IEC TR 62331:2005

TECHNICAL IEC
REPORT TR 62331
First edition
2005-02
Pulsed field magnetometry
Reference number
IEC/TR 62331:2005(E)
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TECHNICAL IEC
REPORT TR 62331
First edition
2005-02
Pulsed field magnetometry
 IEC 2005  Copyright - all rights reserved
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– 2 – TR 62331  IEC:2005(E)
CONTENTS
FOREWORD .4
INTRODUCTION .6

1 Scope and object.7
2 Normative references .7
3 Pulsed field magnetometer (PFM).7
3.1 General principles .8
3.2 Size of test specimen .10
4 Field generator.10
4.1 General .10
4.2 Power supply.10
4.3 Magnetizing solenoid.14
5 Polarization and magnetic field strength sensors (pick-up coils) .14
5.1 General .14
5.2 The polarization sensor (J coil).15
5.3 The magnetic field strength sensor (H coil) .16
6 Transient instrumentation and digitizing hardware.16
6.1 General .16
6.2 Analogue integration and digitization .17
6.3 Digitization and numerical integration .17
6.4 Digitization rate .17
7 Data processing .17
7.1 Data processing elements .18
7.2 Temperature.23
7.3 Magnetic viscosity .25
7.4 Calibration.25
8 Comparison of measurements .29
8.1 Permeameter, “large magnet“ comparison.29
8.2 Extraction method, ”small” test specimen comparison .30
8.3 Comparative measurement conclusions .33
9 Conclusion .33

Bibliography .34

Figure 1 – M’ and H time traces for a permanent magnet.9
Figure 2 – J(H) and B(H) loop for a permanent magnet .9
Figure 3 – Sine wave (decaying) electrical configuration .11
Figure 4 – Unidirectional pulses (1/2 sine wave) electrical configuration .12
Figure 5 – Unidirectional pulses (decaying) electrical configuration .12
Figure 6 –Three arrangements of J coil assembly configurations (drawing with
permission of EMAJ [ref. 30] .15
Figure 7 – M and H time traces and Φ(H) plot of a “zero signal” .19
Figure 8 – J(H) loops of a sintered NdFeB permanent magnet .23

TR 62331  IEC:2005(E) – 3 –
Figure 9 – J(H) loop including eddy currents of a conductive bulk nickel specimen
measurement result from a PFM system .27
Figure 10 – Copper specimen eddy current measurement result.27
Figure 11 – J(H) loop for eddy current “corrected” nickel specimen .28
Figure 12 – Results of a permeameter and a PFM measurement of a “large” specimen.29
st nd
Figure 13 – Detail of the 1 and 2 quadrants of the measurement results shown in
Figure 12 “large magnet” .29
Figure 14 – Comparison of a “small magnet” measured in a super-conducting,
extraction method magnetometer (EMM) compared with a PFM measurement result of
the same magnet [28] .30
Figure 15 – Measurement result of a NEOMAX 32EH NdFeB cylinder of diameter
10 mm length 7 mm on the TPM-2-10 system [34] .31
Figure 16 – Measurement result of a NEOMAX 32EH NdFeB cube of dimensions
7 mm × 7 mm × 7 mm [34].32
Figure 17 – Measurement result of a sintered Sm2Co17 cylinder of diameter 10 mm
and length 7 mm [34] .33

Table 1 – Comparison of methods of generating the magnetic field strength .13
Table 2 – Classification of the influences of eddy currents.21
Table 3 – A comparison of values taken from the measurement results presented in
Figure 11 and Figure 12.30
Table 4 – Comparison of values measured in Figure 14 above (see NOTE) .30

– 4 – TR 62331  IEC:2005(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PULSED FIELD MAGNETOMETRY
FOREWORD
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IEC 62331, which is a technical report, has been prepared by IEC technical committee 68:
Magnetic alloys and steels.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
68/299/DTR 68/303/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

TR 62331  IEC:2005(E) – 5 –
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
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A bilingual version of this publication may be issued at a later date.

– 6 – TR 62331  IEC:2005(E)
INTRODUCTION
In order to measure the full magnetic characterization of magnetically hard (permanent
magnet) materials, it is necessary to apply a magnetic field sufficient to saturate the test
specimen of magnetic material.
The generation of this magnetic field can become a practical limiting factor and can
determine the appropriate measurement techniques.
Super-conducting magnets can generate very high static or slowly changing magnetic fields
but their complexity, high capital outlay and running costs, requiring cryogenic gases make
them far from ideal. It is necessary to change fields slowly to avoid “quenching” the super-
conducting magnet.
Conventionally wound electro-magnets with slowly changing magnetic fields have a
significant heat generation problem through I R loss. This can be alleviated through the use
of a high relative permeability “iron yoke”. However, saturation of the iron prevents maximum
characterization of the loop of rare earth permanent magnet materials to be determined.
A pulsed field system utilizing conventional conductors minimizes heating effects by limiting
field durations and by limiting heat generation to acceptable levels. Fields up to 40 Tesla (T)
can be generated in this way.
Careful consideration, however, must be given to the instrumentation and method to take
account of dynamic effects due to the short duration of the magnetic field.
While work on pulsed field magnetometry is carried out in many parts of the world, the two
main groups are MACCHARETEC [ref. 29] in Europe and EMAJ [ref. 30] in Japan. The
approach adopted in Japan is one of supporting a standard with fixed specimen sizes,
magnetic field strengths and frequencies in a limited number of configurations.
———————
References in square brackets refer to the bibliography.

TR 62331  IEC:2005(E) – 7 –
PULSED FIELD MAGNETOMETRY
1 Scope and object
This Technical Report reviews methods for measuring magnetically hard materials using
pulsed field magnetometers.
The methods of measurement of the magnetic properties of magnetically hard materials have
been specified in IEC 60404-5 for closed magnetic circuits and in IEC 60404-7 for open
magnetic circuits. The measurement result of the magnetic properties of magnetically hard
materials at elevated temperatures is given in IEC 61807.
Pulsed field magnetometers have been developed to provide rapid measurement facilities to
match high speed production rates with 100 % quality control.
The object of this report is to describe the principles and practical implications of pulsed field
magnetometry in order to enable the full potential of the technique to be considered,
including its application using small and large magnets of varying geometries, to various
magnetic field strengths and frequencies.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60404-5:1993, Magnetic materials – Part 5: Permanent magnet (magnetically hard)
materials – Methods of measurement of magnetic properties
IEC 60404-7:1982, Magnetic materials – Part 7: Method of measurement of coercivity of
magnetic materials in an open magnetic circuit
IEC 61807:1999, Magnetic properties of magnetically hard materials at elevated
temperatures – Methods of measurement
IEC 60404-14:2002, Magnetic materials – Part 14: Methods of measurement of the magnetic
moment of ferromagnetic material specimen by the withdrawal or rotation method
3 Pulsed field magnetometer (PFM)
A pulsed field magnetometer consists of the following parts:
a) The magnetic field strength generator consisting of
i) the power supply (usually a capacitive discharge system)
ii) magnetizing solenoid
b) Magnetization and magnetic field strength sensors (pick-up coils)
c) Instrumentation for transient processing and digitizing hardware
i) integration
ii) digitization
d) Data processing facilities to enable the processing of

– 8 – TR 62331  IEC:2005(E)
i) zero signal
ii) M(H) loop positioning
iii) self-demagnetization correction
iv) low band pass filtering
v) calibration factors
vi) eddy current correction.
3.1 General principles
The basic principle of operation of the pulsed field magnetometer depends upon an intense
transient magnetic field being generated by the magnetic field strength generator and being
applied to the test specimen to be measured. The magnetic field strength and resultant
magnetization of the test specimen are recorded and processed.
During a measurement cycle, the test specimen in the J coil increases flux. The output
voltage of this coil is the time derivative of the flux Φ coupled to that coil. This flux is due
largely to the magnetization of the specimen but also to the zero signal (see 7.1.1) and
possible eddy currents (see the eddy current correction techniques in 7.1.6) etc. As a
consequence the coil is usually referred to as the “Jcoil,” or on occasions the “M coil.” It is
however, truly a dΦ/dt coil. In this standard it will be referred to as the “J coil.”
In the case of the H coil, the output voltage is the time derivative of the magnetic flux that is
coupled to that coil and is largely the magnetic field strength applied to the specimen. This
coil is usually referred to as the “H coil,” although it is truly a dH/dt coil.
The outputs of these two coils are integrated (see 6.2). In the case of the integrated signal
from the Jcoil, the zero signal is removed and the result calibrated to generate an M’ signal,
that is, the magnetization of the specimen being measured in an open magnetic circuit. By
combining this with the H signal, an M’(H) hysteresis loop is obtained (see Clause 7).
If the M’(H) loop is corrected for the self-demagnetization of the open magnetic circuit
measurement, (see 7.1.3), the intrinsic M(H) or J(H) loop data can be obtained (or B(H) if
required) by the usual conversion.
The two signal channels, that is, from pick-up coil, through integration, digitization and data
collection and processing within the computer, are generally known as the “J” and “H"
channels.
TR 62331  IEC:2005(E) – 9 –
7 000
6 000
5 000
4 000
3 000
2 000
1 000
0,000 0,004 0,008 0,012 0,016 0,020 0,024
Time  s
6 000
5 000
4 000
3 000
2 000
1 000
–1 000
–2 000
–3 000
–4 000
–5 000
0,000 0,004 0,008 0,012 0,016 0,020 0,024
Time  s
IEC  321/05
Figure 1 – M’ and H time traces for a permanent magnet
The lower trace (above) is the time trace of the magnetic field strength (H) based upon the
field generator configuration discussed in 3.2.2.1. The upper trace represents the time trace
of the specimen magnetization; a specimen of sintered Neodymium Iron Boron; data obtained
after initial integration and digitization of the J and H coil outputs, in arbitrary units [ref. 32].
Remenance –:
–1,321 T
7 000
4,000
Remenance +:
6 000
1,321 T
Coercivity –:
5 000
1157 kA/m
3,000
Coercivity +:
4 000
–1157 kA/m
JHMax.:
3 000
1191,51 kJ/m
2,000
2 000
1 000
1,000
Loop ledgend
Neo27095 h.L_jhs
0,000  0,004 0,008 0,012 0,016 0,020 0,024 0,028
Neo27095 h.L_bhs
Time6  s    6    6 6
0,000
6 000
5 000
–1,000
4 000
3 000
2 000
–2,000
1 000
–1 000
–3,000
–2 000
–3 000
–4 000
–4,000
–5 000
0,000
–2 000 –1 500 –1 000 –500 0 500 1 000 1 500 2 000 2 500 3 000 0,004 0,008 0,012 0,016 0,020 0,024 0,028
Applied field  kA/m 0 Time6  s    6    6 6
IEC  322/05
Figure 2 – J(H) and B(H) loop for a permanent magnet

Magnetization, AD counts
J and B  T Applied field, AD counts

Applied field, AD counts Magnetization, AD counts

– 10 – TR 62331  IEC:2005(E)
The complete hysteresis loop is obtained by plotting the J data against the H data shown in
Figure 1, without using the time domain data. The time domain J and H data are again shown
to the right. [ref. 32]. The inner loop represents the B(H) loop.
3.2 Size of test specimen
As the test specimens are measured in an open magnetic circuit, there is no immediate limit
to the size of specimens that can be tested. Small and large test specimens can be measured
providing that eddy current considerations, and the practical considerations of the
instrumentation, are taken into account (see 7.1.6).
The results shown in this report are for cylinders of a maximum dimensions of 30 mm
diameter and 25 mm length and minimum dimensions of 5 mm diameter and 5 mm length,
although this is not a practical limitation for the PFM technique.
Cylindrical test specimens with diameters less than 3 mm and lengths of 3 mm have been
measured while cylinders of NdFeB of 40 mm diameter and 30 mm length have also been
measured.
The Japanese group EMAJ measure test specimens of a cylindrical shape of 10 mm diameter
and 7 mm length and a cube of 7 mm x 7 mm x 7 mm (see Figures 14–16).
4 Field generator
4.1 General
The field generator consists of a system that enables the magnetic field to be applied to the
test specimen.
This will consist of a power supply and a magnetizing solenoid. The power supply provides
the magnetizing current to the magnetizing solenoid in order to generate the applied
magnetic field.
4.2 Power supply
4.2.1 General
Power supplies normally have the capacity to apply an electrical potential (over the range of
400–10 000 V but more typically 1 000–3 000 V) at currents (with a current range of 1 000–
40 000 A but more typically 5 000–20 000 A), in both positive and negative polarities.
This can be accomplished by one of two methods:
a) capacitive discharge;
b) direct mains supply.
4.2.2 Capacitive discharge
The capacitive discharge arrangement enables electrical energy to be accumulated in
capacitors over an extended period of time, before being discharged in a short time period to
provide high currents from the low impedance source.
The energy storage:
E = ½ CU (1)
where
TR 62331  IEC:2005(E) – 11 –
E is the energy, in joules;
C is the capacitance, in farads;
U is the capacitor voltage, in volts.
For commercial PFM measurement systems, it is necessary to minimize costs and it is
therefore, normally necessary to achieve the required magnetic performance with the
minimum of capacitor energy. The capacitance and energy of the capacitive discharge
system is matched with the magnetizing solenoid to provide the required magnetizing
conditions of peak field strength, field volume, field homogeneity and period. The maximum
magnetic field strength achieved is proportional to the current density; the proportionality
factor being dependent on the geometry of the magnetising solenoid.
The discharge can be applied in the following forms:
a) sine wave (decaying);
b) unidirectional pulses (1/2 sine wave);
c) two unidirectional pulses (with decay).
4.2.2.1 Sine wave (decaying)
C
L
IEC  323/05
Figure 3 – Sine wave (decaying) electrical configuration
The current I(t), and therefore the magnetic field strength is determined by:
U
0 −βt
I(t) = .e sinωt  (2)
ωL
1 R
where ω is given by ω = − (3)
LC
4L
and is given by β = R/2L (4)
Due to the resistive losses in the magnetizing solenoid, the peak field strength created in the
magnetizing solenoid in the reverse direction is reduced, depending on the damping factor β.
It is therefore necessary to apply a higher initial field, in order to achieve the necessary
reverse field.
The sine wave technique has the advantage of a continuous process to apply positive and
negative polarities and to avoid discontinuities. This is important in the testing of conductive
materials where eddy current effects are taken into consideration (see 7.1.6).

– 12 – TR 62331  IEC:2005(E)
4.2.2.2 Unidirectional pulses (1/2 sine wave)

C
L
IEC  324/05
Figure 4 – Unidirectional pulses (1/2 sine wave) electrical configuration
The current is determined by:
U
0 −βt
I(t) = .e sinωt (5)
ωL
1 R
where (6)
ω = −
LC
4L
and β = R/2L (7)
However, after the first half sine wave of the current, the reverse charge that is generated
across the capacitors is not permitted to discharge due to the diode characteristic of the
thyristor.
The resultant current waveform is a half sine wave (0–180°).
By applying an identical pulse with a reverse polarity, a maximum positive and negative field
can be applied with identical peak fields of positive and negative polarities.
The overall measurement is accomplished by two separate discharges. This approach does
have the advantage of achieving the same peak fields on positive and negative pulses.
However, two discrete pulses are applied with their inherent discontinuities of current.
4.2.2.3 Unidirectional pulses (decaying)

C
L
IEC  325/05
Figure 5 – Unidirectional pulses (decaying) electrical configuration
The current discharge is determined by:
U
−β(t - t )
I(t) = .e sinωt (8)
ωL
TR 62331  IEC:2005(E) – 13 –
where t is the time at the start of the discharge.
When the capacitor is completely discharged, the diode becomes conducting and prevents
the capacitor from becoming charged reversely. From this point in time the current is
determined by:
−R.(t +t )
L
I(t) = I .e (9)
where
t occurs when U = 0
1 0
I is the current at the instant in time when the clamping diode becomes forward biased.
As with method 4.2.2.2 a complete positive and negative magnetic field strength period can
be achieved by applying two pulses, one of reverse polarity.
While this technique suffers from the necessity of applying two pulses, it also has two very
different dynamic responses on the rising and falling current waveforms, thereby offering a
lower dH/dt during the period after peak field.
Table 1 – Comparison of methods of generating the magnetic field strength
Fixed Identical positive
Single wave
frequency Continuous and negative
function
fields
Sine wave (decaying)
YES YES YES NO
4.2.2.1
Unidirectional ½ wave
YES NO YES YES
4.2.2.2
Unidirectional pulse
NO NO NO YES
(decaying) 4.2.2.3
The preferred approach is the sine wave (decaying) as described in 4.2.2.1, particularly in
consideration of possible eddy currents in conductive specimens as the resulting applied
magnetic field strength is of a continuous waveform in the case of 4.2.2.1 as opposed to the
discontinuities created by the discrete positive and negative periods required by 4.2.2.2 and
4.2.2.3.
It should be noted that the preferred approach of EMAJ [ref. 30] is 4.2.2.2, unidirectional
pulse (1/2) sine wave.
4.2.2.4 The repeatability of the applied voltage
The repeatability of the voltage applied to the capacitor of the capacitor bank is relatively
unimportant provided that the energy is sufficient to saturate the magnet material of the test
specimen. The repeatability is, however, very important for correcting the zero signal. A
repeatability of ±1 % can be easily achieved and is adequate. A repeatability of ±0,1 % is
more typically available.
The magnetic field strength is dependent upon the resistance of the magnetizing solenoid.
The temperature variation of the magnetizing solenoid can have a small influence on the
result and must be considered.

– 14 – TR 62331  IEC:2005(E)
4.2.3 Direct mains supply
Some high field facilities around the world utilize power supplies that are directly coupled to
mains supplies. These systems are able to create high fields (20–40 T) for periods of around
1 s.
Although these facilities offer a valuable resource, they will not be considered in this report.
It is also possible to construct power supplies of this type on a much smaller scale. The
difficulty with this type of equipment is that it is directly coupled to a mains supply and cannot
comply with mains supply power demand regulations, and therefore is not likely to be of great
significance.
4.3 Magnetizing solenoid
The magnetizing solenoid can be considered as a conventional solenoid.
The design of the solenoid must take into account the following factors:
4.3.1 Peak magnetic field strength
The peak value of magnetic field strength must be sufficiently large to saturate the test
specimen in both positive and negative directions. This should include a margin of +10 %
overshoot.
The required magnetic field strength depends upon the material, the orientation and the
demagnetization factor of the test specimen.
4.3.2 Volume to be magnetized
The volume of the magnetizing solenoid must be sufficiently large to enclose the test
specimen and the pick-up coil system.
4.3.3 Field homogeneity across the volume of the specimen under test
The field homogeneity throughout the entire test specimen volume should be within ±1 %.
4.3.4 Field frequency
Rate of change of field with time, dH/dt, should be kept as low as possible either to avoid
inducing significant eddy currents in conductive specimens, or to be suitable for eddy current
correction.
It should be noted that the preferred approach of EMAS [ref. 31]) is to avoid eddy currents.
5 Polarization and magnetic field strength sensors (pick-up coils)
5.1 General
It is necessary to measure correlated values of polarization J of the test specimen and the
magnetic field strength H during that test.
This is usually achieved using pick-up coils. While it is feasible that other forms of detectors
could be used, such as Hall sensors, such alternative detectors are not considered in this
report.
TR 62331  IEC:2005(E) – 15 –
It may be considered reasonable to utilize a single channel system and measure J and H
transients on successive magnetizing pulses. Apply a field pulse while recording J and when
the magnetizing solenoid has regained its original temperature, apply a second (and
hopefully identical) pulse and measure H. This, however, seems an unnecessary
complication and can be a source of procedural error. Two channels and simultaneous
recording of both channels are required.
Pick-up coils can be configured in a variety of geometries, however all have common
features.
Thus, what is needed is, in principle:
a) the polarization sensor (J coil);
b) the magnetic field strength sensor (H coil).
5.2 The polarization sensor (J coil)
The polarization sensor or J coil assembly consists of two (or more) coils connected in series
opposition. The coils have equal area turns products but different coupling to the test
specimen and so measure only the magnetization of the test specimen and not the applied
magnetic field.
(a)            (b)           (c)
IEC  326/05
Figure 6 –Three arrangements of J coil assembly configurations
(drawing with permission of EMAJ [ref. 30]
As discussed earlier, during a measurement cycle, flux is increased by the presence of the
specimen in the J coil assembly. The output voltage of this coil is the time derivative of the
flux Φ coupled to that coil. This flux signal is due largely to the magnetization of the
specimen but also to the zero signal (see 7.1.1) and possible eddy currents (see 7.1.6) etc.
Considering the J coil assembly in Case 6(a), often known as an “n+n coil,” the two
constituent coils have identical cross-sectional area and numbers of turns (equal area turns
product). The coils are connected in opposition so no coil output results from applying a
pulsed homogeneous field (H) to both coils.

– 16 – TR 62331  IEC:2005(E)
When a test specimen is located in one of the two coils, it will be coupled strongly to that coil
(the test specimen coil) and coupled weakly to the neighbouring coil (the compensation coil).
When a pulsed homogeneous field is applied to the coil assembly, the output will be
proportional to the time derivatives of the magnetization of the test specimen (see NOTE
below).
In the case of J coil assembly, Case 6(b) often known as a “½ n+n+½ n coil,” the
compensation coil is divided into two smaller coils at each end of the J coil assembly. These
two coils each have half the area turns product of the specimen coil that is positioned
between them. The compensation coils are connected in opposition to the specimen coil.
The test specimen is located in the specimen coil and is strongly coupled to it, while only
weakly coupled to the compensation coils. Again, the output will be proportional to the time
derivatives of the magnetization of the test specimen.
In the case of J coil assembly, Case 6(c) often known as a “coaxial coil,” the compensation
coil has a much larger cross-sectional area, but correspondingly fewer turns in order to equal
the area turns product of the specimen coil which is positioned coaxially, inside the
compensation coil. The compensation coil is connected in opposition to the specimen coil.
The test specimen is positioned within the specimen coil and is strongly coupled to it. Due to
the weaker coupling of the test specimen to the compensation coil, once again, the output will
be proportional to the time derivatives of the magnetization of the test specimen. Variations
of these pick-up coil geometries and other geometries are possible.
Ideally, the J coil assembly signal should be uniform over the maximum expected test
volume, i.e. with a homogeneous test specimen material, the response signal should be
strictly proportional to the volume of the test specimen.
Such arrangements permit the measurement of magnetization so that the response, in the
ideal case, is not dependent upon test specimen shape or position. However, systems that do
not have such a pick-up homogeneity must be calibrated with a sample of an identical
geometry to that of the test specimen to be measured.
A value of homogeneity of the pick-up coil of ±1 % or better is typical.
NOTE As the area turns of the two coils are never exactly equal, a suitable correction must be provided. This
may prove to be a mechanical adjustment or additional electronic components. In either case, careful
consideration must be made to avoid compromising the integrity of the coil output during a measurement cycle.
5.3 The magnetic field strength sensor (H coil)
The magnetic field strength sensor may consist of one or more, coils which are coupled to the
magnetic field strength in the region of the test specimen, but not significantly coupled to the
test specimen. These coils are often positioned on the same structure as the J coil.
Also as discussed earlier, the H coil output is the time derivative of the magnetic flux that is
coupled to that coil and is largely due to the magnetic field strength. This coil is usually
referred to as the “H coil,” although it is truly a dH/dt coil.
6 Transient instrumentation and digitizing hardware
6.1 General
The outputs of the pick-up coils are proportional to the time derivatives of magnetic flux.
The magnetic field strength pick-up coil (H) the coil output voltage is:

TR 62331  IEC:2005(E) – 17 –
dH
U ∝ (10)
H
dt
The polarization, J pick-up coil output voltage is:
dJ
U ∝ (11)
J
dt
It is necessary to obtain the integrals of these signals in order to obtain J and H signals.
Two approaches may be used for integration:
a) analogue integration and digitization;
b) digitization and numerical integration.
6.2 Analogue integration and digitization
The signals from the pick-up coils are fed to the input of an analogue integrator and are
incorporated. The output of the integrator is then fed to an analogue-to-digital converter
(ADC) and digitized.
The advantage of this approach is that the analogue integrators have a very wide input
dynamic range and can inherently cope with magnetization time derivatives, from a given test
specimen across a wide range of magnetic field strengths.
6.3 Digitization and numerical integration
The signals from the pick-up coils are fed to the input of an analogue-to-digital converter and
are digitized. A numerical approach is then used to integrate the data.
The disadvantage of this approach is that the ADC must have a very wide input range and
higher resolution than in 6.2 when dealing with the dΦ/dt signals.
This approach does not require an analogue integrator although analogue amplifiers or
attenuators may be required.
6.4 Digitization rate
The digitization process must occur on a rate that is high enough to obtain sufficient points
over the minimum data density region, i.e. close to the coercivity of the test specimen.
This will occur when dΦ/dt is at maximum and the rate will be determined by the applied
dH/dt and the characteristic of the test specimen.
A minimum sample rate of 2 × 10 samples/s for a magnetic field strength with sine wave
period of 50 ms is recommended.
The resolution of measurements should be a minimum of 12 bits.
7 Data processing
The H coil output is proportional to dH/dt. In order to process this data to obtain valid H data
(applied magnetic field strength), it is necessary to integrate the signal and apply an H
channel calibration factor. The H channel calibration factor is specific to the H coil and H
channel integrator arrangement of the individual PFM system.

– 18 – TR 62331  IEC:2005(E)
The J channel signal processing involves more steps:
The J pick-up coil output is proportional to dΦ/dt plus the derivative of the zero signal.
(Where Φ is the flux produced by the magnetization of the test specimen.)
This signal is first integrated and the zero signal (see 7.1.1) removed to obtain an M’ signal,
that is, a signal proportional to Φ. The M’ signal is then multiplied by the J channel calibration
factor and divided by the test specimen volume consideration, in order to obtain M*, that is,
the intrinsic open magnetic circuit magnetization of the test specimen material. By applying a
self demagnetization factor to the M* signal the intrinsic J signal can be obtained (see 7.1.3).
The J calibration factor is specific to the J coil and J channel integrator arrangement of the
individual PFM system. (see 7.1.5).
As the initial magnetization of the test specimen is unknown, it may be necessary to position
the M* signal in the M* domain (see 7.1.2).
The synchronized J and H signals can now be combined to obtain a J(H) hysteresis loop. It is
not uncommon to carry out many or all of the J channel processing steps in the J (H) domain
(i.e. the M’(H), M*(H) or J(H) domains).
The B(H) loop can be obtained by the usual conversion.
In the event that eddy currents have occurred in the test specimen during the measurement
process, eddy current correction may need to be considered (see 7.1.6).
7.1 Data processing elements
The following must be taken into consideration when carrying out data processing:
– zero signal;
– loop positioning;
– self-demagnetization;
– filtering;
– calibration factors/scaling;
– eddy currents.
In order to convert the raw data (“J” and “H”) from the measurement, into a J(H) and/or B(H)
data set, it is necessary to process the data with respect to the following elements:
7.1.1 Zero signal
When a measurement is made on a PFM system, without a test specimen, a signal is
observed. For a well designed and constructed system, this signal will be small compared to
the size of the measured test specimen signal, and very repeatable. This signal is generally
known as the “zero signal.”
The zero signal is caused by eddy currents in conductive materials and other effects in the
region of the applied magnetizing field.
This signal can be expected to change in magnitude, and possible shape, for different
magnetic field strengths.
It is necessary to numerically subtract the zero signal obtained from a prior measurement
without a test specimen, from the real measurement data. The amplitude of the zero signal
can limit the minimum size of the test specimen to be measured.

TR 62331  IEC:2005(E) – 19 –
While a zero signal of say 5 % (compared to the overall measurement signal) might be
acceptable, a variation (from measurement to measurement) in the zero signal of, say 10 %,
will offer a single source of error of 0,5 % in the M measurement process.
A resulting error of 0,1 % should be considered acceptable.
0,005 40
0,003
0,000
–0,002
–10
–0,005
–20
–0,007
–30
0,000 0,004 0,008 0,012 0,016 0,020 0,024 0,028
–0,010 Time  s
–0,012
1 000
–0,015
–0,017
–0,020
–1 000
–0,022
–1 000 –750 –500 –250 0 250 500 750 1 000 1 250 1 500 1 750
0,000 0,004 0,008 0,012 0,016 0,020 0,024 0,028
Applied field  kA/m
Time  s
IEC  327/05
NOTE See Figure 10 for an explanation of “apparent polarization”.
Figure 7 – M and H time traces and Φ(H) plot of a “zero signal”
The measurement result is obtained after the integration and digitization of the J and H coil
signals in arbitrary units (counts). The zero signal recorded above (top right) represents a
peak value of ± 30 counts in a system with a ± 8 192 count range. The time trace, bottom
right, represents the magnetic field strength. The loop (left) represents the zero signal in the
Φ(H) domain [ref. 32] .
7.1.2 Loop positioning
As the pick-up coil sensors are AC coupled systems, the DC portion of the measured signals
is unknown. It is necessary to position the M(H) loop within the M(H) domain.
While the magnetic field strength can be expected to start and end at zero, the magnetization
signal can be related to a magnetic specimen of unknown
...