GB2292223A

GB2292223A - Position sensing system

Info

Publication number: GB2292223A
Application number: GB9519609A
Authority: GB
Inventors: Frank Philip Lund
Original assignee: Millstrong Ltd
Current assignee: Millstrong Ltd
Priority date: 1991-12-20
Filing date: 1991-12-20
Publication date: 1996-02-14
Anticipated expiration: 2011-12-20
Also published as: GB9127109D0; GB2292223B; GB9519609D0; GB2262607A; GB2262607B

Abstract

A positional sensing system comprises a twisted wire pair energised with an alternating signal which generates an elongate magnetic field, the direction of which varies along the length of the wire pair. Orthogonally positioned search coil arrangements are provided for sensing the direction of the magnetic field to give a measure of the position of the search coils within the field. A defect detection system in which the positional sensor may be used is also disclosed (figures 4 to 6). <IMAGE>

Description

POSITION MEASUREMENT The present invention is in the field of position Beasurement, particularly but not exclusively for surface or sub surface measurement of conductive components, for example crack measurement. Our earlier British patent applications Nos. 9112546.8 (11 June 1991), 9117307.0 (9 August 1991) and 9118715.3 (30 August 1991, published as GB 2256713 A) are incorporated herewith in their entirety; in particular, the disclosure of the last of these three will be referred to and corresponding reference numerals will be employed.

The present invention is advantageously employed in an imaging device, or scanning probe, for conductive components, making use of scanned excitation fields.

In another aspect, the invention relates to such a device or probe.

Systems for the inspection of conductive components have been set forth in prior art e.g. G.B.

Patent 2225115A, such systems make use of a combination of excitation and sensor coils which is passed by mechanical or manual means over the surface of the component under inspection. It is desirable for many applications that the inspection is performed by a sensor producing data from a larger area than that inspected under the sensor in the prior art, for example it may be more feasible to implement a mechanical placement system than to implement a mechanical scanning system. However, mere enlargement of the dimensions of the sensor of the prior art will result in a device having worse resolution than the original sensor.

It would be possible to make an array of a large number of sensor coils, whether of the form revealed in the prior art (GB 2225115A) or of the form revealed in our earlier British Appn. 9118715.3 (GB 2256713A).

The resolution of such an array sensor is limited by the separation of the individual sensor coils.

According to the present invention, in one aspect there is provided a positional sensing system comprising means for generating an elongate alternating magnetic field, the direction of said magnetic field varying along its length, and means for sensing the direction of said magnetic field, and hence determining the position of the sensing means within the magnetic field.

Embodiments of the invention will now be disclosed in greater detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates components of a position sensing system according to an embodiment of the invention; Figure 2 illustrates the current flow in the wires of Figure 1; and Figure 3 illustrate current flow over a small portion of Figure 2, to assist understanding of the embodiment of Figure 2.

Figure 4 is a schematic drawing which illustrates a probe which may embody the invention; Figure 5a illustrates a pair of mutually offset elongate sensor coils according to an embodiment of the invention; Figure 5b illustrates the corresponding coil output signals; Figure 6 illustrates an embodiment of the invention including the coils of Figure Sa positioned relative to a magnetic field generator.

In GB 2256713A it has been claimed that the form of sensitivity e.g. defect depth or end detection, may be selected by placing the excitation field at different positions with respect to the sensor coil(s). In this invention is stated that the first form of sensitivity is at a maximum when the second form of sensitivity is at a minimum and vice-versa.

In addition intermediate forms of sensitivity are produced for excitation positions intermediate between those described in that invention. If it is arranged for the excitation field to move relative to the inspected component, for example by variation or switching of the drive to sections of a stationary excitation coil, the actual area under inspection will move with the excitation field.

If this moving excitation field moves over a pick-up system consisting of one or more coils as described in our above-referenced earlier application with reference to the fourth embodiment of Figs 24 and 25 the output of that pick-up system will vary with: - the presence/absence of a defect, - the lift-off of pick-up/field generator from the surface of the inspected component, - the position of the excitation field relative to the pick-up coil or coils.

If a multiplicity of such pick-up systems is employed with the individual pick-up systems mutually placed such that at any point at which an individual pick-up systems yields a zero response in one form of sensitivity, at least one companion pick-up system yields a non-zero sensitivity in that form, it can be ensured that both forms of sensitivity are available over a substantial part of the pick-up assembly. For example, the simple sensor shown in figure 24 of Appn.

9118715.3 may be augmented by the addition of a second set of pick-up coils (41a.d) identical to the original bst positioned such that the centres of its component coils lie over the junctions of the original pick-up coils. The readout system for such a pick-up system composed of multiple pick-up sub-systems would take account of: - the sensitivity profile of the individual pick-up sub-systems, - the position and extent of the moving excitation field, - the outputs of the individual pick-up sub systems.

From these data the readout systems would produce an image of the inspected component in terms of liftoff and defect presence along the line of travel of the moving excitation field. For this field having movement in a single dimension, the axis of the pickup system would be aligned parallel to that direction of movement.

A wider image may be obtained by placing further pick-up systems alongside/overlapping the first and similarly processing the outputs of these additional pick-up systems, taking account of their lateral positioning relative to the first pick-up system.

A practical sensor has been constructed using the above principles having a dual pick-up system as already described and scanning performed by a moving excitation field. This sensor has been demonstrated to be able to display a simulated defect 10 x long x 2mm deep in a steel test plate.

For this sensor the resolution of position of the image is dependent upon the increment of the excitation position which is finer than the pitch of the individual sensor coils comprising the pick-up assembly.

Arranging the pick-up system to exhibit sensitivity in other dimensions and providing a scanned excitation field in those dimensions will enable an image to be obtained which has its resolution in each dimensions controlled only by the fineness of the movement of the excitation field in those dimensions.

Referring to Fig. 4, in an actual embodiment, 4 coils 40a-40d and 4 coils 41a-41d were provided, wound (for convenience) so that each of the coils 41a-d or 40a-d slightly overlaps its neighbouring coils, adjacent an excitation coil 10 50mm long, tapped every 4 turns to provide 21 connections dividing the excitation coil 10 into 20 separately energisable segments. The tapping control 120 means comprises a set of computer control relays selecting one or more og the segments.

One way of providing the excitation coil 10 is to use 80 way 0.635mm pitch ribbon cable, suitably tapped.

Referring to Figs. Sa and 5b, instead of providing a plurality of discrete coils 40a-40e, 41a41e, a pair of coils 40, 41 comprising repeated "figure of '8'" twists may be employed. The two coils 40, 41 are staggered by half a twist relative to one another, so that, as shown in Figs. 2a and 2b, where the sensitivity of one coil is maximum and that of the other is minimum (e.g. 0) and vice versa.

Each of the coils 40, 41 comprises, as in the figure of "8" coils described in the above reference earlier patent application, a number of windings each laid on a serpentine path; corresponding materials and number of turns to the previous figure of "8" examples may be used, but the exact materials and number of coils is determined experimentally.

Referring to Fig. 6, the two coils 40, 41 are laid either just within or just outside the excitation coil 10, comprising for example an 80 turn coil tapped every 4 turns, the tap separation being for example at 2.54mm intervals. The centres of the fields produced by each tapped segment therefore lie at spacings of (n + 0.5) 2.54mm interval spacings from the ends of the coil. It is also possible to produce intervening field centres, by energising several adjacent centres; for example, if two adjacent segments are excited, the magnetic field is centred about the middle tap and consequently field centres of n.2.54mm from the ends of the coil can be produced.

The centres of the lobes of the figure of "8" wound coils 40 are arranged to correspond to the centres of the magnetic excitation field thus produced from the taps.

If two segments of the excitations coil 10 are simultaneously energised, the magnetic field strength produced will be higher than if only a single segment is used. We have found that it can be treated as being twice as high, and therefore when 2 adjacent segments are energised the energising current may be halved or the signals derived from the coils 40, 41 may be halved to correct for this.

It will be recalled from our earlier application GB 2256713 that each figure of "8" winding corresponds to 2 coils connected "back-to-back"; that is in the opposite rotational senses so that their outputs are subtractive rather than additive. Accordingly, the effects of the extended figure of "8" coils 40, 41 can be achieved by providing separate coils as shown in Fig. 4, and either connecting them back-to-back or reading the signals from the coils and then performing a signal processing step of subtracting the coil outputs; thus, an array of separate coils can be processed by subtracting adjacent outputs to provide a corresponding signal.

It will be clear from the foregoing that the general principle of providing a travelling excitation fields scanned electrically along a surface and employing eddy current sensing means along it is itself inventive; particularly if the sensing means comprise two relatively offset sub arrays. Although very preferably conductive coils are used as the sensing means, other eddy current sensing could be used.

The principles of a positional displacement measuring sensor will now be discussed in general terms.

If the excitation current flowing through the two wires of Figures 1 to 3 is an alternating current, the orientation of the field, and thus the orientation of the wire pair, may be determined by orthogonal pairs (X1, X2; Y1, Y2) of search coils.

A single x coil and single y coil may be used but will give a lower sensitivity than pairs of coils (X1, X2; Y1, Y2) . The positioning of such single coils will be more critical than for pairs of coils as they lack tbe compensation action inherent within pairs of coils.

Resolution of the x and y field components allow the direction of the field to be determined thus the position of the search coils may be found in relation to the twists within the pair of wires.

If the rate of twists of the wire pair is known (mm per twist) the position of the search coil assembly relative to a reference position may be found as the search coils are moved axially along the twisted pair without rotation relative to the pair.

dO = direction of field at reference position (degrees) 61 = direction of field at measurement position (degrees) f = length of each twist n = number of cycles sensed

Consider a current flowing in a twisted wire pair such that the current flowing outward in the first wire is returned via the second wire, as shown in Figure 2.

Consider a very short section of the twisted pair, the two conductors may be regarded as being a section of an elongate flat coil having a single turn as shown in Figure 3.

Thus a magnetic field is created having its axis normal to the plane passing through the two sections of wire.

The twisted wire pair may be regarded as consisting of a large number of such sections of parallel wire pairs with each section having its plane rotated slightly relative to its neighbours.

Thus the twisted wire pair, when it carries a current, is a generator of a magnetic field which crosses the axis of the twist but having a direction which is dependent upon axial position along the wire pair. For one complete twist of the wires, the field will progressively rotate through 3600 over the length of that twist.

The above displacement measuring system relies upon the maintenance of rotational orientation of the excitation wires and the sensor assembly.

A reference signal may be produced by incorporating a simple straight excitation wire pair within the excitation lead. The excitation may be rapidly switched between the measurement pair and the reference pair.

Alternatively the reference pair may be twisted in opposite sense to the first excitation pair.

Other excited pairs may be incorporated having other rates of twist so that several outputs are produced with different repeat intervals. Thus there will be unique patterns of output phase which repeat in intervals longer than the interval of any individual pair. For example the output phase from two excitation windings having pitches of 9 units and 10 units will repeat only every 90 units of length.

To sum up, an elongate twisted wire pair, energised with an AC field, may be used to generate a magnetic field the direction of which rotates along the length of the elongate wire. The position of a magnetic sensor element adjacent to the wire may be detected by detecting the rotational direction of the field; where the magnetic sensor is moved along the wire (for example, in a linear scan) , movement of these magnetic sensor may be tracked by counting the number of rotations through which the magnetic field goes (there being one rotation per twist of the twisted wire pair).

The magnetic sensor may comprise a pair of magnetic coils arranged in different planes; conveniently the planes may be mutually normal; conveniently, if the magnetic sensor is being moved along a surface, one may be parallel to the surface and the other may be normal to the surface.

It will thus be seen that this form of positional measuring device is advantageously employed in the above described invention or that described in our earlier mentioned UK Patent Applications, as a means of deriving a probe position when a probe is moved along a weld, for example; in this case, a twisted wire is laid along the weld surface, and a pair of search coils arranged mutually normally are included in the probe, and the probe is moved along the weld surface within the magnetic field of the twisted wire pair. The output of the magnetic sensor coil pair thus can be used to track the position of the probe.

Claims

CLAIMS:

1. A positional sensing system comprising means for generating an elongate alternating magnetic field, the direction of said magnetic field varying along its length, and means for sensing the direction of said magnetic field, and hence determining the position of the sensing means within the magnetic field.

2. A system according to claim 1 in which the elongate magnetic field generator means consists of a twisted wire pair, the wires of the pair being connected so that current passing along one wire is returned via the other.

3. A system according to claim 1 or claim 2 in which the sensing means comprises a pair of orthogonal field sensors.

4. A system according to claim 3 in which the sensing means comprise two said pairs of orthogonal field sensors.

5. A system according to claim 3 or claim 4 in which said field sensors each consist of a sensing coil.

6. A system according to any of claims 1 to 5 in which there are provided further said means for generating further said elongate magnetic fields, having a different rate or rates of variation of direction of said magnetic field along their lengths.

7. A probe array comprising a linearly extending magnetic field generator (10) and means (120) for selectively energising portions of the generator (10) to provide magnetic fields centred at different positions along the length thereof, and sensing means (40, 41) comprising a plurality of portions disposed along the length of the generator (10) each portion being arranged to a respond to a magnetic field at a corresponding position providing from the generator (it).

8. An array according to claim 7 in which the sensing means (40, 41) comprise a pair of mutually offset inductive current sensors.

9. Apparatus according to claim 7 or claim 8, in which at least one of the sensors comprises a plurality of discrete coils (40a-40d) disposed along the length of the generator (10).

10. An array according to claim 7 or claim 8, in which at least one of the sensors comprises a coil provided as a closed serpentine path dividing the coil ;nto a plurality of lobes of opposite rotational senses.