ELECTROMAGNETIC INSPECTION

Abstract: Sensors (1-4) are arranged in a square or a square array. An excitation source is coupled via transformers (30) to twin coils on the sensors. Each transformer (30) has a primary winding (24) and two secondaries (26, 28) and is operable to proportion the excitation energy between the secondaries (26, 28) by a movable element (20). By manipulation of the element (20), a resultant electromagnetic field vector can be produced and scanned in the area bounded by the sensors (1-4).

Description: "Electromagnetic inspection" This invention relates to the inspection of materials by means of electromagnetic induction. The invention may be used in inspecting metals for cracks or other defects and testing the integrity of welds; it may also be used for other purposes, as will be explained below. Eddy-current inspection of metals is a well- established art, but conventional eddy-current inspection suffers from various practical limitations and deficiencies. ' Reference is made to our published European patent application EP-A-O 033 802 in which this background is discussed, and which discloses an improved probe and associated circuitry which overcome some of these disadvantages. One area of interest to which the present invention is addressed is the more detailed inspection of a sample. Another is inspection by means of stationary inductive device. The invention accordingly provides inspection apparatus comprising an array ofr at least four sensors arranged in a square, each sensor having two coils wound on one end of a ferrite core, the array being aligned for fitting said ends adjacent the surface of a test piece; an oscillator for producing a high frequency signal; means for selectively coupling the high frequency signal to selected coils in the array so as to produce a moving electromagnetic field in the test piece; and means for monitoring the impedance of the selected coils to detect non-homogeneity of the test piece. In one form, a multiplicity of said sensors is arranged in a square matrix, whereby selected parts of the sample under the matrix can be inspected without moving the sensors. The invention also provides an identification tag readable by the foregoing apparatus, the tag comprising a non-metallic substrate providing discrete locations for the selective placement of inserts of differing magnetic permeability. The tag is preferably adapted for subcutaneous implantation in an animal. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- 'Fig. 1 illustrates the geometrical relationship of a four-pole arrangement employing the invention; Fig. 2 is a schematic side view of the assembly for one pole; Fig. 3 is a schematic illustration of a coupling transformer used in driving the poles; Fig. 4 is a circuit diagram of an inspection apparatus embodying the invention; Fig. 5 is a schematic perspective view of a joystick arrangement for controlling the coupling transformers of the apparatus of Fig. 4; Fig. 6 illustrates a development of the above embodiment; and Fig. 7 illustrates a tag which can be read by the apparatus of the invention. Referring to Fig. 1, the invention utilises four poles 1-4 arranged in a square. A suitable size for the square is approximately 5mm side. These four locations define the six vectors shown, each of which can be positive or negative (i.e. in the direction indicated or the reverse direction) . The invention is based on energising the poles in phase but with selected energy distribution to give rise to a selected electromagnetic field distribution around the poles. By varying the energy distribution of the applied currents, a stationary four-pole assembly can produce a scanning electromagnetic field in a test piece under inspection. Each of the poles 1-4 comprises, as seen in Fig. 2, a ferrite rod 10 around one end of which are formed interleaved coils 12,14, this end being applied to the test piece 16. Fuller details of a suitable construction are given in the above-mentioned EP-A-O 033 802. The poles are supplied with driving current via coupling transformers, one of which is illustrated in Fig. 3. The transformer comprises a primary coil 24 wound on a ferrite core 20 mounted on a plastics rod 22 and slidable within a plastics tube 18 on which secondary coils 26,28 are wound. The effective coupling of the primary winding 24 to either of the secondary windings 26,28 is determined by the axial position of the ferrite/primary assembly 20,24; in the central position shown a minimum coupling will be obtained. Turning to Fig. 4, the coils 12,14 of each of the four poles 1-4 are connected as shown to be driven by four coupling transformers 30, each of which is as described above. The coupling transformers 30 are arranged in pairs, the ferrite/primary assemblies 2o, 24 of each pair being mechanically ganged together for movement in unison. The primary windings 24 are suitably connected in a bridge circuit (not shown) which replaces the bridge circuit in the circuitry described in EP-A-O 033 802. The pairs of- coupling transformers 30 may suitably be controlled by a joystick arrangement as illustrated in Fig. 5. A gimballed joystick 32 passes through orthogonal slots in plates 34 and 36 to produce x and γ_ motions coupled to respective pairs of the assemblies 20,24. The plates 34,36 also drive potentiometers 38,40 to produce voltages Vx and Vy representing x and y_ excursions. In one form of use, the voltages Vx and Vy can be amplified and applied to the x and y_ deflections of a c.r.t. together with information derived from the poles 1-4 such that movement of the. joystick 32 produces a corresponding cartesian trace so long as the poles are adjacent a homogeneous medium. For example, if the joystick is moved in a straight line, a straight line trace will be produced. In the normal situation, the poles 1-4 will be abutted against a metal test piece. The joystick can then be manipulated to cause the combined electromagnetic field of the poles effectively to scan the area of the test piece within and around the poles. In the event of one sensor being placed upon a sample where local material condition change exists the condition will give rise to an output voltage of lower value than that normally anticipated from a correctly balanced sensor bridge. A rotary motion of the joystick will now give rise to a deformed circle where an ingress towards the centre of the circle will occur representing the distorted field associated with the sensor probe over abnormal material. Where the sensor is close to a surface breaking crack, a rotary motion of the joystick will give rise to a deformed circle with a positive excursion away from the circular indicating that a surface breaking defect exists in close proximity to the affected sensor. The invention thus makes it possible to investigate a test piece without relative motion between the sensor and the test piece. The above principle can be used to examine a larger area by increasing the number of poles to give a matrix array of squares. Such an array comprises a multi- plicity of four-pole sensors, and can be used in a number of ways. For instance, switching gates can be provided to allow the coils of a selected four- pole unit to be driven at a given time. Such an arrangement is illustrated in Fig. 6, in which an array of poles 60 is arranged on a square matrix. An oscillator 62 drives coupling transformers 30 of the type described above whose outputs are selectively gated to the rows and columns by gates 66. When arranged as shown, each pole 60 would require an additional sensing coil to allow its condition to be read free of the effects of the other coils in the same row and column. Alternatively, the matrix could be physically spaced as shown but every coil individually addressed. Although primarily described with reference to materials inspection, the invention has other applications. One is to the reading of data encoded by means of materials of differing magnetic qualities. Fig. 7 shows a tag, intended for use primarily in animals, comprising a plastics strip 70 having bit positions 72 selectively occupied by, for example, stainless steel inserts indicating binary "1" and ferrite inserts indicating binary "0". The strip 70 is inserted subcutaneously and can be read through the skin by the apparatus of the present invention. The controllable nature of the applied field makes it possible to scan across the tag to read the digits.

Claims: CLAIMS 1. Inspection apparatus comprising an array of at least four sensors arranged in a square, each sensor having two coils wound on one end of a ferrite core, the array being aligned for fitting said ends adjacent the surface of a test piece; an oscillator for producing a high frequency signal; means for selectively coupling the high frequency signal to selected coils in the array so as to produce amoving electromagnetic field in the test piece; and means for monitoring the impedance of the selected coils to detect non-homogeneity of the test piece. 2. The apparatus of claim 1, in which a multiplicity of said sensors is arranged in a square matrix, whereby selected parts of the sample under the matrix can be inspected without moving the sensors. 3. The apparatus of claim 1, in which the coupling means comprises a plurality of transformers, each transformer having a primary winding for connection to an excitation source and a pair of secondary windings each connected to one coil of a respective sensor, the inductive coupling between the primary winding and the secondary windings being controllably variable. 4. The apparatus of claim 3, in which each transformer has its secondary windings wound on a tube and spaced apart, the primary winding being slidable within the tube. 5. The apparatus of claim 3 or claim 4, in which each sensor has one coil fed by a secondary winding of a first group of transformers and its other coil fed by a secondary winding of a second group of transformers, and control means are provided for varying said inductive coupling in the first group of transformers simultaneously and in the second group of transformers simultaneously. 6. The apparatus of claim 5, in which said control means comprises a joystick mechanism. 7. The apparatus of claim 2, in which each sensor has a first coil connected in series with the first coils of other sensors in a row and a second coil connected in series with the second coils of other sensors in a column, and means are provided for select- ively gating variable excitation to the rows and columns. 8. The apparatus of claim 7, in which an excitation source is connected to column transformer means and row transformer means, each transformer means having two outputs and being operable to selectively proportion the excitation energy between them, each output being connected to alternate rows or columns via respective gates. 9. An identification tag readable by the apparatus of claim 1, the tag comprising a non-metallic substrate providing discrete locations for the selective placement of inserts of differing magnetic permeability.

Inventors: HALE JOHN CHRISTOPHER, HALE, JOHN, CHRISTOPHER
Applicants: THORBURN TECHNICS INT, THORBURN TECHNICS (INTERNATIONAL) LTD
Application year: 1985
Main classification: G01N27
IPC codes: G01N27