GB2092307A - Field Detector for Underground Conductor - Google Patents

Abstract

The field detector is in the shape of a tooth intended to be fitted to a bucket of a mechanical excavator. According to one arrangement, the tooth may have a body (1) made of steel which contains magnetic field detecting means such as orthogonal coils (3a, 3b) each having a respective core (5a, 5b) passing through diametric apertures (2a, 2b) in the body (1). The end faces of the cores (e.g. 6b) are thus open to the ambient medium. The end portions of the cores are insulated from the body by means of insulating bushes (4b). The body (1), also houses transmitter means which may have respective detecting and transmitting modes to make use of the coils for the dual purpose of detecting a field and transmitting a detected signal. According to another arrangement, the tooth, or part thereof has an extent which operates as an aerial for radiating a carrier frequency signal modulated by a field detector located, together with the transmitter circuitry, within a chamber in the tooth. A receiver in a cab of the excavator vehicle receives the transmitted signal and operates a warning device. The transmitter and receiver may cooperate in that a coding system is used to modulate the detection signal on a carrier frequency to characterise the signal from a tooth of its respective excavator, i.e. where more than one excavator is operating on the same site. A Hall effect device may be used instead of the coils. <IMAGE>

Description

SPECIFICATION Field Detector for Underground Conductor This invention relates to a field detector adapted for use with excavating means, such as a mechanical shovel or bucket excavator, to detect the presence of an underground conductor. The conductor may be a buried cable carrying a pulsating current, or a conductor such as a gas or water pipe carrying an induced pulsating signal.

When a mechanical excavator is used in the vicinity of existing underground services, such as buried cables or pipes, there is a danger of damaging the existing services. For example, the cost of damage caused by mechanical excavators digging trenches to enable public utilities to lay new service runs may be estimated at over 100,000 per day in the UK. The repairs of electrical cables, gas and water pipes which may be necessary following such damage results in unnecessary expenditure of labour and materials, quite apart from the inconvenience suffered by the loss of services to the consumer. Whilst conventional field detectors or metal detectors may be used to detect the presence of buried cables or pipes, such detectors may not be reliable, in use, since the cable or pipe may be buried at the depth outside the range of the detector.Moreover, such portable detectors may not be used, either due to inconvenience, or due to an erroneous belief that the ground to be excavated is already clear of existing 5r rvices.

Therefore, there is a problem of providing a reliable means of detection which is operative during excavation to provide an early and reliable warning of the presence of existing buried services.

The invention provides a solution to this problem, at least to detect an underground conductor which carries a pulsating current or an induced pulsating signal. The solution is to provide a field detector which is adapted for use with excavating means in that it has a body shaped to penetrate ground material. The body contains detecting means responsive to a magnetic field generated by the current flowing or signal induced in an underground conductor, and transmitting means conditioned to radiate a signal when the detecting means responds to said field.

The term "excavating means" is broadly used to define a device or equipment having a member for penetrating the ground and for excavating ground material. The term would cover, for example, a mechanical shovel of the bucket type normally provided with one or more teeth, as well as drills or augers.

According to a particular arrangement, in which the field detector is adapted for use with a mechanical excavator of the type comprising a bucket normally provided with one or more teeth, the field detector has a body in the shape of a tooth which is intended to be fitted to the bucket, for example, as a central tooth in a row of teeth forming a leading edge of the bucket.

Suitably, the body of the field detector is made of abrasion resistant material such as steel. When the body is made of such an electrically conductive material, there is a problem of mounting the detecting means within the body so that it will respond to e.g. a magnetic field generated by the pulsating current or induced pulsating signal in the underground conductor.

According to one embodiment of the invention, this problem is solved by providing an element in which a magnetic field can be induced, which element is largely protected by the metallic body, but is electrically insulated therefrom whilst not being shielded thereby. This arrangement necessitates some compromise between protecting the element from the ambient medium whilst providing sufficient access to the ambient medium to allow it to detect or to respond to the magnetic field generated by the pulsating current or the induced pulsating signal in the underground conductor.

The detecting means may employ a coil or coils wherein the latter-mentioned element is a metal core passing through each coil and through an aperture in the body. The core is electrically insulated from the metallic material of the body in the vicinity of the aperture and the core has at least one end face which is open to the medium surrounding the body. Preferably, a pair of coils are arranged within the body with their longitudinal axes at right angles to one another, respective metal cores passing axially through the coils and through apertures on opposite sides of the body. Suitably, the cores pass diametrically through the body and through aligned apertures.

The end portions of the cores are received in insulated bushes fitted within the respective apertures.

Alternative detecting means may be used, such as a Hall-effect device, which incorporates an element that is unshielded by the material of a metallic body.

The arrangement is also such as to enable the radiation of a signal by the transmitting means when the detecting means responds to a field. In the case of using a coil or coils in the detecting means, the coils may serve the dual purpose of detection and transmission. The transmitting means may then include suitable conventional circuitry which enables detection of the field during a detecting phase, and transmission of a signal (representing a detected field) during a transmission phase. Alternatively, the transmitting means may be connected to a separate aerial which is largely or partly contained by the body and which extends at least to a surface thereof at a point where it is not subjected to the abrasive action of ground material during excavation.

According to another embodiment of the invention the body which is shaped to penetrate the ground, or a part thereof, has a longitudinal extent which is so connected to transmitting means as to operate as an aerial for radiating a signal when the detecting means responds to a magnetic field. For example, said axial extent may be about one half of the wavelength of a carrier frequency which is modulated by the detection signal provided by a magnetic field detector within the tooth. Preferably, the modulation is effected according to a coding system to characterise the detection as belonging for example to the tooth of a respective excavator (where more than one excavator is operating on the same site).

Embodiments of the invention will now be described with reference to the accompanying schematic drawings, in which: Fig. 1 and 2 illustrate, in plan and crosssectional elevation one embodiment of the invention namely, a field detector in the form of a tooth to be fitted to the bucket of a mechanical excavator, and Fig. 3 illustrates a cross-section of a tooth according to another embodiment.

A mechanical excavator or shovel is normally fitted with a bucket having a row of teeth along a leading edge. Usually, there is a large eye tooth at each corner (i.e. at each end of the row of teeth) and several small teeth mounted between the eye teeth, according to the width of the bucket. A bucket which is about 350 cm wide is normally used to dig service trenches and this has only one centre tooth. According to the preferred embodiment of the invention, the size of this centre tooth is increased to accommodate magnetic field detecting means and transmitting means which receive a signal from the detecting means, in response to a change in an ambient magnetic field, for transmitting the detected signal. Figs. 1 and 2 illustrate, in plan and sectioned elevation, such a tooth.

As shown in Figs 1 and 2, the tooth comprises a generally cylindrical body 1 which is made of steel. Transverse apertures 2a, 2b at right angles to one another pass through the body 1. Each of these apertures receives a respective insulated aerial coil 3a, 3b a pair of annular insulating plugs 4a, 4b and a metal core 5a, 5b the core passing through the coil and the plugs. End faces of the core (not visible in the drawing but indicated by reference numeral 6b in the lower core 5b) are flush with the cylindrical surface of the body 1.

A longitudinal bore 7 is provided to accommodate connecting wires 8a, 8b from each of the coils 3a, 3b. These wires are connected to transmitting means 9 housed in a recess 10 in the lower end of the cylindrical body 1. An insulating plug 11 seals the end of the recess 10. The plug 11 may alternatively be made of steel and threaded to engage an internal thread on the outer section of the recess 10.

The transmitting means 10 may include known circuitry to ampiify the signal induced in either 1, or both of said coils and adapted to modulate a carrier signal which is supplied to either or both of said coils for radiating a radio signal which is picked up by a receiver (not shown) in the cab of the mechanical excavator (not shown). The receiver may be connected to visible or audible or both) warning means to indicate to the driver of the excavator that the bucket is in the vicinity of a buried service such as a cable carrying mains current at a frequency of 50 Hz, or a water or gas pipe in which a pulsating signal is induced. Often, water and gas pipes run parallel with electrical cables carrying mains supply current and there is therefore a very good chance that the 50 Hz signal will be induced into the pipes and hence provide a magnetic field.Alternatively, it may be possible to place a coil around a gas or water pipe in the vicinity of an excavation site to induce a signal directly into the pipe. The warning device may be such as to enable the driver of the excavator to be made aware of the approach of the bucket to the buried services whereby he could either use his skill to feel for the service he suspects to be close by, or his power could be shut-off automatically with a view to carrying out hand digging until the sevice is found.

The transmitting means 10 includes its own power supply and hence is preferably of a type with a low current drain. The electronic components of the field detector, including a battery, may be totally encapulated within the body 1 of the metal tooth. The tooth can therefore be supplied to a user ready for use and operating in a listening mode. It may have a continuous operating life or not less than three years, and would normally be discarded by the user either when the wear on the tooth become such that the internal electronics become damaged, or when the power supply is exhausted. Alternatively, it is envisaged that the removal of a conductive pin or plug, or the provision of a suitably protected switch, may enable the power supply to be turned off when the field detector is not required.This would conserve power, particularly when the tooth might be replaced by a conventional tooth when the excavator is used for digging trenches which are not in the vicinity of buried services.

Although the circuitry of the transmitting means 9 is not shown in detail, it preferably includes means which responds to the rate of change of a signal induced in the coils 3a, 3b rather than to an increase in amplitude of a detected signal. When the circuitry is responsive to a rate of change of the detected signal, the response of the detector will be negligible or zero when the bucket of the excavator is digging a trench which is substantially parallel with a buried cable carrying a heavy current and hence radiating a strong magnetic field. Thus, the warning device would not be activated in these circumstances to indicate the presence of a buried cable. However, if there was any change in direction between the path of movement of the shovel and the run of the buried cable, the warning device would be activated.

The circuitry may alternatively include means responsive to the amplitude of a detected field signal. However, such means may also be provided in addition to the circuitry for detecting a rate of change in the detected field signal.

Amplitude response of circuitry is useful to give a warning of the proximity of approach to a buried cable, particularly when a trench is dug parallel to the same. A suitable arrangement may include circuitry for transmitting both rate of change detected signals as well as amplitude detected signals, for example, in separate channels, and a receiver for selectively operating the warning device in accordance with either type, or both types of detected signal. When using the same aerial coil 3a, 3b to detect a change of magnetic field and to radiate a signal from the transmitting means 9, the transmitter includes suitable circuitry for operating in detecting and transmitting modes.

As an alternative to using aerial coils to detect the change of magnetic field, semiconductor devices may be used such as Hall-effect devices.

The object is to encapsulate and thereby protect the electronic components and circuitry as far as possible from the environment, whilst not shielding the magnetic field to be detected and the radio signal to be transmitted. The use of orthogonal coil arrangements, as illustrated in the drawing, improves the response of the field detector, but one or more than two field detecting means may be employed.

According to a further modification, which may be employed to conserve battery current, the circuitry of the transmitting means 9 may include means responsive to a detected field to energise part of the transmitter, such as an oscillator which generates a carrier frequency and a modulator for modulating the carrier with the detected signal. In this case, an amplifier, if used, is permanently connected to the power supply and its output is supplied to a trigger circuit which is also permanently connected to the power supply.

Suitable power supplies may include a cell or cells of the lithium oxide type which have a very long shelf life.

According to a further embodiment of the invention, a hollow tooth of a bucket excavator is shaped and dimensioned such that the length, or part of the length of the tooth is relevant fraction of a wavelength of the radio wave to be transmitted to a receiver in the excavator cab. Fig.

3 schematically illustrates one form of construction of such a tooth.

As shown in Fig. 3, an active or effective portion 1 a of the tooth 1 has a generally triangular cross-section and contains correspondingly triangular-shaped cavity 2. A coaxial feeder 12 is connected to opposite points 1 3a, 1 3b on the interior of the cavity 2, at its respective longituditnal ends. As shown in the drawing, the screen or earth 1 2a of the coaxial feeder 1 2 is connected to the apex part of the cavity, whereas the central conductor 1 2b is connected to the opposite interior wall of the cavity. The longitudinal extent of the cavity, i.e.

in the axial direction of the tooth, in a relevant fraction of the wavelength and, in the example shown, it is approximately (i.e. just less than) one half of the wavelength of the transmitted radio wave. The exact length of the cavity may be determined by experimentation, for optimum results, having regard to the chosen frequency. A transmission frequency of about 1 GHz is preferred, but in practice the transmission frequency may, for example, be in a 1.487 GHz band, or in a 459 MHz band depending on official requirements.

Part 1 b of the tooth shown in Fig. 3 is in the form of a fixing or attachment stub, which is designed to facilitate fitting the tooth to the bucket of the excavator.

The "aerial" tooth 1 of Fig. 3 houses a suitable sensor for detecting e.g. a 50 Hz magnetic field from a mains electricity supply. Suitably the sensor comprises coil 3 wound on a higher core 5.

As shown in Fig. 3, the core 5 is stepped so as to provide a portion 5a which supports the windings of the coil and a portion 5b which is closely adjacent the walls of the chamber 2 inside the tooth 1. With this arrangement, a through aperture or "window" is not required in the tooth as in the embodiment described above with reference to Fig 1 and 2. Such a tooth is clearly more robust. However, it would be possible to employ "windows", if they were required.

Crossed coils, such as those described above with reference to Figs 1 and 2, may be employed to improve the sensitivity from different directions and the arrangement may be such that three core/coil arrangements cross one another at right angles in three dimensions (i.e. x, y, z axes) to provide sensitivity in any direction of movement of the bucket.

The material of the core 5 preferably has a much higher magnetic permeability than the material from which the tooth 1 (Fig 3) is made, although it may be possible to make the core and tooth from the same material if a large signal loss can be tolerated. Preferably, the tooth 1 is made from a non-ferromagnetic material, such as cast manganese aluminium bronze (BS 1400 CMA 2), cast manganese steel (BS 3100 A6) and stainless steel. Cast aluminium bronzes are particularly useful since they are wear and abrasion resistant.

When using cast aluminium manganese bronze to make a non-ferromagnetic tooth, the core 5 may be made or ordinary steel. Alternatively, the tooth could be made of steel and the core of Mu-metal.

The tooth 1 of Fig 3 houses a radio transmitter (not shown) and a power source (not shown). The transmitter is one which is of generally known design, but some particularly useful features are mentioned below. The transmitter operates in conjunction with known circuitry (not shown) responsive to a signal induced in coil 5 so as to suitably modulate the transmitter carrier frequency. This modulated signal, which is at a carrier frequency of about 1 GHz, is radiated to the receiver in the drivers cab and is demodulated so as to provide a warning that the bucket is in the vicinity of the cable.

Preferably, a coding system is used so that the signal which is transmitted and received by the excavator is characteristic of the tooth fitted to its own bucket. Such a coding system enables identification of the excavators own tooth where, for example, more than one excavator is working on a site and each of the excavators are equipped with a field detector according to this embodiment of the invention. The coding system is preferably immune to variations in received signal strength, for example, to account for differences where the tooth may, on the one hand, be buried in wet soil with the excavator arm fully extended, or located near the receiver with the excavator arm fully contracted.

Preferably, that part of the circuitry of the transmitter which is concerned with radio transmission has a 1 :100 duty cycle, because this part of the circuitry consumes the most power.

The other part of the transmitter circuitry is permanently switched on. Such a duty cycle is used, for example, to transmit only 1 msec pulse every 100 msec.

According to one arrangement, the coding systm employs a 459 MHz or 1.487 GHz RF oscillator which is frequency modulated by a modulation oscillator running at a few kHz. The modulation oscillator has its frequency locked, e.g. by a phase locked loop, so that a complete number of cycles n fit into a 1 msec pulse.

Different values of n are programmed into different units, where n varies from, for example 4 to 32 (4 kHz to 32 kHz for the modulation oscillator). If two excavators were working on the same site, and the receivers in the respective cabs listened for a 2 msec "window" period around the time their tooth was expected to transmit its 1 msec burst, the chance of these two teeth having the same n and transmitting during the other tooth window would be very small (i.e. 1 in 1400). The receiver is preferably arranged so as to "learn" its own tooths signature and to align its "window" whenever the receiver was switched on. Such a process might take a minute or less, the "learning time" being less the lower the modulation.

To minimise spurious warnings and sensitivity to interference, the signal (e.g. 50 Hz magnetic field) which is induced into the coil 3 is bandpass filtered, before rectification, by a filter having a Q value which is designed to give reasonable selectivity without causing a long delay.

Avoidance of response time delay is important so as to give the drive adequate warning in time to stop movement of the bucket towards an adjacent buried cable.

Variation in signal is compensated by the use of known AGC (automatic gain control) or compression circuitry at the sensor front end, preferably using transconductance amplifiers. If AGC is used, it must have a slow overall response with a time constant which is of the order of tens seconds for example. If compression is used, this must have a fast response and be clearly defined so as to enable decompression at the receiver.

A suitable receiver for the coding system described above may be based largely on known design. Broadly speaking, however, the receiver is designed to reject spurious signals from teeth fitted to excavators other than its own, to convert the received signals, with the minimum of delay, into a format useful to the driver, to assure the driver of the functional performance of the tooth, at all times, and to warn the driver if the tooth is not working, or is not working correctly.

Preferably, in order to give the driver some idea of signal strength, an audible indication is provided in the form of a continuous tone which indicates the general field strength information and a warning alarm if the rate of increase in field strength exceeds a certain value.

Suitably, the receiver includes a demodulator and an analogue/digital converter whereby a digital signal is provided for processing within a microprocessor. The system functions so as to decompress the received signal, to count the number of cycles (n) in the burst and check that it is correct, to step up the timing of the current and the next "window", to derive the amplitude of the n kHz signal to convert the latter amplitude to an audio frequency and compare its value with a previous value, i.e. to decide whether to provide a "rate of increase" warning and to perform general "housekeeping" and output routines.

These tasks are handled by a microprocessor receiving information from the A/D converter. A/D conversion can either be done fairly rapidly, at full accuracy, to cope with a potential 32 kHz modulation frequency and possibly a high dynamic range, or a discrete analogue peak detector and cycle counting circuit can be used, along with a slow A/D converter.

Whilst radio transmitting and receiving means have been mentioned above, it may also be possible to employ ultrasonic wave or infrared red radiation equipment which is conditioned to radiate a detection signal derived from a magnetic field detector.

Claims (21)

Claims

1. A field detector which is adapted for use with excavating means in that it has a body shaped to penetrate ground material, the body containing detecting means responsive to a magnetic field generated by a current flowing, or signal induced in an underground conductor, and transmitting means conditioned to radiate a signal when the detecting responds to said field.

2. A field detector according to claim 1 adapted for use with a mechanical excavator of the type comprising a bucket normally pro'vided with one or more teeth, the field detector having a body in the shape of a tooth which is intended to be fitted to the bucket.

3. A field detector according to claim 1 or 2 wherein said body is made of electrically conductive material, said detecting means including an element which is largely protected by said body, but which is electrically insulated therefrom whilst not being shielded thereby.

4. A field,detector according to any one of the preceding claims wherein said detecting means comprises a coil or coils and a metal core passing through each coil and through an aperture in the body.

5. A field detector according to claim 4 wherein said core is electrically insulated from said body in the vicinity of the aperture, said core having at least one end face which is open to the medium surrounding the body.

6. A field detector according to claim 4 or 5 wherein said detecting means comprises a pair of coils arranged within the body with their longitudinal axes at right angles to one another, respective metal cores passing axially through the coils and through apertures on opposite sides of the body.

7. A field detector according to claim 6 wherein end portions of said cores are received in insulated bushes fitted within the respective apertures.

8. A field detector according to any one of the preceding claims wherein said transmitting means includes circuitry to enable detection of the magnetic field during a detection phase, and transmission of the signal, representing a detected field during a transmission phase.

9. A field detector according to claim 8 wherein the same coil or coils are used for detecting the magnetic field as for radiating said signal.

10. A field detector substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.

11. Excavating means provided with the field detector according to any one of the preceding claims.

12. Excavating means according to claim 11 and of the type comprising a bucket operated by motive means controlled by a driver in a cab, and including receiving means within the cab for receiving said radiated signal in order to operate means for warning the driver of the presence of the underground conductor.

13. A field detector according to claim 1 or 2 wherein said body or a part thereof has a longitudinal extent which is so connected to said transmitting means as to operate as an aerial for radiating said signal when the detecting means responds to said magnetic field.

14. A field detector according to claim 1 3 wherein said body or a part thereof has an axial length which is about that of a relevant fraction of a wavelength of the signal radiated by said transmitting means.

1 5. A field detector according to claim 1 3 or 14 wherein said body has a chamber therein, the axial length of which is about that of a relevant fraction of the wavelength of the signal radiated by said transmitting means.

1 6. A field detector according to any one of claim 14-1 5 wherein said relevant fraction is approximately one half wavelength of said radiated signal.

1 7. A field detector according to claim 1 5 or 1 6 wherein the detecting means responsive to said magnetic field comprises at least one core made of material having a higher magnetic permeability than the material from which said body is made.

1 8. A field detector according to claim

1 9 wherein said core extends along the axial length of said chamber and has a stepped portion whereby part of the core is closely adjacent the chamber walls and part of the core supports a sensing coil.

1 9. A field detector according to any one of claims 13-18 wherein said body is made of nonferromagnetic material.

20. A field detector substantially as herein described with reference to Fig. 3 of the accompanying drawings.

21. Excavating means provided with the field detector according to any one of claims 1 3-20 and of the type comprising a bucket operated by motive means controlled by a driver in a cab, said body being in the form of a tooth adapted to be fitted to said bucket, the excavating means including receiving means within said cab for receiving said radiated signal in order to operate means for warning the driver of the presence of an underground conductor, the receiving means and the transmitting means being adapted to cooperate with one another according to a coding system, whereby the signal radiated from said transmitting means is modulated when said detecting means responds to said magnetic field, the modulation being characteristic of the tooth which is fitted to the bucket of the excavator and the receiving means being adapted to distinguish such characteristic modulation as belonging to said tooth.