GB2337120A

GB2337120A - Detecting underground conductors

Info

Publication number: GB2337120A
Application number: GB9809783A
Authority: GB
Inventors: Richard William Fling
Original assignee: RADIODETECTION Ltd; Radiodetection Ltd
Current assignee: RADIODETECTION Ltd; Radiodetection Ltd
Priority date: 1998-05-07
Filing date: 1998-05-07
Publication date: 1999-11-10
Anticipated expiration: 2018-05-07
Also published as: GB9809783D0; GB2337120B

Abstract

An excavator vehicle 10 has sensors 12, 13 thereon at different heights. Each sensor has an antenna which is generally horizontal and which detects the magnetic field generated by a current in a buried conductor. By detecting the relative magnitudes of the components of the field detected by the sensors, it is possible to determine whether or not the excavator vehicle is proximate the buried conductor. A warning may be generated, or the excavating tool 11 of the excavator vehicle controlled, to prevent the excavating tool damaging the buried conductor.

Description

2337120 1 - DETECTING UNDERGROUND CONDUCTORS The present invention relates

to a system for detecting an underground object such as a cable or pipe, using a detector mounted on an excavator vehicle.

The proliferation of networks of buried cables and pipes from many different utilities (electricity, gas, telecommunications etc) has meant that an excavation of the ground is likely to be in the vicinity of a buried cable or pipe, and such excavation involves a risk of damage or interference to the buried cable or pipe. This problem is particularly acute when the excavation is carried out by a powered excavating tool such as a back hoe mounted on an excavator vehicle. Because of the separation of the operator of the excavator vehicle from the point of excavation, it is very easy for the excavating tool to make contact with the buried cable or pipe, particularly when the exact location of the buried cable or pipe has not been accurately established and marked in a way which is readily visible to the operator of the excavator vehicle. Moreover, the power of excavating tools is large, so that any contact between the excavating tool and the buried cable or pipe is likely to result in damage to that buried cable or pipe.

It is known to mount a detector on the excavating tool itself, to detect currents in the underground conductor and to generate an alarm or to prevent movement of the excavating tool in a way that would cause the is 1 excavating tool to approach too closely to the underground conductor.

The present invention, on the other hand, proposes that sensors be mounted at different heights on the excavator vehicle itself, with each sensor having at least one antenna generally horizontal relative to the ground. The antenna then detect the magnetic fields generated by currents in the underground conductor.

In such an arrangement, when the excavator vehicle is relatively remote from the underground conductor, the upper antenna detects a larger signal than the lower antenna, because of the difference in angle of alignment of the field with the generally horizontal solenoids. However, as the excavator vehicle approaches the underground conductor, that effect decreases, but the effect of difference in separation between the underground conductor and each solenoid increases. once the antennas are directly above the underground conductor, the lower antenna detects a larger field.

Thus, there is a zone on either side of the underground conductor in which the lower antenna detects a larger field, with the upper antenna detecting a larger field beyond that region. The cross-over points between these two regions may thus be used to trigger an alarm. This means that a detection zone is formed between the cross-over points. The operator of the excavator vehicle may be presented with an alarm when the vehicle such that an underground conductor enters into that detection zone and since most excavator vehicles drag the excavating tool behind them, the excavator tool can be prevented from coming into proximity with the underground conductor. It should be noted that this effect of upper and lower antennas is known per se from US-A-4427942.

The exact positioning of the sensors on the excavator vehicle is not critical, and indeed it is possible for them to have a small displacement in the horizontal direction, relative to each other.

The sensors may provide outputs to a processing apparatus which is frequency selective, so that it is possible to detect the presence of conductors carrying mains power, and/or conductors to which a signal has been supplied at one or more known frequencies.

The antennas may each be in the form of a single coil with a generally horizontal axis, the axes of the coils being generally parallel. However is also possible for each upper and lower sensor to have orthogonal, horizontal, pairs of coils, so that the horizontal orientation of the conductor relative to the sensors does not affect measurements.

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

Fig. 1 is a schematic view of an excavator vehicle embodying the present invention; Fig. 2 shows the relationship between the sensors of the vehicle in Fig. 1 and the magnetic field generated by - 4 an underground conductor; Fig. 3 shows the signals detected by the sensors in Fig. 2; and Fig. 4 is a schematic block diagram of the signal processing arrangement for the embodiment of Fig. 1.

Referring first to Fig. 1, an excavator vehicle 10 has an excavating blade 11 at the back thereof which is used to excavate a trench in the ground, and may therefore strike an underground conductor such as a buried cable or pipe, if the excavating blade 11 comes into contact with that underground conductor. To prevent this, the excavator vehicle has upper and lower sensors 12,13 mounted thereon proximate the front wheels 14, which are used to define a protection zone 15 for the vehicle 10. If the vehicle 10 moves so that an underground conductor comes within the zone 15, this condition is detected by a processor 16 which monitors the outputting of the sensors 12, 13 and triggers an alarm 17 to the operator of the vehicle. The operator may then stop the action of the excavating blade 11, and this occurs well before the excavating blade approaches the underground conductor.

In the simplest arrangement, the sensors 12,13 each comprise a horizontal coil which is generally horizontal, and which is aligned with the frontto-back axis of the vehicle 10. Fig. 2 shows how the sensors 12,13 then intercept the field generated by alternating currents carried by an underground cable 20. When the sensors

12,13 are directly over the cable 20, in the position shown by arrow A, the field is substantially parallel to the sensors 12,13 so that they each detect substantially the whole of the field. However, since the lower sensor 13 is closer to the cable 20 than the upper sensor 12, their outputs will be different, with the upper sensor 12 having a greater output. When the sensors 12,13 are relatively far from the cable 20, as shown by arrow B in Fig. 2, the angles made by the sensors 12,13 relative to the field are different, and the horizontal component in the field detected by the lower sensor 13 is smaller than the horizontal component detected by the upper sensor 12. Thus, at this point, the upper sensor 12 will produce a greater output.

The effect of this is shown in Fig. 3, with the solid line 21 representing the output from the upper sensor 12, and the dotted line 22 representing the output from the lower sensor 13. It can be seen that there is a region 23 within which the output of the lower sensor is greater than that of the upper, and this region 23 can thus be used to define the boundaries of the zone 15. The relative outputs of the sensors 12,13 are detected by a processor 16.

It can be noted that this use of upper and lower sensors is particularly valuable in that it tends to cancel out remote interference, since that will be detected substantially equally by the sensors 12,13. moreover, although the above description has assumed that

6 each sensor 12,13 has only a single coil, an arrangement using orthogonal, but generally horizontal, coils enables an underground conductor to be detected in a way which is independent of the orientation of the conductor to the front-to-back axis of the vehicle 10.

The sensors 12, 13 may be made to detect magnetic fields only at selected frequencies, and it is then possible to detect only certain types of underground conductors. For example, the arrangement may be made sensitive to cables carrying mains power. Alternatively, they may be made sensitive to currents of specific frequencies applied to underground conductors.

Fig. 4 shows the processor 16 in more detail. It also shows a further modification of the embodiment of Fig. 1, in that there are two lower antennas one 13a to the left of vehicle and one 13b to the right. The output of each antenna 12, 13a, 13b is amplified by an amplifier stage 20, mixed 21 with an oscillator signal, and passed through a filter 22 to an analog-to digital converter (ADC) 23. The signals are then passed via a digital signal processor (DSP) 24 to a microprocessor 25. The DSP provides a frequency spectrum. An input 26 to the microprocessor 25 enables the sensitivity of the apparatus to be adjusted if desired. The output of the microprocessor triggers the alarm 17.

The adjustment of sensitivity may be needed in order to adjust to take into account flux distortion due to the materials of the vehicle 10 on which the apparatus is mounted. Alternatively, it may be desirable to reduce the width of the detection zone so that the system can operate next to known conductors which would otherwise be detected, such as when running in a cable right of way parallel to existing services. Moreover, the adjustment may be selective for each operating frequency, so that different frequencies may have different sensitivities.

Another way of adjusting the sensitivity, either as an alternative to, or in addition to, the action of input 24 is to alter the ratio of the gain of the signal from the upper sensor 12 relative to the lower sensor or sensors 13, 13a, 13b. This may be achieved eg by varying the gain of the corresponding amplifiers 20. If there is a relative increase in the gain of the signal from the upper sensor 12, the width of the protection zone 15 is reduced. Another sensitivity adjustment is to alter the bias sensitivity of the signal paths from each sensor 12, 13, 13a, 13b to raise the detection threshold to reduce noise interference.

If the size of the zone 15 is adjusted, this enables the arrangement to be tuned so that the vehicle 10 can run generally parallel to an existing cable, but maintaining uniform spacing therefrom. If the position of the vehicle 10 is adjusted to that the cable is just outside the detection zone 15, the alarm 17 will trigger if the underground conductor moves within the detection zone 15 as the vehicle 10 moves. If the vehicle is then moved so as to maintain the underground conductor just 8 outside zone 15, the line of the trench dug by the trenching blade will be parallel to the underground conductor.

9

Claims

1.1 6. A vehicle according to any one of the preceding claims, wherein the antennas are arranged to be sensitive to magnetic fields of a predetermined frequency. 7. A method of detecting an underground conductor, using sensors mounted at different heights on an excavator vehicle, which sensors each have at least one antenna which is generally horizontal relative to the ground, the method comprising: Causing s current to flow in the underground conductor, so as to generate a magnetic field; detecting the magnetic field using the antennas; and comparing the magnitudes of the magnetic field detected by the antennas; thereby to determine the relationship between the position of the underground conductor and the position of the excavator vehicle. 8. A method according to claim 7, further including generating an alarm when the magnitude of the magnetic field detected by a higher antenna is less than the magnitude of a lower antenna.

1. An excavator vehicle having an excavating tool, the vehicle having a plurality of sensors, each sensor having at least one antenna which is generally horizontal relative to the ground, the antennas being for detecting a magnetic field generated by a current in buried conductors the sensors being at different heights on the vehicle relative to the ground; there being means for comparing the magnitudes of the magnetic field detected by the antennas.

2. A vehicle according to claim 1, wherein each antenna is a solenoid with a generally horizontal axis.

3. A vehicle according to claim 1 or claim 2, wherein the means for comparing the magnitude has a variable sensitivity.

4. A vehicle according to any one of the preceding claims, having amplifiers for amplifying the outputs of the antennas prior to the comparison by the comparison means, the amplifiers being of variable gains.

5. A vehicle according to any one of the preceding claims, wherein each antenna has an adjustable detection threshold.