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