Buried object detector

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.

BACKGROUND

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

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.

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.

Further aspects are described below.

DETAILED DESCRIPTION

FIG. 1shows the components of a buried object detector100in accordance with a first embodiment of the present invention. The buried object detector100includes a marginal oscillator101. The marginal oscillator101is coupled to a loop antenna102. Together, the marginal oscillator101and the loop antenna102form 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).

An output of the marginal oscillator101is fed to a rectifier103. An output of the rectifier103is coupled to feedback amplifier104. The feedback amplifier104acts as a slow control loop for marginal oscillator101. An output of the rectifier103is also fed to detector amplifier105. The detector amplifier105is coupled to voltage-to-frequency converter106which is in turn coupled to tone generator107.

FIG. 2is a circuit diagram for the buried object detector100. The marginal oscillator101comprises two FET devices200A and200B. A suitable FET for this application is the BF245B MOSFET which is produced by various manufacturers. The source of FET200A is coupled to the common collector voltage (VCC). The drain of FET200A and the drain of FET200B are coupled to each other and to resistor201which is coupled to ground. Resistor201may take a value of 1 kΩ. The gate connection of FET200B is also coupled to ground. The source connection of FET200A is also coupled to capacitor202, which may take a value of 10 nF and is also coupled to ground.

The source connection of FET200B is coupled to resistor203, which may take a value of 3.9 kΩ. Resistor203is also coupled to the output of feedback oscillator104. The source of FET200B is also coupled to capacitor204which is in turn coupled to the gate connection of FET200A. Capacitor204may take a value of 4.7 pF. The gate connector of FET200A is also coupled to variable capacitor205which is in turn coupled to ground. Variable capacitor205may take a nominal value of 220 pF. The variable capacitor205may be used to adjust the frequency of oscillation of marginal oscillator101. Loop antenna102is coupled in series between the gate connector of FET200A and the gate connector of FET200B. The gate connector of FET200A acts as the output of oscillator101which is fed to rectifier103. Rectifier103may be a 5082-2835 diode, as manufactured by various companies. The rectifier103converts the output of marginal oscillator101to a DC voltage. The output of rectifier103is fed to feedback amplifier104and to detector amplifier105.

Feedback amplifier104may comprise a TLC271 operational amplifier206. A 1 μF capacitor207is coupled between the inverting input (pin 2) and the output (pin 6) of amplifier206. A resistor208(e.g., 10 MΩ) is coupled in series between the rectifier103output and the inverting input of the operation amplifier206. The non-inverting input of the operational amplifier206(e.g., pin 3) is coupled to a potential divider consisting of resistor209(e.g., 10 kΩ) and a resistor210(e.g., 1 kΩ. Resistor209is coupled to the VCC and the resistor210is coupled to ground. The output of the rectifier103is also coupled to ground by the parallel combination of resistor2119 e.g., 10 MΩ) and the capacitor212(e.g., 1 nF). The feedback amplifier104forms 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 resistor208and capacitor207. The nominal oscillator output level is set by the value of the potential divider combination of resistors209and210.

The output of rectifier103is also connected to detector amplifier105. Detector amplifier105comprises an operational amplifier213which is may also be a TLC271 operational amplifier. A resistor214(e.g., 100 kΩ) is coupled between the inverting input of operational amplifier213(e.g., pin 2) and the output of the operational amplifier (e.g., pin 6). The inverting input of operational amplifier213is also coupled to ground via 1 kΩ resistor215(e.g., 1 kΩ). The output of rectifier103is coupled to the non-inverting input of operational amplifier213(e.g., pin 3). The output of operational amplifier213(e.g., pin 6) is the output of detector amplifier105and is coupled to the voltage-to-frequency converter106.

Voltage-to-frequency converter106comprises a integrated phase-lock loop circuit216. In this example, the phase-lock loop circuit216may be an HEF4046 phase-lock loop integrated circuit. A capacitor217(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 amplifier105may be coupled to pin 9 of the HEF4046 integrated circuit216. Pins 3, 5, 8 and 14 of the HEF4046 integrated may all be coupled to ground. Pin 11 may be coupled to ground via resistor218(e.g., 10 kΩ). Pin 4 of the HEF4046 integrated circuit may be the circuit output which is fed to the tone generator107.

The operation of the buried object detector100will now be described. The loop antenna102forms part of a resonant circuit with the marginal oscillator101. The feedback amplifier104is adjusted so that the DC power level being fed to FET200B is set to fix the oscillator at a nominal output level. When a lossy object is placed near the loop antenna102, the Q factor of the circuit is reduced and the output of the marginal oscillator101dips for a few seconds before the feedback amplifier104compensates. The dip in voltage supplied to the voltage-to-frequency converter106causes a change in frequency supplied to the tone generator107, thereby alerting a user to the presence of an object.

The device operates by monitoring the absorption of a radio frequency magnetic field which is generated by the loop antenna102and the marginal oscillator101. 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 antenna102) 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 oscillator101is 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 amplifier104) controlling the frequency. Absorption of the magnetic field can therefore be measured by monitoring the input power required to maintain oscillation as the loop antenna102is scanned over a lossy object, such as a pipe or wire.

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.

The detector100is most sensitive to lossy non-metallic objects when the object is at the centre of the loop antenna102. However, the detector100is 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 loop102.

The marginal oscillator101is extremely sensitive to all losses. The performance of the buried object detector100is 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.

Stepping the frequency of the marginal oscillator101over 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.

The detection of a buried wire or pipe is sensitive to the orientation of the antenna loop102. 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 detector100therefore produces a signal modulated at the rotation rate. Simple signal processing can easily extract the modulation.

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.

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.

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. 3shows a block diagram of a detector using a simple mechanical scan. A rotating disc300is rotatable by means of motor301. The output of the rectifier is passed through bandpass filter302.

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 inFIG. 4. A coil400is connected across the tuned loop antenna, forming part of the resonant circuit. Two loops401A,401B 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 amplifier403via bandpass filters402A,402B 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.

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.