Towards Enhanced Eddy Current Testing Array Probes Scalability for Powder Bed Fusion Layer-Wise Imaging

Abstract

This work presents a new eddy current testing array probe and readout electronics that target the layer-wise quality control in powder bed fusion metal additive manufacturing. The proposed design approach brings important benefits to the sensors’ number scalability, exploring alternative sensor elements and minimalist signal generation and demodulation. Small-sized, commercially available surface-mounted technology coils were evaluated as an alternative to usually employed magneto-resistive sensors, demonstrating low cost, design flexibility, and easy integration with the readout electronics. Strategies to minimize the readout electronics were proposed, considering the specific characteristics of the sensors’ signals. An adjustable single phase coherent demodulation scheme is proposed as an alternative to traditional in-phase and quadrature demodulation provided that the signals under measurement showed minimal phase variations. A simplified amplification and demodulation frontend using discrete components was employed together with offset removal, vector amplification, and digitalization implemented within the microcontrollers’ advanced mixed signal peripherals. An array probe with 16 sensor coils and a 5 mm pitch was materialized together with non-multiplexed digital readout electronics, allowing for a sensor frequency of up to 1.5 MHz and digitalization with 12 bits resolution, as well as a 10 kHz sampling rate.

Full text

Citation: Barrancos, A.; Batalha, R.L.;
Rosado, L.S. Towards Enhanced
Eddy Current Testing Array Probes
Scalability for Powder Bed Fusion
Layer-Wise Imaging. Sensors 2023,23,
2711. https://doi.org/10.3390/
s23052711
Received: 10 January 2023
Revised: 24 February 2023
Accepted: 27 February 2023
Published: 1 March 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
sensors
Article
Towards Enhanced Eddy Current Testing Array Probes
Scalability for Powder Bed Fusion Layer-Wise Imaging
AndréBarrancos 1, Rodolfo L. Batalha 2and Luís S. Rosado 1, 3, *
1Instituto de Telecomunicações, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
2Instituto de Soldadura e Qualidade, Avenida Professor Dr. Cavaco Silva, 33 Taguspark,
2740-120 Porto Salvo, Portugal
3Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
*Correspondence: luis.rosado@tecnico.ulisboa.pt
Abstract:
This work presents a new eddy current testing array probe and readout electronics that tar-
get the layer-wise quality control in powder bed fusion metal additive manufacturing. The proposed
design approach brings important benefits to the sensors’ number scalability, exploring alternative
sensor elements and minimalist signal generation and demodulation. Small-sized, commercially
available surface-mounted technology coils were evaluated as an alternative to usually employed
magneto-resistive sensors, demonstrating low cost, design flexibility, and easy integration with the
readout electronics. Strategies to minimize the readout electronics were proposed, considering the
specific characteristics of the sensors’ signals. An adjustable single phase coherent demodulation
scheme is proposed as an alternative to traditional in-phase and quadrature demodulation provided
that the signals under measurement showed minimal phase variations. A simplified amplification
and demodulation frontend using discrete components was employed together with offset removal,
vector amplification, and digitalization implemented within the microcontrollers’ advanced mixed
signal peripherals. An array probe with 16 sensor coils and a 5 mm pitch was materialized together
with non-multiplexed digital readout electronics, allowing for a sensor frequency of up to 1.5 MHz
and digitalization with 12 bits resolution, as well as a 10 kHz sampling rate.
Keywords:
surface-mounted coils; single phase coherent demodulation; eddy current testing; sensors
array; metal additive manufacturing; quality control
1. Introduction
Over the past 30 years, additive manufacturing (AM) has evolved from a prototyping
technology to a method that is increasingly employed in final products. Currently, a
multitude of AM methods allow productions with a vast selection of materials including
metal alloys, polymers, and ceramics [
1
]. In powder bed fusion (PBF), energy from an
electron beam (EB-PBF) or a laser beam (LPBF) selectively melts metallic powder layers
while consolidating the part [
2
]. Resolution is mostly constrained by the powder particle
size distribution, layer thickness, and the energy source spot-size, reaching values below
100
µ
m. The design freedom to create lightweight or highly complex parts is one key
benefit to produce aerospace (e.g., aircraft turbine blades) [
3
] and biomedical (e.g., medical
implants) [
4
] parts. Qualifying PBF for broader application requires enhanced quality
control (QC) [5].
PBF QC relies firstly on the monitoring of consumables (metallic powder and inert
gases) and the fine tuning of the part and the process parameter (e.g., the layer thickness,
the laser beam scanning speed energy density) [
5
]. Even with tight control, PBF process
deviations are prone to cause a broad range of defective conditions [
6
]. Optical sensing
was used to gather signatures and provide layer-wise imaging, which can be used to
search for deviations [
7
–
10
]. In addition to non-optical sensing, acoustic emissions [
11
] and
Sensors 2023,23, 2711. https://doi.org/10.3390/s23052711 https://www.mdpi.com/journal/sensors

Sensors 2023,23, 2711 2 of 14
laser ultrasound [
12
] were also tried. Conventional non-destructive testing (NDT) may be
applied post-production; however, diversified limitations arise [13,14].
The previously discussed in situ sensing provides an indirect and thus limited assess-
ment of defective conditions. In contrast, the electrical conductivity difference between
powder and consolidated metal allows for its direct assessment [
15
]. Contactless layer-wise
imaging was demonstrated using eddy currents testing (ECT) sensor array probes installed
on the machinery recoater [16].
ECT [
17
] is well suited for crack detection [
18
] and to perform local conductivity mea-
surements (related to metallurgical characteristics such as grain size or
porosity) [19–21].
ECT array probes have become popular for the generation of imaging results. With the aim
of increasing spatial resolution, an interleaved, 0.826-mm pitch, coil sensor array [
16
] and a
linear, 125-
µ
m pitch, magneto-resistive sensor array [
15
] were used to gather the imaging
results from PBF-processed layers. Although the spatial resolution achieved in [
15
] copes
with PBF imaging requirements, the overall array covers less than 4 mm, and only a single
electronics readout channel is available to multiplex the 32 available sensors [
22
,
23
]. In a
different implementation, even a low 16:1 multiplexing ratio limited the recoater speed to
1/10 of its nominal value [24].
Considering a reasonable sensor pitch of 0.25 mm, an array probe covering the full
extent of the recoater (which may go up to the 0.5 m) requires thousands of sensors. In a
real manufacturing situation, the PBF machinery recoater is swept with speeds that may
reach 250 mm/s. If measurements are needed every 0.25 mm to maintain a regular spatial
resolution, a full array readout needs to be completed periodically and for every 1 ms.
Therefore, sensor multiplexing is highly constrained (i.e., only low multiplexing ratios can
be considered), and a high number of readout channels is required.
This paper presents new approaches on ECT sensor array probes and strategies for
their electronics readout, targeting improved sensor count scalability and readout speed.
The proposed solutions aim at enhancing one-dimensional ECT array probes with the
sensor pitch and readout speed needed to perform layer-wise imaging while being installed
on the recoater units of PBF machinery.
2. Surface-Mounted Device Coils Benchmark
Mass-produced, commercially available discrete surface-mounted device (SMD) coils
are herein proposed as an alternative to custom-made coils and MR sensors. The main
motivation for this was the associated low cost, design flexibility, and easy integration with
the readout electronics. However, an understanding of whether the provided sensitivity
and tolerances were fit for purpose was required.
An initial survey of the available package sizes, inductance values, and coil types
was carried out. The set of coils in Table 1was selected for evaluation using a preliminary
electronics readout circuit described in Section 3. Bearing in mind the need for substantially
high inductance values, only 0805 package coils were selected for evaluation. Wire-wound
coils with a ferrite or ceramic core and with or without shielding were selected together
with multilayer coils with ferrite substrate. The inductance value 22
µ
H was selected for a
baseline comparison between the different construction options.
Absolute ECT coil measurements are preferred to generate imaging results since they
directly relate with the surface conductivity. To cope with the low impedance sensitivity, an
assessment was performed, employing a compensation coil wired in a bridge-differential
configuration (270
Ω
polarization resistors). The evaluation also included the orientation
of the coils in respect to the tested surface. Each coil was soldered in the placeholder to
remain either parallel or perpendicular to the surface. The frequency was chosen to be
equal to 1 MHz, which results in a standard depth of penetration (the depth at which eddy
currents (ECs) are reduced by 1
⁄
e from that at the surface) of around 100
µ
m for copper and
most of the aluminum alloys and around 400
µ
m for stainless steel 316 (1.32
×
106 S/m,
2.25 IACS, εr≈1).

Sensors 2023,23, 2711 3 of 14
Table 1. Evaluated commercial SMD coils.
Commercial
Reference
Inductance
[µH]
Series Re-
sistance
[Ω]
Tolerance
[%] Shield Type Response
Amplitude [µV]
Response
Span [mm2]
1AISC-0805F-220J-T 22 8.0 5 No Wire-Wound, ferrite core 220 2.27
2 AISC-0805-1R0J-T 1 2.5 5 No Wire-Wound, ceramic Not assessed due to low amplitude response
3 AISC-0805F-680J-T 68 17.5 5 No Wire-Wound, ferrite core 220 1.77
4L0805C220MPWST 22 1.7 20 Yes Wire-Wound, ferrite core 70 2.30
5 L0805C101MPWST 100 7.0 20 Yes Wire-Wound, ferrite core 60 2.49
6L0805C220MSMST 22 1.1 20 Yes Multilayer, ferrite core Not assessed due to low amplitude response
7 L0805C330MSMST 33 1.25 20 Yes Multilayer, ferrite core Not assessed due to low amplitude response
The coils were evaluated on a 0.8 mm diameter, 0.8 mm depth reference hole feature
present in a stainless steel 316 LPBF-produced part, as shown in Figure 1. As the 400
µ
m
depth of penetration is higher than the 60
µ
m thickness of each LPBF layer, the results show
the influence of the surface layer together with its consolidation with preceding layers.
Figure 1.
LPBF-produced part. Left side with a 3D hexagonal lattice and right side with a pattern
with different diameters and depth holes (squared holes in the left-most column).
Table 1summarizes the results of the several tested coils in terms of the signal ampli-
tude and achieved response span. From this analysis, it was concluded that the two best
coils were the wire-wound unshielded coils 1 and 2, with 22
µ
H (AISC 0805F 220J T) and
68
µ
H (AISC-0805F-680J-T), respectively. Figure 2shows the reference hole imaging results
for both coils together with the half-amplitude response contour (white marked) and the
concerning enclosing rectangle dimensions.
Figure 2.
Amplitude imaging results measuring (
a
) coil 1 (22
µ
H) and (
b
) coil 2 (68
µ
H) while
performing a two-dimensional scan over the reference hole of the LPBF-produced part.
The results in Figure 2show that both coils have almost the same sensitivity. However,
coil 3 has a slightly better resolution than coil 1 (notice the response span column in Table 1).
Therefore, coil 3 was chosen to be used on the final array probe. To evaluate the overall
potential of coil 3, the hole patterns of the LPBF-produced part were analyzed. The obtained
results are shown in Figure 3, where it can be observed that most of the holes are easily

Sensors 2023,23, 2711 4 of 14
detected with exception of those with diameters of 0.4 mm and 0.2 mm for depths lower
than 0.4 mm.
Figure 3.
Amplitude imaging result measuring coil 3 (68
µ
H) on a two-dimensional scan over the
several holes of the LPBF-produced part.
The previous results were gathered without the powder presence. As referred earlier,
ECT PBF layer-wise imaging relies on the electrical conductivity difference between powder
and consolidated metal. The powder bulk electrical conductivity depends on several
parameters, such as particle size, compactness, and oxidation. Additionally, because of
the geometrical problem itself, bulk electrical conductivity will vary with frequency. For
the usual ECT frequency ranges, its electrical conductivity is expected to be orders of
magnitude lower than the consolidated material [25].
An additional test was performed to validate the negligible influence of the powder
on the ECT results. A scan along the line L1 in Figure 1was performed with and without
the presence of the stainless steel 316 powder. Figure 4clearly shows that the powder has
an unnoticeable influence over the ECT results. Further results throughout the manuscript
were obtained without the powder’s presence.
Figure 4.
Single-coil probe amplitude response for the holes along Figure 1L1, with and without the
stainless steel 316 powder’s presence.
3. Array Probe and Readout Electronics
The goal for the designed array probe is to demonstrate the scalability improvements
arising from the herein proposed approach. This effort resulted in a sensor and a readout
electronics module, which may be replicated with offset/interleaving to increase coverage
or enhance resolution.
A total of 16 sensors were handled using dedicated signal conditioning and demod-
ulation, and an MSP430 microcontroller was shared per each group of the four sensors,
the architecture of which is shown in Figure 5. Besides the main readout electronics, the
probe also includes power management, a shared oscillator, and a universal serial bus
(USB) interface.

Sensors 2023,23, 2711 5 of 14
Figure 5. Array probe block diagram.
Figure 6details the adopted signal processing hardware architecture where several
blocks are implemented using mixed signal peripherals within an MSP430 microcontroller.
Besides the generation of the excitation voltage, the developed circuit is also responsible
for the amplification and demodulation of the sensors’ signal. Coherent demodulation at a
single phase reference is applied to recover an estimate of the input signal variations. Offset
removal and vector amplification are afterwards used to facilitate the digital acquisition of
these two components.
Figure 6. Detailed signal processing hardware architecture.
The design of the several architecture blocks considered constraints that could compro-
mise the overall scalability, specifically the circuits’ cost, size, and single-supply capability.
3.1. Excitation and Demodulation Reference Generation
To avoid sine waveforms, Direct Digital Synthesis generation, together with linear
amplification square waveforms generated using digital timers/counters, are proposed.
A drawback of this is that the excitation harmonic content may limit the applied sensor
preamplification. Yet, given the limited digital output slew rate and the polarization
resistors, the excitation current harmonic content is partially reduced. Figure 7shows the
excitation square waveform together with the coil voltage and current.
Figure 7. Coil excitation voltage (blue), coil voltage (red), and current (yellow).

Sensors 2023,23, 2711 6 of 14
Both the excitation and demodulation reference square waveform signals are generated
using the MSP430 internal timers clocked at 24 MHz. The ability to adjust the reference
phase with respect to the excitation was accomplished by offsetting when the timers’
count begins.
3.2. Preamplification
Preamplification is applied to the difference between the sensors’ measurement and
compensation coils. Instead of using expensive instrumentation amplifiers, the difference
amplifier shown in Figure 8was employed.
Figure 8. Preamplifier simplified schematic.
The amplifier is built around the GS8051 operational amplifier that was selected
because it provided a 250 MHz gain bandwidth product, single-supply operation, rail-to-
rail output, was low in cost, and had a moderate power consumption of around 3 mA. As a
single supply is used, and because the input signals have a zero mean value, the inputs are
AC coupled. A half-supply offset is added at the GS8051 noninverting input to establish
the output mean value.
As R3kR2=R5and R1=R4, the amplifier’s gain is simply
Vout,AC =R5
R4
(VIN+−VIN−)=50 ×(VIN+−VIN−). (1)
3.3. Adjustable Single Phase Coherent Demodulation
Instead of demodulating within the two orthogonal in-phase and quadrature (IQ) com-
ponents, our demodulation incorporated a single phase that was configured to maximize
the concerning component amplitude. This optimization doubles the number of sensors
handled by each MSP430 microcontroller, with minimum impact on the imaging results.
The experimental results provided in Figure 9demonstrate that the ECT signals arising
from presence and absence of the metal remain mostly aligned on a single-phase direction
of the demodulation plane. This characteristic was somewhat expected since the situation
is close to that of the lift-off from a uniform metal.
To maximize the demodulated component, the phase shift between the excitation
and the demodulation reference is adjusted. Figure 9shows the original IQ demodulation
result signals and the effect of adjusting the phase shift of the single-phase demodulation
reference. A simple adjustment procedure can be used to optimize the phase shift value.
Particularly, the phase shift can be set to 0
◦
and 90
◦
, with the sensor positioned in the
presence and absence of a uniform metal surface. With simple trigonometry, the optimal
phase shift value is obtained. With the MSP430 timer clocked at 24 MHz, 24 different phase
shift values can be set for the 1 MHz operation, which results in a 15◦resolution.

Sensors 2023,23, 2711 7 of 14
Figure 9.
Effect of the phase shift in the signal phase demodulation for the signals across the black
dashed line in Figure 2.
A minimalist mixer was built using a keying transistor controlled by the MSP430-
generated demodulation reference (Figure 10). The preamplifier connects to an N-channel
metal–oxide–semiconductor field-effect transistor (MOSFET) through a series resistor.
As the transistor alternates between cutoff and saturation, the drain signal is either the
unchanged preamplifier signal or the one that is close to zero. Therefore, the transistor
multiplies the preamplifier signal using a binary waveform aligned with the demodulation
reference. A simple first-order, low-pass filter is applied to the transistor drain signal,
providing an estimate of the input signal amplitude variation. Bandwidth is set to around
1 kHz, which is enough to accommodate the ECT signals’ dynamics.
Figure 10. Minimalist demodulator circuit schematics.
Characterization results are shown in Figure 11 where (a) shows the demodulator
input (blue) and the transistor drain node voltage, where the binary multiplication result
can be observed. Figure 11b shows the full demodulator output, which is forwarded to the
MSP430 for further processing.
Figure 11.
(
a
) Demodulator input (blue) and transistor drain (red) signals. (
b
) Demodulated signal
output, continuous component, and ripple.
As shown in Figure 11b, the demodulator output has a substantially high continuous
component together with an approximately 5 mV amplitude ripple. This ripple shows up
as the residual of the mixer reference feedthrough (around 2.5 V square waveform) after
being low pass filtered with a 1 kHz bandwidth.

Sensors 2023,23, 2711 8 of 14
3.4. Offset Removal and Vector Amplification
Before digital acquisition, offset removal and vector amplification are applied to the
demodulated signal. These are implemented using the MSP430 Smart Analog Combo
(SAC) peripherals. Each SAC incorporates a 12-bit Digital to Analog Converter (DAC) and
a low-power Programmable Gain Amplifier (PGA) with a gain of up to
×
33. Each SAC is
programmed to operate in the inverting mode as shown in Figure 12.
Figure 12. MSP430 SAC in inverting mode and interconnections.
Vector amplification is ensured by the available PGA, whose gain is changed through
the resistor ladder that is included in the feedback loop. Gain can be selected between
the values
×
1,
×
2,
×
4,
×
8,
×
16,
×
25, and
×
33, and the gain–bandwidth product is equal
to
1 MHz.
The experimental results confirmed that the SAC input impedance was high
enough (>1 M
Ω
) to avoid excessive loading at the demodulation filter output (whose
impedance is already high).
Offset removal is implemented by the DAC, which adds a continuous voltage to the
PGA noninverting input. Offset removal is adjusted by positioning the ECT probe on a
uniform surface. Then, the DAC output is programmed to approach the SAC output to the
ADC mid-range, 1.25 V. The DAC output resolution is around 1.61 mV, which when it is
multiplied by the 33
×
PGA maximum gain results in steps of around 53.2 mV at the SAC
output. After this initial adjustment, the DAC output remains constant during inspection.
3.5. Digital Acquisition and Processing
The MSP430 Analog to Digital (ADC) can be set to a 12-bit resolution at a 200 kSam-
ples/s maximum sampling rate. Each acquisition is triggered by a timer whose rollover
signal fires the ADC sample and hold circuit. This ensures that sampling remains perfectly
periodic and synchronized with the excitation. The timer was set to rollover every 25
µ
s
and to acquire each of the four demodulated signals in the multiplexed ADC channels at
10 kSamples/s. The internal 2.5 V reference established the ADC unipolar input range.
A moving average filter was programmed to filter the acquired stream and remove
high-frequency components arising from the input harmonic content down conversion
and the mixer ripple (feedthrough of the demodulated frequency). The filter has a window
depth of 32 elements, from which results a bandwidth of slightly higher than 100 Hz.
3.6. Auxiliary Hardware
The circuit is powered with a nominal 12 V supply, although the supply voltage can
be as low as 9 V. Three LM317 linear regulators are used to generate 3.3 V for the MSP430
instances; 5 V is needed for the USB communications, and an independent 5 V supply
is required for the analog readout circuitry. The final system has a current consumption
around 250 mA, as well as a 3W power consumption when powered at 12 V.
As frequency match is needed for the excitation of adjacent sensors, and a single clock
reference must be shared between the MSP430 instances. Otherwise, the potential frequency
mismatch would cause crosstalk interferences, resulting in low-frequency beatings in the
demodulated signals. An external oscillator with a 24 MHz nominal frequency was used as
the shared clock reference.
USB-emulated serial communications were used to connect the MSP430 instances
to a computer. To grant connectivity to the four instances, a FTDI FT4232 mini-module

Sensors 2023,23, 2711 9 of 14
was included. This quad high-speed USB-UART bridge is enough to handle the maxi-
mum bitstreams of 640 kbit/s (4 channels, 10 kSamples/s, 16-bit output) generated at
each MSP430.
3.7. Prototype and Specifications
A prototype of the array probe was built and experimentally demonstrated. A four-
layer PCB with overall dimensions of 80 mm by 80 mm holds all the probe elements
(Figure 13). The readout electronics and the sensors’ array take around 80 mm by 40 mm,
with components on only one PCB side. The achieved specifications are summarized in
Table 2.
Figure 13. Array probe PCB prototype.
Table 2. Array probe-achieved specifications.
Overall size 80 mm ×80 mm
Sensors and readout size 40 mm ×80 mm
Excitation frequency Up to 1.5 MHz
Sensors’ number/pitch 16/5 mm
Preamplification gain ×50
Offset removal resolution/steps 12 bits/1.61 mV
Vector amplification gain ×1, ×2, ×4, ×8, ×16, ×25, ×33
Acquisition rate/resolution 10 kSamples/s/12 bits
Power supply voltage/consumption 9 to 12 V, 3 W
4. Array Probe Readout Firmware/Software
4.1. Firmware
Firmware was developed for the MSP430 to perform the necessary configuration and
communication tasks, including:
-
Change the excitation frequency, demodulation frequency, and phase offset configur-
ing the concerning timers;
- Change the offset removal configuring the DAC output;
- Change the vector amplification gain configuring the PGA;
- Set the acquisition rate and perform moving average filtering;
-
Perform the previous tasks when requested through serial communication commands.
Commands are exchanged with a Graphical User Interface (GUI) using a propri-
etary protocol inspired by High-Level Data Link Control (HDLC) protocols. Only the
strictly necessary functionalities were preserved, i.e., frame delimiters, byte stuffing, and
error detection.

Sensors 2023,23, 2711 10 of 14
4.2. Graphical User Interface
The developed GUI computer application front panel is presented in Figure 14, in-
cluding controls to configure the operational parameters such as the excitation frequency
and the vector amplification gain. There are also two dedicated buttons, “Calibration”
and “Compensation”, used to trigger the demodulation phase-shift adjustment and the
offset removal algorithms, respectively. Besides the control functionality, the GUI allows
visualization and saves the results in MATLAB format.
Figure 14. LabVIEW GUI computer application.
Functionality was added to control a mechanized XY table used to move the array
probe over the surface being inspected. Automatic scans are performed after the user sets
the X and Y span and resolution, while 2D intensity charts are continuously updated with
the received data. It is also possible to setup the original probe positioning using dedicated
arrow keys.
5. Experimental Results
The array probe was attached to an available XY table to emulate the LPBF machinery
recoater movement, Figure 15. This table includes two stepper motors to provide motion
along the X and Y directions with resolutions down to 50
µ
m. The vertical along Z probe
position is adjusted manually.
Figure 15. XY table with the array probe over a sample.
The holes pattern of the 316 stainless steel part produced using LPBF was analyzed
using the array probe PCB prototype. The probe lift-off was set to around 0.4 mm and, to
avoid the PCB from scraping the tested part of the surface, it was moved at around 1 mm/s,
allowing it to store one sample every 0.1 mm. As described Section 3, the coils’ excitation

Sensors 2023,23, 2711 11 of 14
voltage is a square waveform with around a 3.3 V amplitude and a 1 MHz frequency fed
through the 270 Ωresistors’ bridge.
The obtained results are shown in Figure 16 and reveal the presence of all the hole
defects whose diameter is greater or equal to 0.8 mm. The result in Figure 16 also highlights
some mismatch in the sensitivity of each sensor channel, which can be observed in the
horizontal lines every 5 mm.
Figure 16.
Amplitude imaging result measuring the array probe on a two-dimensional scan over the
several holes of the LPBF-produced part.
Compared with the results on Figure 3, where the same surface was scanned with
the single-coil probe, a clear signal degradation is observed. Besides the impact of the
previously mentioned channel mismatch, the probe lift-off was substantially higher while
using the full array probe. In fact, estimates indicate around 0.15 mm with the single-coil
probe and 0.4 mm with the array probe. This higher lift-off was necessary to avoid the probe-
wide PCB from interfering with structures on the other zones of the LPBF-produced part.
The reproduced lattice surface was also inspected, generating the results in
Figure 17,
where the inspected surface is overlayed with the imaging result. The black zones represent the
surface layer, whereas the gray zones represent the lattice faces for
lower-positioned layers.
Figure 17.
Amplitude imaging result measuring the array probe on a two-dimensional scan over the
3D hexagonal lattice surface of the LPBF-produced part.
6. Conclusions
New approaches towards the ECT array probes and readout electronics’ scalability
targeting the layer-wise PBF quality control were presented.
Commercially available surface-mounted coils were used as sensor elements to achieve
a low cost, some design flexibility, and automatized assembly with the readout electronics.
Several coil models were tested for optimized sensitivity while preserving the high spatial
resolution. From this effort, it was concluded that wire-wound, ferrite-cored inductors are
effectively a valid ECT sensor implementation option.
The observed sensor characteristics allowed for a significant reduction in the de-
modulation circuit, employing an adjustable, single-phase demodulation scheme with a

Sensors 2023,23, 2711 12 of 14
negligible impact over the imaging result. Minimalist signal generation and demodulation
circuits were implemented, combining external discrete electronics with microcontrollers’
advanced mixed signal peripherals. Scalability adhered to specifications relating to the
circuits cost, size, and ability to operate on a single supply.
The results demonstrated the probe’s ability to assess conductivity changes on a real
LPBF-produced part. However, due to the hand soldering of the sensor coils, a significant
sensitivity mismatch between the channels was verified.
A summary of the different literature-reported approaches and specifications is pro-
vided in Table 3. The purpose of the developed probe was to demonstrate the scalability
improvements from the discussed design optimizations, considering the gathering ECT
data at a nominal recoater speed and the minimum impact on the PBF machinery operation.
Table 3. Comparison with other works.
Sensor Type
Sensor Number
Sensor Pitch [mm]
Array Coverage [mm]
Multiplexing Ratio
ECT Frequency [MHz]
ADC Input
Frequency [kHz]
ADC Resolution
Sampling Rate
[kSamples/s]
Bandwidth [Hz]
Max. Recoater Speed
[mm/s]
Approach/Remarks
[16]Custom
coils 32 0.826
26.4
32:1 ≤5 - - - - - Commercial multiplexer module and
lock-in amplifier.
[15]MR
sensors 32 0.125 4 32:1 1 20 - - - -
Heterodyning MR measurements,
dedicated CMOS multiplexing and
signal conditioning, commercial
lock-in amplifier demodulation.
[24]MR
sensors 128 0.125 16 16:1 1 20 18 500 - 25
Heterodyning MR measurements,
discrete multiplexing and signal
conditioning, offline FFT processing of
20 kHz IF samples.
This
work
SMD
coils 16 5 80 1:1 1 DC 12 10 100 250
Includes 12 bits offset removal and
vector amplification for enhanced used
ADC dynamic range. Single phase
demodulation.
Although this work has not showed a large ECT array probe, the achieved probe and
its electronic specifications achieve the necessary acquisition speed for true online PBF layer-
wise imaging. In future experiments, several instances of the probe will be instantiated
interleaved to increase the spatial resolution. The several microcontrollers’ instances will
be controlled/read through SPI using a field-programmable gate array (FPGA), which will
gather and transmit the data to the personal computer.
Future work will focus on assessing the repeatability of the produced measurements
subject to thermal variations and to the change of other operational conditions as the
probe lift-off and amplification gains. Other future improvements were also identified.
The minimalist mixer with the keying transistor exhibits high reference feedthrough. For
the required high operation frequencies, the proceeding filtering strongly attenuates the
feedthrough component. Nevertheless, a residual feedthrough component was verified
after the acquisition, and an alternative mixer is therefore desired. The sensors matching
will be improved using automatic placement and oven soldering. A great improvement is
expected to enhance the overall channels matching at around 5%. Soldering components
on the two sides of the board will allow for an improvement in resolution by a factor of
two, with no architectural modifications. A second generation of the probe will benefit
from higher performance microcontrollers to increase the number of channels acquired
per instance.

Sensors 2023,23, 2711 13 of 14
Author Contributions:
Conceptualization, L.S.R.; methodology, A.B. and L.S.R.; software, A.B. and
L.S.R.; validation, A.B., R.L.B. and L.S.R.; formal analysis, A.B. and L.S.R.; investigation, A.B. and
L.S.R.; resources, A.B., R.L.B. and L.S.R.; data curation, A.B. and L.S.R.; writing—original draft
preparation, A.B. and L.S.R.; writing—review and editing, A.B., R.L.B. and L.S.R.; visualization, A.B.,
R.L.B. and L.S.R.; supervision, L.S.R.; project administration, L.S.R.; funding acquisition, L.S.R. All
authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by FCT/MCTES through national funds and when applicable
co-funded EU funds under the project UIDB/50008/2020 and grant EXPL/EEI-EEE/0394/2021.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data presented in this study is not publicly available at this time
but may be obtained from the authors upon reasonable request.
Conflicts of Interest: The authors declare no conflict of interest.
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