Abstract:
A device that can reduce the effects of interference on an electromagnetic field emitted from a buried, underground or otherwise inaccessible object is provided. The device generates quality metrics relating to a plurality of sideband signals from one or more input signals, compares the quality metrics and selects one or more sideband signals, dependent on the respective quality metrics. A method of selecting one of a plurality of sideband signals using quality metrics is also disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a device for reducing the effects of magnetic interference on electromagnetic signals from a buried of inaccessible object and a method of performing the same.  
         BACKGROUND OF THE INVENTION  
         [0002]    Guided and unguided underground boring or locating tools are generally monitored in order to determine their location and orientation, in order to allow steering of and/or allowing instructions to be given to the tool.  
           [0003]    It is known for tools to emit signals in order to be monitored, rather than using a physical connection between the tool and the monitor. The emitting device is commonly known as a “sonde”. It is known for sondes to emit electromagnetic fields as signals. Such wireless emission allows greater versatility of the sonde and monitor, for example by allowing the sonde to be located and tracked while otherwise inaccessible. However, use of such electromagnetic field signals means that other electromagnetic fields, such as those generated by power cables, interfere with the signals from the sonde by also being detected by the monitor together with the signals from the sonde.  
           [0004]    It is also known to modulate the signal emitted from the sonde. The modulation carries additional data, such as the orientation (yaw, pitch, roll) of the sonde, and therefore the tool, as well as the basic electromagnetic field allowing tracking of the location of the tool. Ambient electromagnetic fields can corrupt both the overall electromagnetic field and the modulation applied thereto. Such corruption can therefore reduce the effectiveness of the wireless sonde and monitor, especially in areas with high ambient electromagnetic “noise”. There is therefore a need to reduce the effects of such ambient radiation, in order improve the efficiency of the communication between sonde and monitor, for example in order to increase the data transmission rate.  
         SUMMARY OF THE INVENTION  
         [0005]    According to a first aspect of the invention, there is provided a device that can reduce the effects of interference on an electromagnetic field emitted from a buried, underground or otherwise inaccessible object. The device generates quality metrics relating to a plurality of sideband signals from one or more input signals, compares the quality metrics and selects one or more sideband signals, dependent on the respective quality metrics.  
           [0006]    The input signals may be electronic signals from one or more detectors, which detect the electromagnetic field at one or more specified locations. The device may generate quality metrics for sideband signals from more than one such input signal, or from a single input signal. The device may select which sideband signals are to have quality metrics generated, and/or which quality metrics are to be compared. For example, if a sideband signal does not have a sufficient signal amplitude, it may be ignored, either before or after a quality metric is determined, and, if determined, the quality metric in such a case may not be compared with other determined quality metrics.  
           [0007]    In an embodiment, three detectors of electromagnetic fields provide the input signals. In a further embodiment, the three detectors are arranged such that two are horizontal, relative to the ground, and one is vertical. The two horizontal sensors may be orthogonal. The device may additionally comprise such detectors, or the detectors may be separate to the device.  
           [0008]    In an embodiment, each input signal is split into an upper sideband signal (i.e. the signal carried by the upper sideband of the input signal), a lower sideband signal (i.e. the signal carried by the lower sideband of the input signal) and a double sideband signal (i.e. the signal carried by both sidebands when combined). Each of these three signals is a sideband signal. These sideband signals each carry the same modulation, apart from noise added to the input signal before and after detection. There is therefore redundancy in the sideband signals of the input signal. The quality metric of each signal is then generated. The quality metrics of each signal are compared. In an embodiment, the or each input signal is split into more than three sideband signals.  
           [0009]    In an embodiment, the sideband signal corresponding to the highest quality metric, i.e. the quality metric showing the highest quality of sideband signal, is selected, and in an embodiment that sideband signal is output from the device. In an embodiment, the quality metric for at least one of the sideband signals is determined by measuring the ratio of peak voltage to RMS (root mean square) voltage of the signal. In an embodiment, the quality metric for at least one of the sideband signals is determined by measuring the bit error rate of a respective sideband signal. The bit error rate is determined by measuring the ratio between the number of bits that are received that are erroneous in value with the total number of bits received.  
           [0010]    In an embodiment of the invention, the quality metric for at least one of the sideband signals is determined by measuring modulation of the bit width of a respective sideband signal. The modulation of the bit width is determined by analysing the received bit widths against the known correct bit width. The lower the level of modulation, and/or the closer the actual bit width to the expected bit width, the higher the quality metric.  
           [0011]    The output sideband signal(s) may be output to a demodulator, and the signal from the sonde interpreted. The output may be to a switch or multiplexor, to connect the selected sideband signal(s) directly with a demodulator or other device. In an embodiment the sideband signals are converted from analogue to digital before quality metrics are determined, and the sideband signals demodulated before quality metrics are determined. In an embodiment, the input signals are amplified, filtered and digitised using an ADC (analogue to digital converter). In an embodiment, the data in each input signal is recovered and the amplitude and phase of each incoming signal is measured.  
           [0012]    In an embodiment a phase locked loop double sideband demodulator is used to demodulate the double sideband signal. In an embodiment, two Hilbert transformer demodulators are used to demodulate the single sideband signals.  
           [0013]    There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof [that follows] [herein] may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto.  
           [0014]    In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.  
           [0015]    As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Embodiments of the invention will now be described, purely by way of example, in reference to the following drawings, in which:  
         [0017]    [0017]FIG. 1 shows a device according to a first embodiment of the invention;  
         [0018]    [0018]FIG. 2 shows a flow diagram showing a method of operation of the device of FIG. 1;  
         [0019]    [0019]FIG. 3 shows a device according to a second embodiment of the invention; and  
         [0020]    [0020]FIG. 4 shows a flow diagram showing a method of operation of the device of FIG. 3. 
     
    
     DETAILED DESCRIPTION  
       [0021]    A device according to a first embodiment of the invention is shown in FIG. 1. The device comprises an initial processing stage  100 , a signal splitter  110  to split the detected signals and route them to first, second and third demodulators  112 ,  114  and  116 , which demodulate the detected signals after splitting. The device also comprises a quality metric generation stage  120  connected to each of the outputs of the demodulators  112 ,  114 ,  116  to generate quality metrics for the demodulated signals, a comparison logic unit  130  for comparing the quality metrics, selecting a preferred demodulated signal and controlling a multiplexer to output signal corresponding to the chosen quality metric. A main stage  150  comprises all parts of the device apart from the comparison logic unit  130 .  
         [0022]    The initial processing stage  100  comprises an antenna  102 , which detects an electromagnetic field generated by an underground transmitter (sonde). A switched gain amplifier  104 , a bandpass filter  106 , an analogue to digital converter (ADC)  107  and a transformer  108  are connected in series to the antenna  102 . The output from the transformer  108  is connected to the signal splitter  110 . The first, second and third demodulators  112 ,  114 ,  116  respectively demodulate the upper, lower and double sideband signals of the input signal. In the present embodiment, the signal emitted from the sonde is an amplitude modulated (AM) signal, the amplitude modulation producing the sideband signals. The upper and lower sideband signals each carry the same modulated signal. The double sideband signal is the combination of both upper and lower single sideband signals. In this embodiment, the first and second demodulators, demodulating the upper and lower single sideband signals respectively, are Hilbert transformer demodulators. The third demodulator  116  is a phase locked loop double sideband demodulator.  
         [0023]    The quality metric generation stage  120  comprises first, second and third quality metric generators  122 ,  124 ,  126 , which receive demodulated signals from and generate quality metrics for the first, second and third demodulators  112 ,  124 ,  126  respectively. The quality metrics are output to the comparison logic unit  130 , which then controls the multiplexer  140  to output one of the sideband signals. FIG. 2 shows a method of operation of the device of the first embodiment for use in detecting an underground transmitting sonde. An input signal is received at the antenna  102  at S 100 . The signal is subject to electromagnetic noise, especially that caused by mains power cables, which produce fields at multiples of for example, 50 Hz in the United Kingdom, and 60 Hz in the United States of America. It is convenient to assume in the system that the mains interference will be at multiples of 300 Hz as this encompasses both 50 Hz and 60 Hz noise.  
         [0024]    The signal is amplified by the amplifier  104  at S 102 , before being passed through the filter  106  at S 104 . The filter  106  removes components at low frequencies, e.g. mains frequencies, and so has a high pass to reject signals below 60 Hz. The filter also has low pass to reject frequencies above 9 kHz. The low pass is included to avoid the ADC  107  failing to reject multiples of its sampling rate. The ADC  107  converts the analogue signal into a digital signal at S 106  and the digital signal is output to the transformer  108 . The transformer  108  converts the digitised signal into amplitude and phase elements, which can then subsequently be analysed.  
         [0025]    The transformed signal is then split by the signal splitter  110  and a signal is passed to each of the first, second and third demodulators  112 ,  114 ,  116 . The demodulators  112 ,  114 ,  116  demodulate each sideband signal at S 110 . The Hilbert transformer of the first demodulator  112  removes frequency components at and below the carrier frequency. The Hilbert modulator of the second demodulator  114  removes frequency components at and above the carrier frequency. The phase locked loop double sideband demodulator  116  demodulates the combined signal of both upper and lower sideband signals. Each demodulated signal is passed to the multiplexer  140  for selective output at SI  12 .  
         [0026]    The quality metric generation stage  120  generates quality metrics for each of the demodulated signals at S 114 . The quality metric generators  122 ,  124 ,  126  make use of “Crest factor measurement” to generate the quality metrics. This measurement finds the ratio of the peak amplitude of the demodulated signal to the RMS value of the signal. The crest factor is the quality metric in this embodiment and gives an indication of the relative noise in a signal, the higher the value, the lower the noise level. The quality metrics are compared by the comparison logic unit  130  and the highest quality metric is selected at S 116 . The comparison logic unit  130  then controls the multiplexer  140  to output the demodulated sideband signal having the highest quality metric at S 118 .  
         [0027]    The quality metric generators  122 ,  124 ,  126  may also calculate other measures of the quality of the signals. For example, the bit error rate may be calculated for each demodulated signal. The bit width modulation of each signal may also be calculated. The quality metric generators  122 ,  124 ,  126  may produce some or all of these three signal quality indicators and the comparison logic unit  130  may choose the signal to be output by a combination of these metrics with any suitable weighting being used. Other suitable signal quality indicators may also be used.  
         [0028]    If the signals are processed with substantially random noise at a low level, then the double sideband signal will generally have the highest quality metric because only common noise between the upper and lower sideband signals will be in the combined signal. This should lead to a 3 dB improvement in signal to noise ratio for the double sideband signal. However, if there is a noise signal of periodic nature, this will generally appear in one or other of the sidebands only. The sideband which does not contain the periodic noise signal will therefore have a lower noise level and higher quality metric than the other sideband signal. Once the periodic noise reaches a certain level, the non-interfered sideband signal will have a higher quality metric than the double sideband signal, and this single sideband signal will be output from the further multiplexer  240 .  
         [0029]    [0029]FIG. 3 shows an apparatus according to a second embodiment of the invention. The apparatus comprises three main stages  200 A,  200 B,  200 C, comprising the parts shown in the first embodiment of the invention within system  150 . A comparison logic unit  230  receives quality metrics from each of the main stages and sends control signals to the multiplexer in each main stage  200 A,  200 B,  200 C. Each main stage  200 A,  200 B,  200 C outputs a selected demodulated signal to a further multiplexer  240 . The comparison logic unit  230  also sends control signals to the further multiplexer  240 , and the further multiplexer  240  outputs one of the sideband signals it receives.  
         [0030]    [0030]FIG. 4 shows a method of operation of the apparatus of the second embodiment. At S 200 , S 202  and S 204 , the selected sideband signals from each main stage  200 A,  200 B,  200 C are output on the basis of the quality matrices, as described in the first embodiment. Each selected signal is input into the further multiplexer  240  at S 206 . The quality metrics for each selected signal are compared by the comparison logic unit  230  at S 208 , and the comparison logic unit  230  controls the further multiplexer  240  to output a sideband signal selected by the comparison logic unit  230  at S 210 .  
         [0031]    Alternatively, the dotted line in FIG. 4 shows a variation in the method. The quality metrics can be determined from the signals from each main stage  200 A,  200 B,  200 C, rather than using the already generated quality metrics. In this case, comparison logic unit  230  can be two independent comparison logic units, one of which selects a sideband signal for each main stage  200 A,  200 B,  200 C, and one of which selects a signal from the three main stage  200 A,  200 B,  200 C outputs.  
         [0032]    In implementation, the comparison logic unit  130 ,  230  and the quality matrix generation stages  110  of either of the above embodiments may be combined in a single processor. The multiplexers  140 ,  240  of either embodiment may also be combined in the single processor. The signal splitters  110  and/or the demodulators  112 ,  114 ,  116  may also be combined in a single processor. In fact, any suitable combination of the above parts may be placed in one or more processors. The one or more processors may be software controlled, and any software used, in order for the device of the invention to function correctly, is also a part of the invention.  
         [0033]    The chosen signal output from the multiplexer  140  in the first embodiment, or further multiplexer  240  in the second embodiment is then processed to decode the signals encoded into the electromagnetic field emitted by the sonde, in order to determine the pitch, yaw or orientation, for example, of the sonde.  
         [0034]    The second embodiment is particularly useful in detection of underground sondes, because the three main stages can comprise three antennae, wherein two of the antennae are horizontally mounted, perpendicular to one another, and the third is vertical. This configuration allows the antennae used to locate a sonde in three dimensions to also be used to demodulate the additional data carried by the AM carrier sideband signals. Therefore, a total of nine sidebands can be analysed and the strongest signal used for the subsequent decoding of the signals to obtain the data sent from the sonde to the monitor. The apparatus also requires only the three antenna for all of the location and decoding processes. This allows the apparatus to be of a relatively compact size.  
         [0035]    Any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.  
         [0036]    It should be appreciated that further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing description, which is given by way of example only and which is not intended to limit the scope of the invention. In particular, although the invention has been particularly described in relation to AM signals, it could equally apply to frequency modulation (FM) signals and phase modulation (PM) signals, and the invention encompasses the use of any such modulation system. The invention also encompasses systems for analysing input systems comprising more than two different sidebands, for example in FM signal demodulation, and more than three sideband signals may be demodulated from a single input signal, in the same/similar ways as described above.  
         [0037]    The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalisation of any such features or combination, which extends to equivalents thereof. Each feature disclosed in the specification, including the claims, abstract and drawings may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise.  
         [0038]    Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.  
         [0039]    The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.