Abstract:
An object detector comprising: a transmitter arranged to generate a transmit signal; a transmit loop antenna, coupled to said transmitter and arranged to radiate said transmit signal; and a receive loop antenna arranged to generate a receive signal from received radio waves; wherein said received radio waves include said radiated transmit signal and, in the presence of objects within range of the device which reradiate said transmit signal, one or more radio waves reradiated by such objects; said receive signal includes a component corresponding to said transmit signal and, in the presence of reradiated radio waves, one or more components corresponding to said reradiated radio waves; and said object detector further comprises a signal detector, coupled to said receive loop antenna and said transmitter, the signal detector arranged to detect changes in the amplitude and/or phase of said receive signal which are due to changes in said components corresponding to said reradiated radio waves.

Description:
FIELD 
       [0001]    The present disclosure relates to an object detector. In particular, embodiments of the present disclosure relate to an object detector which is suitable for detecting buried elongate conductive objects. 
       BACKGROUND 
       [0002]    Detectors suitable for detecting buried objects are well-known in the art. For example, UK Patent Application published under number GB2280270A, in the name of Roke Manor Research Limited, discloses a detector of buried elongate conductive objects which includes two receive loops. The detector is arranged to cancel background noise which propagates in the horizontal plane, but to detect signals radiated from an elongate buried conductor which propagate in the vertical plane. It is useful when undertaking ground works of any kind, for example, whether it be for utility use, farming, geological, archaeological or other purposes, to know of any existing infrastructure, for example pipes and cables buried in the ground. There is a particular need for detectors which could operate over a range of several metres. Longer detection range means that less surveying time is required for a given area of ground. Furthermore, there is a need for detectors which are less susceptible to electromagnetic interference and which are more reliable in operation, and reduce the probability of false detections. 
         [0003]    It is an object of the present disclosure to provide an object detector which addresses the aforementioned issues. 
       BRIEF SUMMARY 
       [0004]    In a first aspect, the present disclosure provides an object detector comprising: a transmitter arranged to generate a transmit signal; a transmit loop antenna, coupled to said transmitter and arranged to radiate said transmit signal; and a receive loop antenna arranged to generate a receive signal from received radio waves; wherein said received radio waves include said radiated transmit signal and, in the presence of objects within range of the device which reradiate said transmit signal, one or more radio waves reradiated by such objects; said receive signal includes a component corresponding to said transmit signal and, in the presence of reradiated radio waves, one or more components corresponding to said reradiated radio waves; and said object detector further comprises a signal detector, coupled to said receive loop antenna and said transmitter, the signal detector arranged to detect changes in the amplitude and/or phase of said receive signal which are due to changes in said components corresponding to said reradiated radio waves. 
         [0005]    In a second aspect a method of object detection using an object detector comprising: a transmitter arranged to generate a transmit signal; a transmit loop antenna, coupled to said transmitter and arranged to radiate said transmit signal; and a receive loop antenna arranged to generate a receive signal from received radio waves; and a signal detector, coupled to said receive loop antenna and said transmitter, the method comprising: generating and transmitting said transmit signal; receiving, at said receive loop antenna, radio waves including said radiated transmit signal and, in the presence of objects within range of the device which reradiate said transmit signal, one or more radio waves reradiated by such objects; generating a receive signal, at an output of said receive loop antenna, including a component corresponding to said transmit signal and, in the presence of reradiated radio waves, one or more components corresponding to said reradiated radio waves; and detecting, using said signal detector, changes in the amplitude and/or phase of said receive signal which are due to changes in said components corresponding to said reradiated radio waves. 
         [0006]    Further features of embodiments of the disclosure are recited in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present disclosure will now be described, by way of example only, and with reference to the accompanying drawings in which: 
           [0008]      FIG. 1  is a schematic diagram of an object detector in accordance with an embodiment of the present disclosure; 
           [0009]      FIG. 2  is a plan view of the detector shown in  FIG. 1  in use; and 
           [0010]      FIG. 3  is a schematic diagram of an object detector in accordance with a further embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  is a schematic diagram of an object detector  10  in accordance with an embodiment of the present disclosure. The object detector  10  includes a transmitter subsystem  12  and a receive combining subsystem  14 . The transmitter subsystem  12  is coupled to a transmit loop antenna  16  via cable  18 . The receive combining subsystem  14  is coupled to a receive loop antenna  20  by cable  22 . The transmitter subsystem  12  is arranged to generate a high power RF signal which is radiated by the transmit loop antenna  16 . In particular, the transmitter subsystem  12  is arranged to generate a continuous RF signal, rather than a pulsed signal. The transmit loop antenna  16  generates a radio frequency magnetic near-field. This magnetic near-field induces currents in elongate conductive objects. The induced current causes the elongate conductive objects to re-radiate the signal as their own magnetic near-field. The receive loop antenna  20  is arranged to receive both the magnetic near-field generated by the transmit loop antenna, and the magnetic near-field generated by the elongate object. 
         [0012]    Although not shown in  FIG. 1 , both of the loop antennas  16 ,  20  are horizontal, i.e. the plane of each loop is parallel to the ground surface. Furthermore, both loops are circular when viewed from above. The loops are arranged such that their axes are aligned with each other, with the receive loop antenna  20  being positioned vertically above the transmit loop antenna  16 . The distance of separation between the loops will depend on the frequency of operation and the desired range of the device. However, the loops may typically be positioned between 30 cm and 100 cm apart. 
         [0013]    The transmit signal is generated by oscillator  24  and frequency multiplier  26 . The oscillator  24  and frequency multiplier  26  may be adjusted to provide a transmit signal of the required frequency. The output of the frequency multiplier  26  is fed to variable gain amplifiers  28 ,  30  which may be adjusted to provide a transmit signal with the required amplitude. Variable gain amplifier  30  produces a transmit signal at its output which is fed to the transmit loop antenna  16  via line  18 . Variable gain amplifier  28  produces a corresponding transmit signal which is fed to the receive combining subsystem  14  via line  32 . 
         [0014]    The receive combining subsystem  14  includes an RF combiner  34 . The RF combiner  34  is essentially arranged to combine a signal output from the receive loop antenna  20  and the transmit signal received from the transmitter subsystem  12  via line  32 . The received combining subsystem  14  also includes a variable attenuator  36 . The variable attenuator  36  is coupled between line  22  (which is coupled to the receive loop antenna  20 ) and the RF combiner  34 . The variable attenuator  36  is arranged to control the amplitude of the signal which is output from the receive loop antenna  20 . The manner in which the attenuator is controlled will be described below. The receive combining system  14  also includes a variable phase shifter  38 . The variable phase shifter  38  is coupled between the line  32  and the RF combiner  34 . The variable phase shifter  38  is arranged to control the phase of the transmit signal. The manner in which the variable phase shifter  38  is controlled will be described in more detail below. 
         [0015]    The receive combining system  14  also includes a receiver  40 . The receiver  40  is coupled to an output  42  of the RF combiner  34 . The receiver  40  generates a baseband output signal via output  44 . 
         [0016]    The receive combining system  14  includes a feedback mechanism which includes automatic control unit  46 . The automatic control unit  46  is coupled to the output  44  of the receiver  40 . The output of the automatic control unit  46  is coupled to variable attenuator  36  and variable phase shifter  38 . 
         [0017]    The operation of the object detector shown in  FIG. 1  will now be described. 
         [0018]    As noted above, in use, the transmitter subsystem  12  is arranged to generate a continuous transmit signal which is fed to transmit loop antenna  16  in order to generate a radiated signal. The transmit loop antenna  16  therefore generates a continuous magnetic near-field. A corresponding transmit signal is fed via line  32  to the receive combining subsystem  14 . The receive loop antenna  20  receives both a direct signal via the magnetic near-field from the transmit loop  16  and a magnetic near-field signal reradiated from buried elongate conducting objects. In the absence of any elongate conducting objects, the output signal generated by receive loop antenna  20  will be very similar to the transmit signal which is fed to the receive combining subsystem  14  via line  32 . The automatic control unit  46  is arranged to control the variable attenuator  36  and the variable phase shifter  38  such that the output  44  of receiver  40  is minimised. Assuming ideal conditions, with no background noise, it is possible to adjust the signals from the receive loop antenna  20  and the transmitter subsystem  12  such that there is no output from the receiver. As the device  10  moves over the ground, and owing to changes in background noise, the output from the receive loop antenna  20  will vary. The automatic control unit  46  is arranged to continuously adjust the variable attenuator  36  and the variable phase shifter  38  in order to minimise the output from receiver  40 . Once the buried elongate conducting object comes into range of the device, the object will reradiate the radiated signal. This will cause a significant change in the output from receive loop antenna  20 . In such circumstances, the automatic control unit  46  will have to work very hard to minimise the output from receiver  40 . The automatic control unit  46  is arranged so that when the degree of correction required to minimise the output of receiver  40  at either the variable attenuator  36  or the variable phase shifter  38  exceeds a certain predetermined threshold, an alarm is triggered to indicate to the user the likely presence of a buried elongate conducting object. 
         [0019]      FIG. 2  shows a plan view of an open area  48  under which an elongate conductive object  50  is buried. The arrows  52 ,  54  show the area in which the detector  10  may be positioned in order to detect the object  50 . The detector  10  is most effective when positioned halfway along the buried object  50 . Furthermore, the detector  10  is most effective when the object  50  has a length which is half a wavelength, taking into account the dielectric effects of the ground, at the operating frequency of the detector  10 .  FIG. 2  is not to scale, and at best, show approximate dimensions of a buried object  50  suitable for detection by detector  10 . 
         [0020]      FIG. 3  is a schematic diagram of an object detector  110  in accordance with a further embodiment of the present disclosure. The object detector  10  described above uses analogue components. The object detector  110  uses digital components. 
         [0021]    The object detector  110  includes a transmitter subsystem  112 . The transmitter subsystem  112  is essentially the same as the transmitter subsystem  12  described above in connection with  FIG. 1 . The object detector  110  also includes receiver subsystem  114 . The transmitter subsystem  112  is coupled to a transmit loop antenna  116  via cable  118 . The receiver subsystem  114  is coupled to a receive loop antenna  120  by cable  122 . The transmitter subsystem  112  is arranged to generate a high power RF signal which is radiated by the transmit loop antenna  116 . In particular, the transmitter subsystem is arranged to generate a continuous RF signal, rather than a pulsed signal. The transmit loop antenna  16  generates a radio frequency magnetic near-field. This magnetic near-field induces currents in elongate conductive objects. The induced current causes the elongate conductive objects to re-radiate the signal as their own magnetic near-field. The receive loop antenna  120  is arranged to receive both the magnetic near-field generated by the transmit loop antenna  116 , and the magnetic near-field generated by the elongate object. 
         [0022]    Although not shown in  FIG. 3 , both of the loop antennas  116 ,  120  are horizontal, i.e. the plane of each loop is parallel to the ground surface. Furthermore, both loops are circular when viewed from above. The loops are arranged such that their axes are aligned with each other, with the receive loop antenna  120  being positioned vertically above the transmit loop antenna  116 . The distance of separation between the loops will depend on the frequency of operation and the desired range of the device. However, the loops may typically be positioned between 30 cm and 100 cm apart. 
         [0023]    The transmit signal is generated by oscillator  124  and frequency multiplier  126 . The oscillator  124  and frequency multiplier  126  may be adjusted to provide a transmit signal of the required frequency. The output of the frequency multiplier  126  is fed to variable gain amplifiers  128 ,  130  which may be adjusted to provide a transmit signal with the required amplitude. Variable gain amplifier  130  produces a transmit signal at its output which is fed to the transmit loop antenna  116  via line  118 . Variable gain amplifier  128  produces a corresponding transmit signal which is fed to the receiver subsystem  114  via line  132 . 
         [0024]    The receiver subsystem  114  includes a low pass filter (LPF)  134  and a LPF  136 . LPF  136  is coupled to the receive antenna  120 . LPF  134  is coupled to transmitter  112 . The receiver subsystem  114  includes an analogue to digital converter (ADC)  138  which is coupled to the output of LPF  136  and an ADC  140  which is coupled to the output of LPF  134 . The outputs of ADC  138  and  140  are fed to digital processing system (DPS)  142 . The DPS  142  may be implemented as a field-programmable gate array (FPGA), a microcontroller, personal computer, or combination thereof. 
         [0025]    The DPS  142  includes a measurement module  144  which measures the amplitude and phase of the signals output from ADCs  138  and  140 . These measurements are fed to an extraction module  146  which extracts the transmit signal from the receive signal. At this point, the amplitude and phase variations due to system components are cancelled, leaving variations in amplitude and phase due to the environment. The resulting signal is fed to a digital LPF  148  (and/or a high pass filter). The output of LPF  148  is fed to a data logger  150 . The output of data logger  150  is fed to threshold detector module  152 . 
         [0026]    The operation of the object detector shown in  FIG. 3  will now be described. 
         [0027]    As noted above, in use, the transmitter subsystem  112  is arranged to generate a continuous transmit signal which is fed to transmit loop antenna  116  in order to generate a radiated signal. The transmit loop antenna  116  therefore generates a continuous magnetic near-field. A corresponding transmit signal is fed via line  132  to the receiver subsystem  114 . The receive loop antenna  120  receives both a direct signal via the magnetic near-field from the transmit loop  116  and a magnetic near-field signal reradiated from buried elongate conducting objects. In the absence of any elongate conducting objects, the output signal generated by receive loop antenna  120  will be very similar to the transmit signal which is fed to the receive combining subsystem  114  via line  132 . 
         [0028]    Assuming ideal conditions, with no background noise and no shift in the transmitter output, the signal received by threshold detector module  152  will be zero. As the device  110  moves over the ground, and owing to changes in background noise, the output from the receive loop antenna  120  will vary. Once a buried elongate conductive object comes into range of the device, the object will reradiate the radiated signal. This will cause a significant change in the output from receive loop antenna  120 . In such circumstances, a signal will be received by threshold detector module  152 . When the signal exceeds a certain predetermined threshold, or the rate of change of the signal exceeds a certain threshold, a user perceivable alarm is triggered to indicate to the user the likely presence of a buried elongate conducting object. A user perceivable output may a visual output, such as a light, or an audible output, such as an alarm. 
         [0029]    While  FIGS. 1 and 3  show an embodiment in which the transmit and receive loops are positioned one above the other, the detector may also work with the loops positioned side-by-side. It may also be possible for the detector to operate with multiple transmit and receive antenna loops. 
         [0030]    The buried elongate conductive object  50  is also shown in  FIGS. 1 and 3 , although it should be noted that these Figures are not to scale. 
         [0031]    The above-described embodiment is described as one example of the present disclosure. The skilled person will appreciate that variations may be made without departing from the spirit and scope of the claimed disclosure. 
         [0032]    While the claims provide for a particular combination of features, the skilled person will appreciate that other combinations are possible. 
         [0033]    In the above-described embodiments, the transmitter subsystem has been described as generating a continuous signal. In this context, a continuous signal is one that is not pulsed. However, it will be appreciated that the continuous signal may be turned off, and back on, from time-to-time. For example, this may be done as a method of reducing average system power consumption. The present disclosure does not however require the signal to be pulsed in order to function. 
         [0034]    There are three notable differences between the present disclosure and RADAR-type systems. Firstly, in the present disclosure, the continuous transmit signal generates a continuous magnetic field, which may be continuously monitored by the receive combining subsystem. That is to say, the system is transmitting and receiving at the same time on a continuous basis. The reception of the magnetic near-field from the elongate object, and that from the transmit loop, occurs at a similar point in time with negligible delay. This is in contrast to a RADAR-type system, in which a signal pulse is generated, and after a time period corresponding to the return propagation delay, the pulse is received. The propagation delay in such a system is significant and measurable. The RADAR-type system determines the presence of the target from the time delay, and other characteristics, of the received signal. Most notably and in contrast to the present disclosure, the RADAR-type system transmits and receives at separate instances in time. 
         [0035]    Secondly, there is a difference relating to the RF wavelength in use. In a RADAR-type systems, the distance to the target is long compared to the wavelength. This is in contrast to the present disclosure, where the distance between the system and the object is short compared to the wavelength. 
         [0036]    Thirdly, there is a difference in the RF field distribution. A RADAR-type system uses a freely propagating electromagnetic wave, in which the electric and magnetic components exist at a constant ratio, which is independent of distance. This is a consequence of the system operating in the far-field, with several cycles of the wave existing between the system and the target. In the present disclosure, the situation is quite different. Firstly, only the magnetic component of the RF field is used. Secondly, this component is utilised in the near-field region, where the relationship and distribution of the electric and magnetic components of the field varies with position with respect to the loop antennas and the elongate conducting object.