Pulse Induction Metal Detector - 2

Pulse Induction Metal Detector - 2
Pulse Induction Metal Detector - 2
by J. A. Corbyn
The bandpass amplifier in Fig. 11 extracts possible signals from background noise caused mainly by transients in the circuits.
To permit a gain of up to 8000, a narrow pass-band from 0.2 to 0.6 Hz is used with a high-order filter for sharp roll-off. The
circuit also has a limited overshoot with a step function as shown in Fig. 12.
The output is displayed by a voltmeter, and an audible signal is provided by
amplitude modulating a 1400 Hz oscillator for positive signals and a 900 Hz
oscillator for negative signals, see Fig. 13. All of the main timing pulses are
generated by the circuit in Fig. 14. The prototype used a variable c.m.o.s. RC
oscillator with four switched ranges of 40 to 175µs, 160 to 700µs, 640 to 2800µs
and 2.56 to 11.2ms for ∆t. The oscillator drives a counter and decoder which
provide a division of 32 and produce the following waveforms:
A, the receive interval with a duration of 6x∆t and separated from the on pulse by
B, a reversing signal for the synchronous detector and also used to provide the
two on pulses.
C, the last period of the receive interval.
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D and E, alternating on waveforms
to provide the magnetic field pulses.
D and E drive two pulse generators
as shown in Fig. 15 which, with a
BU 326A non-saturating common
emitter output, can supply up to
1.5A. Two transmit coils were used
in the prototype because a rugged
high-voltage p-n-p transistor was
not available at a reasonable price.
The regulated power supply is
shown in Fig. 16. As well as the
capacitors shown, extra decoupling
should be provided on each circuit.
Construction of the metal detector
is not critical and the prototype was
built in module form with jack plugs
and sockets for interconnections.
Selection of damping resistors for
the transmit and receive coils is best
carried out with an oscilloscope,
although I found that the values
chosen were generally in agreement with the theoretical values.
Fig. 14 Generation of timing
waveforms. Due to availability, the
prototype used cmos and ttl
components with a single
transistor interface. One logic
family can be used and the
interface omitted. The complete
circuit is powered from the 7805
regulator and all outputs are
protected by series resistors.
This metal detector is essentially dynamic because it only responds to a target when it is moving in relation to it. In practice this
system is better than the static type because any maladjustments, in connection with the magnetic viscosity effects, are not
important with a reasonably uniform ground. Slow variations of amplifier offsets are also unimportant.
Due to magnetic viscosity effects and possible feedback loops, metal detectors need to be tested in operation to determine their
sensitivity. A 600 mm radius coil assembly, as shown in Fig. 5, satisfactorily detected a piece of brass 50 mm in diameter at a
depth of 750 mm, and a 15 mm diameter brass target at a depth of 50 mm. In both cases a peak transmit current of 1A was used
with a ∆t of 250 µs and a ground speed of 1 m/s.
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