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Flexible PCB-Based Eddy Current Array
Probes for the Inspection of Turbine
Components
Andre Lamarre - OlympusNDT-Quebec City Canada
Benoit Lepage - OlympusNDT-Quebec City-Canada
Tommy Bourgelas - OlympusNDT-Quebec City-Canada
4th International CANDU In-service Inspection Workshop and NDT in Canada 2012, June 2012, Toronto
• Eddy current is a reliable non-destructive inspection method for turbine components. The use of eddy current arrays (ECA) is already known to increase the inspection speed and the reliability of the inspection process.
• One practical limitation of the ECA technology is each ECA probes have to conform to the shape of the inspected components making its use expensive and limited.
• Recently, Olympus NDT has overcome these limitations by introducing the flexible PCB-based ECA probe technology.
• This paper will present an overview of this technology and its applicability to turbine components.
Background
Presentation Plan
• Eddy Current Array (ECA) principles
• Flexible ECA probe for general purpose
• Active shielded flexible ECA probe
What is Eddy Current Array
• Eddy current array technology is the ability to electronically
drive several eddy current sensors placed side-by-side in
the same probe assembly.
• Data acquisition is performed by multiplexing the eddy
current sensors in a special pattern that will avoid mutual
inductance between the individual sensors.
• Most eddy current probes and techniques for flaw detection
can be used with eddy current array probes.
Eddy Current Array – Advantages
• Allows larger coverage in a single probe pass while maintaining a high resolution.
• Improves flaw detection and sizing with C-scan imaging.
• Inspect complex shape with a probe made with to the profile of the parts
Advantages of ECA Testing
Eddy Current Array – Advantages
• Allows larger coverage in a single probe pass while maintaining a high resolution.
• Improves flaw detection and sizing with C-scan imaging.
• Inspect complex shape with a probe made with to the profile of the parts
Inside an ECA probe
• Traditionally with hard
coil sensors
• Brand New: Flexible
Array on PCB film
General OmniScan ECA Features
OMNI-P-ECA4-32
• Conventional and Array ECT
• 4 input channels
• 43 channels
• Frequency range from 20Hz to 6MHz
• Dual Frequency operation
• 2 Encoder input
• 3 Alarm output
• 1 Analog output
ECA Advantages
Olympus OmniScan ECA
• Fast
• Large Coverage
• Easy Imagery
• Data Recording
• Encoded Capability
Multi-purpose ECA Flex Probe
Flexible ECA Probe: FBB-051-500-032
• 32 Coils
• 51mm of Coverage
• Very Flexible
• Two rows of coils to improve resolution
• Detachable cable
Can be configured to be used in absolute or reflection modes
FBB-051-500-032 Probe
• nb channel : 30
• Coil type: Reflexion (transmit-receive) with each channel
being build using an exciter coil and two receiver coils
connected in series.
• Total coverage: 48mm
• Resolution: 1.6 mm
• Frequency range: 100KHz to 4000KHz
• Coil design: 32 X Circular coil, 3.1mm diameter
• Probe design: 2row of staggered coils
Blade Root Inspection
Flexible ECA mounted on a shaped holder
• Gas turbine generator blade
• Notches at different location
• Inspection in one pass
Blade Root Inspection
Cscan representation of the results
Turbine blade Inspection
Active shielded flexible ECA probe
Detecting defects near edges; problem
• Eddy current field spread in area wider than coils footprint
• Eddy current sensors detection capabilities limited near inspected component edges
• Edge signals very important compared to small defects .. Difficult to extract defect information
Detecting defects near edges; conventional approach
• Use of ferromagnetic and/or conductive cup around the coil limit the spread the magnetic field– Improved detection near parts edges
• Applicable to coil array, but expensive to design and build
Detecting defects near edges; conventional approach• Use of finite element simulation tools to illustrate the induced
currents
Unique challenges pertaining to the use of flexible printed circuit board for building eddy current sensor array
• Thin copper layer ; typically less than 18 um
– Integrated eddy current shield offer limited potential
(dop in copper is around 50um @ 2MHz)
• Limited use of ferromagnetic material
• Compact array structure require compact shielding
Solution – Active shielding of coils
• Magnetic field extending outside coil’s is proportional to k*N*I/R^3 for unshielded coils.
• Concentric coils with adequate N1*I1/N2*I2 ratio can generate the same far field but different inner field.
• Opposite field provide shielding out of coil footprint
0 0,5 1 1,5 2 2,5 3
Ed
dy
Cu
rre
nt
de
ns
ity
Radial position (mm)
Inner coil
Outer coil
Inner coil - Outer coil
Inner coil Radius
Outer Coil Radius
k/R^3
First evaluation of the solution with hand wound coils
• Raster scan on 0.8mm long X 0.4mm deep EDM, at the center and edge of a titanium plate.
• Best performance on edge notch obtained with actively shielded coil
Extending the concept to PCB Made coils
• Concept not limited by coil thickness
• Single layer actively shielded coil made by pre-defined number of turn ratio.
• Use of simulation tools to define details of coil structure.
• Easily adapt to non-round shapes
Probe description
26
Qty of channel : 64Coil type : Absolute, bridge (with reference coils
on the probe)Total coverage : 76.8mmResolution : 1.2 mmCoil design : Rectangular shaped (3mm large X 2.2mm long) with active shielding)Probe design : 3 rows of coil for optimized coverage (approx 1.3dB variation between coils vs. at coil center).
27
Sample
0.034 in.
0.017 in.
0.004 in.
0.034 in.
0.017 in.
0.004 in.
0.040 in.
0.004 in.
28
Results
Notches size:
Length: 0.034 in.
Depth: 0.017 in.
Width: 0.004 in.
64 channels
3.15 in. of coverage
0.05 in. of resolution
Summary
• ECA is used in different industries for surface inspection
• ECA is a quick and reliable way to inspect surface
• Flexible ECA probes are suitable for the inspection of turbine components like blades
• Active shielded ECA flexible probe improve the detectionnear the edge of components, creating a lot of possibilities
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