PI Detectors
However the PI works on a different principal than the IB detectors.
A PI detector utilising search head consisting of one loop, works (in a few words) in the following way;
The loop in the search head is used to transmit high voltage (few hundred volts) pulses. After each transmitted pulse the
electronic circuit measures the residual signal left in the loop after the pulse. That residual signal will decay in certain manner.
The most important characteristic of the residual signal is the time constant of the decay.
If a metal target is placed in front of the loop the time constant of the decay will change as a result of the same Eddy currents
described in the case with IB detectors. In this case the Eddy currents will have pulse characteristic as the transmitted signals.
However the basic principal stands, meaning that the metal target placed in some proximity to the loop will alter to some degree
the electro-magnetic properties of the loop, which will cause different decay in the residual signal in the loop.
Again, the most important of all characteristics of the mentioned loop is the Q.
The Q factor will define the capability of any loop (coil) to transmit or receive any signals.
The Q of any loop can be maximized in only one way; by using a capacitor with a calculated value, which will form with the loop a
resonance circuit at the desired operating frequency.
A loop tuned in resonance will always have the highest Q and best performance characteristics, compared with any other loop,
which is not tuned.
A loop can also be partially tuned, meaning that the capacitor value in the resonance circuit will be different than the calculated for
total resonance. In such a case the Q will still be better than a loop not tuned at all, but not as good as a loop tuned in total
resonance.
There is one other important aspect related to the type of search heads consisting of at least two loops or more, which has to be
mention before proceeding further:
This is the actual balance between the TX and RX loops.
If two loops, one TX and one RX, with constant Q for each one of them, are balanced to a minimum offset voltage in the RX at a
certain frequency, they will become a part of a constant electro-mechanical system.
If the RX loop is tuned completely to the frequency transmitted by the TX then the two loops will get in full electro-magnetic
resonance. In this situation the Q of the whole system will be at maximum, which means that the search coil will be with best
detection capabilities.
If the same system is tuned later to work at different frequency that will lead to loss of Q regardless if it is still tuned to a total
resonance or not. The reason for this phenomenon is quite simple. Ones a system of two loops is balanced and dipped in resin,
the loops can not be moved to a different mechanical position. In order to achieve maximum Q at various frequency settings every
two loop system will need some allowance for mechanical repositioning of the loops. This requirement comes as a result of the
fact that for every different frequency, at which the loops in any system are tuned to work they will exhibit different Q. From the well
established electro-technical laws it is known that the Q of a loop (coil) is factor of external and internal diameter of the loop,
cross section of the windings, number of turns and operating frequency. Since the diameter, the
cross section and the number of turns in a loop are mechanical constant, then the only factor which can seriously influence the Q
is the operating frequency. Another factor which can lead to change of the Q is temperature, but providing that metal detectors are
not often used in conditions with excessive thermal changes, we can accept that the temperature is a relatively small problem for
any loop.
How is all of the mentioned above related to comparison between different types of metal detectors?
Following the graphic image on the next page this article will give explanation about Multi Frequency Technology (MFT) type of
detectors.
Firstly, it is important to mention that amongst all detectors based on IB principal, the MFT has the lowest efficiency, concerning
depth penetration capabilities and discrimination accuracy.
The reasons:
Setting a search head to work on various frequencies will define low Q at all of the frequency setting. That will result in average to
poor depth penetration and unreliable discrimination.
Simultaneous frequency transmission from a TX loop is possible, but that kind of transmission will make the work of any IB
detector impossible. Also simultaneous frequency transmission will most certainly result in a very low Q for the whole system.
MFT can not offer any real practical advantage to the IB type of metal detectors as far as depth penetration and discrimination
accuracy is concerned.
However the PI works on a different principal than the IB detectors.
A PI detector utilising search head consisting of one loop, works (in a few words) in the following way;
The loop in the search head is used to transmit high voltage (few hundred volts) pulses. After each transmitted pulse the
electronic circuit measures the residual signal left in the loop after the pulse. That residual signal will decay in certain manner.
The most important characteristic of the residual signal is the time constant of the decay.
If a metal target is placed in front of the loop the time constant of the decay will change as a result of the same Eddy currents
described in the case with IB detectors. In this case the Eddy currents will have pulse characteristic as the transmitted signals.
However the basic principal stands, meaning that the metal target placed in some proximity to the loop will alter to some degree
the electro-magnetic properties of the loop, which will cause different decay in the residual signal in the loop.
Again, the most important of all characteristics of the mentioned loop is the Q.
The Q factor will define the capability of any loop (coil) to transmit or receive any signals.
The Q of any loop can be maximized in only one way; by using a capacitor with a calculated value, which will form with the loop a
resonance circuit at the desired operating frequency.
A loop tuned in resonance will always have the highest Q and best performance characteristics, compared with any other loop,
which is not tuned.
A loop can also be partially tuned, meaning that the capacitor value in the resonance circuit will be different than the calculated for
total resonance. In such a case the Q will still be better than a loop not tuned at all, but not as good as a loop tuned in total
resonance.
There is one other important aspect related to the type of search heads consisting of at least two loops or more, which has to be
mention before proceeding further:
This is the actual balance between the TX and RX loops.
If two loops, one TX and one RX, with constant Q for each one of them, are balanced to a minimum offset voltage in the RX at a
certain frequency, they will become a part of a constant electro-mechanical system.
If the RX loop is tuned completely to the frequency transmitted by the TX then the two loops will get in full electro-magnetic
resonance. In this situation the Q of the whole system will be at maximum, which means that the search coil will be with best
detection capabilities.
If the same system is tuned later to work at different frequency that will lead to loss of Q regardless if it is still tuned to a total
resonance or not. The reason for this phenomenon is quite simple. Ones a system of two loops is balanced and dipped in resin,
the loops can not be moved to a different mechanical position. In order to achieve maximum Q at various frequency settings every
two loop system will need some allowance for mechanical repositioning of the loops. This requirement comes as a result of the
fact that for every different frequency, at which the loops in any system are tuned to work they will exhibit different Q. From the well
established electro-technical laws it is known that the Q of a loop (coil) is factor of external and internal diameter of the loop,
cross section of the windings, number of turns and operating frequency. Since the diameter, the
cross section and the number of turns in a loop are mechanical constant, then the only factor which can seriously influence the Q
is the operating frequency. Another factor which can lead to change of the Q is temperature, but providing that metal detectors are
not often used in conditions with excessive thermal changes, we can accept that the temperature is a relatively small problem for
any loop.
How is all of the mentioned above related to comparison between different types of metal detectors?
Following the graphic image on the next page this article will give explanation about Multi Frequency Technology (MFT) type of
detectors.
Firstly, it is important to mention that amongst all detectors based on IB principal, the MFT has the lowest efficiency, concerning
depth penetration capabilities and discrimination accuracy.
The reasons:
Setting a search head to work on various frequencies will define low Q at all of the frequency setting. That will result in average to
poor depth penetration and unreliable discrimination.
Simultaneous frequency transmission from a TX loop is possible, but that kind of transmission will make the work of any IB
detector impossible. Also simultaneous frequency transmission will most certainly result in a very low Q for the whole system.
MFT can not offer any real practical advantage to the IB type of metal detectors as far as depth penetration and discrimination
accuracy is concerned.
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