5

0

compressive stress σ (MPa)

Fig. 7. The influence of compressive stress σ on the relative permeability µa (σ)/µa (σ = 0) of the of the Co68 Fe4 B13 Si13.5 Mo1.5 amorphous thermally relaxed alloy, for constant values of magnetizing field Hm .

Co68Fe4B13Si13.5Mo1.5 in as-quenched state

6800

f (kHz)

7200

6400 6000 5600

1800

Co68Fe4B13Si13.5Mo1.5 in annealed state

1400

1000

-25

-20

-15

-10

-5

0

compressive stress σ (MPa)

Fig. 8. The functional characteristics of the magnetoelastic sensors with ring-shaped cores made of Co68 Fe4 B13 Si13.5 Mo1.5 amorphous alloy in as-quenched and annealed state (cores were operating in the resonant circuits).

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magnetoelastic, functional properties of the sensing element. These properties determine character and value of output signal from the magnetoelastic sensor. 5.2. Sensors with cores based on the amorphous alloys In a case of magnetoelastic sensing element made of amorphous alloy, the most suitable method of applying stresses in utilization of the ring-shaped cores. In Fig. 6 the influence of compressive stress σ up to 25 MPa on the relative permeability of Co-based amorphous alloy (Co68 Fe4 B13 Si13.5 Mo1.5 ) in as quenched state is presented. In this case, relative permeability increases about 60% for compressive stresses up to 25 MPa. This increase is approximately linear. In a case amorphous alloys (as in a case of ferrites) higher stress induced changes of permeability were observed for lower values of magnetizing field. In Fig. 7 the relative changes of the permeability of the sensing element made of the Co68 Fe4 B13 Si13.5 Mo1.5 amorphous alloy after the thermal relaxation is presented. The thermal relaxation was carried out by the annealing in the temperature 360 ◦ C for 1 h. Under compressive stress up to 25 MPa permeability increase up to 100%. In this case the changes of the relative permeability are higher than changes for as quenched alloy. On the other hand, changes of permeability for annealed alloy is non-linear. The functional characteristics of the magnetoelastic sensor with ring-shaped cores made of Co68 Fe4 B13 Si13.5 Mo1.5 amorphous alloy, which operate in the resonant circuit configuration are presented in Fig. 8. For stresses up to 25 MPa the output frequency of the sensor with annealed sensing core changes 537 Hz. The repeatability of indications is equal 3 Hz. For the same range of stresses and chemical composition of amorphous alloy, but for the sensor with annealed core, output frequency changes 475 Hz. In this case the repeatability of indication is equal 1 Hz. It should be highlighted that for sensing element in as-quenched state significant magnetomechanical hysteresis was detected. On the other hand, sensor with annealed core has magnetoelastic hysteresis lower than repeatability of indication. For this reason thermally relaxed amorphous Co68 Fe4 B13 Si13.5 Mo1.5 alloy is significantly more suitable for construction of magnetoelastic stress sensors.

6. Conclusion Presented results confirmed that high permeability magnetic materials creates new possibilities of practical applications of magnetoelastic stress and force sensors. Described two methods of applying the compressive force to the magnetoelastic sensing elements enable achieving the uniform distribution of stresses at the all length of the magnetic circuit of the magnetoelastic sensing element. In a case of high permeability Mn–Zn ferrites the most suitable is frame-shaped core method. For the amorphous alloys, the

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ring-shaped core method is more useful for construction of magnetoelastic sensing element. Moreover, presented new method creates possibilities of utilizing amorphous alloys as compressive stress sensors. These possibilities of utilization of Co-based amorphous alloys were not presented before. Presented results of the investigation on the influence of the compressive stress on the magnetic properties of high permeability materials indicates that changes of output signal from the magnetoelastic sensors (caused by relative changes of magnetic permeability) are significantly higher than changes of output signal from strain gauge sensors (caused by relative changes of resistance). Moreover, in a case of magnetoelastic sensors, sensing element can be the construction component, and the range of measured force can be adjusted by the dimensions of magnetoelastic core. In the case of annealed, Co-based amorphous alloy magnetoelastic hysteresis is lower than repeatability of indications. Changes of the permeability under compressive stresses enable development of the magnetoelastic sensor which operate in the resonant circuit configuration. Output frequency signal from such sensor is especially suitable for further digital processing.

Acknowledgements This work was supported by Polish State Committee for Scientific Research under grant realized in the years 2002/2005.

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Biographies Adam Bie´nkowski , born in 1937, graduated from the Faculty of Electronics Engineering and Information Technology of the Warsaw University of Technology in 1962. Received his PhD degree and DSc degree in Metrology at the Warsaw University of Technology in 1977 and 1997, respectively. Since 1962 he has been working at Warsaw University of Technology at various positions, Assistant (1962), Lecturer (1969), Adjunct (1977), Professor (2001). He worked at Istituto Materiali Speciali per Elettrinica e Magnetismo MASPEC, Parma (1985). His research interests include metrology of soft magnetic materials chiefly ferrites and amor-

phous, including magnetostrictive and magnetoelastic properties, mechanisms governing magnetization processes under stresses and magnetoelastic Villari effect and possibility of its utilizing for the constructions stress and forces sensors. Recently, the development of the generalization, in whole range of elastic stresses (compressive to tensile) of relationship B(±σ) concerning to the Villari effect with reference to all soft magnetic materials. He is an active Member of New York Academy of Sciences (1995). He has supervised some projects of Polish Committee of Scientific Research. Roman Szewczyk, born in 1976, Warsaw (Poland), graduated from the Faculty of Mechatronics of the Warsaw University of Technology in 2000, in metrology and quality engineering. Received his PhD degree at the same Faculty of Warsaw University of Technology in 2003. His professional interests concern the investigation of the magnetomechanical properties of newly development magnetic materials especially amorphous and nanocrystalline alloys as stress and force sensors. He is also involved in telemetry systems in industrial automation applications.