WO2003058031A1

WO2003058031A1 - Positioning of mechanised mining apparatus

Info

Publication number: WO2003058031A1
Application number: PCT/IB2003/000054
Authority: WO
Inventors: Henri Johnson
Original assignee: Industrial Development Corporation Of South Africa Limited; Foster, Mark, Charles
Priority date: 2002-01-08
Filing date: 2003-01-07
Publication date: 2003-07-17
Also published as: AU2003201067A1; GB2383914A; GB0200331D0

Abstract

A method for Positioning mechanized mining apparatus 1 is disclosed. The method includes providing a transmitter 17 (e.f.a radar) associated with the mining apparatus for transmitting radiation generally towards a surface 27 to be mined (e.g. a coalface) ; receiving radiation including radiation reflected from said surface and generating a signal representative thereof; processing the signal to identify components likely to relate to radiation reflected from said surface to derive a distance Value indicative of the distance between the transmitter. and the surface; and using the distance value to derive a drive value for controlling the mining apparatus for providing a selected depth of cut into the surface by the mining apparatus. The frequency of the transmitted radiation is varied with respect to time. The processing of the signal includes sampling the signal at a plurality of time intervals during the varying of the frequency of the transmitted radiation.

Description

POSITIONING OF MECHANISED MINING APPARATUS

Background of the invention

Field of the invention The invention relates to a method and apparatus for positioning mining apparatus and more particularly, but not by way of limitation, it relates to a method and apparatus that determine the distance from a mechanised miner in relation to the mined coalface to increase mining efficiency, reduce waste and improve the quality of the mined coal.

Description of the pxior art

The field of coal mining has seen the use of both manual and mechanised techniques to break coal from a coal seam in both underground and opencast mining operations. The use of mechanised mining systems has made it possible to increase the scale and rate of coal mining while reducing the use and need for manual labour. Mechanisation has therefore brought greater efficiency and safety in coal mining.

The most common continuous miners make progressive cuts of the coalface using a rotating cutting drum. The cutting cycle starts with an advancing cut into the coalface to a defined depth after which the cutting tool is lowered approximately vertically to remove material from the top to the bottom of the presented seam. Ideally, the depth of the cut throughout the cutting cycle should remain constant to maximize the quality of the resulting product and to minimize waste caused by powdering and shallow cuts. The "sump depth" is the amount initial penetration into the coalface of the cutting drum, which dictates how much material will be removed during the entire shearing down cycle (the lowering of the cutting drum) . A couple of centimeters can make a vast difference to the final volume of material extracted.

Hitherto, one of the problems of practical mining operations is that it is not possible to determine and maintain accurately the distance between the cutting tool and the coalface during the cutting cycle, due to mechanical movement resulting from the miner' s drive mechanisms and the reaction to forces that are present. In manual mining, it is impossible for a human operator to control the sump depth accurately and repeatably due to the distance from the operator to the coalface (typically 25-30m) . The poor visibility due to coal dust and heavy water spray make viewing of the mining process of any kind impossible. Operators use sounds and ^Λλgut feel" to cut currently.

There have been previous attempts at devising methods and apparatus to control cutting depth through measuring distance to the coalface whilst cutting, but these have been largely unsuccessful as a result of the conditions of falling coal, coal dust, water spray, limited space, and other factors that characterize coal mining operations. US patent No. 4430653 relates to a ground probing radar that generates images of defects in the ground behind a coalface, particularly discontinuities that may lie therein.

US patent No. 6161630 discloses an arrangement for controlling an underground boring tool. An above-ground probing and detection unit sends a signal into the ground for reception by the boring tool. The boring tool replies with a signature signal. The probing and detection unit can determine the orientation and location of the boring tool by analyzing the returned signature signal and can adjust the drive of the boring tool to correct its position in accordance with pre-stored information.

Non-limiting summary of the invention

An object of the present invention is to provide an improved arrangement for maintaining the distance between a mining apparatus and a surface to be mined to allow the depth of cut (sump depth) to be consistent.

A further object of the invention is to provide a distance measuring radar system for coal mining that utilizes mechanical and electronic parts and computer software, assembled in a manner that complies with the governing standards of the coalmining industry.

It is also an object to provide a method and apparatus that will be provide measurement information continuously and in real time, so that the mechanised cutting operations will not be inhibited or delayed but could operate unhindered and remain efficiently utilized.

It is also an object that the form and size of the apparatus as well as its requirements for electrical power, repairs, and other forms of maintenance will be easily accommodated within the typical cutting tools and their manner of operation.

According to a first aspect of the present invention, there is provided a method for positioning mining apparatus, the method including providing a transmitter associated with the mining apparatus for transmitting radiation generally towards a surface to be mined; receiving radiation including radiation reflected from said surface and generating a signal representative thereof; processing the signal to identify components likely to relate to radiation reflected from said surface to derive a distance value indicative of the distance between the transmitter and the surface; and using the distance value to derive a drive value for controlling the mining apparatus for providing a selected depth of cut into the surface by the mining apparatus.

The processing of the signal may include band pass filtering the signal. This advantageously removes unwanted high frequency components of the signal to ease subsequent processing of the signal.

Advantageously, the method includes varying a characteristic of the transmitted radiation with respect to time. In the embodiment the frequency is varied with respect to time such that the frequency varies linearly with time, i.e. the frequency varies by a predetermined amount during each of a succession of equal intervals. Advantageously, the transmitted radiation is frequency modulated continuous wave radar radiation.

In the embodiment the processing of the signal includes sampling the signal at a plurality of time intervals during the varying of the characteristic (frequency) of the transmitted radiation.

The characteristic (frequency) of the transmitted radiation may be varied according to a predetermined pattern during a sweep period. In the embodiment the frequency increases linearly from first frequency fi to a second frequency f₂ during the sweep period. Such a variation in frequency allows the calculation of the time that the return signal took from the moment of emission to reception by comparing the received frequency to the frequency being emitted, provided that the sweep rate is known.

In the embodiment the signal is analysed to identify the characteristic (frequency) in the reflected radiation corresponding to a particular time and calculating the difference in the characteristic of the transmitted radiation and the received radiation at that particular time. Further, for a range of values of the difference in the characteristic, the amplitude of the component of the signal corresponding to each difference value of the characteristic is obtained. During a sweep period, an accumulated amplitude value for each of the difference values of the characteristic is generated, the accumulated amplitude values being indicative of the sum of the amplitudes of the signal components having respective ones of the characteristic difference values. The frequency difference with the highest amplitude generally corresponds to the coalface.

Advantageously, the method further includes calculating the mean of the accumulated amplitude values corresponding to each of the characteristic difference values over selected sweep periods, and comparing mean values corresponding to different characteristic difference values to identify the highest mean value. The highest mean value may provide a more reliable indication of which frequency difference value corresponds to the coalface.

Preferably, the selected sweep periods comprise a plurality of consecutive sweep periods, which are stored for analysis, the sampled data of each new period stored replacing the data of the oldest stored period. In this manner, a "running average" may be calculated on an ongoing basis .

According to a second aspect of the present invention, there is provided apparatus for positioning mining apparatus, the apparatus including: a transmitter for association with the mining apparatus for transmitting radiation generally towards a surface to be mined; a receiver for receiving radiation including radiation reflected from said surface and generating a signal representative thereof; means for processing the signal to identify components likely to relate to radiation reflected from said surface to derive a distance value indicative of the distance between the transmitter and the surface; and means for using the distance value to derive a drive value for controlling the mining apparatus for providing a selected depth of cut into the surface by the mining apparatus.

According to a third aspect of the present invention, there provided a mining apparatus positioning system including a transmitter for transmitting a repeating pattern of radiation generally towards a surface and a receiver for receiving reflected radiation including radiation reflected from the surface, the radiation pattern being such that the distance between the point of reflection and the transmitter is derivable from the reflected radiation; means for storing data derived from the reflected radiation during a plurality of said repeating radiation patterns; and means for analysing the stored data to identify the likely distance of the surface from the transmitter.

The embodiment uses microwave technology in the form of a FMCW (Frequency Modulated Continuous Wave) radar to probe the dust and water-laden atmosphere in a continuous coal mining operation, and determines the distance between the apparatus (and hence the continuous miner) and the mined coalface.

According to the embodiment, the system makes use of the reflective characteristics of the coalface and applies signal processing techniques to determine the distance measurement that can be used to position the miner optimally. The system measures the distance between a continuous coal miner system in relation to the coalface which is being mined, and provides control signals that may be used to adjust the tool's position and orientation with respect to the mined coalface. The system is mechanically mounted on the continuous miner in any suitable manner which both fastens and aligns the system with respect to the mining tool. The system of the embodiment includes the ability to determine the distance perimeters from the reflected signal even though it is contaminated and distorted due to mechanical noise and vibrations, the curtain of broken pieces of coal falling between the miner and the coalface, and the coal dust and water spray in the area of cutting. The system of the embodiment operates in the frequency range of 1GHz to 20GHz. The system emits a continuous modulated signal that sweeps from a lower to a higher frequency through an antenna directed towards the mined coalface. The signals reflected from the coalface as well as from other structures and also the conductive dust and other particles in the surrounding air are received, amplified, filtered and the signal amplitude is periodically sampled. The time varying sample values are digitally processed to perform further signal refinement, and parameters, which determined the required distance information, are extracted. The processing of the signal aims to reduce noise or other forms of interference, and to determine variations in the receive signal that are related to the distance between the mining apparatus and the coalface. The distance data is provided in a format that can be used by a control system that directs the continuous miner's servo systems to control the tool's distance from the mined coalface to a required value.

The distance data may also be displayed on a visual display unit.

Although the description below refers to coal mining, it should be understood that the present invention is also applicable to other types of mining operations.

10 Brief description of the drawings

For a better understanding of the present invention an embodiment will now be described by way of example, with reference to the accompanying drawings, in which: -

15 Figure 1 shows a perspective view of a mechanised coal miner apparatus modified according to the present invention;

Figure 2 shows a cross-sectional view of an underground 20 tunnel where the coalface is being cut by the miner;

Figures 3A and 3B show, respectively, a front elevational and side cross-sectional view of radar apparatus according to the embodiment; ^'25

Figure 4 shows a functional block diagram of the radar system;

Figure 5 shows a typical time data series of the signal 30 received by the radar apparatus; and Figure 6 shows a plot of a typical frequency series of the received signal .

In the Figures like elements are generally designated with the same reference numerals.

Detailed description of am embodiment

The continuous miner illustrated in Figures 1 and 2 is of generally conventional form. Briefly, the continuous miner 1 includes a main body 3 housing propulsion means

(not shown) such as an internal combustion engine or electric motor that drives continuous tracks 5. A plurality of rotatable cutting wheels 7 are coupled to an arm 9 which is pivotally mounted with respect to the main body 3. The arm 9 moves in an arc as shown by arrow 11 in Figure 2. After positioning of the miner, it is the movement of the arm 9, in combination with rotation 10 of the cutting wheels 7, that extracts material from a coalface 27. The arm 9 is fitted with an inclinometer used to determine the height to which the cutting wheels

7 are raised or lowered. Mined material is carried away from the region of the coalface 27 by conveyer belt 13.

Those skilled in the art will understand that the elements of the miner 1 described thus far are conventional. It should be understood that different forms of miner could be used in accordance with the present invention. Modifications in accordance with the present invention will now be described. A control unit 15, housing components for controlling movement of the miner 1, including the arm 9, is mounted on or within the main body 3. A radar housing 17, as will be described below in relation to Figure 3, is mounted on the housing 3. The radar housing 17 is mounted on the main body 3 such that transmitted radiation 18 can pass towards the coalface 27 and received radiation 19 reflected from the coalface can be detected with minimum interruption from intermediate objects, such as parts of the miner 1.

Figure 2 shows the miner 1 in use underground. The miner 1 has cleared a tunnel 20 and moves generally forward in the direction of arrow 21. The seam of coal being mined is designated 23. It should be appreciated from Figure 2 that the distance 25 between the miner main body 3 and the presenting surface 27 of the coal seam 23 (the coal face) will control the amount of coal seam 23 cut by the cutting wheels 7 and thus, as described above, the quality of the coal obtained during each arcuate motion of the arm 9.

Referring now to Figure 3, the radar apparatus includes a metal housing 29 within an opening of which a radar- transparent window 31 is mounted. In use, the window 31 is directed generally towards the coalface 27.

The housing 29 contains a radar antenna and radio frequency unit 33, a digital signal processor board 35 and an interface circuit board 37. These components may be accessed by removing access panel 39 at the rear of the housing 29. The housing 29 is mounted to the main body 3 of the miner 1 by four mounting brackets 41.

The radar apparatus 17 is housed in a flame-proof/ intrinsically safe metal enclosure 29 compliant with prevailing standards in the mining industry. The enclosure 29 as well as the entire assembly is robust in order to withstand the extremely harsh environment of coal mining. The enclosure 29 provides the electronic units within with protection against water, particles and externally generated electromagnetic radiation.

The radar apparatus 17 is fed by an external electrical power supply 43 (shown only in Figure 4). The measured distance data and associated timing information is output on a digital data interface for utilization by a digital control system, display devices, or for any other purpose. These connections 43 are provided through armoured electrical cabling passing into the radar apparatus 17.

Figure 4 shows schematically the components of the radar apparatus 17. The radar antenna and radio frequency (RF) unit 33 comprises a sawtooth swept frequency generator 45 and a synthesized microwave signal generator 47, the latter being applied with a clock reference signal 49. The outputs from the sawtooth swept frequency generator 45 and synthesized microwave generator 47 are applied to an RF mixer/filter 51, the output of which is fed to microwave power amplifier 53. The output of the microwave power amplifier 53 is fed to transmit bandpass filter 55 before passing to microwave circulator/ transmitter, receiver isolator 57, the signal from which is fed to planar stripline microwave antenna 59. Electromagnetic radiation from the antenna 59 is transmitted generally towards the coalface 27. This radiation is reflected from the coalface 27 and from various other objects in its path, as discussed above. The antenna 59 receives the radiation reflected from the various sources, this received radiation passing to the microwave circulator/transmitter, receiver isolator 57 and thence to the digital signal processor board 35. On the digital signal processor board 35 a receive bandpass filter 61 receives the signal from the microwave circulator/transmitter, receiver isolator 57, the filtered output being passed to microwave low level amplifier 63. The output of the amplifier 63, together with the signal from the synthesized microwave signal generator 47, is fed to second RF mixer/filter 65, the output of which passes to detector and low pass filter 67. The components of Figure 4 thus far described are generally conventional and are connected together and operated in the conventional manner, as would be known to a person skilled in the radar art. It should be appreciated that the microwave circulator could be dispensed with - two separate antennae could perform the same function

According to the embodiment the signal output from the detector and low pass filter 67, which is still considered to be a "raw" detected analogue signal, is fed to a band pass analogue filter and amplifier 69. The band-pass filtered output is digitized by a high resolution, for example 12 to 16-bit, analogue to digital converter 71, the digitized output of which is processed by digital signal processor 73. A clock generator 75 provides a clock reference signal to the synthesized microwave signal generator 47, as mentioned above, and also a corresponding signal to the digital signal processor 73. Interface circuitry 37 makes available data such as the output of raw distance and time data from the digital signal processor 33 to external devices to be used, for example, in driving the miner 1.

In use, the planar stripline microwave antenna 59 transmits a modulated microwave signal towards the coalface 27. This signal is reflected from the coalface 27 as well as from all other particles and structures (referred to as "targets") in the region that is irradiated with the emitted microwave signal.

The transmitted signal is continuous and varies in frequency according to a set pattern, namely a linear sweep between two defined frequencies, fi and f₂. The sweep occurs over a defined period T. The frequency varies proportionally, preferably directly proportionally, with time.

The signal received at any instant by the radar antenna 59 comprises a combination of reflections from all targets illuminated by the radiation at all ranges continuously. Individual targets contribute signal components with amplitudes that are proportional to the distance from the radar as well as the target's radar cross section, which in turn depends, inter alia, on dimensions and reflective characteristics of the target material .

Signals reflected from targets are also continuous and have varying frequency. The system employs the principles of "continuous wave, frequency modulation" radar. The system determines the time elapsed between the emission and reception of microwave radiation by the radar by comparing the received frequency to the frequency emitted at all instants. From knowing the sweep rate, calculated as (f₂ minus fi) ÷ T (the sweep period), it is possible to calculate the time that the return signal took from the moment of emission to reception. The distance to the target (D) can then be calculated from the known velocity of electromagnetic radiation in air (approximately 2.997 x 10⁸ metres per second) .

Other forms of electromagnetic interference (for example, from other electrical apparatus) , as well as mechanical noise, atmospheric effects (due to humidity and temperature) and signals internal to the radar apparatus may contaminate or distort the signals received by the radar.

The received signal is continuously processed to produce a raw output of the instantaneous value of the received signal as well as a timing reference signal that is derived from the periodic signal that drives the frequency sweep of the transmitted signal.

In order to detect the wanted target (coalface 27) it is necessary to discriminate between signals reflected from it and all other signals (reflected from unwanted targets such as coal dust, water spray, soil particles, rock particles and mechanical structures in the area, and electromagnetic interference) . The radar apparatus employs amplitude detection based on the fact that the coalface 27 produces a reflected signal that has a large amplitude compared to individual other targets. The radar apparatus also employs "averaging" as a technique to discriminate between the wanted target, providing a persistent high amplitude reflected signal, and unwanted targets such as falling pieces of coal that may provide instantaneous high amplitude reflections but disappearing shortly afterwards due to their movement.

The processing is performed in the digital signal processor board 35 in timeframes equal to the sweep period T of the transmitter. Firstly, the raw amplitude signal is band pass filtered by filter 69 to remove high frequency components before being sampled by the high- resolution (12 to 16 bit) analogue to digital converter 71.

The digital signal processor 73 uses a Fast Fourier Transform technique to extract the various frequency components from the return signal of each frequency sweep. As explained above, the return frequency at a particular time in the frequency sweep is indicative of the distance that the radiation producing the signal traveled from emission to detection. The frequency at the time of transmission during the frequency sweep is known. By comparing the return frequency to the transmitted frequency at any time during the frequency sweep, a frequency difference is provided - which is proportional to the distance travelled by the radiation.

The system may include means for obtaining an approximation of the distance between the miner 1 and the coalface 27. For example, pressure and/or vibration sensors in the arm 9 may produce signals which can be analyzed to detect when the cutting wheels 7 come into contact with the coalface 27. Alternatively, or additionally, forward movement of the miner 1 during cutting is characterized by a log_e ( ) function of distance-to-coalface against time. This characteristic is used as a priori information as to distance. This information can be used to discard frequency difference values which correspond to distances which obviously do not correspond to the coalface 27.

The amplitudes of the respective frequency differences (other than any excluded as being obviously not related to the coalface 27) are accumulated during each sweep period and are stored. A plurality of storage locations are provided, one for each frequency difference value. At the beginning of each sweep period, the value in each storage location is set to zero. At each time interval during the sample period, the frequency components of the signal are obtained. The frequency difference for each frequency component is calculated using the timing reference signal (which corresponds to the variation in the output radiation) . The amplitude of each frequency component corresponding to a particular frequency difference is summed with the current value in the relevant storage location for that frequency difference. The frequency difference with the highest amplitude generally corresponds to the coalface 27. However, this will not always be the case because, for example, a falling piece of debris may, for a short time, produce a high amplitude signal. To mitigate this problem, the amplitudes of respective frequency differences for eight consecutive frequency sweeps are stored in eight sets of storage locations to create a table, an example of which is shown below: -

Frequency Sweep

frequency difference

Of course, more or fewer than eight consecutive frequency sweeps (and sets of storage locations) could be used, according to the circumstances.

In the illustrative example of the table, the frequency difference value of "4" corresponds to the distance between the miner 1 and the coalface 27. This is why the amplitudes corresponding to the frequency difference value "4" are generally relatively high. However, occasionally another target will give a high amplitude reflection - for example, in frequency sweep "4" for frequency difference value "7", the amplitude value is 9, higher than the value for the frequency difference value "4" corresponding to the coalface 27. To prevent such a situation resulting in an incorrect calculation of the distance between the miner 1 and the coalface 27, the mean of the amplitudes corresponding to each relevant frequency difference over the eight consecutive frequency sweeps is calculated. The highest mean value provides a more reliable indication of which frequency difference value corresponds to the coalface 27. As can be seen in the example even though targets other than the coalface 27 have caused reflections of significant amplitudes, the mean value corresponding to the frequency difference value "4" is substantially higher than the other mean values, providing a clear indication that the frequency difference value "4" corresponds to the coalface 27. From the frequency difference value "4" the distance between the coalface 27 and the miner 1 can be calculated by assuming a velocity of electromagnetic radiation in air of 2.997 x 10⁸ metres per second.

The table is updated continuously. As data from each new frequency sweep becomes available, this data replaces the data corresponding to the "oldest" frequency sweep stored in the table. In this manner, a "running average" is calculated on an on-going basis.

The analysis of the received radiation is performed in real time, so that the position of the miner 1 can be adjusted continuously, without slowing the mining operation.

Figure 5 depicts typical time data series of the received signal, which is measured using multiple frequency sweeps The variance in the signal amplitude shown is as a result of falling coal, water spray and coal dust in the way of the measuring device.

Figure 6 depicts typical frequency series of the received signal, which is calculated using a Fast Fourier Transform technique within the digital signal processor. The index of the frequency spectrum indicates the distance measured, corresponding to the various targets in front of the radar. This figure shows the distance to the coalface decreasing as the continuous miner 1 is approaching the coalface 27.

The distance measured between the coalface 27 and the radar apparatus 17 provides sufficient information to enable the movement of the miner 1 to be controlled in order to ensure that the thickness of any cut is approximately constant. The known position of the radar apparatus 17 on the main body 3 of the miner and the known length of the arm 9 allows the thickness of the cut into the coalface 27 to be calculated for a given amount of forward movement of the miner 1.

Claims

CLAIMS :

1. A method for positioning mining apparatus, the method including providing a transmitter associated with the mining apparatus for transmitting radiation generally towards a surface to be mined; receiving radiation including radiation reflected from said surface and generating a signal representative thereof; and characterized by processing the signal to identify components likely to relate to radiation reflected from said surface to derive a distance value indicative of the distance between the transmitter and the surface; and using the distance value to derive a drive value for controlling the mining apparatus for providing a selected depth of cut into the surface by the mining apparatus .

2. A method according to claim 1, wherein the processing of the signal includes band pass filtering the signal.

3. A method according to claim 1 or 2, including varying a characteristic of the transmitted radiation with respect to time.

. A method according to claim 3 , including varying the frequency of the transmitted radiation with respect to time .

5. A method according to claim 4, wherein the transmitted radiation is frequency modulated continuous wave radar radiation.

6. A method according to any one of claims 3 to 5, wherein the processing of the signal includes sampling the signal at a plurality of time intervals during the varying of the characteristic of the transmitted radiation.

7. A method according to claim 6, wherein the characteristic of the transmitted radiation is varied according to a predetermined pattern during a sweep period.

8. A method according to claim 7, including analyzing the signal to identify said characteristic in the reflected radiation corresponding to a particular time and calculating the difference in said characteristic of the transmitted radiation and the received radiation at said particular time.

9. A method according to claim 8, including, for a range of values of said difference in said characteristic, obtaining the amplitude of the component of the signal corresponding to each difference value of said characteristic.

10. A method according to claim 9, including, during a sweep period, generating an accumulated amplitude value for each of said difference values of said characteristic, the accumulated amplitude values being indicative of the sum of the amplitudes of the signal components having respective ones of said characteristic difference values.

11. A method according to claim 10, including calculating the mean of the accumulated amplitude values corresponding to each of said characteristic difference values over selected sweep periods .

12. A method according to claim 11, including comparing mean values corresponding to different characteristic difference values to identify the highest mean value.

13. A method according to claim 12, wherein said selected sweep periods comprise a plurality of consecutive sweep periods, which are stored for analysis, the sampled data of each new period stored replacing the data of the oldest stored period.

14. Apparatus for positioning mining apparatus, the apparatus including:

a transmitter for association with the mining apparatus for transmitting radiation generally towards a surface to be mined;

a receiver for receiving radiation including radiation reflected from said surface and generating a signal representative thereof;

means for processing the signal to identify components likely to relate to radiation reflected from said surface to derive a distance value indicative of the distance between the transmitter and the surface; and means for using the distance value to derive a drive value for controlling the raining apparatus for providing a selected depth of cut into the surface by the mining apparatus .

15. Apparatus according to claim 14, wherein the processing means includes a band pass filter.

16. Apparatus according to claim 14 or 15, including means for varying a characteristic of the transmitted radiation with respect to time.

17. Apparatus according to claim 16, including means varying the frequency of the transmitted radiation with respect to time.

18. Apparatus according to claim 17, wherein the transmitted radiation is frequency modulated continuous wave radar radiation.

19. Apparatus according to any one of claims 16 to 18, wherein the processing means samples the signal at a plurality of time intervals during the varying of the characteristic of the transmitted radiation.

20. Apparatus according to claim 19, wherein the characteristic of the transmitted radiation is varied according to a predetermined pattern during a sweep period.

21. Apparatus according to claim 20, including means for analyzing the signal to identify said characteristic in the reflected radiation corresponding to a particular time and calculating the difference in said characteristic of the transmitted radiation and the received radiation at said particular time .

22. Apparatus according to claim 21, including means for obtaining the amplitude of the component of the signal corresponding to each difference value of said characteristic for a range of values of said difference in said characteristic.

23. Apparatus according to claim 22, including means for generating an accumulated amplitude value for each of said difference values of said characteristic during a sweep period, the accumulated amplitude values being indicative of the sum of the amplitudes of the signal components having respective ones of said characteristic difference values.

24. Apparatus according to claim 23, including means for calculating the mean of the accumulated amplitude values corresponding to each of said characteristic difference values over selected sweep periods .

25. Apparatus according to claim 24, including means for comparing mean values corresponding to different characteristic difference values to identify the highest mean value .

26. Apparatus according to claim 25, wherein said selected sweep periods comprise a plurality of consecutive sweep periods, which are stored for analysis, the sampled data of each new period stored replacing the data of the oldest stored period.

27. A mining apparatus positioning system including a transmitter for transmitting a repeating pattern of radiation generally towards a surface and a receiver for receiving reflected radiation including radiation reflected from the surface, the radiation pattern being such that the distance between the point of reflection and the transmitter is derivable from the reflected radiation; means for storing data derived from the reflected radiation during a plurality of said repeating radiation patterns; and means for analysing the stored data to identify the likely distance of the surface from the transmitter.