Journal of Magnetism and Magnetic Materials 254–255 (2003) 318–320

A system for measurement of AC Barkhausen noise inelectrical steels

K. Hartmann, A.J. Moses*, T. Meydan

Wolfson Centre for Magnetics Technology, School of Engineering, Cardiff University, P.O. Box 925, Newport Road,

Cardiff CF24 0YF, UK

Abstract

Barkhausen noise (BN) detection provides a powerful method of non-destructive evaluation of material properties.

Most previous investigation of BN has been carried out at frequencies up to a few Hertz, but this paper demonstrates

that measurements of BN can be made rapidly and accurately at power frequencies. Significant differences in BN are

found in non-oriented steel compared to grain-oriented steel.

r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Barkhausen noise; Non-destructive testing; Electrical steel; Noise reduction

1. Introduction

Barkhausen noise (BN) is a phenomenon that occurs

when a ferromagnetic material is subject to an alternat-

ing external magnetic field. It originates when domain

walls are released from microstructural obstructions

such as grain boundaries, inclusions and surface defects

[1]. Much work has been carried out under DC or low-

frequency magnetisation to derive material character-

istics such as grain size [2], hardness [2] or internal stress

[3] from the obtained BN signal but very little has been

reported on the BN spectra occurring in materials at

power frequency in electrical steel, but some work is

reported on structural steels for traditional NDT

applications [4,5]. Since BN originates from the move-

ment of domain walls, it is expected to be sensitive to

magnetising parameters such as magnetising frequency

and flux density. This paper discusses a system for

measuring BN signals in electrical steels magnetised at

up to 50Hz over a range of flux densities.

The BN measuring system (Fig. 1) comprises, a

magnetising system and a signal detection unit. The

magnetising yokes 167mm length, 32mm width, are

assembled from grain-oriented silicon iron and are

connected as shown to the magnetising source so as to

be able to magnetise electrical steel over the frequency

range 12.5–50Hz and at flux densities up to 1.4T. The

feedback circuit ensured that the time variation of flux

density was sinusoidal with a form factor better than

1.1173% over the measurement range. For the non-

oriented material being tested a form factor within the

given tolerance could only be achieved for a flux density

not exceeding 1.2T and a magnetising frequency up to

25Hz which was sufficient to demonstrate the Barkhau-

sen measurement capability.

Two 80 turns search coils connected in series

opposition and wound around a plastic carrier slid over

the sample provided a differential signal feed to a

National Instruments 4552 AD card with a resolution of

16 bit, a sample rate of 204 and 95 kHz bandwidth. The

output 50Hz signal from each coil was of the order of

200mV at 50Hz and comprises a dominant Faraday emf

component and the low-level (50 mV range) Barkhausen

signal, so by connecting in series opposition in this way

the voltage fed to the PC mainly comprised BN

component which is partly decreased due to the

separation (about 1 cm ) of the coils [6].

Digital signal processing has been carried out with

National Instrument’s software package LabView. A

digital fourth-order Butterworth highpass filter with a

cutoff frequency of 600Hz was used to eliminate the*Corresponding author. Fax: +44-(0)2920-876729.

E-mail address: mosesaj@cf.ac.uk (A.J. Moses).

0304-8853/03/$ - 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 8 1 6 - 8

remaining emf component. Highpass filter types such as

Equi-ripple filter, windowed FIR filter and IIR Butter-

worth filter were tested and it was found that the latter

one performed best in filtering out the remaining emf

component. Finally, a threshold of 74mV was used to

remove all environmental noise from the captured signal

in order to obtain the Barkhausen signal. Data obtained

from the AD card has been stored as datapoints within

an array and comprised two cycles of the dB=dt signal.

The absolute values within the array have been added

together. This was done ten times and the measured

amplitude sum (over 20 cycles) is given by

Amplitude sum ¼Xz¼10i¼1

Xm

k¼1

ðjak jÞ

!i

: ð1Þ

In Eq. (1) variable ‘a’ represents the amplitude of a

measured datapoint, index ‘k’ shows its position within

the measured datapoint array ‘m’. Variable ‘z’ indicates

that this measurement has been taken ten times

consequently. Index ‘i’ displays how often the measure-

ment has been carried out. Finally the total sum of

amplitudes have been measured three times and were

compared subsequently.

A low noise AD card was chosen to take the

measurements to minimize the influence of thermal

noise. In order to reduce environmental noise the yokes,

the sample and the search coil carrier were placed in a

grounded steel box. The computer monitor was placed

over a metre away from the measuring system to avoid

its radiation having any influence on the measurements.

In addition, the signal generator and the feedback circuit

were chosen to be battery driven in order to avoid noise

influence from the mains voltage source. Furthermore,

when the AD card was installed in the PC as much space

as possible was left between the card and other devices

and hardware. All connection leads were coaxial cables.

With this system a noise level lower than �130 dB was

achieved.

2. Experimental details and results

Three Epstein sized samples (300mm� 30mm) were

tested, sample 1 (M103–27P) was 0.27mm thick high

permeability grain-oriented silicon steel, sample 2 was

0.27mm thick conventional grain-oriented steel and

sample 3 was 0.20mm thick 0.1% silicon non-oriented

steel.

Results from numerical analysis show that the total

sum of the BN amplitude varies less than 1% (grain-

oriented material) and less than 3% (non-oriented

material) at frequencies lower than 25Hz over a flux

density range from 0.5 to 1.0T. Fig. 2 displays the

results in graphical form, (error bars indicating the

variations have not been included as they would be too

small to be recognised). Numerical analyses also have

shown that at a magnetising frequency of 50Hz and a

flux density of 1.4 T the variation of the total sum of BN

amplitudes is less than 4% in the worst case (sample 1).

Fig. 1. BN measuring system.

Fig. 2. Sum of BN amplitudes versus flux density at 50Hz

magnetising frequency.

Fig. 3. Sum of BN amplitudes versus frequency at a flux

density of 1.0 T.

K. Hartmann et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 318–320 319

In the case of sample 2 the variation of the total sum of

BN amplitudes is less than 1%.

Fig. 3 displays the total sum of the BN amplitudes

versus frequency at a flux density of 1.0 T. The materials

have clearly distinguished characteristics, the non-

oriented material shows a higher total sum of BN

amplitudes than the grain-oriented materials.

A typical BN signal obtained after IIR Butterworth

filtering can be seen in Fig. 4. The sinusoidal waveform

displayed in the graph is the e.m.f. component about

2000 times smaller. It has been shown [7] by direct

domain observation that the number of domain walls

taking part in the magnetisation process is dependent on

the square root of magnetising frequency. Hence, the

increase of BN amplitude with frequency may be due to

the increased number of domain walls and therefore an

increase of interactions between domain walls and

pinning sites. The grain size in sample 1 is on average

higher than that of sample 2, (13mm diameter compared

to 8mm). According to previous research [2] the BN

amplitude increases with increasing grain size. Also the

domain wall separation in sample 1 will be greater than

that in sample 2 because of the larger grain size so at any

given frequency the average domain wall speed will be

greater in sample 1 leading to an expected increase of

BN amplitude.

Here, the BN amplitude is higher for the material with

a smaller grain size. This contradiction suggests that the

BN process is more complex and the results might be

associated with the materials having different densities

of pinning sites, etc., possibly higher in smaller grain

samples [2].

The reason why the non-oriented material (sample 3)

has the highest BN response is difficult to determine

simply from these measurements but again a higher

number of pinning sites is expected. As BN signals are

affected by both, grain size and number and distribution

of inclusions [2], it has to be noted that the grain size is

typically around 100mm in this material.

3. Conclusion

With the described BN measuring system BN

amplitudes greater than 74mV have been investigated

and the repetitive character of BN amplitudes has been

demonstrated in different types of electrical steel.

References

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als, Chapman & Hall, New York, ISBN 0-412-38630-5,

1991.

[2] R. Ranjan, D.C. Jiles, P.K. Rastogi, IEEE Tans. Magn. 23

(1987) 1869.

[3] K. Mandal, M.E. Loukas, A. Corey, D.L. Atherton, J.

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[4] L.B. Sipahi, D.C. Jiles, J. Magn. Magn. Mater. 104–107

(1992) 385.

[5] L.B. Sipahi, D.C. Jiles, D. Chandler, J. Appl. Phys. 73

(1993) 5623.

[6] J.C. McClure, K. Schr .oder, CRC Critical Reviews in Solid

State Sciences, January 1976, p. 45.

[7] T.R. Haller, J.J. Kramer, J. Appl. Phys. 41 (1970) 1036.

Fig. 4. BN signal at a flux density of 0.4 T, 50Hz together with the sinusoidal e.m.f. voltage (displayed 2000 times smaller).

K. Hartmann et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 318–320320