GB2271425A

GB2271425A - Locating a device underground

Info

Publication number: GB2271425A
Application number: GB9216964A
Authority: GB
Inventors: Peter Michael Flowerdew
Original assignee: AKIZ Ltd
Current assignee: AKIZ Ltd
Priority date: 1992-08-11
Filing date: 1992-08-11
Publication date: 1994-04-13
Also published as: AU4723793A; GB9216964D0; WO1994004947A1

Abstract

The position within a horizontal (x,y) plane of a device under the ground, is ascertained by providing the device with an elongate housing including means for generating an electromagnetic field 2, mounted in the housing for rotation relative to the longitudinal axis of the housing in such a way that the field generated is a solenoidal field the axis of which remains substantially vertical when the longitudinal axis is horizontal even when the housing rotates about the longitudinal axis. Dectecting means including at least one aerial for detecting the electromagnetic field generated by the generating means 2, is placed above the ground in the vicinity of the position directly above the generating means, and the signal detected by the aerial is monitored to ascertain the position in the horizontal (x,y) plane of the generating means 2 relative to the detecting means. The shape of the vertical solenoidal field gives a large variation in the field from a position directly above the generator to a position slightly displaced therefrom.

Description

Underground Location The invention relates to a method and system for ascertaining the position of a mobile device under the ground. The invention is particularly, but not exclusively, concerned with ascertaining the position of a tunnelling machine.

Tunnelling machines of the kind known as trenchless digging machines are well known. Commonly such machines have an elongate tunnelling head that is driven through the ground while high pressure fluid is expelled at the front of the tunnelling head to assist the penetration into the ground. It is desirable to guide the direction of travel of the machine: in a known machine this is done by controlled rotation of the tunnelling head about its longitudinal axis and providing the front of the head with a profile that is not symmetrical about a plane containing the longitudinal axis of the head.As the front of the head is driven through the ground, it tends to be deflected in a predetermined direction transverse to its longitudinal axis and away from the plane of asymmetry; by rotating the tunnelling head, the plane of asymmetry is rotated and therefore the direction of transverse movement altered.

Given that the direction of travel of the machine is controlled it is also desirable to know the position of the machine and for that purpose it has been proposed to generate a solenoidal field whose axis is aligned with the longitudinal axis of the tunnelling head and which is symmetrical about the longitudinal axis of the machine.

By aligning the axis of the solenoidal field with the longitudinal axis of the tunnelling head, the field is unaffected by rotation of the tunnelling head about its longitudinal axis. A detector is used to detect the field above the ground and thereby calculate the position of the head relative to the detector.

Unfortunately a solenoidal field having a horizontal axis does not provide an especially good reference field for ascertaining, by means of a detector above it, the position within a horizontal plane, of the generator of the field. A known method of locating the generator is to move the detector until the reading from the detector suggests that the detector is directly above the generator, but two problems are found to apply to such a method. First, because of the shape of the horizontal solenoidal field there is little variation in the field from a position directly above the generator to a position slightly displaced therefrom, especially at positions relatively far above the generator.Secondly, it is possible to confuse a position spaced far from directly above the generator with a position directly above the generator: directly above the generator the lines of flux are horizontal and as one moves from that point in a given direction parallel to the axis of the field the lines of flux become increasingly inclined until a position is reached at which they are vertical; with further movement in the given direction from that position the lines of flux become inclined (in the opposite direction to that which previously applied) that can therefore suggest that the further movement from the position is towards the generator when in fact it is away from the generator.

It is an object of the invention to provide a method and system for ascertaining the position within a horizontal (x, y) plane of a device under the ground, which mitigates or avoids the disadvantages referred to above.

According to the invention there is provided a method of ascertaining the position within a horizontal (x, y) plane of a device under the ground, comprising the following steps: providing the device with an elongate housing including means for generating an electromagnetic field, mounting the generating means in the housing for rotation relative to the longitudinal axis of the housing in such a way that the field generated is a solenoidal field the axis of which remains substantially vertical when the longitudinal axis of the housing is horizontal even when the housing rotates about the longitudinal axis, placing detecting means above the ground in the vicinity of the position directly above the generating means, the detecting means including at least one aerial for detecting the electromagnetic field generated by the generating means, and monitoring the signal detected by the aerial to ascertain the position in the horizontal (x, y) plane of the generating means relative to the detecting means.

By mounting the electromagnetic field generating means for rotation relative to the housing and by arranging for that field to remain substantially vertical regardless of rotation of the housing, when the longitudinal axis of the housing is horizontal (which it will be approximately during normal use), it is possible to generate a solenoidal field whose axis is substantially vertical. In a vertical field there is a rapid variation in the field from a position directly above the generating means (i.e. on the axis of the field) where the lines of flux are vertical and a position slightly displaced therefrom where the lines of flux are inclined.

Furthermore, there is simply a gradual reduction in the inclination of the lines of flux away from the axis and it is generally possible to arrange for the lines of flux not to reach a horizontal orientation (at which point scope for confusion as to the position of the generating means relative to the detecting means could arise) until a position so far from the generating means that a user is not likely to be confused.

In order to keep the field vertical, even if the longitudinal axis of the housing becomes significantly inclined to the horizontal, the generating means may also be mounted for pivotal movement about a horizontal axis perpendicular to the longitudinal axis of the housing.

Preferably the detecting means includes a plurality of aerials spaced apart in the horizontal (x, y) plane and the signals detected by the respective aerials are compared to ascertain the position in the horizontal (x, y) plane of the generating means relative to the detecting means. Using a plurality of aerials as the detecting means facilitates the determination of the direction in which the generating means is displaced from the detecting means and also enables an especially sensitive determination of the position directly above the generating means to be made: for example, a pair of aerials may be symmetrically positioned about a vertical plane such that the signals from them are the same when the plane of symmetry contains the axis of the solenoidal field.

Although it is possible to calculate the position of the generating means relative to the detecting means when the two are horizontally displaced from one another, a more sensitive determination of position can be obtained from moving the detecting means to a position directly above the generating means. Thus the method preferably includes the step of adjusting the position of the detecting means above the ground until the signals from the respective aerials indicate that the detecting means is directly above the generating means.

As already indicated, in the case where a plurality of aerials are used it is possible to determine the direction in which the generating means is displaced from the generating means and the detecting means preferably indicates to a user the direction in a horizontal (x, y) plane of the generating means relative to the detecting means. Preferably the detecting means indicates to the user the distance in a horizontal (x, y) plane of the generating means from the detecting means. The indication of distance may be simply an indication of which of two or more ranges of distance (for example "near" and "far") apply or a more graded indication may be provided.

As an alternative to moving a single detecting means to keep it above the generating means as the device travels under the ground, a plurality of detecting means may be placed above the ground in proximity in a horizontal (x, y) plane to a route along which the device is to travel. The movement of the device can then be tracked by the detecting means using signals from one or more of the detecting means at a time.

The aerials are preferably directed at an acute angle to the vertical. Thus, for example, if the aerials are in the known form of cylindrical elements most sensitive to lines of flux aligned with the longitudinal axes of the cylinders and least sensitive to lines of flux perpendicular to those axes, then the aerials are preferably arranged with the longitudinal axes of the cylinders extending at an acute angle to the vertical.

Although the lines of flux of the solenoidal field are vertical directly above the generating means they will be inclined to either side of that position. The angle of inclination at a particular distance to either side will of course depend upon a number of factors including the precise shape of solenoidal field that is generated and the depth below ground of the generating means. As will be explained in more detail later with reference to the drawings, there is advantage in the aerials being inclined such that their longitudinal axes intersect above the aerials and there is also a different advantage in the aerials being inclined such that their longitudinal axes intersect below the aerials.

Preferably the aerials are arranged to be most sensitive to lines of flux which are at an acute angle in the range of 300 to 600, preferably approximately 450, to the vertical.

Preferably the aerials include a first pair of aerials spaced apart in a first horizontal direction (the x axis ) and a second pair of aerials spaced apart in a second horizontal direction (the y axis) perpendicular to the first direction. In that case the first pair of aerials can be used to ascertain the x coordinate of the position immediately above the generating means and the second pair of aerials can be used to ascertain the y coordinate of that position.

Each aerial of a pair is preferably directed along the line of a cone whose tip is above or below the detecting means.

Preferably the detecting means also detects the vertical separation of the generating means from the detecting means. Detection of the vertical separation of the generating means from the detecting means enables the z coordinate of the generating means to be ascertained.

For detecting the vertical separation, the detecting means may include a plurality of aerials spaced apart vertically one above another.

Preferably the device also transmits information relating to its angle of tilt to the detecting means and, preferably, also information relating to its angle of rotation (roll) about the longitudinal axis of the housing.

Preferably information from the detecting means is transmitted, for example via a wireless link, to a central control station. Movement of the device under the ground is preferably controlled in dependence upon information received by the central control station.

In different applications different control arrangements may be preferred. In an especially advanced embodiment of the invention the direction of movement of the device is automatically adjusted in accordance with signals received from the transmitting means, whereby a real time automatic closed loop control of the device is provided. The information received from the transmitting means by the central control station may be processed to provide instructions regarding alteration of the route of the device and the device may be steered in accordance with those instructions.

The electromagnetic field generated by the device preferably includes an alternating field having a frequency lying in the range of 100 Hz to 2000 Hz and preferably in the range of 300 Hz to 700 Hz. The use of such a low frequency has previously been regarded as undesirable because of high levels of background fields in such frequency ranges. We have found, however, that by selecting particular frequencies within the ranges indicated, it is possible to avoid those problems and the use of low frequency much reduces the attenuation of the field by metal objects in which eddy currents may be generated. The use of a low frequency also facilitates the use, for a core of the generating means, of a readily machinable metal thereby making it practical to shape the core to any desired shape.

In order to improve the strength of the solenoidal field generated by the generating means it is preferable for the generating means to include a core permeable to magnetic flux, the longitudinal axis of the core being coincident with the axis of the field. A solenoidal winding coaxial with the core closely surrounds the core, which preferably fills the volume available to it in the housing; since the core is rotatable within the housing the volume of the core will in most practical arrangements be maximised by giving the opposite ends and especially the top of the core a curved shape, curved about the axis of rotation of the generating means.

Providing such curvature also has the advantage of spreading the field (the lines of magnetic flux are of course perpendicular to the surface of the core) thereby tending to prevent undesirable reversals in the vertical component of the magnetic field within the range of operation of the detecting means. Where space allows the opposite ends and especially the top of the core may be curved about orthogonal axes so as to define parts of spheres; the magnetic field is then evenly spread in both x and y directions.

The invention is of special advantage when applied to a tunnelling machine but can also be applied to other devices that are to travel underground. For example the invention may be used to track the movement of a device through a pipe. The invention is especially applicable to a tunnelling machine having a tunnelling head arranged to operate with high pressure fluid, the housing in which the generating means is rotatably mounted including longitudinal passageways therethrough to allow the passage of fluid.

According to another aspect of the invention there is provided a system for ascertaining the position within a horizontal (x, y) plane of a device travelling under the ground, the system including: an elongate housing for travelling under the ground with the device, means for generating an electromagnetic solenoidal field, the generating means being mounted in the housing for rotation about the longitudinal axis of the housing such that the orientation of the generating means is unaffected by rotation of the housing about the axis and the solenoidal field generated in use by the generating means has its axis directed substantially vertically upwards when the longitudinal axis is horizontal, detecting means for placing above the ground in the vicinity of the position directly above the generating means, the detecting means including an aerial for detecting the electromagnetic field generated in use by the generating means, and monitoring means for monitoring the signal detected by the aerial to enable the position in the horizontal (x, y) plane of the generating means relative to the detecting means to be ascertained.

The detecting means may include a plurality of aerials spaced apart in the horizontal (x, y) plane. The plurality of aerials are preferably mounted on a common support structure so that they can be moved as a single unit. Preferably, at least one of the aerials is movably mounted on the support structure between a first operative position and a second stored position. It is advantageous for the aerials to be relatively widely spaced apart when in operation but it is also desirable for the detecting means to be relatively compact when it is being transported or stored.

It will be apparent from the description above regarding the method of the invention that the system advantageously includes various other features. Since those will be apparent from the description above, they will not be repeated.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawings, of which: Fig. la is a schematic side view of an assembly including an electromagnetic field generator (transmitter) for mounting in a housing, Fig. lb is a view of the electromagnetic field generator, Fig. 2a is a side view of the housing in which the assembly of Fig. 1 is mounted, Fig. 2b is a side view of a tunnelling head for mounting on the front of the housing of Fig. 2a, Fig. 2c is a side view of an end plug for the back of the housing of Fig. 2a, Fig. 3a is a front view of a detector assembly for detecting the position of the electromagnetic field generator, Fig. 3b is a side view of the detector assembly of Fig. 3a, Fig. 3c is a schematic sectional plan view of the base of detector assembly, Fig. 3d is a view of the direction of the arrow A in Fig. 3b of the top of the detector assembly, Figs. 4a to 4c are examples of displays on the detector assembly for guiding an operator to a position above the assembly, Figs. 5a and 5b are schematic views showing a pair of aerials of a first arrangement in a first symmetrical position (Fig. 5a), and a second displaced position (Fig. 5b), and Figs. 6a and 6b are schematic views showing a pair of aerials of a second arrangement in a first symmetrical position (Fig. 6a), and a second displaced position (Fig. 6b).

The tunnelling machine now to be described comprises an elongate housing 1 shown in Figs. 2a to 2c within which an assembly 2 shown in Fig. Ia is mounted. The assembly shown in Fig. la is divided lengthwise into five sections each of which has a different function. A first section 3 is for ascertaining the angular orientation (roll) of the assembly relative to the housing in which it is located. A second section 4 is for detecting the upward or downward inclination (tilt) of the longitudinal axis of the assembly. The third section 5 houses a printed circuit board (not shown). The fourth section 6 houses a transmitter and the fifth section 7 houses batteries (not shown) which supply power to the various electrical components in the assembly.

The assembly has axles 8a and 8b which are rotatably mounted on the ends of the assembly.

In the first section 3 there is a rotary potentiometer 9 having one part fixed to the axle 8a and the other part fixed to the frame of the first section 3.

In the second section 4 there is a tilt sensor 10 comprising a weight (not shown) pivoted about a horizontal axis perpendicular to the longitudinal axis of the assembly. Another rotary potentiometer or similar device is associated with the pivoted weight.

In the fourth section 6 a transmitter 11 is provided. The transmitter 11 is shown in Fig. 1b and comprises a core 12 of generally circular cylindrical shape, the axis of the cylinder being coincident with the longitudinal axis of the assembly and a coil 13 wound around a central portion 14 of the core 12 that is of reduced cross-sectional area. The coil 13 is disposed in a horizontal plane. The core 12 occupies substantially the whole volume of the fourth section 6.

The assembly 2 is mounted in the housing 1 shown in Fig. 2a. The shaft 8b is fixed in a recess 14 formed in an end plug 15 at the front end of the housing and the shaft 8a is fixed in another recess 16 formed in the end plug 17 shown in Fig. 2c that fits into the other end of the housing.

The assembly 2 is weighted asymmetrically about its axis of rotation in the housing 1 and as a result, under the influence of gravity, remains in the orientation shown in Fig. 2a regardless of any rotation of the housing about its longitudinal axis.

The end plug 17 shown in Fig. 2c has a central threaded bore 18 which communicates at its front end with radial passages 19. Those passages in turn communicate with radial passages 20 in the housing 1 when the plug is fitted into the housing and the radial passages 20 in the housing lead to longitudinal passageways 21 through the housing which then in turn lead to radial passages 22 communicating with an opening 24 at the front end of the housing. A tunnelling head member 25 is fixed to the front end of the housing in the opening 24 and the head has a passageway 27 through which fluid can pass from the opening 24 to the front of the tunnelling head member 25.

As can be seen, the front of the tunnelling head member is asymmetrical about a plane 26 containing the longitudinal axis of the housing with the result that when the housing is pushed forwards the front end tends to move the head in a direction perpendicular to the longitudinal axis and away from the plane 26, the direction being upwards as viewed in Fig. 2b.

Referring now to Figs. 3a to 3c, the detector assembly comprises a central upright elongate body 30 at the bottom of which opposite fixed projecting arms 31a and 31b are provided. The arm 31a has a first part 32a projecting perpendicularly to the body 30, a second inclined part 33a extending outwardly and downwardly from the part 32a and a third inclined part 34a which extends upwardly from the bottom of the body 30 to the outer end of the part 33a. The arm 31b has corresponding parts 32b, 33b and 34b. Pivotally mounted arms 31c and 31d are also provided. The arm 31c has a first part 32c pivotally connected to the body 30 where the first part 32a of the arm 31a adjoins the body 30, and a second inclined part 33c extending-outwardly and downwardly from the part 32c. The arm 31c is movable from a stored position shown in Fig. 3b and in solid outline in Fig.3c in which the parts 32c and 33c lie alongside the parts 32b and 33b respectively of the arm 31b to an operative position shown in Fig. 3a and in dotted outline in Fig. 3c in which the arm 31c is perpendicular to the arm 31b. The arm 31d has parts 32d and 33d corresponding to the parts 32c and 33c of the arm 31c and is mounted in a similar manner to the mounting of the arm 31c.

In each of the downwardly inclined parts 32a to 32d of the arms 31a to 31d a respective cylindrical receiving aerial 35a to 35d is located with the longitudinal axis of the cylinder aligned with the longitudinal axis of the respective part of each arm and inclined at an angle of 450. At the bottom and towards the top of the central body 30 two further cylindrical receiving aerials 36 and 37 are provided, having their longitudinal axes parallel to the longitudinal axis of the body 30. At the top of the body 30 there is a handle 38, a display 39 and a keyboard 40 (see Fig. 3d).

In use, a tunnelling machine incorporating the housing and assembly as described above, but otherwise of conventional form is driven underground. The driving of the machine is conventional and the machine is steered by rotating the housing 1 about its longitudinal axis. As the machine is driven underground, fluid is fed at high pressure to the rear of the end plug 17 from where it passes through the radial passages 19, 20 to the longitudinal passageways 21 through the housing and then through the passages 22, opening 24 and passageway 27 to the front of the tunnelling head. For example if it is desired to move the front of the tunnelling head upwardly, then the housing 1 is rotated to the orientation shown in Figs. 2a to 2c in which the plane 26 is horizontal.If it is then desired to move the tunnelling head to the right (as viewed from behind the head), then the housing is rotated through 900 in a clockwise direction (as viewed from behind the head). Movement downwards or to the left can of course be achieved by rotating the housing to positions 1800 away from those referred to above. During such steering, although the housing 1 may rotate through 3600, the assembly 2 rotatably mounted in the housing and biased by gravity remains in the orientation shown in Fig. la. The transmitter 11 generates an alternating electromagnetic field of three different frequencies determined by components on the printed circuit board housed in the third section 5 of the assembly and powered by the batteries housed in the fifth section 7 of the assembly.

The field is a typical solenoidal field which, when the longitudinal axis of the housing 1 is horizontal, has a vertical main axis. The core 12 of circular cylindrical shape has an upper surface, above its central portion 14, that subtends an angle of the order of 1200. Lines of flux of the electromagnetic field generated by the transmitter extend normal to the surface and thus the overall shape of the field is controlled by the shape of the surface.

The electromagnetic field is an alternating field of relatively low frequency, the precise frequencies being chosen so as to reduce effects of natural background noise which is more prevalent at low frequencies. Much of that natural background noise is generated from mains power supply cables and apparatus and therefore it is desirable to avoid multiples of the mains frequency.

Thus, each frequency of the field is approximately FM X 2n + 1 where F is the mains frequency and n is an 2 integer. The frequency is also preferably in the range of 300 Hz to 700 Hz; keeping the frequency below 700 Hz reduces the effect on the signal of eddy currents which may be generated in any electrically conducting materials buried underground.

Although the solenoidal field generated by the transmitter 11 is only precisely vertical when the horizontal axis of the housing 1 is precisely horizontal, that axis will in normal use remain substantially horizontal throughout a tunnelling operation except during the first and last stages of tunnelling when the machine is leaving or returning to ground level To locate the tunnelling machine underground the detector assembly shown in Figs 3a to 3d is employed with the arms 31c and 31d pivoted out to the operative position shown in dotted outline in Fig. 3c. The field generated by the transmitter 11 in the vicinity of its main (vertical) axis is rotationally symmetrical about that axis.Thus for example, the same field is present at a position 0.5 metre north of the axis as 0.5 metre south and, similarly, the same field is present at a position 0.5 metre north west of the axis as 0.5 metre south east. On the other hand the field changes along any direction away from the axis of the field so that the field 0.5 metre north of the axis is different from the field 1 metre north of the axis.

The aerials 35a and 35d are axi-symmetrically positioned about the central body 30 of the detector assembly so that, when the assembly is positioned with the longitudinal axis of the body 30 coincident with the main axis of the solenoidal field, the same signal will be picked up by the aerial 35a as the aerial 35b and the same signal will be picked up by the aerial 35c as the aerial 35d. On the other hand at any other position of the body 30 different signals are picked up.Furthermore by comparing the signal from the aerial 35a with that from the aerial 35b, it is possible to deduce which aerial is closer to the axis of the field and by approximately how much in the direction along the line of those two aerials (x direction); similarly by comparing the signal from the aerial 35c with that from the aerial 35d, it is possible to deduce which aerial is closer to the axis of the field å by approximately t8W ffluah in the direction along the line of those two aerials (y direction). In that way the approximate x, y coordinates of the axis of the field relative to the detector assembly is calculated in the assembly and the display 39 shows the operator of the assembly the direction, and approximate distance, in which to move towards the axis of the field.When the operator is directly over the axis of the field, the display 39 also indicates that.

Figs 4a to 4c show one example of three displays which may be shown to an operator as he moves the detector assembly to a position over the transmitter 11.

Initially (Fig. 4a) the transmitter 11, indicated by a cross, is shown as being distant and in a direction forward and to the right. As the operator moves towards the transmitter the cross leaves the boundary of the display and becomes mobile within the display area. The distance of the cross from a centre box shown on the display is indicative of the distance remaining in the horizontal plane to the point directly above the transmitter. Fig. 4b illustrates the situation where the operator has moved with the detector assembly to a position in which the transmitter is closer than in Fig. 4a and is further forward and to the right. Finally the operator arrives at the position shown in Fig. 4c, where the presence of the cross inside the central box indicates that the detector assembly is directly on the axis of the field generated by the transmitter 11.

The cylindrical aerials 35a to 35d are, as already described, inclined at an angle of 450. Whilst the lines of flux precisely on the axis of the field are of course vertical (when the housing 1 is horizontal), the lines of flux spaced only a short distance from the axis are inclined. We have found that the sensitivity of the detector assembly is enhanced by inclining the aerials at an angle of 450. There are two options for so inclining the aerials: they may be inclined with their longitudinal axes intersecting above the aerials, as will be described below with reference to Figs. 5a and 5b, or below the aerials, as will be described below with reference to Figs. 6a and 6b and as has already been shown in Figs. 3a to 3c.In each of Figs. 5a and 5b and Figs. 6a and 6b only one of the two pairs of aerials is shown and referred to, for the sake of simplicity.

The arrangement in Figs. 5a and 5b provides maximum detection range by having the axis of greatest sensitivity of the aerials directed along the lines of magnetic flux when the detector assembly is coaxial with the solenoidal field. However, it is also desired that the array be sensitive to movements from the coaxial position in the horizontal plane. Fig. 5b shows the detector assembly displaced by such a movement and it will be seen that as the array is moved away from the position of symmetry of Fig. 6a the aerial 35a which should give the stronger signal to indicate that it is closer to the axis of the field is in a region where its inclination substantially reduces the strength of the signal and this reduces the differential between the signals from the aerials 35a and 35b.The alternative arrangement of Figs. 6a and 6b sacrifices signal strength from individual aerials in the position of symmetry in return for an improved differential performance. When the detector assembly is displaced from the position of symmetry shown in Fig. 6a to the position shown in Fig. 6b, the aerial 35a is in a region where its inclination substantially increases the strength of the signal and this increases the differential between the signals from the aerials 35a and 35b.

In addition to the aerials 35a to 35d which locate the transmitter 11 in a horizontal (x, y) plane, there are also the aerials 36 and 37 which are used to detect the depth (z coordinate) of the transmitter 11 relative to the detector assembly. The manner in which the depth of the transmitter can be detected by such a pair of aerials is known per se and will not be described further here. The depth of the transmitter 11 is also displayed in metres on another part (not shown in Figs. 4a to 4c) of the display.

As already indicated the transmitter 11 transmits an alternating electromagnetic field of three different frequencies. One of those, which is a regular continuous sinusoidal alternating field is used for the location of the transmitter as described above. The other two are frequency switched signals and are used to transmit data to the detector assembly from the roll and tilt measuring sections 3, 4 of the assembly 2. That data is then used to determine what changes, if any, should be made to the orientation of the housing 1 about its longitudinal axis.

As already described, the machine is steered by adjusting that orientation.

Whilst one form of the invention has been described above by way of example with reference to the accompanying drawings, it should be understood that various modifications can be made. In the described system information is gathered and displayed at the detector assembly; it is possible to arrange for that information to be passed to a central control station for example via a unidirectional or bidirectional radio link.

The central control station may be arranged to generate route plans and/or instructions for the operator of the tunnelling machine to adjust its direction of travel. In a still more sophisticated arrangement the central control station could itself be arranged to control the tunnelling machine automatically (without any operator intervention) in accordance with signals received from the detector assembly. Instead of using a single mobile detector assembly it is possible to use a series of fixed assemblies sited above the route that the tunnelling machine is to take.

In the described embodiment the transmitter 11 is mounted for rotation about the longitudinal axis only of the housing. Provided the tilt of the machine will not be very great that is likely to be sufficient but it is possible also to mount the transmitter 11 for rotation about a horizontal axis perpendicular to the longitudinal axis of the housing. Preferably that is accomplished by providing a pivot mounting of the transmitter 11 within the fourth section 6 of the assembly 2. If such an extra degree of freedom is provided then by providing an appropriate gravitational bias to the transmitter 11, the solenoidal field can be maintained in a constant vertical orientation even when the housing 1 is tilted substantially upwards or downwards.

Claims

Claims:

1. A method of ascertaining the position within a horizontal (x, y) plane of a device under the ground, comprising the following steps: providing the device with an elongate housing including means for generating an electromagnetic field, mounting the generating means in the housing for rotation relative to the longitudinal axis of the housing in such a way that the field generated is a solenoidal field the axis of which remains substantially vertical when the longitudinal axis is horizontal even when the housing rotates about the longitudinal axis, placing detecting means above the ground in the vicinity of the position directly above the generating means, the detecting means including at least one aerial for detecting the electromagnetic field generated by the generating means, and monitoring the signal detected by the aerial to ascertain the position in the horizontal (x, y) plane of the generating means relative to the detecting means.

2. A method according to claim 1, in which the detecting means includes a plurality of aerials spaced apart in the horizontal (x, y) plane and the signals detected by the respective aerials are compared to ascertain the position in the horizontal (x, y) plane of the generating means relative to the detecting means.

3. A method according to claim 2, further including the step of adjusting the position of the detecting means above the ground until the signals from the respective aerials indicate that the detecting means is directly above the generating means.

4. A method according to claim 3, in which the detecting means indicates to a user the direction in a horizontal (x, y) plane of the generating means relative to the detecting means.

5. A method according to any one of claims 2 to 4, in which the detecting means indicates to the user the distance in a horizontal (x, y) plane of the generating means from the detecting means.

6. A method according to any one of claims 2 to 5, in which a plurality of detecting means are placed above the ground in proximity in a horizontal (x, y) plane to a route along which the device is to travel.

7. A method according to any one of claims 2 to 6 in which the aerials are directed at an acute angle to the vertical.

8. A method according to claim 7 in which the acute angle is in the range of 300 to 600.

9. A method according to claim 8 in which the acute angle is approximately 450.

10. A method according to any one of claims 2 to 9 in which the aerials include a first pair of aerials spaced apart in a first horizontal direction (the x axis).

11. A method according to claim 10, in which the aerials further include a second pair of aerials spaced apart in a second horizontal direction (the y axis) perpendicular to the first direction.

12. A method according to claim 10 or 11 when dependent upon any one of claims 7 to 9 in which each aerial of a pair is directed at an acute angle to the vertical and directed along the line of a cone whose tip is above or below the detecting means.

13. A method according to claim 12 in which the tip of the cone is above the vertical.

14. A method according to any preceding claim in which the detecting means also detects the vertical separation of the generating means from the detecting means.

15. A method according to claim 14 in which, for detecting the vertical separation, the detecting means includes a plurality of aerials spaced apart vertically one above another.

16. A method according to any preceding claim in which the device also transmits information relating to its angle of tilt to the detecting means.

17. A method according to any preceding claim in which the device also transmits information relating to its angle of rotation about the longitudinal axis of the housing to the detecting means.

18. A method according to any preceding claim in which the detecting means transmits information to a central control station.

19. A method according to claim 18 in which movement of the device under the ground is controlled in dependence upon information received by the central control station.

20. A method according to claim 19 in which the direction of movement is automatically adjusted in accordance with signals received from the transmitting means, whereby a real time automatic closed loop control of the device is provided.

21. A method according to claim 19 in which information received from the transmitting means by the central control station is processed to provide instructions regarding alteration of the route of the device and the device is steered in accordance with those instructions.

22. A method according to any preceding claim in which the electromagnetic field generated by the device includes an alternating field having a frequency lying in the range of 100 to 2000 Hz.

23. A method according to claim 22 in which the electromagnetic field generated by the device includes an alternating field having a frequency lying in the range of 300 to 700 Hz.

24. A method according to any preceding claim in which the electromagnetic field generated includes lines of flux which are substantially normal to a surface defining a top portion of a curved surface of circular cross-section substantially coaxial with the axis of rotation of the generating means.

25. A method according to claim 24 in which the curved surface is a cylinder.

26. A method according to any preceding claim in which the device is a tunnelling machine.

27. A system for ascertaining the position within < horizontal (x, y) plane of a device travelling under the ground, the system including: an elongate housing for travelling under the ground with the device, means for generating an electromagnetic field, the generating means being mounted in the housing for rotation about the longitudinal axis of the housing such that the orientation of the generating means is unaffected by rotation of the housing about the axis and the solenoidal field generated in use by the generating means has its axis directed substantially vertically upwards when the longitudinal axis is horizontal, detecting means for placing above the ground in the vicinity of the position directly above the generating means, the detecting means including an aerial for detecting the electromagnetic field generated in use by the generating means, and monitoring means for monitoring the signal detected by the aerial to enable the position in the horizontal (x, y) plane of the generating means relative to the detecting means to be ascertained.

28. A system according to claim 27 in which the detecting means includes a plurality of aerials spaced apart in the horizontal (x, y) plane.

29. A system according to claim 28, in which the plurality of aerials are mounted on a common support structure.

30. A system according to claim 29, in which at least one of the aerials is movably mounted on the support structure between a first operative position and a second stored position.

31. A system according to any one of claims 28 to 30, in which, at least in an operative position, the aerials are oriented such that they are most sensitive to lines of flux of an electromagnetic field extending at an acute angle to the vertical.

32. A system according to claim 31 in which the acute angle is in the range of 300 to 600.

33. A system according to claim 32 in which the acute angle is approximately 450.

34. A system according to any one of claims 28 to 33 in which, at least in an operative position, the aerials include a first pair of aerials spaced apart in a first horizontal direction (the x axis).

35. A system according to claim 34 in which, at least in an operative position, the aerials include a second pair of aerials spaced apart in a second horizontal direction (the y axis) perpendicular to the first direction.

36. A system according to claim 34 or 35 when dependent upon any one of claims 27 to 29 in which, at least in an operative position, each aerial of a pair is arranged to be most sensitive to lines of flux which are at an acute angle to the vertical and directed along the line of a cone whose tip is above the detecting means.

37. A system according to any one of claims 27 to 36 in which the elongate housing is part of a tunnelling machine.

38. A system according to claim 37 in which the tunnelling machine has a tunnelling head arranged to operate with high pressure fluid, the housing in which the generating means is rotatably mounted including longitudinal passageways therethrough to allow the passage of high pressure fluid.