GB2078968A - Magnetic sensors having misalignment compensating means - Google Patents

Abstract

In magnetic sensing apparatus having a pair of directional magnetic field sensors e.g. fluxgates longitudinally spaced apart in a containing tube (14), mechanical means is provided to compensate for misalignment in the sensor axes. The compensating means takes the form of a non-magnetic member (18) associated with one end of the tube (14) for rotation about the tube axis and a screwed plug (20) of easily magnetizable material contained in a diametrically extending threaded bore (24) formed through the rotatable member (18). Adjustments in the position of the plug (20) relative to the sensors e.g. (A) may thus be made by screwing the plug 20 inwardly and outwardly of the bore (24) to adjust the radial distance of the plug (20) from the axis of the tube and the orientation of the plug (20) relative to the sensors may be adjusted by rotating the member (18) in which it is contained relative to the tube (14). <IMAGE>

Description

SPECIFICATION Magnetic sensors having misalignment compensating means Background of the invention The invention relates to apparatus for measuring or sensing magnetic phenomena, such as magnetic fields, objects or disturbances, and is more particularly concerned with a magnetic locator or gradiometer incorporating mechanically adjustable magnetic compensation for misalignment of the axes of the magnetic sensor elements.

Saturatable core (fluxgate) magnetic locators or gradiometers comprise at least two electrically matched field-sensing elements mounted on a nonmagnetic structure such that their magnetic axes are, theoretically, strictly parallel or coaxial. The output signals of the two sensors are arranged such that they oppose each other. If the structure is oriented in any direction in a uniform magnetic field, the field components existing at each sensor are equal, so that there is no resulting output signal from the combination of the sensors.

If a magnetic object exists within the detection range of the instrument, the magnetic field will generally be stronger at one of the sensors than at the other sensor and as a result, the output signal of one sensor will be greater than that of the other, so that a net difference signal will be produced which is indicative of the presence of the object.

For complete accuracy of operation, the magnetic axes of the two sensors should be precisely aligned.

The precision required in such alignment is of the order of 3 seconds of arc if the error signal due to misalignment of the magnetic axes is to be less than 1 gamma (10-5 gauss) in an ambient magnetic field of 60,000 gammas. If the magnetic axes of the two sensors are not precisely aligned, the component of the ambient magnetic field existing along the magnetic axis of one sensor will not be the same as the component of the magnetic field existing along the magnetic axis of the other sensor. Accordingly, the difference between the two sensor signals will not be zero, and a false signal will be obtained due to the mechanical misalignment of the sensors.

In certain priorartfluxgate gradiometers, alignment of the sensors has been achieved by mechanical adjustment or bending of structural members, such as a tube in which the sensors are mounted.

See, for example, applicant's prior U.S. Patent No.

3,050,679, issued August 1962. With this arrangement, after a certain time, the stress may be relieved and the sensors may become misaligned resulting in inaccuracies and the need for readjustment.

Another approach to the problem of sensor misalignment is to provide the apparatus with misalignment compensating means. Thus, applicant's prior U.S. Patent No.3,488,579, issued January 6, 1970, discloses a system in which misalignment compensation is produced electrically. While this system is capable of compensating for misalignment with high precision, its cost may be higher than is justified when such high accuracy is not required.

It has also been proposed to compensate for extraneous magnetic effects and misaligned cores by providing bodies of easily magnetizable material adjustably positioned adjacent to a gradiometer.

See, for example, U.S. Patent No. 2,966,853, issued January 3, 1961 to Gilfillan et al; U.S. Patent No.

3,012,191, issued December 5,1961 to Milleretal; U.S. Patent No. 2,976,483, issued March 1961 to Moore et al; and applicant's prior U.S. Patents Nos.

3,487,459, issued December30, 1969; and 3,757,209, issued September 4,1973.

The compensating system disclosed in Patent No.

3,757,209, aforesaid, is of particular interest because it consists of a mechanical compensating means which is mounted on the sensor tube itself without being attached in any way to the sensor core, thereby minimizing the possibility of creating stresses in the core resulting in sensing errors. Further, by mounting a mechanical compensator on the sensor tube, the system lends itself to use with portable sensing apparatus.

In one form of sensing apparatus in accordance with Patent No. 3,757,209 the mechanical compensator consists of a nonmagnetic annulus surrounding the tube and having a magnetic body attached thereto. The annulus can be moved both around and along the tube to establish the correct position of the magnetic body to obtain optimum compensation. In another form of the apparatus, the mechanical compensator consists of a non-magnetic cap fitting in one end of the sensing tube and having a magnetic body eccentrically fixed on the inside of the cap. Rotation of the cap within the tube adjusts the position of the magnetic body to obtain optimum sensor compensation. This arrangement is more compact than the previously described annulus but is less flexible since there is no facility for adjusting the position of the magnetic compensating body lengthwise of the tube.

Brief summary of the invention It is an object of the present invention to provide a magnetic sensing apparatus with mechanical means compensating for sensor misalignment, which means combines both compactness with a desirable range of compensation adjustment.

Another object of the invention is to provide apparatus of the type set out in the preceding paragraph which avoids stressing the magnetic core or support structure therefor.

In its broadest aspect the present invention provides magnetic sensing apparatus including at least two sensing elements mounted on a core within a nonmagnetic tube and means for mechanically compensating for misalignment of such elements, the compensating means comprising a nonmagnetic member carried by the tube for rotation about the tube axis and a body of easily magnetizable material carried by the nonmagnetic member in a manner permitting adjustment of the position of this body relative to the nonmagnetic member.

In a preferred form of the invention, the nonmagnetic member forms an end cap for the snsor tube or is attached to the outside of the end cap and the magnetic body comprises a plug threaded into a radially extending bore in the nonmagnetic member.

With this arrangement, adjustments can be made both in the rotational orientation of the magnetic body relative to the tube by suitable positioning of the end cap about the tube axis, and also to the radial distance of the body from the tube axis by adjusting movements of the magnetic body within the threaded bore.

It has been found that this facility for adjusting the radial distance of the magnetic body from the tube axis, which is not available in apparatus as disclosed for example in U.S. Patent No. 3,757,209 contributes significantly to the degree of flexibility offered by the instrument. Moreover, the magnetic body can be made accessible from the exterior of the tube so that adjustments both of this body and of the nonmagnetic member are thereby facilitated.

Brief description ofthe drawings The invention is described, by way of example only, with reference to the accompanying drawings.

Figure 1 is a plan view of a first embodiment of magnetic sensing apparatus in accordance with the invention; Figure 2 is a sectional view of the distal end portion of the apparatus shown in Figure 1; Figure 3 is an end view, with the cover removed, of the tube shown in Figure 2; Figure 4 is a sectional elevation of part of a second embodiment of a magnetic sensing apparatus in accordance with the invention; and Figure 5 is an end view of the apparatus shown in Figure 4.

Description ofpreferred embodiments Referring to Figure 1, reference numeral 10 designates magnetic sensing or detecting apparatus, such as a magnetic locator, employing compensating means in accordance with the invention. The locator which may be employed for locating hidden utility pipes, surveyor's bench marks, etc. comprises a cylindrical case 12, containing electronics, and a nonmagnetic sensor-containing tube 14 extending from the case 12. The apparatus is portable in use and the distal end of tube 14may be swept back and forth along the ground in a scanning motion. A distinctive signal is produced in earphones 16 when a magnetic object is located.

Tube 14 which is formed of nonmagnetic material such as aluminum, for example, contains magnetic field sensors A and B mounted on a suitable core having a friction fit in tube 14 in known manner.

Sensors of the fluxgate type suitable for use in the apparatus are disclosed, for example, in applicant's prior U.S. Patent No. 2,981,885, issued April 25,1961.

The sensors per se and the electronics associated therewith are well known, and will not therefore be described herein in detail. A suitable sensor may, for example, comprise a tubular magnetic core having excitation windings wound longitudinally through the core and having pickup windings wound circumferentially about the core. In operation, the excitation windings are supplied with alternating current by an oscillator. Second harmonic fluxes generated in the sensor cores due to the effect of an external magnetic field acting along the core axes (parallel to the length of the cores) cut the pickup windings and generate second harmonic output voltages therein.

The output voltages from the sensor are applied differentially to suitable measuring apparatus so that if the sensors are matched and, if their axes are perfectly aligned, the meter will read zero in a uniform field. If, however, there is a misalignment of one of the core axes, an error signal will be generated due to such misalignment.

The present invention is not concerned with the precise manner of mounting the sensors and windings therefor, nor with the precise manner in which the signals from the fluxgate sensors are detected. It,, will be apparent, however, to those skilled in the art that the sensors A and B may be mounted substantially coaxially upon a ceramic core, as described in aforesaid U.S. Patent No. 3,487,459 which may then be press-fitted into tube 14 as indicated, with wires from the respective sensors passing through tube 14 and into the case 12 (with the possible interposition of electric plug and socket connections between the tube and case, as indicated), the case containing the oscillator, amplifier, etc.

As previously indicated, to obtain precise aiign mentofthe longitudinal axes of sensors A and B is an expensive and tedious operation. Therefore, in accordance with the invention, means is provided for obtaining mechanical compensation for misalignment of the sensor axes. To this end, tube 14 is provided with an end cap 18 of nonmagnetic material, such as hard synthetic plastic material, this end cap containing a plug 20 of easily magnetizable material, for example Permalloy. An O-ring seal 22 fits between cap 20 and tube 14 and provides resistance to turning of cap 20 so that the cap will stay in a particular orientation to which it is rotated relative to tube 14.

Plug 20 is in the form of a screw contained within a threaded bore 24 extending diametrically through the end cap. Thus, the radial distance of plug 20 relative to the axis of tube 14 can be adjusted by screwing the plug inwardly or outwardly relative to bore 24. Externally, end cap 18 is provided with openings 26 for a wrench or the like which may be used to rotate the end cap relative to tube 14.

In use, the orientation of plug 20 relative to sensors A and B can be adjusted by rotating end cap 20 within tube 14, and the radial distance of the plug relative to the tube axis can be adjusted by screwing the plug within bore 24. These two variables may he used to obtain the correct positioning for plug 20, in order to obtain optimum compensation for misalig1n- ment of the sensors A and B. The theory in obtaining such mechanical compensation will not be described herein in detail and is adequately dealt with in the above-mentioned U.S. Patent Nos. 3,757,209 and 3,487,459, the disclosures of which are incorporated herein by reference. In order to adjust the compensating mechanism of the invention, to obtain optimum compensation for misalignment of sensors A and B, the apparatus 10 may be placed in a fixture (not shown) so that it can be rotated about the axis of tube 14 in an ambient magnetic field perpendicular to this axis. If the axes of the sensors A and B are not precisely aligned with or parallel to the axis of tube 14, a signal will be generated when the assembly is turned about the axis of the tube. The position of plug 20 may then be adjusted either radially in bore 24 or by rotation of cap 18, or both, in order to minimize the generated signal.When the optimum positioning of plug 20 is obtained, the plug may be fixed within bore 24 by cement, to prevent it from turning, and a cup-like cover 28 of resilient material such as rubber is then positioned over the end of tube 14to embrace both the cap and the end of the tube.

A second form of magnetic sensing apparatus in accordance with the invention is illustrated in Figures 4 and 5 and like reference numerals are used to denote the parts thereof corresponding to parts of the previously described embodiment. In this case, tube 14 is provided with a plastic end cap 18a and an O-ring seal 22a is again provided between the end cap and tube 14. Cap 18a may in this case be non-rotatably mounted in the end ofthetube.

The mechanical compensating mechanism in this embodiment takes the form of a cylindrical plastic plate 30 rotatably mounted in a recess on the exterior of end cap 1 8a and held in place by brackets 32 screwed into the end cap by means of screws 34.

Plate 30 is again provided with a diametrically extending threaded bore 24a containing the plug 20a of easily magnetizable material. Plate 30 has external formations 26a and 26b allowing the plate to be rotated relative to end cap 18a when the brackets 32 are suitably loosened.

It will be appreciated that adjustments in the compensating mechanism in this embodiment are made in a similar manner as with the previously described embodyment. Thus, plug 20a may be screwed inwardly and outwardly within bore 24a to adjust its radial distance relative to the axis of tube 14 and may again be cemented in place. The orientation of plug 20a relative to sensors A and B can again be adjusted by suitably rotating plate 30 in end cap 18a and then tightening the plate down by means of screws 34.

It will be seen that the invention provides misalignment compensating means for magnetic sensing apparatus, which means offers a relatively wide range of adjustment while providing a compact structure without internal modification of the sensing equipment and which is readily adaptable to the tube containing such equipment. Further, adjustments of the compensating means can be made wholly from the exterior of the tube without requiring the removal of the end cap, thereby facilitating its use.

While the invention is particularly suitable for use on portable type gradiometers and the like, as shown in Figure 1, it is not limited to this application, and the described compensating means can be used on other types of magnetic sensing apparatus which include a pair of magnetic sensors within a nonmagnetic tube.

While only preferred embodiments of the invention have been described herein in detail, it will be appreciated that numerous modifications may be made within the scope of the attached claims. For example, the compensating means of the invention may, in an alternative form, comprise an annulus mounted for both longitudinal and rotary movement on tube 14 and incorporating a radially extending threaded bore accommodating a screwed plug of magnetic material. This form of the invention offers an additional axis of compensation compared with the previously described embodiments, namely lon gitudinally of tube 14, but the arrangement is not as compact as those previously described.

Claims (12)

1. Mangetic sensing apparatus comprising an elongated nonmagnetictubecontaining a pair of directional magnetic field sensor means longitudinally spaced apart in said tube and having individual magnetic axes approximately aligned with the longitudinal axis of said tube, and means for compensating for misalignment of the axis of either of said sensor means with said longitudinal axis, said compensating means comprising a nonmagnetic member carried by said tube, a body of easily magnetizable material carried by said member, means for adjusting the circumferential orientation of said body relative to said longitudinal axis and means for adjusting the distance of said body from said longitudinal axis.

2. The apparatus defined in claim 1,wherein said means for adjusting the distance of said body from said longitudinal axis comprises means for radially adjusting the position of said body relative to said member.

3. The apparatus as defined in claim 1 or claim 2, wherein said means for adjusting the circumferential orientation of said body relative to said axis comprises means permitting rotational adjustment of said nonmagnetic member on said tube.

4. The apparatus as defined in claim 2, wherein said means for adjusting the position of said body relative to said member comprises a screw threaded bore formed in said member, and said body comprising a threaded plug screwed into said bore.

5. The apparatus as defined in claim 1, wherein said member comprises a cap fitted to one end of said tube.

6. The apparatus defined in claim 5, wherein said cap has a friction fit with said tube permitting rotational adjustment of said cap about said longitudinal axis for adjusting the circumferential orientation of said body.

7. The apparatus defined in claim 5 or claim 6, wherein said cap includes a threaded radially extending bore and said body comprises a threaded plug screwed into said bore.

8. The apparatus as defined in claim 7, wherein said bore is located in a portion of said cap protruding from said tube.

9. The apparatus as defined in claim 5 or claim 6, including a cup-shaped cover embracing said cap and an end portion of said tube.

10. The apparatus as defined in claim 1, including a cap on one end of said tube, said member being mounted on the exterior of said cap for rotational adjustment relative to said cap to adjust the circumferential orientation of said body relative to said axis.

11. The apparatus as defined in claim 10, wherein said member includes a radially extending threaded bore and said body comprises a threaded plug screwed into said bore.

12. The apparatus as defined in claim 10 or claim 11, including releasable bracket means securing said member to said cap for rotational adjustment of said member relative to said cap.