GB2363462A

GB2363462A - Apparatus and method for locating positions

Info

Publication number: GB2363462A
Application number: GB0014275A
Authority: GB
Inventors: Nicholas Robert Shephard; Timothy David Lomax
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-06-13
Filing date: 2000-06-13
Publication date: 2001-12-19
Anticipated expiration: 2020-06-13
Also published as: GB2363462B; GB0014275D0

Abstract

Apparatus (18, 20) and associated method for locating a point (12) through a barrier (14) such as a wall, comprises a permanent magnet (18) to generate a magnetic field being positioned at the point on one side of the barrier. A magnetic field detector (20) is manually movable across the opposite side of the barrier thereby detecting the magnetic field of the magnet and hence determining the position of the magnet and its corresponding point on the opposite side of the barrier. The magnetic field detector (20) comprises a magnetoresistive sensor and is moved around on the barrier surface to find the position where the field sensed is greatest thus indicating the location of the point.

Description

A. 2363462 "k I 11 Position Location Apparatus The present invention

relates to apparatus for locating a point through a barrier and particularly, but not exclusively, relates to apparatus for locating corresponding points on either side of a wall.

When installing cabling, piping, access holes and various structural items, it is often necessary to locate the same position on either side of a barrier, such as a wall, in order to drill a hole through the barrier at a particular point, or to correctly position different components. It can be difficult to locate the required position on each side of the barrier, to ensure that the hole is straight or that the components are at the correct location on either side of the barrier.

The location of a corresponding point on the opposite side of a barrier is currently done by measuring the distance of a point on - one side of the barrier from the top and/or base of the barrier and from one or each edge of the barrier, and then measuring out corresponding distances on the opposite side of the barrier to locate the corresponding point on that side. This is a time consuming process which is not particularly accurate.

According to an aspect of the present invention there is provided apparatus for locating a point through a barrier, the apparatus comprising a magnetic field generator means locatable at the point on one side of the barrier and magnetic field detector means manually movable across the opposite side of the barrier, operable to detect the magnetic field of the magnetic field generator means and to thereby determine the position of the magnetic field generator means and hence the corresponding point on the opposite side.

The magnetic field detector means preferably comprises magnetoresistive sensor means. The magnetoresistive sensor means is preferably operable to detect the direction and magnitude of a magnetic field. The magnet oresistive sensor means is preferably sensitive in each of two substantially perpendicular directions. The magnetoresistive sensor means is preferably an anisotropic magnetoresistive sensor means.

I I JI The magnetoresistive sensor means preferably comprises a magnet oresist or, and most preferably comprises four magnetoresistors arranged in a Wheatstone bridge configuration. The or each magnetoresistor preferably comprises a strip of ferrous material. The magnetoresistor Wheatstone bridge is preferably provided as an integrated circuit.

The magnet field detector means desirably additionally comprises field strength indicator means. The field strength inaicator means is preferably operable to generate an output signal which is variable in relation to the magnetic field strength detected. The field indicator means preferably comprises an audio signal generator means. The frequency of the output audio signal is preferably variable in relation to the magnetic field strength detected, The field strength indicator means may alternatively comprise a visual display means on which an image relating to the strength of the magnetic field detected may be displayed. The display means may be a liquid crystal display.

Desirably, the magnetic field detector means further comprises a processing means operable to process a signal generated by the magnet ore sist ant sensor means. The processing means is desirably a microprocessor. The microprocessor preferably provides analogue to digital conversion means operable to receive and convert the signal generated by the sensor means, and controller means, in communication with the said conversion means, operable to control the field strength indicator means. The analogue to digital conversion means is preferably further operable to generate an output signal following receipt of the signal from the sensor means. The controller means is preferably operable to generate and send a signal to the field strength indicator means following receipt of the output signal from the analogue to digital conversion means.

Desirably, the processing means further comprises amplifier means, provided between the magnet or esist ant sensor means and the microprocessor, operable to amplify the signal generated by the sensor means. The amplifier means preferably comprises a differential amplifier circuit, provided between the Wheatstone bridge of the magnetoresistive sensor means and the microprocessor, operable to compare the output voltages from the Wheatstone bridge and generate a corresponding output signal when the voltages are different. Preferably, the amplifier means further comprises a variable gain amplifier circuit provided in series with the differential amplifier circuit and before the microprocessor, operable to amplify the signal received from the differential amplifier circuit up to an acceptable voltage range for receipt by the analogue to digital conversion means.

The controller means is, additionally, desirably operable to set the gain of the variable gain amplifier circuit.

The apparatus preferably additionally comprises means operable to compensate for the presence of an ambient magnetic field. The said means desirably comprises a sampling circuit operable to sample the output signal from the variable gain amplifier during exposure of the sensor means to the ambient magnetic field and an offset circuit, connected to the sampling circuit, operable to vary one or more characteristics of the magnetic field detector means in order to compensate for the detected ambient magnetic field. The offset circuit is preferably operable to couple a magnetic field onto the magnetoresistor Wheatstone bridge which nulls out the ambient magnetic field.

The magnetic field detector means preferably further comprises means operable to reset the magnetoresistive sensor means, and most preferably the magnetoresistor Wheatstone bridge integrated circuit, in response to detection by the sensor means of a magnetic field the magnitude of which causes overloading of the sensor means.

The magnetic field detector means preferably additionally comprises switch means for terminating power supply to the magnetic field detector means. The switch means is desirably a timer controlled switch means operable to terminate the power supply after a predetermined period.

The magnetic field generator means is desirably a permanent magnet, and is most preferably a Boron Neodymium magnet. Preferably at least two, and most preferably three magnets, are provided. The or each magnet is preferably provided within a housing member. The housing member desirably comprises a casing, having an end wall member and at least one side wall member, which defines a socket within which the or each magnet is receivable. The socket is preferably substantially cylindrical.

Preferably, a mounting means is provided on the side wall member for mounting the housing member and the magnet or magnets on one side of a barrier at a desired point. The mounting means desirably comprises a tab member extending generally radially outwardly from the side wall member.

Preferably, reusable contact adhesive is provided on the or each magnet by which the or each magnet may be attached to the barrier member. Reusable contact adhesive may alternatively or additionally be pTovided on the housing member and/or the mounting means.

The tab member may alternatively or additionally have an aperture provided therein. The aperture is preferably suitable for receiving fixing means by which the housing member may be attached to the barrier member. The aperture may alternatively or additionally be suitable for receiving marker means therethrough for marking the position of the housing member on the barrier.

According to a further aspect of the present invention there is provided a method for locating a point through a barrier, the method including locating a magnetic field generator means at the point on one side of the barrier and manually moving magnetic field detector means across the opposite side of the barrier to detect the magnetic field of the magnetic field generator means, to thereby determine the position of the magnetic field generator means and hence the corresponding point on the opposite side.

The magnetic field detector means is preferably according to paragraphs four to twelve above. The magnet is preferably according to paragraphs thirteen to sixteen above.

The method preferably further includes manually moving the magnetic field detector means across the opposite side of the barrier along a substantially horizontal path until the maximum magnitude magnetic field is detected. The method preferably further includes manually moving the magnetic field detector means across the opposite side of the barrier along a substantially upwardly extending path until the maximum magnitude magnetic field is detected.

The magnetic field detector means is preferably manually moved back and forth across the opposite side of the barrier along one or both of the substantially horizontal and upwardly extending paths.

Preferably, the magnetic field detector means is manually moved back and forth across a first one of the substantially horizontal and upwardly extending paths and is subsequently moved back and forth across the second one of the substantially horizontal and upwardly extending paths. The said second one of the paths desirably crosses the said first one of the paths at the position along the first one of the paths at which the maximum magnitude magnetic field was detected.

The method desirably further includes the step of compensating for the presence of an ambient magnetic field before use of the magnetic field detector commences.

A specific embodiment of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Fig 1 is a diagrammatic representation of apparatus according to the present invention in use on a wall section; Fig 2 is a side view of figure 1; Fig 3 is a plan view of figure 1; Fig 4a is a diagrammatic representation of the magnet housing used in figure 1; Fig 4b is a plan view of the housing of figure 4a; Fig 4c is a sectional view along line H in figure 4b; Fig 5 is a schematic representation of the magnet ore sist ant sensor of figure 1 and the accompanying electronic control circuitry; Fig 6a is a detailed schematic representation of part of the control circuitry of Fig 5; Fig 6b is a detailed schematic representation of -a further part of the control circuitry of Fig 5; Fig 6c is a detailed schematic representation of a further part of the control circuitry of Fig 5; and Fig 6d is a detailed schematic representation of a further part of the control circuitry of Fig 5.

Referring to the drawings, the present invention provides apparatus 10 for locating a point 12 through a barrier. As shown in this example, the apparatus 10 is particularly suitable for locating corresponding points 12, 16 on either side of a wall 14.

The apparatus 10 comprises magnetic field generator means in the form of a permanent magnet 18 locatable at a point 12 on a first side 14a of the wall, and magnetic field detector means 20 which is manually movable across the opposite side 14b of the wall. The detector means 20 is operable to determine the position of the magnet 18 on the first side 14a of the wall, thereby locating the corresponding point 16 on the opposite side 14b of the wall. The detector means 20 is freely movable across the surface 14b of the wall in both the horizontal and vertical directions, i.e. generally left to right and up and down across the wall 14 shown in Fig. 1, as shown by the arrows marked H and V in Figs. 2 and 3.

In this example, the magnet 18 comprises three Boron Neodymium magnets, which are provided in a housing 22, shown in Figs. 4a to c. The housing 22 comprises a casing 24 which defines a socket 26 for receiving the magnets 18. The housing 22 also has a tag 28 on which there is provided reusable contact adhesive (not shown) by which the housing 22 and the magnets 18 may be removably attached to the first side 14a of the wall. The tag 28 also has an aperture 30 provided therein suitable for receiving a nail by which the housing 22 may alternatively be fixed onto the wall 14.

Alternatively, if the housing 22 and magnets 18 are being manually held in position on the first side 14a of the wall, the position of the housing 22 may be marked on the wall 14a by locating a marker, such as a pen, through the aperture 30 in the tag 28, in order to make a mark on the wall 14a.

The magnetic field detector 20 comprises an anisotropic magnet ore si stive sensor 32, in the form of four magnet oresist ors 56 arranged in a Wheatstone bridge configuration. The magnetoresistive Wheatstone bridge 54 is provided in the form of a microcircuit provided in a electronic chip r ( -rm) package 62, such as the HoneywelltMC1021Z. The magnet ore sis tive sensor 32 is operable to detect both the direction and magnitude of a magnetic field, and is sensitive in each of two substantially perpendicular directions.

Various control electronics, shown in Figs. 5 and 6 a-d, are provided within the magnetic field detector 20, associated with the magnetoresistive sensor 32, as follows. Referring to Fig. 5, the control electronics includes a differential amplifier circuit 34, a variable gain amplifier circuit 36 and a microprocessor 38. The microprocessor 38 is programmed to serve as an analogue to digital converter 40 and a controller 42. A magnetic field strength indicator, in the form of an audio speaker 44, is connected to the microprocessor controller 42, and is operable to indicate to a user the strength of a magnetic field detected by the magnet ore sistive sensor 32. A manually operable search switch 46 is provided in connection with the controller 42, for operation by a user to commence detection by the magnet ore si stive sensor 3 2.

An offset circuit 48 is provided within a feedback loop from the output of the variable gain amplifier circuit 36 to the magnet ore sistive sensor 32. The offset circuit 48 is operable to compensate for a background ambient magnetic field. An offset switch 50 is provided in connection with the microprocessor controller 42, for operation by a user in order to activate the offset circuit 48.

A reset circuit 52 is provided in communication with the magnet ore sistive sensor 32, and is operable by the controller 42. The reset circuit is operable to reset the magnetoresistive sensor 32 when the sensor 32 has been overloaded as a result of detection of a magnetic field having a magnitude outside the operating range of the sensor 32.

A timer controlled switch is provided by the microprocessor 38 for terminating the power supply to the magnet ore sistive sensor 32 after a predetermined period of inactivity.

The apparatus 10 operates generally as follows. As the magnetic field detector 20 is moved across the opposite side l4b of the wall, the magnetic field of the magnets 18 is detected by the magnet ore si stive sensor 32. In response to detection of a magnetic field the magnetoresistive sensor 32 generates an output signal, which is passed via the differential amplifier 34 and the variable gain amplifier 36 to the analogue to digital convertor 40 provided by the microprocessor 38. Following receipt by the analogue to digital convertor 40 of the amplified output signal from the magnetoresistive sensor 32, the controller 42 is caused to generate an output signal which is sent to the audio speaker 44. As the magnitude of the magnetic field detected by the magnet oresistive sensor 32 increases or decreases the frequency of the output signal from the audio speaker 44 rises and falls correspondingly, thereby indicating to a user the strength of the magnetic field detected at any particular point.

If there is an ambient magnetic field present, this can be compensated for by operating the offset circuit 48 provided within the magnetic field detector 20. Offset compensation is initiated by actuation of the offset switch 50 by a user.

It will be appreciated that a particular magnet oresistive sensor 32 will have an upper limit magnetic field strength above which it cannot operate. If the magnet oresistive sensor 32 should be exposed to a magnetic field having a magnitude above this upper limit it may be necessary to reset various characteristics of the magnet oresistive sensor 32. Resetting the magnetoresistive sensor 3 2 is achieved by activating the reset circuit 3 2.

The various parts of the control circuitry are shown in more detail in Figs. 6a to 6d. The magnet ore sistive sensor 32 comprises a Wheatstone bridge arrangement 54, having four magnetoresistors 56. The voltages at the mid points 58, 60 of the Wheatstone bridge 54 are fed, via pins I and 8 of the chip 62 to a first buffer amplifier 64 and a second buffer amplifier 66 respectively. The outputs from the buffer amplifiers 64, 66 are fed via resistors R4, R5 to respective inputs of the differential amplifier 34. If the voltages at the two points 58, 60 on the Wheatstone bridge 54 are equal, the output voltage from the differential amplifier will be zero. However, if the voltages at the two points 58, 60 on the Wheatstone bridge 54 are different, then a corresponding output voltage will be generated by the differential amplifier 34.

The output voltage from the differential amplifier 34 is passed to the variable gain amplifier circuit 36, where an appropriate level of gain is applied to the signal in order to amplify it up to a level suitable for acceptance by the analogue to digital converter 40 in the microprocessor 38. The gain of the variable gain amplifier 36 is set by the controller 42 by means of a switch array 68. The switch array 68 is provided as part of an integrated circuit (IC), U3 in Fig. 6 a. The switch array 68 is controlled by the controller 42 in the microprocessor 38. The gain is set by connecting the switch arm 70 to an appropriate one of pins 1, 2, 4 and 5 of the switch IC, to connect a feedback resistor R6 in series with a resistor selected by the switch, between the output and one input of the amplifier 36. In the event that pin 4 is selected, the resistor R6 is connected directly across the input and output.

The variable gain amplifier circuit 36, enables the apparatus 10 to be used on various wall thicknesses. The strength of the magnetic field experienced by the sensor 32 decreases with distance away from the magnets 18. This means that the strength of the magnetic field to be detected by the magnet ore si stive sensor 32 falls as the wall thickness increases. A smaller magnetic field strength to be detected by the magnet ore sistive sensor 32 will result in a lower voltage output signal from the differential amplifier circuit 34, and the signal will therefore require a greater amount of amplification by the variable gain amplifier circuit 36 before it can be accepted by the analogue to digital converter 40.

The output signal from the variable gain amplifier 36 is passed at Y to the analogue to digital converter 40 provided by the microprocessor 38. The digital signal generated by the analogue to digital converter 40 is passed to the controller 42 within the microprocessor, in response to which the controller causes an appropriate signal to be sent to the signal strength indicator 44, shown in Fig. 6c, which in this example comprises an audio speaker 72. The audio speaker 72 then emits an audio signal of an appropriate frequency, the frequency of the audio signal increasing as the strength of the magnetic field detected increases.

The offset circuit 48 is connected to the output from the variable gain amplifier 36. As discussed above, the offset circuit 48 is operable to compensate for any ambient magnetic field. The offset circuit 48 is switched into and out of operation via a switch array 74, provided within the integrated circuit chip U3. The switch array 74 is controlled by the controller 42 provided by the microprocessor 38. The two switch arrays 68, 74 provided within chip U3 operate in tandem, so that as switch 68 moves between pins 1, 5, 2 and 4, switch 74 moves between pins 12,14, 15 and 11.

When switch 74 is connected to pin 12 capacitor C4 is switched into circuit and charges up to a level which drives the offset strap 32a of the magnetoresistive sensor to couple a magnetic field onto the Wheatstone bridge 54 which nulls out the ambient magnetic field. When switch 74 is connected to any of pins 14, 15 and 11 the capacitor C4 holds the offset strap 32a in the nulled state.

As shown in Fig. 6b, an offset switch 50 is provided for operation by a user in order to initiate offset compensation for any ambient magnetic field present. When the switch 76 is depressed by the user, an LED 78 is illuminated to indicate that offset compensation is in process, and the controller 42 is instructed at B to activate the offset circuit 48. The Z output from the microprocessor 38 holds the detector 20 in a "power-on" state. Power is supplied to the detector 20 initially when the calibrate switch 76 is activated, as transistor Q4 is bypassed. After several seconds with _the calibrate switch 76 activated, a timer within the microprocessor 38 sends the Z output high. This turns transistor Q4 on via transistor Q3 and maintains power to the detector 20 for as long as Z is held high. After a period of inactivity, the microprocessor 38 pulls Z low, which then turns off the power to the detector 20.

The reset circuit 52 is operable to supply a signal to the reset strap 32b of the magnet oresistive sensor chip 62, in order to reset the magnetoresistive sensor 32 when the sensor has been overloaded by detecting a magnetic field having a magnitude greater than the upper detection limit of the magnet oresis tive sensor 32.

The operational amplifiers used within the electronic control circuitry shown in Figs. 6a to 6c are bipolar amplifiers, and therefore require a dual rail supply of +5V and -5V. A +9V supply 82 (shown in Fig. 6b) is converted to a +5V by voltage regulator U6. Part of the +5V supply 84 is passed to the circuit shown in Fig. 6d where IC device U7 converts the + 5V supply into a -SV supply 86 to thereby provide the negative power rail.

Before commencing use of the detector 20, any ambient magnetic field must be compensated for, as follows. The detector 20 is held by the user in the general region in which it is to be used, and the offset switch 50 manually actuated by the user. Actuation of the offset switch 50 causes operation of the offset circuit 48, as described above. Once any ambient magnetic field has been compensated for, the apparatus 10 may be used as follows.

The desired point 12 on the first side 14a of the wall 14 is located and the magnets 18 in their housing 22 are placed over the point 12. The housing 22 may be attached to the wall 14 by fixing a nail through the aperture 30. With the magnets 18 in position, the corresponding point 16 on the other side 14b of the wall may then be located as follows.

The detector 20 is held close to the said opposite side 14b of the wall, in the general region of the point 16 which is to be located. operation of the magnet ore sistive sensor 32 is initiated by actuation of the search switch 46 by the user. The detector 20 is then manually moved across the surface 14b of the wall by the user, and the magnetoresistive sensor 32 detects any magnetic field present, and causes the audio speaker 72 to emit an audio signal of an appropriate frequency.

The user first moves the detector 20 back and forth across the surface 14b of the wall in a generally horizontal direction, whilst monitoring the frequency of the audio signal emitted by the audio speaker 72. A maximum frequency audio signal indicates that the maximum strength magnetic field has been detected. From the position along the horizontal sweep at which the maximum magnetic field is detected, the user then moves the detector 20 back and forth across the surface 14b of the wall in a generally vertical direction. The vertical sweeping is continued until the audio speaker 72 emits an audio signal of equal or higher frequency to the maximum signal frequency emitted during the horizontal sweep. The location of the detector on the surface 14b of the wall at the point where the maximum frequency audio signal is emitted by the audio speaker 72 is the location of the point 16 which corresponds to the location of the point 12 on the first side 14a of the wall.

The apparatus provided by the present invention has been used to locate the corresponding point 16 to an accuracy of approximately 1Omm when used on a wall of approximately 45cm to 61cm, in thickness.

it will be appreciated that the above embodiment of this invention provides an apparatus which may be used to accurately locate a point on both sides of a barrier. The apparatus described may be of particular assistance to a user installing cabling, piping, access holes or various structural items through a barrier such as a wall because it enables the accurate drilling of a straight hole through the wall It will be appreciated that the apparatus may be used in connection with barriers of various thicknesses. The gain of the variable amplifier circuit may be changed in order to enable a particular apparatus to be used in connection with barriers of differing thickness. It will be appreciated that the gain of each of the amplifier circuits may be set in order to allow the apparatus to be used on barriers having a maximum thickness within a predetermined range, and that the gain values may be altered in order to cover different ranges of thickness.

Although the particular example given above is described in relation to particular integrated circuit chips, it will be appreciated that other equivalent chips may be used in their place.

it will also be appreciated that the apparatus may be used by a single person, the magnets being, for example, stuck or nailed in position on the first side of the barrier, allowing the user to operate the detector from the other side of the barrier. Alternatively, the magnets may be held in position by a first user, whilst a second user operates the detector from the other side of the barrier. In this instance, the position of the magnets may be marked on the first side of the barrier by passing a pen, or other suitable marker, through the aperture in the tag on the magnet housing.

Although in the example described above the magnetic field generator means comprises three Boron Neodymium magnets, it will be appreciated that a different number of magnets may be used. Also, a different type of permanent magnet may be used, or one or more electromagnets may be used in their place.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

I. Apparatus for locating a point through a barrier, the apparatus comprising a magnetic field generator means locatable at the point on one side of the barrier and magnetic field detector means manually movable across the opposite side of the barrier, operable to detect the magnetic field of the magnetic field generator means and to thereby determine the position of the magnetic field generator means and hence the corresponding point on the opposite side.
2. Apparatus according to claim I in which the magnetic field detector means comprises magnetoresistive sensor means.
I Apparatus according to claim 2 in which the magnetoresistive sensor means is operable to detect the direction and magnitude of a magnetic field.
4. Apparatus according to claims 2 or 3 in which the magnetoresistive sensor means is sensitive in each of two substantially perpendicular directions.
S. Apparatus according to any of claims 2 to 4 in which the magnetoresistive sensor means is an anisotropic magnetoresistive sensor means.
6. Apparatus according to any of claims 2 to 5 in which the magnetoresistive sensor means comprises a magnetoresistor.
7. Apparatus according to claim 6 in which the magnetoresistive sensor means comprises four magnetoresistors arranged in a Wheatstone bridge configuration.
8. Apparatus according to claim 7 in which the magnetoresistor Wheatstone bridge is provided as an integrated circuit.
9. Apparatus according to any of claims 6 to 8 in which the or each magnetoresistor comprises a strip of ferrous material.
10. Apparatus according to any of claims 2 to 9 in which the magnet field detector means additionally comprises field strength indicator means.
11. Apparatus according to claim 10 in which the field strength indicator means is operable to generate an output signal which is variable in relation to the magnetic field strength detected.
12. Apparatus according to claim 11 in which the field strength indicator means comprises an audio signal generator means.
13. Apparatus according to claim 12 in which the frequency of the output audio signal is variable in relation to the magnetic field strength detected.
14. Apparatus according to claim 11 in which the field strength indicator means comprises a visual display means on which an image relating to the strength of the magnetic field detected may be displayed.
15. Apparatus according to claim 14 in which the display means is a liquid crystal display.
16. Apparatus according to any of claims 10 to 15 in which the magnetic field detector means further comprises a processing means operable to process a signal generated by the magnetoresistant sensor means.
17. Apparatus according to claim 16 in which the processing means is a microprocessor.
18. Apparatus according to claim 17 in which the microprocessor provides analogue to digital conversion means operable to receive and convert the signal generated by the sensor means, and controller means, in communication with the said conversion means, operable to control the field strength indicator means.
19. Apparatus according to claim 18 in which the analogue to digital conversion means is further operable to generate an output signal following receipt of the signal from the sensor means.
20. Apparatus according to claim 19 in which the controller means is operable to generate and send a signal to the field strength indicator means following receipt of the output signal from the analogue to digital conversion means.
21. Apparatus according to any of claims 17 to 20 in which the processing means further comprises amplifier means, provided between the magnetoresistant sensor means and the microprocessor, operable to amplify the signal generated by the sensor means.
22. Apparatus according to claim 21 in which the amplifier means comprises a differential amplifier circuit, provided between the Wheatstone bridge of the magnetoresistive sensor means and the microprocessor, operable to compare the output voltages from the Wheatstone bridge and generate a corresponding output signal when the Voltages are different.
23. Apparatus according to claim 22 in which the amplifier means further comprises a variable gain amplifier circuit provided in series with the differential amplifier circuit and before the microprocessor, operable to amplify the signal received from the differential amplifier circuit up to an acceptable voltage range for receipt by the analogue to digital conversion means.
24. Apparatus according to claim 23 in which the controller means is, additionally, operable to set the gain of the variable gain amplifier circuit.
25. Apparatus according to claims 23 or 24 in which the apparatus additionally comprises means operable to compensate for the presence of an ambient magnetic field.
26. Apparatus according to claim 25 in which the said means comprises a sampling circuit operable to sample the output signal from the variable gain amplifier during exposure of the sensor means to the ambient magnetic field and an offset circuit, connected to the sampling circuit, operable to vary one or more characteristics of the magnetic field detector means in order to compensate for the detected ambient magnetic field.
27. Apparatus according to claim 26 in which the offset circuit is operable to couple a magnetic field onto the magnetoresistor Wheatstone bridge which nulls out the ambient magnetic field.
28. Apparatus according to any of claims 2 to 27 in which the magnetic field detector means further comprises means operable to reset the magnetoresistive sensor means, in response to detection by the sensor means of a magnetic field the magnitude of which causes overloading of the sensor means.
29. Apparatus according to any of claims 7 to 27 in which the magnetic field detector means further comprises means operable to reset the magnetoresistor Wheatstone bridge integrated circuit in response to detection by the sensor means of a magnetic field the magnitude of which causes overloading of the sensor means.
30. Apparatus according to any preceding claim in which the magnetic field detector means additionally comprises switch means for terminating power supply to the magnetic field detector means.
31. Apparatus according to claim 30 in which the switch means is a timer controlled switch means operable to terminate the power supply after a predetermined period.
32. Apparatus according to any preceding claim in which the magnetic field generator means is a permanent magnet.
33. Apparatus according to claim 32 in which the magnetic field generator means is a Boron Neodymium magnet.
34. Apparatus according to claims 32 or 33 in which at least two magnets are provided.
35. Apparatus according to claim 34 in which three magnets are provided.
36. Apparatus according to any of claims 32 to 35 in which the or each magnet is provided within a housing member.
37. Apparatus according to claim 36 in which the housing member comprises a casing, having an end wall member and at least one side wall member, which defines a socket within which the or each magnet is receivable.
38. Apparatus according to claim 3 7 in which the socket is substantially cylindrical.
39. Apparatus according to claims 3 7 or 3 8 in which a mounting means is provided on the side wall member for mounting the housing member and the magnet or magnets on one side of a barrier at a desired point.
40. Apparatus according to claim 39 in which the mounting means comprises a tab member extending generally radially outwardly from the side wall member.
41. Apparatus according to claim 40 in which the tab member has an aperture provided therein.
42. Apparatus according to claim 41 in which the aperture is suitable for receiving fixing means by which the housing member may be attached to the barrier member.
43. Apparatus according to any of claims 41 or 42 in which the aperture is suitable for receiving marker means therethrough for marking the position of the housing member on the barrier.
44. Apparatus according to any of claims 32 to 35 in which reusable contact adhesive is provided on the or each magnet by which the or each magnet may be attached to the barrier member.
45. Apparatus according to any of claims 36 to 43 in which reusable contact adhesive is provided on the housing member and/or the mounting means.
46. A method for locating a point through a barrier, the method including locating a magnetic field generator means at the point on one side of the barrier and manually moving magnetic field detector means across the opposite side of the barrier to detect the magnetic field of the magnetic field generator means, to thereby determine the position of the magnetic field generator means and hence the corresponding point on the opposite side.
47. A method according to claim 46 in which the magnetic field detector means is according to any of claims 2 to 3 1.
48. A method according to claims 46 or 47 in which the magnetic field generator means is according to any of claims 32 to 45.
49. A method according to any of claims 46 to 48 in which the method further includes manually moving the magnetic field detector means across the opposite side of the barrier along a substantially horizontal path until the maximum magnitude magnetic field is detected.
50. A method according to claim 49 in which the method further includes manually moving the magnetic field detector means across the opposite side of the barrier along a substantially upwardly extending path until the maximum magnitude magnetic field is detected.
51. A method according to claim 50 in which the magnetic field detector means is manually moved back and forth across the opposite side of the barrier along one or both of the substantially horizontal and upwardly extending paths.
52. A method according to claim 5 1 in which the magnetic field detector means is manually moved back and forth across a first one of the substantially horizontal and upwardly extending paths and is subsequently moved back and forth across the second one of the substantially horizontal and upwardly extending paths.
53. A method according to claim 32 in which the said second one of the paths crosses the said first one of the paths at the position along the first one of the paths at which the maximum magnitude magnetic field was detected.
54. A method according to any of claims 46 to 53 in which the method further includes the step of compensating for the presence of an ambient magnetic field before use of the magnetic field detector commences.
55. Apparatus for locating a point through a barrier substantially as described above with reference to the accompanying drawings.
56. A method for locating a point through a barrier substantially as described above with reference to the accompanying drawings.
57. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.