GB2496191A - A locator for locating a concealed conductor carrying an alternating current having a first and second frequency - Google Patents

Abstract

A locator for locating a concealed current carrying conductor is described. The conductor carries an alternating current having at least a first frequency and a second frequency, wherein the alternating current is produced by at least one dedicated signal generator coupled to the conductor. The locator comprises: at least one magnetic field sensor 3, 5 operable to convert electromagnetic radiation from the conductor into a field strength signal; a digital analogue converter (CODEC 11) configured to generate a digitised signal dependent upon the field strength signals from the magnetic field sensor; a digital signal processor 16 configured to isolate components of the digitised signal resulting from the first frequency and the second frequency; and process the isolated components to generate one or more signals indicative of the proximity of the conductor to the detector; and an output configured to generate an audio and/or visual indication of the proximity of the conductor, wherein in some aspects the isolated signal components resulting from the first frequency signal and the second frequency signal are contemporaneously processed. Other aspects relate to the method of using the locator and to a carrier medium carrying instructions to execute said method using a computer

Description

A locator for locating a current carrying conductor Embodiments of the present invention relate to locators for locating a current carrying conductor.

Before commencing excavation or other work where electrical cables, fibre optic cables or other utilities ducts or pipes are buried, it is important to determine the location of such buried cables or pipes to ensure that they are not damaged during the work. It is also useful to be able to track a path of buried cables or pipes. Current carrying conductors emit electromagnetic radiation which can be detected by an electromagnetic antenna. If fibre optic cables or non-metallic utilities ducts or pipes are fitted with a small electrical tracer line, an alternating electrical current can be coupled into the tracer line which in turn radiates electromagnetic radiation. It is known to use detectors to detect the electromagnetic field emitted by conductors carrying alternating current.

One type of such detector works in one of three modes. These modes are classified as either passive or active modes, the passive modes being power' mode and radio' mode which use signals that are already present -mains power signals and submarine VLF (very low frequency) communications. Each mode has its own frequency band of detection.

Embodiments of the present invention relate to the active mode.

In the active mode, a signal transmitter couples an alternating magnetic field of known frequency and modulation, in a buried conductor. The signal transmitter may be directly connected to the conductor. Where direct connection access is not possible, a signal transmitter may be placed near to the buried conductor and an alternating current signal may be induced in the conductor by an alternating magnetic field produced by the signal generator. The buried conductor radiates an alternating magnetic field corresponding to the signal produced by the signal transmitter.

The choice of signal frequency is an important factor for effective tracing and identification of buried lines, and there is no single frequency that covers all conditions.

For single instruments to be used by relatively non-technical personnel there is no option but to make a compromise, and choose a single frequency high enough to give good performance in the induction mode, but not so high that it will not travel far enough. Active signals between 8 kHz and 33 kHz are commonly used for these applications.

33 kHz is considered to be a good general purpose signal frequency suitable for finding many buried cables and metallic pipes. For short lengths of cable, for example telecom spurs crossing a subscriber's premises, a signal frequency of 33kHz does not provide sufficient signal to give a good quality locate. This is because the signal return path impedance is high, being predominantly capacitive; the shorter the cable, the lower the capacitance to earth and hence the higher the impedance at a particular frequency.

The high impedance results in a small current in the cable.

In this situation a better locate signal quality can be obtained using a higher signal frequency. Multi-frequency locators and transmitters are available having suitable high frequency operating modes, e.g. 66kHz, 83kHz and 131kHz. These products require the operator to select a suitable signal frequency, necessitating a higher degree of operator training and greater expertise than possessed by typical users. Dedicated single-frequency locators exist that are optimised for finding telecoms cables, but these are less well suited to general cable and pipe locating as high frequency signals dissipate rapidly with distance along a typical cable or pipe.

According to an aspect of the present invention, there is provided a locator for locating a concealed current carrying conductor. The conductor carries an altemating current having at least a first frequency and a second frequency, wherein the alternating current is produced by at least one dedicated signal generator coupled to the conductor. The locator comprises at least one magnetic field sensor operable to convert electromagnetic radiation from the conductor into a field strength signal; a digital analogue converter configured to generate a digitised signal dependent upon the field strength signal from the magnetic field sensor; a digital signal processor configured to isolate components of the digitised signal resulting from the first frequency and the second frequency; and process the isolated components to generate one or more signals indicative of the proximity of the conductor to the detector; and an output configured to generate an audio andlor visual indication of the proximity of the conductor, wherein the isolated signal components resulting from the first frequency signal and the second frequency signal are contemporaneously processed.

Locators according to the present invention allow alternating currents in a concealed conductor having two frequencies to be detected. The two frequencies may for example be 33kHz and 66kHz. The frequencies may be detected contemporaneously with each other. Thus an apparatus for locating a concealed conductor may use both frequencies at approximately the same time to locate a conductor. Thus embodiments of the present invention provide for the location of cables or pipes in a wide variety of situations without the need for a user to make adjustments to either the signal generator or the locator. Embodiments of the present invention thus facilitate a robust and accurate system for locating pipes and cables which does not require specialist knowledge or additional training for a user compared with known products.

In an embodiment, the second frequency is a harmonic of the first frequency. In an embodiment, the second frequency is twice the first frequency.

In an embodiment the locator comprises a heterodyrie mixer configured to convert the second frequency to a lower frequency and wherein the digital signal processor is configured to isolate and process the lower frequency signal. This allows frequencies of higher than the Nyquist frequency of the analogue digital converter to be processed in the locator. This has the benefit of facilitating the use of an audio ADC with a sampling frequency of approximately 96k samples per second to be used with alternating currents having a frequency of 66khz or higher.

In an embodiment, the alternating current having the first frequency and the second frequency are produced by one dedicated signal generator.

In an embodiment the alternating current having the first frequency and the second frequency are produced by separate dedicated signal generators.

According to an aspect of the present invention there is provided a system for locating a concealed conductor. The system comprises a locator according to an aspect of the present invention and a signal generator configured to generate the alternating current having the first frequency and the second frequency.

In an embodiment the signal generator comprises a first oscillator configured to generate a first waveform having the first frequency, a first terminal coupled to the first oscillator through a first filter configured to pass signals of the first frequency; a second oscillator configured to generate a second waveform having the second frequency, and a second terminal coupled to the second oscillator through a second filter configured to pass signals of the second frequency.

In an embodiment, the signal generator can be directly connected to the concealed conductor and for connecting the other of the first and second terminals to the ground.

In an embodiment the signal generator can be inductively coupled to the concealed conductor.

In an embodiment, the signal generator further comprise an induction coil for inductively coupling with the buried conductor and a switching circuit configured to vary the current in the induction coil according to a switching waveform having a first component in the first frequency and a second component in the second frequency.

Such an embodiment allows more than one frequency to be efficiently inductively induced in the concealed conductor because the coil is not part of a resonant circuit.

In an embodiment the switching circuit comprises four switching devices in an H-bridge formation.

In a further aspect the present invention, there is provided a method of locating a concealed conductor.

A further aspect of the present invention provides a carried medium carrying computer readable instructions for execution by a processor in a locator for locating a concealed conductor. The instructions cause the locator to operate in accordance with the embodiments of the present invention recited above Embodiments of the present invention will be described by way of example with reference to the drawings in which: Figure 1 shows a schematic view of a signal generator according to an embodiment of the present invention; Figure 2 shows a schematic view of a signal generator according to an embodiment of the present invention; Figure 3A shows a schematic view of a signal generator according to an embodiment of the present invention; Figure 3B shows a power supply circuit for a signal generator according to an embodiment of the present invention; Figure 4A shows a drive waveform for use with an embodiment of the present invention; Figure 4B shows a drive waveform for use with an embodiment of the present invention; Figure C shows a drive waveform for use with an embodiment of the present invention; FigureS shows a locator according to an embodiment of the present invention; Figure 6A shows a schematic view of a locator according to an embodiment of the present invention; and Figure 6B shows a schematic view of a locator according to an embodiment of the present invention.

Figure 1 shows a signal generator 100 which generates an AC signal for coupling to a buried conductor. The signal generated by the signal generator 100 has two frequencies. A first frequency of 33kHz and a second frequency of 66kHz. The signal generator has a first oscillator 102 which generates an AC signal having a frequency of 33kHz. The first oscillator is connected to a first filter 104 which is configured to allow signals having a frequency of 33 kHz to pass and to attenuate any harmonics produced by the first oscillator. A first terminal 106 is connected to the first filter 104. The signal generator 100 has a second oscillator 108 which generates a signal having a second frequency. In this example, the second frequency is 66khz. The second oscillator is connected to a second filter 110. The second filter 110 allows signals having a frequency of around 66 kHz to pass and attenuates harmonics. The second filter 110 is connected to a second terminal 112 of the signal generator 100.

The first and second filters may be for example, low pass filters or band-pass filters.

In use, the signal generator 100 is coupled to a buried conductor by the first terminal 106 and the second terminal 112. The output of the signal generator 100 may be directly connected to the buried conductor. In this case, one of the terminals is connected directly to the pipe or cable at an access point such as a valve, meter or end of the conductor and the circuit is completed by a connection of the other terminal to a ground stake or other ground connection point.

The signal generator 100 may also be inductively coupled to a conductor. This is achieved by the use of an induction clamp. The output from the signal generator is connected to a winding around a magnetic core and the magnetic core is placed around the conductor.

The signal generator 100 thus provides a method of generating a signal having two frequencies in a buried conductor.

Figure 2 shows a schematic diagram of a signal generator 200 for generating a signal having two frequencies. In this embodiment, a controller 202 provides a first waveform having a first frequency and a second waveform having a second frequency. The controller 202 is a complex programmable logic device (CPLD). The first waveform is fed through an amplifier 204 and through a filter 206 to a first terminal 208. The controller 202 also produces a second waveform having a second frequency this is fed through a second amplifier 210 and a second filter 212 to a second terminal 214. The amplifiers 204 and 210 are each formed from a driver 216 which drives two switching devices 218 and 220. The switching devices 218 and 220 are arranged in a half bridge formation.

In use, the controller 202 generates waveforms at a first frequency of 33 kHz and a second frequency of 66 kHz. The waveforms are each selected to suppress signal components at a third harmonic frequency of their fundamental frequency. This waveform is then used by the driver 216 to cause the switching devices 218 and 220 to switch the input of the filter between ground reference and a supply voltage. The filter attenuates harmonics that are present in the waveforms. For example, therefore the filter 206 blocks frequencies other than 33 kHz. Because the driving waveform for the first amplifier 204 is selected to suppress the third harmonic frequency, the largest element that the filter 206 has to block is the fifth harmonic frequency of 33kHz.

When a load is connected between the terminals 208 and 214 the current returns to ground reference through the opposite filter. The impedances of the components of the filters are selected so that the impedance to ground reference of the second filter 212 for a frequency of 33kHz (the frequency emitted from the first terminal 208) is low and impedance to the ground reference of the first filter 206 for a frequency of 66kHz is low.

In the embodiment described above, the half-bridges are controlled by a CPLD. Digital logic (e.g. CMOS), a microcontroller, FPGA or other digital processors could be used in place of the CPLD. In an alternative embodiment, the oscillators controlling the half-bridges could be provided from a pair of crystal oscillator circuits running independently of one another.

Figure 3A shows an embodiment of a signal generator 300 for coupling to a conductor and generating and an alternating current having two frequencies in the conductor.

The signal generator 300 has a controller 302 which controls two signal generating elements. There is a signal generating element for direct connecting 304 which is analogous to the circuit described in relation to figure 2. The signal generator 300 also has an inductive signal generator 306. The inductive signal generator includes an induction coil 308 which is driven by four switching elements in an H bridge formation.

To generate a signal having a first frequency and a second frequency, the controller generates a drive waveform having the first and second frequencies. This drive waveform is used to drive the switching elements in the inductive signal generator 306 and cause the current through the induction coil 308 to vary according to the time integral of the drive waveform.

A current sense point 310 on each of the half bridges is connected to a power supply of the signal generator to regulate the supply voltage if the current between the terminals becomes higher than a threshold.

The power supply 350 is shown in figure 3B. The power supply comprises a battery 352. The battery 352 is connected to a switch on control 354. The battery provides a voltage of 6V to a boost convener 356. The boost converter 356 provides the source voltage for the half bridges and H-bridge shown in Figure 3A. The current sense point 352 is connected to an input of the boost converter 356 and a low pass filter 36C. When the current sensed at the current sense points 310 exceeds a threshold the boost converter lowers the supply voltage this regulates the magnitude of the current through the load.

Figure 4A shows an example of the waveform 402 used to drive the induction coil 308.

The waveform 402 is a rectangular waveform with containing pulses having a 9:23 ratio in duration. Such a waveform has been found to produce first and second frequency components where one frequency is twice the other frequency.

Figure 4B shows an example of the waveform 404 used to drive the 33kHz part of the direct connect circuit 304. The waveform 404 has a high component in F33kHz and a* low component in the third harmonic frequency 3F.

Figure 4C shows an example of the waveform 406 used to drive the 66kHz part of the direct conned circuit 304. The waveform 406 has a high component in 2F66kHz and a low component in the third harmonic frequency 6F.

It is noted that the inductive signal generator 306 is non-resonant. This means that it can generate signals of two different frequencies efficiently. Signal generators for inductively generating a signal in a conductor often comprise a resonant circuit. Such a resonant circuit is effective for generating an alternating current at a frequency close to the resonant frequency of the resonant circuit. However, a resonant circuit is not efficient at generating frequencies outside the resonant frequency bandwidth of the resonant circuit. This means that to generate alternating currents having two frequencies for example 33kHz and 66khz, either a resonant circuit with a broad resonant frequency bandwidth (i.e. a low 0-factor) would have to be used, or the resonant circuit would have to be driven at frequencies a long way from its resonance.

Either case would result in an inefficient energy transfer.

For the Signal Generator, the direct-connection output system described above provides the best power efficiency simultaneously with the best signal quality (lowest harmonic content) over the entire load impedance range from zero ohms towards infinity. Best power efficiency is obtained using class D switching amplifiers. Class B amplifiers have theoretical maximum power efficiency of 76% (r pi over 4) at best when amplifying a sinusoidal waveform. Class 0 improves on this with a theoretical power efficiency limit of 100%. The imperfections of class D are mainly due to switching losses, which become greater as the switching frequency increases, due to repeatedly charging and discharging capacitances in the switching components, resulting in real-world power efficiency of less than 100%. Filtering the output of the switching stage to prevent unwanted switching noise being coupled to the load further reduces power efficiency due to resistive losses in non-ideal inductors and capacitors.

To use uniformly sampled class D PWM requires a switching frequency of at least 10 times and preferably at least 20 times the highest signal frequency. In the present application, a highest signal frequency of 66khz would necessitate a switching frequency of at least 660khz and preferably at least 1.32MHz. This will result is relatively high switching losses in a Class D amplifier. Such an amplifier would be little better than a class B amplifier.

A more power efficient implementation is embodied by the signal generator described above. By switching a half-bridge at the signal frequency the switching loss in the half-bridge is minimised. Use of a switching waveform that eliminates the third harmonic of the fundamental switching frequency of the half-bridge simplifies the output filter design, since the lowest harmonic frequency requiring attenuation is that of the harmonic. Combining two such half-bridge circuits, the first operating at a first frequency (33khz) and the second operating at a second frequency (66kHz) results in a system having minimal power loss, hence maxirnising battery life in a portable battery operated signal generator The signal purity (freedom from unwanted harmonics and noise) is also exemplary.

A locator, or detector for locating conductors carrying an alternating current of two or more frequencies will now be described.

Referring to Figure 5, a detector 1 has two vertically spaced antennae, namely a bottom antenna 3 and a top antenna 5 within an elongate vertically held housing (not shown) configured to be moveable manually by an operator using a handle. The antennae 3, 5 are arranged with their axes parallel and spaced apart so that in use the bottom antenna 3 will be directly below the top antenna 5, their axes being horizontal.

Each antenna 3, 5 produces an electrical signal which is fed into a respective one of two amplifiers 7. The amplifier outputs are field strength signals 9 which are fed into a CODEC 11.

Each of the antennae 3, 5 has a noise floor. Each electrical signal from the antennae 3, 5 is fed to its respective amplifier 7 to lift the noise floor of the magnetic sensor above an intrinsic quantisation noise floor of the CODEC 11. The output of each amplifier 7 is fed into the CODEC ii.

The antennae 3, 5 used are high sensitivity wound ferrite rods. Other magnetic sensors may be used such as Hall effect sensors, flux gate magnetometers, or giant magneto resistance sensors.

The CODEC 11 is a 24-bit stereo delta-sigma analogue to digital converter (ADC).

This is a relatively cheap device which is commonly used in the audio industry. In Radiodetection Limited's product marketed under the RD4000 (RIM)' trade mark, pre-selective filtering, multiple switch gain stages and a phase sensitive heterodyne circuit are used between the antennae and the ADC. In other prior art cable detectors, more sophisticated and consequently more expensive ADCs are used, as the absolute accuracy of the device measurements is important.

The CODEC 11 used in this embodiment has an absolute accuracy of ±5%, however the way that the CODEC 11 is used makes it an ideal ADC for this application. High dynamic range negates the requirement for multiple gain stages The high dynamic range is achieved by massively oversampling the bandwidth of detection -the noise shaping aspect of the audio CODEC 11 being an ideal application for this principal.

Notwithstanding the poor absolute accuracy of this audio-grade stereo ADC, the present embodiment benefits from the fact that the detector 1 calculates the depth of a buried conductor by processing and comparing the signals received from the two antennae 3, 5. Therefore, any absolute inaccuracy in the sampling of the CODEC ills overcome by comparing the two processed signals. Using this CODEC 11 as a ratiometric device provides a significant cost reduction, without compromising overall performance of the detector 1.

The CODEC 11 oversamples the field strength signals 9 at up to 96KHz. The output 13 of the CODEC 11 is fed into a digital signal processing block 15, which is comprised of a digital signal processor 16 (DSP).

The DSP 16 primarily has three tasks. Firstly, it is responsible for defining the selectivity of the detection frequency bands. Secondly, it manages the audio and visual outputs of the detector. Thirdly, it provides general control functions to other components of the detector 1.

More details of the operation of the DSP's tasks are provided in Radiodetection Limited's applications published as WO 031071311, WO 03/069598, WO 03/069769, GB 2400994 and GB 2400674, which are incorporated herein by reference.

Significant benefits are derived from ultra-narrow bandwidth processing, noise typically scaling with the square of bandwidth. The detector 1 processes in several frequency bands simultaneously, allowing ballistic response functions, such as the general locate task, to co-exist with narrow bandwidth functions, such as depth computation. The depth computation task computes in a 1Hz bandwidth at any frequency up to 44kHz, the out-of-band rejection being around -120dB.

Phase tracking allow the narrows bandwidth tasks to lock-on to the carrier frequency when the potential frequency error between transmitter and receiver clocks is in excess of the signal bandwidth. In the case of the active mode, the transmitted signal may be 100% amplitude modulated and the depth calculation task has to position itself exactly on the carrier without cross-talk from the side-bands (located at ±6Hz around the 32,768 Hz carrier).

The phase tracking algorithm is a natural development of processes described in Radiodetection Limited's UK application no. 0407372.2. Signal to noise ratio (SNR) measurements are made on the carrier and side-bands and checks performed to ensure the tracking algorithm does not wander off on any high order harmonics due to power-line transmissions. SNR is quantified from both magnitude and second derivative phase information; all results are correlated from both antennae 3, 5. In the case of an SNR less than 10dB, the depth calculation task is disabled, thus ensuring only accurate information is presented to the user.

The concept of spectral recognition is applied to the active signal when it is in pulsed mode operation. This idea is a simple application of the algorithms described in Radiodetection Limited's UK application no. 0407372.2 and involves a spectral assessment of the carrier and AM side-bands. The assessment is a Discrete Fourier Transform (DFT) convolution and measurement of the SNR. The DFT itself moves with the tracking algorithm and locks on to the carrier frequency.

The combination of these methods ensures that the detector 1 achieves the best possible signal integrity and depth accuracy.

User control of the detector 1 is provided by means of a sensitivity control 17 and a switch 19. The switch 19 is used to set the mode of operation of the detector 1. For example, the detector 1 can be set to operate in radio, power or active mode. The active mode is chosen when a dedicated signal generator is used in proximity to the cable which is to be detected, the signal generator inducing an alternating current in the conductor which re-radiates a magnetic signal. The signal generator operates at a preset frequency and with a preset modulation which is identified by the detector 1. A further position of the switch 19 is avoidance' mode, the operation of which is explained below.

The sensitivity control 17 is used to vary the gradient sensitivity of the antennae 3, 5.

High sensitivity is initially used to detect the presence of a weak signal produced by a current carrying conductor. Once the presence of a conductor has been established, the sensitivity control 17 is varied to decrease the sensitivity of the detector 1 and the detector 1 is used to more precisely determine the location of the concealed current carrying conductor. This method of profiling the locate window as a function of sensitivity is described in Radiodetection Limited's application published as US 6777923, which is incorporated herein by reference.

A liquid crystal display (LCD) 21 is provided in the housing surface to display such information as the mode of operation of the detector, the battery status, the depth of a conductor and/or the strength of the detected signal. Other user display devices can be used, as will be apparent to the skilled person.

The detector I also comprises a flash ROM 23, in which software is stored, and a power supply unit (PSU) 25. A key requirement of the detector 1 is that it must be portable. Therefore, batteries 26 are used to power the detector 1, in this case two 0'-type batteries, each providing a nominal 1.5V.

In use, the detector 1 is powered up and software is loaded from the flash ROM 23 into the digital signal processing block 15. A user adjusts the switch 19 to select the mode of operation. The selection will be either radio mode, power mode, active mode or avoidance mode. A depth threshold alarm function is active in power mode, active mode and avoidance mode. In avoidance mode the depth threshold alarm function operates on frequencies in the frequency bands of power mode and active mode. The depth threshold alarm function is detailed below.

When the detector 1 is in proximity to a current carrying conductor, a current is induced in the bottom and top antennae 3, 5. The current induced in each of the antennae 3, 5 is amplified by a respective amplifier 7. The outputs 9 from the amplifiers 7 are field strength signals of the two antennae 3, 5. These signals are input to the CODEC 11 which samples these signals at up to 96 kilo samples per second. The digitised signals 13 are fed to the digital signal processing block 15. The DSP 16 of the digital signal processing block 15 isolates signals of target frequency bands, depending on the mode of operation. If the DSP detects the presence of a current carrying conductor an audio and/ar visual alarm is triggered on the speaker 22 andIor indicator 21.

Figures 6A and 6B show a more detailed block diagram of the detector 1 showing the dual frequency mode system, which is implemented in the detector 1. As mentioned above, two frequencies 33kHz and 66kHz are induced in the conductor being detected.

The detector 1 of this embodiment processes the two frequencies of 33kHz and 66kHz simultaneously. The pair of antennae 3, 5 receive signal components of both frequencies. The system also has a common detection indicator 21 and speaker 22 which provide an indication of the depth of the buried conductor calculated from both of the frequency components. The detection sensitivity 30 is normally set to maximum, but can be set at a lower level.

The stereo CODECs 11 are clocked at 73.242 Kl-Iz. In order to process the 66KHz signal, using such a CODEC, a heterodyne system 31 of a heterodyne oscillator and two heterodyne mixers is used to convert the 66Khz signal into an intermediate frequency 6KHz signal. As shown in Figure SA, an oscillator with a frequency of approximately 60KHz is used, and the heterodyne mixers reject the signal resulting from the sum of the 60KHz oscillator and the 66KHz signal and pass only the difference signal having a frequency of approximately 6 KHz. This is within the Nyquist frequency range of the ADC. The DSP 16 processes the field strength signals produced by the antennae 3, 5 and simultaneously isolates signals of each of the two frequency bands in two mode selectivity functions 43, 45.

In an alternative embodiment, the heterodyne system 31 is omitted and an analogue digital converter having a higher sample rate which places the Nyquist frequency above 66KHz is used.

Signal outputs from the DSP 16 corresponding to the different frequencies are fed into automatic gain controllers 47 (AGCs), such as the AGC described in Radiodetection Limited's application published as US 6777923, which is incorporated herein by reference. The output of each of the AGCs 47 is converted to a detection signal in comparators 49. The detection signals are combined and used to provide an audio output from a speaker 22 and/or a visual signal on an indicator 21, for example on the LCD.

The detector 1 continually calculates the estimated depth of a buried conductor. If the depth of a buried conductor is calculated as less than a preset threshold, e.g. 30cm, an audio and/or visual alarm may be triggered to alert the operator of a shallow conductor.

Such shallow conductors are of particular interest as there is an increased risk of hitting a shallow conductor when excavating an area.

In order to optimise the user interface of the detector, when calculating the depth of a conductor, the DSP 16 processes signals in two frequency bands simultaneously to tailor the manner in which information is presented to the user. The depth of the conductor is calculated in a 1Hz bandwidth; the visual display is processed in a 10Hz bandwidth so that the flicker of the display is at an acceptable level; and the processing of the audio alert is performed at 35Hz, to ensure that the pulsing tone is clearly audible.

Embodiments of the present invention may be implemented in combination with a depth threshold alarm, and/or with an avoidance mode' as described in UK Patent Application no. 2427473 the contents of which are incorporated herein by reference.

While in the embodiments described above, the two frequencies are induced in a conductor by a single signal generator, embodiments of the invention are envisaged in which the two signals are generated by different signal generators. Indeed embodiments of a locator are envisaged in which the different frequencies are induced in different conductors, for example with a different frequency being induced in different types of utility. In such an embodiment, the locator may provide an indication of which frequency is dominant) thereby giving an indication of the utility which is present.

The digital domain signal processing may be implemented in FPGA, DSP or microcontroller devices, or split across some combination of the aforementioned devices.

Aspects of the present invention can be implemented in any convenient form, for example using dedicated hardware, or a mixture of dedicated hardware and software for the processing of the signals. The processing apparatuses can comprise any suitably programmed apparatuses such as a general purpose computer, personal digital assistant, mobile telephone (such as a WAP or 3G-compliant phone) and so on.

Since the processing of the present invention can be implemented as software, each and every aspect of the present invention thus encompasses computer software implementable on a programmable device. The computer software can be provided to the programmable device using any conventional carrier medium. The carrier medium can comprise a transient carrier medium such as an electrical, optical, microwave, acoustic or radio frequency signal carrying the computer code. An example of such a transient medium is a TCP/IP signal carrying computer code over an IP network, such as the Internet. The carrier medium can also comprise a storage medium for storing processor readable code such as a floppy disk, hard disk, CD ROM, magnetic tape device or solid state memory device.

Claims (1)

1. <claim-text>CLAIMS: I. A locator for locating a concealed current carrying conductor, the conductor carrying an alternating current having at least a first frequency and a second frequency, wherein the alternating current is produced by at least one dedicated signal generator coupled to the conductor, the locator comprising: at least one magnetic field sensor operable to convert electromagnetic radiationfrom the conductor into a field strength signal;a digital analogue converter configured to generate a digitised signal dependent upon the field strength signals from the magnetic field sensor; a digital signal processor configured to: isolate components of the digitised signal resulting from the first frequency and the second frequency; and process the isolated components to generate one or more signals indicative of the proximity of the conductor to the detector; and an output configured to generate an audio and/or visual indication of the proximity of the conductor, wherein the isolated signal components resulting from the first frequency signal and the second frequency signal are contemporaneously processed.</claim-text> <claim-text>2. A locator according to claim 1 wherein the second frequency is a harmonic of the first frequency.</claim-text> <claim-text>3. A locator according to claim 2, wherein the second frequency is twice the first frequency.</claim-text> <claim-text>4. A locator according to any preceding claim further comprising a heterodyne mixer configured to convert the second frequency to a lower frequency and wherein the digital signal processor is configured to isolate and process the lower frequency signal.</claim-text> <claim-text>5. A locator according to any preceding claim, wherein the alternating current having the first frequency and the second frequency are produced by one dedicated signal generator.</claim-text> <claim-text>6. A locator according to any preceding claim, wherein the alternating current having the first frequency and the second frequency are produced by separate dedicated signal generators.</claim-text> <claim-text>7. A system for locating a concealed conductor comprising a locator according to any preceding claim; and a signal generator configured to generate the alternating current having the first frequency and the second frequency.</claim-text> <claim-text>8. A system according to claim 7, wherein the signal generator comprises a first oscillator configured to generate a first waveform having the first frequency, a first terminal coupled to the first oscillator through a first filter configured to pass signals of the first frequency; a second oscillator configured to generate a second waveform having the second frequency, and a second terminal coupled to the second oscillator through a second filter configured to pass signals of the second frequency.</claim-text> <claim-text>9. A system according to claim S further comprising a connector for connecting one of the first and second terminals to the concealed conductor and for connecting the other of the first and second terminals to the ground.</claim-text> <claim-text>10. A system according to claim 8 claim further comprising an inductive coupler for coupling the first and second terminals.</claim-text> <claim-text>11. A system according to any one of claims 7 to 10, the signal generator further comprising an induction coil for inductively coupling with the concealed conductor and a switching circuit configured to vary the current in the induction coil according to a switching waveform having a first component in the first frequency and a second component in the second frequency.</claim-text> <claim-text>12. A system according to claim 11, wherein the switching circuit comprises four switching devices in an H-bridge formation.</claim-text> <claim-text>13. A method of locating a concealed conductor, the method comprising: applying an alternating signal to the conductor, the alternating signal having at least a first frequency and a second frequency, using at least one magnetic sensor local to the conductor and above ground to generate a field strength signal proportional to the strength of an electromagnetic field generating a digitised signal dependent upon the field strength signal from themagnetic field sensor;isolating components of the digitised signal resulting from the first frequency and the second frequency; and processing the isolated components to generate at least one signal indicative of the proximity of the conductor to the detector; and generating an indication representing the proximity of the conductor, wherein the isolated signal components resulting from the first frequency signal and the second frequency signal are contemporaneously processed.</claim-text> <claim-text>14. A method according to claim 13 wherein the second frequency is a harmonic of the first frequency.</claim-text> <claim-text>15. A method according to claim 14, wherein the second frequency is twice the first frequency.</claim-text> <claim-text>16. A method according to any one of claims 13 to 15 further comprising a converting the second frequency to a lower frequency, wherein the lower frequency signal is isolated and processed.</claim-text> <claim-text>17. A method according to any one of claims 13 to 16, wherein the alternating current having the first frequency and the second frequency are produced by one dedicated signal generator.</claim-text> <claim-text>18. A method according to any one of claims 13 to 16, wherein the alternating current having the first frequency and the second frequency are produced by separate dedicated signal generators.</claim-text> <claim-text>19. A carrier medium carrying computer readable instructions for execution by a processor in a locator for locating a concealed conductor, the conductor carrying an alternating current having at least a first frequency and a second frequency, wherein the alternating current having the first frequency and the second frequency is produced by at least one dedicated signal generator coupled to the conductor, the locator having at least one magnetic field sensor operable to convert electromagnetic radiation from the conductor into a field strength signal, wherein the instructions comprise instructions for controlling the processor to generate a digitised signal dependent upon the field strength signal from themagnetic field sensor;isolate components of the digitised signal resulting from the first frequency and the second frequency; and process the isolated components to generate at least one signal indicative of the proximity of the conductor to the detector.</claim-text>