Instrumentation & Measurement Magazine 25-6 - 6

Fig. 1. Positioning set-up for NDT application. (a) An image of the complete developed set-up; (b) Schematic representation of the positioning system.
operation of two systems both operating with low-frequency
magnetic fields: the positioning system and the ECT system.
The positioning system consists of 8 rx coils operating as
anchors and a tx coil operating as a tag. Both the rx and tx coils
are circular in shape and wrapped in air and are placed on
the same plane (coil plane), connected in parallel to a suitable
capacitor to obtain a resonance condition. The resonant frequency
common to all coils is about 200 kHz. In this way, the
tag powered through a Hewlett Packard 33120ATM
waveform
generator transmits a variable magnetic field at 200 kHz. On
the anchors, an electromotive force is induced and amplified
through an Analog Device AD8421 instrumentation amplifier
connected in parallel to the anchors powered by an AGILENT
E3631ATM
about the anchors' positions and the size of the mobile tag
domain.
To compare the obtained performances between the autopower
supply. Finally, all the amplified voltages are
acquired through a KEYSIGHT U2331A data acquisition unit
and transmitted via USB interface to the PC for processing.
The ECP set-up is composed of: an excitation coil powered
at a 5 kHz frequency sinusoidal wave generated by a FLUKE
397 waveform generator and amplified by KEPCO BOP 20-20
bipolar operational amplifier; and an AAH002-02 Giant Magneto
resistive magnetic sensor connected to an Analog Device
AD620 instrumentation amplifier. Both the magnetic sensor
and the instrumentation amplifier are powered through
an AGILENT E3631ATM
power supply. The adopted magnetic
sensor is characterized by a sensitivity value equal to 150
mV/V/mT and a wide band between 0 and 1 MHz [7].
Fig. 1a and Fig. 1b show a real picture and a schematic representation
of the realized set-up respectively. Fig. 1a shows
the fundamental components of both systems, such as the rx
and tx coils of the positioning system and the ECP. There is
also a metal plate to be used for carrying out ECT tests. In addition,
to avoid the non-linear effect due to the metal plate
on the induced voltages of the rx coils, a distance of 7 cm was
considered between the coil plane and the metal plate. The
mutual influence between the positioning system and the
ECT system is negligible since the two systems work with two
very different frequencies (i.e., 200 kHz for the positioning
system and 5 kHz for the ECT system). Fig. 1b shows a schematic
representation of the positioning system with details
6
matic scanning system and those obtained with the magnetic
positioning system, two distinct ECT tests were performed. In
the first test, the positioning system is kept off and the ECP position
is provided by the automatic scanning system. Instead,
in the second test the magnetic positioning system is turned on
and provides the estimated ECP position. In both tests a metal
plate with a central surface defect of 5 mm length and 0.1 mm
width was considered. The size of the metal plate was 20 cm x
20 cm x 0.2 cm. To perform the tests, the scans were carried out
on a number of points equal to 1271 equally distributed within
the metal plate.
Fig. 2 shows the result of the ECT with the positioning system
switched off. In detail, the figure represents the scan map,
showing on the z axis the Signal to Noise Ratio computed with
reference to the measured field by the magnetic sensor of the
ECT system. The colored area in the center of the map indicates
the clear presence of the defect on the metal plate. It is important
to note that the ECP positions used for the construction of
the map are provided by the automatic scanning system.
Subsequently, the same scan was repeated with the positioning
system turned on. The obtained scan map is shown in
Fig. 2. Obtained ECT map with the positioning system turned off.
IEEE Instrumentation & Measurement Magazine
September 2022

Instrumentation & Measurement Magazine 25-6

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 25-6