GB2389909A

GB2389909A - Inductively coupled eddy current probe

Info

Publication number: GB2389909A
Application number: GB0214446A
Authority: GB
Inventors: John Calvert; John Hansen
Original assignee: HOCKING NDT Ltd
Current assignee: HOCKING NDT Ltd
Priority date: 2002-06-22
Filing date: 2002-06-22
Publication date: 2003-12-24
Also published as: WO2004001406A1; AU2003240072A1; GB0214446D0

Abstract

A probe for non-destructive eddy current testing of a surface comprises at least one winding (2, 3) inductively coupled by way of a transformer (4) to connection means linked to transmit/receive apparatus. The apparatus includes an oscillator to drive the winding (3) and an amplifier to process a signal from the sensing winding (2). The connection means may be a cable or a cordless link. The transformer coupling improves the probe sensitivity and enables the probe to have fewer turns on the coils (2, 3) which makes the probe more robust. The probe may be used for surface flaw detection such as cracks or material composition, stress or ageing, in applications such as railway tracks, bridges and oil rigs.

Description

Non-destructive Testing Probe, ADoaratus and Method This invention relates

to a probe for non-destructive testing of a surface using eddy currents, and to testing apparatus incorporating such a probe. It also 5 concerns a method of making such a probe and a corresponding method of non destructive testing. The invention is useful for the inspection of surfaces for various types of discontinuity such as cracks or variations in metallurgical composition, and one particularly important application is in the scanning of railway track.

The present invention is widely applicable in non-destructive testing systems, 10 but it is particularly intended for use for the detection of surface breaking cracks in ferritic (steel) components. Such components are of course widely used in the railway industry, for example on railway track.

However, the invention may also be used in the detection of surface and sub-surface flaws in a wider range of conductive materials. This is useful in 15 manufacturing industry in relation to aluminium, brass, copper, titanium, silver and their related alloys, for example. The invention may also be used for the verification of bulk properties of conductive materials, and for the detection of flaws, both surface and sub-surface. It may be used for precise flaw location as well as for sorting different grades of product in this environment.

20 The invention may also be used for the inspection of manufactured products in manufacturing industries such as in aerospace, petrochemical or automotive industry and in bar, rod, pipe and wire manufacture. It may also be used in the inspection of products whilst they are in service by the end users of materials and products as part of maintenance, repair and overhaul, e.g. in power generation 25 plants, petrochemical and automotive industries. In this environment, the invention can be used for the detection of surface and sub-surface flaw conditions, including life cycle fatigue effects, stress corrosion, material ageing and traumatic environmental damage such as impact or heat damage. The invention is also intended for use on safety critical installations and infrastructure including bridges, 30 oil rigs, holding tanks and amusement park rides.

Eddy current non-destructive testing is well known, and is described for example in British Patent No. 1378711 which discloses a probe with two windings around separate coaxial cores. In systems of this general type a probe is scanned across the surface whilst a driving coil or winding in the probe radiates an alternating 35 magnetic field which induces eddy currents in the surface layer. These eddy

currents are measured by picking up the electromagnetic field re-radiated by the

/ eddy currents, using a sensing coil which may be the same winding as is used for driving, or may be a separate winding.

Typical apparatus comprises a probe connected by a cable to remote instrumentation, and much effort has been devoted to optimising the winding of wire 5 into coil arrangements within the probe. The connecting cable is considered to be an active part of the eddy current system, as losses or capacitance in the cable result in signal attenuation which reduces the signal to noise ratio in the system.

The overall signal to noise ratio, and the ability to discriminate useful signals to determine the location and nature of surface irregularities, are influenced by the 10 design of the coil, in relation to the number of sums, the gauge of wire, the use of material such as ferrite in the coil core, the layout of the individual windings and cores in the probe. Using empirical or theoretical techniques, skilled practitioners have been able to determine appropriate inspection frequencies for a given inspection type with a given coil design, depending on the material properties and 15 the instrument characteristics. However, practical limitations in the manufacturability and useability of the probes mean that a compromise has always had to be drawn between performance and physical space limitations. Even with the optimum frequency selection, skilled practitioners have found it hard to detect responses from some flaw conditions on surfaces under test, using conventional probes.

20 The purpose of the invention is to provide a probe with improved sensitivity.

Accordingly, the invention provides a probe for eddy current testing a surface, comprising, all within a housing, at least one winding coupled by way of an inductive coupling to coupling means; the coupling means being for connection, in use, to transmiVreceive apparatus for driving the probe with a drive signal and for 25 analysing a sensed signal from the probe. The connection may be by cable or by a cordless signal transmission such as by radio.

As will be described in more detail below, the inductive coupling or inductive couplings are selected so that, for the desired operating frequency of the probe, the input and output impedances is matched, for optimal efficiency. Preferably, the 30 inductive coupling or couplings are matching transformers.

The invention allows the manufacture of probes with a much lower turn count than the equivalent conventional probes, by virtue of the use of inductive couplings.

This reduction in the number of turns has a significant impact in the fabrication of practical probes, as the same electrical characteristics can be achieved using heavy 35 gauge windings. This results in a robust design that is far less prone to accidental damage in operation: any break in a winding of the probe results in an open circuit,

/ effectively rendering the probe useless. In addition, the lower number of turns can significantly reduce the size of the physical package. The ability to use larger diameter wire for windings reduces the effect of parasitic capacitance, increasing the signal to noise ratio. Further, the location of the inductive coupling or couplings 5 close to the probe windings means that the connecting cable, when such a cable is used, is effectively decoupled from the probe assembly, so that losses due to the length or material of the cable are minimised.

The invention also provides non-destructive testing apparatus for eddy current testing comprising a probe according to the invention described above, 10 whose coupling means are connected via a cable to the transmiVreceive apparatus, which is remote from the probe, for driving the probe with the drive signal and for analysing the sensed signal from the probe.

Further, the invention provides a method of testing a surface for defects or other irregularities, comprising using a probe according to the invention described 15 above to scan the surface whilst driving the winding, or one of the windings, with a drive signal and detecting a sensed signal from the winding or one of the windings to provide an indication of the location and characteristics of the irregularity.

Further still, the invention provides a method of non-destructive testing a surface for irregularities, comprising using a drive signal to drive a winding through 20 an inductive coupling and detecting a sensed signal, induced in a winding by eddy currents in the surface, through an inductive coupling, and scanning the surface using the or each winding and the or each inductive coupling adjacent the surface.

The invention also provides a method of making a probe according to the invention described above, comprising converting an existing probe by inserting an 25 inductive coupling to couple a winding in the probe to coupling means; the coupling means being for connection, in use, to transmiVreceive apparatus for driving the probe with a drive signal and for analysing a sensed signal from the probe.

Further, the invention provides a method of making such a probe, comprising placing in a housing an existing winding taken from a conventional probe, and 30 connecting it to coupling means by way of an inductive coupling, the coupling means being for connection, in use, to transmiVreceive apparatus for driving the probe with a drive signal and for analysing a sensed signal from the probe.

In order that the invention may be better understood, conventional arrangements and also embodiments of the invention will now be described, by way 35 of example only, with reference to the accompanying schematic drawings, in which: Figure 1 is a conventional instrument input circuit;

Figures 2, 3 and 4 are examples of standard coil configurations used in probes connected to the input circuit of Figure t; Figure 5 is a diagram of testing apparatus embodying the invention; Figure 6 is a circuit diagram of an alternative probe; 5 Figure 7 is a circuit diagram of a further alternative probe; and Figure 8 is a perspective view of a further alternative coil configuration in a probe embodying the invention.

With reference to Figure 1, non-destructive testing apparatus has a front end which is coupled by way of a cable to a remote probe, and the terminal connections 10 for the cable are shown on the left-hand side of the drawing. An oscillator provides an oscillator output for driving a drive coil in the probe at a predetermined frequency: this frequency may be selectable to suit the specific probe connected, for a specific testing purpose. Positive and negative inputs from the sensor coil of the probe are fed to an amplifier and then to further instrumentation (not shown) for analysing the 15 received signal. The sensing coil in the probe provides a signal representative of the electromagnetic field re- radiated by the eddy currents in the surface under test.

Phase and amplitude information is determined from the sensed signal, by comparison with the drive signal, and the result is displayed on the instrument to provide an indication of the location and the nature of the irregularity in the surface, 20 whilst the probe is scanned across the surface.

One example of a conventional probe winding circuit is shown in Figure 2, in which the connections on the right-hand side are intended to match those on the left-hand side of Figure 1. Thus a cable would connect the windings directly to the terminals shown in Figure 1. Altemative coil arrangements are shown in Figures 3 25 and 4.

The alternating current applied to the coil assembly of Figures 2, 3 or 4 may be a single frequency or a waveform that comprises multiple frequencies. These frequencies may be either simultaneous or multiplexed in the time domain, depending on the instrument characteristics. A sensing coil 2 and a driving coil 3 30 are wound in parallel with each other and in series with respective bridge completion resistors 1. In Figure 2, instead of a drive coil 3 there is a dummy, i.e. non-sensing, balance load 3 which is typically an inductor mounted in the probe or in the remote instrument. In the examples of Figures 3 and 4, there are two sensing coils 2A, 2B.

There may also be more than one drive coil 3. In Figure 4, the sensing coils 2A, 2B 35 are wound as two separate coils in anti-phase with an unconnected centre point,

including a figure-of-eight winding, but alternatively the centre point may be connected. This configuration is known as a reflection probe.

A first embodiment of the invention is shown in Figure 5, with an absolute reflection probe. The instrumentation is conventional, although in this example it is 5 possible to connect the input positive connection to earth and to use only a 3-core cable to communicate with the probe. The sensing coil 2 (wound in ant-phase) is connected to a matching transformer 4 with a turns ratio of m:n. This provides an inductive coupling to the signal output terminals on the right-hand side of the drawing of the probe, which are connected by the cable to the input negative and 10 input positive terminals of the instrumentation. Correspondingly, a separate matched transformer 4 couples the drive coil 3 inductively to the corresponding terminals of the probe for connection to the oscillator output and the input positive terminal of the instrumentation. In this example, the matching transformers 4 are identical, but this need not be the case.

15 In an alternative coil arrangement shown in Figure 6, for Absolute" sensing with a single coil, there is just one winding 2 and one matching transformer 4. The positive and negative terminals for connection to the differential amplifier in the instrumentation are in this example reversed, for convenience. The uppermost terminal of the probe' marked "drive", for connection to the oscillator output, is 20 connected by way of a 50 Ohm bridge completion resistor to each of the positive and negative terminals. The negative terminal is connected by way of a load resistor to one side of the transformer 4 winding, and the positive terminal is connected directly to the other side of the transformer 4 winding, on the output side of the transformer. This arrangement allows the same winding to be used for 25 driving and sensing.

The probe of Figure 7 is a differential bridge probe, and in this example matching transformer 4 of the sensing coil 2 has its output side connected directly between the earth terminal and the input positive terminal. The oscillator output is connected through bridge completion resistors both to the input positive terminal 30 and to the input negative terminal, as in Figure 6. The matching transformer 4 of the drive coil 3 has its output side winding connected between the earth terminal and the input negative terminal.

The physical layout of windings in a probe of the type shown in Figure 4, the reflection probe, is shown in Figure 8: coils with such few windings are only 35 possible with the benefit of the present invention. The coil configuration of Figure 8 is suitable for a wide scan width reflection type probe with a central driver 3 and two

differentially-connected pickups 2A, 2B. In this example, the drive coil 3 has just one tum, and the sensing coils 2A, 2B each have just two sums.

In a typical probe embodying the invention, the number of turns m on the sensing winding or drive winding side of the transformer is one or two, and usually 5 no more than four, although it is possible to have more than four. The number of turns n on the other side of the matching transformer is selected to give the required matching impedance, so that the impedances of the coils match that of the instrument and cable; a typical range of n is from 12 to 25.

The wire size may be large, 1mm in diameter, for example. The matching 10 transformer or transformers 4 are preferably placed very close to the windings 2,3, typically less than 50mm apart. The transformers, typically toroidal, or equivalent to toroidal, range from 15mm to 40mm in diameter, and the probe itself may provide a working surface scanning width ranging from 20mm to 600mm. The inductance of the windings is typically in the range of 1 to 20 pH, dependent on configuration.

15 The probe of Figure 8 is particularly useful for the rapid scanning of railway track, and with the very low number of windings in a heavy gauge wire, the probe is particularly robust and sensitive at the same time.

There need not be a cable connecting the probe to external instrumentation.

Instead, the signals could be transmitted and received by radio or some other form 20 of cordless connection.

Conventional probes may be adapted, to make a probe embodying the inventions, and this may alter the optimum operating frequency to a desired frequency. The existing winding may be removed and re-used in a new probe, by putting it in a housing and connecting it to coupling means, such as terminals, by 25 way of an inductive coupling, such as a toroidal transformer. Altematively, an existing conventional probe may be adapted by the insertion of the inductive coupling, to couple its winding(s) to the coupling means (terminals).

There are several ways of scanning the surface using the probe. The probe may be moved over the surface, or the probe may be held stationary over the 30 surface and the surface moved, or it may be held stationary over the surface whilst scanning is done electronically by time division multiplexing the windings, spaced across the surface, or applying varying levels of sensitivity, using transformers, to separate windings, as is already known.

Claims

- 1. A probe for eddy current testing a surface, comprising, all within a housing, at least one winding coupled by way of an inductive coupling to coupling means; the coupling means being for connection, in use, to transmiVreceive apparatus for 5 driving the probe with a drive signal and for analysing a sensed signal from the probe.
2. A probe according to Claim 1, comprising one such winding which is for both

inducing eddy currents in the surface and sensing a signal derived from those eddy currents. 10
3. A probe according to Claim 1, comprising one such winding for inducing eddy currents in the surface and at least one adjacent such winding for sensing a signal derived from those eddy currents.
4. A probe according to Claim 3, in which each winding is connected to an inductive coupling or to a respective one of plural inductive couplings, for coupling it 15 inductively to the transmiVreceive apparatus.
5. A probe according to Claim 3 or Claim 4, in which there are at least two such windings for sensing.
6. A probe according to any preceding claim, in which the or each inductive coupling is a transformer.

20
7. A probe according to Claim 6, in which the transfommer or each transfommer is toroidal or equivalent to toroidal.
8. A probe according to any preceding claim, in which the winding or windings for sensing have no more than four sums.
9. A probe according to any preceding claim, in which the winding or windings 25 for sensing have no more than two sums.
10. A probe according to any preceding claim, in which the winding or windings for driving have no more than four sums.
11. A probe according to any preceding claim, in which the winding or windings for driving have no more than two sums.

30
12. A probe according to any preceding claim, in which the winding or windings for driving have one tum.
13. Non-destructive testing apparatus for eddy current testing of a surface, comprising a probe according to any of Claims 1 to 12 whose coupling means are connected via a cable to the transmiVreceive apparatus, which is remote from the 35 probe, for driving the probe with the drive signal and for analysing the sensed signal from the probe.
14. A method of testing a surface for flaws or other irregularities, comprising using a probe according to any of Claims 1 to 12 to scan the surface whilst driving the winding, or one of the windings, with a drive signal and detecting a sensed signal from the winding or one of the windings to provide an indication of the location 5 and characteristics of the irregularity.
15. A method of non-destructive testing a surface for irregularities, comprising using a drive signal to drive a winding through an inductive coupling and detecting a sensed signal, induced in a winding by eddy currents in the surface, through an inductive coupling, and scanning the surface with the or each winding and the or 10 each inductive coupling adjacent the surface.
1ff. A method of making a probe according to Claim 1, comprising converting an existing probe by inserting an inductive coupling to couple a winding in the probe to coupling means; the coupling means being for connection, in use, to transmiVreceive apparatus for driving the probe with a drive signal and for analysing 15 a sensed signal from the probe.
17. A method of making a probe according to Claim 1, comprising placing in a housing an existing winding taken from a conventional probe, and connecting it to coupling means by way of an inductive coupling, the coupling means being for connection, in use, to transmiVreceive apparatus for driving the probe with a drive 20 signal and for analysing a sensed signal from the probe.
18. A probe substantially as described herein with reference to Figures 5 to 8 of the accompanying drawings.
19. Non-destructive testing apparatus substantially as described herein with reference to Figures 5 to 8 of the accompanying drawings.

25
20. A method of non-destructive testing a surface, substantially as described herein with reference to Figures 5 to 8 of the accompanying drawings.