Online Inspection System Inclusion Detection System IDS

Whitepaper
Online Inspection System
Inclusion Detection System (IDS)

CONTENTS
05
Measurement Results
01
Introduction
02
03
Development of
IDS Measuring System
3.1 Sensor Technology
05 - 08
5.1 Laboratory Measurements
16 - 17
5.2 Operational Measurements
18 - 22
5.3 Calibration Coil
23
02
Task
03 - 04
3.2 Structure of the Sensor Block
3.3 Structure of the
Overall System
09 - 11
12- 13
06
Summary
24 - 25
04
Technical
Production Design
14 - 15
2
3

01
Introduction
02
Task
The exact control of production processes
in the manufacture of strip steel and flat
products is crucial for the production of
high-quality products with homogeneous
properties over their complete length.
For verification, it is necessary to determine
various material parameters over the
complete length and not just selectively at
the ends. Non-contact and non-destructive
testing methods are necessary.
The demands placed on steel as a material
are growing continuously due to the
ongoing development of the material and
processing methods. This also applies to
the degree of purity. In the cold strip sector,
even the smallest purity deficiencies or
inclusions have negative effects.
The inclusions remain in the material
despite the subsequent rolling processes
and take on elongated shapes through
deformation. If the inclusion does not reach
the surface of the material, detection by
optical systems is not possible. The relative
magnetic permeability of such inclusions
is significantly lower than that of the
surrounding material. This means detection
During steel production and the subsequent
casting process, inclusions get into
the melt during the metallurgical process.
Some of these non-metallic particles
remain in the products. The quantity,
particle size and spatial distribution of the
inclusions define the purity of the steel.
Non-metallic inclusions in flat steel
products can cause damage to the product
being manufactured and to the tools used
in further processing operations, such as
forming. This report describes the development
and operational results of a detection
system for online inspection of cold-rolled
strip for internal inclusions.
Figure 1: Schematic illustration of the magnetic field profile in the case of an internal inclusion
2
INTRODUCTION
TASK
3

y magnetic flux leakage is possible. The tested material is
magnetised, whereby a homogeneous magnetic field also
results on its surface if its structure is homogeneous.
If there are local areas in the material with significantly
lower permeability, caused by, for example, non-metallic
inclusions, cracks and indentations on the surface, the
magnetic resistance increases at this point. A part of the
magnetic flux is thus forced to the surface of the material.
The difference between the relative permeability of the
material and the surrounding air results in pronounced
magnetic refraction. The extent of the stray field emerging
from the material surface is therefore significantly greater
than the causal defect, which enables its detection.
03
Development of an IDS
Measuring System
3.1 Sensor Technology
The flux leakage detection system developed
for the detection of internal inclusions
offers the functionality of a complete flux
leakage test during ongoing production
operation. Electromagnets are used to
magnetise the material. Their power can
be adapted to the properties, structure and
geometry of the material being inspected
and can be switched off for maintenance
and cleaning purposes. The stray fields are
detected by means of GMR sensors.
The sensors used are GMR differential
sensors (gradiometers). They consist of 4
individual GMR sensors that are connected
in the form of a Wheatstone measuring
bridge. Two of these sensors each are
linked spatially as shown in Figure 2. A
differential signal is formed in dependence
on the difference in magnetic field strength
between the two sensitive areas.
The sensitive areas of the sensors used
have a centre-to-centre distance of 1 mm.
V0
Exit 2
Exit 1
0
Figure 2: GMR differential sensor
4
TASK
DEVELOPMENT OF AN IDS MEASURING SYSTEM | SENSOR TECHNOLOGY
5

The use of gradiometers enables a significantly
higher amplification compared
to absolute sensors (magnetometers) as
external fields have no influence on the
sensor signal. This means that particularly
small local magnetic field inhomogeneities
can be detected.
In the IDS measuring system, the material
is magnetised transversely to rolling direction.
This direction of magnetisation was chosen
on the basis of laboratory measurements
with artificial defects. These defects were
defined as through holes of different
diameters as well as grooves with a length
of 1 mm, a width of 100 µm and variable
depth. The internal inclusions occurring in
cold-rolled strip normally take on elongated
shapes due to the strong deformation.
Grooves, therefore, correspond most
closely to the defects that actually occur.
For holes, which correspond to compact
defects, approximately equivalent results
were obtained with all magnetisation
directions. For grooves, significantly better
signal-to-noise ratios were obtained with
magnetisation transverse to the rolling
direction. This can easily be explained by
the larger defect cross-section in this direction.
Figure 3 and 4 show the signal-to-noise
ratios for a through hole with a diameter of
100 µm and a groove with a depth of 25
µm, a width of 100 µm and a length of
1 mm.
- Magnetization
- Magnetization
Figure 4: SNR of a 100 µm through hole at different magnetisation directions and measuring distances
Magnetisation at a 45° angle to rolling magnetic field homogeneity, the yoke width
direction is not a compromise, as this was chosen so that a 48 mm wide area is
worsens the signal-to-noise ratio for measured in each case.
compact defects compared to parallel Consequently, multiple magnets are necessary
in order to cover the different material
magnetisation, without improving the
signal-to-noise ratio for elongated defects. widths. Since no measurement is possible
in the area of the pole shoes of the
With magnetisation transverse to the rolling magnets, the sensor modules are arranged
direction, the maximum yoke width of the in two rows for continuous coverage.
magnet is limited. Due to a suitable
Figure 3: SNR of a groove 25 µm deep, 100 µm wide and 1 mm long at different magnetisation directions and measuring distances
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DEVELOPMENT OF AN IDS MEASURING SYSTEM | SENSOR TECHNOLOGY
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Figure 5 shows the dependence of the
signal-to-noise ratio on the distance
between the sensor and strip surface.
Starting from a minimum air gap of 200
µm, the ratio decreases almost linearly
by about 1 dB / 100 µm additional air gap as
the distance increases. In practice, an air gap
of 700 µm is sufficient for this application and
resolution.
20 μm groove
40 μm groove
70 μm hole
150 μm hole
03
3.2 Structure of the Sensor Block
One magnet and the sensor line inside each magnet were combined to form a compact
sensor module. This guarantees easy maintenance, repair and scalability of the measuring
system.
Figure 6 shows an opened module with its key components. A module contains the
sensors, the amplification and filtering of the sensor signals, AD converters as well as
the control of the electromagnet and stabilised voltage supplies. The sensor modules are
installed quickly and reproducibly on the measuring system using quick-release clamps
in conjunction with fitting screws. Mechanical adjustment is not necessary thanks to the
precise manufacture and assembly of the individual module components. The GMR sensors
are encapsulated into a sensor block within a protective and stabilising aluminium frame.
This hinders adhesion of dirt and provides protection against mechanical damage.
The sensor modules comply with protection class IP 64, which enables direct use in
harsh environments.
Figure 5: Signal-to-noise ratio of grooves and holes in dependence on the measuring distance
Figure 6: Sensor module
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DEVELOPMENT OF AN IDS MEASURING SYSTEM | SENSOR TECHNOLOGY
DEVELOPMENT OF AN IDS MEASURING SYSTEM | STRUCTURE OF THE SENSOR BLOCK
9

Figure 7: Sensor block open
Figure 8: Sensor block encapsulated
Figure 10: Signal amplitude of a 1 mm x 100 µm x 30 µm groove depending on
its position.
For compact small defects, for example holes, the stray field is optimal for detection
with this sensor arrangement.
Figure 9: Replacement of sensor block
Compared to smaller sensors with higher spatial resolution, the sensors used are more
sensitive. This means that particularly weak signals of small defects can be detected
better. The defects detected by a sensor are determined via the signal amplitude.
The modular design ensures easy replacement
of the sensor block without
adjustment work.
A sensor module has an outer width of 95
mm, which allows two sensor module rows
to cover the material without gaps. Each
sensor module contains 48 GMR differential
sensors in the centre of the magnet.
The distance between the sensors
transversely to rolling direction is 1 mm.
This is advantageous as the size of the
stray fields of the smallest defects is also
above 1 mm. Figure 10 shows the signal
amplitude of a 1 mm long, 100 µm wide
and 30 µ deep groove.
Figure 11: Signal amplitude of different defects depending on their volume
10 DEVELOPMENT OF AN IDS MEASURING SYSTEM | STRUCTURE OF THE SEN-
DEVELOPMENT OF AN IDS MEASURING SYSTEM | STRUCTURE OF THE SENSOR BLOCK
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03
3.3 Structure of the Overall System
The overall system is structured hierarchically. The individual levels work on a task basis
and are interconnected via fast network technology. The sensor signals are converted
from analogue to digital with a sampling rate of up to 187.5 kHz at a resolution of 15 bits.
Length-dependent scanning takes place with a constant longitudinal resolution (rolling
direction) of 0.1 mm.
The digitised sensor signals from up to eight sensor modules are fed to a common
Gig-E hub and converted to the Gig-E camera standard.
The Gig-E hubs are connected to a camera computer. This camera computer has
the following tasks:
Figure 12: Hardware structure
1. Signal pre-processing
2. Detection of defects
3. Feature computation
4. Classification
5. Control and adjustment of the sensor modules
The database server is superimposed on this camera computer. The database server
stores the defect images and contains the production and training database.
Visualisation of the defects as well as the connection to the customer database is
effected via the database server.
Figure 13 shows the signal processing with the detection, classification and
post-processor functions.
Figure 13: Signal processing
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04
Technical Production Design
After extensive laboratory tests and use of a pilot system, a measuring system with 28
sensor modules (maximum material width 1344 mm) was installed and commissioned
in a tinning line in the first half of 2020.
Product data:
Strip thickness
0.1 – 0.6 mm (max. 1 mm)
Strip width
600 – 1,250 mm (measuring width 1,344 mm)
Strip speed
Max. 1000 m/min. at full resolution
Measuring distance > 0.5 mm
Measuring accuracy:
Detectable defect size Hole: 70 µm diameter; groove: 10 µm deep, 100 µm wide,
(substitute defect) 1000 µm long in strip 250 µm thick
The minimum defect size must be determined depending
on the application.
Distance influence 1 dB / 100 µm
Reproducibility >98%
Figure 14 IDS measuring system in operation
The measurement takes place on a deflection
roller with two rows of sensor modules.
The sensor modules can be adjusted to
different material thicknesses with the help
of servo motors.
To ensure mechanical stability, the sensor
module rows and their carriers are kept at
a constant temperature by means of water.
The water cooling also serves to dissipate
the waste heat of the sensor modules.
The measuring system also has a pneumatic
drive to move the sensor rows on
and off the strip surface completely. The
drive is activated automatically for quick
retraction of the measuring system from
the strip surface if there is a risk of collision
with the material being measured.
The distance of the sensor modules to the
strip surface is monitored permanently by
three capacitive distance sensors per sensor
module row. As an additional safety device,
the measuring system has an optical
wrinkle detector. This is a laser light barrier
which is installed in the strip run 10-20 m
in front of the measuring point. In the case
of wrinkles, the sensor rows are swivelled
away.
The measuring system can be moved to
park position outside the system. In this
position, the sensors are adjusted automatically
and maintenance work can be
carried out while the line is running. During
the adjustment, all sensors are standardised
to a defined magnetic sensitivity and
defective sensors are detected.
14 TECHNICAL PRODUCTION DESIGN
TECHNICAL PRODUCTION DESIGN
15

05
Surface defects on the opposite side of the material to the measuring system can also be
detected. Figure 17 shows the raw signal of grooves measuring 1 mm x 100 µm x 25 µm
in 200 µm thick steel strip. The first groove is on the side of the material facing the measuring
system, the second on the opposite side.
Measurement Results
5.1 Laboratory Measurements
The following figures show the raw signals of artificial defects: a hole with a diameter of
100 µm (Figure 15) and a surface groove measuring 1 mm x 100 µm x 30 µm in 200 µm
thick steel strip at a measuring distance of 500 µm and a speed of 500 m/min.
Figure 18: Groove signal profile, top and bottom strip side
Signal curve of a 100 µm
through hole
Signal curve of a 30 µm deep,
100 µm wide and 1 mm long groove
As can be seen in Figure 19, the speed has only little influence on the signal-to-noise
ratio of defect signals.
Signal to noise ratio at variable material speed,
100 μm through hole
Signal curve of a 10 µm deep,
100 µm wide and 1 mm long groove
Speed
Figure 19: Signal profile at different speeds
Figures 15 – 17:
Signal profiles of a 100 μm hole and 10 μm and 30 μm deep,
100 μm wide and 1 mm long grooves
16 MEASUREMENT RESULTS | LABORATORY MEASUREMENTS
MEASUREMENT RESULTS | LABORATORY MEASUREMENTS
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05
5.2 Operational Measurements
The internal inclusions measured are evaluated as follows:
1. Based on defect size (amplitude/volume)
2. Based on classification
e.g. shells, cracks, overlaps, M-defects, scratches, indentations
Figure 21: Large defects such as scratches and wrinkles
Some inclusions detected were located precisely by means of magnetic
particle inspection and examined subsequently by making 3 sections each
transverse to the rolling direction.
Figure 20: Compact defects, various representations
Figure 22: IDS image of defect 1 Figure 23: Magnetic particle image of defect 1
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Figure 29: 1st section, defect 2 Figure 30: 2nd section, defect 2 Figure 31: 3rd section, defect 2
Figure 24: 1st section, defect 1 Figure 25: 2nd section, defect 1 Figure 26: 3rd section, defect 1
Figure 27: IDS image of defect 2 Figure 28: Magnetic particle image of defect 2
Figure 32: IDS image of defect 3
Figure 33: Magnetic particle image of defect 3
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MEASUREMENT RESULTS | LABORATORY MEASUREMENTS
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05
5.3 Calibration Coil
The complete measuring system, including the classification
performance, is checked cyclically by means of a
calibration coil. The calibration coil should contain as
many defects as possible. Cold strip from the production
of transitional slabs is suitable for this. A reproducibility
of >98% was achieved during the cyclical check.
Figure 34: 1st Section, defect 3
Figure 35: 2nd section, defect 3 Figure 36: 3rd section, defect 3
22 MEASUREMENT RESULTS | LABORATORY MEASUREMENTS
MEASUREMENT RESULTS | CALIBRATION COIL
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06
Summary
Using high-quality measuring electronics and
advanced image processing, a high-resolution
measuring system for internal defects
and external material damage has been
developed.
The IDS measuring system is used for complete
evaluation of purity. The maximum strip
thickness at full sensitivity is 1 mm and is
scalable to any strip width. The measuring
system can be adapted to customer specifications.
The image processing with feature computation
and classification used distinguishes the
defects based on their size and type. The
classification is adapted to the respective
material and the customer specification.
The IDS measuring system avoids the delivery
of defective material and ensures perfect
product quality for the end customer. In addition,
the measurement results are used to optimise
the pre-material stages. By improving
quality and output, resources are conserved
and costs reduced.
24 SUMMARY
SUMMARY 25

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Phone: +49 (0) 2056 / 975 – 0
Fax: +49 (0) 2056 / 975 – 140
Mail: info@ims-gmbh.de
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