ELECTROMAGNETIC INSPECTION PROBE

Abstract: The probe comprises a casing (10) having a pointed end within which is positioned a ferrite rod core (14) having wound therearound a pair of coils (18). The coils are wound bifilar-fashion, and have a predetermined difference in impedance, suitably about 1 mu H.

Description: "Electromagnetic Inspection Probe" In our prior patent EP-0 033 802 is described an electromagnetic inspection probe and associated circuitry which give greatly improved performance compared to equipment previously available. The present invention is concerned with improve¬ ments in the probe of EP-0 033 802 ("the prior patent"). The prior patent relies on a layered arrangement of two coils which has been found to be disadvantageous if the probe is applied to a surface significantly hotter or colder than ambient, since differential heating or cooling of the coils results. It has also been found that the probe of the prior patent requires a relatively high frequency drive (typically about 1.25 MHz) which limits the inspection depth. An object of the present invention is to eliminate or mitigate these drawbacks. Accordingly, the invention provides an electro¬ magnetic inspection probe including a ferrite core and a pair of coils wound on the core, and is characterised by the fact that the coils are formed to have a predetermined difference in impedance, preferably about 1/uH. Preferably also, the coils are bifilar wound. The probe may suitably have a tip of conical or chisel shape, the coils being positioned adajcent one end of a rod-shaped ferrite core and constrained to the tip shape. Preferably also, the coils form part of a bridge circuit driven at a frequency of 50 kHz - 1 MHz. Embodiments of the invention will now be described, by way of example, with reference to the drawings, in which:- Fig 1 is a cross-sectional side view of one probe embodying the present invention; Fig. 2 is a cross-sectional view on the line 2-2 of Fig. 1; Fig. 3 is a cross-sectional end view of another embodiment of probe; Fig. 4 is a view on the line 4-4 of Fig. 3; and Fig. 5 is a cross-section of a further embodiment of probe. Referring to Figs. 1 and 2 , a probe comprises a plastics housing 10 with a generally conical tip 12. A ferrite rod 14 is mounted in the housing 10 by a spacer 16 and is secured (e.g. by adhesive) with one end flush with the tip 12. A winding 18 is positioned around the lower end of the rod 14. The winding 18 is a bifilar winding, i.e. it is formed by winding two wires simultaneously around the rod 14, to form two coils whose ends 20a, 20b, 22a, 22b are brought out through the probe. The winding 18 can be formed in a generally cylindrical shape and caused to assume the form shown by being pressed into the housing 10. Figs. 3 and 4 show a generally similar probe, but in this case the tip 24 has a chisel shape. As shown in Figs. 1 and 3, the electromagnetic field pattern produced by the probes has a central lobe 26 and side lobes 28. This is similar to the field pattern of the prior patent, but the side lobes are closer to the central lobe. The probe shapes shown in Figs. 1-4 are particularly suitable for external inspection of structural welds since they can be used on flat surfaces and also into angles. In the prior patent, it was thought essential to have the windings positioned around one end of the ferrite rod. This is still preferred for the majority of applications such as the inspection of external welds In some uses, however, it may be preferable as shown in Fig. 5 to have a ferrite rod 30 surrounded by a bifilar winding 32, the test piece indicated at 34 being coupled with the field at one side of the rod 30. Other forms of ferromagnetic core such as toroids or interrupted toroids could also be used. An important feature of the invention is that the coils of the winding are of similar impedance but with a controlled difference in impedance. For most purposes, a difference of 1)»H inductance is suitable, e.g. with iron or ferritic steel. For inspection of less magnetically susceptible materials, e.g. austenitic steel, a slightly greater difference may be desired. An impedance difference of this nature is necessary to allow unambiguous detection of changes in amplitude and phase produced by traversing a defect while remaining within the operating limits of practical amplifiers. The use of such a controlled out-of-balance also makes it possible to utilise a much lower frequency than in the prior patent. The frequency can be chosen according to the material to be inspected and the desired depth of penetration in accordance with the formula where P is the depth of penetration, P is the conductivity of the material, and •μ- is the permeability of the material. Typically, the frequency will be in the range 50 kHz to 1MHz and penetration about 0.01" to 0.001". The probes described above may be used with the circuitry of EP-0 033 802 and similar results will be achieved, namely defects may be detected by gross changes in phase whereas surface roughness will produce principally changes in amplitude. The use of a bifilar winding substantially eliminates temperature effects. It should be noted that the apparatus may be used on both ferrous and non-ferrous materials.

Claims: CLAIMS 1. An electromagnetic inspection probe including a ferrite core (14;30) and a pair of coils (18;32) wound on the core; characterised in that the coils are formed to have a predetermined difference in impedance 2. The probe of claim 1, in which said impedance difference is about 1 U.H. 3. The probe of claim 1 or claim 2, in which the coils are bifilar wound. A. The probe of claim 1, the probe being of conical or chisel shape, said ferrite core being rod-shaped and having said coils positioned adjacent one end thereof, the coils being constrained to the tip shape. 5. The probe of claim 1, in which said coils form part of a bridge circuit driven at a frequency between 50 kHz and 1 MHz. 6. A method of producing an electromagnetic inspection probe, characterised by the steps of: providing a rod-shaped ferrite core, winding a pair of coils bifilar-fashion around one end of said core providing a casing having a substantially closed end of reducing cross-section in a conical or chisel shape, and pressing the core and coil assembly into the casing to cause the coils to occupy said end and assume the shape thereof.

Inventors: HALE JOHN CHRISTOPHER, HALE, JOHN, CHRISTOPHER
Applicants: THORBURN TECHNICS INT, THORBURN TECHNICS (INTERNATIONAL) LIMITED
Application year: 1986
Main classification: G01N27
IPC codes: G01N27