The general forms of most metal detectors that interrogate soil for detecting an electrically conducting target are either hand-held battery-operated units, conveyor-mounted units, or vehicle-mounted units. Examples of hand-held products include detectors used to locate gold, explosive landmines or ordnance, coins and treasure. Examples of conveyor-mounted units include fine gold detectors in ore mining operations, and an example of a vehicle-mounted unit includes a unit to locate buried land mines.
These electronic metal detectors usually consist of transmit electronics generating a repeating transmit signal cycle of a fundamental period, which is applied to an inductor, for example a transmit coil, which transmits a resulting alternating magnetic field sometimes referred to as a transmit magnetic field. Time domain metal detectors usually include switching electronics, within the transmit electronics, that switches various voltages from various power supplies to the transmit coil for various periods in the repeating transmit signal cycle.
Metal detectors contain receive electronics which processes a receive signal from a measured receive magnetic field to produce an indicator signal, the indicator signal at least indicating the presence of at least some metal targets within the influence of the transmit magnetic field.
Time domain metal detectors include pulse induction (PI) or PI-like metal detectors. It is possible for this technology to transmit and receive using the same inductor, often called a “mono-loop” coil. This requires some sort of transmit/receive switch, sometimes called a “T/R switch”.
There are several problems with the conventional T/R switches used in PI metal detectors. One common method that avoids a conventional T/R switch, is to connect a damping resistor of the transmit/receive coil (mono-loop) to an inverting input of a low-noise preamplifier of a detector, wired up so this input is a virtual earth. However, as this damping resistor is typically roughly 500Ω for typical suitable contemporary coils, the Johnson noise in this damping resistor is approximately 3 nV/sqrt(Hz), whereas the input noise of contemporary preamplifiers is about 1 nV/sqrt(Hz) and about 2 pA/sqrt(Hz), and hence the total input equivalent noise of this system is approximately 3.3 nV/sqrt(Hz). In contrast, if a low on resistance “isolating” T/R switch is connected between the transmit/receive coil and input to the preamplifier as is the case in some commercial products, the damping resistor being connected to ground, the total noise due to the T/R switch series resistance is typically insignificant, and hence for the above figures, a signal-to-noise ratio improvement of about 3 times is possible using a low on resistance isolating T/R switch rather than connecting the damping resistor to an inverting input of the preamplifier. The disadvantage of using an isolating T/R switch is that the turn-on charge injection pulse of the T/R switch induces another decaying signal, after the back-emf decaying signal, into the transmit/receive coil (mono-loop). This charge injection pulse first needs to decay to an acceptably low level before the commencement of synchronous demodulation or sampling during a receive period, and hence a system using a isolating T/R switch requires a longer delay between the cessation of transmission and commencement of demodulation compared to the arrangement with the damping resistor feeding to an input of the preamplifier.
This series T/R switch usually consists of small signal FETs that are in a switched off state during transmission, and in a switched on state during receive periods, once the back-emf has decayed to about a volt or less, thus switching the coil to the preamplifier.
While a nulled coil arrangement (such as the well-known “double D”) requires no isolating T/R switch, it is often desirable to use a transmit/receive mono-loop coil as it has a superior detection range.
It is often desirable to detect fast time constant metal targets such as fine gold nuggets, low-metal content mines containing small bits of metal, fine gold chains etc. In PI detectors, this requires that receive sampling or synchronous demodulation commence as soon as possible following a termination of the “back-emf” or transmit high-voltage period and the commencement of a receive period.
WO2009/155668 discloses PI-like waveforms where the transmit coil current is controlled to be approximately zero during a zero-voltage receive period, without the use of a T/R switch. However, that arrangement has to use a separate transmit and receive coil, as the transmit coil is always being driven by the transmit electronics. Another limitation of that disclosed is that the zero current, during a zero-voltage receive period, is limited in accuracy by the accuracy of the electronics. The invention described herein may also produce zero transmit coil current during zero-voltage periods when a shunt T/R switch is switched off, but the arrangement may also facilitate the use of mono-loop coils where the receive and transmit windings are the same winding, rather than separate receive and transmit windings.