EDDY CURRENT PROBES (SENSORS) 

 B P C Rao

 
 
  INTRODUCTION

Eddy current (EC) testing is used to ensure pre-service quality and to assess in-service health of industrial components made of electrically conducting materials, by way of deteection and characterisation of defects or discontinuities. This technique works on the principles of electromagnetic induction and test phenomenon can be explained using the Maxwell’s equations. In this technique, as shown in Fig. 1, a coil (also called probe) is excited with sinusoidal alternating current (frequency, f) to induce eddy currents in the component under test and the change in coil impedance is measured. Defects such as cracks, inclusions, notches, microstructure variations etc. cause a discontinuity in electrical conductivity, and/or magnetic permeability, hence, distort the eddy current flow and in turn, change the coil impedance. The measured impedance change is correlated with defect parameters e.g., length, depth, location, and orientation etc. The locus of impedance change during the movement of an EC probe is usually called an EC signal or the impedance-plane trajectory. Eddy current test phenomenon is controlled by the skin effect, according to which the depth of penetration (also standard depth of penetration [SDP]), depends on frequency and material properties (see Fig.1). Due to skin-effect, the detection and characterisation of surface defects is more reliable as compared to buried or sub-surface defects. Popular industrial applications of eddy current  testing include defect detection, material property measurement, alloy sorting, and material as well as coating thickness measurements. It is also used for proximity sensing, level measurements, metal particles/debris in non-conducting media (cardboards, bakery products, currency notes, undreground mines, insulators etc. )

Eddy current probe is the main link between the eddy current instrument and the component under test. Success of eddy current testing for a specific inspection applcation depends on sensor, instrument and optimisation of test parameters. The probe plays two important roles: it induces the eddy currents, and it senses the distortion of their flow caused by defects. Design of probe / sensor is an important task and a variety of aspects such as component geometry, impedance matching, magnetic field focussing, and environment etc. need to be considered for its design and development. In this contribution, some important aspects concerning probe design and development are covered.


TYPES OF EDDY CURRENT PROBES / SENSORS

Design and development of eddy current probes is very important as it is the probe that dictates the probability of detection and the reliability of characterisation. In general, defects that cause maximum perturbation of eddy currents are detected with high sensitivity. The shape, cross-section, size and configuration of coils are varied to design an eddy current probe for a specific application. Depending on the geometry of the component three types of eddy current probes viz. surface pancake, encircling and bobbin probes shown in Fig.2 are employed. The three types of probes can be operated in absolute, differential or send-receive modes. In absolute mode only one coil is used for exciting and sensing eddy currents. The differential probes with two coils usually wound in opposite direction, and the send-receive probes with separate receiver coils, employ different bridge circuits. The absolute and differential modes exhibit different characteristics (Table.1) and selection depends, primarily on inspection requirement.

Surface (Pancake) Probes

Surface probe or Pancake probe, usually a spring mounted flat probe or a pointed pencil type probe, allows determining the exact location of a defect. The probe may be hand held, may be mounted on automated scanners or may even be rotated around to get e.g. a helical scan in tube/rod inspections. Surface probes possess directional properties i.e. regions of high and low sensitivity (Table.2). Usually ferrite cores (absolute cylindrical as well as split-D differential types) and shields are used for enhanced sensitivity and resolution. Besides ferrites, copper coils are used for shielding purpose. Surface probes are extensively used in aircraft inspection for crack detection in fastener holes and for detection of corrosion/exfoliation in hidden layers. When the component geometry is complex, it is not uncommon to use probe guides, shoes, centering-mechanisms to maintain uniform lift-off and detection sensitivity. Surface probes were developed for EC imaging, for measurement of liquid sodium level in steel tanks and also for measurement of thickness of coatings.

Encircling Probes

Encircling probes are used to inspect rods, tubes and wires. In an encircling probe the coil is in the form of a solenoid into which the component is placed. In this arrangement, the entire outside circumferential surface of the component covered by the coil is scanned at a time, giving high-inspection speeds.These probes may not detect circumferential defects (Table.2) as the edy currents flow parallel to them without getting distorted. Popular industrial application of encircling probes is high-speed inspection of tubes from outside during the manufacturing stages. Encircling probes were developed NDE of thin-walled cladding tubes and thick-walled steam generator magnetic tubes.

Bobbin Probes

These probes are the most widely used ones in eddy current NDE. Bobbin probes consist of a coil arrangement in the form of a winding over a bobbin, which passes through components such as tubes and  scans the entire inside surface in one-go. Popular application of bobbin probes is high-speed multi-frequency inspection of heat exchanger tubes in-situ for detection of cracks, wall thinning and corrosion in tubes as well as under support plate regions. The directional properties of these probes are identical to encircling probes. In some instances, bobbin type probes are employed for inspection of bolt holes. For inspection of critical coomponents, phased-array probes are slowly replacing the traditional bobbin/encircling probes as regards to detection and location of circumferential and short defects.


DESIGN OF EDDY CURRENT PROBES

In most EC instruments excitation current is kept constant (in a few tens of mA range) and the inductance may vary by a factor of one thousand. The usual input impedance could range between 20 and 200 ohms. The number of turns and wire gauge (between SWG 30 and SWG 45) are fixed such that the coils fill the available cross sectional space in uniform layers and turns per layer so that inter-winding effects are minimal. In some situations, it may be necessary to use a number of bridge circuits as well as probes operating simultaneously, essentially to cover larger area. For good sensitivity to small defects, small diameter probes are used. Similarly, in order to detect sub-surface and buried defects, large diameter high throughput probes are necessary. As a general rule, the probe diameter should be less than or equal to the expected defect length and also comparable to the thickness of the component. The sensing area of a probe is the physical diameter of the coil plus an extended area governed by magnetic field spread. Hence, it is common to use ferrite cores/shields (high permeability and low conductivity) to contain the lateral extent of magnetic fields without affecting the depth of penetration.

It is essential to operate EC probes below the probe/cable resonance frequency, especially while using long probe cables and at very high frequencies. The probe bodies are usually made of non-conducting plastics. Wear of probes is normally be reduced by giving wear resistant coating to the probe heads or tips. It must be noted here that such coatings add to the built-in lift-off of probes and tend to reduce signal amplitudes. Temperature stability of probes is usually accomplished by using coil holder material with poor heat transfer characteristics. Most common commercial copper wires are used upto about 150 deg. C. For temperatures above this, silver or aluminium wires with ceramic or high temperature silicon insulation or MIC are used. The probe material must be chemically compatible with the component. In brief, probe design is usually done considering the following:

• Geometry of the component e.g. rod, tube, plate etc.
• Type of discontinuity expected e.g. fatigue cracks, conductivity variation etc.
• Likely location of defect e.g. surface, sub-surface
• Coil impedance and its matching with the bridge circuit of the EC instrument
• Frequency range of the probe i.e. for simultaneous multi-frequency excitation
• Inspection requirement e.g. detection, evaluation of length, depth etc.
• Material characteristics e.g. ferromagnetic or non-ferromagnetic
• Coil response to a notch, drilled hole or other reference discontinuity
• Field distribution in space and eddy current flow distribution in the material
• Shape and dimensions of core, coil /coils and lift-off characteristics
• Environmental characteristics such as wear, temperature and chemical attack


As many factors need to be considered, three different approaches viz. experimental, analytical and numerical are often resorted to for designing eddy current probes.

Experimental Approach

This approach usually involves trail and error fabrication of probes suiting the geometry. In this approach, the coil dimensions and the test frequency are usually optimised by comparing the detection sensitivity of artificial reference notches as well as natural cracks if available. This approach was usd to design encircling EC probes for inspection of stainless steel cladding tubes of Fast Breeder Test Reactor (FBTR) and also to design probes for Cr-Mo steam generator tubes of Prototype Fast Breeder Reactors (PFBR). In another instance, in order to minimise low-sensitivty zones of phased-array eddy current probes for inspection of heat exchanger tubes, tandem probe was developed.

Analytical Approach

Analytical approaches for probe design involve analysing the eddy current testing phenomenon and calculating the coil impedance and examining the operating point on the impedance plane as well as the effect of variations in coil radius r, shape, material conductivity, thickness t and test frequency f. Two popular impedance plane diagram based methods are 1) calculation of characteristic parameter, Pc introduced by Deeds and Doods for planar geometris and 2) calculation of characteristic frequency ratio f/fg, where fg is the characteristic frequency introduced by Förster for tubular geometries. Using these two methods, coils are designed such that the operating point is in the “knee” region on the normalised impedance plane diagrams.
 
 
  Char. Freq.  
 
  Numerical Approach

Eddy current testing phenomenon can also be analysed numerically using finite difference, finite element (FE), boundary element (BEM) and other methods. In this approach, coil and core dimensions are varied systematically and signals are predicted for a reference defect and the dimensions that result in maximum detection sensitivity are chosen. Not only signal amplitude, but phase angle from lift-off is also considered for decision making. A few typical applications of
axi-symmetric FE model, are discussed elsewhere [96 BPCRao]. In this model, the region consisting of EC probe and component, is discritised into triangular elements and variational principles are applied to compute the vector potential at the vertices of the elements. From the vector potential, the probe impedance is calculated and in turn, the impedance plane trajectories. This model has been used to optimise eddy current probes for location of garter springs in the coolant channels of Pressurised Heavy Water Reactors (PHWRs) [96 BPCRao].


RECENT TRENDS IN EDDY CURRENT PROBE DESIGN

Detection of cracks eminating from edges and corners of components is very important. Often, strong signal from the edges mask the small/weak signal from a potentially harmful crack. Focussed surface probes are being explored and likewise appropriate signal processing methods are being incroporated to suppress edge contributions. In the case of heat exchanger tubes, rotating surface probes or array probes with multiplexing are preferred for detection and characterisation of defects along the tube circumference (location). For detection of defects at roll joints special array probes are being tried. In order to inspect components with complex geometries, flexible probes are being tried. These probes can be mounted/scanned over a region for inspection purpose and be easily removed. Similarly, for detection of sub-surface and deep-seated defects in multi-layer and other structures eddy current probes are mounted and integrated with Hall probes, SQUIDs, GMR and AMR sensors. The main objective in these strategies is to detect the weak magnetic fields from defects, rather than the traditional impedance changes. When more than one sensors is used and data fusion methods are adopted to combine the sensors data to form a comprehensive gloabl picture of investigated regions. At times, it may be beneficial to combine information of a single sensor, but operating at different frequencies to get enhanced information of defects. Such an approach has been used in an intelligent imaging scheme to obtain accurate and quick 3-dimensional pictures of defects.

Inspection of ferromagnetic tubes is difficult due to high and varying magnetic permeability. For testing such tubes from outside, encircling D.C. saturation coils are used, where as remote field eddy current probes and permanent magnet based probes are used for testing from tube inside. Optimisation of frequency and location of receiver coil (usually about 3 to 4 tube diameters away from exciter) in the remote field eddy current testing method is very important. FE model and experimental based approaches have been successfully used this purpose.

When surface EC probes are scanned in a ratser and the impedance data is displayed, Eddy current C-scan images of defects can be formed. EC images provide valuable information of defects. However, these images are blurred due to distributed point spread function of the probe. FE model based approach was used to optimise ferrite-core probes for eddy current imaging.  In case of heat exchangers and steam generators, probes have to negotiate U-bend regions and detect defects, if any, in those regions. Design of flexible probes that are insensitive to bend regions is very challenging. For inspection of bend regions in ferromagnetic steam generator tubes, flexible remote field eddy current probe, with WC rings on either sides, was developd and wavelet transform based signal processing method was incorporated to suppress disturbing signals from bend regions.
 
 
  Table 1 Comparison of absolute and differential eddy current probes.

Characteristic                  Absolute Mode                                      Differential Mode

Detection response         Respond to both abrupt & gradual            Only for abrupt changes
                                       changes in properties and dimensions

Temperature drift             Prone to drift                                             Immune to drift due to
                                                                                                          differential nature of
                                                                                                          coils

Signal interpretation         Relatively simple                                       Signal interpretation is
                                                                                                           difficult

Long defect detection       Possible                                                    Detects only the defect
                                                                                                           ends due to differential
                                                                                                           nature

Lift-off / wobble                 Highly sensitive                                         Less sensitive
 
 
       Table 2. Characteristics of surface and encircling / bobbin probes

     Surface Probes                                        Encircling or Bobbin Probes


-
Coil is mounted with axis perpendicular to the     -Coils are parallel to the circumference component surface                                                 of the component    

-Surface defects are detected with high                -Longitudinal or transverse defects are
sensitivity as compared to buried defects             detected

-Poor sensitivity for laminar defects                      -Poor sensitivity for circumferential
                                                                              defects

-Sensitivity decreases with depth                          -Sensitivity is zero at the centre of rods

-Popular applications include aircraft inspection   -Popular applications include heat
for fatigue cracks, corrosion etc. and coating        exchanger tube testing, material sorting,
thickness measurement                                         dimensional measurements

-Characteristic parameter, Pc is used for sensor  -Characteristic frequency f/fg is popular
design (scale modelling)                                         for Thin walls–1, Thick walls-4,
                                                                               Cylinders-10

-Lift-off, distance between coil and component,    -Fill factor, ratio of square of diameters of defines the magnetic coupling and very small       coil and component, defines the magnetic lift-off is preferred for better detection sensitivity   coupling and moderate to high fill factor                                                                                   is preferred

-Field focussing using ferrite cores and cups,       -Field focussing is difficult and not
for enhancing sensitivity is possible                       followed

-Ferromagnetic material testing difficult in plate    -Remote field, saturation, permanent
geometries                                                            magnet based methods are possible for
                                                                              tubes
 
 
 
 
 
  Fig.2 Three types of EC probes. Also shown are absolute and differential modes of a bobbin probe.  
 
  TIPS FOR SELECTION OF EDDY CURRENT PROBES  
 
 
 
 
  SEQUENTIAL STEPS INVOLVED IN EDDY CURRENT
PROBE DESIGN


1. Decide coil shape and operating mode depending on geometry

2. Decide frequency using skin-effect relation depending on thickness and detection sensitivity

3. Decide shielding & core depending on resolution & sensitivity requirements

4. Optimise coil, shield dimensions, inter-coil spacing following numerical or experimental appraoch

5. Carefully fill coil cross-sectional area with suitable gauge wire and number of layers and turns and ensure frequency characteristics and impedance matching

6. Experimentally establish the detection sensitivity and completely conceal the coils to withstand wear, temperature, irradiation, and corrosion.
 
 
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