METAL DETECTORS - HOW DO THEY WORK? (3).
Sampling Circuit
The amplified signal coming from the receiver is connected to a switching
circuit which samples the reflected portion of the pulse as it reaches zero.
The reflected pulse up to this point references in actuality a series of
pulses at the transmit frequency. When a metal object nears the coil the
transmit portion of the signal will remain unchanged while the reflected
pulses at the transmit frequency. When a metal object nears the coil the
transmit portion of the signal will remain unchanged while the reflected
portion of the pulse will become wider. The metal object stores some of the
electrical energy from the transmit pulse and increases the time it takes
for the reflected pulse to reach zero. An increase in duration of a few
millionths of a second is enough to allow the detection of a metal target.
The reflected pulse is sampled with an electronic switch controlled by a
series of pulses which are synchronised with the transmitter.
The most sensitive sampling point on the reflected pulse is as near as
possible to the point where it reaches zero. This is typically about 20
millionths of a second after the transmitter shuts off and the reflected
pulse begins. Unfortunately, this is also the area where a PI can become
unstable. For this reason most PI models sample the reflected pulse at a
decay of 30 or 40 millionths of a second, well after it decays to zero.
Integrator
In order for an object to be detected the sample signals must be converted
to a DC voltage. This task is performed by a circuit called an integrator.
It averages the sampled pulses over time to provide a reference voltage.
This DC reference voltage increases when metal nears the coil, then
decreases as the object moves away. The DC voltage is amplified and controls
the audio output circuitry which increases in pitch and/or volume to signal
the presents of metal.
The time constant of the integrator determines how quickly the metal
detector will respond to a metal object. A long time constant (in the range
of seconds) has the advantage of reducing noise and making the metal
detector easier to tune. Long time constants require a very slow sweep of
the coil because a target might be missed if it passes quickly by the search
coil. Short time constants (in the range of tenths of a second) respond more
quickly to targets. This allows a quicker sweep of the loop however, it also
allows more noise and instability.
Discrimination
PI metal detectors are not capable of the same degree of discrimination as
VLF metal detectors.
By increasing the time period between transmitter shut-off and the sampling
point (pulse delay), certain metal items can be rejected. Aluminium foil
will be the first to be rejected followed by nickel, pull tabs and gold.
Some coins can be rejected at very long sample delays however, iron cannot
be rejected.
There have been many attempts to design a PI that can reject iron however
these attempts have had limited results. Iron is detectable at very long
time delays however, silver and copper have similar characteristics. Such
long time delays also have a negative affect on detection depth. Ground
mineralization will cause some widening of the reflected pulse as well,
changing the point at which a target responds or rejects. If the time delay
is adjusted so that a gold ring doesn't respond in an air test, that same
ring may respond in mineralised ground. Mineralised ground thus changes
everything regarding the time delays and discrimination of PI metal
detectors.
Ground Balance
Ground balancing, while very critical on VLF metal detectors, is not
necessary with PI circuits. Average ground mineralization will not store any
appreciable amount of energy from the search coil and will not usually
produce a signal. Such ground will not mask the signal from a buried object.
On the contrary, ground mineralization will add slightly to the duration of
the reflected pulse increasing the depth of detection. The term "automatic
ground balance" is often applied to PI instruments because it will normally
not react to mineralization and there are no external adjustments for any
specific ground conditions.