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importance of the electromagnetic test methods used by FOERSTER was
recognized as early as the thirties by the company founder, Friedrich
Förster, and was corroborated scientifically and mathematically
and
incorporated time after time in industry-standard equipment in the
fifties.
The
term"electromagnetic
methods" covers the following areas:
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With the
magneto-inductive method, it is
possible to investigate the core and surface selectively by varying the
frequency. Excitation is either with one single frequency or with
several frequencies (serial or simultaneous), depending on the testing
task.
The hardened passenger-car crankshafts are supplied to the automatic
tester after heat treatment. The MAGNATEST S detects inadmissible
deviations from the required surface hardness by conducting a magneto
inductive test. |
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Magneto-inductive methods
which cover the frequency band from a few Hz to approx.1,000 Hz
conventionally with absolute coils and which are generally used to
determine integral, technological material properties, such as grain
structure, hardness, case-hardening depth and layer thickness.
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Semi-finished
products, such as wires, bars and tubes, are tested for local flaws in
the form of cracks and holes by encircling through-type coils.
The surface of
semi-finished products or components is scanned with scanning probes.
This allows maximum flaw resolution.
Example application: Valve spring wire is tested on a compact testing
line with encircling, through-type coil (DEFECTOMAT) and rotating
scanning probes (CIRCOGRAPH+ROTATING HEAD). The testing line also
contains a demagnetization system and a flaw marking system.
Example application: Two scanning probes scan defined surface zones of
a rotating brake disk.
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Eddy-current methods
methods which conventionally cover the frequency band up to approx. 10
MHz with differential coils and which are generally used for testing
for surface flaws. |
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The
high-energy, alternating-field, leakage-
flux method concentrates the magnetic flux at the material surface and
is thus particularly suitable for detection of small surface flaws
upwards of a depth of 0.1 mm.
Two rotating yokes generate the magnetic flux locally; the integrated
magnetic field sensors detect the leakage flux emerging at the flaws.
CIRCOFLUX testing line at Aichi Steel Works, one of Japan's most
important steel bar manufacturers.
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Magnetic leakage flux probe method
used with DC-field or AC-field magnetization preferably on tubes and
steel bars with round cross-section and hot-rolled surfaces. |
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Object detection with ec (eddy current) method:
The
active detection sensor of the eddy current technology is generating an
electromagnetic field. This causes eddy currents in the object to be
detected. These eddy currents generate a secondary electromagnetic
field which is detected and evaluated by receiver sensor.
Magnetic field interferences caused by Fe objects and detection signals:
The sensor of the magnetometer technology is a passive sensor, this
means no signal is generated by the sensor. The sensor measures field
gradients, generated by ferromagnetic objects in the earths magnetic
field. Dipole evaluation signals are the classical result.
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Metal
detection using the eddy current principle which generates with one or
two frequencies an electromagnetic field. This field is used for the
detection of hidden metal objects, even objects with very low metal
content. All metals can be detected. Object detection with ec (eddy
current) method: The active detection sensor of the eddy current
technology is generating an electromagnetic field. This causes eddy
currents in the object to be detected. These eddy currents generate a
secondary electromagnetic field which is detected and evaluated by the
receiver sensor.
Magnetometer principle Which uses highly sensitive magnetic field
sensors for the measurement of smallest magnetic fields and magnetic
field gradients on earth and in space. Magnetic field interferences
caused by Fe objects and detection signals: The sensor of the
Magnetometer technology is a passive sensor, this means no signal is
generated by the sensor. The sensor measures magnetic fields and
magnetic field gradients, generated by ferromagnetic objects hidden in
the earths magnetic field. Dipole evaluation signals are the classical
results.
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