Abstract:
A buried object detector comprising: a loop antenna and an RF source, the RF source coupled to the loop antenna and arranged to feed the loop antenna with an RF signal, the detector further comprising a detector circuit coupled to the loop antenna and arranged to detect changes in the quality factor of a resonant circuit formed by the loop antenna, wherein the loop antenna is arranged to magnetically couple with a buried object, thereby reducing the quality factor of the resonant circuit.

Description:
BACKGROUND 
       [0001]    Hand held metal detectors are well known and may be used to detect metallic objects hidden in the ground or under a surface. Metal detectors operate by transmitting an alternating magnetic field into the ground or other surface. The reflected magnetic field is detected by a detector coil. Changes in the magnetic field due to the presence of metallic objects can then be detected. Metal detectors for the commercial and domestic markets have been available for many years. 
       SUMMARY 
       [0002]    In a first aspect, the present invention provides a buried object detector comprising: a loop antenna and an RF source, the RF source coupled to the loop antenna and arranged to feed the loop antenna with an RF signal, the detector further comprising a detector circuit coupled to the loop antenna and arranged to detect changes in the quality factor of a resonant circuit formed by the loop antenna, wherein the loop antenna is arranged to magnetically couple with a buried object, thereby reducing the quality factor of the resonant circuit. 
         [0003]    In a second aspect, the present invention provides a method of detecting a buried object, comprising: feeding a loop antenna with an RF signal; and detecting changes in the quality factor of a resonant circuit formed by the loop antenna; wherein the loop antenna is arranged to magnetically couple with a buried object, thereby reducing the quality factor of the resonant circuit. 
         [0004]    Further aspects are described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention will now be described by way of example only and with reference to the accompanying drawings, in which: 
           [0006]      FIG. 1  is a block diagram showing the components of a buried object detector in accordance with a first embodiment of the present invention 
           [0007]      FIG. 2  is a circuit diagram of the buried object detector shown in  FIG. 1  according to various aspects; 
           [0008]      FIG. 3  is a block diagram showing the components of a buried object detector in accordance with a further embodiment of the present invention; and 
           [0009]      FIG. 4  is a block diagram showing the components of a buried object detector in accordance with a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  shows the components of a buried object detector  100  in accordance with a first embodiment of the present invention. The buried object detector  100  includes a marginal oscillator  101 . The marginal oscillator  101  is coupled to a loop antenna  102 . Together, the marginal oscillator  101  and the loop antenna  102  form a tuned resonance circuit. The circuit is tuned to oscillate at a predefined frequency (e.g., 15 MHz). The antenna may consist of an aluminium strip bent to form a loop of a predefined diameter (e.g., 15 cm). 
         [0011]    An output of the marginal oscillator  101  is fed to a rectifier  103 . An output of the rectifier  103  is coupled to feedback amplifier  104 . The feedback amplifier  104  acts as a slow control loop for marginal oscillator  101 . An output of the rectifier  103  is also fed to detector amplifier  105 . The detector amplifier  105  is coupled to voltage-to-frequency converter  106  which is in turn coupled to tone generator  107 . 
         [0012]      FIG. 2  is a circuit diagram for the buried object detector  100 . The marginal oscillator  101  comprises two FET devices  200 A and  200 B. A suitable FET for this application is the BF245B MOSFET which is produced by various manufacturers. The source of FET  200 A is coupled to the common collector voltage (VCC). The drain of FET  200 A and the drain of FET  200 B are coupled to each other and to resistor  201  which is coupled to ground. Resistor  201  may take a value of 1 kΩ. The gate connection of FET  200 B is also coupled to ground. The source connection of FET  200 A is also coupled to capacitor  202 , which may take a value of 10 nF and is also coupled to ground. 
         [0013]    The source connection of FET  200 B is coupled to resistor  203 , which may take a value of 3.9 kΩ. Resistor  203  is also coupled to the output of feedback oscillator  104 . The source of FET  200 B is also coupled to capacitor  204  which is in turn coupled to the gate connection of FET  200 A. Capacitor  204  may take a value of 4.7 pF. The gate connector of FET  200 A is also coupled to variable capacitor  205  which is in turn coupled to ground. Variable capacitor  205  may take a nominal value of 220 pF. The variable capacitor  205  may be used to adjust the frequency of oscillation of marginal oscillator  101 . Loop antenna  102  is coupled in series between the gate connector of FET  200 A and the gate connector of FET  200 B. The gate connector of FET  200 A acts as the output of oscillator  101  which is fed to rectifier  103 . Rectifier  103  may be a 5082-2835 diode, as manufactured by various companies. The rectifier  103  converts the output of marginal oscillator  101  to a DC voltage. The output of rectifier  103  is fed to feedback amplifier  104  and to detector amplifier  105 . 
         [0014]    Feedback amplifier  104  may comprise a TLC271 operational amplifier  206 . A 1 μF capacitor  207  is coupled between the inverting input (pin 2) and the output (pin 6) of amplifier  206 . A resistor  208  (e.g., 10 MΩ) is coupled in series between the rectifier  103  output and the inverting input of the operation amplifier  206 . The non-inverting input of the operational amplifier  206  (e.g., pin 3) is coupled to a potential divider consisting of resistor  209  (e.g., 10 kΩ) and a resistor  210  (e.g., 1 kΩ. Resistor  209  is coupled to the VCC and the resistor  210  is coupled to ground. The output of the rectifier  103  is also coupled to ground by the parallel combination of resistor 211 9e.g., 10 MΩ) and the capacitor  212  (e.g., 1 nF). The feedback amplifier  104  forms a very slow control loop which has a predetermined time constant (e.g., approximetly 10 seconds). The value of the time constant is set by resistor  208  and capacitor  207 . The nominal oscillator output level is set by the value of the potential divider combination of resistors  209  and  210 . 
         [0015]    The output of rectifier  103  is also connected to detector amplifier  105 . Detector amplifier  105  comprises an operational amplifier  213  which is may also be a TLC271 operational amplifier. A resistor  214  (e.g., 100 kΩ) is coupled between the inverting input of operational amplifier  213  (e.g., pin 2) and the output of the operational amplifier (e.g., pin 6). The inverting input of operational amplifier  213  is also coupled to ground via 1 kΩ resistor  215  (e.g., 1 kΩ). The output of rectifier  103  is coupled to the non-inverting input of operational amplifier  213  (e.g., pin 3). The output of operational amplifier  213  (e.g., pin 6) is the output of detector amplifier  105  and is coupled to the voltage-to-frequency converter  106 . 
         [0016]    Voltage-to-frequency converter  106  comprises a integrated phase-lock loop circuit  216 . In this example, the phase-lock loop circuit  216  may be an HEF4046 phase-lock loop integrated circuit. A capacitor  217  (e.g., 22 nF) may be coupled between pins 6 and 7 of the HEF4046 integrated circuit. Pin 16 the HEF4046 integrated circuit may be connected to VCC. The output of detector amplifier  105  may be coupled to pin 9 of the HEF4046 integrated circuit  216 . Pins 3, 5, 8 and 14 of the HEF4046 integrated may all be coupled to ground. Pin 11 may be coupled to ground via resistor  218  (e.g., 10 kΩ). Pin 4 of the HEF4046 integrated circuit may be the circuit output which is fed to the tone generator  107 . 
         [0017]    The operation of the buried object detector  100  will now be described. The loop antenna  102  forms part of a resonant circuit with the marginal oscillator  101 . The feedback amplifier  104  is adjusted so that the DC power level being fed to FET  200 B is set to fix the oscillator at a nominal output level. When a lossy object is placed near the loop antenna  102 , the Q factor of the circuit is reduced and the output of the marginal oscillator  101  dips for a few seconds before the feedback amplifier  104  compensates. The dip in voltage supplied to the voltage-to-frequency converter  106  causes a change in frequency supplied to the tone generator  107 , thereby alerting a user to the presence of an object. 
         [0018]    The device operates by monitoring the absorption of a radio frequency magnetic field which is generated by the loop antenna  102  and the marginal oscillator  101 . A buried wire or pipe is tightly coupled to the ground around it at RF frequencies. The ground has a resistive loss at RF frequencies and therefore absorbs a proportion of any RF signal carried by a wire or pipe. A tuned loop (such as loop antenna  102 ) placed above the ground in which a wire or pipe is buried, magnetically couples with the wire or pipe. The resistive loss in the ground is transferred via the wire or pipe to the tuned loop. As a result, the quality factor of the tuned loop is reduced by the loss and the impedance across the ends of the tuned loop is reduced at its resonant frequency. Magnetic coupling occurs with both metallic and non-metallic objects, and the device may therefore be used to detect both metallic and non-metallic objects. The operating point of the marginal oscillator  101  is adjusted to be near its oscillation threshold by controlling its feedback gain. One feature of a marginal oscillator is that its oscillation level is extremely sensitive to the quality factor (Q) of the tank circuits (in this case the feedback amplifier  104 ) controlling the frequency. Absorption of the magnetic field can therefore be measured by monitoring the input power required to maintain oscillation as the loop antenna  102  is scanned over a lossy object, such as a pipe or wire. 
         [0019]    A marginal oscillator is one example of how the present invention may be implemented. A buried object detector in accordance with an embodiment of the present invention may detect an object by directly measuring changes in certain measurable parameters, such as the impedance across the ends of the tuned loop, the RF voltage across the tuned loop, or the current running through it. It will be appreciated by the skilled person that various changes and modifications may be made to the buried object detector within the scope of the claims. 
         [0020]    The detector  100  is most sensitive to lossy non-metallic objects when the object is at the centre of the loop antenna  102 . However, the detector  100  is most sensitive to buried metallic wires or pipes when the loop is at right angles to the plane of the ground and in line with the pipe or wire. In this position, the loop is least sensitive to losses in the ground itself and therefore it is a preferred configuration for pipe and wire detection. Even a short piece of buried pipe or wire is affected by the surrounding lossy ground and therefore absorbs some of the RF power that is received from the detector loop  102 . 
         [0021]    The marginal oscillator  101  is extremely sensitive to all losses. The performance of the buried object detector  100  is therefore determined mainly by the discrimination between the loss due to the target and the loss due to the wall or ground. Examples of effects that can be used to achieve more discrimination are given below. 
         [0022]    Stepping the frequency of the marginal oscillator  101  over a range of frequencies can supply additional information to discriminate between different non-metallic materials and the ground or wall. The loss characteristics or loss profile of each material varies differently with frequency. Sweeping widely over a buried object tends to produce sharp changes in the signal as the object is swept over and much slower changes from the surrounding ground. Discrimination can therefore be obtained just by filtering the detector output or shaping the time constant of the oscillator control loop in order to favour faster changing signals. 
         [0023]    The detection of a buried wire or pipe is sensitive to the orientation of the antenna loop  102 . If the plane of the loop is at right angles to the pipe or wire, there is a complete null in the signal and therefore no detection. In a further embodiment, the loop antenna may be mounted on a mechanically rotating disc. The detector  100  therefore produces a signal modulated at the rotation rate. Simple signal processing can easily extract the modulation. 
         [0024]    In an alternative embodiment, two crossed antenna loops may be used. Each loop is connected to a separate detector system, each operating on slightly different frequencies. The output from the two detectors may then be added or subtracted to reduce the effects orientation or ground effects. Orientation effects can also be reduced using two crossed loops that form two independent tuned circuits coupled together, both feeding a single system. The main requirement of the latter option is that the RF signals radiated from the loops should be at phase quadrature to one another to prevent cancellation (at some loop orientations) within the pipe/wire. This can be achieved by connecting one tuned loop directly to the marginal oscillator and energising the second tuned loop with light inductive coupling to the first. 
         [0025]    The systems described above may have problems with unwanted detection of long grass and poor performance in very wet ground. Poor electromagnetic coupling in the presence of local high level signals is also a problem. An alternative approach is to use a horizontal loop. Loss detection due to coupling between a horizontal loop and a wire is at a maximum when the loop (in any orientation) is placed either side of the wire. There is a distinctive very sharp null in the absorption when the loop is symmetrically over the top of the wire. Unfortunately, in this arrangement the loop is also very sensitive to the losses in the ground, with or without the target, and therefore, except at very short range, the detector performs badly because ground effects mask the wire loss. One method of reducing all these limitations is to make the detector sensitive only to the null when the centre of the horizontal loop passes over the wire. This may be done as follows. 
         [0026]    The electrical centre of the loop antenna could be rapidly scanned, electronically or mechanically, in a circular pattern. There are several ways of doing this. The circular scan could be easily achieved at a rate of at least a few tens of rotations per second. Using this system, when the loop is directly over a buried wire, the absorption null is passed twice for each complete revolution of the loop centre. The oscillator therefore produces an output modulated at double the frequency of the rotation, due only to the presence of the null. Ground and other unwanted effects due not produce a sharp null and only produce modulation at the fundamental frequency of the rotation. Note that the output from the detector can be a correlation of several rotations as the detector passes over the wire in a broader sweep of the ground. This results in much improved sensitivity.  FIG. 3  shows a block diagram of a detector using a simple mechanical scan. A rotating disc  300  is rotatable by means of motor  301 . The output of the rectifier is passed through bandpass filter  302 . 
         [0027]    The marginal oscillator is a very sensitive device and it includes a large loop antenna. It is therefore very vulnerable to interference from strong local signals at any frequency. The oscillator circuit should therefore include filters to suppress signals outside its operating frequency range. An example of how this can be done is shown in  FIG. 4 . A coil  400  is connected across the tuned loop antenna, forming part of the resonant circuit. Two loops  401 A,  401 B are placed either side of the coil such that any coupling between the loops is predominantly via the resonant circuit. The loops are connected to the input and output of an amplifier  403  via bandpass filters  402 A,  402 B that are centred on the resonant frequency of the loop system. Oscillation occurs at the frequency of maximum coupling between the input and output of the amplifier (i.e. the resonant frequency of the loop system) provided that the input signal to the amplifier is in phase with its output. The polarity of the two loops and the delay characteristics of the filters must be taken into account in the circuit design to achieve this. 
         [0028]    While the present invention has been described in connection with the detection of wires and pipes buried in the ground, it may also be used to detect other objects buried in a lossy material such as the ground. For example, it may be used to detect objects buried within the walls of a building.