GB2405211A

GB2405211A - Locating the position of a reference point on the opposite side of a wall

Info

Publication number: GB2405211A
Application number: GB0418334A
Authority: GB
Inventors: Hugh Richard Marsh
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-08-19
Filing date: 2004-08-17
Publication date: 2005-02-23
Anticipated expiration: 2024-08-17
Also published as: GB2405211B; GB0319483D0; GB0418334D0

Abstract

A method and system for locating a reference point on one side of a structure 2 from the opposite side of the structure 2. A transmitter unit 1 placed on one side of the structure 2 emits an electromagnetic signal from a transmitter coil 42 to identify the reference point. A receiver unit 4 is moved around on the opposite side of the structure 2 to locate the transmitter unit 1. The receiver unit 4 may contain two orthogonal receiver coils 43, 44 with axes of winding which, in use, are in a plane perpendicular to the axis of winding of the transmitter coil 42. The receiver unit 4 indicates that it is positioned opposite the transmitter unit 1 when a minimum voltage is induced across the two receiver coils 43, 44. The receiver unit 4 also contains a phase coil 45 to detect the phase of the locator signal and enable the receiver unit 4 to indicate the direction in which it should be moved to be opposite the transmitter unit 1. The voltage induced across the phase coil 45 may also be used to verify that the locator signal is being properly received and prevent false indications. The receiver may operate with just one of the receiver coils and the phase coil.

Description

240521 1

POSITION LOCATOR

The present invention relates to a system and method for locating a reference point on one side of a structure from the opposite side of the structure.

It is often useful to locate a reference point on one side of a structure from the opposite side. For example, in the construction industry, when installing wiring or plumbing, it is often helpful to locate a conduit on one side of a wall or other structure so that a hole drilled from the other side will line up correctly. The location of such a reference point becomes essential, for example, when space is restricted on one side of the structure so that drilling apparatus can be used only from the other side.

Traditionally, such reference points have been located by identifying a datum, such as an intersecting structure or existing hole, which can be identified on both sides of the structure. Measurements are then taken from this datum on one side of the structure and used on the other to locate the reference point. However, this method is often inaccurate and complicated and it relies on the availability of a convenient datum.

US 6,441,599 describes a reference point locator for residential and commercial construction. This system comprises a transmitter unit and a receiver unit. The transmitter unit emits an electromagnetic signal, the intensity of which can be measured by the receiver unit and displayed on a suitable visual, audio or tactile indicator. The transmitter is positioned on one side of the structure in a position that defines the reference point. The receiver unit is moved around by an operator on the other side of the structure. The reference point is identified by the position of the receiver unit when a maximum intensity of the electromagnetic signal is indicated.

However, such apparatus does not indicate the reference point with great precision.

The intensity of the electromagnetic signal varies only slightly when the receiver is moved slightly out of alignment with the transmitter and this change is difficult for the operator to perceive. A farther problem is that. the operator has to determine the direction in which the receiver should be moved in order to increase the intensity of the signal by trial and error.

A method and system for providing a more precise indication of the position of a reference point on one side of a substantially planar structure from the other side of said structure would be desirable. An indication of the direction in which the receiver should be moved in order that its position more closely corresponds with the reference point would also be desirable.

in accordance with a first aspect of the present invention, there is provided a method for locating a reference point located on a first side of a substantially planar structure from a second side of the structure, the method comprising: generating an alternating electromagnetic signal on the first side of the structure with magnetic flux lines through a reference point substantially perpendicular to the plane of the structure; placing a first receiving coil with a winding axis substantially parallel to the plane of the structure in the electromagnetic field on the second side of the structure; placing a phase coil with a winding axis substantially perpendicular to the plane of the structure in the electromagnetic field on the second side of the structure; sensing the voltage induced across the first receiving coil and the phase coil at different positions on the second side of the structure; comparing the voltage induced across the first receiving coil with that induced across the phase coil in order to provide an indication of the direction in which the receiving coil needs to be moved along its axis of winding in order to minimise the voltage induced across it; and identifying the reference point as the position at which the induced voltage across the first receiving coil is a minimum.

Preferably, the method further comprises: placing a second receiving coil with a winding axis substantially parallel to the plane of the structure and substantially orthogonal to the first receiving coil in the electromagnetic field on the second side of the structure; sensing the voltage induced across the first receiving coil and the second receiving coil when placed together at different positions on the second side of the structure; comparing the voltage induced across the first receiving coil and the voltage induced across the second receiving coil with that induced across the phase coil in order to provide an indication of the direction in which the receiving coils need to be moved in a plane parallel to the plane of the structure in order to minimise the voltage induced across them; and identifying the reference point in two dimensions as the position at which the induced voltage across both receiving coils is a minimum.

in accordance with a second aspect of the present invention, there is provided a receiver unit for use in a locator system comprising: a contact plate for contacting the receiver unit against a flat surface; a first receiving coil having an axis of winding substantially in a plane parallel to the plane of the contact plate; a phase coil having an axis of winding substantially perpendicular to the plane of the contact plate; and electronic circuitry, wherein the electronic circuitry is adapted to determine the polarity of a voltage induced by an electromagnetic signal across the first receiving coil relative to the polarity of a voltage induced by said electromagnetic signal across the phase coil.

Preferably, the receiver unit further comprises a second receiving coil, substantially orthogonal to the first, having an axis of winding substantially in a plane parallel to the plane of the contact plate, wherein the electronic circuitry is adapted to determine the polarity of a voltage induced by an electromagnetic signal across the first receiving coil and the second receiving coil, relative to the polarity of a voltage induced by said electromagnetic signal across the phase coil.

Preferably the first coil and the second coil are symmetrical about a common axis perpendicular to the plane of the contact plate.

Preferably the voltage induced across the coils is an alternating voltage and voltages alternating approximately in phase are determined to be of the same polarity and voltages alternating approximately 180 out of phase are determined to be of opposite polarity.

Preferably the receiver unit further comprises a display panel comprising four direction indicators, wherein: an up indicator and a down indicator are controlled by the voltage induced across the first coil; and a left indicator and a right indicator are controlled by the voltage induced across the second coil. Preferably the receiver unit further comprises a signal indicator controlled by the voltage induced across the phase coil.

Preferably the signals to the direction indicators are strobed using a pulse with a frequency and phase determined so as to avoid errors arising from small phase differences in the alternating voltages induced in the coils.

Preferably the coils are positioned so as to avoid interference generated by electrical noise.

In accordance with a third aspect of the present invention, there is provided a locator system comprising a transmitter unit and a receiver unit in combination. Preferably the transmitter unit comprises an oscillator and a coil that co-operate to generate an electromagnetic signal. Preferably the electromagnetic signal has a frequency of approximately 8kHz. Preferably the transmitter unit further comprises means for temporarily attaching it to a flat surface so that the transmitter signal is directed perpendicularly into said flat surface.

Other aspects and preferred features of the present invention will become apparent from the following description, claims and drawings.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a reference point locator system according to a preferred embodiment of the present invention.

Figure 2 shows an external view of a transmitter unit of the reference point locator system shown in Figure 1.

Figure 3 shows an external view of a receiver unit of the reference point locator system shown in Figure 1.

Figure 4 shows an arrangement of coils within the reference point locator system shown in Figure 1.

Figure 5 is a simplified schematic circuit diagram of the electronic circuitry used in the receiver unit shown in Figure 3.

Figure 6 illustrates the relationship between the magnetic flux lines and a coil in the receiver unit shown in Figure 3 when the receiver is aligned with the reference point.

Figure 7 illustrates the relationship between the magnetic flux lines and a coil in the receiver unit shown in Figure 3 when the receiver is not aligned with the reference point.

Figure 8 is a detailed circuit diagram of the electronic circuitry used in one embodiment of the transmitter unit shown in Figure 2.

Figure 9 is a detailed circuit diagram of the electronic circuitry used in one embodiment of the receiver unit shown in Figure 3.

Figure 10 shows the strobe timing signals in the receiver circuit of Figure 9.

Figure 1 shows a reference point locator according to the present invention in use. A transmitter unit 1 has been temporarily attached to one side of a structure 2 (illustrated as a wall) and switched on. An operator 3 is holding a receiver unit 4 against the other side of the structure 2. The receiver unit 4 indicates the direction in which it should be moved in order to align more closely with the transmitter unit 1. The operator 3 moves the receiver unit 4 as indicated until the receiver unit 4 indicates that it is aligned with the transmitter unit 1. The operator 3 then marks the position of the receiver unit 4 using a marker 5. The receiver unit 4 and transmitter unit 1 are both then removed from the structure 2. The operator 3 may then drill a hole through the structure 2 at the correctly identified and marked position.

Figure 2 shows the transmitter unit 1 in more detail. The transmitter unit comprises a transmitter for emitting a locator signal mounted within a case. The case has a flat end 25 to orientate the transmitter against the structure 2 and markings 21 on the other end to indicate the reference point. Alternative forms of indication, including for example an axial bore through the case, are possible. In the embodiment shown, the case is cylindrical in shape but cases of other practical shapes, such as for example cuboid, may be used. There is a switch 22 for turning the transmitter on and off and an LED 23 which lights to indicate that the transmitter is switched on. In one embodiment, the transmitter will switch itself off automatically after a predetermined period, for example 5 minutes after it is switched on. The flat end 25 of the case incorporates suitable fixing means 24 to hold the transmitter unit 1 in position against a structure 2. The fixing means 24 may comprise any suitable temporary fixing means such as rubber suction pads, spikes, gum, blue tack, sticking tape or the like and may be replaceable between uses. Fixing means 24 may also comprise holes through which pins or nails may be inserted into the structure 2.

Figure 3 shows the receiver unit 4 in more detail. It comprises a receiver mounted within a cylindrical case. The diameter of the receiver unit 4 is about 80mm so that it can be comfortably held between the thumb and fingers of an adult hand. In an alternative embodiment a handle is fitted to facilitate manipulation of the receiver unit 4. One end of the receiver unit 4 has five LED indicators 31, 32 positioned in a cross formation. The central LED 31 is green and, in use, lights when a locator signal is received from the transmitter unit 1. The remaining four LEDs 32 are red and, in use, light to indicate the direction in which the receiver unit 4 should be moved in order to more closely align with the reference point indicated by the transmitter unit 1. There is also a button 33 for turning the receiver on and off and a LED 34 which lights to indicate that the receiver unit 4 is on. The button 33 and LED 34 are positioned on the circumference of the receiver case for ease of use but may be on the same end as the indicator LEDs 31, 32. The other end of the receiver case forms a contact plate 35 comprising a smooth, flat surface to facilitate easy movement over a structure 2 and to correctly orientate the receiver unit 4.

The receiver case has two pairs of diametrically opposed marking points 36 around its circumference. When the LEDs indicate that flee position of the receiver unit 4 corresponds with the reference point, a conventional marker can be used to mark the position of the four marking points on the surface of the structure 2. Lines joining these marks will intersect at the position of the centre of the receiver case, which will indicate the reference point. If, as in a preferred embodiment, the receiver is mounted centrally in the receiver case, the intersection will be precisely aligned with the reference point.

In an alternative embodiment, a marker is embedded at the centre of the contact plate and a control provided to push the marker onto the surface of the structure 2 when the reference point is located. Yet another alternative is to place one or more markers in a ring shaped holder around the circumference of the receiver unit 4. The holder can be rotated around and moved axially relative to the receiver case. When the reference point is located, the ring may be moved axially so that the markers contact the surface of the structure 2 and then rotated to mark a circle indicating the reference 1 5 point.

In one embodiment, the transmitter unit 1 and the receiver unit 4 are adapted to be joined together for storage. A circumferential recess in the receiver unit 4 fits into an extended rim in the transmitter unit 1. The recess and the rim may incorporate corresponding screw threads. The transmitter unit 1 and the receiver unit 4 may switch off automatically when the two units are engaged.

in another embodiment, the receiver unit 4 may stay switched on only while the button 33 is pressed or for only a defined period, for example two minutes, after the button 33 is pressed in order to conserve power.

Figure 4 illustrates schematically some internal elements of the transmitter unit 1 and the receiver unit 4. The transmitter comprises an oscillator 41 coupled to a transmitter coil 42. The oscillator is powered by a 9 Volt battery and generates an alternating current which is passed through the coil 42 and thereby generates an alternating electromagnetic field. The coil 42 comprises typically 400 circular turns with a diameter of 70mm formed from wire of typically 0.2mrn in diameter. The coil 42 is generally short in length to facilitate accurate orientation with the flat face 25 of the transmitter case. Increasing the number of turns or the diameter of the wire used can reduce power consumption, as can tuning the coil 42 with a parallel capacitor. The transmitter coil 42 is orientated so that its axis of winding 46 is perpendicular to the flat end 25 of the case of the transmitter unit 1. The transmitter coil 42 is positioned within the transmitter unit 1 so that its axis of winding 46 passes directly through the marking 21 on the case indicating the reference point. The transmitter coil 42 may be wound around a ferrite core to provide the same inductance using fewer turns of wire.

The transmitter uses a frequency of around 8kHz although the system will operate over a wide range of frequencies. Lower frequencies are desirable to reduce errors caused by eddy currents in nearby metal objects and, since the cost of amplifiers generally increases with the frequency of operation, to reduce the overall cost of the system. However, larger coils are required at lower frequencies and electrical noise can be a problem, particularly around mains frequency of 50Hz. The frequency of around 8kHz is considered to be an optimal compromise between these requirements.

The receiver unit 4 comprises a first receiver coil 43 and a second receiver coil 44 substantially orthogonal to the first. The axes of winding of both coils 43, 44 are in a plane parallel to the plane of the contact plate 35.

As illustrated, for best results, the two receiver coils 43, 44 are each orthogonal to the transmitter coil 42. Therefore best results are obtained when the sides of the structure 2 through which the reference point is to be located are parallel - i.e. the structure 2 is of substantially constant thickness over the width of the fixing means 24 and the contact plate 35.

The two receiver coils 43, 44 are similar to each other. Both have approximately 400 circular turns with a diameter of 70mm. Square turns enclosing equal area (shown in Figure 4) may be used to facilitate the internal arrangement of the components of the receiver unit 4. The wire thickness affects the frequency response or "Q" of the receiver circuit. A thicker wire (having a lower resistance) will make the tuning more critical so that the receiver must be precisely tuned to the transmitter frequency; a thinner wire (having a higher resistance) will make the tuning less critical but the rejection of unwanted frequencies will not be so good. A copper wire of 0.2mm diameter is preferred.

The windings of the two receiver coils 43, 44 overlap along an axis 46 perpendicular to the plane of the contact plate 35. In use, it is the intersection of this axis 46 and the plane of the structure 2 which corresponds to the location of the reference point. The receiver coils 43, 44 are of equal size and are symmetrical about this axis 46 to provide equal sensitivity in all directions but electronic compensation of unequal signals is possible. To facilitate accurate alignment of the coils 43, 44, each coil 43, 44 is short axially. A cylindrical coil of 70mm diameter and 1 Omm in axial length has been found to have its magnetic centre within lmm of its physical centre if it is wound evenly. Both receiver coils 43, 44 are substantially the same distance from the contact plate 35 but it is possible that one may be positioned behind the other, disadvantageously resulting in extra case height and reduced sensitivity in the coil further from the contact plate 35.

The receiver unit 4 of the illustrated embodiment also comprises a phase coil (so called because, in use, it detects the phase of the signal transmitted by the transmitter coil 42), which is orthogonal to both the first receiver coil 43 and the second receiver coil 44 and has an axis of winding perpendicular to the plane of the contact plate 35.

In use, it is therefore orientated to correspond as nearly as possible with the orientation of the transmitter coil 42. The phase coil 45 may be identical to the first receiver coil 43 and the second receiver coil 44 but it has been found sufficient to use only 100 turns since the signal received by this coil is stronger than that received by the other two. The phase coil 45 is preferably positioned along the axis 46 of intersection of the first receiver coil and second receiver coil. The precise positioning of the phase coil 45 is not fundamental as long as it can receive a strong signal from the transmitter unit 1. However, best results are obtained when the phase coil 45 lies broadly on the axis 46 of intersection of the other two coils as the signal it receives will be strongest there when the receiver is aligned with the transmitter. In one embodiment, all three receiver coils 43, 44, 45 are formed around a single box like forming structure (not shown).

The three receiver coils 43, 44, 45 of the illustrated embodiment are not wound on ferrite cores since the cores would distort the electromagnetic field generated by the transmitter. However, use of ferrite cores may provide the advantage of smaller coils if the system were adapted to compensate for this distortion.

The three receiver coils 43, 44, 45 are each connected to electronic circuitry which senses the voltage across them. The circuitry drives a display comprising five LED indicators 31, 32 positioned in a cross formation. The electronic circuitry is powered by a 9 Volt battery but in an alternative low power embodiment may be powered by the current induced in the receiver coil 45, in which case a Liquid Crystal Display may be used instead of LEDs to reduce power consumption. In one embodiment, the battery is housed in a handle so that it is positioned sufficiently far away from the coils not to interfere with the magnetic field around them.

A schematic diagram of the circuitry is shown in Figure 5. Each receiver coil 43, 44, is connected to a corresponding amplifier 51, 52, 53. Each amplifier may be coupled with a comparator (not shown in Figure 5) which provides a digital output signal when a voltage is sensed across the coil.

Since the transmitter unit 1 generates an alternating electromagnetic signal, the voltage across the coils 43, 44, 45 will alternate at the same frequency and the outputs from the amplifiers 51, 52, 53 will oscillate with the same frequency.

The output of the third amplifier 53 from the phase coil 45 is connected to the central LED indicator 31 on the display. This LED 31 lights when the locator signal is being properly received.

The remaining four LEDs 32 are each driven by logical AND gates 54, each gate having two inputs. The first input of the first gate 54A and the first input of the second gate 54B are driven by the output of the first amplifier 51 connected to the first receiver coil 43. The first input of the third gate 54C and the first input of the fourth gate 54D are driven by the output of the second amplifier 52 connected to the second receiver coil 44. The second input of the first gate 54A and the second input of the third gate 54C are driven by a signal derived (as described below) from the output of the third amplifier 53 connected to the phase coil 45. The second input of the second gate 54B and the second input of the fourth gate 54D are driven by a signal derived (as described below) from the inverse of the output of the third amplifier 53 connected to the phase coil 45.

Consequently, the first LED 32A will light when the first receiver coil 43 and the phase coil 45 both have alternating voltages across them that are in phase, the second LED 32B will light when the first coil 43 and the phase coil 45 both have alternating voltages across them that are out of phase, the third LED 32C will light when the second coil 44 and the phase coil 45 both have an alternating voltage across them that is in phase and the fourth LED 32D will light when the second receiver coil 44 and the phase coil 45 both have an alternating voltage across them which is out of phase.

As a result of slight phase shifts in the received signals, the four LEDs 32 may tend to flicker. The signals input to the second inputs of the AND gates 54 may therefore be timed using the output of a strobe unit 56.

[Figures 6 and 7 depict in one dimension how the position of a receiver coil (for example, the second receiver coil 44) relative to a transmitter coil 42 can be determined using the apparatus described above. l

The transmitter coil 42 generates an alternating electromagnetic field represented by the magnetic flux lines 61. These magnetic flux lines are symmetrical about the axis of winding of the transmitter coil 42. As shown in Figure 6, when the receiver coil 44 is perfectly aligned with this axis, very few flux lines pass through the receiver coil 44 and thus minimal voltage is induced across it. Even if, as a result of the finite length of the coil, a small voltage is induced in one end of the coil, it will be cancelled by the small voltage induced at the other end of the coil because of the symmetrical nature of the magnetic field. Therefore, as long as the centre of the receiver coil 44 is directly aligned with the axis of winding of the transmitter coil 42, neither of the two LEDs 32C, 32D (shown in Figure 5) associated with the receiver coil 44 will light.

If the receiver coil 44 is moved out of alignment with the axis of winding of the transmitter coil 42, as shown in Figure 7, more flux lines pass through the coil and a detectable voltage will be induced across the receiver coil 44. Thus, depending on the phase of the induced voltage, one of the associated LEDs 32C, 32D will light.

Thus it can be seen that a very precise indication is given when the receiver coil 44 lines up with the winding axis of the transmitter coil 42. It is only with precise alignment that no voltage will be induced in the receiver coil 44 and neither of the LEDs 32C, 32D will light.

As represented in Figure 7, the receiver coil has moved below the axis of winding of the transmitter coil 42. The magnetic flux lines flowing from the transmitter coil 42 therefore pass through the receiver coil 44 from top to bottom. If the coil were moved above the axis of winding of the transmitter coil 42, the flux lines flowing from the transmitter coil 42 would pass through the receiver coil 44 from bottom to top. This difference results in the voltage induced across the coil being of the opposite polarity, or since the induced voltage is alternating, the induced voltage varying in phase by 180 from the transmitted signal.

If the phase of the original signal is known, it can be determined whether the coil 44 is above or below the axis of winding of the transmitter coil 42. In the present invention, this phase is sensed by the phase coil 45. This enables the generation of an indication of the direction in which the receiver coil 44 should be moved using, for example, the electronic circuitry described above. One LED 32C might indicate that the receiver unit 4 must be moved up and the other LED 32D might indicate that the receiver unit 4 must be moved down.

In order to locate the reference point in two dimensions, a similar arrangement is required to indicate that the receiver unit 4 must be moved to the left or to the right.

The precise relationship between the phase of the signal and the required direction of movement depends on the sense- in which the receiver coils 43, 44, 45 are wound.

One skilled in the art can easily deduce which LED indicates which required movement and arrange them accordingly on the display.

By way of example, the electronic circuitry used in a specific embodiment of the transmitter and the receiver will now be described with reference to Figures 8 to 10.

The transmitter circuit shown in Figure 8 comprises a Colpitts Oscillator with a delay allowing it to run for a predetermined period after an actuation button S1 is pressed.

This period is determined by the values of capacitor C1 and resistor R1, one or both of which may be variable. The oscillator waveform is clipped so that it has a flat bottom for phase identification in the receiver. The inductor L1 is the coil that provides the electromagnetic locator signal.

The receiver circuit shown in Figure 9 can be seen to broadly correspond with the simplified circuit of Figure 5. The main difference is that the AND gates 54 of Figure have been replaced by comparators 9c, 10c, 1 lo, 12c and an implementation of the strobe generator 56 is provided.

The signal from coil L3 (equivalent to phase receiver coil 45) is used to generate pulse signals "A" and "B". The signal from coil L3 is amplified in amplifier 3A and the amplified signal (Figure lOa) is fed into comparator 5C which outputs a square wave (Figure lob). This signal is applied to integrator 4A which outputs a triangular wave (Figure lOc) which will swing from -IVolt to +lVolt. Comparators 6C and 7C provide a strobe signal which is high when the triangular wave is between -0.4Volts and +0.4Volts but low throughout the rest of the cycle. The strobe signal therefore corresponds approximately with the peaks and troughs in the signal from coil L3.

Pulse signals "A" and "B" are both held low when the strobe signal is low(via diodes D5 and D6). When strobe signal is high, pulse signal "A" will be high if the output of comparator 5C is high and pulse signal "B" will be high if the output of comparator 5C is low. Thus pulse signal "A" is approximately synchronised with the positive peak of the signal from coil L3 and pulse signal "B" is approximately synchronized with the negative peak of the signal from coil L3.

The pulse signals "A" and "B" are biased to range from -4Volts (Low) to OVolts (High). The outputs from the amplifiers (1A, 2A) on the directional receiver coils (L1, L2 corresponding to first and second receiver coils 43 and 44) range within lVolt of the supply rail, i.e. between -3Volts and +3Volts. Each comparator 9C, lOC, 11C, 12C therefore outputs a low signal (to light the appropriate LED) when the voltage across the relevant receiver coil is less than OVolts and the relevant strobe pulse (A or B) is at OVolts (High). This occurs only if the voltage across the relevant receiver coil and the relevant strobe pulse (A or B) are in anti-phase. The comparators therefore provide a similar function to the AND gates 54 in Figure 5.

The negative part of the locator signal waveform is clipped. Phase coil L3 is connected in a reverse sense to the transmitter coil and therefore, in correct operation, a sine wave with its positive part clipped is induced across it. Tbis clipping causes the output of comparator SC to pulse positive for a longer period than it pulses negative.

In the embodiment shown the signal pulses positive for approximately 70, us and negative for approximately 50ps. This causes the triangular wave output of integrator 4A to move towards the negative rail. Comparator 8C limits the excursion to -IVolt by applying a current to the integrator 4A. This also lights a LED (corresponding to LED 31) indicating that the receiver unit 4 is receiving the correct signal. In order to ensure the correct timing of the strobe signal, the time constant of integrator 4A is set to provide an output waveform of 2Volts peak to peak centred on zero when the lVolt excursion limit is reached.

If coil L3 is moved a sufficient distance from the target reference point, it may receive a phase reversed signal because the magnetic flux is a complete loop. In this case, the output of integrator 4A will be a triangle wave between +3 Volts and +1 Volt. This condition does not light the LED 31 or provide a strobe signal and therefore no LEDs on the receiver unit 4 will light.

The direction sensing receiver coils L1, L2 are tuned by capacitors C3 and C4, although this is not critical since the coil resistance results in a relatively low "Q".

The phase coil L3 does not need to be tuned but capacitor C5 and resistor R6 filter out high frequency interference.

Amplifiers 1A, 2A, 3A are identical Texas Instruments TL074 operational amplifiers and are integrated in a quad package. Each is arranged to provide a gain of 100 but the gain is not critical and can be optimised for the desired performance. Capacitors C6, C7 and C8 prevent amplification of offset voltage and reject 50Hz mains hum.

The TL074 package is preferred because it has a gain bandwidth of 3MHz, enabling a gain of 100 at 8kHz with little phase shift. The fact that the output of this amplifier can only swing to within about lVolt of either supply rail is advantageous in that it enables the comparators 9c, 10c, tic, 12c to function as both AND gates and comparators simultaneously. Other amplifiers may be used instead and may be cheaper but, for example, the maximum gain at 8kHz using a National Semiconductor LM324 operational amplifier would be only about 30 times, resulting in reduced sensitivity.

A resistor R7 between the positive input of amplifier 3A (associated with phase coil L3) and a-4V rail provides a bias voltage of-20mV to overcome the amplifier offset voltages and ensure that all LEDs are offwhen no locator signal is being received.

Comparators 5C to 12C are National Semiconductor LM339 comparators. They are integrated in two quad packages.

Since the strobe signal is only high for about 20ps in a 125,us cycle, capacitors C1 l, Cl2, C13 and C14 are used to hold the LEDs on between strobe pulses. These capacitors also reduce signal frequency noise that would otherwise interfere with system operation.

It has been found that the apparatus above works best with walls of less than 300mm thick. The accuracy diminishes with the locator signal strength which is inversely proportional to the square of the distance from the transmitter unit 1. It has been found that when using a 9Volt battery in the transmitter unit 1 the maximum range is about 500mm. However, if operation over greater distances is necessary, the power of the locator signal can be increased by using a more powerful transmitter unit 1.

The accuracy is reduced by large metallic objects within the structure 2 which interfere with the magnetic field generated by the transmitter unit l although small objects such as nails and screws do not significantly affect operation. If the unit is being used to locate a point to drill a hole, the inability to locate accurately may provide a warning that there is a pipe in the structure near the reference point and that a hole should not be made.

In alternative embodiments, the information provided by the receiver coils 43, 44, 45 may be used in different ways.

For example, a single indicator lamp may be used. This may be brightly lit when the signal from the first receiver coil 43 or the second receiver coil 44 is strong but extinguished when the reference point is located and the signals from the coils 43, 44 are weak. Such an indicator may flash at a varying rate rather than lighting with varying intensity. The receiver unit 4 may also produce a sonic or tactile indication of proximity to the transmitter unit 1.

In other alternative embodiments of the receiver unit 4, the second receiver coil 44 is omitted. Such an embodiment preferably has two direction indicator LEDs 32 preferably positioned along the axis of winding of the first receiver coil 43 and two marking points 36 preferably positioned in the plane perpendicular to the axis of winding of and half way along the first receiver coil 43. En use, the receiver unit 4 is placed approximately over the transmitter unit 1 and moved along the axis of winding of the first receiver coil 43 in a direction indicated by the direction indicator LEDs 32.

When both LEDs 32 switch off, the position of the marking points 36 is marked on the surface of the structure 2. A straight line is drawn through the two marked points.

The receiver unit 4 is then rotated, preferably through an angle of approximately 90 and the process repeated. The lines through the marked points will intersect at a position directly opposite the transmitter unit 1. Such an embodiment may include any one or more of the adaptations of previous embodiments described herein.

One skilled in the art will readily conceive of many alternative embodiments of the invention described above. The present invention includes all such alternatives which fall within the scope of the following claims.

Claims

CLAIMS: 1. A receiver unit for use in a locator system comprising: a

contact plate for contacting the receiver unit against a flat surface; a first receiving coil having an axis of winding substantially in a plane parallel to the plane of the contact plate; a phase coil having an axis of winding substantially perpendicular to the plane of the contact plate; and electronic circuitry, wherein the electronic circuitry is adapted to determine the polarity of a voltage induced by an electromagnetic signal across the first receiving coil relative to the polarity of a voltage induced by said electromagnetic signal across the phase coil.
2. A receiver unit as claimed in claim 1, further comprising a second receiving coil, substantially orthogonal to the first, having an axis of winding substantially in a plane parallel to the plane of the contact plate, wherein the electronic circuitry is adapted to determine the polarity of a voltage induced by an electromagnetic signal across the first receiving coil and across the second receiving coil, relative to the polarity of a voltage induced by said electromagnetic signal across the phase coil.
3. A receiver unit as claimed in claim 2, wherein the first receiving coil and the second receiving coil are symmetrical about a common axis perpendicular to the plane of the contact plate.
4. A receiver unit as claimed in either claim 2 or claim 3, further comprising a display panel comprising direction indicators, wherein: an up indicator and a down indicator are controlled by the voltage induced across the first receiving coil; and a left indicator and a right indicator are controlled by the voltage induced across the second receiving coil. it,
5. A receiver unit as claimed in claim 4, wherein the signals to the direction indicators are stroked using a pulse with a frequency and phase determined so as to avoid errors arising from small phase differences in the alternating voltages induced across the coils.
6. A receiver unit as claimed in any one of claims 1 to 5, wherein, when the voltage induced across the coils is an alternating voltage, voltages alternating approximately in phase are determined to be of the same polarity and voltages alternating approximately 180 out of phase are determined to be of opposite polarity.
7. A receiver unit as claimed in any one of claims 1 to 6, further comprising a signal indicator controlled by the voltage induced across the phase coil.
8. A locator system comprising a transmitter unit and a receiver unit as claimed in any one of claims 1 to 7.
9. A locator system as claimed in claim 8, wherein the transmitter unit comprises an oscillator and a coil that co-operate to generate an electromagnetic signal.
10. A locator system as claimed in claim 9, wherein the electromagnetic signal has a frequency of approximately 8kHz.
11. A locator system as claimed in any one of claims 9 to 10, wherein the transmitter unit further comprises means for temporarily attaching it to a flat surface so that the transmitter signal is directed perpendicularly into said flat surface.
12. A method for locating a reference point located on a first side of a substantially planar structure from a second side of the structure, the method comprlsmg: generating an alternating electromagnetic signal on the first side of the structure with the magnetic flux lines through a reference point being substantially perpendicular to the plane of the structure; placing a first receiving coil with a winding axis substantially parallel to the plane of the structure in the electromagnetic field on the second side of the structure; placing a phase coil with a winding axis substantially perpendicular to the plane of the structure in the electromagnetic field on the second side of the structure; sensing the voltage induced across the first receiving coil and the phase coil at different positions on the second side of the structure; comparing the voltage induced across the first receiving coil with that induced across the phase coil in order to provide an indication of the direction in which the receiving coil needs to be moved along its axis of winding in order to minimise the voltage induced across it; and identifying the reference point as the position at which the induced voltage across the first receiving coil is a minimum.
13. A method as claimed in claim 12, further comprising: placing a second receiving coil with a winding axis substantially parallel to the plane of the structure and substantially orthogonal to the first receiving coil in the electromagnetic field on the second side of the structure; sensing the voltage induced across the first receiving coil and the second receiving coil when placed together at different positions on the second side of the structure; comparing the voltage induced across the first receiving coil and the voltage induced across the second phase coil with that induced across the phase coil in order to provide an indication of the direction in which the receiving coils need to be moved in a plane parallel to the plane of the structure in order to minimise the voltage induced across them; and identifying the reference point in two dimensions as the position at which the induced voltage across both receiving coils is a minimum.
14. A locator system substantially as hereinbefore described with reference to Figures l to S.
15. A locator system substantially as hereinbefore described with reference to Figures 8 and 9.
16. A receiver unit substantially as hereinbefore described with reference to Figures 3, 4, 5 and 9.